Not just a pretty face...

 

Technical descriptions of UK test cards

IF YOU'VE arrived here from Test Cards, Tuning Signals, Idents and Clocks zip up your anorak and prepare to get your teeth into some real technical stuff. Each of the elements in a well-designed test card has a reason for being there and this page, which includes extracts from BBC and IBA information sheets and articles derived therefrom, attempts to explain exactly what their purpose is.

Please note that some descriptions on these pages were written a decade or so before the UK analogue terrestrial network closedown in October 2012, when digital transmission and widescreen were still in their infancy, and most tellies were traditional 4:3 standard definition crt models. The almost universal adoption of 16:9 HD flat screen displays has made much of what I wrote about the practical aspects of resolution and aspect ratio obsolete, but I have avoided re-editing the text in order to give a flavour of the confusion that reigned in that era.

A note on nomenclature: 'Test Cards' are cards or transparencies placed in front of a camera or in a telecine machine containing multiple tests of that equipment or other equipment downstream. 'Test Patterns' are electronically-generated waveforms usually designed to test a single parameter. Of course modern test cards are in the main generated purely electronically, but if they look like cards, I call them 'Cards'. Similarly, 'Tuning Signals' were either simple electronic waveforms or simple cards/transparencies. I call them all 'Signals'.

It's just the way I am.


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Contents

 

Not just a pretty face

Test Card History

Non-UK Test Cards


|Top | Aspect Ratio | Resolution | SMPTE | C | D & E | F | PM5544 | ETP1 | J, W & HD | Colour Bars | Bookmarks |

  Test Card C was introduced in January 1948, when the transmission aspect ratio was still 5:4, and it was based on a 10x8 square grid pattern.

Test Card C 5:4

  When it was redesigned in preparation for the change to 4:3 on 3 April 1950 the basic chart remained the same, while the top and bottom castellations were moved nearer to the centre and the corner gratings were repositioned slightly so that they still lay upon the diagonals, as they did in the 5:4 chart.

Test Card C 4:3

Aspect Ratio

 

THE ASPECT ratio of a picture is its width compared to its height. A television picture is made up of a number of parallel lines created when a spot of light moves from side to side and down the screen to form a 'raster'. The waveforms that create this movement can be varied independently in amplitude - that is, the height and width of the picture can be varied. As long as the relationship between height and width of the raster in the receiver is the same as in the camera, the picture will be the correct shape and circles will be displayed as circles.

The book 'Television Up-To-Date', by R.W.Hutchinson, published in 1935, second edition 1937, states that the aspect ratio of the EMI 405-line standard was set at 5:4, the same as half-plate photographs, while the Baird 240-line standard used the slightly wider 4:3 (the so-called Academy Ratio used by the film industry). According to Edward Pawley, the BBC's technical historian, in 'BBC Engineering 1922-1972' the 5:4 aspect ratio of the 405-line standard was changed on 3 April 1950 to 4:3.

It is not clear how this change was effected. The original specification for the 405-line standard had a blanking period per field of 'at least ten lines', but later it was standardised at 14 lines. If this change was made at the same time as the aspect ratio change it doesn't account for it entirely. Reducing the number of active lines per frame from 385 to 377 gives a change in aspect ratio of 1.25:1 (5:4) to 1.28:1 rather than 1.33:1 (4:3). The number of active lines on that basis would have to be 361, so it has to be assumed that the camera scan amplitudes were adjusted as well.

Theoretically all the domestic receivers would also have needed adjustment, but in practice the stability of the circuitry of the day meant that the height and width would vary by more than that in an evening's viewing. However, television display tubes (cathode ray tubes, or 'crts') continued to be made to the old 5:4 aspect ratio until the nineteen seventies, when the 'flatter squarer tubes' started to make an appearance. That meant that the 'sixties set had to be adjusted either to give the correct width and reduced height, or correct height and excessive width. In order to avoid black bars appearing at the top and bottom, the latter approach was used.

Test Card F has castellations with arrows to enable the dealer to adjust the scan amplitudes and thus ensure that the picture just fills the screen whilst circles remain the correct shape.

Test Card F on a 5:4 tv On the left is the display of a correctly adjusted receiver having a 5:4 aspect ratio crt from the sixties... Test Card F on a 4:3 tv
..and on the right a 4:3 tube from the seventies. The slightly wider screen allows the castellations down the sides to be displayed as well as those along the top and bottom.

I was surprised to find that few of the BBC test cards that I have unearthed dating from before 1950 show signs of having been designed for the 5:4 aspect ratio. Indeed two, dating from 1936 and 1949 complete with circles and castellations, are most definitely 4:3 in aspect ratio and they cannot be cropped or reshaped to 5:4 without producing an elliptical circle or missing castellations. I have also found adverts for tv sets and display tubes from the period 1936-48 that quote picture sizes as 10"x7.5", etc; in other words 4:3, despite the tubes themselves being circular or perhaps 5:4 in shape.




What of 15:9?

15:9 is the aspect ratio of Super 16mm flm stock which has been used widely (pun intended) in television for many years and it was the original aspect ratio proposed in the 1980s at around the time widescreen MAC (multiplexed analogue component) pictures from DBS (direct broadcasting by satellite) were being developed. Later, the specification was changed to 16:9, which also became the standard for PALPlus and digital anamorphic transmissions


It would appear that many of the smaller (up to 22-inch) 'widescreen' LCD (liquid crystal display) television screens currently available, (as well as 'widescreen' computer monitors and laptops) are in fact 16:10 - somewhere between 14:9 and 15:9 - something that can be verified before purchase if the pixel resolution is known, since they use square pixels and the numbers in each direction need to be in the ratio 16:9 if 16F16 pictures are to be correctly diplayed without cropping, stretching or letterboxing. Proper 16:9 screens have pixel counts of 1366x768 or 1920x1080. Those having 1440x900 or 1680x1050 pixels are 14.4:9 and not 16:9.

It seems sad that having suffered 14L12 pictures on analogue telly for two decades, with only 4:3 or 16:9 tellies to display then on, now that the analogue transmissions have been closed down, we are being offered odd-shaped tellies on which to display 16:9 or 4:3 digital pictures.

At the start of the twenty-first century there are currently two aspect ratios (and therefore shapes of display screen) in common use - 4:3 (which can also be expressed as 12:9 or 16:12) and the so-called widescreen 16:9. That is the ratio that defines the shape of the raster (that is the grid of lines scanned out by the electron beam), and on many modern sets the raster can be displayed in either shape, resulting in deep black bars along the top and bottom of a 'square' set or down each side of a 'widescreen' set when the 'wrong' shaped raster is displayed. Since black bars are now in fashion, the broadcasters use them to blank out parts of the 12:9 or 16:9 coded frame in order to present more (or in some cases less) of the original picture on both traditional and widescreen sets.

There is a shorthand way of describing the aspect ratio of a television picture in the form of a letter between two numbers. The first number defines the shape of the picture (4:3, 14:9 or 16:9) and is the numerator of a fraction whose denominator is 9. The letter indicates whether the picture fills the screen (F), is letterboxed with black bars top and bottom (L) or is pillarboxed with black bars left and right (P). The second number indicates the shape of the coded frame and is 12 for the traditional 4:3 (12:9) shape and 16 for the new widescreen 16:9 'anamorphic' shape. Shapes currently used in the UK are 12F12, 12P16, 14L12, 14P16, 16L12 and 16F16.

5% and 10% overscan templates Overscanning is something that his been with us throughout the history of television, but it has started to confuse the issue with multiple aspect ratios. A professional-grade television monitor (and indeed most computer visual display units, now also called monitors) are designed to project a picture (called a raster) that is wholly displayed within the face of the display tube, whatever the shape of either may be, so that the whole of the broadcast picture is visible. Domestic receivers however, almost invariably project a raster that is larger than the display screen and a significant border of the transmitted picture may be cut off by the edge of the tube or a mask surrounding the front of the tube. The reasons for this are manifold: early cathode ray tubes (crts) were small and rounded (or indeed circular) and so to obtain the largest possible picture consistent with displaying most of what was broadcast a degree of overscan was employed. Also, circuits were not very stable to begin with, and height, width, centring and linearity were all liable to vary over a few hours. Overscanning helps to avoid the danger of black bars appearing at the edge of the picture - which would almost certainly involve a call-out to put right.

This was all very well when the transmitted picture was roughly the same shape as the screen, but today we have two shapes of screen and many more shapes of picture. The raster is still overscanned, but because the picture transmitted sometimes doesn't reach the edges of the raster - the broadcaster adds an area of black as padding - one pair of edges gets cropped while the other pair doesn't, which results in a displayed picture that has a different aspect ratio from the transmitted one (even if the geometry is correct - that is circles appear circular). This means that in the case of 12P16 pictures the digital widescreen viewer sees the full width of the original picture but perhaps not the full height, whereas with 16L12 pictures the analogue viewer sees the full height of the original picture but probably not the full width. Of course in the case of 14L12/14P16 the original 16:9/4:3 pictures are cropped for width/height by the broadcaster.

Digital widescreen viewers may notice that the pictures in 12P16 (eg sports from overseas) and 14P16 (eg news from overseas) appear wider than their 12F12 and 14L12 analogue counterparts and assume that the broadcasters are somehow cutting out something at each end of the lines in the latter cases, but this is not so. Setmakers have become so fond of overscanning that they have employed a trick to use it when a widescreen set is displaying a reduced-width raster, such as would be used for 12F12 or 14L12. They electrically chop a margin off the left and right of the picture (called 'blanking') so that less than the whole width is displayed. This gives a vertical straight edge to the picture with no danger of 'foldover' which might be visible if blanking were not used.

Safe title area

The overscan on a domestic receiver can be in the order of 5%-10%. For this reason broadcasters specify a 'graphics safe' area within which the text of captions should appear. The 'forbidden zone' was originally made rather generous to allow for the excessive overscan (and curved display tubes) found in early television sets but it has been relaxed in recent years. In the graphic on the left, the magenta inner curved line shows the Safe Title Area specified in 1966, and the outer cyan rectangle is the 1998 Graphics Safe Area for 4:3 pictures.

The following graphics represent how some currently transmitted formats in the UK might appear on widescreen and 4:3 sets of equivalent size (32" 16:9 and 25" 4:3 models for example). I have added a template to indicate the visible area with 5% and 10% overscanning. 12F12, 14L12 and 16L12 pictures displayed on a widescreen set will have the same cut-off as shown for the 4:3 set. The 14:9 pictures are instructive - with 10% overscan, no black bars are visible so that such 4:3 viewers rarely notice that most shows are now 14L12, and many 16:9 viewers think that 14P16 shows are true widescreen.

Video frequencies and line standards

On this page and elsewhere I have calculated the frequencies of the gratings of test cards displayed on various line standards. People seem to get in a twist about this, trying to work out how the different scanning frequencies affect the video frequency. In fact it is very simple: the frequencies on the two standards are in the same ratio as the active line periods (not the total line period, or line frequency). In fact this is quite intuitive. Imagine a pattern of vertical lines being scanned by a multi-standard camera. The frequency generated by scanning the lines is dependent purely on the speed of the scanning spot, in other words the time it takes to traverse the pattern for each standard the camera is switched to.

For reference, the active line periods of various line standards are as follows:
405/50: 80.3µs
525/60: 52.9µs
625/50: 51.95µs
819/50: 39.44µs
720p: 17.24µs
1080i: 25.86µs
1080p: 12.93µs

The same rationale may be applied to aspect ratio conversion of pictures using the same line standard, so with a 12F12 picture converted to 12P16 the frequencies would be higher by a factor of 16/12, and for a 16F16 picture converted to 14L12, they would be lower by a factor of 14/16.

Since the cards I've used to demonstrate the aspect ratios below have frequency gratings that have been scaled, I've shown the actual frequencies in MHz that would be found in the pictures shown. Bear in mind that although the theoretical maximum frequency transmitted in the digital domain is 6.75MHz, in analogue System I the highest luminance frequency carried (for example by the PAL encoded composite baseband or uhf output of a set-top box) would be 5.5MHz. The figures therefore give some idea of the amount of original detail lost in the Aspect Ratio Conversion (ARCing).

Test Card J on a widescreen tv [2.00] [3.33] [4.67] [5.33] [6.00] [7.00]

12P16 (AFD=0, 1 or 7)

A 4:3 picture coded into a 16:9 frame (also as a 4:3 12F12 transmission should be correctly displayed on a widescreen set). 4:3 action contributions to BBC 16:9 sports shows (eg Grandstand) often appear like this, with 16F16 half-time expert analysis. If the show is ARCed to anything other than 12F12 for analogue broadcast, the result on a 4:3 set is a border all around as shown in "Floaters" below.

Test Card W letterboxed on a 4:3 tv [2.00] [3.33] [4.00] [4.67] [5.33] [6.00]

16L12 (AFD=2)

A 16:9 picture coded into a 4:3 frame (as widescreen feature films are sometimes transmitted on analogue).

Test Card J 14:9 on a widescreen tv [1.71] [2.88] [4.00] [4.57] [5.14] [6.00]

14P16 (AFD=3)

A 4:3 picture zoomed to 14:9 and coded into a 16:9 frame (as a 4:3 source on CBBC, CBeebies, News 24, and inserts into entertainment shows, documentaries national and local news programmes etc, would be displayed on a widescreen set).

Test Card W 14:9 on a 4:3 tv [1.75] [2.91] [3.50] [4.08] [4.66] [5.25]

14L12 (AFD=6)

A 16:9 picture zoomed to 14:9 and coded into a 4:3 frame (as most 16:9 shows, excluding most sports and some feature films, are transmitted on analogue).

