Re: Question for other Highway converter owners

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

On Apr 5, 3:32 pm, Steven Hirsch <snhir...@gmail.com> wrote:
> I just barely finished setting up my Highway 100 RGB-->VGA converter with the
> IIgs, connected by a cable from Roger Johnstone.  First impressions are not so
> good, however.  Instead of a solid blue background on the desktop, I'm seeing
> violet/blue vertical stripes and the entire display is periodically
> interrupted with jagged flashes.
>
> Is this expected?  Are others seeing these artifacts?  Perhaps I'm expecting
> too much, but for > $200 this just doesn't look so hot :-(.

The IIgs desktop background is a 640-mode dithered blue/white
background.  It is the "poster child" for solid areas that will show
beat frequencies with the converter's sampling clock.

It is actually an 8MHz square wave, so any sampling frequency
not a multiple of 16MHz is going to produce some artifacts.

The good news is that such bad cases are not too common--
the bad news is that anything that "fixes" this problem will
likely cause ordinary Apple II graphics (14.3MHz dot clock)
to look even worse.

An "excellent" solution to this problem would require a
converter specifically designed for and locked to the Apple
video dot clock (14.3MHz and 16MHz), and, as you might
expect, there are no such devices on the market.

Something that would help considerably is to put a low-pass
filter in all the video lines with a cutoff at 8MHz (an anti-
aliasing filter, actually).  This would greatly reduce the
amplitude of the banding artifacts, but would also reduce
the "sharpness" of the converted video.  80-column text
would still be quite readable, but not "crisp".

-michael