[Coco] screencast presentation re: NTSC 8-bit Color on the Color Computer 3

[Gene Heskett]

On Tuesday 29 April 2014 23:14:20 Joel Ewy did opine:

> On 04/29/2014 06:23 PM, John W. Linville wrote:
> > I made a screencast version of the presentation I gave at the 23rd
> > Annual "Last" Chicago CoCoFEST! about the CoCo3's NTSC-only 8-bit
> > color mode.  If you didn't get to see it (or want to see it again),
> > then now is your chance -- enjoy!
> > 
> > 	https://www.youtube.com/watch?v=gc6Eu7IansE
> > 
> > John
> 
> Thanks for posting this, John.  This was helpful in understanding
> artifact colors a little better.  In addition to emulator support (which
> would really encourage the wider use of 8-bit artifacts), being able to
> calculate artifact colors programmatically would be useful for, say, GIF
> display programs on the CoCo 3, though a look-up table would probably be
> faster.  But if 16 palette registers and the 'A' bits yield different
> palettes than just the 'S' bits alone, then you could select the most
> representative palette for a given image algorithmically, assuming all
> that can eventually be worked out.
> 
> One thing I'm still not clear on is this business of 160 color
> transitions per line.  If that is inherent to the NTSC signal, then the
> effective horizontal resolution of all NTSC video is only the analog
> equivalent of 160 "pixels" (plus overscan?).  Somehow my video cassettes
> look better than that, even as low-def as they are. Is it just that the
> Luma can vary quicker and is more visually important?  Or am I
> misunderstanding something.

Your understanding is fairly accurate.  The luminance signal has the most 
bandwidth as it can use the whole nominally 4.2 mhz of the 6 mhz channel 
(in ntsc)

The chroma signal was split into two parts, with the I being allowed 1.2 
mhz, and the q being allowed .6 mhz.  So the chroma is not that sharp.
The I however uses + and - .6mhz, centered on the color subcarrier which is 
at 3.595454545 mhz above the visual carrier.  Theoretically q uses half 
that, but by the middle 80's, most color encoders used the same filtering 
in both the I & Q generator circuits.

These frequencies are all mathematically locked together such that each 
even scan line's color signal is reverse phased from the odd lines color, 
in this way, leakage into the luminance, causing a dot pattern is blended 
and smoothed. Maintaining this mathematical lock is easy with digital 
circuitry, but the older tube type tvs weren't that good and interlacing 
suffered greatly depending on the condition of the power supply filters.

It gets more complex than that, but it also serves to point out that where 
your monitor can use the RGB video output, the pix will be much sharper 
because all 3 signals are separate, and can use as much bandwidth as the 
circuits can actually generate.  In some of the high end graphics in the 
80's & 90's, the effective video bandwidth was often 20+ megahertz.  But in 
analog, you can't stuff 60 megahertz worth of color information into a 6 
mhz channel.

> Regarding the relative importance of luma over chroma, I have considered
> combining 4-color 640 pixel wide grayscale images derived from the Y
> component with 160 pixel color images mapped to one of the artifact
> palettes in order to get more visual detail in these 8-bit artifact
> color images.  Undoubtedly because of the artifacting there would be
> some additional color error, but I think it's worth trying.

Because the coco's color output is not interlaced, that, while you may be 
able to pull it off in the mixing using framestore techniques, the results 
will not be "impressive".  In live tv, motion artifacts when there are 
moving objects in the picture, can be pretty distracting because the 
luminance moves, then the color moves past the luminance and that 
positional jitter will drive your eyeballs berzonkers after a few minutes.
 
> JCE
> 
> 
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Cheers, Gene
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