Re: Apple IIGS monitor replacement

["Michael J. Mahon" <mjmahon@aol.com>]

Mitchell Spector wrote:
>  heuser.marcus@freenet.de wrote:
>
>
> > On Apr 29, 2:00 am, Michael Black <et...@ncf.ca> wrote:
> >
> >
> > > ANd of course, the problem isn't that the IIGS didn't output VGA, it's
> > > that the design is over twenty years old now, so what was easy to
> > > get back then may no longer be the case.  But history doesn't intend
> > > to be historical, so it didn't matter at the time what was used in
> > > the IIGS.
>
>
>     Interestingly enough, the Macintosh II was introduced 6 months 
> after the Apple IIgs and supported a VGA-type mode with it's NuBus 
> card (640x480 with 256 colors out of 16.7 million). The IIgs doesn't 
> pre-date VGA graphics by _that_ much.
>
>
> > I remember reading that Apple didn't want to use 400-line
> > monitors for the GS for several reasons (upgrade path for
> > Apple II users, higher resolutions being reserved for the Mac).
>
>
>     What I recall was the cost of non-interlaced monitors being very
> costly back then, so Apple reasoned it made no sense supporting
> more than 640x200 in the GS's hardware. Ironically, the AppleColor 
> RGB was *still* very expensive, and, does support 640x400 interlace. 
> As a matter of fact if you have a Video Overlay Card (which duplicates 
> the same VGC video chipset from the IIgs) you can display 640x400 
> on the AppleColor RGB as-is. Made me wonder why Apple couldn't 
> at least update the GS to display interlace video if its companion
> display supported it. If the VOC did it, how difficult would it be for
> the ROM 3?

There are two problems with this approach.

First, interlaced displays produce very bad small-detail flicker,
since adjacent lines are refreshed on alternate fields.  This makes
it a non-starter for text displays unless a long-persistence phosphor
is used--and that makes for terrible motion smear.

Second, doubling the vertical resolution requires doubling the size
of all video frame buffers--something that is completely out of the
question for Apple II compatibility.

SHR is a kind of nod in that direction, and completely different from
the Apple II video design, but they chose to use the doubled frame
buffer size to increase color depth, not resolution--a very reasonable
choice given the tradeoffs.

To both increase depth *and* double vertical resolution would have
required quadrupling the frame buffer size--essentially dedicating
a "bank" of IIgs memory space to each "pair" of buffers.  (Of course,
this would be completely untenable in Apple II-compatibility mode.)

>     It would've been doable to produce an updated VGC that could
> support 640x400 with 256 colors by 1987-88 (there were rumours
> it had been done even) but if you updated the GS Finder to make
> use of that mode, you'd have an exact duplicate of the Macintosh
> Finder. Probably why Apple never updated the GS's video...or it's
> speed for that matter.

Given the atmosphere at Apple in the late 1980s, I find it very
believable that there was a real desire to keep the IIgs out of
the Mac's competitive neighborhood.

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

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