X/Z color demodulation.

[old_tv_nut]

Originally Posted by benman94

That's just difference demod: R-Y, B-Y, derive G-Y, mix luma in the gun. I'm pretty positive that X/Z has some difference, but Pete's site and Ed's old site don't say what exactly is different.

As you know, the X-Z demod and matrix circuit that RCA invented and used for many years has color difference output tubes that are connected at the cathodes to matrix R-Y, G-Y and B-Y from the X and Z inputs. Because of the connected cathode circuits, most of the X input shows up in the R-Y output and most of the Z shows up in the B-Y, but some negative Z shows up in the R-Y output and some negative X shows up in the R-Y output. To compensate this, the X and Z angles are adjusted so that the X demod output contains mostly R-Y plus a little B-Y, that is the demodulator angle is between normal R-Y and B-Y angles, but closer to R-Y. Similarly for Z, its angle is moved a bit towards R-Y from B-Y.

The nominal angles to get out pure R-Y and B-Y after the matrix are:
X: 74.8 degrees (instead of 90);
Z: 15.5 degrees (instead of 0).

Pure R-Y and B-Y outputs are actually not optimum for CRT's with sulfide green, and I believe RCA eventually changed the angles and the matrix at some time in later years.

There are more things to know about this circuit:
It is known as low-level demodulation because of the amplification in the matrix section. Sometimes this term is confused with I/Q non-equiband demodulation, which also was always low level. However, X-Z low-level demodulation is equiband, but it does not suffer from the worse bandwidth limitations that are likely to occur with high level (single stage) demodulation (without separate amplification).

A clever thing about the matrix circuit that is never explained in texts:
The matrix circuit has the X and Z signal inputs capacitively coupled. This by itself would be a minor disaster, since AC-coupled color difference signals drift to the opposite background color of whatever is prominent in the picture: e, g, performer enters in red shirt, and the whole picture drifts toward cyan. However, the matrix has negative horizontal blanking pulses coupled into the cathodes. This draws grid current in the three matrix tubes during retrace, making each of them a DC restorer. The circuit is stable not only against scene content, but also against any DC drifts in the X and Z demodulators.