[Coco] Monitor Adapters was >Re: *COOL* Look what I found! (VideoMod for CoCo2)

[John Eric]

Fedor,
Your post got me to thinking about monitor adapters for the CoCo. Looking at the 6847 data sheet, it appears that the 0a and 0b (theta a and theta b) signals of the VDG are R-Y (theta a), and B-Y (theta b), along with Y (Luminance). These are the signals normally mixed by the MC1372 to generate the composite video. The CoCo's 1 and 2 all have a 1372 either on the motherboard, or later, within the RF Modulator itself. You could tap the composite video from the MC1372 thru a buffer and an amp (two transistors) and achieve full color direct composite video, or you could make an adapter that included it's own MC1372. If you did this, then you could use a switch that allowed the reversing of the diode across pins 13 and 14, the effect of which should provide inverse video (inverse video can also be obtained by inverting the signal entering pin 32 of the VDG.)
However, even more interesting than all of that is that since we have luminance, r-y and b-y, we should be able to create an RGB monitor adapter for the CoCo 1 and 2. Let me explain, If we feed  those three signals into properly configured high speed operational amplifiers we can generate the Red and Blue signals for an RGB monitor, then by mixing and buffering the Red and Blue signals in proper proportion, we can generate the Green signal. We would then end up with an RGB color signal, with Sync On Green. Sync could be seperated if the rgb monitor in question does not support sync on green. In theory, this sounds achievable - four high speed op-amps (single, dual or quad packaging should all work), about 7 caps and a handful of resistors. Okay, time for me to place another parts order because I REALLY want to try this. If I can calculate the outputs correctly so that they don't exceed .7 volts, I should be able to bring my CoCo 2 up on my CM-8 monitor.
 I need to determine which op-amps to use - they need to have a relatively high bandwidth and a high slew rate, and should operate off a single 5v supply (since that's all a CoCo 2 has). I'll keep you posted as to whether this pans out or not. -JEric





________________________________
From: Fedor Steeman <petrander at gmail.com>
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Sent: Thursday, January 8, 2009 6:37:52 AM
Subject: [Coco] *COOL* Look what I found! (VideoMod for CoCo2)

Hi all,

While going through the huge lot of copied CoCo Manuals I inherited from
Michel Collette, a former Dutch CoCo-society regional coordinator, I
discovered the following schematics for making a video buffer mod for
the CoCo2, enabling a CoCo2 to be coupled up any television or video
equipment with a composite video input:

http://www.steeman.dk/CoCoManuals/VideoMod_(KoenVaartjes).pdf

How cool is that!?

It is all in Dutch, written by someone called Koen Vaartjes, but if people
are interested I could translate, clean it up, perhaps redo the schematics
in a graphics editor and put it up on a webpage!

Some of the equipment i inherited from Michel Collette actually included a
CoCo2 with this video mod. I tried it out when I first discovered it and it
worked flawlessly then. After having transported the CoCo one time, however,
something went wrong, because after that all I got was a black screen and no
sound. I believe there must be a loose contact somewhere, because every now
and then when I try it, it works again.

The final page looks like a schematic of a scart cable (
http://en.wikipedia.org/wiki/Scart) that is widely used in Europe at least.
I do also have a modified SCART cable that can be used to hook up the
RGB-output of a CoCo 3 to a normal television with a scart input. That too
once worked, but not anymore. Maybe if someone could help me understand the
schematic, and figure out what wires of a RGB-cable should be connected to
what pin of the scart, I could try to fix the cable or make a few more for
anyone interested... This would mean that a CoCo3 could be connected from
the RGB output directly to a television with scart input.

Cheers,

Fedor

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