Re: RGB to VGA on a IIGS (again)

["Michael J. Mahon" <mjmahon@aol.com>]

mdj wrote:
> On Jun 24, 9:48 am, "Michael J. Mahon" <mjma...@aol.com> wrote:
>
> > mdj wrote:
> >
> > > On Jun 24, 12:18 am, "Bryan Parkoff" <nos...@nospam.com> wrote:
> >
> > > > "mdj" <mdj....@gmail.com> wrote in message
> >
> > > > 1182580110.751279.80970@g37g2000prf.googlegroups.com">news:1182580110.751279.80970@g37g2000prf.googlegroups.com...
> >
> > > > > On Jun 23, 3:57 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
> >
> > > > > > mcph...@yahoo.com wrote:
> >
> > > > > > > I know this has been asked before, but I was wondering if there were
> > > > > > > any new RGB to VGA upscaling converters out that were particularly
> > > > > > > good for this task. The XRGB-2 is no longer available (and was
> > > > > > > pricey).
> >
> > > > > > > I would like to do RGB rather than composite to VGA with a GS and a
> > > > > > > viewsonic LCD.
> >
> > > > > > Right--if you start with composite, then the result can never be
> > > > > > any better than that, no matter what format you convert to.
> >
> > > > > The 'simple' solution is to use an LCD TV with a SCART input on it,
> > > > > and use Roger's IIgs->RGB cable. The results will depend on the
> > > > > quality of the TV.
> >
> > > > > I tried this a while back with my TV (A 32" model, a bit inconvenient
> > > > > placing atop an Apple II) and the results were quite acceptable. On my
> > > > > Apple II though, I've now decided to regress to a green phosphor
> > > > > monochrome display. For a non-gamer, this gives the best possible
> > > > > results, but I would certainly like to find a small LCD TV that worked
> > > > > well.
> >
> > > >   I would be surprised to see Video (NTSC) to VGA Converter athttp://www.ramelectronics.net/html/video-vga-gvm-2000.html.  I am sure 80
> > > > column is unreadable on VGA screen unless digital filter is used isntead of
> > > > analog.  Do you agree?
> >
> > > I'd imagine pretty much all these converters use some form of digital
> > > storage.
> >
> > The issue is how the composite video is decoded, and what the bandwidth
> > of the resulting luminance signal is.  For NTSC decoding, it is almost
> > impossible to preserve a luminance bandwidth of more than 3.5MHz, which
> > isn't enough for good 80-column text.  Your monochrome monitor no doubt
> > has (luminance, of course!) bandwidth exceeding 10MHz.
>
>
> It's a lovely old Sanyo unit that looks wonderfully retro beside the
> pinball machines downstairs :-) Bandwidth is 14Mhz, so yes it produces
> a very clean picture.

I know the Sanyo monitor--it's a fine unit.

The symptom of marginal bandwidth in a text display is that the
vertical parts of characters are dimmer than the horizontal parts.

> > The AppleColor Composite monitor, when in monochrome mode, switches its
> > luminance bandwidth to about 8MHz--a capability almost unique in
> > composite video processing, even in the digital age.
>
>
> 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.

> > > 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.

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.

-michael

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