[sdiy] Modular Video?

[Grant Richter]

Hello,

Raster based video synthesis is more challenging than audio, but not
impossible. It helps to have an understanding of the black and white video
signal. Since color video is an extension of the black and white standard,
you need to know it first.

The black and white signal is composed of three parts, luminance
(brightness) blanking and sychchronization (synch). The standard video
signal is 1 volt peak to peak into 75 ohms. When DC coupled, the positive
voltages from 50 to 700 millivolts will vary the brightness from black to
white. At zero volts the beam is turned off so you don't see it during
retrace, this is called "blanking". Negative voltages from zero to -300
millivolts are synch signals and tell the monitor when to start the sweep
across the tube.

Video signals can be handled with available op-amps. Because the high
frequency content contains the edge detail or "sharpness" a bandwidth of 4
megahertz or better is needed. LM318 op-amps will work and aren't too hard
to find. The outputs must be buffered with current boosters in order to
drive 75 ohm inputs. There are newer op-amps specifically designed for video
with high current outputs. I believe Maxim makes them, among others.

Basically, as long as the blanking and sync information are present, and as
long as the signal stays between +50 and +700 millivolts, ANY signal can be
displayed on a TV monitor. You can build a video formatter from a synch
stripper chip like the LM1881 and a CD4066, just send a broadcast video
signal into the LM1881 to provide synch, then use the CD4066 to switch
between the normal broadcast input and your funky signals. An LM319
comparator can be used to extract blanking information. It's just a few
dollars worth of parts. Don't let your signal go below 0 volts or you will
screw up the synch.

All your regular synthesizer modules can be used to generate signals. Send
the vertical or horizontal synch signal to the sync input of the VCO so the
image won't drift. Video vertical synch runs at 60 Hz so a 120 Hz oscillator
will show 2 horizontal bars. Horizontal synch runs at 15,750 hertz so a 32
Khz oscillator is needed to show two vertical bars. If you combine two
oscillators you can draw "checker boards" using a CD4070 XOR gate. You can
see that in the images shown on the Spectrum video synth from EMS.

For color just send three black and white signals to the inputs of an RGB
monitor.

The Wiard modules are "roughed in" for video synthesis. With part changes,
the bandwidths can be expanded to 4 Mhz and the oscillators are designed to
run in the megahertz region by changing the timing cap. It's probably not
going to happen, but it was in the back of my mind during design.
> 
> Audiovisualizers.com seems to indicate that old Buchla synths could be used
> for video synthesis.  It sais something about they have to be video synced.
> If someone could find out how that was done with old buchla modules,
> couldn't the same thing be done with our Moog filters, and our OTA VCAs and
> our oscillators, and our....?  I've often thought that video synthesis (If I
> could get spaced out pictures and patterns) would be just as fun as audio
> synthesis.  In some ways DIY makes even more sense for video synthesizers
> since there is NOTHING available commercially to make video synthesis
> (unless its computerized and d*g*t*l, which I'm assuming is not comparable
> to most of the cool looking and knobified video synthesis stuff on
> audiovisualizers, like the Rutt/Etra and the Spectron) .
> 
> -=<Jonathan Pratt>=-
> (Phdinfunk at hotmail.com)