Originally Posted by old_tv_nut
Analog NTSC receivers continued to have modified decoding until the end.
The major decoding adjustment is increased R-Y gain to compensate for the excess of effective red content in the yellower green phosphor. Because the yellowish green was like having extra red whenever the green was turned on, it reduced the hue shift between red and green. The R-Y signal controls the balance of red in a given color, so increasing R-Y means that the difference in red as the transmitted hue changes is emphasized. This only works up to a point with the non-linear CRT and new phosphors, because 1) it can't really change the hue of the pure yellow-green phosphor when pure green is called for, and 2) it adds too much red on bright red colors.
R-Y, G-Y, and B-Y should really be all primed, e.g. R'-Y', because they are derived from R', G', and B' in the encoder, but the primes are usually omitted, just like they are for I and Q. They are the color difference signals that are derived from the chroma signal in the receiver, and can be obtained either by wideband IQ demodulation and matrixing or by equiband direct demodulation on the appropriate three different axes. These three color difference signals are then added to Y' in the receiver to get R', G', and B' drives for the picture tube. The CT-100 did the adding in external matrix circuits and then drove the 15GP22 grids. In many tube receivers that followed, the final addition was done in the picture tube by applying Y' to all three cathodes and R-Y, G-Y or B-Y to the appropriate grid (G1). Later tube designs that did not have separate grids required adding the Y' and color difference signals in the circuits before driving the picture tube cathodes.
R-Y, G-Y and B-Y are not independent, and any one can be derived from the other two, just as any one can be derived from I and Q. So some tube sets had R-Y and B-Y demodulators with a matrix for G-Y, a few had a different choice of the two axes and matrix, and a few had three separate demodulators and needed no matrix.
RCA chassis from CTC-7 onward for several years used a clever matrix that included DC restoration for the signals, but had some cross coupling between R-Y and B-Y outputs, so the demodulator axes were adjusted to compensate, and were called X and Z axes. The signals to drive the CRT grids then came out to be R-Y, G-Y and B-Y, but were adjusted further in later chassis to get approximate compensation for the newer phosphors, as discussed in previous posts.
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