[Coco] For those of you following the RGB2VGA FPGA

[Gene Heskett]

On Monday 29 September 2014 04:03:47 Nick Marentes did opine
And Gene did reply:
> The quality of your card is very good. Very sharp and clean. It's
> obviously been tailored to suit the CoCo3 video.
> 
> It's color spectrum is it's biggest problem.
> 
> I think that for most OS-9 users though, it's ability to represent the
> 8 digital colors very well will be enough. Just don't plan on running
> many of the games such as Koronis Rift, Thexder and the Sierra
> adventures... although the Sierra adventures are mainly 8 digital
> colors with 2 levels of brightness so they may be ok (EGA 16 colors).
> 
> I have an Amiga and have a flicker fixer installed. I tried connecting
> an LCD monitor to it and although it works fine, the quality of
> upscaling from a non native resolution to the LCD panel resolution is
> downright ugly.
> 
> In the end, I found a very cheap (free!) LG CRT 15" monitor with high
> quality built in stereo speakers and the image (and sound) quality is
> brilliant! The Amiga works best on VGA LCD if you have a proper video
> card that can deliver native LCD resolution.
> 
> And likewise on the CoCo3, I'll stick to CRT here too. I have a Tandy
> CM-8 (which is a pretty low grade RGB monitor) but connecting it to a
> Amiga 1084 yields better results. I'm happy with my original CM-8
> though.
> 
> A CoCo 1/2 to VGA converter would be good. It only has the 8 colors so
> your VGA adapter would be good.
> I assume that a VGA adaptor for this will require tapping to the
> internals to derive the necessary luma and chroma signals.
> 
> Nick

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.

> On 29/09/2014 4:17 PM, Luis Antoniosi (CoCoDemus) wrote:
> > You can buy one for 80 dollars.
> > 
> > But I'm not aiming any production batch or commercial product. What I
> > wanted to do is to master the technology and not be slave of any
> > other device. Also I want to make a VGA converter for the coco2.
> > 
> > It will be hard to get the full coco3 color spectrum but I would be
> > very happy If I could remove most of the noise. What I know now from
> > where is it coming.
> > 
> > 
> > Luis Felipe Antoniosi


Cheers, Gene Heskett
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