Test Card W on a widescreen tv [2.00] [3.33] [4.00] [4.67] [5.33] [6.00]

16F16 (AFD=0, 2, 6 or 7)

A full-height anamorphic 16:9 picture as displayed on a widescreen set.

Test Card W centre-cut-out on a 4:3 tv [1.50] [2.50] [3.00] [3.50] [4.00] [4.50]

12F12 (AFD=7)

A 16:9 picture zoomed to 4:3 and displayed on a 4:3 set (as most sports shows are transmitted on analogue).

Test Card J on a widescreen tv [1.50] [2.50] [3.50] [4.00] [4.50] [5.25]

"Smart" or "Just" mode

A 4:3 picture displayed on a widescreen set with special non-linear horizontal scan designed to eliminate 'black bars'.

Most widescreen television receivers have a special display mode that attempts to squeeze a 4:3 picture into the full 16:9 screen. This is achieved by cropping the top and bottom off the picture and progressively increasing the slope of the horizontal timebase so that the edges af the picture are stretched out to reach the sides of the display tube, whilst keeping the centre of the picture roughly the correct shape. It is most effective in landscape shots - the widening of the edges is similar to the effect of using a very wide-angle lens - which compensates for the fact that the 4:3 picture is actually 'tighter' than a similar 16:9 shot.

Across the centre of the picture I have superimposed a sliver from Test Card J zoomed linearly to the same height in order to show how the 'smart' function fattens the picture towards the edges.


Floaters

This is what happens when producers of 16:9 shows reduce the size of a 16:9 or 4:3 picture in order that all viewers shall see the entire frame.

Test Card W made '14:9-safe' [2.33] [3.89] [4.67 [5.45] [6.22] [7.00]

In this case a 16F16 picture has been reduced in size so that when the picture is ARCed to 14L12 for analogue transmission the full width of the card is seen. On a 4:3 set the picture looks identical to 16L12 above, but on a widescreen set displaying a 16F16 anamorphic picture there is a border all around. With some decoders it's possible to select a 14L12 output, in which case the picture can be zoomed by the set to fill the screen.

Advertisements are occasionally broadcast like this - advertisers like to put their obligatory text as near to the edge of the screen as allowed by the ITC, but in some cases this would result in it being cropped off when ARCed to 14L12 unless the picture size is reduced.

The effect may also be seen on "The World", a news programme broadcast simultaneously in 16F16 on BBC4 and 14L12 on BBC World. As is now standard on BBC News, perfectly good 4:3 pictures are butchered to produce 14:9 versions as a compromise that means that both 16:9 and 4:3 viewers lose the top and bottom of the picture. Although the studio part of this show is made in 16F16 with the sides sliced off for the the 14L12 version, when it comes to 16:9 contributions they shrink the picture so that the whole frame can be seen in the 14L12 version of the show, while 16:9 viewers see the postage stamp version depicted above.

Test Card J displayed on a 4:3 set [1.71] [2.88] [4.00] [4.57] [5.14] [6.00]

Here a 4:3 picture has been incorporated into a 16:9 production which has then been ARCed to 14L12 (above) and 16L12 (below) for analogue transmission. On a 16:9 receiver the results are the same as 12P16 above, but on a 4:3 set the result is a border all around. There is nothing the analogue viewer can do, but a digital decoder could be switched to 12F12 to give a full-screen picture. Producers often put coloured 'curtains' each side of the 4:3 image when using this technique.

Test Card J displayed on a 4:3 set [2.00] [3.33] [4.67] [5.33] [6.00] [7.00]

This effect will also be seen on 4:3 transmissions from the BBC and five when using a terrestrial decoder that does not understand AFDs (see below) and is set to display letterbox pictures on a 4:3 set. That is because those broadcasters do not send full-width 4:3 pictures on terrestrial digital, but 'pillarbox' them in the middle of a 16:9 coded frame.


Wot? no AFDs?

Active format descriptors (see below) enable a 4:3 telly to give its best shot at displaying a compromise widescreen picture of maximum size, with minimum black bars, and including as much of the intended picture as possible, automatically for each show. However, some terrestrial receivers, and all Sky digiboxes, ignore AFDs and give the 4:3 viewer a fixed choice of centre cut-out or letterbox only. Such viewers, who have 'upgraded' their 4:3 sets via this route are stymied when it comes to watching ITV1 regional and national news, for those in charge of digital transmissions in most regions have chosen to slice off the top and bottom of the 4:3 pictures and transmit them in 14P16, in order to con 16:9 viewers into thinking that they are widescreen. In order to avoid losing even more of the picture, or having a small 'floating' picture, the only option is to tune back to analogue, where the original 4:3 pictures are broadcast (at least there is this choice - the BBC never offers it).

ITV1 News letterbox on digital

ITV1 News letterbox on digital

ITV1 News on analogue

ITV1 News on analogue

ITV1 News centre cut-out on digital

ITV1 News centre cut-out on digital

This sequence highlights another problem with widescreen. Subtitles, which are generated by the receiver at a fixed position within the raster, often do not appear where the subtitler intended when the underlying picture format is changed (in most cases they are designed to fit correctly in the 14:9 analogue version, as here).


W R O N G !

Test Card J incorrectly displayed on a widescreen set

A 4:3 picture stretched to fill a widescreen receiver (as often set up in the home by the 'installer', or seen in shop showrooms).

There are special non-linear screen modes available on most widescreen sets that attempt to squash a 4:3 picture into the 16:9 frame (see above). For those who dislike black bars, that is an arguably more elegant solution than putting up with uniformly fat actors.

Test Card W incorrectly displayed on a 4:3 set

A 16:9 picture as received on a digital set-top box or DVD player set to '16:9' or 'widescreen' and displayed on a 4:3 set without the capability of reducing the raster height to 16:9.

This is also the shape in which digital 4:3 viewers see the reduced-size pictures on BBC Parliament and the BBCi newsloops, since they cannot be rescaled to 4:3 by the receiver.





Widescreen Signalling

 

How does the receiver know what shape the transmitted picture is? In the days of analogue transmissions and 4:3 receivers when the transmitted picture was always 4:3, the broadcaster simply blanked off part of the picture in order to produce an active field of the required shape. The receiver didn't know or care about this - it just continued to churn out a 4:3 raster. Viewers cared though - the black bars looked like a fault in their telly and so transmitted pictures that completely filled the screen were the order of the day for many years.

The first time anything needed to be done was when PAL Plus was introduced, giving pictures that were intended to be displayed on a tube with a 16:9 aspect ratio. Such a receiver needed to know when a PAL Plus transmission was being received so that it could switch in its decoding circuitry and present the picture as a full-height-anamorphic 16F16 raster. A system called WSS (Wide Screen Signalling) was devised, comprising a teletext-like signal occupying the first half of scanning line 23 (this is officially the last part of the field blanking period in 625/50 analogue transmissions). The signal comprises four cycles of clock run-in (833kHz) followed by a framing code and then fourteen bits of data which carry information about the picture.

WSS signal

The first group, comprising four bits, B0-B3, carries information about the picture shape, while the remaining groups are concerned with other characteristics of the PAL Plus transmission.

Contents of Group 1 of WSS signal
3 message bits (LSB first) + odd parity
WSS Aspect Ratio B0 B1 B2 B3
0 Full-width 4:3 ("12F12") 0 0 0 1
1 Letterbox 14:9 Centre ("14L12") 1 0 0 0
2 Letterbox 14:9 Top 0 1 0 0
3 Letterbox 16:9 centre ("16L12") 1 1 0 1
4 Letterbox 16:9 Top 0 0 1 0
6 Letterbox deeper than 16:9 1 0 1 1
6 Full-height 16:9, framed to be "14:9-safe" 0 1 1 1
7 Full-height 16:9 ("16F16") 1 1 1 0

Here are some full-size 'square pixel' versions of WSS signals. If you download the files and paste them into the top left-hand corner of a 768x576 pixel picture they should produce the appropriate switching action when fed via a video board to a standard widescreen receiver that can decode WSS. The files are one pixel high, but I have asked your browser to fatten them for display here to make them more visible.

WSS = 0 WSS = 0 (12F12)

WSS = 1 WSS = 1 (14L12 Centre)

WSS = 2 WSS = 2 (14L12 Top)

WSS = 3 WSS = 3 (16L12 Centre)

WSS = 4 WSS = 4 (16L12 Top)

WSS = 5 WSS = 5 (Deeper than 16L12)

WSS = 6 WSS = 6 (16F16/14:9-safe)

WSS = 7 WSS = 7 (16F16)

With digital transmissions, part of the data stream is devoted to information relating to the shape of the picture. There are two methods of signalling, the first simply indicating whether the coded frame is 4:3 or 16:9 ("anamorphic") in shape. A digital receiver equipped with scart connections sets the voltage on pin 8 of the scart connected to the tv set to either +6V for a 16:9 picture or +12V for a 4:3 picture. (0V tells the tv to display its own off-air picture.)

This is fine as far as it goes, but in the case of 4:3 coded frames there are several available letterboxed formats, and many widescreen receivers are capable of displaying them full-height rather than with a small picture floating in a sea of black.

A second digital signalling system called AFD (Active Format Descriptor) is used to tell the receiver (or indeed, any other relevant equipment in the broadcasting chain) what the current picture shape is, and what the required display shape is to be. This is important because unlike an analogue receiver, the digital one is able to change the shape of the picture under the control of the broadcaster and/or the viewer to produce an appropriate video signal for the type of display device that is connected.

For example the BBC currently distribute all their digital pictures as 16F16 or 12P16 - that is, in 16:9 coded frames only. An AFD sent with the pictures instructs the ARC (Aspect Ratio Convertor) in the satellite chain to convert 12P16 pictures to 12F12 before transmission. On the terrestrial network the pictures are broadcast 12P16 and the home receiver is expected to interpret the AFDs and generate the 12F12 picture itself. Any receiver that failed to do this would diplay 4:3 pictures as a postage-stamp picture with broad black borders all around on a 4:3 receiver that is set up to display 16:9 pictures in deep letterbox.

However, a terrestrial receiver equipped with comprehensive AFD decoding facilities can not only produce correct 4:3 pictures, but can render 16F16 pictures in an appropriate shape as dictated by both the broadcaster (who knows the optimum display shape for the programme material) and the viewer (who knows what he likes). Thus a 16:9 show could be displayed automatically as 16L12, 14L12 or 12F12 as dictated by the broadcaster, or the viewer could opt to change to another of those formats.

It is interesting to note that while both BBC and ITV Sport generally choose to transmit 12F12 pictures on analogue, their 16F16 digital transmissions carry AFD7 (BBC, 4:3-safe) and AFD6 (ITV, 14:9-safe).

While the majority of digital receivers continue to communicate the required aspect ratio to the display device by means of the scart switching signal, a number also use WSS signals inserted on line 23 so that widescreen receivers can display a full-height picture. This is relevant when for example a film is broadcast as 16L12 or 14L12, as occasionally happens with old telecine transfers. It also means that recordings (including VHS) played through a suitable 16:9 display will assume the correct picture shape automatically.

How pictures with different AFDs are output from the decoder
    Intended output when decoder is set to...
(CCO=Centre cut-out,
ie top/bottom or L/R of
transmitted picture discarded)
4:3 tv
4:3
16:9 tv
16:9
..and transmitted coded frame is...
AFD number AFD description Test Card J 12F12
4:3
Test Card W 16F16
16:9
Test Card J 12F12
4:3
Test Card W 16F16
16:9
0 Same as coded frame Test Card J 12F12
12F12
Test Card W 16L12
16L12 [1]
Test Card J 12F12
12F12
Test Card W 16F16
16F16
1 4:3 only Test Card J 12F12
12F12
Test Card W 12F12 CCO
12F12 CCO
Test Card J 12F12
12F12
Test Card W 12F12 CCO
12F12 CCO
2 16:9 only Test Card W 16L12
16L12
Test Card W 16L12
16L12 [1]
Test Card W 16L12
16L12 [2]
Test Card W 16F16
16F16
3 14:9 only Test Card W 14L12
14L12
Test Card W 14L12
14L12 [1]
Test Card W 14L12
14L12 [2]
Test Card J 14P16
14P16
4 Reserved: decoders should behave as if AFD=0 were being transmitted.
5 4:3 (12F12) coded image framed to be "14:9-safe" Test Card J 12F12
12F12
- Test Card J 14L16 CCO
14L12 CCO [2] [3]
-
6 16:9 (16F16) coded image framed to be "14:9-safe" - Test Card W 14L12
14L12 CC0 [1]
- Test Card W 16F16
16F16
7 16:9 (16F16) coded image framed to be "4:3-safe" - Test Card W 12F12 CCO
12F12 CC0 [1] [4]
- Test Card W 16F16
16F16

Notes:

[1] In these instances the decoder may often be set to output a different shape of picture, from full height to deep letterbox, under the control of the user.

[2] Widescreen displays are often capable of 'zooming' 14L12 and 16L12 pictures to the full screen height, either under the control of the user or a WSS signal from the decoder.

[3] In this instance the decoder might not add extra blanking the to 12F12 picture, leaving the widescreen display to mask the picture by zooming it to '14P16'.

