User Guide

Control Voltage - Control voltages (also called CVs) are used in analog synthesizers
to affect changes in the sound. In the case of pitch, pressing a key on the
keyboard sends a control voltage that determines the pitch of the oscillators. The
pitch can also be changed by a voltage provided from a panel control, such as an
oscillator tuning control. Every panel control on the Voyager produces a control
voltage that is routed to the circuit that the knob or slider is designed to change. An
oscillator with pitch varied by the voltage from the keyboard or tuning control is a
Voltage Controlled Oscillator, or VCO. A filter whose cutoff frequency is determined
by a voltage provided from the cutoff control is a Voltage Controlled Filter, or VCF.
An amplifier whose amplification is determined by a CV is a Voltage Controlled
Amplifier, or VCA.
To illustrate the idea of control voltages, let’s look at a theoretical voltage
controlled synth with a VCO, VCF (lowpass) and VCA. Let’s suppose that the VCO
frequency and VCF cutoff frequency change one octave for every volt applied to
their control inputs. Let’s also suppose that the VCA output level is at 100% when +5
Volts is supplied to the control input, and is at 0% when 0 Volts is applied. In figure
8a, The VCO oscillates at 500 Hz with a +1 V CV, the VCF has a cutoff frequency of
16 kHz with a +5 V CV, and the VCA produces 100% output with a +5 V. In figure
8b, we reduce the VCO CV to 0 V, a change of 1 Volt. Notice that the frequency is
halved – a change of one octave. The VCF CV is dropped 6 volts to –1 V. Notice
that the cutoff frequency then drops 6 octaves, and at 250 Hz, only allows the
fundamental tone through. Finally, the VCA CV is reduced by 3 Volts. Notice the
amplification is reduced to 40% of the maximum level.
Modulation - Modulation is the use of a CV to affect a voltage-controlled circuit.
Modulation has a source, destination, and amount. This could be as simple as the
filter cutoff of a VCF (a modulation destination) being changed by the front panel
cutoff control (the source), or as complex as mixing multiple CVs together to