[Coco] For those of you following the RGB2VGA FPGA

Gene Heskett gheskett at wdtv.com
Tue Sep 30 09:08:53 EDT 2014


On Tuesday 30 September 2014 07:07:50 Christopher R. Hawks did opine
And Gene did reply:
> 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

I have one of their vga upscalers, had to cobble inverted & mixed sync to 
get it to work.  Its not as sharp as Roy's later version.  However, this 
would be one way to make use of the S-VHS input on a modern digital tv.  
How well "registered" the color would be is probably a test that needs to 
be done.  It could be slide sideways a fraction of a microsecond. 
> 
> Christopher R. Hawks
> HAWKSoft


Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
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-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>
US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS


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