>PAL is specified as 4.43361875 MHz which implies ~2.2x10^-9.
>This is about 6 msec per month or about 70 msec per year.
>IF the TX is coupled to a freqency standard

I don't know if they still do, but I believe when the PAL colour stations
were first set up they used a rubidium standard for generating this, as a
secondary standard (using a national Caesium standard as the primary), so it
was possible to use the colour burst as a highly accurate standard. However
with modern digital signal transmission I believe that the requirement for
tight accuracy of the colour burst has been relaxed somewhat as at the
digital-analogue interface all signals are resynced to the "local" (at that
point) reference. This means that the high accuracy is not needed right
across the studio network, so a lower accuracy standard (perhaps an ovened
crystal) can be used as a local standard.

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