[4] The DTG in its document DIGITAL RECEIVER IMPLEMENTATION GUIDELINES and Recommended Receiver Reaction to Aspect Ratio Signalling in Digital Video Broadcasting (160KB file) recommends that decoders output a 16L12 picture when 16:9/AFD7 is transmitted, presumably because those viewers whose decoders have the option of only letterbox/full-height in 4:3 would have no other way of choosing to display a 16L12 picture.




Other Aspect Ratios

 

Feature films shown on UK television are generally now cropped to 12F12 or 16F16/16L12 (using a technique known as 'pan and scan' in order to keep as much of the action as possible within the narrower screen), with a few films with original aspect ratios near to 4:3 or 16:9 being shown with thin black bars either at the top/bottom or sides. The problem with these odd aspect ratios is that it is sometimes impossible to avoid a black border all around the picture on one or other of the tv set formats, and the broadcasters rely on overscan in the receiver to mask these. This is also true of the 14:9 analogue television format, but because that is so prevalent in the UK, widescreen setmakers now include a 14:9 zoom mode so that analogue 14L12 pictures may be displayed at their full height. DVD film releases often strive to preserve the original cinema aspect ratio, and present the film with black bars in either a 4:3 or 16:9 coded frame. Here are some common examples:

1.66:1 (15:9)

15:9 film frame displayed on a widescreen set

15P16

15:9 film frame displayed on a 4:3 set

15L12

15:9 is roughly the aspect ratio of a super-16mm negative. The actual image size is 11.65 x 7.00mm, so the precise aspect ratio is 1.66:1, or 14.98:9. Much television material has been shot on this stock for many years, but was generally framed and cropped for full-screen 4:3 showing. For widescreen television it is now framed 14:9-safe and cropped for full-screen 16:9 showing (14L12 on analogue). Occasionally the full frame is shown, generally letterboxed into a 4:3 frame for television, perhaps where the negative has been blown up to 35mm cinema prints. Television films are frequently shown in this 15L12 format in France.


1.85:1 (17:9)

1.85:1 film frame displayed on a widescreen set 1.85:1 film frame displayed on a 4:3 set

1.85:1 is the cinema format nearest to television's 16:9 (1.78:1). In fact, given the tolerances in film production and telecine framing, they can be considered equivalent. The thin black bars top and bottom of the 16:9 television frame (11 scanning lines each - half the height of a teletext row), even when they are transmitted, are very likely to be hidden by the crt overscan.


2.35:1 (21:9)

2.35:1 film frame displayed on a widescreen set

21L16

2.35:1 film frame displayed on a 4:3 set

21L12

2.35:1 is an ultra-widescreen cinema format, almost always cropped to 16:9 on UK television, but often shown deep-letterboxed in 4:3 frames in mainland Europe, even though half the tv lines then contain no picture. 2.35:1 DVD releases are generally in 16:9 coded frames, though it's almost impossible to tell from the sleeve labelling information whether or not this is so.





Because the 625-line and 525-line standards share approximately the same line repetition frequency (15.625kHz and 15.734264kHz respectively) a decision was made to use a common sampling frequency of 13.5MHz when digitising either standard. Using this sampling frequency, a whole number of samples per scanning line is produced in each standard - 858 samples in the 525-line 63.556µs line period, and 864 in the 625-line 64.000µs line period. Only the active portion of each line need be digitised and the EBU proposal of taking 720 samples per active line has been adopted so that the whole of the active line, even in the presence of timing errors, is sampled, these errors being corrected either in the digital domain or on conversion back to the analogue domain.

However, the number of sampling periods contained in the actual active line is smaller than 720 in each case. For the 52.90µs active line period of the 525-line standard this figure is 714.15 sampling periods, while for the 51.95µs period of the 625-line standard it is 701.325. In each case the figure is often rounded up or down to 704, a multiple of 16, which is desirable for MPEG encoding purposes. Indeed, 704 samples represents an active line period of 52.15µs, which is just within tolerance for both standards. However, this has led to some confusion over the actual active length of the line that contributes to the aspect ratio. The 625-line 4:3 (or 16:9) picture area comprises 702 x 576 samples, while the 525-line 4:3 (or 16:9) picture area comprises 715 x 480 samples.



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Resolution

 

Resolution, or definition, is a measure of the amount of fine detail contained in a picture. On a photographic film it represents the finest grid of lines that can be distinguished on the final product before it merges with the background grain. In a digital medium such as your computer it's limited by the number of discrete picture elements (pixels) that go to make up the display. An analogue television picture is a combination of both. Comprising discrete scanning lines, in the vertical direction the picture cannot show a grid finer than those lines, but in the horizontal direction the resolution depends on the maximum vision frequency handled by the transmission system and this has a smoother roll-off.

Resolution is expressed as 'lines per something', the lines being alternately black and white. In photography it would be lines per unit length. In television a convenient measurement is 'lines per screen height' - that is the maximum number of vertical lines that can be distinguished in a square the same height as the picture. One would expect the horizontal resolution to match the vertical - in computer terms the pixels would be square - but that's not the case.

Because of the discrete scanning lines, and the fact that they are not displayed progressively, but interlaced - that is lines from one field fit in the gaps between the lines from the previous one - the vertical resolution is somewhat less than the number of displayed scanning lines would suggest. If a camera were to be pointed at a card printed with a grid of horizontal lines that exactly concided with the tv scanning lines the picture would flash alternately white and black at field rate, or if the lines on the card fell between the scanning lines the result would be an even grey. In between there would be a mixture of the two. Similarly if the camera zoomed in slightly the grid would not be reproduced correctly - there would be interference between the grid and the raster producing strong moiré patterns, called aliasing. The apparent vertical resolution is given by multiplying the number of active scanning lines by a 'Kell factor' and it's this modified figure that the horizontal resolution should be designed to match.

The Kell factor is entirely psychological and subjective and so cannot be determined by calculation or measurement, but a figure has emerged of 0.7 for still pictures, rising to 0.9 for pictures with fast movement and falling to 0.5 to take into account 2:1 interlace. You pays your money and takes your choice... Certainly a Kell factor of 0.7 was specifically allowed for in the NTSC 525/60 system at the time of its design in 1940, whereas the 405-line system was over-endowed with bandwidth by comparison.

In the analogue domain the horizontal resolution is a function of the scanning frequencies and the video bandwidth of the system. The faster the spot traverses the screen, the higher the bandwidth needed to maintain the same resolution. For example, in the 405-line standard the active line period is 80µs and the maximum modulation frequency is 3MHz. 240 cycles of that frequency can be displayed across the screen, and since each cycle comprises a positive-going (light) and negative-going (dark) half, that means 480 lines may be accommodated, equating to a resolution (with a 4:3 aspect ratio) of 360 lines per picture height, comparable to the 377 active display lines.

In a digital television picture the horizontal lines are sampled at discrete intervals instead of being continuous, and it's this sampling frequency that defines the maximum horizontal resolution. In both the European 625/50 and the US 525/60 systems the sampling frequency is 13.5MHz as that is a convenient value that coincides with requirements of both systems. The 525/60 system has 715 samples per active line while the 625/50 system has 702, though the number of samples to be coded per line is 720 for each system. That's equivalent to a maximum vision frequency (given by Nyquist) of 6.75MHz - half the sampling frequency.

Since the scanning line is divided into discrete samples, some sort of Kell factor should be applied when assessing the horizontal resolution of digital pictures, though in the horizontal direction there's no interlace to consider and on a crt at least, the line is produced as a continuous stream rather than discrete steps, so there are no possible gaps or overlaps. The value of an appropriate Kell factor could be anywhere between between 0.7 and 1.0 depending on whom you ask.

Resolution comparison

Early 405-line transmissions were viewed on sets with screen sizes around 12 inches diagonal. This is about the same resolution (just over 20 lines per centimetre vertically) as a 15-inch 525-line set, a 19-inch 625-line set or a 42-inch high-definition (1920 x 1080) widescreen set.

Below is a list of the horizontal resolutions, expressed as lines per picture height, and the Kell factors of various television systems past and present. A Kell factor of unity represents 'square pixels', while less than 1 means 'fat pixels' and greater than 1 means 'tall pixels'.

It would be instructional first of all to apply these calculations to the Baird 30-line standard, in which a raster of aspect ratio 3:7 was produced by thirty vertical scanning lines with a picture repetition frequency of 12.5 per second. This gives figures of fH = 12.5Hz and fV = 375Hz. As the raster is produced by a spiral of holes punched at the rim of a rotating disc, there is no line or field flyback period (nor are there any synchronising pulses!). For a 100% vertical resolution of 30 lines per picture width (70 lines over the full picture height) a vision bandwidth of 13.125kHz would be needed. With a Kell factor of 0.7 applied, that requirement falls to 9.1815kHz, within the nominal 10kHz audio bandwidth of the radio channels that were used to transmit Baird's pictures.

The system letters in the table below are those adopted by CCIR (Comité Consultatif International des Radio Communications - the International Radio Consultative Committee). Note that with digital systems, the number of active lines/samples is usually given, rather than the total lines per frame period as with analogue.

Horizontal resolution of CCIR Systems
System No of Scanning Lines per frame/Fields per sec Horizontal Resolution in Lines per Picture Height Kell Factor No of Active Scanning Lines Vision Bandwidth (MHz)
or
No of Samples per Active Scanning Line
A 405/50 360 0.95 377 3.0
B/B1/C/D1/G/H 625/50 390 0.68 575 5.0
D/K/K1/L 625/50 468 0.81 575 6.0
E 819/50 599 0.81 737 10.0
F 819/50 300 0.41 759-761 5.0
I 625/50 429 0.75 575 5.5
L[1] 819/50 360 0.49 737 6.0
M 525/60 343 0.71 485 4.2
N 625/50 327 0.57 575 4.2
Digital 625 4:3 625/50 526 0.91 576 702
Digital 625 16:9 625/50 395 0.69 576 702
Digital 525 4:3 525/60 536 1.12 480 715
Digital 525 16:9 525/60 402 0.84 480 715

Notes:

[1] For a time, the French Band I/III 819-line monochrome programmes of TF1 were duplicated on a few low-powered UHF transmitters, also on 819-lines, but fitted into System L 625-line channels with a horizontal resolution similar to that of the 405-Line Standard A.

It should be noted that when a PAL, NTSC or SECAM colour signal is carried by any of the above analogue systems, the horizontal resolution is reduced - by the visible presence of the colour subcarrier when viewed on a monochrome receiver and by filtering of the luminance channel on a colour receiver. The chrominance resolution also has to be considered separately - in PAL, SECAM and DVB the vertical resolution of the chrominance is half that of the luminance. In digital systems the horizontal resolution is also half that of the luminance, while in analogue systems it depends on the bandwidth allocated to the double-sideband modulated coded chrominance signal - in System I that's 1.3MHz per sideband, so the horizontal resolution of the recovered chrominance signal is a quarter of that of the (unfiltered) 5.5MHz-wide luminance signal.

Although the sampling rate at the analogue-to-digital and digital-to-analogue stages at the start and end of the DVB system is 13.5MHz, or 720 samples per active line (702 for 625/50), in the actual transmission path the number of samples can be any one of 352, 480, 544, 702 or 720. 'Short' lines are resampled by the receiver to give a full-width picture of reduced horizontal resolution. Many, if not most, UK "Sky" digital transmissions are of reduced sampling rate. The BBC and five (the rebranded Channel 5) use 702 samples, while ITV and Channel Four use 544, giving three-quarters of the full resolution, and many others use the half-resolution of 352 samples. (ITV and C4 have recently crammed more channels into their satellite and terrestrial multiplexes and appear to have reduced the sampling rate to 480 - or possible 352 - in order to do so.) However, both the BBC and five distribute all the their pictures in 16:9 coded frames. 4:3 shows are padded with 88 black-level samples before and after the 526-sample active 4:3 line which is expanded to fill the full 702 samples just before satellite transmission (the complete 'pillarboxed' picture is sent on digital terrestrial, with the receiver instructed to expand the central portion to full-width). So in fact the resolution on all five 'terrestrial' stations is about the same for 4:3 shows but worse on ITV and C4 for 16:9 shows.

Using test patterns to assess resolution

Optical Test Slide
A detail from a 35mm projector test slide showing the square-wave resolution gratings
Frequency Gratings from Test Card W
A detail from Test Card W showing the sinusoidal resolution gratings and anti-aliased transitions

Generally, optically generated test cards had square-wave frequency gratings and sharp lines, relying on the optical transfer device and filters in the transmission chain to smooth off the edges so that they passed through the system without distortion - see the (non-television) test slide on the left - though Test Cards D and E are an exception to this, having sinusoidal frequency gratings printed onto the slide. Most electronic patterns have sinusoidal frequency gratings and vertical edges shaped precisely to match the system characteristics. Many have their horizontal edges shaped as well - in computer parlance they are 'anti-aliased' - as in this detail from Test Card W. In some BBC digitally generated cards the frequency gratings do not extend from white to black - their peaks are around 15% and 85% of peak white.

You might think that by defocussing or filtering a pattern of sharp black and white stripes you would end up with a nicely rounded black and white sinusoid, but the reality is a little more complicated, as the original designers of test cards discovered. A square wave, whether it's synthesised electronically, or produced by scanning black and white stripes on a card, comprises a sinusoid of the fundamental frequency, which gives the pattern its pitch, combined with a series of odd multiples of the fundamental which gives the waveform its shape. If the amplitude of the fundamental is 1, the amplitude of the resultant square wave is pi/4, or about 0.79.

