This may not relate to the USA but a couple of years back I had an
interesting chat with a BBC engineer at an NPL Time and Frequency gathering
at Teddington. His comment was along the lines of....."....well there is as
you realise not need to sync lines and frames nationally now because all the
network content travels digitally even though we have only just started a
phased change-over. The frames and line syncs broadcast will be probably
derived from a "good clock" but it will not be a single traceable unit as it
was. There is no telling how good the clock is, though it will probably be
better than absolutely necesary.

Alan G3NYK
----- Original Message -----
From: "Hal Murray" hmurray@megapathdsl.net
To: time-nuts@febo.com
Sent: Saturday, June 13, 2009 11:18 PM
Subject: [time-nuts] Time/freq from digital TV

clock?

https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

On Sat, Jun 13, 2009 at 03:18:16PM -0700, Hal Murray wrote:

I don't believe there is any standards grade time or frequency

that one can derive from a FCC legal standards conformant ATSC
transmission in digital format.

There are fairly loose specs for ATSC symbol rate (by time-nuts

standards) and carrier frequency - not dead nuts on in any broadcast
plant unless someone engineering it really cared for some reason.

And nothing in the ATSC stream establishes a precise epoch - the

stream consists of 188 byte transport stream packets... sent from a
queue when a time slot is available... with no particular discipline
that a  particular packet be sent at a particular exact time.

There is, however, a timing mechanism in MPEG2 TV based on a 27

MHz clock that controls and coordinates video and audio rendering in
digital TVs ... and part of this is that some packets carry a timestamp
in ticks of this clock (called the PCR) so a receiver can lock its own
27 MHz video timebase to this clock so video frames and audio samples
are output at the correct times - mostly relative to each other and the
video source, of course, and not any absolute UTC time.

Broadcast plant may or may not lock its master 27 MHz PCR

reference to a GPSDO or rubidium - there is no requirement to do so.
And no particular expectation that some epoch of the PCR clock matches a
particular UTC epoch.

Likely the choice is to use the OCXO in a house master sync

generator or ATSC multiplexer  as the reference or use an external clock
input from some kind of frequency standard.  I suppose there ARE
stations that use a decent GPSDO or other high grade frequency source
for this... how one knows this is true in any particular case, however,
is not clear.

Most network broadcast distribution is via satellite, and

satellites move around in their box in the sky so any time or frequency
derived from satellite downlinked signals is subject to Doppler shifts
over the course of a day as the satellite completes its figure eight
pattern in the sky  (most operational birds are slightly inclined, thus
the figure 8 - none are dead nuts on the equator and in perfectly
circular orbits).  This means that no network timing from a satellite
signal is stable by precise metric standards... even if the uplink signal
is right on.

What all this means in practice is that there is no longer any

precise broadcast TV signal that can be depended on as really accurate.
Of course this has already been true for at least the past 20 years with
analog NTSC transmissions from ubiquitous digital broadcast plant... for
the most part the timing for the old analog NTSC transmissions was
derived from the OCXO (or even just TCXO) in either a master sync
generator or the digital NTSC modulator just before the transmitter
modulator analog input... long long gone are the 1970s era days of a
completely analog and un frame buffered path between a stable analog
terrestrial microwave based link to a master rubidium or cesium network
clock in NYC and the input to the transmitter...

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."

The digital transmitter could have a GPS based frequency reference.
I am the CE for a local station (WCIV).
We do not use any standard other than what is installed in the transmitter.

I have not verified it, but, I think that the UHF transmitter
frequency is controlled by an OCXO.

I will try and remember to check on this Monday.

I do know that our Analog transmitter used an ovenized crystal  for
frequency control. We were on channel 4.

We definitely are not slaved to the network for frequency control.

73
Glenn
WB4UIV

At 06:18 PM 6/13/2009, you wrote:

On Sat, Jun 13, 2009 at 09:59:50PM -0400, Glenn Little WB4UIV wrote:

I'd bet a lot of current digital exciters use a digital

synthesizer locked to a reference rather than a special OCXO crystal cut
to a particular TV channel frequency. This COULD allow a 10 Mhz in lock to
a GPSDO, rubidium or better...

I found a couple of the Boston analog stations were very

accurately on frequency using my spectrum analyzer signal counter with a
10 MHz GPSDO reference... so some folks somewhere must bother to lock
signals.

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."

Glenn: Please forgive a slight deviation from the time, but do you know
what's happening to transmitters in the industry?
Thanks very much
Don Latham
----- Original Message -----
From: "Glenn Little WB4UIV" glennmaillist@bellsouth.net
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Saturday, June 13, 2009 7:59 PM
Subject: Re: [time-nuts] Time/freq from digital TV

David I. Emery skrev:

You are correct, until the time when an ATSC network is operated in
Single Frequency Network (SFN) mode, in which case the transmitter
signals is coordinated with the aid of GPS receivers. The ATSC SFN specs
is in a separate document. As I recalled it when I fast read it a few
years back was that it seemed like they had made a few thought errors
which caused overspecing but still possible.

Cheers,
Magnus

Alan Melia skrev:

Actually, this is still far from the thinking I have heard from other
sources. They essentially want everything traceable to TAI/UTC.

Digital doesn't help, it makes it more acute actually.

Cheers,
Magnus

Hi Magnus that may be the case in some countries I suppose. Surely the
accuracy is only required at the  end of the transmission link, the frames
from different sources are then resynchronised, and it is not "necessary" to
transmit nationwide accurately synced frames?? The UK is "privatised" and
content distribution network and transmitters are no longer owned by the
programme companies, and they probably have little say in this aspect except
that it will depend upon the price they are willing to pay.......... hence
the BBC engineer did not think it a "reliable form of frequency distribution
in the UK"....this does not mean it might not be useful!

Alan

----- Original Message -----
From: "Magnus Danielson" magnus@rubidium.dyndns.org
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Sunday, June 14, 2009 12:31 PM
Subject: Re: [time-nuts] Time/freq from digital TV

gathering

as

the

a

as it

be

https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

In message 4A34DF9B.4010306@rubidium.dyndns.org, Magnus Danielson writes:

Make sure you ask an exact question.

For source material, they are very keen on traceability.

On the transmission side they don't care much because the network
delay these days can approach tens of seconds...

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Never attribute to malice what can adequately be explained by incompetence.