Re: IIgs RGB to VGA experiment

["js@cimmeri.com" <js@cimmeri.com>]

I wired up the "CGA" converter and am having some mixed results.   I
also have questions for you more knowledgeable folks.  :-)

The converter is nicely made, and has two inputs.. both of which are
ANALOG RGB.  The "CGA" wording is being uniquely used by the
Australians, and does not mean the IBM digital RGB CGA standard.

The first input is for RGB+composite synch (eg. IIgs) and is very
limited in what scanning frequencies it can handle; the second input is
for RGBHV, and is multisync capable.

Studying the converter's RGB+composite sync specs, it accepts 15.75khz
horiz and 60hz vert, and outputs based on that 30.9khz(!) by 60hz.

Now, I could not find any IIgs RGB video output port specs, but the
IIgs applecolor RGB monitor input port specs are 15.734khz horiz x 60hz
vert.   So I figure the IIgs is putting out 15.734khz x 60hz, and
that's close enough to work with this CGA unit.

So I tried the setup with an older IBM VGA non-multisynch monitor.
Turning everything on produced quite a buzzing racket from the monitor.
  I checked its specs.  Turns out the monitor only goes down to
31.5khz... which is what I thought was normal VGA non-interlaced horiz
freq (exactly 2 x 15.750).

Next, I tried on a Nec Multisynch that goes down to 31khz.   Here, I
got a sharp picture, good contrast, with very nice color.   However,
the whole screen image shimmys around a bit at what appears to be 60hz.
  Not stable.   Not sure what is causing this -- whether it be the
30.9khz output (versus the Nec's low end 31khz), or the external power
supply I'm using to drive the converter.. might be introducing some
60hz AC into the 5V DC it takes to operate it.   I will try with a lab
power supply to see if that improves the shimmying.

Bottom line is, though, that using its RGB+composite synch input, this
unit does not put out the correct 31.5khz VGA horiz scan freq... and
thus won't work with many VGA monitors.     It DOES have another
jack... RGBHV which from the specs appears that it will correctly
convert 15.75 -> 31.5.

So my questions are:
- Does the IIgs itself (as opposed to its mate monitor input) put out
15.734khz horiz scan?   If so, why 16hz less than the standard
15.75khz?   Does this slight difference make any difference at all?  In
other words, can a 15.734khz source drive a 15.75khz monitor?      I
notice that the Commodore 1084 monitor I have is also 15.734khz input
for its RGB jack.

- I've seen notes on going from RGBHV-> RGB composite synch.    Does
anyone know how to convert from RGB+composite synch back to RGBHV such
that the more flexible converter input could be utilized?

Onwards,
  ~ J