[Coco] Artifacting on LCD?

[Arthur Flexser]

Very nice to have someone on the list who was involved in the design
of the original CoCo's VDG!

Perhaps, then, you can answer a question I'm a little curious about.
Did the designers realize from the outset the value that the artifact
colors would have for enhancing the graphics that the VDG was capable
of?  Or, was it more the case that it was seen as an unavoidable
annoyance under NTSC that conceivably programmers might find uses for?

I do recall that the original CoCo manual (and the VDG spec sheets?)
made no reference to artifact colors.  PMODE 4 was described as a
2-color mode and there was no support for artifact colors under BASIC.
 Perhaps that was done so the same manual could be used in countries
using the PAL standard?

I guess the value of artifact colors for graphics would have been
fairly obvious if there were well-established micros prior to the CoCo
where programmers made profitable use of them.  Was that the case,
anybody know?

Art



On 4/8/09, LinuxRules <Linux-Rules at austin.rr.com> wrote:
> Robert Gault wrote:
>
> > Allen Huffman wrote:
> >
> > > Are artifact colors caused by the CRT or the composite/RF modulation?
> > >
> > > If I get an LCD TV or monitor with composite inputs, will it show
> artifact colors?
> > >
> > > Sent from my iPhone
> > > Follow me on Twitter - http://twitter.com/allenhuffman
> > >
> > >
> >
> > Artifacts are not cause by the CRT but rather the NTSC format. The rapidly
> changing pixels appear to the composite demodulators as color information as
> the rate of change has the same frequency as the color burst. Difference in
> phase with the color burst gives the color.
> >
> > So if the LCD TV has a composite input which conforms to NTSC standards,
> you ought to see artifact effects.
> >
>  What he said!!! That's it exactly.
>
>  And as best I can remember with failing memory cells, the early 6847 design
> would power up
>  in either of two states with the clock phase different. So the same data
> would produce a different
>  artifact color depending on how it powered up.
>
>  I don't recall if any of those ever went into production. (Several "patch"
> mask sets were made
>  early on, before and during early production.....) It was kind of strange
> to watch, though......
>
>  cheers,
>  john dumas
>  (Motorola circuit designer engr for 6847...retired)
>  JohnDumas at austin.rr.com
>
>
>
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