Re: How does the apple2 RGB card work?

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

mdj wrote:
> Michael J. Mahon wrote:
>
> > mdj wrote:
> >
> >
> > > The AppleColour Monitor 100 actually had such a thing internally, since
> > > it was actually an adapted analog display, but it simply mimiced the
> > > digital colours.
> > >
> > > IIRC, there was a switch inside this monitor that could disable the
> > > digital decoder, making it compatible with the IIgs RGB monitor. This
> > > information was provided to purchasers of the IIgs upgrade kit for the
> > > IIe.
> >
> > Very interesting!  Does anyone have more info on this?
>
>
> It was just a simple resistor/zener diode network. All you need to do
> to convert digital RGB to analog is level adjust the r,g,b digital
> output to full intensity analog, which I think it 1V, then allow for
> 1/2 level output dependant upon the intensity level.

Presumeably only the attenuation is required, and the zener is just to
prevent overloads?  With a 5:1 attenuation required, any DC level shift
would be very easy just with resistors.

I had expected some matrixing of the RGB signals to produce "Apple"
colors instead of RGBI colors.

> Pretty simple
> circuit, I've used a similar thing to adapt my Ramworks digital RGB
> output to drive televisions that have SCART connectors. Provides a nice
> plentiful source for Apple II displays, and I can have either true
> composite colour via the composite input, or nice, clear RGB via the
> SCART ;-)
>
> Of course as has been noted before, digital RGB produced this way is
> only an approximation of the real Apple II colour palette, but it's
> reasonable. You get the 16 possible digital RGB colours instead. You
> could do a more accurate conversion I suppose by decoding the digital
> signal into a 12 bit RGB representation, and using either an actual
> DAC, or a poor mans version with resistors.

From this group I got the idea that there was special "Apple"
non-standard matrixing in the AppleColor RGB 100 that created a
pretty good approximation of the composite colors, rather than the
typical RGBI colors.

Frankly, it always seemed strange to do this matrixing in the monitor
instead of on the RGB card.

You also mentioned that there is a *switch* inside the monitor that
bypasses the digital conversion, making it an analog RGB display...?
(Getting screwdriver...)

-michael

Parallel computing 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."