Fourier synthesis of a square wave
Fourier synthesis of a square wave
Fourier synthesis of a square wave

If vertical black and white stripes are scanned by a camera with a sufficiently wide frequency response, they have a squarewave p-p amplitude of 0.7V, but when the harmonics are filtered out to leave the fundamental on its own, as happens when its frequency is higher than one third of the system bandwidth, the p-p amplitude rises to 0.9V (-0.1V to +0.8V), causing overmodulation of the transmitter. That is not a helpful characteristic of a test signal, so the amplitude (contrast ratio) of all the squarewave frequency gratings has to be reduced accordingly in order to provide a realistic indication of frequency response.

In the diagrams, the blue waveform represents the idealised squarewave (with an infinite number of harmonics), while the brown, orange, green and violet waveforms are the fundamental with successive odd harmonics added, up to the seventh in this instance. The relative amplitudes of the harmonics are: first (fundamental) 1, third 1/3, fifth 1/5, seventh 1/7, and so on, and they are all in phase at 0° of the fundamental.

The top diagram is symmetrical about 0V, as with an audio signal for example, but the bottom ones show a video waveform with the squarewave sitting between black and white levels. In the middle diagram, with peak white/black gratings, the reduced-bandwidth waveforms are clearly overshooting above white and below black levels, which would cause distortion in equipment further down the chain. The solution is to reduce the amplitude of the squarewave by making the gratings dark/light grey. The fundamental in the bottom diagram now has an amplitude of 100% of peak white and there is no overshoot. Note that even when sinusoidal gratings are used, their amplitude is often less than 100%.


This chapter has concentrated on the luminance, or black-and-white, resolution of television standards. In colour television the chrominance (hue and saturation) resolution is generally much lower than that of the luminance. This, and its implications on picture quality, are discussed in the chapter on Colour in the World TV Standards section of this web site.


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SMPTE Test Card

 

SMPTE Test card
525-line optical monochrome SMPTE Test Card
SMTE Test Card, BBC version
625-line optical monochrome SMPTE/BBC Test Card

ALTHOUGH IT was designed primarily for 525-line use, the SMPTE (Society of Motion Picture and Television Engineers) test card was adapted for use on 625-lines by the BBC for tests prior to the introduction of a specially designed 625-line test card. However, when one was produced in the form of Test Card E, there were aesthetic problems with the frequency gratings and so a 625-line version of Test Card C was pressed into service instead. However, Test Card D, the 405-line twin of Test Card E, was used on both BBC1 and ITV between 1964 and 1969.

Despite having been designed for the NTSC system with its relatively low resolution, the frequency gratings on the SMPTE card extend to 500 lines (per picture height), well beyond the horizontal resolution of all the 625-line standards, though the BBC replaced the wedges with finer ones, reading up to a rather optimistic 600 lines of resolution. (The vertical resolution of the 625-line standard is 575 lines, or 400 lines using a Kell Factor of 0.7.) The points on the wedge scales corresponding to the Test Card F frequency gratings would be: 1.5MHz - 115, 2.5MHz - 195, 3.5MHz - 275, 4.5MHz - 350 and 5.25MHz - 410. The NTSC System M with 4.2MHz bandwidth would resolve the gratings at 345. This description of the BBC version of the SMPTE test card comes from a BBC press release of 1962.

  1. Picture Size: The test card should just fill the viewing aperture with the tips of the centring arrows indicating the picture limits.

  2. Bandwidth and Resolution: The four wedges of converging lines are for judging resolution. The vertical wedges are for judging the horizontal resolution which, in the British 625-line standard, extends to 5.5 Mc/s. The numerical scales next to the vertical resolution wedges are related to the video frequency of the 625-line television system as follows: 4 Mc/s corresponds to 310 on the wedge scale, 5 Mc/s to 390, and 5.5Mc/s to 430.

  3. Contrast: A 5-step contrast pattern appears on each side of the test card.

  4. Scanning Linearity: The small circles at each corner of the test card should be of equal size.

  5. Low-Frequency Response and Reproduction of Edges: In the centre of the test card is a black circle within a white circle, which may be used to check the low-frequency response.

  6. Uniformity of Focus: The wedge patterns within each circle at the corners of the test card should be resolved uniformly.




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Test Card C

 

Click on an area in the test card to jump to an explanation of its function.

Test Card C
405-line optical monochrome Test Card C Corner stripes Corner stripes Corner stripes Corner stripes Letterbox Frequency gratings Frequency gratings Greyscale White circle Black needle pulse White needle pulse Right-hand arrowhead Right-hand side castellations White grid lines on grey background Castellations and arrowheads

THE FOLLOWING account is taken from the Collins Radio Diary, 1968, probably quoting from a BBC information sheet of two decades earlier, judging by the Cholmondly-Warner style of prose. [I have added amendments and notes in square brackets.]

Test Card C was introduced in 1949 and continued in use on 405-lines until 1964 when it was replaced on both BBC and ITA transmitters by Test Card D.

Test Card "C". The various patterns on this card are designed to assist [assessment/adjustment of] certain characteristics of the system thus:-

  1. Aspect Ratio: The concentric black and white circles should appear truly circular when the width to height ratio of the picture is adjusted to a ratio of 4:3. Back to picture.

  2. Resolution: Within the circle are two groups of 5 frequency gratings having black and white stripes corresponding to frequencies of 1.0, 1.5, 2.0, 2.5 and 3.0 Mc/s. In a good receiver the 3.0 Mc/s grating must be clearly reproduced. Back to picture.

  3. Contrast: the 5-step contrast "wedge" in the centre of the card has its top square white (100% mod.) and the lowest square black (30% mod.). The intermediate squares should be pale, medium and dark grey. Back to picture.

  4. Linearity: The mesh of white squares forming the backgound of the card should reproduce as equal squares in all parts of the picture. Back to picture.

  5. L.F. Response: [In the letterbox above the circle] The black rectangle should not, in a perfect system, streak into the white. Certain imperfections at present prevent this, but it should be possible to judge whether reproduction is normal. Back to picture.

  6. Reflections: [In the black and white 'ears' each side of the circle] Narrow vertical bars, representing a pulse of 0.25 microsecs., should be reproduced without positive or negative images to the right-hand side of them. Back to picture.

  7. Focus: The four diagonal areas of black and white stripes (corresponding to 1 Mc/s) should be resolved evenly. Back to picture.

  8. Picture into sync: Should video break into the line timebase through poor sync separator and/or poor video response the line timebase will trigger early on white castellations at right-hand side, displacing picture to the right corresponding to each white vertical castellation. Back to picture.

  9. [Picture size: The transmitted picture is slightly smaller than the test card. Its limits are indicated by the points of the opposing arrowheads in the border. As most receivers have a display area with an aspect ratio of about 5:4, it is usual to adjust the receiver so that the top and bottom of the display area coincides with the arrowheads and the side castellations of the test card just appear in the display area of the receiver.] Back to picture.



Test Card G
625-line international version of optical monochrome Test Card C (Redrawn by Dantus and Alan Pemberton)
625-line Test card
625-line BBC version of optical monochrome Test Card C

These two variants were modifications of Test Card C for 625-line operation. The one marked G was used in various 625-line countries, but not the UK. The one marked BBC2 was used instead of Test Card E (which had been the intended BBC2 test card) on the BBC2 monochrome network before Test Card F took over in 1967. It continued to appear occasionally after that, as did a version with the ident "BBC 1" when that channel was duplicated on uhf 625-lines in 1969. There are minor design differences from Test Card C - the circle on G for example and the arrowheads in the castellations of the BBC cards, but the chief difference was in the frequency gratings. On Test Card G they were 1, 2, 3, 4 and 5MHz, while on the BBC cards they were 1.5, 2.5, 3.75, 4.5 and 5.25MHz (equivalent to 1.0, 1.6, 2.4, 2.9 and 3.4MHz when radiated on 405-lines, where only the first four were within the System A vision passband).



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Test Cards D and E

 

Test Card D
405-line optical monochrome Test Card D
Test Card E
625-line optical monochrome Test Card E

TEST CARDS D and E, introduced in early 1964, were updated versions of Test Card C designed jointly by the BBC, ITA and BREMA, the British Radio Equipment Manufacturers Association, and were more suited to the 'swinging sixties'. Whilst most of the tests are the same, the technical specifications were tightened up and the design made more aesthetically pleasing. Obvious changes are the use of arrowheads in the castellations, corner stripes at 45° instead of on the diagonal, small, slightly brighter dots in the white and black squares of the greyscale stepwedge to assess white and black crushing and a large space at the bottom for station identification. The background grid of grey squares and white lines was rationalised so that with the castellated borders - each half the width of one square - there is a 12x9 grid, the same as the aspect ratio. All these features have prevailed through the subsequent test cards F, J, W and X (High Definition), with the widescreen versions having a 16x9 grid.

Test Card C gratingsTest Card C 
Test Card D/E gratingsOriginal Test Cards D & E 
Test Card F/J/W gratingsAll Later Test Cards 

The two cards D and E are identical to each other apart from the frequency gratings - Test card D was designed for 405-line use while Test Card E was for 625 lines. We saw above that early test cards used gratings of 100% amplitude squarewaves (that is, plain black and white stripes) that could cause equipment to be overmodulated when they were bandlimited to fit the transmission system. Test cards D and E demonstrated a departure by having sinusoidal frequency gratings of 50% amplitude (ie with excursions reaching 25% and 75% of peak white). These look much softer on the screen and were not entirely liked by the trade. Test Card E had a further innovation - instead of a peak white border around the gratings it had one the same 75% brightness as the peak of the sine waves, and each row of gratings started and ended at this brightness level. The BBC decided that the visual effect was unacceptable, and so Test Card E was rarely transmitted by them and the 625-line version of Test Card C was used instead until the start of colour transmissions in July 1967. However, the Portuguese broadcaster RTP seemed pleased with the pair and regularly radiated both D and E interchangeably, despite the fact that Test Card D was only designed for the 405-line standards and Portugal has only ever used 625 lines.

Test Card D Detail

During its life Test Card D was modified slightly, in order to appease the trade who demanded that the frequency gratings should look 'brighter' on their receivers, and to identify the new version a white dot was placed in the centre of each square to the side of the one containing the letter D. Later test cards have all used sinusoidal frequency gratings having an amplitude just below 100%.

Characteristics of Test Cards D & ETest Card DTest Card E
Frequency gratings (MHz)1.01.51.52.5
2.02.53.54.0
2.753.04.55.25
Width of black & white needle pulses (µs)0.30.2
Frequency of corner stripes (MHz)Approx 1Approx 1.5


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Test Card F

 

Click on an area of either test card to jump to a description of its purpose.
Test Card F Original
625-line optical colour Test Card F
Test Card F Digital
625-line electronic colour Test Card F Corner stripes Corner stripes Corner stripes Corner stripes Black outline Black outline Black outline Black outline Black outline Black outline Black outline Black outline Black outline Black outline Black outline Black outline Chalk Cross Centre Picture White circle Letterbox Frequency gratings Greyscale Arrowhead Arrowhead Arrowhead Arrowhead Right-hand side castellations Top castellations White grid lines on grey background Left-hand side and bottom castellations

TEST CARD F was introduced to BBC2 at the start of the experimental colour service in July 1967, having been designed jointly by the BBC, ITA, EEA and BREMA.

The original Test Card F comprised two transparencies - one colour and one black and white - sandwiched into a 35mm slide mount. The image above left is taken from a high-definition scan of such an example. In 1984 a digital version was produced (above right) using computer technology to draught the card and scan in the original central photograph. There are several differences between the two and a few error crept in at the pixel level.

This account of Test card F is taken from BBC Information Sheet 4306(1).

  1. Decoder performance: The top castellations start four lines after the end of the field-blanking period. The first four lines of each field scan are the standard electronic colour bar signal of 100% amplitude and 95% saturation. The position chosen permits the signal to be viewed easily on an oscilloscope triggered from the field scan.

    Faults or adjustment errors in a receiver show up as changes in shape, asymmetry of the waveform, or a difference in amplitude from line to line on the waveform of the red, green and blue components of these colour bars as seen at the output of the decoder. In addition, if the colour bars are observed on the face of the tube, an assessment of the decoder performance can be obtained by switching off individual guns of the tube. Back to picture.

  2. Reference generator: The top border castellations (cyan, which has a high luminance component) show up decoder errors effectively. Their location also enables the recovery period of the reference generator after the field-sync period, when the reference bursts are absent to be seen as variations in the saturation of the castellations. The bottom castellations (green) provide a means of assessing the reference generator performance at the end of the field as compared with the start.

    The red and blue castellations on the left-hand side will give rise to the greatest disturbance of the regenerated colour picture if the gating circuits permit picture information to pass to the reference generator. Thus, if this fault is present in the receiver, bands of saturation changes or "Venetian Blinds" will be visible on coloured areas across the picture depending on the decoding circuits employed. Back to picture.

  3. Sync separator: The right-hand castellations (yellow and white) provide a check on sync-separator performance in the presence and absence of the sub-carrier. Any malfunctioning of the sync-separator circuits will appear as variations in the position of the picture content on the extreme right. See also Line Synchronisation. The spacing of the left- and right-hand castellations has been staggered to make it clear from which side any disturbance arises, and to give the maximum possibilty of phase change arising from a gating error. Back to picture.

  4. Resolution: The bars in the frequency gratings are of square-wave form. Allowance is made to control the modulation depth, to avoid over-modulation resulting from the parameters of the system. Back to picture.

  5. Convergence: Certain lines in the grid have been outlined in black to assist in checking display tube convergence. Others have no outlining in order to avoid the confusion which might otherwise result from the low-frequency ringing seen when certain types of quadrature distortion are present in a receiver. The blackboard and white cross also provide a check on static convergence, allowing this to be set correctly in the central areas of the screen. Back to picture.

