[time-nuts] TV Signals as a frequency reference
Glenn Little WB4UIV
glennmaillist at bellsouth.net
Sun Apr 1 22:51:11 EDT 2018
I was a chief engineer for a TV station during the transition to digital.
I am now a transmitter supervisor for two digital TV transmitters.
None of the stations in my area have a frequency standard.
Time in injected into the digital stream usually from a pc clock.
This pc may or may not be allowed to communicate with a ntp server.
The way you determine the frequency of a digital tv station is to look
at the spectrum and determine the frequency of the pilot which is the
highest peak in the spectrum.
There is no requirement for us to inject time of any prescribed accuracy
into the digital stream.
The tv transmitter may or may not be locked to a gps derived source.
The exciter is usually synthesized with the reference as a crystal that
may or may not be stabilized by any accurate means.
At a previous UHF station, the frequency was controlled by a TO-5 cased
crystal in a very small oven.
I would not rely on a signal from a tv station as a time or frequency
standard unless you know how their timing and frequency is derived.
On 3/31/2018 9:38 AM, Bob kb8tq wrote:
> My comments really were a bit brief…. indeed there *are* clocks in the modern
> signals. Those clocks come over as part of the signal you get. The must be a
> way to build something that would get at those clocks.
> You still have the same basic issue as with the “old” signals. Does it go through
> a satellite link? Does it come straight from a Cs based studio? Does it get regenerated
> against an OCXO or a TCXO? All of that will make it a good reference “some of
> the time”. Working out when that is …. good luck.
> You probably would do better to build a gizmo to pull timing off you local cell towers.
> The hardware to do it is relatively well documented. As long as you are careful about
> which system you use, the timing should be GPS based ….
> Ok, so the issue is an alternative to GPS? Well one of the “likely sources” for
> a modern TV broadcast setup would be a GPSDO. The same thing for the modern
> digital FM broadcast setups. I have good reason to make this claim ….. :)
>> On Mar 31, 2018, at 1:43 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>> As noted earlier, color burst references were a big deal a long time ago.
>> Thanks. I was fishing for something modern, maybe a bit clock out of the
>> digital receiver.
>> I'm assuming that the digital stream is locked to the carrier. That may not
>> be correct.
>> These are my opinions. I hate spam.
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Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv at arrl.net AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI LM NRA LM SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
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