World Analogue Television Standards and Waveforms

 

Line Standards


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THE INFORMATION presented in this section has been compiled from several modern and historical sources and, errors and omissions excepted, the intention is to give a summary of the various standards at the time that they were current. Nevertheless, it is hoped that present-day standards are also accurately accounted for, and to this end any corrections would gratefully received (please E-mail me with any comments). Thanks are especially due to Mark Carver, Steve Palmano and Peter Vince for help and advice. Written sources consulted include [Electronics and] Wireless World and [Practical] Television magazines, textbooks by Benson KB and Whitaker JC, Carnt PS and Townsend GB, Holm WA, Hutson GH, Kerkhov F and Werner W, and technical publications from BBC, EBU, IBA and ITU.

On this page, reference is also made to drawings made by Alan Blumlein in 1935 of the 405-line pulse waveform and to a set of drawings from around 1950 of the four standards 405, 525, 625 and 819 lines, as they exhibit significant differences compared to the most recent specifications.

I am particularly indebted to Peter Vince for recently spotting certain anomolies in the ITU document BT.470-6 from which many of the details in these pages were taken. It has been superseded by BT.1700 and BT.1701, and the values quoted in these pages are now verified by those, and by SMPTE 170M-1999 in relation to the NTSC standard. Many of the NTSC parameters feature recurring decimal fractions, and I have indicated these throughout with square brackets, for example fSC = 3 579 545.[45]Hz



Contents

 

World Analogue Television Standards and Waveforms

Page 1:
Overview

This page:
Line Standards

Page 3:
Colour Standards

Page 4:
Transmission


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Line Standards

 

History

How many lines?

The choice for the number of lines per frame is not easy. Because the interlaced systems require accurate placement of scanning lines it is necessary to make sure that the horizontal and vertical timebases are in a precise ratio. This is achieved by passing the one through a series of electronic 'divider' circuits to produce the other. Each division is by a small number, usually odd and prime. 243 for example is 3 x 3 x 3 x 3 x 3, and 343 is 7 x 7 x 7. The 30-line standard, being purely mechanical, has no oscillators to synchronise together.

Prime factors for the line standards mentioned in the text are as follows:.

  1. 2 x 3 x 3 x 5
  2. 2 x 2 x 2 x 2 x 2 x 3
  3. 2 x 2 x 3 x 3 x 5
  4. 2 x 2 x 2 x 2 x 3 x 5
  5. 3 x 3 x 3 x 3 x 3
  6. 7 x 7 x 7
  7. 3 x 5 x 5 x 5
  8. 3 x 3 x 3 x 3 x 5
  9. 2 x 2 x 2 x 5 x 11
  10. 3 x 3 x 7 x 7
  11. 2 x 3 x 3 x 5 x 5
  12. 5 x 7 x 13
  13. 3 x 5 x 5 x 7
  14. 5 x 11 x 11
  15. 5 x 5 x 5 x 5
  16. 3 x 3 x 7 x 13

THERE HAVE been four 'high-definition' (having more than 400 lines - now called 'standard definition') line standards that have been in common use throughout the world for more than a few years, of which two remain. The survivors are the US 525-line standard and the German 625-line standard, the others being the French 819-line standard and the British 405-line standard, which was originally developed in 1935 by EMI and used in arguably the first all-electronic 'high definition' television service in the world, that broadcast from the Alexandra Palace studios and transmitters in London by the BBC.

Prior to that there had been many more lower-definition experimental transmissions including at least two systems designed by John Logie Baird using mechanical scanning devices. One, his 30-line 5 (later increased to 12.5) pictures per second standard, was little more than a demonstration, and though receivers - 'Televisors' - were sold to the general public, transmissions by the BBC (on the frequencies of two of their London medium wave radio services after their nightly closedown) were purely experimental.

Baird and his company eventually developed a 240-line, 25-picture-per-second, standard based on mechanical scanning (for announcements etc) and intermediate film (for programming, whereby the scene was filmed by a special 17.5mm ciné camera whose exposed stock passed directly into an automatic developing tank and was scanned within just over a minute by a 'flying spot scanner'). This alternated week by week with the EMI 405-line standard for the first three months or so (from 2 November 1936 until 8 February 1937) of the BBC's official programme service but was abandoned as being unworkable, despite Baird's introduction of all-electronic cameras based on an American design (the Farnsworth Image Dissector).

Meanwhile, in the USA, following experiments with a 240-line standard in 1933, the RCA 343-line standard in 1936 and the RMA 441-line standard in 1939 (at around this time there were also proposals from Philco for 605-line 24 frames per second and Du Mont for 625-line 15 frames per second standards), a 525-line all-electronic standard was developed, with regular transmissions starting in 1941. A potted history of the early developments in the US and an explanation of why there is no channel 1 is on Missing Channels?.

