Subject: Composite video to TV

From: Robert Mueller <[email-masked]>
Date: Wed Jul 2 19:23:51 2003

I have tried this twice with some luck in both cases.   In both cases the 
computer was an Atari 800XL.  One monitor started as a Black and white TV 
able to operate from 12 volts or line power.  It was easy because it 
generated 12 volts with a transformer when running from the line.  The 
signal went into the contrast control, not the wiper terminal but the "hot" 
stationary terminal.   If you have enough signal level this will work 
directly but you can try getting in before the video amplifier or simply 
put in a small amplifier between the new video input and the contrast 
control (trivial with today's fast opamps)  You can steal the power from 
somewhere in the TV.

The other case was more interesting because it involved one of those nasty 
TV s running without a proper line isolating transformer.  Here I went to 
an optical coupler based on a fast HP model coupler for analog signals (I 
believe these reach 10 MHz with some effort.)   I needed a tiny transformer 
to provide drive for the LED on the computer side of the optocoupler (the 
computer output is not enough).  This was setup to use a slight bias 
current at zero input voltage from the computer and increasing light level 
(more drive to the LED) with increasing video level.  The main trick in all 
this is some extra frequency compensation to delay the fall-off of the 
signal at the useful higher frequencies  (you should preserve as much as 
possible at least to 6 MHz and even a bit more if possible).   The 
receiving end of the optocoupler was configured to serve as an amplifier to 
get enough signal for the contrast control.

The B&W version gave a quite good picture containing most of the sharpness 
you might expect from a proper  monochrome monitor and I liked it much 
better than the green things I was using at the time with other computers.

The color version with the optocoupler was fairly satisfactory and I was 
happy to use it when a color monitor was simply too expensive to 
justify.  Text was adequately sharp at 40 characters per line.   Don't 
forget, the color still had to go through a PAL decoder with loss of 
sharpness.  Incidentally, PAL is inferior to NTSC for this kind of job 
because the fundamental idea of PAL is the mixing of two adjacent lines to 
correct for level dependent phase shifts in transmitted pictures.  The idea 
is great for TV but poor for computer work.  Narrow horizontal lines simply 
suffer from this mixing.  Still, the overall result was "good 
enough".  HOWEVER; PLEASE OBSERVE ALL WARNINGS IN EARLIER MESSAGES.  These 
transformerless sets kill   the foolish.  If you live in 230 volt land, the 
risk is still worse.

Bob