World Analogue Television Standards and Waveforms

HE 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.

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 It has been superseded by BT.1700 and BT.1701, and the values quoted in these pages are now verified by those, and by SMPTE 170M-1999 in relation to the NTSC standard.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 f_SC = 3 579 545.[45]Hz

HERE ARE currently three colour coding standards in common use: NTSC, SECAM and PAL, though several others have been proposed, including SECAM IV and MAC. John Logie Baird (of course) was the first to demonstrate colour television using a field-sequential version of his 30-line standard, but during the war he also had a hand in developing high-quality electronic systems and came up with his 'Telechrome' two-and three-gun display tubes. George Valensi, a Frenchman, was the first to propose a compatible system based on luminance and chrominance signals.

However, most of the early work on colour in the United States of America was based on field-sequential systems that were incompatible with monochrome transmissions. CBS developed a 343-line 120 fields per second system in 1940 and followed it with a 525-line 144 fields per second system that required a 12MHz vision bandwidth in 1945. This latter system was transmitted for a while in 1951, but quickly abandoned, and the search for a compatible system was intensified.

Experimental colour systems:

System A/NTSC: Transmitted by BBC on Bands I and IV in London between 1955 and 1960
^[1] These values were used with E'_Q signals having slightly different bandwidths during early System A/NTSC experiments.
System I/NTSC: Transmitted by BBC on Band IV in London from 1963-64
System A/SECAM: Transmitted by ITA in Band III in 1962
System M/SECAM: Considered by US and Japan for recording onto unmodified monochrome video tape recorders in 1965 (transcoded to/from NTSC)
System A/PAL: Demonstrated by BBC and ABC in London from 1965-66

NTSC is named after the National Television Systems Committee which had been set up in 1940 by the RMA (Radio Manufactureres Association) and was asked in 1949 to find a compatible colour television standard for the USA to replace the incompatible field-sequential colour system that had been used since 1948. RCA took up the challenge and devised an entirely new system from start to finish - from camera, through coder, decoder and shadowmask display tube - that could be also received in black and white on an existing receiver. It was submitted for approval on 25 June 1953 to the FCC (Federal Communications Commission) who authorised it for use on 1 January 1954, and has only ever been adopted for use (other than experimentally) on 525-line systems.

Effect of positive and negative phase errors on NTSC colour bars (correct at centre)

In it the two colour difference signals (see the section on colour) are amplitude modulated in quadrature (QAM) onto a subcarrier of 3 579 545.[45] ±10Hz. A burst of subcarrier is placed on the back porch (between the end of the line synchronising pulse and the start of video) in order to synchronise the local oscillator required to demodulate the signal in the receiver. Unfortunately, during propagation, the phase relationship between the burst and the picture information can drift, resulting in incorrect hues that cannot be corrected automatically, and so a manual front-panel 'hue' control is incorporated in receivers which the user adjusts for 'most natural' colours.

In addition, crosstalk between the two colour difference signals resulting from poor separation during decoding also leads to incorrect hues and the orginal RCA proposal made use of colour phase alternation (CPA) in order to reduce it. However, with no delay line available for electrical cancellation of the errors strong 'Venetian blinds' appeared as a crawling line structure on the picture and this was considered unacceptable, so the I/Q method of modulation as described below, which also reduced crosstalk, was used instead. Ironically, since the introduction of glass delay lines for PAL systems elsewhere, NTSC decoders have begun to use them in 'comb filters' which eliminate crosstalk between the luminance and chrominance signals, improving the quality of both, though without removing hue errors.

Because of the relatively low vision bandwidth (4.2MHz) of the 525-line NTSC system, the choice of colour subcarrier frequency is a compromise. It would be impracticable to transmit the U and V signals [ E'_U = 0.493(E'_B - E' _Y) and E'_V = 0.877(E'_R - E' _Y) ] without seriously distorting them due to restriction of the upper sideband, so instead a further property of human vision is exploited. Green-magenta vision is less acute than orange-cyan, so it was arranged for the axes of the chrominance information to be rotated by 33° in order that the orange-cyan (I) signal may be assigned about twice the bandwidth of the green-magenta (Q) one. Therefore, the Q signal is transmitted with equal sidebands, while the upper sideband of the I signal is truncated.

There are several options available to the designer of the NTSC decoder. The chrominance signal may be filtered so that both I and Q have the same bandwidth and then either the I and Q or U and V signals may be demodulated using suitably phased referenced oscillators, or the the wider lower sideband of the I signal may be exploited by using single-sideband demodulation. The green colour-difference signal may be obtained from either the I and Q or U and V signals, whereby:
(G-Y) = -0.28I - 0.54Q = -0.575U - 3.94V = -0.509(R-Y) - 0.194(B-Y)
or it may be demodulated directly using a reference subcarrier signal having a phase angle of 55.8° wrt the +(B-Y) axis. The resultant is then attentuated to 70% of its level to obtain the correct (G-Y) value.

Although never used in service, experimental 405-line colour transmissions were made in the 1950s and 1960s by the BBC using NTSC coding. The subcarrier frequency was 2 657 812.5Hz. Tests were also made with E'_Q signals of slightly higher and lower bandwidths than the one finally used. These necessitated subcarrier frequencies of 2 809 687.5Hz and 2 505 937.5Hz. The BBC also made tests with 625-line NTSC, in which the subcarrier frequency was 4 429 687.5Hz.

NTSC coding is used on System M only.

The instantaneous voltage E_NTSC of the complete encoded NTSC signal is given by:
E_NTSC = E'_Y + E' _Q sin(wt + 33°) + E' _I cos(wt + 33°)
where:
w (omega) = 2 pi times the subcarrier frequency f_SC
E'_R, E'_G and E'_B are the gamma corrected primary colour signals
E'_Y = 0.299E'_R + 0.587E' _G + 0.114E'_B
E'_Q = 0.74(E'_R - E' _Y) - 0.27(E'_B - E' _Y)
E'_I = 0.48(E'_R - E' _Y) + 0.41(E'_B - E' _Y)

The modulation mode of the colour subcarrier is suppressed-carrier amplitude-modulation of two subcarriers in quadrature (QAM).

