[sdiy] Q: oscillator bank

[Grant Richter]

This is exactly how the EML-101 was done. There are two uA726s, one for the
VCOs and on for the filter. But everything is linear voltage responding.

It works, a bit fussy though.

>> My thoughts on this:
>> - sum the various control voltages: keyboard key, plus modulation, plus
>> pitch-bend
>> - apply to the exponential converter
>> - use the converted Hz/V value to drive several oscillators
>> - each oscillator has octave switching via a resistive divider
>> - each oscillator (but one) has scaling to allow relative tuning +/-
>> some interval
>>