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Re: IIgs RGB to VGA experiment
js@cimmeri.com wrote:
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.
They're probably just using it (incorrectly) to mean CGA video sync
frequencies.
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.
I would be very suspicious of the power supply, especially if it's not
isolated from the earth. I had a customer with one of my SCART cables
who was having interference problems on his TV when using it with the
IIGS. After several emails back and forth trying all sorts of things he
finally found it went away when he unplugged the TV antenna!
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.
Most displays have enough leeway that it shouldn't matter. I have an old
Kaga RGB monitor I use on my IIGS and a PAL-only TV I test SCART cables
with. Both of them can sync whether the IIGS is set to 50 or 60Hz output.
--
* Roger Johnstone, Invercargill, New Zealand -> http://roger.geek.nz
* PS/2 Mouse Adapter for vintage Apple II or Mac
* SCART RGB cable for Apple IIGS