Archive of the former Yahoo!Groups mailing list: MOTM

I thought this was interesting for those not on the SynthDIY ML...
from Grant Richter...

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.

I have a number of questions I was hoping the group could address.

1. How closely could this technique be replicated using a MIDI sequencer and
MIDI to CV converter? I understand the convenience of quantized voltages for
pitch, but how well would high resolution smooth voltages be handled? MIDI
continuos controllers only handle 0-127 and the zipper noise is obvious from
experiment. Are there other methods with higher resolution DAC output
supported by sequencer programs?

2. Do any sound cards or interfaces directly support storage of DC voltages
as audio tracks?

3. I experimented with using 2 x CD4046 as encoder and decoder. With some
work the VCO can be made fairly linear. The phase locked loop decoder had
surprising DC accuracy for a 40 cent chip, but the transient handling was
very poor. A 100 Hz square wave showed very poor recovery. If the PLL filter
was optimized for 100 Hz recovery, then 10 Hz recovery was poor.

What were the original techniques used for DC instrumentation recorders,
such as the multi-channel models made by Tascam? Are there any circuits on
the web?

Thank you for any help,

Grant