On Wed, 11 Jan 2006, Alan B. Pearce wrote:

>> 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.

In studio mode digital actually *very* high accuracy is needed because 
these people use the network at full bandwidth with switching and so on. 
The digital studio network is completely synchronous afaik, and has very 
accurate timing (unlike Ethernet). Also if they connect to foreign 
networks then they almost certainly use at least a GPS derived clock (at 
both ends). There is nothing as annoying as a TBC that drops frames once 
every 10 seconds when the accumulated errors sum up to a full frame or 
two and the TBC resyncs to the top of the next quad frame, when 
something important is being passed over the link. The digital link is 
at least as fussy although it hides the mistakes better.

Peter
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist