XPost: comp.sys.sinclair, comp.sys.atari.8bit, comp.sys.apple2   
   XPost: comp.sys.cbm   
   From: eric@brouhaha.com   
      
   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.   
      
   Eric   
      
   --- SoupGate-Win32 v1.05   
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