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Originally Posted by TVtommy
Gee, it's a shame I don't live near the Illinois area Old Tv nut, I would really love to pick your brain more intensely! An old friend of mine who just recently passed was a long time employee and xmtr super for our local CBS afilliate. When I got eat up with the vintage color bug in the 80's, we talked at lenght about the challenge of getting a color signal through those old RCA 25kw coffee cans and on the air. Seems as though there was an issue with dfferential phase distortion that reeked havoc with satisfactory hue reproduction and that it was more apparent on I/Q sets. BTW, was RCA the only man. to try and reintroduce I/Q demods in the 80's and didn't this coincide with their introduction of real comb filters? Shame they didn't have those in "54. Sorry if I'm off thread topic.
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There are three kinds of phase distoriton that can happen in an NTSC transmitter. The "indcidental phase distortion" is a non-linear effect where the chroma phase is changed according to the luma it's riding on - so bright parts of a face get a different hue compared to the shadows. Then there are two linear distoritions: if the tramsmitter doesn't have a flat frequency response, it will roll off the upper chroma sidebands - this makes the Q sidebands non-symmentrical, causing crosstalk into the I channel of an I/Q set. The receiver IF can do the same thing. I think that's part of what I was seeing in the receiver. Third, the transmitter can have a poor phase response near the band edge (due to sound diplexer), which causes color transients (edges) to be wrong. Probably some of that in the receiver too. NTSC specifies the group delay response of the transmitter be precorrected to match the NTSC committee's idea of what a receiver IF would do. (Called a "Friedendall" precorrection filter after the guy who managed to design a workable one.) PAL does not use a precorrection, and lets any phase (hue) distortion be cancelled by the alternating phase.
As far as comb filters, reasonable ones came in in the mid 60's with the invention fo the glass delay line for PAL, so this wasn't really tied to the abortive I/Q revival.