Re: Apple Hi-Res RGB

[rubywand@aol.com (RUBYWAND)]

In article <4meagm$pjv@blackice.winternet.com>, irsman@winternet.com (Ian
Schmidt) writes:

>
>Bzzzzt!  (the sound you'd hear moments before the HV/horizontal circuitry
>in the VGA monitor went critical with such a setup).  The GS's vertical
>sync is not the problem.  The GS's HORIZONTAL sync is.  The GS outputs
>RS-170A
>compliant NTSC video, with a horizontal scan rate of around 15.75 kHz or
so.
>VGA/SVGA and Mac monitors, on the other hand, run at a _minimum_
horzontal
>scan
>rate of 31.5 kHz, or about double the GS's rate (this is why
fixed-frequency
>Mac monitors will show an image when connected to the GS, but doubled
>horizontally).
>-

     "Bzzzzt!" Bad news, indeed. It is a shame if the Mac monitor requires
a minimum 31.5kHz Horizontal Sync.

      It would be interesting to test some standard VGA monitors and see
if any can sync at 15.75kHz. If not, then some kind of frame-grabber with
the necessary VGA outputs may be the IIgs-to-VGA solution-- not a very
elegant one and likely to be pretty expensive.

      Some line-grabbing scheme like David's might get the job done at
much less cost. The hardware input does A/D and stuffs line info into a
LIFO stack. The hardware output sends an 'off' line and then begins to
pull line info off the stack, do D/A and send it. After 200 "off" and 200
displayed lines, the hardware repeats the process except that displayed
lines appear where 'off' lines were sent on the last pass. You would be
interlacing two 200-line frames but, at 30Hz, it would look like one
400-line frame.  

     Another non-IIgs monitor candidate is the RGB monitor used with the
Commodore 128. These probably sync fine at 15kHz and, perhaps with minor
plug conversion, should work on a IIgs.
    

Rubywand