Re: RGB and the Apple II

[mjmahon@aol.com (Michael J. Mahon)]

Joel wrote:

<excellent summary of Apple II/III video pinouts snipped>

>The AppleColor composite monitor gives a pretty good picture. It also has a
>black and white switch which can improve the display quality on text and
>monochrome graphics. The Laser 128 has a mono/color switch which can often
>improve the display for text and graphics when set to mono.

The function of the switch on the Laser 128 is to eliminate the color burst
from the composite signal.  All Apples and compatibles incorporate color
burst supression for text modes, but, of course, the color burst is on for
all graphics modes.

The reason for this is that composite color is an "artifact" of isolated,
precisely timed graphic "dots", and text is made up primarily of just such
dots.  As a result, a text display with color burst on will show lots of
distracting color fringes.

Since many applications and environments present text in a graphics
mode, having a manual switch to disable the color burst is quite handy.

The AppleColor Composite monitor is a very special case.

Virtually all composite monitors have a luminance (intensity) bandwidth
which is limited to about 3MHz, because the chrominance information
is carried on a suppressed-carrier 3.58MHz subcarrier signal.  A better
(and more expensive) way to filter the subcarrier from the luminance
signal is to use a comb filter, but these are very unusual in older
composite color monitors.

Because of this luminance bandwidth limitation, most composite
color monitors display "fuzzy" text and graphics, and are incapable
of sharp 80-column text display.

The AppleColor Composite monitor has a special luminance filter
circuit which limits the luminance bandwidth to 3MHz _only_ when a
color burst is present.  When the color burst is absent, or when the
monochrome button on the front panel is pressed, the luminance
bandwidth is increased to 6 or 7 MHz (IIRC--I can't find my manual).
This allows 80-column text to be cleanly, if not crisply, reproduced.

-michael

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