[sdiy] chaotic sound/analog computer

[David G. Dixon]

> The crucial part of the diagram that I can't work out how to cube the
> signal. What would work for something like that? Something using an
> OTA? Two four-quadrant multipliers? Can anyone point me to a suitable
> circuit for getting the cube?

Wouldn't two multipliers do the trick?  First square the signal, then
multiply this result by the signal again to cube it.

Yes, an OTA would work, but only for two-quadrant multiplication.  If you
wanted the cubic to exist over the entire coordinate plane, you would need
four-quadrant multiplication.  The best thing for that would be a proper
analog multiplier.  Alternatively, you could use two OTAs (i.e., a complete
LM13700 chip) to rig four-quadrant multiplication.  I've done this in my
"Multiplying Modulator".  Email me for details.  Another, slightly more
expensive but superior option, which I've also used in my Multiplying
Modulator, is to use two linearized 2164 VCAs (requiring the entire quad VCA
chip) to do the same thing.  Hence, one could build a virtually perfect,
four-quadrant cubing circuit with two 2164 quad VCAs and two quad opamps.
Again, email for details.

Ian probably has a much simpler and more elegant solution.