Re: Newbie Apple IIgs questions

[Mitchell Spector <mitch2gs@hotmail.com>]

"Michael J. Mahon" <mjmahon@aol.com> wrote:

>Mitchell Spector wrote:
>>     Ahh, that finally explains a little mystery about the Video Overlay Card.
>> When I first picked one up a couple of years ago, I noticed text and 
>> graphics were noticeably blurred. I thought it was a defect of my card,
>> I had no idea it was doing hardware based anti-aliasing.
>> 
>>     I've also noticed the VOC seems a bit less color saturated and there's
>> faint and subtle vertical banding appearing in the image. To this day I
>> cannot be sure if it's my specific card or a standard "feature". 
>
>This is necessary to keep the chrominance signal levels within
>standard limits.  The usual Apple II color is quite intense.

    Another interesting thing I just noticed, the VOC has trouble displaying 
any animation effects in the border area. In the GS-IRC Demo from 1992,
the scrolling text in the border disappears when output to composite, and
appears only the top portition of the screen when the VOC is connected to 
RGB (it is supposed to appear on the bottom). I'm sure it has to do with
timing issues.

    There also seems to be slight visual glitches with other games and demos
that were hard-coded instead of following Apple programming guidelines.
I'd really have to pull out my Second Sight card to be sure, I know it causes
all sorts of incompatibility issues even when it's not being used. Despite
that, it's still vastly preferrable when using composite output on a IIgs,
such as recording video to video-casette or even digitally capturing.

>> I dug up my post from 4 years ago that mentioned most of these issues
>> in greater detail...
>> 
>> http://groups-beta.google.com/group/comp.sys.apple2/msg/85c4efe85f3f6d5a?dmode=source
>
>This certainly explains the blurring (which is bandwidth-limiting to
>reduce the energy in the chrominance bands).
>
>The vertical "banding" you describe sounds like digital noise getting
>into the video.  If there are about 40 visible bands, then the noise is
>at 1MHz, the processor/memory clock frequency.

    I counted the vertical bands, there's about 49. I believe they can
_faintly_ be seen using the standard composite IIgs output, but you
have to push the brightness to maximum on the display (and again,
it's nowhere as visible). 

    The banding also appears on my RGB display, if I connect the VOC
directly to the monitor (overall, the output from the GS's built-in RGB
port is superior quality than the RGB-out connector on the VOC).

Mitchell Spector