When
playing up and down a scale the control voltage from a keyboard would
consist of a series of discrete steps. This would give rise to equally
discrete changes of pitch, the minimum change in pitch being one semitone
interval on the tempered scale, as with any other keyboard instrument.
However, many instruments are characterised by the ability to make continuous
(smooth) changes of pitch, one example being the trombone with its slide.
This style of playing is known as `portamento'.
The addition of this circuit between the keyboard control voltage and the VCOs allows the user to vary the degree or portamento applied.