[Coco] For those of you following the RGB2VGA FPGA

[Christopher R. Hawks]

On Mon, 29 Sep 2014 23:30:59 -0400
Gene Heskett <gheskett at wdtv.com> wrote:

> 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

    Nope, mine was a regular RGB to S-Video converter. You can get a
commercial model for less money from:
http://www.jammaboards.com/store/rgb-to-ntsc-pal-tv-converter-pcb-cv-04.html


Christopher R. Hawks
HAWKSoft
-- 
DON'T LET YOURSELF BECOME PROFICIENT IN VI. You will regret it in the 
morning. There is some knowledge that is not meant for the human mind 
to know.					      -- David Packer

(Rats! too late...)