[QuadList] VR-1200B in operation

[Dennis Degan]

			On Dec 31, 2008, at 9:06 PM, Ted Langdell wrote:

 > Not sure when AT&T piggybacked the sound with the picture—called 
"multiplexing" as I recall—but David Crosthwait has mentioned the 
relatively inferior quality of recordings made "off the line" at West 
Coast points (including network delay centers in LA and Burbank) 
because of the limited frequency response of the separate audio lines.

			I offer:

	I worked at a TV station in Spartanburg, SC in 1977 (WSPA-TV, a CBS 
affil) when mulitplexing was introduced.  Telco came in with this 
rack-mounted unit with a brushed-aluminum front cover.  Wideband video 
goes into the back, baseband video and audio come out.  I don't know 
who made the box, but it improved the audio quality noticeably over the 
old system.  An added bonus to the system was that there were TWO card 
slots available inside the box for audio; meaning that stereo was 
possible for the first time (although as I recall, our unit had only 
one audio card installed at the time).

			Ted sed:

 > The telco folks also had to work out some sound/picture sync issues 
because the sound and picture had different propagation characteristics 
due to the technology used for each.

			I reminisce:

	When I worked at WIS-TV in Columbia, SC in 1975-76 (yes, I worked at a 
lot of TV stations in those days), I saw a sync test being performed 
on-line by engineers at NBC.  The test was being done in the early 
afternoon during time when the network was not on-air.  The engineers 
had this little light device which they held up to the camera.  I think 
it was a neon bulb of some kind because it was orange.  Neon would be a 
good choice because it could flash very quickly and brightly.  A 
standard incandescent bulb might appear to turn on quickly but it 
actually glows brighter over a brief time as it is switched on and dims 
out equally slowly when switched off.
	This neon light device apparently also generated an audio 'pop' when a 
button was pressed, which flashed the light at the same moment.  
Audio/Video sync could be measured at the receiving end (I'm not sure 
how they measured it; might have been simply by eye), allowing the 
engineers to align the timing between the two (it's also not clear how 
timing could be adjusted in those days; frame synchronizers were rare 
and expensive.  Obviously if the audio was early, it was more easily 
delayed than the video).

			Ted:

 > After they learned to multiplex the audio with video and delivered it 
on—want to say 110ohm balanced copper—things got better in terms of 
frequency response, and audio-video out of sync issues.

			I say:

	I think it was 150 Ohms, balanced video line, the same line that was 
used formerly for the video alone.  The real difference was in the IF 
amplifiers along the path.
	The reason why multiplexing wasn't used prior to the '70's was that 
Telco had limitations on the video bandwidth and noise of the 
amplifiers along the path from the origination point to each TV 
station.  Those added audio carriers were well above the video 
baseband; something like 6.8MHz and above.  This would reduce the 
overall S/N for the IF as it was transmitted over microwave and coaxial 
cable.  With the introduction of newer lower-noise, wideband 
amplifiers, multiplexing became possible.

			Ted:

 > I googled a while to find some links to the multiplexing process, but 
didn't come up with anything.  As I recall, the video went as baseband, 
and the audio was on a carrier above the video.

			Me:

	That's how I remember it too.

			Dennis Degan, Editor-Consultant-Knowledge Bank
	  				NBC Today Show, New York