Colour synchronisation of the oscillator in the NTSC decoder is by reference to a burst of nine cycles of subcarrier inserted into the back porch of the line blanking period. The p-p amplitude of the burst is 4/10 of the luminance (blanking to peak white) amplitude and the phase is 180° relative to the (E'_B - E'_Y) axis. It is important that the burst amplitude remains in the correct proportion to the chrominance amplitude, since the burst amplitude is used as a reference for automatic gain control in the receiver and errors would affect the saturation of the displayed picture.

Subcarrier frequency and chrominance bandwidth

For M/NTSC the field frequency f_V and line frequency f_H (nominally 60Hz and 15.750kHz for monochrome transmissions) are derived from the subcarrier frequency f_SC = 3 579 545.[45] ±10Hz as follows:
f_H = 2f_SC / 5x7x13 Conversely, f_SC = (455/2) f_H
f_V = 2f_H / 3x5x5x7
These values may also be expressed as follows:
f_SC = 5 000 000 x 63 / 88 = 3 579 545.[45]Hz
f_H = 5 000 000 x 63 / 88 / 227.5 = 15 734.[26573 4]Hz
f_V = 60 x 1000 / 1001 = 59.94[005 994]Hz

The attenuation of the colour difference signals before modulation in System M/NTSC is as follows:
E'_I - <3dB @ 1.3MHz, >20dB @ 3.6MHz
E'_Q - <2dB @ 0.4MHz, <6dB @ 0.6MHz, >6dB @ 0.6MHz

For the experimental System A/NTSC transmissions radiated by the BBC between 1956 and 1963 the field frequency f_V and line frequency f_H (nominally 50Hz and 10.125kHz for monochrome transmissions) were derived from the subcarrier frequency f_SC = 2 657 812.5Hz as follows:
f_H = 2f_SC / 3x5x5x7 Conversely, f_SC = (525/2) f_H
f_V = 2f_H / 3x3x5x7

The attenuation of the colour difference signals before modulation in System A/NTSC was as follows:
E'_I - <3dB @ 1MHz
E'_Q - <3dB @ 0.34MHz

In addition, two other values for the bandwidth of the E'_Q signal were tested by the BBC. Each required a different subcarrier frequency.
E'_Q - <3dB @ 0.22MHz, f_SC = 2 809 687.5Hz
E'_Q - <3dB @ 0.46MHz, f_SC = 2 505 937.5Hz

For the experimental System I/NTSC transmissions the field frequency f_V and line frequency f_H (nominally 50Hz and 15.625kHz for monochrome transmissions) were derived from the subcarrier frequency f_SC = 4 429 687.5Hz as follows:
f_H = 2f_SC / 3x3x3x3x7 Conversely, f_SC = (567/2) f_H
f_V = 2f_H / 5x5x5x5

Frequency spectrum of a System M/NTSC vestigial sideband transmission

The frequencies marked are relative to the vision carrier. The levels and slopes of the curves are stylised for clarity. The attenuation on the 'plateaux' is <3dB and is >20dB where the curves meet the x-axis. The colour subcarrier is supressed - the vertical magenta line indicates its frequency.

Colorimetry

In 1953 the CIE coefficients for the primary colours, and for the reference white of colour display devices (where E'_R = E'_G = E'_B), of the NTSC standard were chosen as follows:

However, over the years the phosphors used in colour display tubes have changed, and they no longer match those for which the coefficients were chosen. Nevertheless, the same primaries continue to be used in coding NTSC signals in the United States of America, and correction circuits are now in use in studio monitors and domestic receivers in order to improve the colorimetry of the display. The white reference for monitors is now specified as Illuminant D₆₅ (x = 0.313, y = 0.329). In Japan, studio monitors are adjusted to display Illuminant D, 9 300K.

Twitter

Thanks to interlace, the overall flicker rate of 525/60 pictures is 60Hz. However, along horizontal edges there is 'twitter' at frame rate - ie 30Hz - which can be very noticeable when the adjacent areas have significantly different luminance values. The same frame-rate twitter pattern is visible on a monochrome receiver displaying any colour signal, but the effects on a colour receiver depend on the encoding standard. Here is what happens in the NTSC colour system across four fields of video:

Let us take as an example a transition between a red area and a blue area, as in the detail shown on the left, which shows both fields superimposed (or a non-interlaced 'progressive' version, if you like). The colours chosen lie precisely along the (R-Y) and (B-Y) axes for ease of calculation. When scanned in the 525/60i NTSC colour standard the transition would appear as in the diagram on the right, over four successive fields of video. Since the two colour difference signals are transmitted simultaneously on each line and the decoder derives both colour difference signals from the current line, there is red/blue twitter at 30Hz, as with monochrome.

SECAM, Système en Couleur à Mémoire, sometimes known as Séquentielle Couleur à Mémoire, or Sequential Colour avec Mémoire, is used on 625-line standards in some Francophone and former USSR countries, though many of the latter began to change to PAL following the fall of communism in the nineteen nineties. It was a French development (by Henri de France) of NTSC in which the colour difference signals are separately frequency modulated on two subcarriers of frequencies 4 406.25 ±2kHz and 4 250.0 ±2kHz ^[1] and transmitted on alternate lines and stored in a steel or glass acoustic delay line in the receiver. Its advantages in robustness and immunity to phase-shift errors are outweighed by its higher visibility on monochrome receivers, poorer colour quality and greater complexity at the broadcasting end of the chain. A different system, technically similar to an abandoned BBC proposal of 1963, NIR or SECAM IV, was developed in the USSR and then the USSR and France worked together to develop SECAM III which was tested during 1966/67 and launched simultaneously in both countries on 1 October 1967.

^[1] Originally both subcarriers had the same undeviated frequency of 4 437.5kHz with a symmetrical deviation of ±770kHz, but like many of the characteristics this was later 'optimised' in order to improve some of the shortcomings of the system.

In the autumn of 1962 the Independent Television Authority (ITA) in the UK made tests using System A/SECAM in order to assess monochrome receiver compatibility. The subcarrier frequency was 2 660kHz with a deviation of ±250kHz. From 1963 the ITA made 625-line SECAM colour tests, as did the BBC.

