Re: IIgs Analog Video Circuitry

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

Kevin Dady <kevin@hackaday.com> wrote:
> On Sep 30, 7:46 pm, Daniel Kruszyna <d...@krue.net> wrote:
>> Kevin Dady <ke...@hackaday.com> wrote:
>>> I have never even seen a IIGS in person (I know I know) so I am just stabbing
>>> in the dark, do you have any idea what of what timing that monitor is
>>> expecting? I would guess its much faster than what the GS is putting out
>>> (somewhere close to VGA?) and that starts to get pretty tricky (though if you
>>> feel the need to blow a small pile of money there are scan converters
>>> available)
>> 
>> The monitor is expecting exactly what the IIgs is outputting, since it is the
>> standard IIgs monitor :) Timing is ~15kHz lines and 60Hz frames.
>> 
>> It looks like this module shifts the rgb components to the correct voltage
>> range, and adds in the sync. Why would it need the sync if there is already a
>> composite sync signal on the monitor port?
>> 
>> -- Daniel
> You need the sync cause if there is stuff on the lines while the
> monitor should be syncing it wont sync properly, this is usually
> handled by whatever is creating the signal and its just simpler to
> send it though a chip (or just a passive dac)  rather than to lock two
> signals together.
> 
> What I am confused about is what are you hooking up to this screen? I
> get that your doing something to output a RGB signal but from where
> and how?

Sync only needs to be present where a monitor expects it.  It could be a
separate composite sync line or it could be added in (to the "blacker than
black") voltage range of one or more color signals--green is a common
choice. 

With proper sync separation, the video content is irrelevant, since sync
pulses are only present during blanking time. 

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