Re: IIgs RGB to VGA continued

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

jsnospam@cimmeri.com wrote:
> > In other news, I am also testing a lower cost, prototype unit for
> > RGB->VGA conversion using the Averlogic chipsets.  Results here also
> > look promising so far in all modes up to 320x200.   Still having
> > trouble with 640x200.
>
>
> Hi, all.   I'd like feedback from those of you who might know what's
> going on here in the following photos.
>
> We've got a prototype VGA converter going using some of the Averlogic
> chips.
>
> All modes except 620x200 are great, but I don't understand yet what's
> going on in 640x200.
>
> First photo:
> http://members.aol.com/mdnttrain/vga1.jpg
> This photo shows GSOS bootup.  What are you seeing here?
>
> I'm seeing two things:
>
>   1) seems like every other vertical line is black.
>
>   2) seems like alternating vertical groups of lines are in color, and
> not.  In the photo only (not when viewing), the groups that are in
> color don't show the in-between blackness as much so you might have a
> hard time seeing.
>
> Second photo:
> http://members.aol.com/mdnttrain/vga2.jpg
> This photo shows the color bands quite well in the GSOS desktop colors
> area.  But note that not every alternate line is black.. sometimes it's
> a colored line.. as in the yellow/pink area.
>
> Third photo:
> Another desktop view
>
> Fourth photo:
> Booting into Platimum Paint in 320x200 mode, colors are solid and
> perfect.
>
> Fifth photo:
> Same program, switching into 640x200 mode.. reveals all the same
> desktop flaws.  Note again the yellow/pink alternating area is solid..
> suggesting these alternating lines are deliberate and are "dithering."
>
>
> One theory we have for the alternating black lines is that the
> converter and monitor have too much bandwidth to dither.   A SVGA
> monitor shows the dithering that the Apple uses to create colors
> whereas an Apple RGB monitor does not have the bandwidth -- it simply
> shows a solid color because it can't react fast enough to display it
> (done on purpose).
>
> A low pass filter might lower the bandwidth of the SVGA
> monitor/converter combo to equal that of those old monitors.
>
> However, the color banding problem persists.  Would this be some kind
> of color reference signal lock problem?
>
> Thanks in advance for any feedback.

I'd take a look at how the converter is sampling the IIgs video.

I'm guessing that the RGB signals are sampled very quickly, at a rate
that is not an even multiple of the IIgs dot clock (14MHz).  So if any
signal is alternating pixel-to-pixel, the clock samples one value for
several samples, then samples the other value for several samples,
resulting in banding.

Is it possible to adjust the sample rate and see what happens to the
picture?

Ideally, the sample rate would be phase-locked to the Apple dot clock.

-michael

New, faster SUDOKU v2.0 solver for Apple II's!
Home page:  http://members.aol.com/MJMahon/

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