SECAM coding is used on Systems B, D, G, K, K1 and L.

The instantaneous voltage E_SECAM of the complete encoded SECAM signal is given by:
E_SECAM(R) = E'_Y + G cos 2pi (f' _0R + df_0R f' ₀ D'_R* dt)
E_SECAM(B) = E'_Y + G cos 2pi (f' _0B + df_0B f' ₀ D'_B* dt)
where:
(R) refers to the lines carrying (R-Y) information
(B) refers to the lines carrying (B-Y) information
E'_R, E'_G and E'_B are the gamma corrected primary colour signals
E'_Y = 0.299E'_R + 0.587E' _G + 0.114E'_B
It is permitted to attenuate E'_Y within the chrominance passband (as a function of its amplitude) in order to reduce interference distortions between the luminance and chrominance signals.

The chrominance signals are built up as follows:

Firstly the two colour difference signals are scaled and signed:
D'_R = -1.902(E'_R - E' _Y)
D'_B = 1.505(E'_B - E' _Y)

Then low-frequency pre-correction is applied, with a frequency response rising from 0dB at 10kHz to a peak of 9dB at 750kHz and falling to 0dB at 1.5MHz:
D'_R* = A_BF(f) D' _R
D'_B* = A'_BF(f) D' _B
where:
A_BF(f) = (1 + j ( f / f ₁ )) / (1 + j ( f / 3f ₁ ))
f = signal frequency (kHz)
f₁ = 85kHz

The two chrominance components are then frequency-modulated onto the two colour subcarriers, whose instantaneous amplitudes are given by:
cos 2pi (f'_0R + df _0R f'₀ D' _R* dt)
and
cos 2pi (f'_0B + df _0B f'₀ D' _B* dt)
where:
f_0R = 4 406 260 ±2 000Hz [B-Y subcarrier rest frequency]
f_0B = 4 250 000 ±2 000Hz [B-Y subcarrier rest frequency]
df_0R = 280 ±9kHz [nominal deviation of R-Y subcarrier]
df_0B = 230 ±7kHz [nominal deviation of B-Y subcarrier]

The amplitude of each subcarrier is frequency-weighted by a factor G by means of a 'bell filter' (rf pre-emphasis) before being added to the luminance component:
G = M₀ (1 + j 16F) / (1 + j 1.26F)
where:
2M₀, the p-p subcarrier amplitude, is 23 ±2.5% of the luminance (blanking level to peak white) amplitude
F = f / f₀ - f ₀ / f
f₀ = 4 286kHz
f is the instantaneous subcarrier frequency

By this means, the p-p amplitude of the subcarriers is maintained at 23% of peak white at the centre frequency of the bell filter (4.286MHz) around which most of the low-luminance colours are found, and 70% of peak white at the limits of modulation (3.9MHz and 4.756MHz) which are only reached during colour transients. Thereby the visibility of the subcarriers on monochrome receivers is reduced somewhat. In the receiver a reciprocal bell, or 'remise-en-forme' filter, giving an attenuation of 0dB at 4.286MHz and 12dB at 3.750 and 4.822MHz (the tolerance at all points is ±0.5dB) restores the correct amplitudes before passing the signal through the 64µs delay line.

This diagram from the SECAM colour bars section of the Test Cards page shows the frequencies and amplitudes (relative to peak white) of the two subcarriers for a set of 75% colour bars, and also shows the locus of the subcarriers during the field interval 'green bottles' ident signals, where each starts with undeviated carrier during the back porch of line blanking and progresses in a linear fashion to its respective limit of modulation.

The two coloured areas under the curve represent the deviation of the two subcarriers during period of constant colour (D'_B at the bottom and D'_R at the top). The white areas are those occupied by 'overshoot' of the subcarriers on colour transients due to the video pre-emphasis applied to the colour difference signals before modulation. The colour bar names reading upwards refer to lines carrying D'_B and those reading downwards to lines carrying D'_R.

The field frequency f_V and line frequency f_H (nominally 50Hz and 15.625kHz for monochrome transmissions) are derived from the unmodulated subcarrier frequencies f_0R = 4 406 260 ±2 000Hz and f_0B = 4 250 000 ±2 000Hz at the beginning of each line as follows:
f_H = f_0R / 2x3x47 = f _0B / 2x2x2x2x17 Conversely, f_0R = 282 f_H and f_0B = 272 f_H
f_V = 2f_H / 5x5x5x5

The attenuation of the colour difference signals before modulation and before low frequency pre-emphasis is applied is as follows:
D'_R and D'_B - <3dB @ 1.3MHz, >30dB @ 3.5MHz

Frequency spectrum of a System L/SECAM vestigial sideband transmission

The frequencies marked are relative to the vision carrier. The levels and slopes of the curves are stylised for clarity. The attenuation on the 'plateaux' is <3dB and is >20dB where the curves meet the x-axis.

Colorimetry

Originally, the same CIE coefficients for the primary colours, and for the reference white of colour display devices, as for the NTSC standard were chosen. However, over the years the phosphors used in colour display tubes have changed, and they no longer match those for which the coefficients were chosen. For that reason, the following CIE coordinates are now used in SECAM countries (though the older NTSC coordinates are also permitted):

Synchronisation of SECAM colour transmissions

SECAM decoders do not have a crystal reference oscillator, nor do they require an accurate amplitude reference, but there is a line-by-line commutating switch which needs to be set correctly so that the D'_R and D'_B signals may be sent to the appropriate demodulators. Either of two colour synchronisation methods may be used:

• Line identification: by chrominance subcarrier reference signals inserted into the back porch of the line blanking period, or
• Field identification: by chrominance subcarrier reference signals occupying nine lines of field-blanking period on Lines 7 to 15 in Fields 1 & 3 and Lines 320 to 328 in Fields 2 & 4

In both methods these identification signals comprise subcarrier of deviation +350kHz on D'_R lines and -350kHz on D'_B lines of 500mV p-p amplitude, where the luminance (blanking level to peak white) amplitude is 700mV. When decoded, these signals give a particular polarity of (G-Y). This is compared by the decoder with the expected polarity, and if it is incorrect, the commutating switch is held up for a line to re-establish correct synchronisation.

