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PAL colour subcarrier frequency

This is for pure curiosity, can anyone explain? I'm wondering what the situation is in regard to the PAL colour subcarrier, which reportedly requires extreme accuracy... IIRC some of the TV stations have, or had, atomic standards for generating the CSC. In fact, as I understand it, the frequency of the subcarrier is calculated as some sort of multiple of frame and line frequencies, arranged so that the errors cancel from frame to frame. So what you'd really want is for the colour subcarrier to be locked to whatever frequency source generates your frame and line controls. Either the TV station transmits all 3, in that case the accuracy of the CSC is fairly irrelevant because everything is locked together at the source, or one or more of them is generated at the receiver end in which case the whole setup is governed by the quality of the oscillators in the receiver.

Or is the problem something to do with phase change during transmission?

Or is the whole question archaic because it's all going digital anyway?

Reply to
bruce varley
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TVs have to reconstruct the subcarrier very accurately (exact phase) for colour purity, from a very short burst, ten cycles, or 2us worth, every horizontal line. So they have a high-stability crystal oscillator that can be gently pulled into phase. High stability means it can't be pulled very far, or very fast, so it's important that it's tuned correctly, and that the TV station broadcasts exactly on the right frequency.

Reply to
Clifford Heath

In the UK the analogue shutdown is scheduled to begin in a couple of years time and be complete by 2012.

If you still want to learn about the intricacies of PAL, seek out the "Q&A" series of books - I'll try to find the copy I bought many years ago back in the days of hybrid sets and see if the ISBN number helps you any.

PAL of course stands for "Phase Alternate Line", the phase of the colour signal is reversed line by line and reversed again (back to normal phase) by the decoder - the whole point of which is that phase errors during transmission/propagation are cancelled out.

It is rumoured that the British engineer who cracked GPS without buying the decryption key from the US used the same principles and similar decoder design - The GPS satellites apparently use some form of swinging phase so it could only be used with a decryption key - which the PAL principle rendered obsolete!

Note that the US standard NTSC which does not use phase error cancelling has been dubbed "Never Twice the Same Colour"!

Reply to
ian field

"bruce varley"

** The ABC was alleged to be doing something like that.

However, a Google search turns up the fact that synch generators used by most broadcast organizations do not have or need the accuracy or stability of a Caesium or Rubidium reference.

This example probably used a temp controlled crystal time base with about

0.25 ppm accuracy ( or 1 Hz ) and stability over the room temp range.

formatting link

........ Phil

Reply to
Phil Allison

And PAL then became "Perfect At Last".

--
John G

Wot's Your Real Problem?
Reply to
John G

The frequency is 4.4336.......MHz right down to .75 of a cycle The phase is changed by (r-y) and (b-y). Both these are reduced (this avoids over modulation, on some colors).

Of course on PAL (r-y) is changed by 180° on alternative lines, to reduce phase errors the NTSC system used to suffer.

Or is the problem something to do with phase change during transmission?

Or is the whole question archaic because it's all going digital anyway?

Reply to
Eric

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