[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Newbie Apple IIgs questions
Mitchell Spector wrote:
"Michael J. Mahon" <mjmahon@aol.com> wrote:
Mitchell Spector wrote:
Good example: Look at the fonts displayed inside a Finder window
on a TV set and the text actually appears in heavy bold (by no means
subtle) bands of green, blue, purple and orange. The whole screen is
quite literally a rainbow colored mess. Now switch over to output from a
Video Overlay Card (which produces a corrected composite signal)
and the black text appears greyish black. Any traces of color shifting
is subtle and faint, you really need to stare closely at the screen.
The video overlay card does two things: it creates standard sync and
blanking signals (correcting the minor timing differences) and it does
anti-aliasing of the chroma signal, so that there is less color artifact
on, for example, dot pairs. If a single dot is displayed, and is not
shaped/stretched, then it must appear colored, since it represents
an appreciable amount of chroma energy at that position.
The "graying" of the black is a sign of anti-aliasing to reduce
chroma artifacts. This is appropriate in a screen known to be
monochrome, but it is not appropriate if the screen is supposed
to be color. Normally, the "intent" is reflected in the presence
or absence of a color burst on the "back porch" of the horizontal
blanking pulse, but most monitors require many scan lines to respond
to a change in color burst status, so entire fields are either color
or monochrome, not partial fields.
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.
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.
Perhaps the noise it getting in on the power, because of incomplete
decoupling. Try a few 0.1 microfarad capacitors near the edge
connector, and maybe a 25 microfarad electrolytic, as well, and see
if that changes the bands.
I've used my Commodore 64 with both the split chroma-luma (essentially
S-Video) and regular composite, and even the latter produces a superior
video signal compared with the IIgs's composite-out. The same is true with
my Amiga, Nintendo, Super Nintendo or any other composite device.
The C=64 and most video games use a technique of creating color video
that is more like the Apple II Lo-Res color, or Apple II DHR color,
based on 4-bit patterns shifted out at 14.3Mhz.
That would make sense, though what about the Amiga outputting
composite video? Some of its graphic modes are very similar to the IIgs
and yet there is no major color shifting or bleeding. It looks respectable.
I'm not familiar with how the Amiga generates its composite video.
It is entirely possible that a higher-clock-rate approach is being
used, or even multi-level pulses (Apple II video pulses are always
full-on (white) or full-off (black), sometimes above the frequency
response of any ordinary composite monitor, so that the "dot structure"
of the color pixels is not visible.
-michael
New Applesoft BASIC interface for NadaNet networking!
Home page: http://members.aol.com/MJMahon/