[time-nuts] Using digital broadcast TV for timing?

Hal Murray hmurray at megapathdsl.net
Wed Feb 8 04:49:02 UTC 2012


albertson.chris at gmail.com said:
> The plan is to try and phase lock a local oscillator and use a very long
> time constant on the loop filter.   I bet the TV transmitters are locked to
> GPS and over a long enough time are as good as GPS.  Also in many cities
> there are many TV transmitters, should be able to take advantage of that.

>  Before I try some experiments anyone want to tell me why I'm wrong? 

In the old days, the whole TV network was locked to a cesium in network 
headquarters.  I think it was back in the late 70s when a friend showed me a 
neat booklet from NBS (it wasn't NIST yet) that described using NBC (I think) 
to distribute time.  I forget the details.  It basically described where to 
look in the signal for the time-sync info.  All the local stations needed it.

HP did the calibration for the Silicon Valley area.  Every month they would 
publish the delay from New York to HP.   I think you could get to a ms or so. 
 The main problem was that the phone company might reroute the signal.


When did frame buffers arrive?  They eliminated the need for stations to be 
in sync.  That means that your local TV station might not be in sync these 
days.

Then came digital TV and compression.  That introduces variable display.

I think it would be fun to find the time info on a local TV station and 
measure the offset to a local GPS.  I'll bet it's not very good.



-- 
These are my opinions, not necessarily my employer's.  I hate spam.






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