PAL colour subcarrier frequency

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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?



Re: PAL colour subcarrier frequency


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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.

Re: PAL colour subcarrier frequency



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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"!



Re: PAL colour subcarrier frequency



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And PAL then became "Perfect At Last".
--
John G

Wot's Your Real Problem?



Re: PAL colour subcarrier frequency



"bruce varley"

Quoted text here. Click to load it


** 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.

http://www.courtyard.co.uk/CY490%20Short%20Form.pdf#search=%22pal%20colour%20subcarrier%20%20stability%20%22




........  Phil



Re: PAL colour subcarrier frequency


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.




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?




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