Line identification is now the preferred method, and in France all receivers manufactured from 1 December 1979 had to be able to use this method of synchronisation. The so-called 'green bottles', however, are still used on certain transmissions, for example the French first, second and third programme analogue satellite transmissions from 5.0°W.

The 'green bottles' comprise subcarrier that starts undeviated and moves to the maximum deviation for -D'_B and +D'_R. The amplitude of the subcarrier also increases according to the bell curve above. The line ident signals are similar, but are compressed into a few cycles during the back porch of the line blanking period.

During normal picture lines the subcarrier from the current line together with that from the previous line (from the output of the ultrasonic delay line) is fed to the (R-Y) and (B-Y) demodulators via a commutating switch that changes state after every line. The recovered (R-Y) and (B-Y) signals are then matrixed to give the (G-Y) signal. In the case of the ident signals, if the commutating switch is correct, the resultant will be in the third quadrant and the (G-Y) signal will be positive. If the ident is incorrect, the (G-Y) signal will be negative. Thus the polarity of the (G-Y) pulse may be used to reset the ident if required. This process is the same for line ident signals during the back porch of the line blanking period as well as the full-line 'green bottles' in the field blanking period.

These two oscilloscope traces show eight 'green bottles' (lines 8-15 and 321-328) sharing the vertical blanking interval with teletext (6, 19-21, 318-320 and 332-335) and VITS (vertical interval test signals, 16-18 and 329-331) on odd and even fields in a recent French SECAM analogue satellite transmission.

Correct SECAM decoding requires that the subcarrier be present throughout the active line, and not just in coloured areas. If an attempt is made to inlay text, as illustrated here where an analogue satellite receiver's menu is being displayed, the loss of subcarrier during the white characters causes the automatic gain control on the decoder to set the colour gain to maximum with the result that severe coloured noise is present on the text and for a short while afterwards until the gain has settled back to the correct value.

It is partly for this reason that the French devised the SCART, or Peritel, interconnection system in which inlaid text from decoders or video recorders is carried as separate red, green, blue and keying signals rather than encoded SECAM. Similar problems occur in production studios and so component (YPbPr) or PAL encoded pictures are used, the SECAM transcoding taking place either at the station output, or at the transmitter.

Twitter

Thanks to interlace, the overall flicker rate of 625/50 pictures is 50Hz. However, along horizontal edges there is 'twitter' at frame rate - ie 25Hz - which can be very noticeable when the adjacent areas have significantly different luminance values. The same frame-rate twitter pattern is visible on a monochrome receiver displaying any colour signal, but the effects on a colour receiver depend on the encoding standard. Here is what happens in the SECAM colour system across four fields of video:

Let us take as an example a transition between a red area and a blue area, as in the detail shown on the left, which shows both fields superimposed (or a non-interlaced 'progressive' version, if you like). The colours chosen lie precisely along the (R-Y) and (B-Y) axes for ease of calculation. When scanned in the 625/50i standard the transition would appear on an RGB monitor as in the diagram on the right, over four successive fields of video. Since the two colour difference signals are transmitted sequentially on alternate lines and the decoder derives one colour difference signals from the current line, and the other from the previous line via a delay line, there are inaccuracies at the horizontal transition.

The diagram on the right shows the actual chroma signals being transmitted, superimposed upon a grey pedestal for clarity. As the shade of red chosen has no (B-Y) content, lines of (B-Y) subcarrier show as grey during the lines of the red colour. The same is true of the (R-Y) signal during the lines of the blue colour. Notice that during two fields there are adjacent lines of full red and full blue chroma, while in the other two fields there are adjacent lines of no chroma. The decoder interprets the former as red + blue = magenta, and the latter as grey.

In the decoded display shown on the right, there is twitter at 12.5Hz comprising two fields of magenta and two fields of grey at the colour transition. This is in addition to the 25Hz luminance and red/blue twitter.

Although I have chosen a saturated red/blue transition, the effect happens wherever one or other of the colours appears, and is particularly noticeable with text on a white background, as in this example (far right) where the SECAM display is compared to an RGB display (near right). Between them is an animated sequence illustrating the SECAM 12.5Hz twitter in slow motion.

PAL, Phase Alternation by Line, is used on 625-line standards and was developed in 1962 by Dr Walter Bruch of Telefunken in West Germany as an offspring of NTSC and SECAM. The QAM subcarrier frequency is 4 433 618.75 ±5Hz (±1Hz for I/PAL), and the sense of the (R-Y) colour difference signal is inverted on alternate lines. When this signal is re-inverted in the receiver and averaged with the signal from the previous line, which has been stored in a glass acoustic delay line, phase errors are cancelled out so that hue errors are eliminated and appear instead as less-noticeable amplitude (saturation) errors. The two colour difference signals are recovered simply by adding and subtracting the 'live' signal and that from the delay line:
(E_{PAL(Line n)} + E_{PAL(Line n+1)})/2 = E'_U; (E_{PAL(Line n)} - E_{PAL(Line n+1)})/2 = E'_V.
A secondary advantage of adding and subtracting the direct and line-delayed signals electrically is that a comb filter action is produced, resulting in excellent separation of the two colour difference signals.

The hue of a given point on the screen is defined by the phase of the subcarrier at that point with reference to the phase of the carrrier insertion oscillator which is synchronised to a burst of subcarrier at the start of each line. Hue errors occur when the phase of the picture subcarrier has shifted during transmission with respect to the phase of the reference burst. On the phasor diagram, this shows up as a rotation of the phasors with respect to the burst and the (R-Y) and (B-Y) axes. Because the (R-Y) signal is inverted on alternate lines, the resultant phasors of one line are a mirror image of those from the previous one, reflected about the (B-Y) axis. A clockwise rotation on one line becomes an anti-clockwise rotation on the next, following re-inversion of the (R-Y) signal and so the hue error can be cancelled out electrically by means of a delay line, or optically in the case of a PAL-S (simple PAL) receiver that does not use a delay line.

When used on System N, in which 625-line signals are fitted into narrower 525-Line Standard M channels, the subcarrier frequency used is 3 582 056.25 ±5Hz. Uniquely, in Brazil, PAL is used on System M and the subcarrier frequency is 3 579 611.[88811 1] ±10Hz.

