Re: DHGR Video Riddle Solved!!!

[david@uow.edu.au (David Wilson)]

Matt Jenkins <mdj@bilby.cs.rmit.edu.au> writes:

>In comp.sys.apple2 David Wilson <david@uow.edu.au> wrote:

>> But only if you have an RGB card. If you use composite video on a //e bit 7
>> is ignored in DHR mode.

>Sure? On my Composite color equipped //e programs like Publish It, or
>Multiscribe/Beaglewrite still manage to remaine black and white when the
>monitor is in 'color' mode, granted things are hard to read and fringy, but
>on-off raster patterns don't suddenly change into colors.

Yes, I have the //e Tech Reference Manual and Jim Sather's "Understanding the
Apple IIe". In HiRes mode bit 7 determines the color set by causing a 1/2
pixel shift in the output bitstream. In Double HiRes you are putting pixels
out at 14MHz already so no shifting can take place. You only have 560 pixels
to display and that is 7x80.

With composite video you effectively always have a line of 560 pixels of
black or white. The color burst tells the monitor to interpret this as color.

With RGB cards the otherwise unused bit 7 in DHR can be used to modify
how the color is generated by the RGB card.
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia