PAL colour subcarrier frequency

[bruce varley]

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?

 

[Clifford Heath]

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...

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.

 

[Phil Allison]

"bruce varley"

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

** 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 Short Form.pdf#search="pal colour subcarrier stability "




......... Phil

 

[John G]

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"!

And PAL then became "Perfect At Last".