In 1965-66 ABC Television undertook colour television demonstrations using System A/PAL at its Teddington, Middlesex, studios. The BBC is also understood to have made similar tests. The subcarrier frequency was 2 660 343.75Hz.

PAL coding is used on Systems B, D, G, H, I, K, K1, M and N.

The instantaneous voltage E_PAL of the complete encoded PAL signal is given by:
E_{PAL(Line n)} = E'_Y + E' _U sin wt + E' _V cos wt
E_{PAL(Line n+1)} = E'_Y + E' _U sin wt - E' _V cos wt
where:
Line n refers to the odd lines of Fields 1 & 2 and the even lines of Fields 3 & 4
Line n+1 refers to the even lines of Fields 1 & 2 and the odd lines of Fields 3 & 4
w (omega) = 2 pi times the subcarrier frequency f_SC
E'_R, E'_G and E'_B are the gamma corrected primary colour signals
E'_Y = 0.299E'_R + 0.587E' _G + 0.114E'_B
E'_U = 0.493(E'_B - E' _Y)
E'_V = 0.877(E'_R - E' _Y)

The modulation mode of the colour subcarrier is suppressed-carrier amplitude-modulation of two subcarriers in quadrature (QAM) with the sign of the E'_V component inverted on alternate lines.

Colour synchronisation of the oscillator in the PAL decoder is by reference to a burst of ten cycles of subcarrier inserted into the back porch of the line blanking period. The p-p amplitude of the burst is 3/7 of the luminance (blanking to peak white) amplitude and the phase is ±135° relative to the (E'_B - E'_Y) axis, alternating line by line, the sign being the same as that of the (E'_R - E'_Y) signal on the same line. It is important that the burst amplitude remains in the correct proportion to the chrominance amplitude, since the burst amplitude is used as a reference for automatic gain control in the receiver and errors would affect the saturation of the displayed picture.

Subcarrier frequency and chrominance bandwidth

For all systems except M/PAL and N/PAL the field frequency f_V and line frequency f_H (nominally 50Hz and 15.625kHz for monochrome transmissions) are derived from the subcarrier frequency f_SC = 4 433 618.75Hz as follows:
f_H = 4f_SC / (5x227 + 4 / 5x5x5x5) Conversely, f_SC = (1135/4 + 1/625) f_H
f_V = 2f_H / 5x5x5x5

For A/PAL the field frequency f_V and line frequency f_H (nominally 50Hz and 10.125kHz for monochrome transmissions) were derived from the subcarrier frequency f_SC = 2 660 343.75Hz as follows:
f_H = f_SC (3x5x5x7/2 + 1/4 ) Conversely, f_SC = (1051/4) f_H
f_V = 2f_H / 3x3x3x5

For M/PAL the field frequency f_V and line frequency f_H (nominally 60Hz and 15.750kHz for monochrome transmissions) are derived from the subcarrier frequency f_SC = 3 579 611.[88811 1] ±10Hz as follows:
f_H = 4f_SC / 3x3x101 Conversely, f_SC = (909/4) f_H
f_V = 2f_H / 3x5x5x7
These values may also be expressed as follows:
f_SC = 5 000 000 x 63 / 88 x 909 / 910 = 3 579 611.[88811 1]Hz
f_H = 4 / 910 x 5 000 000 x 63 / 88 = 15 734.[26573 4]Hz
f_V = 2 / 525 x 4 / 910 x 5 000 000 x 63 / 88 = 59.94[005 994]Hz

For N/PAL the field frequency f_V and line frequency f_H (nominally 50Hz and 15.625kHz for monochrome transmissions) are derived from the subcarrier frequency f_SC = 3 520 562.5 ±5Hz as follows:
f_H = 4f_SC / (7x131 + 4 / 5x5x5x5) Conversely, f_SC = (917/4 + 1/625) f_H
f_V = 2f_H / 5x5x5x5

The attenuation of the colour difference signals before modulation is as follows:
M/PAL: E'_U and E'_V - <2dB @ 1.3MHz, >20dB @ 3.6MHz
N/PAL: E'_U and E'_V - <3dB @ 1.3MHz, >20dB @ 3.6MHz
All other systems: E'_U and E'_V - <3dB @ 1.3MHz, >20dB @ 4.0MHz

The bandwidth (-3dB points) of the modulated chrominance signal prior to modulation on the vision carrier is as follows:
Lower sideband:
All systems: f_SC - 1.300MHz
Upper sideband:
D/PAL and K/PAL (in People's Republic of China only): f_SC + 1.300MHz
I/PAL: f_SC + 1.066MHz
M/PAL: f_SC + 0.600MHz
N/PAL: f_SC + 0.620MHz
Other systems/territories: f_SC + 0.570MHz

Frequency spectrum of a PAL vestigial sideband transmission

The frequencies marked are relative to the vision carrier. The levels and slopes of the curves are stylised for clarity. The attenuation on the 'plateaux' is <3dB and is >20dB where the curves meet the x-axis. The colour subcarrier is supressed - the vertical magenta line indicates its frequency. The attenuation of the luminance signal by the optional notch filter is 6dB maximum at frequencies near f_SC. The width of the notch at the 3dB points is 400kHz.

Colorimetry

Originally, the same CIE coefficients for the primary colours, and for the reference white of colour display devices, as for the NTSC standard were chosen. However, over the years the phosphors used in colour display tubes have changed, and they no longer match those for which the coefficients were chosen. For that reason, the following CIE coordinates are now used in PAL countries:

Resolution of PAL colour signals

The diagrams below emulate various ways of displaying a small portion of an electronically generated colour signal comprising saturated red, green and blue areas in close proximity. If you switch your monitor to greyscale you will see that each diagram is identical in luminance.

Here is the signal displayed on an RGB monitor where the R, G and B signals have the same high bandwidth as the encoded Y or luminance signal.

Here it is displayed on an NTSC or PAL-S (Simple PAL, with no delay line) monitor where the (R-Y) and (B-Y) signals have about a quarter of the bandwidth of the Y signal, resulting in a smearing of the coloured transitions, although they are still defined by the high-bandwidth Y signal.

