Re: Apple RGB monitor- VGA interface

[Rubywand <rubywand@swbell.net>]

pubpc1@library.ucla.edu writes ...
> 
> Rubywand wrote
> ....
> >      Concerning the addition of new display features, it does appear
> > that the best place to plug in the converter is the VGC socket. Once we
> > accept that this would be the way to go, we suddenly have a bunch of
> > interesting options.
> 
> The main problem with this route is that the ROM 3 has a
> soldered VGC (it also has a soldered Ensoniq for that
> matter, preventing a sound upgrade).  How about building
> the interface into a box that plugs to the video connector
> outside of the GS?  Such a device could have separate
> adapters for GS, Amiga, and Atari ST and heck, maybe SNES
> as well, expanding the market for this device and thus
> lowering costs!  :)
>

     Other platforms! Yes; that could boost volume and lower costs.

     Since the VGC socket is out as option, the external box may be out
best bet. On the GS, it would avoid using up a Slot-- an important
consideration if there is a later Slot card to add new display modes.

     The downsides of the box vs. a Slot card are that we lose access to
one useful signal (the 7M 7mHz clock) and do not have a +5V source. (We
will need to add a regulator to get +5V from the +12V available at the
GS monitor socket.)

     It looks like any non-VGC socket approach to a VGA interface is
going to require 'undoing' signals available at the GS monitor socket.
For instance, we shall need to separate H and V syncs. Also, we shall
need to do a 4-bit A/D conversion on each color output (R, G, and B). 

     Given the box's marketing advantages, it looks like the best design
to go for. The tricky part may be syncing an in-box 14mHz clock with
incoming RGB 'dots'. H-sync may be enough; but, it seems likely that
there will need to be an adjustable delay (e.g. a 1-shot or count-down
chain) to allow each user to get the best display. We can try using a
14mHz dot input rate for both 640 and 320 modes. 320 mode dots would be
sampled twice.

     For 30kHz Horizontal output, the output dot clock will be around
28mHz. Since we need a 14mHz input clock, we can save by going with a
28mHz osc and dividing to get 14mHz. Probably, once a prototype is
built, there will be several places where costs can be cut.

     By the way, one thing about looking at the GS video circuitry is
that it drives home how easily the original design could have been
modified to produce 256 colors in both 320 and 640 mode. Mainly, another
512kB of RAM would have been required. It's too bad Apple did not use
the release of ROM-03 to add more colors, more lines, and VGA monitor
compatibility.

     Anyway, it looks like an external box VGA converter is doable. It
could have inputs to fit other platforms and to allow expansion to new
GS display modes when a new modes Slot card is ready.  


Rubywand