  6. Colour picture: The colour picture in the centre circle contains areas of flesh tones and of bright colours to facilitate overall picture quality assessment and the correct setting of saturation. The blackboard and white cross provide a check of static convergence. The circle containing the colour picture is outlined in white to avoid any "optical illusion" effects which might occur if the coloured areas abutted directly on to the background grey and white grid. Test Card "F" is intended primarily for technical purposes and is only secondarily a demonstration picture. It is not intended that the colour test card should be regarded as a substitute for the grid-pattern generator when setting convergence. Back to picture.

  7. Aspect ratio: The central white ring should appear truly circular when the width and height of the picture are adjusted to the standard aspect ratio of 4:3. Back to picture.

  8. Picture size: The transmitted picture is slightly smaller than the test card, its limits are indicated by the points of the opposing arrowheads in the border. As most receivers have a display area with an aspect ratio of about 5:4, it is usual to adjust the receiver so that the top and bottom of the display area coincides with the arrowheads and the side castellations of the test card just appear in the display area of the reciver. Back to picture.

  9. Contrast: To the left of the centre circle of the test card is a column of six rectangles with a contrast range of about 30:1 between the top and bottom squares. The differences in brightness between adjacent rectangles should be constant on a correctly adjusted receiver. Within the top and bottom rectangles are small lighter spots; white or black crushing is shown by the merging of the top or bottom spot into its surrounding area. Back to picture.

  10. Resolution and bandwidth: At the right of the centre picture are six gratings each consisting of vertical stripes corresponding to the following fundamental frequencies: 1.5, 2.5, 3.5, 4.0, 4.5 and 5.25Mc/s [equivalent to 1.0, 1.6, 2.3, 2.6, 2.9 and 3.4MHz when radiated on 405-lines, where only the first five were within the System A vision passband]. The gratings are equivalent to square-wave signals, and on a correctly adjusted receiver will appear to extend in value from white to black, with their surrounding area white. Back to picture.

  11. Scanning linearity: The white lines in the background should enclose equal squares and the central white ring should appear truly circular. Back to picture.

  12. Line synchronisation: The border of the test card is a pattern of alternate rectangles in white and colours with a higher luminance value. On black and white receivers these rectangles appear in tones from black, grey to white. The right side of this border serves as a test signal to check the line sync. Faulty line sync shows as horizontal displacement of those parts of the picture on the same level as the lighter toned rectangles in this side; it will also give the central ring the appearance of a "cog-wheel". Back to picture.

  13. Low-frequency response: Low-frequency response can be checked by means of the black rectangle at the top centre of the test card. Poor l.f. response shows as streaking at the right-hand edges of these areas and also of the border castellations. Back to picture.

  14. Reflections: Reflections of the TV signal, from hills or large buildings, may result in displaced "ghost" images. This effect will be most readily seen as displaced images of the white or black vertical lines, particularly where these are adjacent. Back to picture.

  15. Uniformity of focus: In each corner of the test card there is a diagonally-disposed area of black and white stripes; the focus of these areas and of the central area of the test card should be uniform. Back to picture.


Widescreen version of Test Card F

An interim widescreen version of the electronic Test Card F file was produced. It is simply the 4:3 version with four extra columns of the background grid inserted between the circle and the greyscale/frequency response columns. It is not as aesthetically pleasing as Test Card W and does not incorporate the extra digital tests of TCJ and TCW.



|Top | Aspect Ratio | Resolution | SMPTE | C | D & E | F | PM5544 | ETP1 | J, W & HD | Colour Bars | Bookmarks |

Test Card 'G' and PM5544

 

Click on a feature on either card to jump to a description of its purpose.

PM5544 BBC version
625-line BBC 'Test card G' version of electronic colour pattern PM5544
PM5544
625-line international version of electronic colour pattern PM5544
Burst Gating test (R-Y) (B-Y) White needle pulse White-black-white rectangles 150kHz square wave Centre cross Colour Bars Horizontal centre white line Frequency gratings around 4.4MHz Frequency gratings Greyscale White-black-white rectangles Yellow-red-yellow rectangles White grid lines on grey background Castellations Burst Gating test (R-Y) (B-Y) White needle pulse White-black-white rectangles 150kHz square wave Lower centre black square Centre cross Colour Bars Horizontal centre white line Frequency gratings around 4.4MHz Frequency gratings Greyscale White-black-white rectangles Yellow-red-yellow rectangles Right-hand side castellations White grid lines on grey background Castellations

THE PHILIPS PM5544 pattern generator was designed as a test for PAL receivers and was soon taken up by broadcasters as a useful alternative to optical test generators. There have been many variations over the years, and when the BBC began using it for insertion at regional transmitting centres on the rare occasions that the main transmission feed from London was being used for other purposes, they called it Test Card G, and it contained many features that were based on Test Card F. This account has been compiled and paraphrased from various magazine articles.

Luminance and geometry checks

 
  1. Aspect ratio and picture centring: The aspect ratio of the pattern is 4:3. When the height and width of the receiver are correctly adjusted the castellations should be visible along the top and bottom equally, and down each side of the picture equally. The castellations at the sides are slightly wider than the height of those along the top and bottom. The amount of each displayed will depend on the precise shape of the display tube in the receiver. Back to picture

  2. Picture geometry: The large centre circle and the grid of grey squares enable the height, width and linearity to be adjusted precisely. Back to picture

  3. Convergence: Static and dynamic convergence may be checked by observing the white lines in the grid of squares and in particular the central white line and cross on the black background. It may help to reduce the saturation and brightness and increase the contrast to make the white lines more visible. It is not recommended that adjustments to convergence be made other than with a cross-hatch pattern generator however. The width of the vertical white lines on the PM5544 pattern is 230ns, though in the BBC version these pulses are band-limited and will appear broader. The horizontal lines comprise pairs of scanning lines, one per consecutive field, in each version. Back to picture

  4. Horizontal synchronisation: The side castellations comprise alternate rectangles of black level and peak white and provide a check of sync separator operation. If any video signal is passed by the sync separator to the line timebase it will result in horizontal displacement of those areas on the same rows as the white castellations. In receivers employing flywheel synchronising circuitry the result will not be a pronounced 'cogging' as was the case with earlier designs with direct sync. Back to picture

  5. Vertical synchronisation: The top and bottom castellation provide a check of field synchronisation pulse separation and field synchronisation. A particular problem with interlaced scanning is 'line pairing' which occurs when the lines of one field do not fit precisely at the centre point between those of the previous field. The horizontal white lines in the pattern are arranged in pairs, one from the odd field and the next from the even field. However the lines along the centre of the pattern on the black background have this reversed - the upper line is from the even field and the lower one from the odd field. When viewed on the screen all the horizontal lines should appear the same thickness. Any tendency towards line pairing will be highlighted by the central lines appearing thicker or thinner than the rest. Back to picture

  6. Grey scale: The steps of the grey scale wedge in the lower part of the circle have the values 0%, 20%, 40%, 60%, 80% and 100% of peak white. The change in brightness of each to the next should appear the same. There should be no coloured cast in any of the steps, each grey rectangle appearing a neutral grey. Back to picture

  7. PLUGE: There is an option for the lower square in the central black cross to be set 3% below black level for picture monitor line-up purposes. Back to picture

Frequency response

 
  1. Low-frequency response: The large black rectangle at the top of the circle is intended as a test for lf response. There should be no smearing to the right of the vertical edges at either side of the rectangle. The larger black rectangle at the bottom of the pattern may also be used, though either rectangle will often contain a station identification. Back to picture

  2. Reflections: The large white rectangle below the one for the lf check contains a 230ns-wide black pulse. There is sometimes a similar white needle pulse in the lower black rectangle when that is not being used for identification purposes. Reflections of the transmitted signal from hills or large buildings will appear as displaced 'ghosts' to the right of each needle pulse. The BBC version has band-limited pulses that are wider than 230ns. Back to picture

  3. Transient response: Below the white reflection test rectangle and just above the colour bars is a set of light and dark rectangles created by a 250kHz square wave signal. The vertical transitions should be free of overshoot, preshoot and ringing effects. The actual amplitude levels of the square waves depend on the style of the colour bars below. In the PM5544, using 75% EBU bars, the levels are 0% and 75% of peak white. In the BBC pattern which uses 95% bars, the levels are 25% and 100% (see the Colour Bars section for an explanation of these terms). This 250kHz square wave is also used in the matrixing test described below. Back to picture

  4. Resolution and bandwidth: The frequency gratings just above the grey scale wedge comprise sine waves of full amplitude on a grey background corresponding to frequencies of 0.8, 1.8, 2.8, 3.8 and 4.8MHz. These frequencies were chosen to avoid multiples of 500kHz which would result in vision buzz on the television transmission. In the SECAM version the five multiburst frequency gratings have the values 0.8, 1.8, 2.8, 1.8 and 0.8 MHz. The BBC version has the six frequencies used by Test Card F: 1.5, 2.5, 3.5, 4.0, 4.5 and 5.25MHz and the amplitude is 71.4%. Back to picture

Colour checks

 
  1. Delay line circuit phase and gain adjustments and reference oscillator phase: The coloured 'ears' or square brackets [ ] at each side of the circle and the columns of grey squares between them and the side castellations contain colour subcarrier superimposed upon a mid-grey pedestal. The left-hand bracket contains negative and positive R-Y (phasor 270° and 90°) in the upper and lower upright halves, while the right-hand one has negative and positive B-Y (phasors 180° and 0°). The smaller return boxes contain the phasors at which G-Y=0, ie 326° and 146°.

    The two uncoloured columns contain subcarrier which is of the opposite sense to standard PAL. That is, on the left is unswitched R-Y and on the right switched B-Y. In a correctly functioning PAL decoder this subcarrier will cancel out line by line and the columns will appear the same grey as the other squares. (See below for the effect on PAL-S decoders).

    However, an amplitude error in the decoder will result in 'Hanover blinds' (Venetian blinds) in these areas whereas a phase error will appear as blinds in the coloured brackets as well as in the magenta and cyan colour bars.

    If there is a reference oscillator phase error the colourless areas will appear tinted - blue or green on the left and pink or yellow on the right.

    On the BBC version the subcarrier in the outer two columns of the grid is not transmitted, apart from in two squares near the top of the left-hand column, and so the above tests cannot be performed Back to picture

  2. Colour balance/drive/matrixing: The colour bars below the 250kHz squarewaves are 75% EBU bars in the PM5544 and 95% BBC bars in the BBC version (see the Colour Bars section for an explanation of these terms). The bars can be used in conjunction with the 250kHz squarewaves to set up the colour difference drive and saturation of the receiver by eye, or an oscilloscope may be used to examine the waveforms at the colour difference outputs. In the case of a receiver that employs RGB drive, only the saturation should be adjusted by eye.

    Firstly, set up the greyscale tracking, brightness and contrast for a correct monochrome display. Then turn off two of the crt guns to leave a single colour display. When the saturation and drive controls are correctly set each colour bar will be the intensity of either the light or dark half of the squarewave above it. Repeat for the other two guns. For an RGB-drive receiver it is usual to make a single saturation adjustment with only the blue gun turned on.

    This 250kHz square wave is also used for assessing transient response as described above. Back to picture

  3. Colour fit: The bottom segment of the central circle is coloured yellow with a central 2.9µs pulse of red. The colours are the same as their equivalents in the colour bars above. If the luminance-chrominance delay is correct the red area should sit precisely vertically above the grey square below it. Back to picture

  4. Colour bandwidth: The 3.8 and 4.8 frequency gratings, being within the passband of the chrominance demodulator, will appear coloured. If the bandwidth of the chrominance channel is symmetrical, the colouring should be similar on each grating. On the BBC version the three right-hand gratings will exhibit colouring, and it will be strongest on the 4.5MHz set. If the reference oscillator is locked to the line frequency the coloured moiré pattern will be stationary. If the relationship with line frequency is lost (for example in a VHS playback) the pattern will be moving. Back to picture

  5. Burst gating: The 'colourless subcarrier' in the second and third rows of the first column of grey squares (between the left-hand castellations and the left-hand bracket) extends into the castellations. If the burst gating is late and picture information passes to the reference oscillator altered colours and desaturation will occur on those two rows of the pattern. On the BBC version (which does not have the full columns of colourless subcarrier) these two squares may be used to assess phase and delay errors. Back to picture


"Colourless" subcarrier

Colourless (R-Y) subcarrier Colourless (B-Y) subcarrier

When viewed on a PAL-S (simple PAL, not having a delay line) receiver, the 'colourless' subcarrier decoder test part of the pattern is not colourless at all. Alternate pairs of lines (one from each field) contain colours which are electrically complementary, and therefore produce grey in a PAL-D (delay line) decoder, but are not cancelled out in the human eye/brain combination. The diagrams show enlarged squares of the PM5544 pattern containing 'colourless' (B-Y) and (R-Y) subcarrier. The smaller diagrams show the squares actual size. See Colour Standards: PAL for a description of PAL-D and PAL-S decoding. Back to picture

Colourless (R-Y) subcarrier Colourless (B-Y) subcarrier

Variations

PM5544 MPEG version
625-line animated digital version of electronic colour pattern PM5544

The intervening years have spawned many variations on the PM5544 pattern, and on the satellite channels it is rare to find any two identical. This version is designed for digital use and presumably carries no colourless subcarrier information since it would be irrelevant to a digital transmission. However, as a test of the integrity of the transmission circuit, an animated feature is included. If the path of a digital circuit fails at any point the usual result is a still picture of the last available frame. Since a test card is a still picture anyway such an event is unlikey to be noticed.

