XPost: comp.sys.sinclair, comp.sys.atari.8bit, comp.sys.apple2   
   XPost: comp.sys.cbm   
   From: mjmahon@aol.com   
      
   Eric Smith wrote:   
   > 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.   
      
   Thanks for the explanation.   
      
   Now that I know it's a genlock card, I understand both the motivation   
   and the niche market support for such a complex modification.   
      
   In 1979-1980, I lived in Austin, TX, and the local TV station used   
   a Apple equipped with a genlock card for the weather forecast graphics.   
   Sounds like it might have been this system.  ;-)   
      
   -michael   
      
   Music synthesis for 8-bit Apple II's!   
   Home page:  http://members.aol.com/MJMahon/   
      
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