Help Needed with Voltage Controlled Synth Circuit
Analog synths are built around a voltage-controlled oscillator. For compatibility between keyboards and oscillators and to allow hooking of synths together a standard control of 1 V per octave became standard. This gave a reasonable range of 8 V for an 8-octave keyboard. So, if, for example 3.732 V gave a certain frequency then 4.732 V would give one octave higher. The oscillators were tricky to design as they needed to behave linearly over a wide range and have excellent temperature stability. On the other hand it made the keyboards simple as the same resistor value was used between each pair of keys.

Keyboards aren't the only pitch controllers, however. Two other common methods are pitch-bend controllers (traditionally a wheel on the left end of the keyboard, sprung to centre) and a vibrato wheel (also on the left end of the keyboard) which introduced a low-frequency vibrato to the oscillator.

Now your circuit should become clearer.



Figure 1. VCO (voltage-controlled oscillator control inputs.)

IC2 is configured as an (inverting) summing amplifier. The output to the VCO will be the sum of signals 1, 2 and 3.


"Base" would be used to tune the oscillator with other instruments.
I suspect CV1 would be used to modulate the frequency as the potentiometer can be used to attenuate the effect. It wouldn't make any sense to modify the keyboard signal in this way.
CV2 is most likely the keyboard control voltage input.