[Coco] RGB --> VGA for CoCo: A success story

[Gene Heskett]

On Saturday 31 January 2009, Steven Hirsch wrote:
>On Sat, 15 Nov 2008, J.P. Samson wrote:
>>>> On Nov 15, 2008, at 11:24 AM, Steven Hirsch wrote:
>>>>> This morning I finally got motivated and built an adapter to run RGB
>>>>> video from my CoCo 3 into the Wei-Ya ACV-011 RGB-->VGA converter.  I'm
>>>>> pleased to report that it works quite well!
>>>>
>>>> Any plans to do a write-up on how you put this together, Steven?  How's
>>>> the quality?  Have you ever used Roy's converter to be able to compare
>>>> results?
>>>
>>> There's not a whole lot to write up...  It's just a matter of building a
>>> cable with a 10-pin ribbon cable header on one end.  If you are not
>>> sharing the converter with other classic machines, you just splice the
>>> other end to the header supplied with the converter board.  The signals
>>> are just 1:1, connect Gnd, R, G, B, H-sync and V-sync and you're good to
>>> go.  The pinout for the bottom of the CoCo 3 is widely available.
>>
>> Nice and easy, that's for sure.  I'd guess that Roy's converter does a
>> better job, since it is specifically designed for the CoCo 3 and includes
>> a filter to absorb some of the vertical banding that is an artifact of the
>> computer's RGB output.  Roy's adapter has difficulty clearly resolving
>> some color combinations, though, making such text hard to read.
>
>A quick followup on the subject:  I built an RLC low-pass filter based on
>Roy's design (thanks for the schematic, Roy!) and gave it a try.  Results
>in my situation were not very good.  It did not eliminate visible banding
>(which was very slight to begin with), but did throw away enough in-band
>video signal to blur the text display noticably.
>
>Disclaimer:
>
>This is not to be taken as a comment on the performance Roy's converter
>box (which I have never used)!  The input impedance of the Wei-Ya RGB
>board may vary from his design and that can have a sizable effect on the
>filter characteristics.  I know that many folks are happy with Roy's
>converter and I'll take it on faith that it does a fine job.
>
>I just picked up another SCART cable, so the next thing to try will be
>running the CoCo into my Highway 100 converter.
>
>Steve

I did some checks on that myself, Steve because the blue in particular is not 
a usable text screen color here, never has been.  I have traced the rolled 
off response all the way back through the gime's own rgb output pins, and 
nothing is lost in the cc3's rgb buffering, nor in Roy's converter.  The 
conclusion I came to was that my gime was just plain slow.

Blue text on a black background never reaches more than 5% of white, and the 
reversed situation of blue text coming down from white is also only about 5% 
amplitude due to very very slow rise and fall times compared to the 'dot 
clock' itself, with the overall amplitude being nstc gain trimmed by the time 
the gime outputs it.  That alone restricts the voltage swing of the blue 
signal to under 100 millivolts right there.  Roy's converter doesn't throw a 
thing away that the gime doesn't do to a far worse degree.  If it were 
possible to bypass the gime's 2 bit output d/a's easily, I'd do it in a heart 
beat.  But I'm not sure how one would go about capturing the memory read 
intended to feed the d/a's and do that.  All in the latch decoding and timing 
I expect, and an 8 bit data latch with 4, 2 resistor groups of 2+2r could 
handle the rest, at a far more usable bandwidth delivered to the monitor.

Roy, might that be the next project?  2, maybe 3 all cmos chips I'd think.  
Cutting it into the coco3's circuitry, once the thing is ready as a kit, 
would be a bit scary for the soldering iron challenged, but again, not 
impossible.  I'll be a guinea pig any day as I have a spare cc3 and I know 
which end of the soldering iron gets hot. Hint, hint.

-- 
Cheers, Gene
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