Re: RGB to VGA on a IIGS (again)

Intersil makes a bunch of different chips that are great for this.
I've been working on a converter for old computers (and other devices)
that takes the various res's and syncs it to a n RGB type monitor,
with an option to adjust the sync for multisync monitors.

Then intersil came out with a chip that will do the same thing, but
convert it to Componet form, which I realised is probably better for
todays TV's.

I recently picked up an LCD screen for a Xbox 360 (I don't have one)
because it has componet vid in. Granted it's only a 9.2" monitor, but
it gives me a componet viewing source to experiment on.

I have a GS, but no monitor for it. I did pick up some older mac
monitors, but i'm not sure they will work. But very few of my old
computers actually have the monitors for them.

anyways, i'm hoping to get back to my electronics work soon, so if you
let me know what the monitor is you want to hook your GS up to, i'll
see about making a device for you to test for me (and then keep a
working version for the testing, of course).

Plus any input onto what sort of monitors people are trying to connect
their older computers too would be nice. If everyone is going
componet then that makes things easier.







On Jun 26, 12:46 am, "Michael J. Mahon" <mjma...@xxxxxxx> wrote:
mdj wrote:
On Jun 25, 8:05 pm, "Michael J. Mahon" <mjma...@xxxxxxx> wrote:

I love my AppleColor Composites too, but I'm finding my eyes are
starting to struggle with long exposure to anything less that pristine
text. Whether this is a function of getting old, or a side effect of
now spending so much time in front of higher quality displays I'm not
sure :-)

I used a monochrome monitor for programming for years, with a 10" TV
for color, but a few years back, I set up an AppleColor Composite and
it was good enough that I never bothered to set up a monochrome monitor
in addition.

It helps that I keep my monitor behind my "open faced" Apple //e, so
when I'm in position to type, my outstretched arm can barely touch
the monitor screen more than two feet away. At that working distance,
the dot structure of the shadow mask is not distracting.

My physical layout is similar, but as I'm naturally far-sighted I can
see the effect. It's a little bit like when audiophiles talk of
'listening fatigue' ;-)

Ah--there's the difference. For me, that distance falls into the
"too far, and yet too close" zone of my bifocals, so I see it with
a little pleasant blurring. ;-)



80 column text tends to look a little odd, with alternate scanlines
interpreting the pixel widths slightly differently. Readable, but not
particularly clear.

This is a result an interaction between the monitor's sampling of
what it thinks is an interlaced video signal, but isn't.

I figured it'd be something like that, or alternatively it's that the
upsampling algorithm is very simplistic and doesn't cope at all well
with the Apple II's slightly odd clock rate.

Actually, some multiple of the color subcarrier frequency is a quite
rational choice for sampling frequency, but that doesn't match the usual
640- or 720-pixel value.

The active time of the Apple II video line is 40/1.0205 microseconds,
while the active time of a standard NTSC signal is 52.6 microseconds,
for an NTSC-to-Apple II ratio of about 1.34:1.

So the number of NTSC horizontal pixels corresponding to 280 Apple
pixels is about 375, and to 560 Apple pixels, 750. The equivalent
resolution for 640 pixel SHR mode is 858 pixels. So none of the
"usual" VGA-derived resolutions correspond well with Apple resolutions.

The result is a "beat" between the sampling frequency and the Apple
pixel clock. For a 640-pixel NTSC sampling, the sampler "lines up"
with an Apple pixel about every 10 "280-pixel" pixels, or about
every 20 "560-pixel" pixels. This should result in a quite visible
pattern of vertical stripes with those spatial frequencies.

There is a *very* minor amout of this present on my display.

Your monitor is doing a good job of supressing it.

The fact that your sampling alignment changes from even to odd lines
is the tipoff that it's interlace (or the absence of interlace)
related.

"Possibly" :-) The effect seems bound to Apple scanlines rather than
underlying video scanlines, so I figured it's related to limited luma
processing causing the shift in areas where there is only one lit
pixel, like vertical lines in the characters, a sort form of
simplistic anti-aliasing in the upscaling algorithm which is probably
quite effective for "video" signals, but less so for computer imagery.
I'd have to hook it back up and again and study it more closely to be
sure.

I see what you mean--this is a likely explanation with digital
processing. I was putting too much responsibility on the sampling
and not enough on the post-processing.

I think this problem will plague us indefinitely until either somebody
designs a custom solution, or physical display resolution becomes so
damn high than the effects disappear into the 'noise'

Right. "Smart" upsampling to 1080p should do pretty well. I haven't
tried that yet...

-michael

NadaNet file server for Apple II computers!
Home page: http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it's seriously underused."


.

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