[sdiy] "Ghost" Electronics

[Terry Michaels]

Message text written by INTERNET:grichter at asapnet.net
>Morton Sobotnick and Don Buchla pioneered a technique that Subotnick
called
"Ghost" Electronics. This is a method of storing control voltages on a
recording media for playback. This allows the performer to store his
"performance" as control voltages on tape (for example) and frees his/her
hands for knob twiddling during performance. It allows performance of very
complex pieces in a live situation. For example, "4 Butterflies" was a live
performance piece.

Originally the control voltages were stored using amplitude modulation
techniques. This makes the encoder a VCA and the decoder an envelope
follower. Later, I understand they switched to frequency modulation
techniques for more precision. That makes the encoder a linear VCO and the
decoder a frequency to voltage converter.<

Hi Grant:

The technique of recording analog voltages for later playback has been
thoroughly addressed in the laser lightshow industry.   The goal was to
record analog signals ranging from DC to about 3 KHz for driving laser
displays at planetariums and other places.    Laser graphics displays use
an X and a Y signal, with one or more intensity signals, usually R, G and
B.  Two more channels carry a stereo music track.  Around 25 years ago,
technology was developed to store these signals on a reel to reel 8 track
recorder.  Because an audio tape recorder does not have response down to
DC, each channel of control signals would   frequency modulate an audio
oscillator with a center frequency of around 8 KHz or so.  One channel
carried a fixed pilot tone, this was used to correct for DC shifts in the
analog signals when played back because of variations in tape recorder
speed.  Each channel of analog voltage information was demodulated by a
conventional PLL for show playback.

A company called Image Engineering developed a system that used ADC's to
convert the analog signals to digital, the digital information then
amplitude modulated a NTSC video signal.  The video signal was recorded on
an off-the-shelf 3/4" or VHS video tape recorder.  Playback involved
running the video through a corresponding decoder unit that demodulated the
digital information in the video signal, and applied it to DAC's to
recreate the original analog signal.

The current technique is the use of an Alesis ADAT recorder.  The original
ADAT's were easily modified for recording and playback of signals down to
DC, this required nothing more than jumpering a few DC blocking capacitors
in the input and output circuitry.  The later XT version of the ADAT have a
ADC that has an input voltage range that is not centered around 0 volts DC
like the original ADAT, so modifying these units requires adding a small
circuit board containing opamps for level shifting the analog signals as
needed.  Pangolin Laser Systems makes and sells the circuit cards for this
modification.

These ideas could easily be applied to electronic music production and
performance, the CV and gate signals could be recorded and edited for live
playback at a later date, with the possibility of real time manipulation of
the sound modification modules in the system by the artist.  This would
make every performance of the same composition unique.
Terry Michaels