On Wed, 11 Jan 2006, Olin Lathrop wrote:

> I think he's already seen that the power line isn't good enough at any one
> point in time.  I can think of several choices:

I have been thinking some more so here is another choice:

TV color burst would be very nice, but it is locally derived. The 
horizontal (and vertical), however, aren't. They should be related to 
the signal rate from the digital feed, and that is supposed to be 
accurate. So one could say that the TV's H and V rate should be 
traceable to a good frequency standard. They should be significantly 
more accurate then the mains in any case, probably a few ppm or less.

So why not use a TV's V rate (equal to mains frequency but more 
accurate - beware of scan converting TVs) to drive a normal mains 
powered clock and see what happens to the accuracy ?

Similarly, one could tune to 15625 Hz horizontal line freq. anywhere 
near a TV (certainly in a house and withing at least 10 meters of any 
working normal size crt TV), using a ferrite antenna not unlike tuning 
into a LW atomic clock frequency. In fact I have built such receivers 
before (receiving TV line freq. radiated as H field by the deflection 
coil assembly), but for a different purpose. A clock referenced to this 
frequency should be as good as the (digital) clock of the transmitter 
chain, and better than the color subcarrier probably. Of course the TV 
will have to be on air (cable box or sattelite). Watching DVDs or 
Nintendo won't work. I don't think that any digital sattelite or cable 
box can get away with an unsynched data stream. It would frame drop like 
crazy (this is very annoying to spectators).

Peter
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