[sdiy] "Ghost" Electronics

[Crystal]

this has been an interest of mine for quite some time. what i envision is a
real time sequencer that can put control voltages into memory, then play
them back on demand. no serious editing capabilities needed. maybe 4 or 8
channels. with a battery backup this unit could store the CV patterns
indefinetly, although i am more interested in using the output patterns in a
live situation, where i can continuosly update them. i was thinking a simple
ad/da with cmos ram. only needs to sample at about 300hz or so.


>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
>
>