[time-nuts] TV frequency control

Murray Greenman murray at rakon.co.nz
Mon Oct 8 13:33:39 EDT 2007


Hi,

Jose's correct. Co-channel TV stations are accurately offset by a
fraction of the line frequency. Interestingly enough (and for this
reason) independent TV stations often have very accurate transmitter
frequencies, while potentially having quite poor (1e-6 or so) control of
the picture timing, while larger stations (without co-channel problems),
are the other way around.

In larger TV networks, the picture timing is maintained with very high
precision. This allows very clever mixing desks at studios hundreds of
km apart to fade between pictures from different sources, without even a
change in picture hue, never mind a picture shift or roll. I understand
that our biggest network, TVNZ, uses Trimble Thunderbolt GPSDO units and
Neve digital desks, which can digitally advance or retard video to allow
for the transmission delay between sites, which the desks negotiate
between each other.

In my shack, in honourable retirement, is TVNZ's first network reference
- a venerable HP 5065A Rubidium, dating from about 1975. This unit has
the 4.43361875MHz colour subcarrier option. The divider in this option
(from the days before DDS) requires the 5MHz to be offset slightly for
the colour subcarrier to be correct. It last had a new tube in 1978, and
is still running fine, although the power supply required an overhaul
when I first got it. Electrolytics had dried out, and the resulting hum
confused the oven controller terribly, it overheated and blew the
protection fuse.

Regards,
Murray ZL1BPU



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