In France, 30-line transmissions had begun in 1929, with 441-lines being demonstrated in 1931 and officially begun in 1935 from the Eiffel Tower (46MHz vision, 42MHz sound). Although the service was closed down in 1936, a year later a 455-line service was started from the same spot, though it was closed down during the war. 180-line experiments were also carried out in France pre-war, as were 455/50 (two versions), 450/50 and 375/50, and there was even a 405-line station at Montrouge in 1939. In Italy the 441-line standard was in use for a while following experiments with 90-line and 375/50 standards, and in The Netherlands the German 180-line standard was superseded by tests with 405 lines by Philips of Eindhoven.

In March 1935, having tested on 96 lines in 1928 and on 90 lines in 1932 the German Post Office began a 'high' definition service from a transmitter in Berlin. This was on 180 lines 25 pictures per second, but since no receivers were available the only way to view was at public screenings in theatres and cinemas. A fire destroyed the transmitter a few months later in August, and when the service reopened in January 1936 it was firmly in the hands of the Nazis who saw it as a propaganda tool. They produced coverage of the Berlin Olympic Games (1-14 August 1936) using three cameras, one of which was manned by Walter Bruch, later to be the pioneer of the 625-line standard and PAL colour. Fernsehsender Paris Ident The system relied heavily on the use of intermediate film, of which several rolls have survived and have been transmitted by European broadcasters recently. On 1 September 1939 the change was made to 441 lines using electronic cameras, and a service was planned to cover the whole country but the war intervened. The Berlin transmitter was destroyed by allied bombing in November 1943, but for over two years the ex-French station on the Eiffel Tower in Paris radiated 441/50 programmes from a local studio called 'Fernsehsender Paris', mainly for the Wehrmacht occupation troops, but including French-language shows for the locals, until the Americans arrived on 16 August 1944.

Following the Second World War, the 525-line standard continued in use in the USA and was later introduced in Japan and other countries that used a 60Hz mains supply. In the UK the 405-line system was brought out of mothballs (television transmissions had ceased two days before the start of the war), despite EMI's recommendation for a 605-line standard (there was also a proposal for a 1 000-line standard for use in cinemas). In France a high-definition 819-line standard was introduced in December 1949, though the 441-line standard survived until January 1956, and in most of the rest of Europe and the 50Hz world Dr Bruch's 625-line standard was adopted. The first 625-line service was from Hamburg on Bands I and III starting in July 1950, with Ireland (1962), France (1963), the UK (1964), Belgium, Monaco and Luxembourg adopting the system on vhf and/or uhf in tandem with their 405/819-line monochrome services which were eventually dropped.

Russia was also a pioneer in television services, starting with 30-line tests in the mid-thirties, changing to higher definition standards in 1937, and the situation there too seems to have become rather chaotic. At one time, around 1950, The Moscow centre was busy changing over from 343 lines to 625 lines, whilst the Leningrad (St Petersburg) studios were still on the pre-war 240-line standard and preparing to convert to 440 lines. Moscow was an early adopter of colour however - regular programmes started in 1963, probably on System D 626/50 in SECAM IV, the brainchild of the research institute NIR.


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Line and Field Numbering

in interlaced line standards

These points are covered elsewhere in the text, but I have highlighted them here as they seem to cause confusion, particularly to authors of web sites.

Field numbering

In an interlaced standard there are two fields in every frame. In monochrome standards they are numbered 1-2. In colour standards, the fields are numbered 1-4 because there are patterns of colour subcarrier phase and burst blanking which repeat every four fields. In the PAL standard there is a relationship between subcarrier phase and line sync pulses which repeats every eight fields, but the fields are still numbered 1-4.

Field 1 is defined as the field in which the leading edge of the field sync pulse is coincident with the leading edge of the line sync pulse. Field 2 is therefore the field in which the field pulse occurs mid-way through a line.

In the 625-line standard the lines of Fields 1 and 3 appear on the screen above those of Fields 2 and 4, (ie the half line at the top belongs to Field 1 or 3) whilst in all other standards they are displayed below (ie the half line at the top belongs to Field 2 or 4).

In all line standards a full frame comprises Field 1 followed by Field 2 (or Field 3 followed by Field 4). Abrupt changes in picture content (ie 'cuts') should occur between Field 2 and Field 3 of Frame n, or Field 4 of frame n and Field 1 of Frame n+1. Where the picture information contained in two consecutive fields comes from the same time period (eg when originated on film) these should be Field 1 and Field 2, or Field 3 and Field 4, of Frame n (this is often not possible in 60fps standards, where the '3:2' pull-down form of telecine is used).

Odd/Even

Alternate fields are labelled as Odd or Even. The definition is such that in the 625-line standard Fields 1 and 3 are 'Even', whilst Fields 2 and 4 are 'Odd'. This is because of the odd number of equalisation pulses (5 of them) preceding each field sync pulse in the 625-line standard. In all other standards Fields 1 and 3 are 'Odd' whilst Fields 2 and 4 are 'Even'

The odd fields are defined as the ones that end in a half-line and even fields are those ending in a full line.

