[Coco] High Color Display Mod for MM/1?

[Joel Ewy]

Ok, you've all gotten me thinking about the MM/1 again.  I've long
wanted to experiment with restuffing all, or more likely only part of
the palette registers on the MM/1 to get more colors per line, like
what's already been done for high color displays on the CoCo.  So I
needed to know the address of the RAMDAC.  I was looking at the
schematics Bob uploaded instead of falling asleep, and then got curious
and went searching for a datasheet for the Brooktree BT478.  Before I
found it, I came across a datasheet for the Brooktree BT481, which
appears to be an updated, pin-compatible replacement for a number of
RAMDAC parts, including the BT478.  What intrigues me about this chip is
that it has high-color and true-color modes where it latches serial
bytes to feed the video DACs directly, bypassing the palette.  There is
one 15-bit (5-5-5) mode that looks particularly promising. 

Now the problem with this is that, at the very least, the horizontal
resolution would be halved.  So the interlaced overscan 384x480 8-bit
mode would become a rather odd 192x480 15-bit color mode.  Would it be
worth doing?  I don't know.  Probably not, but it's certainly
interesting to think about, and I'm definitely going to keep my eyes
open when I next look through my boxes of old video cards to see if
there's a BT481 out there that can be liberated from a socket or gently
heat-gunned off its board.

Any thoughts?

JCE