Re: apple ii lineage?

["Michael J. Mahon" <mjmahon@aol.com>]

Mad Scientist Jr asked:

>Thanks for the reply... I have another question - what colors did the
>IIe support? I visited apple2history.org and was suprised to learn the
>II only could display green, violet, black & white?? Even the Atari
>2600 had better color support than that. And why would Woz choose such
>an off color as violet?!

The Apple II uses a trick of timing to generate colors as
"artifacts" in the NTSC video standard.  By choosing a
clock frequency that is a multiple of the NTSC color
subcarrier frequency, it becomes possible to create
artifact colors on a TV display by generating pulses
with a 3.58MHz component, interpreted as color information.

This clever approach allows the frame buffer to contain only
monochrome data--pixels on or off--while combinations of pixels
can represent color.

But the colors available are quite limited with a 7Mhz video clock
(as in hi-res):  only two colors can be generated unless the phase
of the 7MHz clock can be shifted relative to the subcarrier.  The
two colors are green and purple--180 degrees apart on the hue circle.

That modification--a 90 degree phase shift, or one 14.3MHz clock--
was the change in the later Apple II boards that permitted two more
colors 90 degrees away on the hue circle from the two original
colors, giving orange and blue.

Black results when two or more adjacent pixels are off, and white
results when two or more adjacent pixels are on.

The full range of hues can be generated by "dithering" across lines
and, to some extent, within lines, but, of course, at the cost of
resolution.

When double hi-res was introduced with the //e, the video clock
was increased to 14MHz, allowing 4-pixel groups to represent 16
colors (including black and white).

Of course, lo-res graphics always had these 16 colors, since the
bits were re-used and did not require more memory bandwidth than
the 1MHz bus could support.

Though the artifact color approach is non-traditional, it is
extrememly economical in its use of memory and memory bandwidth,
allowing very colorful, moderately detailed images to reside in
just 8KB.

Note that this was achieved with only simple TTL circuitry,
no custom chips.

And when used with a monochrome display, the full 280x192 or
560x192 resolution was available.

-michael

Music synthesis for 8-bit Apple II's
http://members.aol.com/mjmahon