Re: How does the apple2 RGB card work?

["mdj" <mdj.mdj@gmail.com>]

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. 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.

Matt

Matt