An RCA Engineer explains the Colorplexer which preceeded Color Bars

As a result of my article on Color Bars at VideoUniversity, I received an email on Aug 16, 2006 from Bob Thomas who wrote:

Hal Landen’s article on monitor adjustment with color bars is informative for those who have recently arrived on the scene and a good reminder for old timers.

I joined the RCA Broadcast Division at Camden in November 1951. I can’t remember if there was a CB (color bar) generator in the lab when I arrived, but there certainly would have been one soon afterward. Initially the CB Gen consisted only of a pulse generator that produced RGB outputs of 1.0V pulses with width and position corresponding to saturated component color bars. It did not generate composite bars with subcarrier as we know them today; that was done by a separate unit called a Colorplexer. The Colorplexer consisted of a crystal-controlled S.C. generator (not 3.58 Mhz at that time), I and Q modulators, and filters. It had two selectable inputs, one for camera RGB component video signals, the other for the C.B. generator pulses. When you wanted to output bars from the “camera,” you switched the Colorplexer input to the component output of the bar generator to make composite bars, using the same circuitry that produced composite camera video from camera RGB signals. Neat, but it meant a separate bar generator was required for every camera; there were very few standalone bar/colorplexers in the system because they were too costly.

Hal’s reference to use of a blue filter for bar setup recalls an amusing incident. In its infancy, nobody ever achieved correct colorimetry throughout their system. Cameras were typically matched by setting them up individually, and then tweaking their Coloroplexers for closest match. That often entailed compensating tweaks of individual camera monitors until acceptable overall color reproduction was achieved. Yes, you could easily walk yourself to oblivion! We had a free spirit engineer named Norm Kellaway and at an NAB Convention in the early color days, Norm delighted in walking around the exhibit floor with his filter (I forget the Wratten Number, but it widely used in film and TV) to check validity of monitor setup. He would gleefully grab passersby to show them how far out of whack our competitors’ monitors were in an attempt to make good pictures.

Your site is very interesting, to say the least!

Regards,

Bob Thomas

Blue Bell, PA

Thank you, Bob and we all wish you a speedy recovery from your recent surgery.

The first nation-wide color TV broadcast was The Tournament of Roses Parade on January 1, 1954.

Send feedback on this article to me, Hal Landen, at Email

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