24 Oct
2013
24 Oct
'13
1:19 p.m.
Dave,
If the internal screen is fine then the syncs are at
or close to the
proper frequency, so any problems are going to be around the NTSC
modulator, which would be fun to debug without a scope...
I do have a scope- poking about down in the chassis is difficult- I don't have
an ISA extension card. The composite trace drops down to the back side of the
card, comes up through a resistor, goes to the light-pen header, through another
resistor and vanishes into the middle layer. I could start just prodding with my
meter to see where it re-surfaces... but, well, yeah. A schematic would make
that easier.
The image on composite "tears" horizontally. I made a video of it, hooked up to
my TV set in the living room: http://youtu.be/a4jcKiSwUos
On the other hand whilst I have never tried the
Composite on CGA, but I
have tried comparable modes on several other computers, including my
rather old Atari STE, and a very modern Raspberry PI and it sounds like
its working (almost) just fine. The composite out on CGA cards always
was pretty useless, and "legible, just" pretty much describes any
80-colum output on composite. Try 40 a column mode (2 or 3)...
That's in 40-col mode, above. In 80-col it goes into a text mode rather like
MDA, which I haven't scoped to see if it's not present at the jack, or if the TV
refuses to display it. It's remarkably crisp, considering.
On a regular TV set (older, cheaper, no-name set)-
http://www.flickr.com/photos/philandrews/8657681921/
Above was trying to display this-
http://www.flickr.com/photos/philandrews/8660917851/
Admittedly that was all unbalanced, but I'm sure it should be a little better
than that.
(Oh and my experience is with PAL, NTSC would
probably be worse.....)
If you have a TV with a SCART then the circuit below would work..
http://www.electroschematics.com/377/
but I guess you are in the US and and your TV will have CYMG inputs.
Actually that circuit would probably work with CYMG but the colours
would be wrong.
Yeah, I should have said. I moved out here to the States a number
of years ago
now. My TV sets have all the usual "modern" inputs for here, CATV (NTSC
modulated), Composite-in, YPbPr component, HDMI and the one in the bedroom has a
VGA-in.
If have a modern LCD TV with a VGA input you could try
something
similar, but leaving the syncs separate and feed it into the VGA. This
might sound daft but often LCD TVs will sync to normal TV on the VGA. IT
doesn't work with monitors, but it does with some TVs. As a quick and
dirty test you could just use 470 ohms on all leads and omit the "I"
line....
I can give that a try, but it's a band-aid to the symptom. I've got another
machine with CGA out and the TV displays it nicely. I'd like to be able to use
the composite-out because the machine has several games on that make use of the
timing inconsistencies in NTSC to create a larger color palette than the
standard 4. That would, however, be useful for other applications. It's been a
while since I've seen a SCART connector..!
Dave
--
Dave Wade G4UGM
Illegitimi Non Carborundum