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

[Tom Holmes]

A thought  and an observation related to getting timing from broadcast DTV
signals.

BC DTV uses a scheme called 8VSB, which includes a pilot 'carrier' at the
low frequency end of the signal. This is used to help the receiver lock to
the TX so that the phase information can be accurately decoded. It seems to
me that this might be a possible source of a useful frequency reference,
although there is no guarantee that it is traceable to anything or even very
accurate.

The observation is that one of my local stations simulcasts the same
programming in both 1080 and 780 formats, and when I switch from the 1080
channel to the 780, there is a very obvious 1 to 2 second delay in both the
audio and video of the 780 format. These are being transmitted as separate
streams on the same carrier. Definitely suggests that there is a lot of
'processing' and buffering going on while putting the complete bit stream
together.

Tom Holmes, N8ZM
Tipp City, OH
EM79


> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of jerryfi
> Sent: Wednesday, February 08, 2012 7:38 PM
> To: time-nuts at febo.com
> Subject: [time-nuts] Using digital broadcast TV for timing?
> 
> A bit off topic, but historically related....  back in the 70's, I tapped
off the color
> burst
> 
> oscillator in my TV (a Heathkit) to get a 3.579545.... MHz  (315/88 MHz)
source to
> 
> calibrate my homebrew frequency counter. The TV's color burst oscillator
was
> phase
> 
> locked to the color burst signal on the broadcast signal  (which was on
the "back
> 
> porch" of the hori sync signals).  Supposedly, the networks were locked to
Cesium
> 
> standards traceable to NBS for LIVE broadcasts, such as news and sports.
> Taped
> 
> programs, of course, were not usable as an accurate source.  In any case,
that
> signal served my purposes at the time (providing a reference for
calibrating my
> counter that was more accurate than anything else available to me).
> 
> I'm not sure if, what, or where analog TV is still broadcast, but I think
there are still
> a
> 
> few stations (low power) around.  You might still be able to use that
signal, IF you
> can
> 
> dig it out of your old analog TV.  ;-)  I do have analog tv's hooked up to
my cable
> 
> box - I suspect that live broadcasts would still have an accurate color
burst, so
> maybe....
> 
> I think the other methods discussed here (ie, GPS) would provide easier
and more
> 
> reliable timing sources. ;-)
> 
> 
> Trying to locate the appropriate signal(s) in a digital TV today would be
> interesting.
> 
> Just as a historical aside.....
> 
> 
> 
> Jerry Finn
> Santa Maria, CA
> 
> 
> 
> > Date: Tue, 7 Feb 2012 18:01:26 -0800
> > From: Chris Albertson <albertson.chris at gmail.com>
> > To: Discussion of precise time and frequency measurement
> >     <time-nuts at febo.com>
> > Subject: [time-nuts] Using digital broadcast TV for timing?
> > Message-ID:
> >
> >
> <CABbxVHvb3SKzuMx+bDyKTtesGzuf2k5HSJWYpdKK+RQOArxRgA at mail.gm
> ail.com>
> > Content-Type: text/plain; charset=ISO-8859-1
> >
> > GPS requires a good view of the sky,  Hard to do in say the 7th floor
> > of a 40 story building if you have no windows.   I'm wondering about
> > using the new digital TV signals for timing.
> >
> > I'm pretty sure there is time code in the signal and I'm pretty sure
> > the bits are clocked at a very accurate rate.   Also TV receivers are
> > very easy to find and put "hooks" into.      I'd bet the broadcast TV
> > signal could be almost as good as GPS.
> >
> > 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?
> > --
> >
> > Chris Albertson
> > Redondo Beach, California
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