[Coco] cm-8, the next generation (fwd)

Marty Goodman martygoodman at worldnet.att.net
Fri Apr 23 11:37:40 EDT 2004

Kevin asked:

> From: Kevin Diggs <kevdig at hypersurf.com>
> Subject: [Coco] cm-8, the next generation

> Has anybody ever hooked a tre up to a multi-sync to see what
> actually happens? I know that if you over drive a multi-sync, the def-
> lection circuits start snarling and spewing "colorful metaphors". But
> what happens if you under drive them? Vertically everything should be
> fine since 60 is within the vertical sync range for a multi-sync.
> Horizontally will it not sync or will you just see the left half of
> the picture (cauz it just can't scan that slow)?

Although it's been a long time since any common computer display used 15.75
KHz horizontal sync, in the old days it was common for "multisynch" VGA
monitors to be made to sync to that speed, primarily because the old CGA
video standards used a 15.75 KHz H sync rate.  Secondarily to accommodate
the Commodore Amiga, Atari ST, and lastly TRS 80 color computer market.

Thus the first two or three (in some cases even four or five) generations of
NEC, Sony, Nanao, and other brand and non brand name mutlisync monitors
would accommodate CoCo-type analog video (tho sometimes you had to use a
logic gate or two to provide inverted or inverted and combined sync signals
to the sync input).

The original NEC multisync and the multisync 2 in all its versions supported
15.75 KHz, and some if not all of the Multisync 3 versions did the same.  I
vaguely seem to recall that some of the last NEC models (3D?  4 of some
sort?) to support 15.75 KHz actually had only a 15 pin DB VGA type
connector, unlike the earlier ones that had a separate 9 pin DB connector.

Some of these appear to this day at thrift stores, surplus shops, etc.

Note that support for analog RGB  video and 15.75 KHz still has, I believe,
some applications (or at least did until much more recently than for
computer monitors) in the world of broadcast video, so used equipment from
that market may include monitors that will work with the CoCo 3's RGB analog

I believe if you search enough, and if your pockets are deep enough, you can
find gadgets that are black boxes of electronics that plug into a VGA
monitor and provide it with an NTSC input. I wonder if similar widgets are
available on the market (for professional broadcast video situations?)  that
will work with analog 15.75K Hz RGB video?  They probably are expensive, if
they exist.

If you find a compatible monitor, I can assist you with making a cable for
the CoCo.  I've still lots of those 10 pin dual row crimp on connectors
required for the CoCo's analog video output port.  And pin outs, of course.
And experience in making up thousands of video cables for Color Computer 3's
to allow their use with what then were widely available Magnavox, Sony, Nec,
Nanao, and many other brand of "multisync" monitors that supported 15.75 KHz
analog video input.

By the way, some old Sony televisions (like the KV1311CR, the immediate
predecessor of the XBR series) have 34 pin dual row  connectors on them, and
THESE can accept analog RGB video at 15.75 KHz H sync rate, and even audio,
thru those connectors.  Tho you need to use a 74LS02 NOR gate to combine the
H and V sync from the CoCo to make composite negative sync for the Sony.

NEC and Nanao and Magnavox RGB monitors (those that also supported 15.75 KHz
sync, that is) conveniently used separate positive H and V sync signals,
tho, so no active electronics were needed with them.  Very oddly, the Atari
ST monitors used separate NEGATIVE sync signals.  I recall having to use two
74LS04 inverter gates to adapt CoCo video for one of those.

> This becomes more interesting for an LCD monitor since there
> is no "electron beam" to push around. What would control the lower
> sync limits? I have an LCD monitor that can be used as a TV. So it
> can definitely sync at 15khz.

I suspect you'd run into similar problems, because while the LCD, as you
write, does NOT use an electron beam, the input video is designed for
driving an electron beam (has H and V sync and analog video levels), so
circuitry inside the LCD monitor must make it emulate the responses of a

But, it's worth a try if you have a LCD VGA monitor that also has a NTSC
video input. Because just MAYBE the sync processing circuitry is all in once
place for both the NTSC and the VGA inputs, meaning the durn thing just
MIGHT support CoCo 15.75 KHz H sync analog video right out of the box!

Then again, the conversion might be at some other point... such as NTSC to
VGA... in which case you'd be out of luck.


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