[time-nuts] Using digital broadcast TV for timing?
jerryfi_99 at yahoo.com
Thu Feb 9 03:48:39 UTC 2012
Thanks Paul. You and Bob Camp provided some good updates/info. It may present enough of a challenge/reward for someone to examine further. I'm with you on the available time front - too many other projects/commitments to pursue further myself. I'll be interested if Chris, or someone else, can make some headway though.
From: paul swed <paulswedb at gmail.com>
To: jerryfi <jerryfi_99 at yahoo.com>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Wednesday, February 8, 2012 7:07 PM
Subject: Re: [time-nuts] Using digital broadcast TV for timing?
All gone these days in the US.
Indeed I can speak to the CBS network it was driven by CS references in the 80s and 90s.
I used CBS for aligning my references Xtal oven oscillators that were never ever turned off in a large facility that uplinked all 8 CBS regions and 22 other cable networks.
Unfortunately few could get to that color burst signal as devices called frame synchronizers came into play from the 80s to the 90s. They would strip off that burst and insert the local reference of generally much lower quality.
As far as todays digital TV signals they can contain significant jitter. But its actually trickier then that and I honestly have to say I am not sure that you might not be able to get something useful.
Several interesting points. Many of the television transmitters do use GPS referenced sources. Its an interesting exploration. I simply don't have the time though.
On Wed, Feb 8, 2012 at 7:38 PM, jerryfi <jerryfi_99 at yahoo.com> wrote:
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.....
>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?
>> <CABbxVHvb3SKzuMx+bDyKTtesGzuf2k5HSJWYpdKK+RQOArxRgA at mail.gmail.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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