Apple ][ to VGA monitor adapter board - looking for best color decoding method

I've been working on a board to double each line of the Apple //e's
video output so that it can be viewed on a normal VGA monitor. My
goal was the perfect picture quality. To yield it, I sampled the Apple
video signal with the original Apple clock, stored it in RAM and
displayed each line twice with double speed. This works so far (except
for one timing problem that can be fixed by touching the correct
line... this is going to be fixed) and produces an extremely crisp
picture. Especially the 80-col mode is way better readable because the
lines aren't that far apart as on a monochrome monitor.
So far, so good. Now the problem starts: color. The signal needs to be
converted to RGB somehow.
I tried:

1. Latch the video signal 4 times at different phases to COLOR
REFERENCE. (7M rising, 14M falling with 7M high, 7M falling, 14M
falling with 7M low - note that COLOR REFERENCE becomes 7MHz after the
conversion). These 4 signal are mixed to form the 3 colors. This
method gives exact reproduction of the 15 LORES colors (I designed it
to do so) but blurs the picture in HIRES.

2. decode the color as above, but gate it with the monochrome signal,
so that a green horizontal line actually becomes a dotted line. With
some displays this looks really nice, but it sometimes produces ugly
color fringes on white areas.

The Apple //e's PAL encodes uses a circuit with a quad D-FF, but I
don't know how it is wired.
So I'm looking for the schematic of the //e's PAL encoder or the PAL
color card for the ][.

BTW: I'm missing both the Eurapple jumpers and the 14M/Graphics time
signals on Slot 7 in the Apple //e PAL. I had to jumper them directly
from the main board.

Ferdinand Meyer-Hermann
.

Relevant Pages

  • Re: Apple ][ to VGA monitor adapter board - looking for best color decoding method
    ... video output so that it can be viewed on a normal VGA monitor. ... goal was the perfect picture quality. ... The Apple //e's PAL encodes uses a circuit with a quad D-FF, ... signals on Slot 7 in the Apple //e PAL. ...
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