Roger Johnstone wrote:
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!
There is getting to be a market in coax isolators that break the ground circuit... BTW, it should be fine if the ground of the power supply is common with the Apple (as it must be anyway for the RGB signals). If the monitor cannot be connected to the same ground, then it is not a suitable monitor (rare).
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.
I completely agree. Only the newer digital monitors are picky. -michael Music synthesis for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."