The function or method that determines, from the control input to an oscillator or filter, what frequency the oscillator or filter should operate at. There are two basic formulas for scaling this input, known as the volts/octave and volts/Hz methods, and within both of these, there is some variation in the exact conversion parameter. Scaling is important not only to make the synth basically play in tune, but also to ensure that it remains in tune at all notes within the instrument's span.
This can sometimes get complex, particularly in the scaling of analog VCOs and VCFs. A characteristic of many VCO circuits is that their response to a control voltage is not electrically "perfect"; even if the control voltage is totally within the proper scale, the VCO may play flat in the higher range. This is inherent in the circuit, and experienced VCO designers will add high frequency compensation circuits. These must be tuned during the calibration process so that the VCO will play in scale across the whole span.
Some synths have controls allowing the performer to vary the scaling during performance. Many recent model synths allow for selecting "stretched" or "shrunk" scaling. In stretched scaling, higher notes play slightly sharp and lower notes play slightly flat (which mimics the way pianos are usually tuned). Shrunk scaling is the opposite. They may also allow for half-scale, in which the distance between two adjacent keys is a quarter step and there are 24 notes to the octave. The EML 101 was unusual in having a panel control allowing the scaling to be continuously varied between about 10 notes and 60 notes per octave.