On this pattern the white needle pulse in the lower black rectangle swings from side to side - in this time exposure you can see it on two consecutive fields. In addition, as the pulse passes the two outer of the three white pips from left to right the audio tone in the left-hand channel is interrupted, thus providing an indication of sound-vision synchronisation and an identification of the stereo sound channels.

PM5544 on 525 lines

PM5544 NTSC 525-line version
525-line version of electronic colour pattern PM5544 used by NHK in Japan (Redrawn by William Brown)

The PM5544 test pattern has also been used in 525-line countries. The picture above, and the two below, were redrawn by William Brown of New York who has provided the following technical details:

Where the 625-line pattern has a grid of grey squares having a height of 40 scanning lines, the 525-line version has grey squares 32 lines high (separated in each case by two white lines, making each row 42 or 34 lines high, respectively). The height of the centre circle is 504 lines in 625 lines and 408 lines in 525 lines. The grey background is 48% of peak white and the colour bars are 75%.

In the Japanese version the frequency gratings are 0.5, 1.0, 2.0, 3.0 and 4.0MHz. The square wave above the colour bars lies between 0% and 75% white, with a frequency of 300kHz. At the top left, where BBC Test Card G has 'colourless' subcarrier, this version sometimes has extended 180° burst.

In the United States, the multiburst frequencies are the FCC standard of 0.5, 1.0, 2.0, 3.0, 3.58 and 4.2MHz. In Canada, 0.5, 1.25, 2.0, 3.0, 3.58 and 4.2 MHz have been used. The square wave frequency in North America is usually 307.5kHz.

SInce the 'colourless' subcarrier test of unswitched (R-Y) and switched (B-Y) is meaningless in NTSC, the two extreme left and right hand columns of grey squares sometimes carry instead unswitched (R-Y) on the left and unswitched (B-Y) on the right as a subcarrier phase check. The Canadian version omits the 'ears' carrying the subcarrier phase checks altogether and also includes 180° colour burst superimposed on the greyscale, which has eleven steps instead of five.


US 525 Line PM5544
525-line version of electronic colour pattern PM5544 used by WBOY-TV in Clarksburg, West Virginia (Redrawn by William Brown)
Canadian 525 Line PM5544
525-line version of electronic colour pattern PM5544 used by SRC [Radio-Canada] in Montreal (Redrawn by William Brown)

ETP1

 

Click on any feature of the pattern to jump to an explanation of its purpose.

ETP1
625-line electronic colour pattern ETP-1 Yellow-red-yellow rectangles White needle pulse White-black-white rectangles 150kHz sqaure wave Colour Bars Frequency gratings Greyscale Right-hand side castellations Top castellations Left-hand castellations White grid lines on grey background Bottom castellations

Prototype ETP1
The IBA ETP1 electronic colour pattern in development, photographed in 1978

The IBA introduced its new ETP1 electronic test pattern in 1979 in preparation for the subsuming of the fourteen existing control centres around the country into four regional operations centres (ROCs). It was necessary to employ a test card generator that once installed required no adjustment at all. The pattern met with some resistance from the trade however because of its lack of a circle, picture, arrowheads and square grid.

On the left is a rather unusual photograph of the ETP1 pattern in development. It appears to comprise only the black, white, magenta and cyan components of the pattern. More fundamentally, only the five sets of frequency gratings of the PM5544 are present - later a sixth was added and the Test Card F frequencies adopted.

The prototype test pattern was transmitted from the Rowridge, Isle of Wight transmitter and its dependants for evaluation purposes in 1978. The rack-mounted equipment that generated ETP1 was notoriously unreliable. Boards would come loose, or edge connectors become noisy, resulting in components missing from the transmitted pattern, similar to the prototype pictured left.

The following description together with the colour picture above is taken from IBA Engineering Information Service information sheet number EIS 120 9/79. [I have added explanatory notes in square brackets.]


Features of the IBA test pattern include:

  1. Crosshatch pattern - for convergence check. This may best be seen with the colour control turned down to give a monochrome picture. So far as possible, the white grid should be free from colour fringeing. In practice, most receivers tend to give some slight fringeing, particularly near the edges of the picture. Back to picture.

  2. EBU colour bars (75% amplitude, 100% saturation). [See the Colour Bars section for an explanation of these terms.] Back to picture.

  3. Grey scale - 0%, 20%, 40%, 60%, 80%, 100% amplitude. The difference in luminance signal voltage between adjacent rectangles should be approximately constant. Back to picture.

  4. Multiburst - for bandwidth/resolution check. Six sets of sine-wave gratings corresponding to the following frequencies (MHz):

    625-line UHF 1.5 2.5 3.5 4.0 4.5 5.25
    405-line VHF 1.0 1.6 2.25 2.6 2.9 3.4*
    *[This last frequency (3.4MHz) falls outside the System A 3MHz passband and is not transmitted on 405-lines - the block appears grey.]

    It is normal for a colour receiver to exhibit a bluish-yellow pattern (known as 'cross-colour') on the 4.5 MHz and 5.25 MHz gratings. Because of a special filter incorporated in colour receivers, which prevents the colour subcarrier from appearing on the screen, the 4.5 MHz bars are likely to be indistinct. Also, they are likely to be indistinct on most 405-line receivers. [This is because a similar filter is applied at the transmitter prior to standards-converting the signals to 405 lines, in order to eliminate any visible dot crawl pattern from the colour cubcarrier which would be converted to 2.8791855MHz - within the 3MHz System A passband.] Back to picture.

  5. 150 kHz squarewaves - for transient response check. Just above the colour bars there is a train of 150 kHz squarewaves (0% and 75% amplitude). This is to facilitate a check on any ringing, overshoot or preshoot. Ideally, there should be sharp transitions between the black and white rectangles, without 'smudging'. The transmitted transitions are as fast as the UK 625-line standard permits. Back to picture.

  6. Black rectangle within white rectangle - for low frequency response check. Low frequency response can be assessed by the appearance of the black rectangle within the white rectangle near the top of the pattern. Poor low frequency response shows as streaking at the right-hand edges of the rectangles, and from the border castellations. Back to picture.

  7. White needle pulse - for reflection check. Any reflections of the television signal, from hills or large buildings, can result in displaced 'ghost' images. The effects of short-term reflections are revealed by secondary images of the white needle pulse within the black rectangle. Back to picture.

  8. Yellow-red-yellow rectangles - for chrominance/luminance delay check. The redness of the rectangle near the top of the pattern should fit snugly between the yellow rectangles. Back to picture.

  9. Line synchronisation castellations - the left, right and bottom borders are formed by a pattern of alternate rectangles in black and colours with high luminance value and with a white rectangle in each corner. On monochrome receivers these rectangles appear either as black or as various lighter tones ranging from grey to white. The right-hand side border serves as a test signal for checking the line synchronisation of receivers - faulty line synchronisation shows as horizontal displacement of those parts of the picture on the same lines as the lighter toned rectangles on this side. These castellations, being yellow and white, provide a check on sync separator performance in the presence and absence (in 625-line transmissions) of the colour sub-carrier. The spacing of the left-hand and right-hand castellations has been staggered to identify the side from which any disturbance arises. Back to picture.

  10. Colour receiver reference oscillator castellations. The coloured border castellations can be used in checking for correct decoding; top: cyan, bottom:green, left-hand side: red and blue, right-hand side: yellow. Back to picture.

  11. Picture centring castellations. The width of each border castellation along the sides of the picture is the same as that of one of the grey rectangles within the crosshatch grid. Similarly, the height of the castellations along the top and bottom is equivalent to the height of the grey rectangles within the crosshatch grid. The picture size on receivers would normally be set for some slight overscan at the edges, but castellations should be clearly visible along all four sides of the picture. Back to picture.

The average picture voltage has been set (nominally) at 50% of the white level voltage.



Test Cards J, W and HD

 
Test Card J
625-line animated digital 4:3 electronic colour Test Card J
Test Card W
625-line animated digital 16:9 electronic colour Test Card W
High-defiition Test Card
1080-line animated digital 16:9 electronic colour high definition Test Card

TEST CARDS J, W and HD are the current standard definition (625-line) and high definition (1080-line) patterns used by the BBC, for internal if not external consumption. They each feature most of the tests carried by Test Card F plus some others required for digital distribution and transmission systems. All the cards were designed by Richard Russell. Most of the information in this section is taken from the now-defunct website of Peter Vince (Barney Wol), who worked with Richard Russell on the production of the digital patterns.

The frequencies of the gratings for all three patterns are given below. Since Test Card W could be displayed or distributed in 12F12 centre cut-out or 14L12 as well as its native 16F16 format, all three sets of frequencies are given.

Frequencies of gratings in MHz
Test Card F,
Test Card J
Test Card WHD Test Card
12F1212F12
CCO
14L1216F1616F16
1.501.501.752.00  5.00
2.502.502.903.3310.00
3.503.003.504.0015.00
4.003.504.084.6720.00
4.504.004.665.3325.00
5.254.505.256.0030.00

Test Card W

Test Card W was the first of the test cards to have been brought up to date for the digital age at the turn of the century. Clearly based upon Test Card F, it was the far superior final version of several rather poor attempts to 'stretch' TCF gracefully from 4:3 to 16:9. As it was not required to be used on the analogue network, it is missing a few of the features of TCF specific to analogue reception, and contains some digital tests of its own. Many of the new features and improvements are common to all three Test Cards J, W and HD.

The original TCF slide
The 1998 scan of the original 1967 slide of Carole Hersee and the doll she stitched from a kit

The central (!) improvement is the rendition of the colour picture. The original 6x6cm slide was borrowed from George Hersee, the BBC engineer who had taken the 1967 shot of his 9-year-old daughter Carole and the rag clown doll she had made up from a kit, and a 5000x5000 pixel 16+16+16 bit scan of it was made by Masterlith, of Mitcham in Surrey, on an Agfa Selectscan 2000 scanner using special software imported from the manufacturers in the USA to handle the 48-bit colour depth.

The colour was graded more accurately than with the original TCF version, and the image was placed with the blackboard cross in the precise centre of the card and then zoomed in and out until a pleasing composition was obtained.

Test Card W Top Arrow RGB
Test Card W Top Arrow with chroma delay
Test Card W Top Arrow blue gun only
The green square inside the black letterbox is for showing up luma/chroma delay. On TCF the clown's yellow buttons were intended for that purpose, but the effect was always unclear, and on the digital version of TCF the luma/chroma registration is itself incorrect! On the new cards, luma/chroma delay shows up as a grey edge to the green block.

The top cyan castellations of TCF have been replaced by colour bars (as they always were in practice, and as incorporated in the digital TCF). In the new cards however, the bars are 100% instead of 95%, since the card is not intended for transmission in PAL on analogue circuits, where the subcarrier would extend beyond specified limits. The white arrowhead in the magenta bar may be used to spot luma/chroma delay errors, and it also doubles as a colour saturation check on monitors, where examination of the red or blue displays should show the arrow blending into the background at the same brightness level.

The left- and right-hand castellations on TCF contained coloured blocks that tested for sync separator and burst gating problems on analogue systems, and since TCW is for digital distribution only, these features have been dispensed with with. Instead, the right-hand border contains 100% non-anti-aliased colour bars (that is, the colours change abruptly from line to line). Any averaging caused by subsequent processing will show up as a blurring of the transitions. The left-hand border comprises blocks of 50% grey superposed with ±(R-Y) and ±(B-Y) components at amplitudes such that they produce the maximum allowed excursions of red and blue when decoded.

The green castellations on the bottom border of TCF have been replaced by pairs of luminance and chrominance ramps. The bottom left ramp runs from 240 down to 12 digital units, that is from whiter-than-white to blacker-than-black (black and white are specified as 16 and 235 units out of a range of 0 - 255). The other ramps are shallow (just over a quarter of the range) luminance, (R-Y) and (B-Y), intended to show up quantising errors.

To aid the setting up of aspect ratio convertors and monitors, extra arrowheads are included along the centre line to indicate the edges of 4:3 and 14:9 centre cut-out pictures. Their aesthetic positioning emphasises the wisdom of the choice of the size and positioning of the white grid, which changed between TCC and TCD.

Castellations have not entirely disappeared - there is a single line of them at the top and bottom of the card, at a similar frequency to the basic 833kHz WSS data, as a check that these lines have not been blanked whilst passing through equipment such as synchronisers or other frame stores.

The greyscale blocks have changed too. TCF had lighter dots in the darkest (0%) and lightest (91%, with a 100% dot) blocks to show up black and white crushing. In the new cards, the top and bottom blocks are at 100% and 0% (the same as the black and white grid lines surrounding them), each containing a pair of dots at ±7% relative to them. In addition the dots flash on and off once per second as an indication that the signal is 'live' and not frozen in a frame store somewhere.

The active line period in the digital domain is slightly longer than its analogue equivalent. This arises both from a desire to use the same sampling frequency (13.5MHz) for both 525- and 625-line standards that is a common multiple of both line frequencies, and a requirement for MPEG coding that pictures have a multiple of 16 pixels on each side. This led to a common active line period for 525/60 and 625/50 of 720 samples, which encompasses the 702 sample active line of 626/50 and the 714 samples of 525/60.

