I wrote:
And if you install the Video Associate Labs VB3 Microkeyer in your
Apple II or Apple II+, you get some added video modes including linear
mapping.
Michael J. Mahon wrote:
That's a pretty amazing mod, since video generation is so
pervasive in the Apple II design.
How did it do that?
The VB3 consisted of two boards. One was a rather long slot card, and
one was a big rectangular card that sat over the power supply. The
two were connected by a ribbon cable. You pulled out about a dozen
of the TTL chips in the Apple, and installed small ribbon jumpers between
them and the VB3 boards. The slot card was mostly digital, and the
card that sat over the power supply was mostly analog.
The main purpose of the VB3 was to act as a gen lock and proc amp, make
the video fully compliant with the NTSC spec (and thus FCC broadcast
requirements) and do simple video overlay and keying. The linear mapped
hires graphics mode was a bonus; since they replaced the entire video
timing chain anyhow, it was relatively easy to add.
Schematics weren't available, and I wasn't inclined to try to reverse-
engineer it. I suspect that the most amazing part of the design was
getting the color phase correct. One of the most serious deviations of
the Apple II video from NTSC spec was that it used 228 cycles of the
color carrier per scan line, rather than 227.5. Woz did that so that
the color phase would be the same on all lines. But that doesn't work
if you're generating true NTSC.