[time-nuts] Measuring TV delays
Magnus Danielson
magnus at rubidium.dyndns.org
Fri Jan 3 14:26:56 UTC 2014
On 03/01/14 15:05, David J Taylor wrote:
> There seems to be a mis-perception that folk might estimate a fixed
> delay and somehow use the data as a source of time. That was certainly
> never my thinking, but simply to observe the delays, and see what values
> might be obtained. I've contributed one, rather uncertain, data point.
> It would be interesting to know whether there is any timing data in the
> various digital streams we get, either via terrestrial or via satellite
> sources.
There are several potential time-signals, but I would say it would be a
bit hard to verify their traceability, which makes the exercise difficult.
The MPEG-2 Transport Stream have PID timestamps for every channel.
However, there is really no good TAI/UTC traceability in them. Also,
re-stamping is used to cover up for delay variations.
VITC might be present, and might provide local time, but for NTSC
countries there is a frequency error only partly covered in the
drop-frame compensation which causes a deviating display-time from
actual local time.
Caption can also carry time, but there is no guarantee it has a sane source.
Cheers,
Magnus
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