Video Ramblings #4

Generating Psychedelic Video from Music Sources Inexpensively
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Over the last months I've been researching on the Internet
and from video expert friends how to do precisely what the
title says.  In every instance, I've chosen low-price over
quality.

If you've been reading these ramblings for a while, you'll know I 
was looking into ways to generate a video signal and
have sound/music affect it.  But over the last few months,
I've discovered it can be cheaper to begin with an existing 
video signal and modify it than to generate a video signal from scratch.  

One of the most useful components I've learned about is
called the LM1881.  Just mention the name and your buddies who
know about video electronics will say, "Oh yeah!  the LM1881."
It supposedly can take a color or b/w video signal from a VCR or
video camera (aka composite NTSC output), and generate a
signal which has *all of the sync information* but absolutely
nothing in the *video* area.  So... you can fill
in anything you want for the video area.

What I want to do is devise a simple circuit which
turns audio into video frequencies, adds it to the LM1881's
signal so I can see pretty colors on a TV screen that change 
in response to the music.

Here are some possible examples of cheap circuits which would
use inexpensive and commonly-available parts, e.g. at radio shack:
1) a 555 oscillator running @1MHz with audio applied to the CV input
2) a transistor used to invert the incoming video signal in response
   to music
3) a transistor used to distort or clip the video signal in response to
   music
4) xsistor used to filter certain frequencies out of an incoming video
   signal, in response to music
5) xsistor used to disable or make black video in response to music
6) the LM1881's odd/even output and horizontal sync outputs could be used
   to trigger CMOS/TTL counter circuits which affect the video signal.
   the horizontal sync will pulse about 17,000 times in a second.  the
   Odd/Even will be a 29.97 Hz square wave.

The LM1881 is not available at Radio Shack but they are quite common
from places like Newark, Mouser, Jameco, Digi-Key, and my personal
favorite Future-Active electronics.  It's only got 8 pins, so it does
not require much soldering.  It costs about 5-8 dollars.

So far one issue I forsee
is that the LM1881's output doesn't seem to include the color burst 
signal that the composite sync output needs to create a color signal.
I think it could be easy to rememdy.  What needs to be done is the 
color burst output signal needs to be used to recover the chroma burst
from the input signal.  Hopefully I can find a circuit somewhere to do
this.    

Of course I wish I had a nice schematic to show you but I don't, I've
just barely learned about this stuff.  So for now, I went to Future-Active
in Cambridge, and bought an LM1881.  I put it on my breadboard, and I'm
going to at least observe that its output signal drives a monitor dark
black.  If that looks good, I'll attempt to mix in some audio and
see what it looks like on the screen!

-Shifty

if you have any ideas e-mail me at shifty@gweep.net