Re: Apple RGB monitor- VGA interface

[Rubywand <rubywand@swbell.net>]

Scott G. writes ...

> .... I searched the net and found dozens of
> companies that make little devices that will take a 30kHz
> (VGA/SVGA) output and transform it to 15kHz for connection
> to "TV" analog RGB devices (NTSC rate).  These typically
> sell for $25 and are designed to connect PC and Mac computers
> to projection displays.
> 
> If such a device is possible for so little money, would it
> not be possible to build the reverse device?  One that would
> take analog RGB output from a GS or an Amiga and pipe it to
> a VGA/SVGA/Mac monitor?  There is an market for such a thing.
> It would be different from the Second Sight too, because it
> will allow the display of ALL of the GS's modes ....
>

     For sure, there would be a market if the device could be priced
below $75. The interface plus a VGA/SVGA monitor could become the
standard GS monitor setup.

     The main challenge is finding a way to double the dot rate. This
would seem to require capturing at least each horizontal line and
resending it. Basically, I guess, you would read-in the first (top) line
to Buffer-A during the monitor Vertical Blanking period and, afterward,
send it twice while loading in the next line to Buffer-B. You, then send
Buffer-B twice while loading Buffer-A, etc..

     An alternative is to just capture and re-send whole screens. You
would need more buffer memory; but, the design could be easier because
timing would be less critical.

     Either approach could display the usual 200-line GS screen as 400
lines on a 30KHz VGA/SVGA monitor. It should look very nice and, as you
mention, all GS display modes would be available.

     Of course, once you've gone to the trouble of developing such an
interface, there would be a temptation to add embellishments (e.g. a way
to display 400 unique lines, more colors, etc.).  It would not be too
surprising to discover that Second Sight began as a 'simple' VGA monitor
interface. 

     In quantities of a thousand or so, the basic GS-to-VGA monitor
interface circuit might be commercially doable at a sale price below
$75, especially, if it's an Apple club project or other such situation
where no one actually expects to be paid much for the time he/she
contributes.


Rubywand