Re: Help needed: Emulation of AppleII Graphic modes

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

Holger Picker asked:

>I wonder if someone could help me, please, and tell me a bit more on how the
>AppleII graphic circuits generate the colour pixels.
>
>While writing the graphic emulation code for my little emulator, I came
>across a rather strange problem I cannot solve with the limited knowledge of
>mine. I tried to write code to generate a "blurry" output like ApplePC does
>and succeeded in that with the exception of the left and right border.
>ApplePC treats double hires or lores like a row of 560 pixels whose colour
>is determined by 4 consecuting bits.
<snip>

Perhaps it will help to note that the Apple II does not generate "colors", but
simply a video bitstream with a bitrate at a 4x multiple of the color burst
frequency.  This means that the color that appears on the screen of an
NTSC color monitor is a function of those bits as a continuous stream
across the line.  When the bits stop, the "luminance" of the signal goes
to zero, and the "chrominance" becomes irrelevant.  So, at the edge of
the screen, in a time determined by the luminance bandwidth (typically
a little over 3MHz) the signal decays to black.

How one simulates this "artifact" NTSC color on a PC display is a matter
of some art.  ;-)  Generally, you would be safe to assume that the bits
"shifted in" before and after the ends of the line are 0 (black).

-michael

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