[Coco] For those of you following the RGB2VGA FPGA

[Gene Heskett]

On Monday 29 September 2014 23:00:42 rcrislip did opine
And Gene did reply:
> On Mon, 29 Sep 2014 10:40:46 -0400
> Gene Heskett <gheskett at wdtv.com> wrote:
> 
> <snip>
> 
> > Absolutely Nick. The reason for that is that the color subcarrier out
> > of the composite jack if installed, or the rf jack to feed a tv, is
> > not properly interlaced.  The subcarrier frequency is generally ok,
> > but the H line length is an even number of clock cycles, whereas its
> > even + 1/2 in real NTSC. A tv with or w/o a comb filter to separate
> > it again will have atrocious dot crawl because that lack of timing
> > emphasizes it as opposed to trying to cancel it, AND will destroy the
> > luminance sharpness in its attempts. So picking them off and shipping
> > them out as RGB must be done.
> > 
> > The only semi-workable alternative would be to translate to S-VHS,
> > putting the luminance on one pin, and the color subcarrier modulated
> > signal on another pin which never mixes the color into the
> > luminance.  There are places in the coco1-2 where that can be done,
> > and possibly in the coco3 too, under the modulator shielding of
> > course. I have no clue if the signal levels are usable but most S-VHS
> > circuitry had an automatic gain control based on the amplitude of the
> > incoming color synch signal, usually called the burst, sitting on the
> > back porch of the H synch signal.
> > 
> > Trying to cobble a color signal into the NTSC b&w signal was the
> > biggest cobble and con job ever perpetrated on the american public.
> > But was forced on us by the then limited bandwidth of a 6Mhz wide
> > channel. Mathematically it should have worked well, and I have seen
> > it work well, but the Conrac monitors that made it look good were
> > also $6000 a copy.  In the consumer affordable products, the math
> > used would have forced a whole new meaning into the phrase "fuzzy
> > math".
> > 
> > There, I've said it, since NTSC, which paid me well for decades is
> > officially dead except for your old vhs and dvd players.
> > 
> > But, an internal adapter, picking off the signals and making them fit
> > the S-VHS plug, sure seems like a worthwhile project to me.  For the
> > coco3, a combiner to mix the rgb into luminance, possibly with a net
> > positive gain for the blue might work to make the S-VHS luminance.
> > That can be gotten from the RGB output transistors, or possibly from
> > inside the modulator can.  The color then would be a connection to
> > the color modulator chips output taken before it went any place
> > else.  It should not be that hard technically, or expensive but would
> > probably need someone handy with a soldering iron to hook it up. Even
> > if a video speed opamp or 4 are needed, those have been available
> > from the likes of TI for at least 10 years at less than a buck a copy
> > prices.  And they'll run on 5 volts!
> > 
> > Gotta run, need to go skinny dipping since I spent the day yesterday
> > cutting and machining steel, and go take my Toy in to get some
> > recalls taken care of.
> 
> <snip>
> 
> 
> Didn't Chris Hawk have something like that? I pretty sure he did.

ISTR someone had one, and it could have been Chris quite easily.  Chris?

Cheers, Gene Heskett
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