Introduction to Sound Synthesis Pitch
One volt per octave
One volt per octave is a pitch standard for control voltages. Each volt added to an oscillator's pitch input doubles its frequency and raises the pitch by one octave, so a twelfth of a volt is one equal-tempered semitone. Oscillators, keyboards and sequencers from different makers that follow it play in tune together.
The chapter on pitch and frequency showed that the ear hears frequency by ratio: every octave is a doubling. For a pitch control to give the same interval for the same movement anywhere in its range, equal steps of control have to give equal ratios of frequency, so that a step of one volt raises the pitch by an octave whether the oscillator is at 50 Hz or 2 kHz. One volt per octave, written 1V/oct, is the standard that fixes that step at one volt.
The tool below converts a voltage to a frequency. At 0 volts it plays the note set by Tune, in the same way that the tuning control on an oscillator sets its pitch with nothing connected. The plot shows the conversion as a curve, with the frequency axis either in hertz or in octaves.
Volts to pitch
A control voltage from 0 to 6 volts sets the pitch of an oscillator. The plot shows how frequency rises with voltage under 1V/oct, and under the Hz/V standard for comparison.
Listen for Each +1 V step is an octave, the same size of step at the bottom of the range as at the top. Under Hz/V the same +1 V step is a large interval at low voltages and a small one at high voltages, because the frequency rises by a fixed number of hertz for each volt.
An exponential conversion
Under 1V/oct the frequency is the tuned frequency multiplied by two to the power of the voltage. Plotted against hertz that is a curve that gets steeper as it rises, because each volt adds twice as many hertz as the one before. Plotted against octaves it is a straight line, so the control is linear in the way pitch is heard. A sequencer step or a keyboard key one twelfth of a volt higher plays one equal-tempered semitone higher, and a quantizer rounds a voltage to the nearest of a chosen set of these steps, or to any other set of pitches you give it.
Inside an analogue oscillator the frequency is set by a current, and the relation between current and frequency is linear. The part of the circuit that turns 1V/oct into that current is the exponential converter, usually built from a matched pair of transistors. Transistors change their behaviour with temperature, which is why analogue oscillators drift as they warm up and why many designs add temperature compensation.
Other standards
Not every analogue synthesiser used 1V/oct. The Korg MS-20 and Yamaha's CS series used hertz per volt, where frequency is proportional to voltage and the octave is a doubling of voltage. Buchla instruments use 1.2 volts per octave, which makes each semitone a tenth of a volt. Instruments on different standards need a converter between them to play in tune. In MIDI and in software, pitch is sent as a note number or a value in semitones rather than as a voltage, and the same exponential conversion to frequency happens inside the synthesiser.
Tracking
Two oscillators step up through five octaves together, one volt at a time. Both are tuned to the same note at 0 volts. The second oscillator's scale can be set slightly away from one octave per volt, so that it responds to each volt with a little more or a little less than an octave.
Listen for With the scale at 0% the two oscillators stay in unison all the way up. With a scale error of 1% they are in tune at the bottom and beat against each other faster with each octave they climb. By the fifth octave the second oscillator is 60 cents sharp, and the two sound like separate notes.
Tracking and calibration
An oscillator tracks well when one volt gives exactly one octave across its whole range. If the scaling is slightly off, say 1.01 octaves per volt, the error is zero at the voltage you tuned it at and grows with every volt away from it. At five volts it is five hundredths of an octave, 60 cents, which is more than half a semitone. Two oscillators that are meant to play in unison or in fifths drift apart as a melody climbs, and you hear the drift as beating that speeds up.
Most analogue oscillators have a scale trimmer for this, usually on the circuit board, and calibrating one means adjusting it until octaves played from a known voltage source measure as exact doublings. Digital oscillators that read 1V/oct through an analogue-to-digital converter are calibrated the same way, in software. Manufacturers usually state tracking as an error in cents over a number of octaves.
Common questions
What does 1V/oct mean?
1V/oct stands for one volt per octave: each volt added to an oscillator's pitch input doubles its frequency, which raises the pitch by one octave. A twelfth of a volt, about 83 millivolts, raises it by one equal-tempered semitone. It is the pitch standard used by Eurorack and by most analogue synthesisers with control voltage inputs.
What is Hz/V?
Hz/V stands for hertz per volt, a different pitch standard in which the frequency is proportional to the voltage, so doubling the voltage raises the pitch by an octave. The Korg MS-20 and Yamaha's CS synthesisers used it. A Hz/V instrument and a 1V/oct instrument cannot play in tune together without a converter.
What voltage is middle C?
1V/oct fixes the size of an octave but not where the scale starts, so each oscillator's tuning control sets the pitch it plays at 0 volts. VCV Rack uses 0 volts for C4, middle C.
What is tracking on an oscillator?
Tracking is how accurately an oscillator follows 1V/oct across its range. An oscillator that tracks well over five octaves plays every octave within a few cents of where it should be. One that tracks badly plays in tune at the note you tuned it to and goes progressively sharp or flat further away.
Why do analogue oscillators drift out of tune?
The circuit that converts volts per octave into frequency, the exponential converter, is built from transistors whose behaviour changes with temperature. As the circuit warms up after switching on, the pitch moves. Designs compensate with matched transistor pairs and temperature-compensating resistors, and some heat the transistors to a fixed temperature.
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