Please don't ignore this page. It could help you solve any tricky faults you encounter!
As under 'The Basics'. I have assumed a good grounding of electronics and AC theory. The pointers here apply specifically to TV receivers.
On UHF transmissions, the Vision and Sound Carriers are frequency stabilisesd at the transmitter, the gap between them, 6Mhz, will remain constant. Both the vision and sound carriers are amplified together at I.F. and the non linearity of the vision detector diode will result in 6Mhz intercarrier 'beat' product. This can be extracted either immediately after the vision detector or in some cases it is further amplified by the vision stages before extraction.
This is why the vision Modulation level at Peak White - Negative AM Modulation on 625 Lines remember! - is set at 20% and should never go below 18%. If this signal dropped to zero, the intercarrier beat signal would be lost, hence No Sound!
It can be seen from this that the level of this 6Mhz intercarrier beat product will vary with picture content and vision modulation.
Faulty IF alignment can result in AM sound content on this carrier as well, as the FM is 'slope demodulated' by the response curve of the I.F. stages of the receiver at I.F. Similarly, Intecarrier Buzz, also called Vision on Sound or 'Caption Buzz' can occur if the IF stages 'over favour' the sound channel. Before being demodulated, this carrier is amplitude limited to eliminate any A.M. content. This usually occurs in the ratio detector or the discriminator valve.(Often an EH90)
This was first introduced in the 1950s by Ferguson to minimise the effect of electrical interference on the sync. of the receiver. Put very simply this is a line scanning oscillator whose frequency is voltage controlled. This waveform is compared with the sync pulses and the mistiming builds up and error voltage. This voltage is used to correct the scanning frequency and lock the two together. Because this voltage builds up gradually, random mistimings as a result of noise will even out and therefore have little effect. Initaially it was used mostly in 'Fringe Area' sets.
Each individual frame does not actually have 405 or 625 lines. It only has half that. Two separate frames are scanned, one starting the first line at the top left of the screen. The other starts in the cente and 'fits inside' the other. The full 405 or 625 lines are scanned over TWO frames. Most sets can be set up for optimum interlacing - check the manual!
In order to represent colours accurately, the correct balance of Red, Green and Blue content is required. In order to achieve this, the Black and White picture must not have any tendancies to any colours except for Black, White or the intermediate shades of Grey. The 'Shade' of white that is used as a reference is referred to as ILLUMINANT 'C'. This is a slightly warmer white than seen on Black and White tubes.
The amounts of each Primary Colour, i.e Red, Green or Blue, must vary in unison to maintain the essential grey shade. This is what is known as Grey Scale Tracking.
BASIC 'PAL' COLOUR DECODER THEORY.
When Colour was first introduced to 625 Line UHF transmissions in the 1960s, one of the conditions was that the extra information to be transmitted did not interfere with existing Black and White reception. This is why the sub carrier frequency was set at 4.43361875 Mhz, a multiple of the line frequency. in practice, a very faint dot pattern is visible on Black and White receivers, but you have to get up close and look hard to see it! Amazingly, some people did just that...!!
Two signals are generated at this frequency, with a phase difference of 90 degrees. The one that is 'In Phase' is modulated with the Red-Luminance, or R-Y signal. The other, with a 90 degree lag, carries the Blue-Luminance, or B-Y signal. Both of the carriers are suppressed to mimimise interference to the Luminance signal. When the two sets of sidebands are added, a sub carrier is generated. This is the Colour Subcarrier which appears 4.43 Mhz above the vision carrier.
This subcarrier varies in both Amplitude and Phase. The higher the amplitude, the more saturated the colour. i.e. in a transmission where there is no colour content, the subcarrier will disappear. The colur displayed on the screen, the HUE, is determined by the phase of this subcarrier.
But!!, I hear someone cry, the original carriers have been suppressed! How do you get the right phase and then the right colour?
Also transmitted, at the beginning of evey line, between sync pulse and picture info, is a brief 4.43 Mhz Colour Burst. This is a sample of the original suppressed carrier. A Crystal Oscillator in the decoder, also at 4.43 Mhz, is phase locked to this burst to 'recreate' the missing carrier.If this local oscillator is not in phase with the original carrier, therefore, incorrect hues will result.
Green is not transmitted, as this can be recovered my matrixing in the receiver.
On the PAL system, the R-Y signal is reversed on alternate lines. This has the effect of correcting any errors during transmission. A Phase or Lag error on one line is cancelled out by an equal and opposite error on the next line.
On a Simple PAL receiver, this is done visually, by 'tricking' the human eye. However, due to field interlacing, adjacent pairs of lines will show the same hue error. This effect is what is known as HANNOVER BARS.
Therefore, the problem is solved electronically. An Ident Signal of 7.8 Khz is derived from the swinging colour burst. This switches a Bistable which reverses the phase on alternate lines in step with the transmitted R-Y phase reversals. The overall effect of this is as if the R-Y signal was constant.
The above is only a very basic explanation of the PAL decoder. If you wish to take the matter further, you should now find the explanations in most textbooks easier to understand. This is a fascinating subject and is ideal for those of you wishing to make friends and impress the opposite sex in the local watering hole.