In a message dated 1/25/01 4:01:41 PM Eastern Standard Time,
[email protected] writes:
<< I can remember in
the early days of TV here in the states that we would occasionally see such
things --- when they were not synched. Now you just see a stable ghost of
the
'interfering' station in the background. >>
This may be true in some major television markets, but it is not the case
nationwide. Most television stations are not synched to anything but a local
reference oscillator, usually a TCXO. This is a big improvement over the
situation in the 1960s and early 70s, but it is not nearly as good as that
period in the late 70s to early 80s when all networks' atomic-referenced sync
pulses were passed directly by local stations. Back then, it was literally
true that "ghost" sync bars would stand still at the same position day after
day. (In locations where it was possible to overlay the video from three
stations carrying the same network program, it was great fun to try to deduce
either the spacing between the stations or one's own position relative to
them.)
Now, frame synchronizers re-sync incoming network signals, the output of
videotape machines, and all other sources to the local studio reference. The
network owned-and-operated stations may use atomic standards or GPS-based
references, but most others don't. Sync drift is generally very slow, but it
does occur in most of the country.
73,
John
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