Quote:
Originally Posted by Synchrodyne
...There is no suggestion in the deliberations that the choice of one affected the choice of the other. So positive vision with FM sound and negative vision with AM sound were both technical possibilities....
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Negative vision with AM sound is certainly a possibility, but positive vision with FM sound is somewhat of a no-no, for the following reasons:-
Systems using FM sound for TV have mostly (always??) used an intercarrier system for the sound channel, for definite reasons largely associated with local oscillator drift in the receiver. If you don't use intercarrier sound, then the local oscillator drift has to be kept within much tighter limits, which are easy enough in Band I but get increasingly problematic for Band III, becoming impossible for Bands IV/V. Hence the use of intercarrier sound, which largely eliminated the problem because the intercarrier interval (6MHz for UK) was constant and was set at the transmitter end, and was independent of receiver local oscillator frequency. A big plus, then , but there was a drawback.
One got one's intercarrier signal by mixing the sound and vision IFs in the vision detector, thus producing the constant intercarrier frequency. But to make this mixing work, there always had to be some residual vision signal present at all times, 'cos if it dropped below a certain level the intercarrier mixing failed and the result was colossal vision buzz on the sound channel. So, with negative modulation systems, the vision carrier never dropped below 20% of peak signal for white, rising to 76% or so for black and 100% for sync tips.(I'm neglecting the effects of colour subcarrier signals in this description, but in practice they made little difference to the operation).
Try to work the intercarrier stunt on positive modulation systems, and you start running into problems. Here, sync tips are represented by 0% modulation depth, so an intercarrier system would be in a continuous state of failure. True, you could make the system work by tweaks: you could put your sync tips at 20% carrier, black level would need to rise to 41% of peak carrier to preserve the 7:3 picture:sync ratio of the standard signal. You'd be throwing away almost half of your transmitter's dynamic range, pushing up the mean transmitter power requirements thereby increasing your electricity bills, and pushing the video part of the signal further towards that region of the transmitter's characteristic which is traditionally the most non-linear, thereby increasing the amounts of linearity pre-correction required.
I suspect that most TX engineers thought that sticking with AM sound on a positive-modulation system was far the easiest option....