Line numbering

Lines are numbered consecutively in time, eg from 1 to 625, and not in the order in which they are displayed down the screen. Odd fields do not contain only odd numbered lines. Even fields do not contain only even numbered lines. In an interlaced line standard the second field starts half-way along the middle line in the series, eg halfway along Line 313.

Line 1 always occurs in Field 1 (and therefore also Field 3), though its actual position depends on the line standard.

Half lines...

..why have them?

All electronically scanned, interlaced, analogue television standards have half a line of video at the top and bottom of the picture, since each two-field frame has to comprise an odd number of lines for interlace to work. As the diagrams around these pages show, however, these half lines are not superflous. All the scanning lines slope downwards to the right slightly. The start of a full line at the top left of the screen is at exactly the same height as the start of the half line 'above' it, helping to preserve the picture as a rectangle and not a parallelogram. It would be feasible to blank the picture information on these half lines, but that would go against another essential property of the video signal - that the field blanking period should be identical on odd and even fields.

With modern pixel-based imaging and display devices the pixels are in perfectly horizontal rows, and so the active pictures officially comprise an even number of whole lines, but as long as the interlaced system prevails, half the top and half the bottom lines will have to remain blank.

Naming of Parts

 
 Key to Amplitudes  A Maximum excursion with chroma
B Peak white level
C Positive peaks of burst
D Black Level
E Blanking level (Reference Point)
F Negative peaks of burst
G Minimum excursion with chroma
H Sync tip level
 Key to Line Blanking Period 
I Time reference point 0H (Horizontal Time Datum)
J Line blanking period
K Line blanking rise time
L Front porch
M Line sync pulse width
N Line sync rise time
O 0H to start of burst
P Subcarrier burst duration
Q Burst envelope rise time
R 0H to end of line blanking
S Back porch
 Key to Vertical Blanking Period 
T Time reference point 0V (Vertical Time Datum)
U Field blanking period
V Pre-equalising duration
W Pre-equalising pulse width
X Equalising pulse rise time
Y Field sync duration
Z Field sync broad pulse width
aa Field serration pulse width
bb Field pulse rise time
cc Post-equalising duration
dd Post-equalising pulse width

Notes:

The Field Blanking Interval (U) includes the Front Porch (L) of the final line of active video in the previous field plus the Line Sync Pulse (M) and Back Porch (S) of the first active line of video in the new field. In the case of half-lines of video the Field Blanking Interval starts one half line period after the start of the Line Blanking Interval of the previous full line and one half line period before the end of the Line Blanking Interval of the next full line. Thus the Field Blanking Interval has the same duration in each field.

The Pulse Rise Time is defined as the time taken for the pulse to rise from 10% to 90% of its full amplitude.

The Pulse Width is defined as the time between the 50% amplitude points on the leading and trailing edges of the pulse.

In the NTSC standard the Start of Burst is defined as the zero crossing (positive or negative slope) that precedes the first half cycle of subcarrier that is 50% or greater of the burst amplitude. The End of Burst is defined as the zero crossing (positive or negative slope) that follows the last half cycle of subcarrier that is 50% or greater of the burst amplitude.

Pulse with raised cosine profile The preferred shape of all pulses is Raised Cosine (that is the portion of a sine wave between -90° and +90° for positive slopes and between +90° and +270° for negative slopes).


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Units

 

DIFFERENT OFFICIAL bodies prefer different units. The American Society of Motion Picture and Television Engineers (SMPTE) divides the video waveform between blanking level and peak white into one hundred IRE units, while the Comité Consultatif International des Radio Communications (CCIR) and European Broadcasting Union (EBU) refer to the actual voltages on a standard video waveform of amplitude 1Vp-p into 75ohm. In his original sketches of the 405-line standard Alan Dower Blumlein defines levels as a percentage of the peak to peak amplitude (peak white to sync tips), and as the four waveform diagrams I have used here did the same, I have retained the percentage figures, as they are easily converted to millivolts (1% = 10mV). Time values are universally quoted in sub-multiples of seconds, except that Blumlein perversely used fractions of a line period instead. SMPTE uses cycles of colour subcarrier to define the position and length of the NTSC colour burst, but I have converted these to microseconds for easy comparison.

I have redrawn the waveform diagrams as scalable vector graphics in the form of Adobe portable document format files. You will need to install Adobe Acrobat Reader to view them, but doing so will allow you to zoom in to details of portions of each waveform.


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Amplitudes

 
Key:    Key to Amplitudes  A Maximum excursion with chroma
B Peak white level
C Positive peaks of burst
D Black Level
E Blanking level (Reference Point)
F Negative peaks of burst
G Minimum excursion with chroma
H Sync tip level

For amplitudes of modulated carriers please see the Transmission Systems page.

n/a = not applicable 405 Lines 525 Lines 625 Lines (I/PAL) 819 Lines
mV IRE mV mV mV
A Maximum excursion with chroma n/a +131 +936 +933 n/a
B Peak white level +700 +100 ±1 +714 ±7 +700 ±20 +700
C Positive peaks of burst n/a +20 ±0.5 +143 ±3.5 +150 ±4.5 n/a
D Black level as blanking level +7.5 ±1 [1] +54 ±7 [1] as blanking level [2] +50 [2]
E Blanking level (Reference Point) 0 ±30 [3] 0 0 0 0
F Negative peaks of burst n/a -20 ±0.5 -143 ±3.5 -150 ±4.5 n/a
G Minimum excursion with chroma n/a -23 -164 -233 n/a
H Sync tip level -300 -40 ±1 -286 ±7 -300 ±9 -300

Notes:

[1] In the 525-line standard in Japan black level is the same as blanking level.

[2] Both the 625- and 819-line systems are shown in the c.1950 diagrams as having 'set up' (ie black level sits on a pedestal above blanking level) as in the 525-line standard. I have been unable to ascertain whether the 819-line or the original 625-line transmissions in Europe had this pedestal, but the 625-line transmissions in the UK have always has black and blanking levels the same, as in the 405-line standard. The set-up figure given for 625 lines is 3-6.5% (30-65mV). In System N the 525-line set up level of +7.5 ±1mV is used, except that N/PAL in Argentina has no set up.

[3] The tolerance for the 405-line black and blanking levels is taken from the original Blumlein specification of 1935, in which levels are referred to zero carrier. Sync level is given as 0%, peak white as 100% and black level as 30% ±3% during one transmission and a further ±3% day-to-day.


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Horizontal Timing

 
Key:    Key to Line Blanking Period 
I Time reference point H0 (Horizontal Time Datum)
J Line blanking period
K Line blanking rise time
L Front porch
M Line sync pulse width
N Line sync rise time
O H0 to start of burst
P Subcarrier burst duration
Q Burst envelope rise time
R H0 to end of line blanking
S Back porch

The fully annotated pulse waveform graphic for this section is a Portable Document Format file showing the line blanking period of all four line standards to the same scale. You can zoom and pan to any detail to read the measurement.

n/a = not applicable 405 Lines 525 Lines 625 Lines (I/PAL) 819 Lines
  Line frequency 10.125kHz Mono: 15.750kHz
Colour: 15.[734265]kHz ±0.0003%
15.625kHz ±0.02%
(Colour: ±0.0001%)
20.475kHz
  Total line period 98.765µs Mono: 63.492µs
Colour: 63.556µs
64.000µs 48.840µs
  Active line period 80.3µs 52.9µs 51.95µs 39.44µs
J Line blanking period 18.5µs [2] 10.7 +0.3 -0.2µs [3] 12.05 +0.4 -0.1µs 9.4 ±0.1µs
K Line blanking rise time (10%-90%) 250-500ns 140 ±20ns 250 +150 -100ns 200 ±30ns
L Front porch (between 50% points) 1.7µs 1.5 ±1µs 1.65 +0.4 -0.1µs [4] 0.5 ±0.1µs
M Line sync pulse width (between 50% points) 9 ±1µs 4.7 ±1µs 4.7 ±0.2µs 2.5 ±0.1µs
N Line sync rise time (10%-90%) <=250ns 140 ±20ns 250±50ns 120±20ns
O Time reference point [1] to burst start (between 50% points) n/a 5.3µs (19 cycles) 5.6 ±0.1µs n/a
  Subcarrier burst phase wrt time reference point n/a 0° ±10° 135°/225° ±10° [5] n/a
P Subcarrier burst duration (between 50% points) n/a 2.5 ±0.28µs (9 ±1 cycles) 2.25 ±0.23µs (10 ±1 cycles) n/a
Q Burst envelope rise time (10%-90%) n/a 300 +200 -100ns 250±50ns n/a
R Time reference point to line blanking end (between 50% points) 16.8 ±0.5µs 9.2 +0.2 -0.1µs 10.4µs 8.9µs

Notes:

[1] The Horizontal Time Reference Point is the 50% amplitude point of the leading edge of the line sync pulse

[2] The c.1950 diagrams give the line blanking period of the 405-line standard as 16.5 ±0.5µs, rather than 18.5µs. Blumlein defines it as 1/200 + 1/10 + 1/20 of a line period = 0.494 + 9.88 + 4.94 = 15.314µs.

[3] The 525-line standard line blanking period used to be 10.9µs (the c.1950 document gives it as 10.8µs min). The figure of 10.7µs is from a 1999 document (SMPTE 170M-1999).

[4] The front porch of the I/PAL waveform was increased from 1.55µs to 1.65µs to facilitate the use of 2" quadruplex (four transverse rotating heads) video tape recorders, whose head switching was performed during this part of line blanking, though this was not taken up in Europe or elsewhere. The c.1950 document defines the front porch as 1.3-1.8µs.

