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

["Tim Haynes" <timhaynes@alumni.NO.SPAM.PLEASE.uwaterloo.ca>]

Michael, I think this group needs to sponsor a couple of months of your time
so you could finally build us all the IIgs2VGA box/dongle/card/whatever that
everybody keeps fantasizing about.  :-) :-)

Cheers
Tim

"Michael J. Mahon" <mjmahon@aol.com> wrote in message
20030513193743.28006.00000333@mb-m05.aol.com">news:20030513193743.28006.00000333@mb-m05.aol.com...
> 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/