User Guide

8
- A lowpass filter is one in which frequencies above the cutoff frequency are removed and all
frequencies below the cutoff are passed through.
- A highpass filter is one in which frequencies below the cutoff frequency are removed and
frequencies above the cutoff are passed through.
- A bandpass filter has two cutoff frequencies that define a frequency band, outside of which the
frequencies are removed (figure 7).
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 makes 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 CV. 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