On Tue, 10 Jan 2006, Jinx wrote:

>> NZ does have a time standard source - it is a Caesium Standard
>> that drove a 2.5MHz transmitter that used to be just down the
>> corridor when I worked at the DSIR in Gracefield. I believe it is
>> used to derive the time pips used on the radio
>
> Ah, that's quite true
>
> http://msl.irl.cri.nz/services/time/
>
> "Colour Burst" looks interesting. A little less "fuzzy" than the others,
> although I guess the pips could be detected with one of the under-
> used LM567s lazing about here
>
> I can feel an impending exercise comparing MSL to Transpower.....

With PAL B as you (and we) have you have to account for the phase jumps 
in the carrier burst (every other line). Most PLLs for color TV use 
follow the phase jump (the oscillator jumps phase every other line). 
This reduces the available accuracy somewhat. A relatively easy way to 
fix it is to slave a second PLL on the same frequency to the first, with 
a long (100 seconds ?) time constant in the loop. That should give a 
pretty stable signal for most normal purposes. 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 
(such precision is good even for metrology - and I don't think that the 
TV stations are trying very hard to apply it - cable even less, with the 
color carrier being prepared in the set top box from a low precision 
quartz). Anyway with 70msec per year error one can do some interesting 
astronomy (e.g. earth rotation period disturbances and such).

Peter
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