Re: Commodore 1901 monitor for IIgs?

["william strutts" <wrstrutts1@home.com>]

"ralmin" <xbiber@bigpond.net.au> wrote in message
news:AzhW6.133137$ff.998557@news-server.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.


Interleaved and interlacing is basically the same thing.
It means to overlap.  I recognized that I used the wrong phrase
after I sent it. Sorry.

> 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.

I know that dude.  The IIgs would use the Amiga display or so I am told.

The Amiga 1000 supported the following:

"up to 64 colors among 4096  (EHB mode) (32 with 320 x 200
/ 16 with 640 x 200). The Amiga can display 4096 colors
(HAM mode) but only for static display."

"From 320 x 200 to 640 x 512"

The Amiga 500:

"320 x 256 / 320 x 512 / 640 x 256 / 640 x 512"

"32 (for 320 x X modes), 16 (for 640 x X modes) among 4096
+ 2 Special modes : EHB 64 colors and HAM 4096 colors on
static display."

According to this site:

http://www.computingmuseum.com/site/page_museum.htm

> 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.

It increased color depth by artifacting.  Making two pixels of different
colors
reside next to each other causes them to make a different color than what
was supposed to be there. The Amiga has 4096 possible colors and limited
pallets depending upon the graphics mode but using the artifacting principle
the color depth could be larger.  I believe they used some kind of dual
paging for the graphics memory from what I remember and they could
overlay one on the other to increase the color depth.  I never quite
understood completely how that worked since I never owned an Amiga.
My best friend in college did and he would always tell me about the
great things his Amiga could do.

> 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 has a special processor to handle graphics so it wouldn't
burden the main CPU with the graphics processing.  For the extreme case
you're talking about, they had a couple of static graphics modes
according to that web site I quoted from.

> > 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.

You mentioned that the monitor had a CGA type connector that someone
was trying to say was a CGA monitor adapter.  I was responding to
that claim.  The Amiga monitor was a 1085(??) or something like that
from the computer history sites that I visited.  I did verify that it had
a composite output but very few people I know who had Amigas
used a composite output.  It might have been a selling feature for people
with the Amiga 500 since it was billed as a gaming machine.

Beyond that, your guess is as good as mine.  I never worked with one
long enough to know all of its features or limitations.  I was just
commenting on that the monitor that we are talking about doesn't
sound like the Amiga monitor that most Amiga users bought.  If
you're an Amiga expert or something, I appologize for my lack
of knowledge about its inner workings.  I guess that is another
"guru meditation number" to think about. (to use an Amiga
phrase) :-)

--
--
William R. Strutts - wrstrutts1@home.com - Whatever!

C'est moi!  http://www.facelink.com/wrstrutts

Just hacking away...