Crystal for 540B Display board

[bxdanny]

It would be very strange if a difference of 0.1% in the crystal frequency produced a visible difference in the output, and of course it does not.

I think the crystal frequency gets divided by 768 to produce the horizontal scan frequency. If that's right, then the 12.096 MHz crystal produces a horizontal frequency of 15,750 Hz, exactly what the original (black and white) NTSC standard called for. The revised color standard reduced this by a factor of 1.001 to about 15,734 Hz.

The 540B schematic specifies the crystal value to 4 places, calling for 12.08 MHz on a color board, and 11.79 MHz on a black-and-white board. It looks like the those values are meant to optimize the horizontal and vertical frequencies, respectively, so that one or the other will be as close as possible to the NTSC standard. The ratio of the two frequencies is always non-standard, being 256 instead of 262.5.

The original crystal value of 12.083915 MHz came even closer to the NTSC standard of about 15,734 Hz than the 12.08 MHz listed on the schematic would, coming closer than any other potential value that is a multiple of 5 Hz. But there is just no need for that kind of precision.