Re: IIgs Analog Video Circuitry

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

Daniel Kruszyna <dan@krue.net> wrote:
> MdntTrain <js@cimmeri.com> wrote:
>> It's possible that the IIgs added sync to its colors just for compatiblity
>> with more monitors, because, according my Applecolor RGB monitor manual, it
>> syncs off the separate composite sync signal, negative going, 3volts min.
>> peak-to-peak into a 2200 ohm termination, with the video lines of course
>> going into 75 ohm termination.
> 
> Is that information from the "Appendix: AppleColor RGB Monitor specifications"
> section on page 12? Because, if so, the previous sentence says that it accepts
> "Red, green, blue video signals composited with negative going sync 1.0 +- 0.5
> volts peak to peak, internally terminated with 75 ohm resistors."
> 
> For the record, I did some measurements, and sync is indeed combined with all
> three color components. There is no separate blank signal. The digital RGB
> outputs from the VGC all go to 0 during blanking instead.

This is almost always the case. The separately *generated* blanking signal
is simply used to gate the video. It is very seldom provided as a separate
output. 

>> In this case, you would not need to worry about exactly duplicating the
>> IIgs's output structure... just provide standard RGB+S.
> 
> I hope that this works, since the chip I plan on using for the RGB interface
> does not support sync on all channels.

Fortunately, it is trivial to remove the sync, since the video is
positive-going and the sync signal is negative with respect to ground. A
series diode biased to conduct at ground potential will pass the video but
not the sync. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon