Re: RGB to VGA on a IIGS (again)

[mdj <mdj.mdj@gmail.com>]

On Jun 26, 5:46 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
> mdj wrote:
> > On Jun 25, 8:05 pm, "Michael J. Mahon" <mjma...@aol.com> 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...

My HDTV is a 32" model, 1366x768 resolution - I didn't see much point
going for 1080p unless the screen size was above 40", and that'd be
too large for my lounge.

It does a glorious job of deinterlacing and upscaling 480i or 576i
component source material from the DVD player. The composite
processing is somewhat less amazing, but handles 40 column Apple II
admirably, and the 80 column text is as I mentioned above, but not
quite as bad as the cheapie "portable" TV I was testing on.

Unfortunately it's a little too large to use as a monitor, unless I
build myself a wireless keyboard adapter for the IIe and integrate it
into the home entertainment rack ;-)

Of course, the resolution of this device is actually worse than a
smaller display (I prefer the dpi==resolution interpretation of
'resolution'), but I'm sure at some stage economies of scale will
bring us 10"-12" 1080p+ monitors :-)

Matt