Colour TV
The UK/PAL system is very complex and has lots of special features. Here we will ignore most of them and just concentrate on the essential details. To obtain colour pictures we need, in effect, to transmit three pictures — one red, one blue, and one green. These are then overlaid by the TV to produce a colour image. As with mono/stereo radio this needs to be done in a way ‘invisible’ to those who can only afford a black and white TV. As with stereo, DSBSC modulation is used.
The UK uses the Phase Alternate Line (PAL) system to multiplex the colour information. As with stereo we don't just send the RGB (Red-Blue-Green) patterns but rearrange them into more convenient forms. The black and white signal described above is now used to send an R+G+B signal. Two other Chrominance signals are then sent, one providing G-B information, the other R-B information. A colour TV can recover these signals and use them with the ‘sum’ signal of R+G+B to recover all the colour details it requires. A black and white set ignores these extra signals and just uses the R+G+B signal as its brightness pattern.
UK PAL uses a pair of DSBSC signals both of which use the same subcarrier frequency, 4·43361875 MHz. Although they share the same frequency, the G-B and R-B subcarriers are arranged to be in quadrature. Hence the TV can recover and separate the two signals by using two DSBSC subcarrier demodulators. The local oscillator input for both can come from a single oscillator, but the input to one demodulator is shifted in phase by 90 degrees before being used. In effect, we use two properties of the complete subcarried signal — its amplitude and phase — to convey two pieces of colour information.
One initially puzzling feature of the subcarrier frequency chosen is that it lies inside the luminance bandwidth! For this reason we can expect that some colour information may appear as unwanted brightness patterns and some luminance information may produce unwanted colour patterns. This does, in fact, occur. You may well have seen ‘herringbone’ patterns of fine lines (usually red ones) appearing on the screen when someone is wearing clothing which has a close pattern. This is caused by luminance signals near 4·4 MHz being interpreted by the TV as colour!
This effect would be far worse but for the careful choice of subcarrier frequency. In most TV pictures there is a tendency for adjacent lines to be similar. The periodic (64 microseconds per line) nature of the signal, combined with this line-to-line similarity means that the video signal spectrum has a fairly distinct ‘hooped’ or ‘comb’ shape with minima at those frequencies which would cause subsequent lines to differ markedly. 4·43361875 MHz is one such frequency. This means that the chrominance signal spectrum tends to ‘fit into the gaps’ spectral left by the luminance signal. It also means that any colour information leaking into the luminance tends to have opposite effects on adjacent lines. Hence, when viewed from across the room, these effects usually go unnoticed.
As usual with DSBSC modulation we need to give the receiver some help with identifying the precise frequency and phase of the missing subcarrier. In a stereo radio this is done with a continuous pilot tone. In colour TV it is done using a colour burst. The transmitter sends a short burst of the subcarrier it is using during the back porch section of the video signal. (This is a ‘black level’ part of the blanking period between the end of the line synch pulse and the start of the picture information.) The TV can receive this burst and use it to correct its local subcarrier oscillator, resetting its phase and frequency ready for the next line.
The PAL system is based upon the American NTSC system (National Television System for Colour). The USA wanted to be first with colour TV so they devised a system which could be mass produced as early as possible. The technique used is essentially as described above, although the frequencies, line rates, etc, differ from the values given for UK TV. In fact — as anyone who has watched American TV will agree — it is virtually impossible to make their system work correctly. For this reason TV engineers often refer to NTSC as “Never Twice the Same Colour”.
The main problem with early colour TV's was that it was difficult to build a local subcarrier oscillator which oscillated at precisely the correct frequency, even given the once-per-line ‘hint’ of the colour burst. An oscillator running at slightly the wrong frequency tends to drift out of phase as the line is scanned. As a result, by the time the line scan reaches the right hand end the colour decoder is confusing G-B information with R-B information and vice versa. Hence the picture shows a graded colour shading change from side to side. Other problems, such as interference, also tend to upset the oscillator as it free runs in between being resynchronised by the colour bursts. Hence USA colour TV's tend to provide very poor colour.
The PAL system — as the name Phase Alternate Line implies — alternates the phase relationship from one line to the next. For one line the G-B subcarrier is 90 degrees ahead of the R-B. For the next line is is switched to being 90 degrees behind. Then, for the next, 90 degrees ahead again, and so on, alternating the relative phase line by line. The effect of this alternation is to ensure that any colour error produced by oscillator drift has opposite effects on adjacent lines. If one line has too much red at its right hand end the next will have too little, then too much, etc. Viewed from across the room our eyes tend to average away these effects. As a result, unless the colour errors are very gross they become unnoticeable. This simple change of alternating the phase between the colour signals means we don't have to make a perfect TV to get good colour pictures.
Various other systems have been invented to provide colour TV. The French devised a system called SECAM, the UK Independent TV labs developed a time division multiplexed system called MAC, etc. We won't take any interest in the details of these methods, but they all use multiplexing to convey the colour picture information and are based upon raster scanning to capture a moving picture for transmission as a serial signal.
Summary
You should now know that a DSBSC subcarrier system is used to transmit Stereo Radio on the FM band. That this system used a 38 kHz subcarrier and a 19 kHz Pilot Tone to help the receiver. That the Left & Right information is rearranged into ‘sum and difference’ form to provide a basic signal which is still compatible with cheaper monophonic radio receivers.
You should also now understand how a TV uses Raster Scanning to convey a two dimensional moving picture as a serial data stream. That the PAL (and NTSC) system uses a pair of DSBSC subcarrier signals to convey the extra Chrominance information which, combined with the monochrome Luminance signal, can be used to produce colour pictures. That the UK system use subcarriers at a special frequency of just over 4·4 MHz, chosen to fit the colour data ‘in the gaps’ of the luminance spectrum which arise because of the regular scan period of the raster. That PAL uses subcarrier phase alternation to avoid the problems which arise with the more basic NTSC system.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit3 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.