On Tue, 10 Jan 2006, Jose Da Silva wrote:

> Here's something else to ponder in terms of colourburst....
> Studio cameras are indeed accurate for the colour burst, if they get the
> colourburst off of some atomic clock, but what would you make of
> handycams and other portable cameras? They are portable, and therefore
> not as accurate as you would like.

Studio cameras are not very accurate wrt color burst. They become 
accurate because they are either genlocked (to a time standard derived 
signal distributed throughout the studio) or passed through a TBC which 
genlocks them. If the processing is all-digital then they do not *have* 
a color carrier signal, sending D2MAC or similar digital data through a 
high speed switched dedicated network.

> Now imagine you are watching the news and they show an on-the-spot
> segment recorded from a handy-cam type camera.  I would think that you
> are getting colourburst according to the portable camera and not
> according to studio-quality colourburst.

Any magnetic recording will have the color carrier generated (not 
regenerated!) at the last player. So its quality determines the 
stability. Since this will often be a studio Betacam or DVPro machine 
likely genlocked to the studio timebase it will be good no matter what. 
Unfortunately this cannot fix wannabe movies shot with mini DV cams or 
such but even then the image quality will be bad but the timing will be 
perfect (well, almost).

> I think the colourburst would be accurate for studio stuff (generated on
> sight) or even film type shows and movies, but now you get a lot of
> media recorded digitally, VCR tapes, etc, and pumped out via plain
> VCRs, DVD machines, etc, ...all of which produce their own colourburst.
> We may possibly be seeing the sunset on colourburst as an accurate
> reference sent out by the TV services.

Probably true. The odd part is that cable and sattelite digital framed 
signals are very accurately timed, probably traceable to a rubidium 
standard somewhere, and the cable or sattelite box loses that and uses a 
lousy 50ppm crystal to generate the output subcarrier. The reason the 
cable and sattelite signals are very accurate is, afaik, the fact that 
they have to keep a large switched network operating. For sattelite they 
also have sideband and intermodulation concerns. E.g. I do not know what 
stability the *carrier* of a sattelite transmission has but it should be 
very very good imho. Maybe that is locked to a rubidium standard 
somewhere. The same should be true for the bit rate in the digital cable 
signal.

Perhaps the future of precision timing for the masses will require a 
PLLd LNB on a sattelite dish and little else ? (An ordinary LNB can be 
PLLd for sure, some already do this and the 2nd IF can be divided down 
directly for frequency measurement). Now that would be nice. 10GHz with 
+/-0.1Hz over 24 hours would be really nice even if weather doppler 
would cause some problems.

Peter
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