Re: Apple Hi-Res RGB

[irsman@winternet.com (Ian Schmidt)]

In article <4mdqrc$g51@newsbf02.news.aol.com>,
RUBYWAND <rubywand@aol.com> wrote:
>In article <4mce59$hn4@atlantis.atlantis.actrix.gen.nz>,
>dempson@atlantis.actrix.gen.nz (David Empson) writes:
>>
>>Hm... a thought just occurred to me: how hard would it be to make an
>>external box which plugged into the IIgs RGB port, sampled each video
>>line and regenerated it twice, at double the scan frequency?  This
>>would allow the IIgs to work with most modern analog RGB monitors.

>
>     Based upon a comparison of standard VGA and GS monitor inputs, the
>solution may be fairly simple. VGA monitors would be happy enough with
>GS's 60Hz Vertical Sync.

Yes, but read on.

>     The main problem seems to be that GS uses a composite Sync while VGA
>uses separate Vertical and Horizontal Sync signals. A IIgs-to-VGA
>converter box could handle plug conversion and include a sync seperator
>plus any signal level adjustments which might be required.

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).
--
Ian Schmidt - irsman@winternet.com - http://www.winternet.com/~irsman/
    "Where would I be without IBM?" - P. Robb, Information Society