Why Did They Change?

[old_tv_nut]

Originally Posted by Tom9589

Thanks for all the inputs.

Let me slightly expand the question: Why did RCA (and others) go to the X-Z demodulators? Was it something about better flesh tones? Or less sensitivity to phase shift with the color sub-carrier?

Neither. The resulting grid drive signals were exactly NTSC (at least at first).
The X-Z demods with following matrix/amplifier stages was a clever design to maximize the DC stability of the color difference signals. The three amplifier stages are coupled at their cathodes, which generates G-Y in the third tube. But at the same time, some minus (R-Y) is coupled into the B-Y output, and some minus (B-Y) is coupled into the R-Y output. This is compensated by changing the demodulator axes to "X" and "Z." By changing the angles, the X demodulator output is mostly (R-Y), but with a little (B-Y) added in to counter the minus (B-Y) in the R-Y amplifier. Similarly with the Z demod and (B-Y) amplifier.

The improved DC stability of this circuit comes from having the amplifiers also act as DC restorers as well as blanking amplifiers. A negative blanking pulse applied to the three cathodes causes a negative blanking pulse on the CRT grids. At the same time, it draws a bit of grid current in each amplifier tube to set the DC voltage on the coupling capacitors that feed in the X and Z signals. If you look at the G-Y amplifier, you see an input coupling capacitor, which doesn't go to a demodulator but just sets a constant DC voltage.

The X and Z angles were changed in later years to provide some compensation for the non-NTSC phosphor colors.