On 26 feb, 19:19, "Michael J. Mahon" <mjma...@aol.com> wrote:
I apparently don't have a IIgs schematic, so I'd like to ask about
video generation on the IIgs. (Wouldn't it be great to have an
"Understanding the Apple IIgs"?)
The traditional (non-IIgs) video modes all fit their video into
a 40-cycle "window", based on a 140-/280-/560-pixel width, and have
a video dot clock of 7MHz or 14MHz.
The IIgs introduces the SHR modes, which have 320- and 640-pixel
widths.
Is the display "window" for the SHR modes still 40 microseconds
wide--implying a different pixel rate--or does the "border" shrink
for the SHR modes to allow room for more pixels at the same rate(s)?
-michael
NadaNet networking for Apple II computers!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
tool--and it's seriously underused."
If the pixels were clocked at the same rate the image would grow by
320/280=640/560=1,1428 in the horizontal direction, that's a ~15%
wider image.
OTOH, if the image had exactly the same width, the pixels would have
to be shifted out at 7*1,1428=8.0 Mhz. or at 14*1.1428=15.999MHz.
These figures is quite suspicious... isn't it. So I'd say that the
image is probably of the same width if it were not because 8Mhz/16MHz
isn't an even multiple of the color carrier, and I guess that this,
from an NTSC point of view, may mess up the things a little bit. Or
not ?