All musical instruments give musicians some types of control, including control over which notes are played as well as what sounds the instrument produces. With electronic instruments, control signals can be sent from one place to another — either within the instrument or from one instrument to another. When a control signal is received, the sound of the instrument will change in some way.
Today, most electronic instruments are digital, so digital control signals, which are often automated, are the norm. MIDI (the Musical Instrument Digital Interface) provides a widely used method for controlling instruments digitally.
FIG. 1: The Korg MS-20 monophonic synth had a patching matrix, which is reproduced here in Korg''s new software version of the same instrument.
Early electronic instruments, however, were analog, not digital. In an analog synthesizer, the audio signals within the instrument and the control signals are in the form of analog voltages. (A voltage is a type of electrical signal.) The control signals used in an analog synth are called control voltages, often abbreviated as CV.
A number of companies still make control-voltage-based instruments, but they have become specialty items as other types of synthesis are more versatile and affordable. In addition, manufacturers of some digital instruments use the word voltage to refer to digital control signals. (Propellerhead Reason, for example, has rear-panel control jacks that are labeled “CV.”) Though the latter usage is incorrect, it's easy for musicians to understand. In this column, I'll explain the concept of CV, which you should find useful, even if you're using only digital synths.
Turn the Knob
With modular analog synthesizers, separate modules (oscillators, filters, envelope generators, and so on) perform unique tasks yet work together to create the sound we hear. The oscillators generate raw audio signals, the filters filter out portions of the audio signal, and so forth. Audio signals and control voltages are typically routed from one module to another using patch cords. A cord is plugged into an output jack on the front panel of one module and into the input jack on another module (see Fig. 1).
In addition to input and output jacks, most modules have one or more control knobs. For example, the oscillator has knobs for controlling its pitch and waveshape, and the filter has cutoff and resonance knobs for adjusting the filter characteristics. A good way to think of control voltages is that the voltage “turns” the knob for you while your hands are busy doing something else. Although the knob isn't motorized and therefore doesn't physically rotate, the musical result is the same.
For instance, when the voltage level at an oscillator's pitch CV input increases, the oscillator's pitch rises just as if the pitch knob had been turned. When the voltage level drops, the pitch falls. If the module being controlled is a voltage-controlled amplifier (VCA), increasing the level of the CV will cause the amplifier to open further, thus increasing the amplitude of its output. Assuming that an audio signal is passing through the VCA, the output signal will get louder as the voltage increases.
With real analog hardware, a voltage can change smoothly from one value to another. As it increases from 1 to 2V, for instance, it will pass through all of the intervening values — theoretically, an infinite number of them. With digital music systems, signals are always stepped rather than continuous. Because of that, if you turn a knob on a digital synth, you may hear a grainy digital artifact called stair stepping. One reason that musicians prize real analog synths is that their response to control signals can be absolutely smooth.
As with MIDI, you can use control-voltage signals to connect equipment from different manufacturers. The standards for interfacing weren't as well developed in the 1960s as they are now, but many analog synths use a system known as one volt per octave, or 1V/oct. If the CV is applied to an oscillator's pitch-control input in a 1V/oct instrument, increasing the voltage by 1V will raise the frequency of the oscillator by one octave. In other words, the frequency will double. Most new analog synths built today use the 1V/oct standard, but the Buchla 200e system is calibrated to 1.2V/oct so its modules can interface more easily with older Buchla modules, which use that value.
FIG. 2: A gate signal is used to start and stop an ADSR -envelope generator.
To control the operation of envelope generators, analog synthesizers use voltage signals that are called gates and triggers. A gate is a signal that starts when a key is pressed and ends when the key is released. A trigger is a short, sharp spike in the voltage. In practice, the leading edge of a gate signal can usually function as a trigger, so we need to talk only about gates. An envelope generator such as an ADSR is controlled by a gate (see Fig. 2).
Two different standards are used for gates. Instruments from ARP and other companies use a gate signal of 5 to 10V to indicate that a key has been pressed; when the key is released, the voltage falls back to 0. That type of signal is called a voltage trigger. Moog synths use a competing system called switch triggers, or S-triggers, which works the other way around: a continuous signal of 5 or 10V drops to 0 with each key press, and rises when the key is released. As a result, using a Moog keyboard with an ARP envelope generator or vice versa requires an extra piece of hardware — a voltage inverter.
Getting Hooked Up
Three types of cables are commonly used for patching voltage-controlled synths. Some have ¼-inch phone plugs, some have ⅛-inch miniplugs, and some have unshielded banana plugs (see Fig. 3). If you have hardware that uses two different connectors, you may be able to link the modules in a larger system by using adapter jacks, but you'll need to look into grounding and other issues, such as whether you need to convert voltage triggers to S-triggers.
FIG. 3: The three types of connectors most often used for control voltage are 1/4-inch, 1/8-inch, and banana connectors (L to R).
In many voltage-controlled synths, the distinction between control voltages and audio signals is arbitrary: A low-frequency oscillator, for instance, would normally be used as a CV source, but after cranking its frequency up into the audio range (higher than 20 Hz), you could just as easily plug its output voltage into the audio signal path and listen to it. When working this way, it's advisable to keep an eye on whether your synth is introducing a DC offset into the audio signal path. A DC offset is a voltage that rises above or falls below 0V and stays there rather than fluctuating back and forth between positive and negative values. If you send a signal with a DC offset to your speakers, they may not work efficiently, and the dynamic range of your synth will be reduced.
More often, analog synthesists use audio signals for control purposes, rather than using control signals for audio. Modulating the pitch of an oscillator with the output of a resonant filter, for instance, can create an unstable, organic sound that might be perfect for a special effect.
Analog modular synthesizers are large, heavy, and expensive, and they lack such modern refinements as programmable memory. But there's a thrill in creating your own sound by plugging in a bunch of patch cords and twiddling a few knobs. Control voltages threw open the doors of sonic exploration in the 1960s, and when that happened, the world of music changed forever.
Jim Aikin is the author of Power Tools for Synthesizer Programming (Backbeat Books, 2004) and Chords and Harmony (Backbeat Books, 2004). His first synthesizer, purchased in 1980, was a Serge Modular, but these days his favorite instrument is his Jensen 5-string electric cello.