Re: IIgs to arcade monitor experiment

["Michael J. Mahon" <mjmahon@aol.com>]

MdntTrain wrote:
> I'm my seemingly never-ending dallying with with IIgs video, I thought
> I'd try interfacing my IIgs to an analog 15khz 19" RGB Electrohome
> "Go7" arcade monitor I have here.. just to see how well it works (or
> not).
>
> I could use some insight on impedance matching and video signal level
> from you resident masters of electron manipulation.  :-)
>
> Here's the situation.
>
> The IIgs video has these output specs:
>   RGB:  1 Vpp @ 75 ohm impedance
>   Sync:  4 Vpp @ 2k impedance
>
> An arcade monitor (non VGA type) typically has these input specs:
>   RGB: 4 Vpp @ 5k impedance
>  Sync: 2 - 4 Vpp @ 35k impedance.
>
> Video: these numbers don't look well matched, and I'm not sure what to
> do about it yet.    If I connect the IIgs RGB directly to a higher
> impedance than 75, the signal voltages are higher.. not as high as
> 4V... but maybe enough for sufficient contrast.   However, does the
> IIgs video signal get screwed up somehow if I don't load it with 75
> ohms?
>
> Sync: looks like a better situation.. as the voltages start out in the
> right range.   I'm not sure but my guess here is that the 35k sync
> input impedance won't be a problem for signal integrity ... as either
> 2k or 35k are "high impedance" inputs.
>
> Any thoughts are greatly appreciated as always.

The sync level and impedance is fine--for short lines (and anything
over 500 Ohms has to be a short line ;-)--the only requirement is
that the load (the monitor) not load down the source, so any load
impedance more than 5x the source impedance is fine.

The RGB signals are more interesting.  4vpp is a pretty high level
for an analog input, so I'd check to be sure that it isn't actually
TTL.

If it is analog, then you'll need some gain (and possible a polarity
inversion).  The right answer is three video op-amps, but since the
load impedance is so high, you don't actually need much current, just
voltage gain.  I'd start with a quad op-amp with a unity-gain frequency
in the hundreds of megahertz (which isn't too hard), and build either
non-inverting or inverting gain-of-4 amplifiers.  You can probably
find these in a single package and operating with a single (+5) supply.

Again, for short cables, you don't really need to terminate the RGB
cables, which will typically provide you with 2x the terminated
voltage, so you may actually need only 2x amplifiers--so make the
feedback resistors easy to change or even variable to set the
contrast.

Op-amp compensation may turn out to be necessary to prevent oscillation,
but with so little gain needed, that shouldn't be hard.

-michael

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