MULTI FUNCTION VCF:

Although the VCF implemented in the 3372 is a standard 4-pole low
pass, the additional circuitry made of UX14, UX13, UXI5, UX17 and the one
percent resistor array on the left of UX17 allows 15 different filter modes
including high pass, band pass, notch and all pass.
Understanding how this magic trick happens requires a little theory.

The figure below shows a simplified representation of the circuitry.

The complex representation of the transfer function of  a 1-pole low pass filter is:

Therefore, the transfer function of the network will be:

Selecting specific values for A, B, C and D allows different 
transfer functions. For example, selecting A=1, B=1, C=O and D=O 
transforms the above equation into:

which is the transfer function of a 2 pole band pass filter.

Getting different responses from this network will be just a 
matter of adjusting the coefficients A, B, C end D in order to transform 
the transfer function into the desired one.For the high pass and all pass
filter modes, it is necessary to cancel the first cell of the filter.
This is accomplished by opening the switch S1.

In practice, S1 is implemented by one cell of the anolog switch UX14,
the buffers b1, b2 and b3 are the four op amps of UX13, b4 is the op amp
UX15 (output pin 7) and the summation is performed by the second cell
of UX15 (output on pin 1). The coefficients A, B, C and D are set by
the value of the summing resistors. The analog multiplexer UX17 allows
to select from eight different resistor combinations providing eight
different filter modes. Seven additional modes are obtained by selecting
the previous modes and opening S1