[Coco] RGB video on CoCo3
snhirsch at gmail.com
Sat Feb 14 17:26:36 EST 2009
On Sat, 14 Feb 2009, Phill Harvey-Smith wrote:
> Steven Hirsch wrote:
>> On Wed, 11 Feb 2009, Phill Harvey-Smith wrote:
>> The main problem you're facing is that RGB SCART requires composite sync,
>> while NTSC CoCo 3 systems (and maybe PAL, but I've never seen one) have
>> separate H & V sync. I ran into this trying to interface my CoCo to a scan
>> converter that uses SCART. If you're handy, there's a simple circuit to
>> recombine H & V to composite. I plan to knock this together Real Soon Now
>> and play with it further.
> Ok some progress, I made up some leads to go from CoCo to veroboard and
> veroboard to SCART so I can do some patching. So far I have :-
> RGB on CoCo direct connected to RGB on SCART (plus grounds connected as
> H & V Sync on the CoCo XOR'd together with an LS86, and the the output
> inverted with an LS04 and passed to CSYNC on the SCART.
That's probabably close enough for a contemporary monitor. The circuits
I've seen go a bit further to condition pulse widths.
> This produces a stable picture that is pretty crisp *BUT* in shades of almost
> white, Ii.e. the colour is all wrong :( I tried putting some resistors in
> line incase the voltage out of the CoCo was the wrong level, seemed higher
> than 0.7V when measured with my scope, looked more in the 2V range.
A simple series resistor is not the way to go, since it plays havoc with
impedance matching. The source Z looking into the monitor should be as
close to 75-ohms as practical. A proper pad preserves both the load and
source impedance, but a a simple divider will probably give reasonable
results and tell you if you're on the right track.
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