[5] Subcarrier along the -(B-Y) axis (the mean phase of the alternating bursts) should pass through 0° ±10° at the 50% amplitude point of the first broad pulse of Field 1 of the eight-field sequence. The burst is at 135° on the odd lines of Fields 1, 2, 5 & 6 and the even lines of Fields 3, 4, 7 & 8, and is at 225° on the even lines of Fields 1, 2, 5 & 6 and the odd lines of Fields 3, 4, 7 & 8.


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Vertical Timing

 

The following pulse waveform graphics for this section are available in Portable Document Format:

Obsolete standards

405-lines

System A (monochrome, obsolete)

Republic of Ireland until 23 Nov 1982
United Kingdom until 2 Jan 1985

819-lines

System E (monochrome, obsolete)

France until 1984, Monaco until 1985

System F (monochrome, obsolete)

RTB Belgium until February 1968
Luxembourg until 1 September 1971

Current standards

525-lines

System M (NTSC colour)

United States of America, Japan and most other territories with 60Hz mains

625-lines

Systems B, D, G, K, K1 and L (SECAM colour)

France from 1982
Luxembourg
People's Republic of China

Systems B, D, G, H, I, K, K1 and N (PAL colour)

Most other territories.



Start lines of each field

This is a depiction of a raster identifying the fields and the start and end lines of each active field. It shows the line numbers at the top and bottom of each picture (there are additional lines above the top of the picture that are either at blanking level or carry test or teletext data and in some standards there are a few blank lines below the bottom of the picture that carry equalisation pulses).

The alternate fields are shown in red and blue, and the horizontal flyback lines are shown as darker, dashed, lines. In the case of NTSC and PAL colour signals Fields 1 & 3 and Fields 2 & 4 are identical as shown except that there is a four-field sequence of burst phase and/or blanking that makes Field 1 distinguishable from Field 3, and Field 2 from 4. However, in the PAL system the initital relationship between colour subcarrier phase and line sync pulse is repeated only once every eight fields.

Because, in the interlaced system, there is an odd number of lines in each frame, it follows that each field contains a number of full lines plus one half line. The fields that have the half line at the end are called 'odd' and those with a full line at the end are called 'even'. This nomenclature is unfortunate because it is intuitive to suppose that fields given the numbers 1, 3, 5, 7 etc are 'odd' and the intervening ones are 'even', when this is not the case for all standards. In fact in the 625-line standard Field 1 is 'even' and Field 2 is 'odd'. The same is true of the Belgian 819-line standard in which the active lines of Field 1 run from half-way through Line 27 to Line 405, and those of Field 2 from Line 436 to half-way through Line 816.

In this diagram and some of the pulse waveform diagrams 'odd' fields are shown in blue while 'even' fields are in red.

End lines of each field

In all standards Field 1 is defined as the one in which the leading edge of the first field sync broad pulse coincides with the leading edge of a line sync pulse. 'Line 1' of the frame appears in this field. In all standards except 525-lines, Line 1 is defined as the line whose start is coincident with the start of the first field sync broad pulse in Field 1. In the 525-line standard Line 1 is defined as the first line in the field blanking period of Field 1.

Field 1 is also termed the 'dominant' field. Changes in picture information (in other words 'cuts' or 'edits') should occur at the start of Field 1. However, it is not possible to ensure this in the 525-line standard where filmed material is scanned using the '2:3 pull-down' principle.

Field order in 625-line standard

Because of the distribution of equalising pulses (2.5 eq, field sync, 2.5 eq) in the 625-line standard, (3.5 eq, field sync, 3.5 eq in the Belgian 819-line standard), the first active line on Field 1 of these two standards is a half-line, and likewise the last active line of the frame in Field 2. The other standards all have a full line as the first and last active lines in the frame, with the half lines coming at the bottom of the first field and top of the second. Thus in the 625 and Belgian 819 line standards Field 1 lines lie vertically above their corresponding Field 2 lines, whereas in the other standards they lie below. Note that to obtain a correctly interlaced display, the 'half' lines should start or end at precisely the same height on the screen as the adjacent 'full' lines.

Field order in other standards

Field order in the 625-line and Belgian 819-line standards

Field order in the 405-, 525- and French 819-line standards




Key:

Key to Vertical Blanking Period 
T Time reference point 0V (Vertical Time Datum)
U Field blanking period
V Pre-equalising duration
W Pre-equalising pulse width
X Equalising pulse rise time
Y Field sync duration
Z Field sync broad pulse width
aa Field serration pulse width
bb Field pulse rise time
cc Post-equalising duration
dd Post-equalising pulse width
n/a = not applicable 405 Lines 525 Lines 625 Lines (System I/PAL) 819 Lines
System E (France) System F (Belgium)
  Field frequency 50Hz Mono: 60Hz; Colour: 59.94[005 994]Hz 50Hz 50Hz 50Hz
  Total field period 20ms Mono: 16.667ms; Colour: 16.6833ms 20ms 20ms 20ms
  Total frame period 40ms Mono: 33.333ms; Colour: 33.3667ms 40ms 40ms 40ms
  Active field period 188.5 lines 242.5 lines [1] 287.5 lines [2] 368.5 lines 379.5-380.5 lines
U Field blanking period (N lines plus line blanking) 15 lines 20 lines [3] 25 lines [4] 41 lines 29-30 lines
V Pre-equalising duration n/a 3 lines
(6 narrow pulses)
2.5 lines [5]
(5 narrow pulses)
n/a 3.5 lines
(7 narrow pulses)
W Pre-equalising pulse width (between 50% points) n/a 2.30 ±0.10µs 2.35 ±0.10µs n/a 1.70 ±0.10µs
X Equalising pulse rise time (10%-90%) n/a 140 ±20ns 250 ±50ns n/a 200±100ns
Y Field sync duration 4 lines
(8 broad pulses)
3 lines
(6 broad pulses)
2.5 lines [5]
(5 broad pulses)
0.5 line
(1 broad pulse)
3.5 lines
(7 broad pulses)
Z Field sync broad pulse width (between 50% points) 40 ±2µs 27.1µs 27.3 ±0.2µs 20±1µs 21µs
aa Field serration pulse width (between 50% points) 10±2µs 4.70 ±0.10µs 4.70 ±0.10µs n/a 3.60 ±0.20µs
bb Field pulse rise time (10%-90%) <250ns 140 ±20ns 250 ±50ns <200ns 200±100ns
cc Post-equalising duration n/a 3 lines
(6 narrow pulses)
2.5 lines [5]
(5 narrow pulses)
n/a 3.5 lines
(7 narrow pulses)
dd Post-equalising pulse width (between 50% points) n/a 2.30 ±0.10µs 2.35 ±0.10µs n/a 1.70 ±0.10µs

Notes:

In all line standards, certain lines in the field blanking interval between the end of the field synchronising broad pulses and the first active picture line are available for test signals and data, otherwise they should remain at blanking level. However, in some SECAM encoded 625-line standards certain of the lines in the field blanking interval carry colour subcarrier for receiver synchronisation purposes.

[1] In the 525-line standard Line 21 (the first active line) is sometimes used for Closed Caption data which is regarded as programme material and should not be blanked, except where video processing would destroy their usability.

[2] In the 625-line standard the active part of Field 1 starts halfway along Line 23. In the PAL-Plus system (an analogue '4:3-compatible' 16:9 system in which the central 432 active lines form a letterbox picture and 'helper' signals are encoded on the black lines above and below, from which a suitable receiver may reconstruct the intermediate missing lines to display a full-resolution 575-line 16:9 picture - see Steve Hosgood's web site) the first half of that line is used to transmit a widescreen switching (WSS) signal and this is now used more generally by broadcasters for in-house use and by digital receivers for communicating aspect ratio information to display devices. In general, this signal should be blanked by equipment (at both transmitting and receiving ends of the chain) where it is no longer relevant to the output signal (eg following aspect ratio conversion). In the analogue domain the first half of Line 23 (and the second half of Line 623, which contains the first equalising pulse) are considered to be in the field blanking period, but in the digital domain both lines are regarded as fully in the active period (even when they contain blanking level), there being 576 complete lines instead of 2 x 287.5 = 575.

[3] In the 525-line standard the final line in the field blanking period (Lines 20 and 282) may not be blanked by certain component equipment, resulting in a 19-line field blanking period when signals from such equipment are coded into NTSC.

[4] In the 625-line standard, Lines 7 to 22 inclusive and 320 to 335 inclusive may contain teletext, identification, control or test signals.

[5] At the VIth Plenary Meeting of the CCIR in Geneva in 1951 the 625-line standard was specified as having six pre-equalising pulse, six broad field sync pulses and six post-equalising pulses, as in the 525-line standard, with a note that the Swiss delegation had proposed a change to five of each pulse. This change, which was later adopted, has put the 625-line waveform out of kilter with the others, since the field reference point is now shifted half a line with respect to other systems, and the frame reference point by a whole field. 625-line field order is now even-odd in each frame instead of odd-even as in the other standards, so that the first and last lines of each frame are half instead of full length.


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Line and Field Synchronisation

 

in 2:1 interlace systems

When we draw the field blanking periods of two adjacent fields one below the other, we can align vertically either the line sync pulses or the field sync pulses, but not both. It appears that there is a half-line discontinuity in one or the other. In fact this is an optical illusion. The line sync pulse train is continuous and the pulses are equally spaced throughout time, as are the field sync pulses. It is the fact that there is an odd number of lines per field that makes the field sync pulse coincide with a line sync pulse on one field and occur halfway through a line on the next.

