Re: Commodore 1901 monitor for IIgs?

["ralmin" <xbiber@bigpond.net.au>]

> A non-interleaved monitor refreshes the screen display
> on one pass.  An interleaved monitor refreshes in two
> passes.  On the first pass it refreshes the odd lines and
> on the second pass it refreshes the even lines.  This allowed
> the Amiga to increase the color depth of the monitor.
> I believe the Amiga was able to display only 16 colors
> per scan and by using interleaving it could have a second
> set of 16 colors.  It would then be able to increase the
> number of colors it could display on the screen.  I think
> a lot of IIgs programmers utilized this concept in programming
> more colors on the IIgs.

You're combining two different concepts.

Interlaced video is one... I've never heard it called interleaved, although
that is equally good symantically. Interlaced video does as you say,
refreshing odd and even lines on separate passes. Television uses interlaced
video, so that a 60 Hz refresh rate actually only gives 30 complete frames
per second. It's a way to get more vertical resolution without increasing
the horizontal frequency. All television-like display technologies,
including the Apple II through IIgs, Amiga, etc use a horizontal frequency
of around 15.75 kHz. That doesn't mean they all actually interlace the
video; the Apple II doesn't take advantage of the extra vertical resolution,
and displays the same thing on both passes. VGA monitors have double the
horizontal frequency and so do not need interlacing to get their 480 line
vertical resolution.

All of this has nothing to do with colour depth. All analogue monitors have
an infinite colour depth, it makes absolutely no difference whether they are
operating in interlaced mode or not. Depth limitations are a feature of the
video generation hardware in the computer, which generally had a limited
about of video RAM and therefore could not store a complete 24-bit
representation for each pixel. A limited number of colours are chosen,
called a palette, and each pixel can be one of those colours. You might have
a 16-colour palette, in which case you can fit two pixels in each byte of
memory. If you have a 256-colour palette then you can only fit one pixel per
byte. The table which maps each position in the palette with its actual
colour information can be changed by the software at any time.

The programming trick which you talk of is to change this palette allocation
on every scanline. This means that each line is limited to 256 different
colours, but the colours can be different on each line. Taken to the limit,
(assuming a vertical resolution of 400 lines) this allows up to 256 * 400
colours on screen in each frame. Doing all that palette swapping consumes a
fair amount of processor resources, so it is not suitable for general use on
the computer in a graphical user environment or an action game. But it is
useful for displaying or previewing graphics and photos because you can get
very good quality results if the palettes for each line are pre-calculated
optimally.

> The Amiga I don't believe supported CGA PC type monitors
> as someone mentioned.  It had a special type of monitor
> for it which met the requirements mentioned above.

If you're using the composite output, there is no special requirement; all
composite monitors should be able to display interlaced graphics since
television is interlaced. I suppose some that were built especially for a
non-interlaced computer system might not support it. In the case of analogue
RGB output, you need a monitor which supports the television scan rate of
15.75 kHz, most VGA monitors do not support that horizontal sync frequency.

Ralmin.