[sdiy] Re: [AH] Video Processing

[Grant Richter]

A VGA display uses 700 millivolt peak RGB video, but horizontal and  
vertical synth are on separate pins, not combined with the luminance  
signals.
A video synthesizer is going to separate the composite color  
information into monochrome RGB signals and separate the synch  
information.
That is all done for you already at the VGA interface.

I have a lot of old VGA monitors left over from computer systems that  
I upgraded.

Interestingly, audio synthesizer modules have a 1K resistor on the  
output (usually) and a 10 volt output, VGA monitors have a 75 ohm  
resistor on the input. These form a voltage divider that attenuates  
the audio 10 volt output level to (10/1075 = 0.0093 x 75 = 0.697  
volts). VGA signal are spec'ed at 700 millivolts so 697 millivolts is  
perfect.

You can run video through a Vactrol just fine. It is only a resistor  
and will handle 10 Megahertz just fine.

I made a thing I call a "Chromatrope" after a type of Magic Lantern  
slide.

It is a color organ for a VGA display.

A lowpass, band pass and highpass filter (like the three outputs of a  
state variable) control the current into LEDs for three Vactrols  
wired in series with the RGB signals to the VGA. If the state  
variable filter has 1K resistors on the outputs, you can connect them  
directly to the LEDs in the Vactrols. The Vactrols themselves act as  
envelope followers, because of the decay time of the photoresistor  
phosphor.

It really makes a music visualizer program like G-Force come alive,  
when the colors pulse in response to the frequency content of the  
music, in addition to all the visualizer oscilloscope type stuff.

The Vactrols are Allied Part Number 699-3012, cost is $1.42 ea. Their  
resistance goes down to much less than 100 ohms and you can wire them  
inside the cable, or with a little 15 pin to 15 pin bridge connector.  
You don't need a buffer or any active parts, just wire the Vactrol  
resistors in series with the analog RGB signals.

So three cheap Vactrols, an old VGA monitor and a synthesizer filter  
will make a pretty fun video synthesizer for music display.

Too bad I don't have time to do video modules at Wiard. I have a lot  
of working prototypes of video modules, but it is a whole other  
marketplace, and the audio modules keep me busy enough already.

Have Fun!