Brick wall panel, 'even'

Let's build a small brick wall using the familiar 'stretcher bond'. The joints between the bricks are staggered from course to course, but the ends of the wall are straight because we have used a half brick at one or other end of each course. We have added a damp-proof course in blue which serves to highlight one course of the wall, and we will concentrate our attention on this. In this case there is a full brick at the right-hand end of the damp-proof course - this is an 'even' wall panel.

To build a larger wall, we could prefabricate several small panels and join them together on site, alternating 'odd' and 'even' panels. Allowing our eye to follow the damp-proof course we see that the pattern of whole bricks is uninterrupted, though there is a mortar joint between two half bricks at certain points. (We are ignoring the practicality that the mortar ruins the damp-proof course here.)

Brick wall panel, 'odd'mortarBrick wall panel, 'even'mortarBrick wall panel, 'odd'mortarBrick wall panel, 'even'

The thin vertical lines of mortar running the height of the wall represent field sync pulses, while the vertical mortar between individual bricks represents line sync pulses. You can look at the wall in two ways: by considering a single panel (having the width of 'one field') you can see how bricks on one course are displaced by half their length with respect to those in adjacent courses, showing how the scanning lines on one field fit in between those of the next; by considering a course horizontally across several panels you can see that the bricks are regularly spaced, as are the panels.

The line and field scanning timebases in a television receiver run continuously and are synchronised by means of line and field sync pulses within the 'composite' (meaning containing both picture and synchronising information) video signal. Both sets of pulses are carried in the same portion of the video signal (between 0 and -0.3V) - in modern communications parlance they are 'multiplexed' - and have to be separated out by the receiver. This is done by 'differentiating' the short line sync pulses and 'integrating' the longer field sync pulses. In practise this means that the line oscillator is triggered by the slope of the leading edge of the line sync pulse and the vertical timebase by the accumulated 'area' under the field sync pulse. In modern receivers neither timebase is actually triggered directly by the pulses, but indirectly via 'flywheel' circuitry of varying complexity. However, early receivers used 'direct' sync, and it was for these that the system was specified.

There are two constraints that have to be put on the two sets of pulses. Firstly the train of line sync pulses must not be interrupted, even during the field sync pulse, and secondly the format of the field sync pulses must be identical on each field, even though the line sync pulses occur at different times on each, otherwise the timing of the vertical retrace - and hence the integrity of the interlace - would be compromised.

Field blanking period of 625-line standard

Here is the pulse waveform diagram of part of the field blanking period of the 625-line line standard, which uses equalisation pulses and a train of broad field pulses. The sawtooth waveform of the receiver horizontal scanning circuitry is superimposed as a pecked red line, and the edges that trigger it are shown in the same colour. The even fields have a red backgound and the odd fields blue. The portion of the field syncs that is identical on both fields is indicated by a white background and line numbers are in grey.

The equalisation pulses, and the line sync pulses during the broad field syncs, appear at twice the line frequency. The horizontal timebase ignores those that arrive halfway through the forward scan.

The equalisation pulses have half the width (time duration) of normal line sync pulses. This is so that line-for-line they carry the same energy as sync pulses. If both sets of pulses were the same width and the output of the field sync pulse integrater were to double suddenly it might result in false field triggering. Note that where the line sync pulses occur during the broad field sync pulses they appear to be inverted. It is the falling edge (0V to -0.3V) that triggers the timebase.

As an analogy, consider a railway or tramway that moves temporarily from an open rail/sleeper/ballast section to a solid level roadway section or vice-versa. The contruction of the actual rails is entirely different (the roadway ones appear to be in bas-relief) but from the point of view of the wheel bogies the essential feature is a pair of vertical and horizontal surfaces separated by the gauge of the system and level with the rest of the track. At a complicated junction there may be other rails criss-crossing ours, but they are effectively ignored by our wheels.



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The 405-Line Standard

 

Summary of features

Although the 405-line standard was a development of previous systems, and its requirements seem modest by today's standards, it was touch-and-go whether the system would actually work properly. It was the first standard to employ interlace, which, together with the 3.0MHz vision bandwidth effectively doubled the resolution of the Baird system in the horizontal, vertical and temporal dimensions. In fact because the Kell effect was unknown at the time, the horizontal resolution was made greater than that required to match the vertical resolution - especially since many receivers were incapable of displaying 100% accurate interlace (resulting in 'line pairing' which reduced the apparent vertical resolution and emphasised the coarse line structure even further). The choice of horizontal and vertical blanking times was generous to give receiver timebases plenty of time for the flyback stroke to occur while the signal was at black level.

