[sdiy] "Ghost" Electronics

[Happy Harry]

Hi Grant: inline
>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.

>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.

I would not use PLL techniques. If you could square up the recovered
audio, use the PV-1 pitch to voltage converter... Penalty of divide by 2 in 
frequency... but it will track very accurately and with the minimum of 
ripple. It "could" staircase at low frequencies, but you could filter that 
with only 10% of what a loop filter would need to be. TomG still has some 
circuit boards for the unit. I never thought
of using it like this. Great Idea.
>
>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
>
>

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