Re: Blech! How can you stand 640x200??

[dempson@actrix.gen.nz (David Empson)]

Mr. Boffo <mister_boffo@hotmail.com> wrote:

> So anyway I've been trying out stuff like Hyperstudio for the IIgs,
> and the one thing which keeps coming to mind is: "Boy, this screen is
> just godawful!"
> 
> What exactly was the point of limiting the IIgs to a maximum of
> 640x200?

The screen resolution of every Apple II is designed to be compatible
with standard television, but without interlace (which would cause
flicker).

This sets the horizontal scan frequency (about 15.75 kHz), refresh rate
(50 or 60 Hz depending on locality) and hence the number of lines of
vertical resolution (192 or 200, depending on video mode).

To get a higher vertical resolution requires either

(a) Interlacing the image at half the scan rate.  This would produce
excessive flicker on a computer monitor.

(b) Use a higher frequency monitor.  This would eliminate compatibility
with standard television.

The second part of the equation is the address space required for the
screen image.  The IIgs Super Hi-res graphics mode uses 32KB, and due to
hardware implementation details, has to reside within a particular
memory bank, which only has about 40KB of address space potentially
available for the video buffer.

The same video chip can support interlaced video (640x400) by using two
32KB buffers in adjacent banks (which means that the video memory is no
longer linear), but this is only supported on Apple's Video Overlay
Card, not in the IIgs implementation.

Apple could have chosen to support higher resolutions by doing a
significantly different hardware implementation, or by reducing the
number of colours available for each pixel.  The system they ended up
using was a good compromise for colour range without introducing
significant compatiblity issues.

> This reminds me of something that's always bothered me.. how is it
> that the image appears to really only have 200 lines, when NTSC is
> really 525 lines?

NTSC is 525 lines, but it interlaced with two fields, each of which
displays 262.5 lines.  There are 59.94 fields per second, or 29.97
complete frames per second.

The IIgs generates non-interlaced video with frames approximately at the
field rate of NTSC, i.e. about 60 frames per second.  The vertical
resolution available is that of the NTSC field (262.5 lines), not the
NTSC frame (525 lines).

The original Apple II video modes use 192 lines, though I'm not sure why
this particular value was chosen, except as a side effect of the memory
map chosen for the video modes.

For the IIgs Super Hi-res mode, the revised memory map allowed enough
room for a further eight lines of video.

> It would seem that those 200 image scanlines are double-thick, yet
> when you look at the graphics on an NTSC monitor you can't tell that a
> pixel is two scanlines tall. Even with the old hires 280x192 grahpics,
> a pixel appears to be made up of a single scanline, with a definite
> separation from the next pixel above and below but no visible split in
> the pixel itself. Is this really a "normal" NTSC resolution at all?

If you had a "normal" NTSC signal, there would be another 200 scan lines
interleaved with the 200 existing lines, but half a frame later in time.