Re: IIgs RGB to VGA continued

["jsnospam@cimmeri.com" <js@cimmeri.com>]

Repost with all photo links.

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.

All photos below are of a VGA screen, using converted RGB->VGA video
(no composite!).


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:
http://members.aol.com/mdnttrain/vga3.jpg
Another desktop view


Fourth photo:
http://members.aol.com/mdnttrain/vga4.jpg
Booting into Platimum Paint in 320x200 mode, colors are solid and
perfect.


Fifth photo:
http://members.aol.com/mdnttrain/vga5.jpg
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. 

~ J