Re: Let's pay a developer to make a VGA output card

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

Mark McDougall wrote:
> Michael J. Mahon wrote:
>
> > I understand your preference, but I think you would find it much
> > simpler in reality to build an "Apple scan doubler" that grabbed the
> > dot clock from the Apple (AUX slot?), sampled the composite video
> > signal (it's digital, after all), and "played it back" at 2x the scan
> > frequency for a VGA monitor, using an appropriate RGB conversion.
> > This would produce an "emulator style" VGA display, which, I expect,
> > would satisfy many users--though perhaps none perfectly.  ;-)
>
> Ummm, I admit I've not much experience with video processing but I'm not
> sure it's so "simple" to build a scan doubler.
>
> The chrominance information in a composite signal is encoded via a phase
> offset from the colour burst. This is not trivial to recover, and
> certainly can't be done by sampling at the dot clock rate. It is
> possible to generate a limited number of colours using 'digital' logic
> but that requires a clock 4 times (for example) the burst frequency.

I think the sensible approach is to reproduce the artifacts from the
underlying video signals that generate it, effectively reproducing the
Apple II's colour burst generator in RGB. Trying to revert it back when
the hardware technique used to generate it in the first place is a bit
of a waste of effort.

> In any case, if you're sampling at the dot clock rate, then you can't
> reproduce any analogue artifacts such as blurring, enlarging due to
> contrast, etc since you only have a single value sampled at the 'centre'
> of the pixel.

I think you can, the colour burst on the Apple II is an artifact of the
raster signal, essentially, and easily recreated.

> At least that's how I understand it.
>
> Regardless of all this, my decision to do it this way was heavily
> influenced by the fact that I have a few other features (video and
> non-video related) that I'd like to incorporate on an expansion card.
>
> So I'd be mucking about with an FPGA in an Apple slot...

I thought that was the plan? I agree sampling it from the composite
output isn't a very clever way to go, but then constructing a bandpass
filter then sampling it with a DAC isn't really *that* complex, but
since the solution is going to be tied to Apple II's anyway there
little reason to make it harder than it actually is...

Matt