Re: IIgs video modes...

["bieling@terra.es" <bieling@terra.es>]

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 ?

Jorge.