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Re: Apple Monitor IIc horizontal sync
option8 wrote:
I'm in the midst of putting together another of my Frankintoshes -
this time wedging a G4 Mini into the body of an Apple //c. One of the
finishing touches I'd like to add is video out to an original Apple
monitor. The model I have is a 9'' green monochrome like this one:
http://upload.wikimedia.org/wikipedia/commons/4/48/Apple_IIc_with_monitor.jpg
Using schematics from this site - http://www.geocities.ws/podernixie/htpc/cables-en.html#vgavideo
- I built an adapter which seems to work at 640x480@60Hz, with one
glaring problem.
As you can see from this photo - http://www.flickr.com/photos/charles_mangin/6926854065
- i'm having issues with the horizontal sync. The picture seems to be
clear, but shifted horizontally.
Does anyone here have a suggestion for how to adjust either the
monitor or the adapter I've built to connect the Mini's output to
monochrome RCA? The monitor has Vsync and Vcenter knobs but nothing
for horizontal. Maybe there's something internal i could look for?
Your problem will not be remedied by an adjustment.
The monitor is locking to the VGA's 31kHz horizontal sweep at
15kHz--that's why you see two copies of the desktop.
There is no trivial way to convert a 31kHz VGA video signal to
a 15kHz NTSC composite video signal. This conversion will require
an external "box" that saves every other frame of the VGA video
and plays it back as NTSC video at half speed.
In fact, unless the box can generate interlaced video (which you
may find quite annoying for text), you will only be able to display
about 200 lines per frame, so another approach to conversion would
be to digitize every other line of the VGA video and then play it
back at half speed. This approach uses much less memory in the box,
but it also skips every other line.
To use an NTSC monitor in non-interlaced mode (best for text and
other fine details), you'll need to generate no more than 200 lines
of video. Otherwise, skipping lines will result in missing a lot
of fine detail (again, like in text).
In summary, this is not a very practical conversion. You'd do
better to find a small VGA-capable monitor--an NTSC monitor is not
up to it.
-michael
NadaNet 3.1 for Apple II parallel computing!
Home page: http://home.comcast.net/~mjmahon/
"The wastebasket is our most important design
tool--and it's seriously underused."