This does not mean that pictures are any wider - only the inner active samples should be displayed - but it makes sense to put information in the extra samples of test signals. In Test Card W the spare nine samples beyond each edge of the card simply contain a reflection of the the edge of the card. This is not a new concept in test card technology - most physical cards or transparencies had features beyond the arrowheads that indicated the picture limits in order to ease setting up of the originating camera or scanning equipment.


Test Card J

Test Card J was created a year after TCW as a replacement for TCF in the 4:3 analogue environment. It is basically a cleaned-up version of TCF that uses many of the new features of TCW apart from those that are only suitable for the digital domain.

The coloured castellations of TCF are retained, apart from the top border, where the cyan castellations are replaced by colour bars, as in TCW. The right- and left-hand borders contain a useful test of line blanking, using the extra nine samples available in the 720-sample digital line. White and magenta blocks are included in the black castellations, and they have the same rise time as the line blanking pulse. They are positioned such that when correct analogue line blanking is applied, the result is a 700mV 423ns-wide raised cosine pulse, which may be observed on a waveform monitor.

The top and bottom lines of the card contain a different waveform from that on TCW, but the purpose is the same - to highlight when these lines have been cut off while passing through synchronising equipment.

A 525-line version of Test Card J was also designed, for use in NTSC territories. It is identical to the 625-line version in all but two features. The frequency response gratings are 1.5, 2.5, 3.0, 3.5, 4.0, and 4.2 MHz, to suit the lower video bandwidth of System M transmissions, and the left- and right-hand borders contain a different test. The active analogue line period takes up 714 samples, so there was no room to fit the line blanking test in the 'spare' pixels beyond the edges of the card. Instead standard NTSC I and Q subcarrier - that is 40 IRE units (286mV) of subcarrier at -57° and +33° - was added to the black castellations.

TCJ 525-line version
525-line version of animated digital 4:3 electronic colour Test Card J

HD Test Card

The HD Test Card - which does not appear to have an identifying letter - is 1920 x 1080 pixels in size. In all respects it is simply a higher-definition version of Test Card W, except that there is no information beyond the edges of the picture, since the blanking period anomaly is not present in HD transmissions.

'3D' versions of the card have appeared, with certain features (the centre picture, or additional lettering) in a different plane from the rest of the card. There are some examples in the HD section of the main test card page.




Colour Bars and PLUGE

 

A COLOUR television picture comprises three separate pictures of red, green and blue superimposed. If the red, green and blue signals are switched fully on and off in a certain pattern across each line (in fact square waves with the approximate frequencies fH, 2fH and 4fH for green, red and blue channels respectively) the result is vertical blocks in each of the primary and secondary colours plus black and white - the same saturated colours used in the teletext display. This particular test signal is called 'colour bars' and when sent through a television circuit it makes an excellent test of the performance from camera to display tube. Unfortunately, it is impossible to code the information for these saturated colours and send them to a transmitter, so compromises have to be made. As you can imagine, different broadcasters adopted different compromises. The BBC liked to reduce the saturation of the colours while leaving the amplitudes the same, resulting in bars that are slightly lighter. The ITA and EBU (European Broadcasting Union) took the view that keeping the coloured bars fully saturated but reducing their amplitude by 25% gave signals most like those found in average programme material. Since various adjustments are made by observing the display of the bars on vectorscopes, waveform monitors or tv screens, it's essential to know which sort one is dealing with.

The two diagrams below show the vectors of all the chrominance components of a 100% colour bar pattern, excluding the colour burst signals which appear on the back porch of the line blanking period, since they differ from standard to standard as follows:

(Line n refers to the odd lines of Fields 1 & 2 and the even lines of Fields 3 & 4; Line n+1 to the even lines of Fields 1 & 2 and the odd lines of Fields 3 & 4.)

Colour bar vectors, weighted

For 95% and 75% bars, the angles are the same, but the lengths of the vectors of the bar colours are reduced by a quarter.

The diagram on the left represents the vectors of the chrominance signals after weighting, ready to be modulated onto the two subcarriers. Weighting is necessary because the amplitude of subcarrier that would be obtained by modulating the pure (R-Y) and (B-Y) signals would exceed the allowed parameters of the transmitted signal. It would be possible merely to reduce the modulated subcarrier amplitude, but it is more efficient to scale the individual (R-Y) and (B-Y) signals before modulation, since this makes full use of the permitted subcarrier excursions. The weighted values are called U [=0.493(B-Y)] and V [=0.887(R-Y)].

In the SECAM and PAL standards the U and V values themselves are transmitted (on alternate lines in SECAM, and simultaneously with V alternating line by line in polarity in PAL), but in NTSC the values as projected onto the I and Q axes are transmitted instead, in order that the bandwidth of the Q signal may be reduced to about half that of the I signal. The values of I and Q are I=0.74(R-Y)-0.27(B-Y) and Q=0.48(R-Y)+0.41(B-Y), and these weightings ensure that the maximum chroma excursions are within 33% above white level (for the 100% yellow and cyan bars) and 33% below black level (for the red and blue bars).

In the receiver the demodulated U and V signals must be rescaled to give the correct amplitudes of (R-Y) and (B-Y) from which the R, G and B signals are then derived. The (G-Y) signal may be recovered either by matrixing the other two colour difference signals or by directly demodulating the subcarriers along the (G-Y) axis and then rescaling by multiplying by 0.7. (G-Y)=-0.509(R-Y)-0.194(B-Y). In an NTSC receiver (R-Y)=0.61I+0.62Q, (B-Y)=-1.1I+1.7Q and (G-Y)=-0.28I-0.64Q. See Colour Standards for more details.

Colour bar vectors, unweighted The colours along the U and V axes are included as ramps in the bottom castellations and, saturated, in the left-hand side castellations of Test Card W and in the 'ears' of the PM5544 pattern. The colours along a line normal to the (G-Y) axis (at 145.8° and 325.8°) are also included saturated in the PM5544 'ears'. Subcarrier corresponding to -I and +Q is included in the two squares flanking the peak white square at the bottom of SMPTE bars, but since it is centred on black level the correct RBG hues are not displayed (since the negative excursions of those waveforms are clipped at black level).

The unweighted colour difference vectors are shown in the diagram on the right. The colour values within the coloured hexagon are the only ones that can be reproduced by the television system, the outline of the hexagon representing the saturated colours (being defined as those where either one or two of R, G or B is 0). The values for (G-Y) are found by projecting a line parallel to the line joining the points where (G-Y)=0 onto the (G-Y) axis (these are not perpendicular in the unweighted diagram).


Values of components for 100% (100.0.100.0) colour bars (normalised)

(Amplitudes and angles of the resultant vectors are marked on the U,V diagram, above left)
  White Yellow Cyan Green Magenta Red Blue Black
R 1.00 1.00 0 0 1.00 1.00 0 0
G 1.00 1.00 1.00 1.00 0 0 0 0
B 1.00 0 1.00 0 1.00 0 1.00 0
Y 1.00 0.89 0.70 0.59 0.41 0.30 0.11 0
(R-Y) 0 +0.11 -0.70 -0.59 +0.59 +0.70 -0.11 0
(G-Y) 0 +0.11 +0.30 +0.41 -0.41 -0.30 -0.11 0
(B-Y) 0 -0.89 +0.30 -0.59 +0.59 -0.30 +0.89 0
U 0 -0.440 +0.150 -0.290 +0.290 -0.150 +0.440 0
V 0 +0.097 -0.620 -0.520 +0.520 +0.620 -0.097 0
I 0 +0.3217 -0.5990 -0.2773 +0.2773 +0.5990 -0.3217 0
Q 0 -0.3121 -0.2130 -0.5251 +0.5251 +0.2130 +0.3121 0


Below I have recreated the four main types of bars used in Europe - 100% PAL, BBC PAL (95%), IBA/EBU PAL (75%) and EBU SECAM (75%) together with the SMPTE (75%) bars used in NTSC countries. Because of the way your computer displays colour the rendering is not going to be terribly accurate - the results will depend on the colour palette of your browser and the gamma of the screen. If you are viewing in 256 colours, the results are going to be dreadfully misleading, I'm afraid. An accurate way of adjusting the saturation, brightness and contrast of a colour receiver is to observe the bars on the screen with just one colour lit (usually blue) and tweak until all the lit bars are the same brightness, so I have included the blue-gun display for each pattern as well. You may have noticed that test cards such as the PM5544 have dark and light non-coloured blocks adjacent to the colour bars like so:

Colour bars from PM5544

These have brightnesses corresponding to the lit and unlit states of each bar which makes comparison and adjustment slightly easier when only one colour is displayed on the screen.

Colour bars from PM5544, blue gun only

Test Card W Top Arrow RGB The white arrow at the top centre of Test Cards J and W may also be used as a peak-white reference, if it has not been cropped off the screen by overscan. Test Card W Top Arrow Blue

The waveforms displayed beneath each set of colour bars depict the PAL composite (sync = -300mV, black level = 0mV and peak white = +700mV) and the blue channel (black = 0mV, peak blue = +700mV) waveforms as displayed on an oscilloscope with a 100mV/cm vertical sensitivity and measured across 75 ohms. For a negatively modulated carrier, the top line of the scale would represent zero carrier, and the bottom line (sync level) 100% carrier.

The table given for each set of colour bars shows the luminance, R, G and B amplitude, and colour subcarrier p-p amplitude and phase for each colour. E'Y is the luminance amplitude. E'U is the chrominance modulating voltage used to create a modulated subcarrier component lying along the reference axis. E'V is the chrominance modulating voltage used to create a modulated subcarrier component at ±90° to the reference axis. S is the amplitude of the resultant subcarrier phasor (so 2S is the peak-peak subcarrier voltage). S Phase(n) is the angle of the resultant subcarrier phasor on odd lines of the first and second fields and even lines of the third and fourth fields. S Phase(n+1) is the angle of the resultant subcarrier phasor on the remaining lines. E'R, E'G and E'B are the amplitudes of the red, green and blue colour separation voltages.




100% full-colour bars100% Bars blue display only

100% full-colour bars composite waveform100% full-colour bars blue channel waveform

100% Bars (PAL)

 

100% amplitude, 100% saturation with white and black

Often referred to as 100.0.100.0 after the maximum and minimum percentage values of the components of the non-coloured and coloured bars. Sometimes only the last two figures (corresponding to the levels of the coloured bars) are used - 100.0.

These fully-saturated bars can be used from a test generator, but cannot be sent through a normal analogue terrestrial transmitter without distortion, and would probably wreak havoc with an analogue satellite transponder, due to the high amplitude, high frequency subcarrier which would be increased still further by the pre-emphasis network before frequency modulation.


Amplitudes and phases for 100% (100.0.100.10) PAL colour bars

  Burst White Yellow Cyan Green Magenta Red Blue Black
E'Y (mV) 0 700 620 491 411 289 209 080 0
2E'U (mV) 212 612 206 405 405 206 612 212 0
2E'V (mV) 212 0 140 861 721 721 861 140 0
2S (mV) 300 0 627 885 827 827 885 627 0
S Phase(n) (°) 135 - 167.0 283.5 240.5 060.5 103.5 347.0 -
S Phase(n+1) (°) 225 - 193.0 076.5 119.5 299.5 256.5 013.0 -
E'R (mV) 0 700 700 0 0 700 700 0 0
E'G (mV) 0 700 700 700 700 0 0 0 0
E'B (mV) 0 700 0 700 0 700 0 700 0



95% full-colour bars95% Bars blue display only

95% full-colour bars composite waveform95% full-colour bars blue channel waveform

95% Bars (PAL)

 

100% amplitude, 95% saturation with white and black

Often referred to as 100.0.100.25 or just 100.25 or 'BBC bars'.

The blue channel waveform shown on the left has three voltage levels, including black (blanking) level. EMax and EMin are the voltages of the coloured bars, referred to blanking level.

The saturation percentage of set of colour bars is given by the expression:

Saturation % = [ 1 - (EMin / E Max ) gamma ] x 100

In this case saturation = [ 1 - ( 0.175 / 0.7 )  2.2 ] x 100 = 95.26%.

More generally, the saturation percentage of any colour is given by the expression:

Saturation % = [ (Max - Min) / Max ]  x 100
where Max is the normalised (ie fraction of peak white) value of the highest of the colour separation signals and Min is the normalised value of the lower of the other two, both values being before gamma correction.

In this case Max is (0.7/0.7)2.2 = 1.0 and Min is (0.175/0.7)2.2 = 0.047 for all the coloured bars, so that:

Saturation % = [ (1.0 - 0.047) / 1.0 ] x 100 = 95.26%.

For a brief discussion of gamma correction, please visit World TV Standards.


Amplitudes and phases for BBC 95% (100.0.100.25) PAL colour bars

  Burst White Yellow Cyan Green Magenta Red Blue Black
E'Y (mV) 0 700 640 543 483 392 332 235 0
2E'U (mV) 212 0 459 155 304 304 155 459 0
2E'V (mV) 212 0 105 646 541 541 646 105 0
2S (mV) 300 0 470 664 620 620 664 470 0
S Phase(n) (°) 135 - 167.0 283.5 240.5 060.5 103.5 347.0 -
S Phase(n+1) (°) 225 - 193.0 076.5 119.5 299.5 256.5 013.0 -
E'R (mV) 0 700 700 175 175 700 700 175 0
E'G (mV) 0 700 700 700 700 175 175 175 0
E'B (mV) 0 700 175 700 175 700 175 700 0



75% full-colour bars75% Bars blue display only

75% full-colour bars composite waveform75% full-colour bars blue channel waveform

75% Bars (PAL)

 

75% amplitude, 100% saturation with white and black

Often referred to as 100.0.75.0 or just 75.0 or 'EBU' or 'IBA' bars.

