Chapter Four: Synthesis

4. Analog Synthesis Concepts

The Modular Voltage-Controlled Synthesizer | Page 2

Functions of Control Voltage (c.v.)

As mentioned above, viewing voltage as a variable force applied to a task, such as changing the frequency of an oscillator, or making a sound louder or softer works well…the more voltage, the greater the effect. While the sound output we ultimately hear from a synthesizer is also output as audio-rate fluctuations in voltage, control voltage serves a different function, which is to modify the parameters of various synthesis modules, sometimes at audio-rate, sometimes at sub-audio rate, and sometimes as a fixed single value.

Control voltage (c.v.) can be applied to affect many sound parameters. In the simplest patch (what we refer to as the basic patch on the following pages) c.v. controls the pitch of an oscillator via a keyboard, it controls the cutoff frequency of a filter, and it controls the amplitude of a sound over time (including the start and end of a note).  C.v. can also control pulse width, filter resonance, and much more.  Control voltages can be summed with other c.v.’s.  For example, vibrato is often created by summing control voltage, patched to a signal oscillator, from a keyboard providing the center pitch c.v. with a low frequency oscillator creating the pitch fluctuation around the center pitch.

control voltage module
Module control voltage input section

Some modules have several types of control voltage inputs. For VCO's and VCF's, it is common to have three c.v. input types: 1 volt per octave (1 V/oct), linear, and exponential. For both of those modules, 1 V/oct would change the oscillator or filter cutoff frequency by an octave for each volt's change in c.v. and was not adjustable. Classic analog synth keyboards were calibrated to output a 1-volt change per octave to match this input. For example, if a VCO was tuned to output a C3 when it received 3 volts, it would output C4 when it received 4 volts. The VCO would respond with equal tempered note spacing when 1 V/oct was used. Linear moved the module's function an equal number of Hz up or down around a center frequency (crucial to Chowning-style audio-rate FM we will cover later), while exponential moved these parameters an equal musical interval up and down. Unattenuated exponential was equivalent to 1V/oct, while attenuated exponential would create microtonal equal temperament. For example, if the exponential input of a VCO was set to attenuate the voltage by 50%, a change of an octave on the keyboard would change the pitch only by a tritone, half the musical interval of an octave.

Some early synths, like the Moog modulars were designed to condition the control voltage before it arrived at a module, so you would not find these three choices on the Moog VCO's for example.

Signal Paths vs. Control Paths
When studying synthesis, it is important to keep in mind which connections are routing the audio signal, i.e. the signal we will ultimately hear at the end of the line, vs. control connections, which we will not hear as audio. Control connections affect what we hear, for example the pitch, but we don't hear their signal directly as audio output. Some synthesizer modules can be used as a source for both audio signal AND control signal depending on how they are patched. The voltage-controlled oscillator, for example can provide either audio as part of the signal path, or be a source of control for tremolo or vibrato if it is patched to control inputs rather than the signal path.

Sources and Types of Control Voltage

AC control voltage—cyclic or noise-based control voltage from oscillator, frequently an LFO (low frequency oscillator), or noise generator

DC control voltage—single value or non-cyclic single stream control voltage from sources like a keyboard, envelope generator, gate, sample and hold module and envelope follower.

Bipolar control voltage—voltage that has both a positive and negative component crossing 0 volts. For example, most analog LFO's, VCO's and Noise generators output AC voltage that transited between −5 to +5 volts.

Unipolar control voltage—a control source that is usually 0 volts to the system maximum. For most analog synths, this would be between 0 and +5 volts. Envelope Generators are almost always unipolar. Bipolar signals, such as those from VCO's can be reduced by half and then offset to 0 and above to create a unipolar signal.

Common Synthesizer Controls
The following two terms are essential to understanding 99% of a voltage-controlled synthesizer's settings and are common to most signal-path modules such as oscillators, filters and amplifiers:

Offset: an offset sets the initial setting of a module, such as the initial frequency of a VCO, the initial cutoff frequency of a VCF or the initial amplitude of a VCA. This is where the module's parameter begins before control voltage is applied.* For example, if you play middle C on a synthesizer keyboard and that sends out three volts to the frequency control input off an oscillator, you can offset or tune the oscillator to whatever pitch you want, not necessarily middle C. If you then play a C an octave higher, and the oscillator receives one additional volt, if the control input is set to respond 1 volt per octave (1V/oct), the oscillator will output an octave above of whatever you set with the offset.

An interesting example of an offset was Irving Berlin's transposing piano, which had a shift lever to move the mechanism to whatever key he wanted to play in, as the famous, but self-trained composer could only play in the key of F#. A guitar capo is another analog of an offset.

*The Absynth soft-synth we use takes a different approach, whereby the offset is actually calibrated to be the maximum value of a parameter at maximum c.v. rather than the initial value at 0V c.v.

Attenuator: an attenuator cuts down on the effect of applied control voltage. For example, a filter won’t sweep as wide a frequency range when an envelope generator is applied to it.  On current soft-synths, it is often called depth. In the above offset example, while playing an octave higher on a keyboard may send an additional 1 volt, if it is attenuated, the module will only receive a portion of that voltage change, and therefore only move a fraction of the octave.  If the attenuator reduced the 1-volt change to .5 volts, then the oscillator would shift a tritone rather than an octave. The result would be a microtonal scale rather an equal tempered one. If the control voltage is completely attenuated, it won’t change frequency at all. In classic synths, many control inputs (except the fixed ones labeled 1V/oct) had attenuator knobs directly above or below the c.v. jacks, as depicted in the c.v. section above.