[time-nuts] Digital TV signals
james.p.lux at jpl.nasa.gov
Wed Apr 16 00:33:54 EDT 2008
Quoting "David I. Emery" <die at dieconsulting.com>, on Tue 15 Apr 2008
09:18:37 PM PDT:
> On Tue, Apr 15, 2008 at 04:50:29PM -0700, Stanley Reynolds wrote:
>> From: david brown <davidarbrown at bigpond.com>
>> >From a complete amateur, is there any useful timing info to be gained
>> from the newer format of digital tv transmission(Australia) to replace
>> that available from current analogue transmissions.? My recently
>> repaired TV derived frequency standard looks to be becoming obsolete!
> > GPS based standard would be the most common now.
> FWIW my understanding is that in the US few local broadcast
> facilities make any very serious attempt to lock their signal timing to
> high accuracy standards or GPSDOs. Much digital TV broadcast equipment
> is designed so this COULD be done, but there is no current reason to
> spend the money and engineering effort to actually do it - more likely
> than not signal timing on the output signal is determined by at most a
> medium grade OXCO calibrated every once in while and possibly just a
> TXCO not much better than barely meeting the FCC spec.
If they want to get better coverage, they can transmit from multiple
sites, all on exactly the same frequency. The receivers have adaptive
equalizers (e.g. RAKE) in them and the multiple identical signals
just look like multipath, and it coherently combines them.
> So it is not clear that TV signals are good time or frequency
> references any more - though there is little doubt that if there was
> some purpose to doing so both time and frequency could be rather closely
> locked to a GPSDO at the transmitter - it is just with the entire system
> designed to deal with small time, frequency and rate errors at many of
> its interfaces most stations haven't seen fit to implement high accuracy
> frequency or time control on their actual transmitted signal.
It's the multiple transmitters transmitting identical signals
scenario.. quite unusual in the traditional broadcast environment.
I don't know that anyone is actually doing this, but that's the
motivation for the synchronization to a gnat's eyelash. I think there
are some systems in Europe doing it.
More information about the time-nuts