Quote:
Originally Posted by andy
I believe RCA made a chassis in the early 80's with I Q processing. I think it was a CTC13X chassis, but I forget the exact chassis number.
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RCA developed a clever way to drive the required I delay line single ended, thus saving a pin on the analog IC. However, when I measured the performance of this chassis for comparison with Zenith sets of the time, the I channel high frequency response was low, not much better than the residual high frequency chroma in the Zenith narrowband demods. The reason, I believe, is that it was too difficult to get the required flat and phase-distortion-free chroma response even in the Q channel, partly due to the difficulty of making an IF that was both sharp-cutoff at the sound carrier and phase-distortion free. So, I think RCA turned the I channel high frequencies down (or didn't peak them to normal level) to avoid annoying hue effects on edges. The other part of the problem was that the NTSC never specified the encoder Q filter closely enough. Some Q filters that would meet the NTSC/FCC spec would still leave visible Q sidebands beyond +/-0.5 MHz that would be unbalanced by the sound carrier trap in the receiver. By the time this RCA set was around, it was known how to make good encoder Q filters that would have a zero at 4.5 MHz and symmetrically at 2.7 MHz, but of course the specs would never be changed in the thousands of encoders out there.
Experiments I did with "ideal" I/Q pictures (using baseband filters and never going to RF) showed that the increased saturation on small bright details and increased sharpness of red roses, barns and rocks was noticeable in side by side comparisons, but also visible were things like the edges of typical yellow movie-title letters turning orange. With a 27 inch or larger set, you couldn't keep people from getting too close, the way a 12-inch console would. Zenith concluded that there was no way to get a good compromise between visibility of red details and visibility of artifacts, and that it would be hard to actually show any of this to customers. If you couldn't turn up the red detail, there was no point in going to I/Q demodulation.