Re: Double hires mode color artifacts

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

Dirk Thierbach wrote:
> Michael J. Mahon <mjmahon@aol.com> wrote:
>
> > Linards Ticmanis wrote:
>
>
> > > As regards the actual Apple hardware, double hires simply drives out a
> > > high voltage for every pixel that's set and a low voltage for every
> > > pixel that's not set, using seven pixels from each byte, at a speed of
> > > (theoretically) 14,318,181.8181... pixels per second.
> > > That's all there is to it. The End.
>
>
> I was beginning to suspect this, but wasn't sure. Thanks, that was the
> information I needed.
>
>
> > > I hope I have amused you, if not I hope I have confused you. ;-)
>
>
> Neither, you were quite helpful :-)
>
>
> > This analysis is what would happen in an NTSC monitor if the two chroma
> > channels had infinite bandwidth.  In fact, they have quite limited
> > bandwidths, on the order of 1MHz, and the two channels have different
> > bandwidths!
>
>
> > In addition, the bandwidth of the luminance channel is also limited, but
> > to a larger value, on the order of 3MHz.
>
>
> Is the limit implemented in the monitor?

Yes.

> I tried to do some calculations
> before going to bed, but must have confused something -- the standard DVD
> format contains 720 pixels, so the TVs should be able to display this 
> many different luminance samples. The output signal in the active area
> is 52us long, makes 0.072us per pixel. If pixels alternate between
> black and white, that's a bandwidth of about 7MHz for luminance. Or is
> it just impossible to display alternating pixels at that resolution? I
> must be making some mistake here...

We are discussing analog color monitors of the 1980s--and they were
generally built without comb filters, so their luminance bandwidth
was generally limited by the color subcarrier at 3.58MHz, and the
filters were not particularly sharp.  All US broadcast analog video,
regardless of processing, is limited by the sound subcarrier at
4.5MHz.

Bottom line, only digital video processing of *non-broadcast*, non-
composite video can deliver luminance bandwidth in excess of 4.25MHz.
(This would be S-video or component video.)  For analog composite
monitors, the 3MHz limit stands.

> > The result is that a larger window (more FIR samples) must be considered
> > to adequately simulate the analog processing--in fact, closer to 14
> > (which would correspond to about 1 microsecond of time).
>
> [...]
>
> > Since the effect of preceding and following dots has a rapidly
> > diminishing effect, 
>
>
> And that gets to the core of the problem: How strongly do the extra
> samples influence the hue? Because...

Enough to make the RGB-ized displays look decidedly different from
the composite displays that most games and graphics were designed for.

> > I'm not aware of anyone taking the time to do this, but it would be
> > appropriate for a machine-independent NTSC monitor emulation.  At
> > one time, the MAME project seemed to be heading in this direction.
>
>
> .... I'm just in the process if re-implementing double hires for the
> "apple-emul" or "apple2" emulator on Linux (after being fed up with the
> fact that it only run under svga, or under X with 8bpp). I've implemented
> hires in a reasonably fast way by using tables which guarantee 4 bit
> samples for every pixel, and at the moment I am wondering if this is
> enough for double hires, or if I have to make the window a bit larger
> to be able to display the color artifacts with reasanble accuracy.

It isn't just double hi-res, it's lo-res and even normal hi-res.  All
4-bit renditions of these displays are unfaithful to the rendition of
an analog NTSC monitor.  All Apple II video can be processed in exactly
the same way, regardless of "resolution".  (Actually, IIgs 320 and 640
modes are an exception, but we weren't talking SHR.)

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

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