A PAL-D decoder averages the chroma from adjacent lines, though these are adjacent in time, not in position on the screen. The diagram represents two interlaced fields. If the lines are lettered A, B, C, etc from the top, the lines pairs that are averaged are A/C, B/D, C/E, D/F, etc. There is further smearing of the coloured boundaries because in this test signal the content of the adjacent lines is significantly different. Note that the first pair of blue lines is not only coloured incorrectly because it is mixed with the green of the previous lines, but that the shape of the 'point' of the green area is repeated in the blue lines, resulting in a visible loss of vertical resolution.

Here is a Simple PAL (PAL-S) display fed with a signal containing a large phase error. The error appears as different hues on adjacent pairs of lines, known as 'Hanover' or 'Venetian' blinds. With a PAL-D decoder employing a delay line, the colours would be the correct hue, but uniformly desaturated.

Twitter

Thanks to interlace, the overall flicker rate of 625/50 pictures is 50Hz. However, along horizontal edges there is 'twitter' at frame rate - ie 25Hz - which can be very noticeable when the adjacent areas have significantly different luminance values. The same frame-rate twitter pattern is visible on a monochrome receiver displaying any colour signal, but the effects on a colour receiver depend on the encoding standard. Here is what happens in the PAL colour system across four fields of video:

Let us take as an example a transition between a red area and a blue area, as in the detail shown on the left, which shows both fields superimposed (or a non-interlaced 'progressive' version, if you like). The colours chosen lie precisely along the (R-Y) and (B-Y) axes for ease of calculation. When scanned in the 625/50i standard the transition would appear on an RGB monitor as in the diagram on the right, over four successive fields of video. Since the two colour difference signals are transmitted simultaneously on each line but the PAL-D decoder averages colour difference signals from the current line and the previous line via a delay line, there are inaccuracies at the horizontal transition.

The actual chroma signals being transmitted are as shown in the RGB diagram (above right). Notice that at the transition there are adjacent lines of red and blue chroma. The decoder interprets this as a gross phase error and averages it as (red + blue)/2 = magenta/2; that is, desaturated magenta. In the decoded display shown on the right, there is twitter at 25Hz comprising a line of magenta at the colour transition, in addition to the 25Hz luminance and red/blue twitter.

Although I have chosen a saturated red/blue transition, the effect happens wherever saturated colour transitions appear, and is particularly noticeable with text on a white background, as in this example (far right) where the PAL-D display is compared to an RGB display (near right).

SECAM IV was a colour standard developed by the Russian research institute NIR (Nautschnuiu Issledowatelskaya Rabota). In fact two standards were developed: Non-linear NIR in which the square root of the amplitude of the chroma signals is transmitted (in a process analogous to gamma correction) and Linear NIR which omits this process. SECAM-IV as described below is the Linear version of NIR.

The USSR subsequently entered into an agreement with the French to produce an improved common SECAM standard from their two individual ones. However, the one chosen was the present standard described above (sometimes called SECAM III or SECAM Opt (optimised). Colour test transmissions began in 1963 from Moscow on System D, probably using SECAM IV, or something similar, before changing to SECAM III in a simultaneous launch with France on 1 October 1967.

News of the new Soviet colour system arrived in the west around 1966 at which time the BBC were quoted as saying "It is of interest to note that this proposal appears identical with one made by Mr. B. W. B. Pethers, a BBC engineer, in April 1963, but which was not pursued because at the time it was thought its advantages, with respect to the other systems, were not sufficiently attractive." Pethers' original system was similar to Non-linear NIR, and he also developed two variants.

Tests were made using NIR by the ITA in the UK, and there was a strong lobby for adopting the standard throughout Europe before the various nations polarized into a PAL-SECAM split.

Although derived from NTSC, SECAM IV is quite unlike either PAL or SECAM. It uses a 'third way' to reduce hue errors.

On one line a PAL-like supressed-carrier quadrature amplitude modulated subcarrier is transmitted, and on alternate lines a similar signal is sent, but this time with a constant phase as a reference. Thus both the information and the reference carriers undergo similar phase shifts in transmission and the demodulated signals are free of phase errors. A similar idea is used in the 'colour-under' system found in domestic videotape recorders. The high-frequency chrominance signal is heterodyned to a much lower frequency range and along with it a reference signal is recorded. On playback this reference is used as a beat frequency oscillator to restore the chrominance to its proper frequency and since both suffer the same variations in tape speed the recovered chrominance is jitter-free.

SECAM-IV/Linear NIR has two deficiencies that are absent from the other systems (NTSC, PAL and SECAM III) and which arise from the use of the transmitted reference signal in its raw wideband form rather than a locally generated reference signal. Firstly, because both the picture chroma and the reference signal from adjacent lines are applied to a ring demodulator, any interfering signal which is present on both inputs will demodulate itself, producing a dc component on the output. Depending on the frequency of the interfering signal this gives either an overall colour cast or a coloured venetian blind pattern. Secondly, the effect of noise on the chroma signal is to reduce its demodulated amplitude, resulting in desaturation which is particularly noticeable on flesh tones.

The subcarrier frequency used in 625/50 SECAM IV is 4 433 618.75Hz as in the PAL standard.

The instantaneous voltage E_SECAM-IV of the complete encoded SECAM IV signal is given by:
E_{SECAM-IV(Line n)} = E'_Y + E _S1
E_{SECAM-IV(Line n+1)} = E'_Y + E _S2
where:
Line n refers to the lines carrying S1 (chrominance) information
Line n+1 refers to the lines carrying S2 (reference) information
E'_R, E'_G and E'_B are the gamma corrected primary colour signals
E'_Y = 0.299E'_R + 0.587E' _G + 0.114E'_B
E_S1 and E_S2 are the instantaneous amplitudes of the subcarrier contained on alternate lines.

The modulation mode of the colour subcarrier is suppressed-carrier amplitude-modulation of two subcarriers in quadrature (QAM).

