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Re: How does the apple2 RGB card work?
Jorge Chamorro Bieling wrote:
Michael J. Mahon <mjmahon@aol.com> wrote:
Basically, yes. All Apple II color is "artifact" color resulting from
video signal pulses which contain 3.58MHz "color" information, in
various amplitudes and phases, causing an NTSC monitor to display color.
various amplitudes ?
Yes.
Since the video pulses for lores graphics and DHR are sent at 14MHz,
each cycle of 3.58MHz covers 4 pulses. Any "asymetrical" pattern
will contain a 3.58MHz component (in a Fourier sense).
So the patterns 1100, 0110, 0011, or 1001 are maximum amplitude
(saturation) 3.58MHz signals, while the patterns 1000, 0100, 0010,
0001, 1110, 0111, 1011, and 1101 are half-amplitude chroma signals.
The patterns 1010, 0101, 1111, and 0000 do not contain any 3.58MHz
component, and so are colorless.
As you can see, the phase of the 3.58MHz chroma signal is a function
of the patterns, so they produce different hues, though some hues are
the same. For example, 0100 has the same hue and saturation as 1110,
but a different luminance (1 and 3 respectively).
The luminance of the pixel group as a whole is equal to the number
of "1"s: 0,1,2,3,4. Of course, a high-resolution rendering of the
lumnance channel will cause light to be produced only at the "1"
positions in the group, though the overall hue and saturation of
the group is set as a 4-pixel group. This causes saturated colors
to be obviously composed of discrete vertical lines.
The "sliding window" algorithm treats each 4-bit group in this
way to determine hue and saturation, sliding by just one bit per
DHR pixel (560 across). This is a DSP technique to approximate
the response of analog chroma processing--though it is only an
approximation.
The algorithms used by the common Apple II RGB cards and by the
IIgs RGB conversion are not fully understood yet, but Bryan is
working on it.
-michael
Parallel computing for 8-bit Apple II's!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
tool--and it is seriously underused."