As early analog synths were modular, each synthesizer component (e.g. LFO, VCF) could be connected to another component by means of a patch cable that transmits voltage, with changes in that voltage causing changes to one or more parameters of the component. The most popular example of this technique is a keyboard that transmits a signal with two components:
While the concept of CV (Control Voltage) was fairly standard on analog synths, the implementation was not. For pitch control via CV, there are two prominent implementations:
The following example table demonstrates some notes and their corresponding voltage levels in both implementations (this example uses 1 V/octave and 55 Hz/V):
|Volts per octave scheme, V||1.000||2.000||3.000||3.167||3.250||3.417||3.583||4.000||5.000|
|Hertz per volt, V||1.000||2.000||4.000||4.491||4.745||5.345||6.000||8.000||16.000|
Generally, these two implementations are not critically incompatible; voltage levels used are comparable and there are no other safety mechanisms. So, for example, using a Hz/Volt keyboard to control a Volts/Octave synthesizer would eventually produce some sound, but it will be terribly out of tune.
On synthesizers, this signal is usually labelled as "CV", "VCO In", "Keyboard In", "OSC" or "Keyboard Voltage".
Gate (Trigger) also has two implementations:
Depending on the voltage level used, using the wrong combination of triggering mechanism would either yield no sound at all or would reverse all keypress events (i.e. sound will be produced with no keys pressed and muted on keypress).
On synthesizers, this signal is usually labelled as "Gate", "Trig" or "S-Trig".
Since the publishing of the MIDI standard in 1983, usage of CV/Gate to control synths has decreased dramatically. The most criticized aspect of the CV/gate interface is the allowance of only a single note to sound at a single moment of time.
However, the 1990s saw renewed interest in dated analog synthesizers and various other equipment, notably the Roland TB-303. In order to facilitate synchronization between these older instruments and newer MIDI-enabled equipment, some companies produced several models of CV/Gate-MIDI interfaces. Some models target controlling a single type of synthesizer and have fixed CV and Gate implementation, while some models are more customizable and include methods to switch used implementation.
CV/Gate is also very easy to implement and it remains an easier alternative for homemade / modern modular synthesizers. Also, various equipment, such as stage lighting sometimes uses CV/Gate interface. For example, a strobe light can be controlled using CV to set light intensity or color and Gate to turn an effect on and off. With the advent of non-modular analog synths, the exposure of synth parameters via CV/Gate provided a way to achieve some of the flexibility of modular synths. Some synths also could generate CV/Gate signals and be used to control other synths..
Some software synthesizers emulate control voltages to allow their virtual modules to be controlled as early analog synths were. For example, Propellerheads Reason allows a myriad of connection possibilities with CV, and allows Gate signals to have a "level" rather than a simple on/off (for example, to trigger not just a note, but the velocity of that note)...