Re: Adapter to use IIGS's RGB output on modern VGA/SVGA monitors (idea?)

[mjmahon@aol.com (Michael J. Mahon)]

ToasterKing replied:

>In article <20030512180919.24954.00000344@mb-m06.aol.com>, Michael J.
>Mahon <mjmahon@aol.com> wrote:
>
>> Some signal processing is required, but perhaps only simple
>> analog addition and subtraction.
>> 
>> There are quad video op-amps available that will certainly
>> do the job of generating YPbPr from RGB, but you'll have
>> to get some power to them, since they drive 75 ohm loads.
>> 
>> The Y signal is a weighted sum of R, G, and B signals,
>> where the weights are a standard.  Pb is B minus Y and
>> Pr is R minus Y.  All signals have standard polarities and
>> peak-to-peak amplitudes.
>> 
>> -michael
>> 
>> Check out amazing quality 8-bit Apple sound on my
>> Home page:  http://members.aol.com/MJMahon/
>
>That sounds like possible a little more complex design than I was
>hoping for, especially since my circuit design skills are a little
>rusty.  Since I don't know exactly what the specifications of YPbPr
>call for, and how to manipulate the signal from RGB, I'm not really
>sure where to start here.  If you happen to have any of this knowledge,
>could you possibly point me in the right direction?  Otherwise, can you
>think of any other easily-achievable design that's a little more
>elegant than conversion to first YPbPr, then to VGA?

There's a Linear Technology Application Note that has a
circuit for converting RGB to YPbPr:

http://www.linear-tech.com/pdf/13989f.pdf

It requires two dual video op amps and some resistors.
It does not show adding composite sync to the Y signal.
That will require another op amp between the Y output
shown in the diagram and the real Y output, to add (or
subtract--I couldn't quickly confirm the IIgs sync polarity)
the composite sync to the Y signal in the right proportion
(about 1/4 of the Y peak-to-peak amplitude).

The circuit assumes +5v and -5v supplies, which is
a bit awkward.  The circuit could probably be adapted
to work from a single-ended supply, since I believe all
the signal components are positive(?), but levels would
have to be shifted some to keep the video linear.

Assuming that 75 ohm impedances are used throughout,
the current requirement from each supply should be less
than 100ma, so, though it's not a power hog, it will need
access to "real" power.

The reason that this "transcoding" is so simple is that
we need not do anything to change the timing of the
signals, only the mixing of the components.

-michael

Check out amazing quality 8-bit Apple sound on my
Home page:  http://members.aol.com/MJMahon/