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Re: IIgs RGB to VGA experiment

js@cimmeri.com wrote:




Strangely, IIgs to the same monitors' composite ins really performed quite poorly, with or without color.




Actually, that would be expected.  It would be quite unusual to detect the absence of the color burst and then expand the processing bandwidth of the video signal (as the


Applecolor //e monitor does).  The Y component input, however, is known
not to contain a chroma subcarrier, and so can be uniformly processed
with high bandwidth.

**** I understand now.  Does make sense.




  I do note that regardless of the size of the TV, the output still looked great.. even at 50".   Others have expressed the concern that a SMALL screen is required.   Apparently not true -- these TVs are *sturdying up* the lines and making them look good at large sizes.




The lines *really are* that wide.  The effect is just one of "magnification".  If you don't mind the relatively low pixel count, then it will look fine--crisp but coarse.



**** I have to disagree with something here, though not sure what.
The lines are in fact much fatter than the actual screen resolution.  I
know this because -- I can create finer lines from other sources, and
also can see the pixels making up the lines.

For instance, the LCD monitor has a native res of 1280x1024.  I run a
PC with it at this res normally.    With the IIgs composite->Y
connector, the gs image fills the whole screen... even though the gs
res is much lower than the native.  So the screen appears to be
fattening up all the IIgs pixels by making pseudo fat pixels from
several native ones.  Same thing occurred on the Toshiba HDTV
widescreen.


Yes, I understand.  The lines are produced by video pulses that are
multiples of 70 ns.  HGR pulses, for example, are 140ns wide, which
will look pretty chunky on a hi-res display.

I was just saying that it's *not* the monitor, it's the Apple video.


What is the minimum resolution needed to correctly display all IIgs modes, up to SuperHires 640x200?




The real answer will have more to do with the interpolation/scaling algorithm of the TV than the native resolution, since any digital display, even a CRT one, will pixellate the display.   Since it is *very* unlikely that the TV will attempt to lock to the Apple dot clock, results may be a little unstable unless the TV pixels provide more than 2:1 oversampling of the Apple video--which would be a *very* high end display.  ;-)



**** I'm confused (not hard to do).   As I imagine this, say I run the
IIgs at 320x200.   If I have an LCD that has a res of 1280x800, it
already has a pixel density 4 times greater than that IIgs res.   Is
that not equivalent to 4x oversampling?   The LCD wouldn't need to sync
to the II dot clock.. since one or more of those 4 pixels is going to
hit any one 320x200 pixel.   Until I run more extensive tests, I'm not
sure how this LCD would handle that particular case.. but it easily
displays the 640x200 res, for instance.. with no beat shadowing.


Then you are golden.  All Apple display modes except SHR are composed
of pulses of a 14MHz clock, and so are multiples of 70ns.  If you
translate 70ns to displayable monitor pixels in 39+ microseconds,
it works out to 750 pixel resolution across the whole horizontal
line.  In the 640x? case, it works out to 858 pixels across the
whole line.

So if your monitor can handle, say, 2x those numbers, then you
will have 2x oversampling of the smallest details for the respective
graphic formats.

If you have less than 2x, then you may be able to see some "squirming"
of the thinnest vertical lines due to unsynchronized sampling, but
much depends on the characteristics of your particular monitor.


But still, I'm wondering about the IIgs RGB monitor, or even the
Commodore 1084.   If the IIgs 640x200 mode is displaying in the middle
of the screen with large borders, and IF you could, in theory, get each
of the 640x200 pixels visible individually, that would indeed mean that
the actual # of pixels for those monitors was somewhat greater than
640x200, correct?


Right--and that was the computation I did above.

The active area of the Apple screen is 40 cycles, or about 39.2
microseconds.  The active area of an NTSC line is about 52.5
microseconds, for a ratio of about 1.34:1.  That is, the full
horizontal width of a line can hold 1.34 times the number of
Apple pixels in the center section of a line.


Yet, I think what you are saying is that the approximate res of those
screens is 640x200... but that the IIgs is scrunching it's own version
of 640x200 into a smaller portion of those monitor's space.... using
less than 640x200.. such that one SCREEN PIXEL is actually a
combination of several IIgs pixels.   Do I have that right?   Should be
easy to find this out.


640 Apple pixels wide corresponds to 858 TV pixels wide.
560 Apple pixels wide corresponds to 750 TV pixels wide.

-michael

Parallel computing for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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