The chrominance signals are built up as follows:

Firstly the two colour difference signals are scaled:
D'_R = (E'_R - E' _Y) / 1.14
D'_B = (E'_B - E' _Y) / 2.03
These baseband colour difference signals have a bandwidth of >1.5MHz

Then the colour difference signals are modulated onto a subcarrier whose instantaneous amplitude on alternate lines is given by:
E_S1 = ( ( D' _R² + D'_B ² )^1/2 + E_p ) cos ( w ₀t + phi(t) )
E_S2 = ( ( D' _R² + D'_B ² )^1/2 + E_p ) cos ( w ₀t + phi₀ )
where:
E_p is a DC voltage equal to 10% of the maximum signal
phi(t) = arctan ( D'_B / D'_R )
phi₀ is the positive (B-Y) axis
w₀ is 2 pi times the subcarrier frequency.

In the receiver the ident switch is synchronised by 40µs trains of chrominance subcarrier occupying six lines of field-blanking period on Lines 6 to 11 in Field 1 and Lines 319 to 324 in Field 2. The p-p amplitude of the subcarrier trains is 30% of the luminance (blanking to peak white) amplitude and their phase is 180° on odd lines and 90° on even lines (relative to the (B-Y) axis. The colour difference signals D'_R and D'_B are recovered by multiplying the chrominance signal from the current line with that from the previous line which has been stored in an acoustic glass delay line (whose tolerance has to be twice as tight as that of a PAL delay line). The E_S2 signal is used as a reference oscillator for the E_S1 signal which contains the chrominance phase information and so a reference crystal oscillator is not required in the receiver. The E_p component of the modulated chrominance signals ensures that the subcarrier never falls below 10% amplitude, so that the reference signal is always present. The E_S2 signal should have an amplitude 10 - 20 times that of the E_S1 signal when applied to the demodulator.

For SECAM IV the field frequency f_V and line frequency f_H (nominally 50Hz and 15.625kHz for monochrome transmissions) are derived from the subcarrier frequency f_SC = 4 433 618.75Hz as follows:
f_H = 4f_SC / (1135 + 4/625) Conversely, f_SC = (1135/4 + 1/625) f_H
f_V = 2f_H / 625

MAC, Multiplexed Analogue Component was a system devised in the UK by the Independent Broadcating Authority (IBA) for delivering high quality colour pictures via direct broadcast satellites that would be independent of European countries' individual choice of (terrestrial) colour coding standard.

In satellite transmissions the vision signal, including colour and sound subcarriers, is frequency modulated onto the main carrier with a channel bandwidth of 27MHz. FM has a particular characteristic in that the level of noise that is inevitably added to the signal rises with modulation frequency. This is countered by increasing the amplitude of higher modulation frequency components on transmit (pre-emphasis) and reducing them on receive (de-emphasis) to mask the noise introduced by the modulation process. This works well with audio and monochrome video signals which have few large high-frequency amplitudes, and so raising their level does not cause problems with over-modulation. However, encoded colour video signals contain a great deal of high-amplitude high-frequency components thanks to the colour subcarrier, and pre-emphasising them can result in noisy and distorted colour signals.

For that reason a new non-compatible colour system was devised that transmitted baseband luminance and colour signals. The luminance signal is time-compressed by a factor of 3:2 and the two colour difference signals are compressed by 3:1 and transmitted individually on alternate lines prior to the luminance signal. Instead of a line synchronising pulse, the remaining 10-12µs of the line period is filled with a digital signal which contains audio, sync information and other housekeeping data. The 25 lines of each field synchronising period are available for other data such as teletext. The MAC receiver stores the separate components and from them assembles a combined video signal to pass on to the receiver. This can be in the form of component (RGB or YPbPr), S-video (Y + C) or even CVBS (Colour/Video/Blanking/Syncs - the standard encoded video signal, which could also be UHF modulated to feed a standard receiver). A representation of how a 'raw' MAC picture might be displayed on a conventional receiver (with height reduced and infinitely fast line flyback) is shown below.

Other advantages of MAC over encoded pictures are the wider luminance and chrominance frequency responses that can be accommodated and the absence of any cross-colour effect on the picture. Various versions of MAC have been proposed, the latest being an 'extended' version that allows 16:9 pictures to be transmitted, but the system was quickly superseded in the early years of direct-to-home satellite broadcasting by entrepreneurs who realised that a cheap and cheerful system of transmitting existing format PAL, SECAM or NTSC pictures to unmodified television sets would generate more revenue more quickly, despite the poorer technical quality, and MAC transmissions are now few and far between.

The main differences between the various 'flavours' of MAC concern themselves with the way the 12µs of audio/data information is modulated and the resultant bandwidth of the whole signal. In some variants the audio, data and video signals may be combined at baseband, whereas with others the modulation mode is different for the vision and sound/data. Other variants are designed for non-satellite use such as cable distribution or studio-studio links.

A-MAC carries the digital information - sound, data (teletext etc) - on an fm subcarrier at 7MHz. Since the vision bandwidth of a standard MAC signal is 8.4MHz, the horizontal resolution on A-MAC has to be reduced to make room for the 7MHz carrier. A-MAC has not been used in service.

B-MAC uses teletext-style non-return-to-zero (NRZ) signalling with a capacity of 1.625Mb/s. The video and audio/data signals are therefore combined at baseband. This system has been used in Australia.

C-MAC is the variant approved by the European Broadcasting Union (EBU) for satellite transmissions. The digital information is modulated using 2-4PSK (phase-shift keying), a variation of quadrature PSK where only two of the phasor angles (±90°) are used. The data capacity is 3Mb/s, but the data has to be sent to the transmitter separately from the vision, and the transmitter switches between fm (vision) and PSK (sound/data) modulation during each television line period..

D-MAC is a reduced bandwidth variant designed for transmission down cable. In it the data is duo-binary coded with a data burst rate of 20.25Mb/s so that 0° as well as ±90° phasors are used, resulting in a bandwidth of 8.4MHz instead of the 27MHz of C-MAC. However, most continental cable systems work on 7MHz channel spacing for which even D-MAC is too wide, hence D2-MAC:

D2-MAC uses half the data rate of D-MAC (ie 10.125Mb/s) and a reduced vision bandwidth in order to squeeze the whole signal (which is combined at baseband) into 5MHz.