The blue channel waveform shown on the left has three voltage levels, including black (blanking) level. EW is the voltage of the white bar, and EMax the voltage of the coloured bars, referred to blanking level.

The amplitude percentage for a given set of bars is derived from the expression:

Amplitude % = (EMax x 100)/EW

In this case amplitude = (0.525 x 100)/0.700 = 75%

These bars were at one time referred to at the BBC as 53% bars, since an RGB amplitude of the coloured bars of 53% of peak white before gamma correction yields an amplitude of 75% after gamma correction.


Amplitudes and phases for IBA/EBU 75% (100.0.75.0) PAL colour bars

  Burst White Yellow Cyan Green Magenta Red Blue Black
E'Y (mV) 0 700 465 368 308 217 157 060 0
2E'U (mV) 212 0 459 155 304 304 155 459 0
2E'V (mV) 212 0 105 646 541 541 646 105 0
2S (mV) 300 0 470 664 620 620 664 470 0
S Phase(n) (°) 135 - 167.0 283.5 240.5 060.5 103.5 347.0 -
S Phase(n+1) (°) 225 - 193.0 076.5 119.5 299.5 256.5 013.0 -
E'R (mV) 0 700 525 0 0 525 525 0 0
E'G (mV) 0 700 525 525 525 0 0 0 0
E'B (mV) 0 700 0 525 0 525 0 525 0



75% SECAM bars full-colour75% SECAM bars blue gun only

75% SECAM bars waveform on DR lines75% SECAM bars waveform on DB lines
SECAM composite waveforms of D'R and D'B lines

SECAM bell filter
Graph showing subcarrier frequencies and p-p amplitudes (relative to peak white) in SECAM 75% colour bars. Colour names reading upwards are on lines carrying D'B and those reading downwards are on lines carrying D'R.

75% Bars (SECAM)

 

75% amplitude, 100% saturation with grey (75% of white) and black

Often referred to as 75.0.75.0 or just 75.0 or 'EBU SECAM' bars.

SECAM bars have a 75% white bar instead of the peak white bar of 75% PAL bars, though in other respects the component form of the bars is similar. In SECAM coding the (R-Y) and (B-Y) signals are frequency modulated onto two subcarriers which are carried on alternate scanning lines. A delay line in the receiver presents the current and previous lines to the decoder so that the red, green and blue signals may be recovered.

There is no colour burst as such in the original SECAM, but undeviated subcarrier is carried in the back porch of the line blanking period in order that the decoder circuitry may be stable by the start of the active line. In the later system using line identification instead of the full-line field blanking interval identification signals, subcarrier at the limit of modulation (40756MHz on D'R lines and 3.900MHz on D'B lines) is present on the back porch. The chrominance signals are subject to video pre-emphasis before being frequency modulated and then the modulated subcarrier has rf pre-emphasis applied so that low-deviation carrier has a lower amplitude than that carrying saturated colours in an attempt to make the dot pattern on monochrome receivers less noticeable.

In the 'bell filter' diagram, the coloured areas under the curve represent the subcarrier excursions on D'B lines (bottom) and D'R lines (top) for large areas of colour. However, because of the video pre-emphasis, there is overshoot on colour transients and the subcarrier frequency may then move into the white areas under the curve. This also creates amplitude overshoot that may be clearly seen in the composite waveforms above. The thick blue and red arrowed lines running along the 'bell curve' represent the full-line ident signals that are transmitted in the field blanking period of some SECAM transmissions. The idents comprise subcarrier that starts undeviated and moves to the modulation limit as indicated by the arrows.

There is more on SECAM coding and decoding in World TV Standards.


Amplitudes and frequencies for EBU 75% (75.0.75.0) SECAM colour bars

  Back Porch [1]  White Yellow Cyan Green Magenta Red Blue Black
E'Y (mV) 0 525 465 368 308 217 157 060 0
D'B Freq (MHz) 4.250 (3.900) 4.250 4.020 4.328 4.100 4.402 4.172 4.480 4.250
D'B Dev (kHz) 0 (-350) 0 -230 +78 -150 +152 -78 +230 0
2S (D'B lines) (mV) 240 (500) 240 560 230 450 310 360 400 240
D'R Freq (MHz) 4.40625 (4.756) 4.40625 4.36125 4.68625 4.64125 4.17125 4.12625 4.45125 4.40625
D'R Dev (kHz) 0 (+349.75) 0 -45 +280 +235 -235 -280 +45 0
2S (D'R lines) (mV) 260 (500) 260 230 550 500 390 410 290 260
E'R (mV) 0 525 525 0 0 525 525 0 0
E'G (mV) 0 525 525 525 525 0 0 0 0
E'B (mV) 0 525 0 525 0 525 0 525 0

Notes:
[1] Figures in brackets refer to transmissions with Line Identification rather than Field Identification.




SMPTE full-colour barsSMPTE Bars blue display only


75% SMPTE Bars (NTSC)

 

75% amplitude, 100% saturation with grey (75% of white) and black

Similar 75% bars are used in the USA, where the white bar is also reduced by 25% in amplitude. They are specified as 77.7·5.77.7·5 because of the 7.5% black-level pedestal. With the US NTSC (National Television Systems Committee) coding system the receiver has a 'hue' control as well as brightness, contrast and saturation. Using the blue display, the saturation control is adjusted to make the white and blue bars appear the same brightness, followed by the hue control to match the magenta and cyan bars. This recreation of the SMPTE (Society of Motion Picture and Television Engineers) pattern has a peak white block at bottom left, flanked by subcarrier at -I and +Q (303° and 33°) centred at black level, and a small 'PLUGE' (Picture Line-Up Generating Equipment) display at bottom right, though the latter is probably too dark to see on your monitor, due to the presence of bright coloured areas, and is therefore not a great deal of use for setting up the correct black level of the display.

SMPTE -I and +Q blocks The -I and +Q blocks are a standard NTSC colour test signal, though because they are superimposed on black level, their true hues are not reproduced, since the colour separation signals derived are clipped below black level. Were they to be superimposed on mid-grey, as most PAL subcarrier test signals are, they might appear as shown on the right.





Luminance rampLuminance staircase

Ramp waveformStaircase waveform

Ramp and Staircase

 

Monochrome test signals

"Get it right in black-and-white" was one of the slogans that appeared at the same time as colour television. These ramp and staircase patterns are used to assess circuit linearity and greyscale tracking of receivers and monitors, amongst other things. The ramp (or 'sawtooth', since a display of several lines on an oscilloscope resembles a saw) is primarily an electronic test, and so the waveform that generates it is a straight line. However, the staircase may also used for optical assessment, and in this example the 'risers' are unequal in size, but on the display each step looks twice as bright as the previous to the eye, thanks to the square-law characteristics of the cathode ray tube.

However, linear steps are used where circuit linearity assessment is the prime objective, and colour subcarrier may be added to measure any differential gain and phase errors present. Such a waveform is sometimes inserted as a single line in the vertical blanking interval (VBI) and several different test signals, called VITS (vertical interval test signal) may be present. Along the broadcasting chain, the received waveforms may be measured automatically and adjustments made to correct for any errors that have arisen.


Luminance ramp with +U subcarrierEncoded Blue ramp

Ramp + subcarrier waveformEncoded blue ramp waveform

100% ramp with 0° 300mV p-p subcarrier 100% blue ramp encoded

Here (far left) subcarrier of amplitude 300mV p-p (same as burst) of phase 0° (along the +U axis) has been added to a standard 100% luminance ramp in order to assess the differential gain characteristics of the transmission path. The amplitude of the recovered subcarrier should be 300mV p-p throughout.

Compare that signal with a 100% ramp in the blue channel only which has been PAL encoded (near left).

Notice that generally the convention is for monochrome test waveforms to rise in value with time, while colour bars fall in luminance value, so that the two types may be easily distinguished.





PLUGEPLUGE Lifted
Standard PLUGE displayed correctly... ..and with the brightness control advanced.

PLUGE

 

Picture Line-Up Generating Equipment

The standard PLUGE display is used for setting up the brightness and contrast of receivers and monitors to a common standard. The background of the pattern is black level[1] - by convention a composite video signal has a peak white level of +700mV (+714mV in NTSC) and a blanking level of 0V. The synchronising pulses extend from 0V to -300mV (-286mV in NTSC). In the Unites States, the NTSC 525-line waveform has a black level of a few percent above blanking level - in other standards black level and blanking level are both 0V. On the left of the pattern two vertical bars are present, one a few percent brighter than black and the other a few percent darker. With the brightness control advanced (as simulated by the right-hand picture), both bars should be visible against the (now grey) background. By reducing the brightness it should be possible to make the left hand bar disappear into the background while the right hand one is still visible, albeit only under low ambient lighting conditions. That is the correct setting of the black level (brightness) control.

[1] It has been suggested to me that BBC PLUGE has the darker bar at black level, and the background a few percent brighter in order to avoid waveforms with 'illegal' blacker-than-black values. In this case the monitor brightness would be adjusted so that the left hand bar (black) is just visible agains the dark grey field.

The blocks on the right hand side are for adjustment of contrast. The contrast should not be so high that the brightest block appears larger than the others (blooming). A bank of monitors may be adjusted by means of a light meter to ensure that they all display the same picture. The three grey blocks should remain neutral and not have a colour cast. If they appear coloured, then the grey-scale tracking adjustments should be performed using suitable test patterns and measuring equipment.

High-brightness PLUGEHigh-Brightness PLUGE lifted
High-brightness PLUGE displayed correctly... ..and with the brightness control advanced.

Standard PLUGE has a low overall brightness content in order that the adjustments at near black level can be made accurately. This alternative display has a 1/3-screen peak white bar instead of the grey scale blocks so that the performance of the monitor can be assessed at higher beam currents. The average brightness level of this pattern is still only 33%, compared with 50% for other test patterns and cards and most broadcast pictures.


PLUGE in Test Cards J & W

As mentioned above, the SMPTE test pattern includes a small area of PLUGE, as do the UK test cards J and W. The 0% black square in the contrast stepwedge contains small spots of +7% and -7% brightness. These are flashed on and off at one second intervals to make them more visible and also as a test of data integrity in the case of a digital transmission. The 100% white square also contains flashing dots at 107% and 93% brightness to assess white clipping.

It is possible that either or both of the 106% and -6% dots may be removed by clipping during transmission.




RGB bars fed to component display
100% colour bars
RGB bars fed to a component display...

Component colour bars fed to RGB display
RGB colour bars
..and component bars fed to an RGB display

RGB staircase fed to component display
Component Luminance staircase
RGB luminance staircase
fed to a component display...

Component staircase fed to RGB display
Component Luminance staircase
..and component staircase
fed to an RGB display

RGB TCJ fed to component displayComponent TCJ fed to RGB display
RGB TCJ fed to a component display... ..and component TCJ fed to an RGB display

RGB and Component displays

 

When connections go wrong...

If you ever see a display showing strange colours like these, the chances are that a component signal is being fed to a display designed to take RGB, or vice versa. By component, we mean separate luminance (Y) and colour difference (Pb) and (Pr) signals, though strictly speaking, RGB is also component. See World TV Standards for details of the component signals.

Now that digital receivers, recorders and players are available that can supply component and/or RGB signals via their scart sockets, there is considerable scope for confusion. The diagrams assume that the connections incorrectly made are Y to G, Pr to R and Pb to B, and vice versa, though strange displays will appear if any two of the component or RGB signals are transposed anywhere in the equipment. A little applied brainpower is likely to result in a correct diagnosis more quickly than random lead-swapping.

The top pair of pictures shows 100% colour bars and the pair below shows a luminance staircase, each applied to the 'wrong' display. The correct display is shown as a strip at the bottom of each graphic. RGB and component signals can exceed the acceptable range for the 'wrong' inputs. When calculating the values for the resulting colours displayed I have assumed that the Pb and Pr inputs are clipped at ±350mV and the RGB drives to the screens are clipped at 100%.

The two lower pictures give a rough idea of how Test Card J might look if displayed in a similar manner. The pictures were manipulated in a graphics package, and the resultant colours are not as accurate as I might have wished.





Active colours  Active colours
Red only  Red only
Red missing  Red missing
Green only  Green only
Green missing  Green missing
Blue only  Blue only
Blue missing  Blue missing
(R-Y) inverted (wrong PAL ident phase)  (R-Y) inverted (wrong PAL ident phase)
(R-Y) missing  (R-Y) missing
(G-Y) missing  (G-Y) missing
(B-Y) missing  (B-Y) missing
Luminance only  Luminance only
Luminance missing  Luminance missing
Luminance missing - brightness advanced  Luminance missing - brightness advanced

Effects of missing signals on displays

 

as shown up by colour bars






This patchwork quilt shows how the absence of certain signals within the decoder may be diagnosed by displaying a colour bar pattern. Again, the colours are only an approximation to what would be displayed on the screen.


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Bookmarks

Television Website Bookmarks

 

Mike Brown/MB21/Ether.net
Andrew Emmerson/Paul Stenning/405 Alive/British Vintage Wireless Society
François Frappé, a French DX enthusiast from the 1960s and 70s
Keith Hamer
Darren Meldrum
Richard Russell
Justin Smith/Aerials and TV
Andrew Wiseman/625 Room
Bill Wright


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Compiled by Alan Pemberton
Sheffield, South Yorkshire, England
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