4-tube cameras and illegal signals

[old_tv_nut]

I just got a copy of the October 1966 RCA Broadcast News, with articles touting the TK-42 4-tube studio camera and TK-27 four tube telecine camera.

You may have heard of illegal signals being generated when computer image files and video are converted among different standards, but it turns out this was a problem with four tube cameras back then, because the separate luminance channel generally would have a too-high signal output on saturated colors compared to a correctly matrixed three-tube camera.

This issue has two pages of cartoon diagrams and explanation of special non-additive matrix circuits that were used with the TK-27 to drive the waveform monitor so that these out of bound signal conditions could be detected and corrected (but they don't say how - just iris/gain down?) before putting out signals that might overload the transmitter.

Four-tube cameras avoided loss of detail in the luma due to tube misregistration, but it was a really dumb idea for color reproduction. Of course, there are lots of testimonials in this issue of how much the purchasers of TK-42s like the color reproduction. Blecch.