E-MAC (Extended MAC) is a widescreen version of C-MAC. Originally designed for 15:9 pictures, it later adopted the 16:9 aspect ratio. The point about E-MAC is that all the 4:3 information is transmitted exactly as in C-MAC so that C-MAC receivers are still compatible. The extra luminance and chrominance information is tucked away in the field blanking interval and parts of the line blanking interval that are made available by reducing the sound/data capacity. A 'steering' signal is transmitted to indicate to the 16:9 receiver whereabouts the 4:3 picture belongs in relation to the two pieces of extra side information and the receiver then stitches the three seamlessly together to produce a 16:9 picture.

S-MAC ('Studio MAC') is a system used mainly in the USA where handling component signals gives better results than manipulating NTSC directly. It is not possible to mix standard MAC signals in the studio environment because the (R-Y) and (B-Y) components are carried on alternate lines, so in S-MAC, the luminance is compressed by 2:1 and the two chrominance signals by 4:1 so that all three may occupy the same line. The vision bandwidth required is therefore 11MHz, and S-MAC can be carried on a single circuit and converted losslessly to and from C-MAC at any stage.

Extended PAL was the BBC's counter-proposal in 1982 to the IBA's MAC system for direct broadcast to home (DBH) via satellite. The BBC believed that agreement on standards other than PAL or SECAM was unlikely to be reached in time for the planned start of DBH.

One of the drawbacks of PAL (and all the other compatible coded colour systems) is that the chrominance signal, modulated onto a subcarrier, occupies the same frequency space as part of the luminance signal. The BBC argued that since standard PAL decoders filter out most of the luminance information above about 3.5MHz in order to reduce the visibility of the chrominance subcarrier as crawling dots or hatching, and since the presence of these high luminance frequencies in the transmission inevitably results in false colour artifacts when they find their way into the chrominance channel of the decoder, it would be better if they were not transmitted at the same frequency as the chrominance signals. Instead, in Extended PAL they are frequency shifted by an amount equal to the colour subcarrier frequency (4.43361875MHz) and transmitted between around 8.5-10.5MHz. This leaves a 2MHz gap between the chroma information and shifted luma (at 6-8MHz) which is available for digital audio and data carriers.

Thus, an Extended PAL receiver would separate out the two portions of luminance signal and the chrominance signal, shift back the higher luminance frequencies to their proper position by using the colour reference oscillator as a beat-frequency oscillator, and produce a colour picture with full luminance bandwidth (up to 5 or 6MHz) and no false colour or dot-crawl artefacts. A standard PAL receiver would simply ignore the high-frequency luminance component of the signal and produce a colour picture with 3.5MHz bandwidth but with no false colour or dot-crawl artefacts.

Frequency spectrum of an Extended-PAL baseband signal

The vision signal and subcarriers are frequency-modulated onto the satellite carrier with a transponder channel width of 27MHz. The levels and slopes of the curves are stylised for clarity. The attenuation on the 'plateaux' is <3dB and is >20dB where the curves meet the x-axis.

PAL Plus is not a different colour system as such. It is a widescreen system designed to transmit 16:9 aspect ratio pictures that are compatible with standard 4:3 receivers within normal terrestrial analogue channels. Full-height anamorphic (FHA) pictures are generated at the studio with 16:9 aspect ratio and using all 575 available television lines. The pictures are then converted to 'letterbox' format with so called 'helper' signals transmitted on the unused black lines above and below the picture which are intended to be invisible to the 4:3 viewer. A PAL Plus receiver uses these helper signals to reconstruct the missing vertical information, and presents a full-resolution 575-lines high picture to the widescreen viewer.

Although the colour standard is essentially identical to normal PAL, some optimisation and processing of the signal is done at the transmitter and receiver to improve some of the shortcomings, such as chroma and luma resolution and cross-colour artefacts. These techniques are called 'Clean PAL' encoding and decoding or 'Colour Plus' and improve the colour pictures on both standard and PAL Plus receivers.

Here is a summary of the main relationships between the signals used in the coding and decoding of analogue colour pictures.

Coding

Y = 0.299R + 0.587G + 0.114B
(R-Y) = 0.701R - 0.587G - 0.114B
(G-Y) = -0.299R + 0.413G - 0.114B
(B-Y) = -0.299R - 0.587G + 0.886B

U = 0.493(B-Y)
V = 0.877(R-Y)

I = 0.74(R-Y) - 0.27(B-Y)
Q = 0.48(R-Y) + 0.41(B-Y)
or
I = 0.60R - 0.28G - 0.32B
Q = 0.21R - 0.52G + 0.31B

Decoding

(R-Y) = 0.96I + 0.62Q
(G-Y) = -0.28I - 0.54Q
(B-Y) = -1.10I + 1.70Q
or
(R-Y) = 1.13U
(G-Y) = -0.575U - 3.94V = -0.509(R-Y) - 0.194(B-Y)
(B-Y) = 2.03V
or
(R-Y) = 1.13[resultant of subcarrier sampled at 90.0 deg wrt +(B-Y) axis]
(G-Y) = 0.70[resultant of subcarrier sampled at 55.8 deg wrt +(B-Y) axis]
(B-Y) = 2.03[resultant of subcarrier sampled at 0.00 deg wrt +(B-Y) axis]

R = Y + (R-Y)
G = Y + (G-Y)
B = Y + (B-Y)

The diagram on the left shows the vectors and phasors involved in encoding NTSC, PAL and SECAM signals. The I and Q axes relate to NTSC signals only. The background colours and the (G-Y) axes are shown for (Line n) only in the case of PAL signals. In the case of SECAM signals the vectors relate to the baseband U and V signals only.

The diagram below shows the gamut of saturated colours. The horizontal axis shows the phase of the NTSC subcarrier, or the the PAL subcarrier on (Line n).

Television Website Bookmarks   Mike Brown/MB21/Ether.net Andrew Emmerson/Paul Stenning/405 Alive/British Vintage Wireless Society Keith Hamer David Laine/Vintage Broadcasting Richard Logue/irishtv Darren Meldrum Richard Russell Peter Vince/Barney Wol Andrew Wiseman/625 Room Bill Wright

Back To Top

Pembers' Ponderings

Compiled by Alan Pemberton
Sheffield, South Yorkshire, England
Email me