Early 1970's adventures (long winded post)

[Grant Richter]

Preface: Rich Nelsons mention of the ARRL handbook got me reminiscing about
some early electronics adventures. This post has no point (other than to
ramble on about some early electronic music experiments).

I got involved with electronics when I was a kid, building Allied Radio
kits. I was nuts for anything with lots of
knobs, an oscilloscope was my holy grail until I saw a picture of a
Minimoog in 1968, which had more knobs and was therefore even cooler. So
there I am stomping around Wisconsin in 1974 trying to find schematics
to build a music synthesizer.

I started by hitchhiking to visit every University electronic music studio
in Wisconsin - not much going on with the exception of Madison which had a
big studio. When I smoozed my way into the studio and told them I was
trying to build a modular, there wasn't a dry eye in the place (cause they
were laughing so hard). Daniel Harris was the director of the studio and
Wayne Weber was his technical assistant. Wayne took me seriously and let me
hang around trying to pick up information. But they didn't even have
schematics at that time, so not much luck there.

What does stick with me is the number of experiments they were conducting
in "multimedia". That was the fashionable term for collaborative
performances between the music, theater and dance departments. One of the
main things Wayne was working on was controlling "galvos" directly with
synthesizers. The "galvos" were an assembly of two mirrors with actuators
which were used to sweep a laser beam in X-Y coordinates. Like a really big
oscilloscope. You could use synthesizer oscillators, VCAs and especially
filters to drive the DC coupled "galvos" and produce huge laser art in the
theater next door to the studio. Optical Electronics Corp. (now make
frequency counters) even made a line of modules specifically for the
purpose of controlling "galvos" to a very high level.

One experiment that I saw performed went like this: The theater stage has a
black back drop. Set up in front of it was a laser with the "galvos" and a
high speed photodetector with a lens. A sequencer is used to step an
oscillator, the DC from the sequencer drives the X "galvo", the oscillator
drives the Y "galvo". This produces a series of vertical lines across the
stage like this |  |  |  |  |  |  | now each of these lines is at a
different frequency because the oscillator pitch is being changed by the
sequencer. A dancer in a white leotard gets up on stage. Now wherever the
laser beam touches the dancer, its reflected back to the photodetector,
amplified and put out through a speaker. So if the dancer places one hand
in the beam, a single pulse is output at the frequency of that line. Two
hands, two pulses and so on.

Now the cool thing come when the dancer moves through the lines, this mixes
the pitches and the body intersecting at different widths causes pulse
width modulation. Very cool in a meditative "OM" kind of way. Wayne showed
me how the same thing could be done with any oscilloscope. Take a sawtooth
signal (for example) and feed it into the external sweep (X) input on a
scope. Put the output of a control voltage pedal into the regular input
(Y). Place a high speed photo detector in front of the scope screen, it
must be a photo transistor because a photoresistor is too slow. Now invert
and amplify the output of the phototransistor. This makes the output high,
when it goes dark.

You can draw different shapes onto acetate with India ink and tape them to
the screen with transparent tape. Take a drawing of a tree like structure
that starts with a single line, splits, splits again and so forth. With the
CV pedal at the bottom, the signal sweeps through a single interruption and
outputs a single pulse per sweep. As you increase the control voltage it
sweeps the two branches and outputs two pulses per sweep etc. For such a
simple gadget it really has amazing capabilities as a waveshaper.

The above is only operating in the digital domain, that is the output is
only on and off. If you consider the case of placing a gradated filter in
front of the screen, that goes from black to clear smoothly, and assuming
you could calibrate the optical sensor, the system could become somewhat
linear. That is you would get your sawtooth back out. What about the case
encoding a waveform in the transparency by varying the density. Lets say we
had a transparency that varied in a sine curve fashion, a sawtooth in would
get a sine wave out. So in theory, any waveform could be stored optically.


Grant Richter
Wiard Synthesizer Company
http://www.wiard.com/