Re: Apple IIe TV output quality

[dempson@actrix.gen.nz (David Empson)]

Cyril Thibout <info@pulsar-informatique.com> wrote:

> actually only one character out of two has its last pixel column missing.
> This is not due to the monitor since the TV has the very same problem.

That modifies my earlier post slightly.

Numbering columns from 1 to 80, all the odd ones come from auxiliary
memory (on the 80-column card) and all the even ones come from main
memory (on the motherboard).

Despite this, it doesn't indicate a problem reading memory, because the
data in memory is merely an ASCII character, which is translated into a
bit pattern via the video ROM. Each memory location is read 8 times to
get each row of pixel data. Clearly the character data going into the
video ROM is correct, but one column of the pixel data coming out of the
video ROM is getting lost around the shift register, but only in
80-column mode.

The interaction could be between bit 5 of the video ROM (pin 4 of the
LS166 shift register) and some other signal which is in a certain state
when fetching data from auxiliary memory (or main memory, depending on
which columns are affected).

-- 
David Empson
dempson@actrix.gen.nz