The pulses were fairly primitive, there being no equalising pulses nor pedestal, often called 'set up'. Narrow horizontal synchronising pulses were detected by a 'differentiating' circuit in the receiver and made to trigger the flyback of the line timebase which was set to free-run slightly slowly. Similarly, several broad field pulses were detected by an 'integrating' circuit, directly triggering the vertical timebase. The choice of positive modulation, with sync tips at 0% and peak white at 100% modulation levels meant that 30% of the vision carrier modulation was devoted to video. However, with positive modulation it is difficult to implement an automatic gain control in the receiver that is independent of the video level, and impulse interference results in white spots which are noticeable and annoying.


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The 525-Line Standard

 

Summary of features

The 525-line NTSC standard was developed in the USA which has a 60Hz mains electricity supply and so was given a 60 field-per-second interlaced repetition rate. Apart from the improved vertical, horizontal and temporal resolutions, there are several features not found in the 405-line standard.

Firstly, a set of equalising pulses at twice the line frequency was added at the end of each field, allowing the field pulse integrator to fire at precisely the same moment on both odd and even fields, allowing much better interlace in the receiver. Black level was made higher than blanking level, which means that during line and field flyback the beam current is always cut off and no flyback lines are visible on the display tube face. The vision bandwidth of 4.2MHz was chosen so that the horizontal and vertical resolutions would be the same after Kell adjustment. Negative modulation was chosen which allows receivers to measure the carrier level simply and accurately by sampling the sync level which is always 100% modulation. However, interference pulses, while black and therefore less visible, confuse the sync separater circuits causing false triggering of the line and field oscillators. This was less of a problem when 'flywheel' rather than direct synchronisation began to be used.

Peak white level is set to give 10% carrier level, though colour subcarrier excursions take the level lower. Only around 60% of the carrier modulation is devoted to video. The sound carrier is frequency modulated, which allows the 'intercarrier sound' technique to be used, making fine tuning much easier and less critical. When the NTSC colour system was introduced, it became impossible for the original line and field frequencies to co-exist with the 4.5MHz vision-sound carrier spacing, and so slightly different timebase frequencies were chosen. In fact the field and line frequencies were each multiplied by (1000/1001) so that there was no longer an integral number of frames per second.

Because of the necessarily low colour subcarrier frequency the resolution of the NTSC colour system (both luminance and chrominance) is severely limited when compared to the 625 line standard. Also the colour system itself often led to hue errors that could not be corrected automatically. Later decoders featuring 'comb' filters largely overcame these limitations.


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The 625-Line Standard

 

Summary of features

The 625-line standard was developed in Germany after the second world war as a 50Hz version of the US NTSC standard. The horizontal timebase frequencies are almost the same, but because of the higher number of scanning lines, the vision bandwidth was increased to 5.0 (CCIR), 5.5 (UK) or 6.0MHz (OIRT). Generally no pedestal is used, and black level is the same as blanking level. Most countries used the NTSC method of negative vision modulation and frequency modulated sound, but France (and Belgium originally) opted for positive modulation and amplitude modulated sound. Intercarrier sound therefore cannot be used in France, though where Nicam digital sound is present, the vision modulation depth is reduced so that there is at least 5% carrier during sync pulses.


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The 819-Line Standard

 

Summary of features

The 819-line standard was developed in France after the war as a high-definition alternative to the 441-line standard used previously. Because it had more than twice the number of lines of the 405-line standard, and therefore over twice the horizontal timebase frequency, it required four times the vision bandwidth (10MHz). Otherwise, the standard was very similar to 405-lines. There were no equalising pulses or pedestal in the French version, and instead of a series of broad field sync pulses there was just a single one, half a line period in length, though the Belgian version differed (in this respect only) by having seven broad field pulses preceded and followed by sets of seven equalising pulses in a similar way to the 625-line standard. Positive modulation vision and amplitude modulated sound were used, together with the usual vestigial sideband, though a mixture of upper and lower sideband channel allocations was devised whereby adjacent channels in Band III overlapped, allowing more transmitters to be accommodated in the same spectrum.

It could be said that 819-lines was a High Definition standard before its time. In modern digital parlance it would be described as '800x738 50 2:1', though the aspect ratio was 4:3 and the pictures were monochrome only. For comparison, the coarsest current (16:9) digital HD standard is '1280x720 50 1:1', of which the centre 4:3 portion would be 960x720 pixels. Modern HD standards use 'square pixels' rather than Kell-factor elongated ones since they are expected to be displayed on dot matrix rather than rasterised displays, and so the horizontal and vertical resolutions are compromised equally by being separated into discrete pixels in both directions instead of only the vertical.


Related sections: | E-mail me | Home Page | 405-Line Standard | Test Cards | Teletext |
World TV section: | Overview | Line Standards | Colour Standards | CCIR Systems | Radio Channels |
This page: | Top | Contents | History | Naming of Parts | Units | Amplitudes | H Timing | V Timing | Syncs | Summary | Bookmarks |

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Bookmarks

Television Website Bookmarks

 

Mike Brown/MB21/Ether.net
Andrew Emmerson/Paul Stenning/405 Alive/British Vintage Wireless Society
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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