[mythtv-users] Electrical power in my cable line? Is that bad?
beww at beww.org
Fri Sep 7 15:10:13 UTC 2007
Jay R. Ashworth wrote:
> On Fri, Sep 07, 2007 at 07:21:24AM -0600, Brian Wood wrote:
>> The reason 60 cycle AC hum moves up the screen is that the NTSC frame
>> rate is not really 60/sec, but 59.something (sub-multiple of 3.579545
>> color subcarrier which, if it were 3.600000 would result in a precise 60
>> Hz. frame rate).
> 59.97. It used to be 60, back in the days when black and white
> programming actually moved with no chroma subcarrier.
Precisely. Some B/W systems actually used the AC power line for a
> Fun fact: the 3.58 oscillator in a TV used to be the most accurate
> frequency source a general consumer could get their hands on -- *when
> the TV was tuned to a network color program*. The local affils used to
> pass through the actual subcarrier from the network.
The original standards the networks used for the color subcarrier were
rubidium vapor controlled oscillators. If you were picking up the
network stations in NYC you actually had access to the original signal.
If an affiliate was passing the live signal, received via cable or
microwave, you had the "real thing", but if the video had been recorded
on tape the accuracy was questionable.
Rubidium is still a secondary standard (though a good one). A quartz
(actually usually tourmaline) oscillator phase locked to WWV was
actually a better standard, assuming you corrected for propagation phase
problems etc., as WWV used (and still uses I believe) an array of Cesium
standards, which are in fact primary standards.
These days, GPS receivers give you access to cesium-based time (and thus
frequency) standards as well.
LORAN-C also gives you access to cesium-based standards, though those
devices are not used much these days since GPS is so cheap, and they are
useful only in their limited coverage areas (mostly coastal areas).
> Clearly, they don't anymore. :-)
Again, if you have a "live" network feed the accuracy should be pretty
darned good. At least for the "real" networks (CBS,NBC,ABC). Some of the
cable-only networks seem to use more questionable standards (like $2
Timex watches :-))
Then there was the "Broadcaster-Controlled Color" idea of the VIRS
signal. Theoretically a good idea for a live network feed, but what an
early VTR (or even a modern VCR) could do to that signal was pretty
horrid, and even phase or group delay problems in microwave links could
render the system pretty useless.
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