[time-nuts] TV sync
John Ackermann N8UR
jra at febo.com
Thu Oct 11 20:53:14 EDT 2007
Jeffrey Pawlan said the following on 10/07/2007 12:03 PM:
> All larger TV stations use Cs standards. What you don't know is that the FCC
> assigns SLIGHTLY different scan frequencies to each station on the same
> channel in a close area so when you are in a fringe area between two stations on
> the same channel, you will intentionally see both pictures superimposed with a
> rolling of the scan lines. Otherwise they would be locked and you would see
> only black and white bars.
(Sorry this is replying to an old post; I'm just catching up after being
out of town.)
This is a little different than my understanding. I thought that the
carrier frequencies, not the scan frequencies, were offset. The options
were either on frequency, +10khz, or -10kHz. In fact, one of my friends
does TV DXing and he identifies stations by their carrier offset.
When I was a kid, I lived in an area where co-channel interference was
very common in the summer. We had channel 2, 5, and 11 in Green Bay,
Wisconsin, and all three of those channels were also used in Chicago,
about 200 miles down Lake Michigan. We lived about 70 miles north of
Green Bay, and both sets of stations were within the beamwidth of our TV
antenna. Due to ducting effects along the edge of the lake, in the
summer channel 2 was unwatchable much of the time, channel 5 sometimes,
and channel 11 a couple of times.
Back in the '60s, an attempt was made to reduce this problem by
controlling the carrier frequency much more tightly than usual. This
may have been before the -10/0/+10 offset scheme. I believe the goal
was to keep the two carriers so close in frequency that the beat
frequency would be a small fraction of a Hertz. In any event, the Green
Bay and Chicago stations bought very stable oscillators and WWVB
tracking receivers. The experiment lasted a few years and didn't make
much difference. Ultimately, all the high-stability stuff was scrapped
and they went back to the standard oscilltors.
As for us, when the interference got bad we turned the antenna to watch
stations in the Upper Peninsula of Michigan.
Regarding scan frequencies, back in the old days, the scan and
colorburst were controlled by the networks, and Rb oscillators were used
for stability. The FCC monitored the network stations and reported on
their offset (using the colorburst). However, with digital techniques
things like frame synchronizers came along and the ability to trace sync
or colorburst back to the networks disappeared. Today, there is a
master clock in each station, but its quality is an unknown factor;
almost certainly, it's not calibrated against anything except whether
it's close enough to make everything sync up (this last bit was passed
on to me by a local TV station chief engineer).
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