[time-nuts] Time/freq from digital TV
magnus at rubidium.dyndns.org
Sun Jun 14 04:41:43 EDT 2009
David I. Emery skrev:
> Most network broadcast distribution is via satellite, and
> satellites move around in their box in the sky so any time or frequency
> derived from satellite downlinked signals is subject to Doppler shifts
> over the course of a day as the satellite completes its figure eight
> pattern in the sky (most operational birds are slightly inclined, thus
> the figure 8 - none are dead nuts on the equator and in perfectly
> circular orbits). This means that no network timing from a satellite
> signal is stable by precise metric standards... even if the uplink signal
> is right on.
> What all this means in practice is that there is no longer any
> precise broadcast TV signal that can be depended on as really accurate.
You are correct, until the time when an ATSC network is operated in
Single Frequency Network (SFN) mode, in which case the transmitter
signals is coordinated with the aid of GPS receivers. The ATSC SFN specs
is in a separate document. As I recalled it when I fast read it a few
years back was that it seemed like they had made a few thought errors
which caused overspecing but still possible.
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