CV/Gate

CV/Gate (an abbreviation of Control Voltage/Gate) is an analog method of controlling synthesizers, drum machines and other similar equipment with external sequencers. The Control Voltage typically controls pitch and the Gate signal controls note on/off.

This method was widely used in the epoch of analog modular synthesizers, beginning in the 1960s and up to the early 1980s. It was mostly superseded by the MIDI protocol, which is more feature-rich, easier to configure reliably, and more easily supports polyphony. The advent of digital synthesizers also made it possible to store and retrieve voice 'patches' - eliminating patch cables and (for the most part) control voltages. However, numerous companies – including Doepfer, who designed a modular system for Kraftwerk in 1992 Buchla, MOTM, Analogue Systems, and others continue to manufacture modular synthesizers that are increasingly popular and rely primarily on analog CV/Gate signals for communication. Additionally, some recent non-modular synthesizers (such as the Alesis Andromeda) and many effects devices (including the Moogerfooger pedals by Moog as well as many guitar oriented devices) include CV/Gate connectivity. Many modern studios use a hybrid of MIDI and CV/Gate to allow synchronization of older and newer equipment.

Basic usage

In early modular synthesizers, each synthesizer component (e.g., LFO, VCF, etc.) can be connected to another component by means of a patch cable that transmits voltage. Changes in that voltage cause changes to one or more parameters of the component. This frequently involved a keyboard transmitting two types of data (CV and Gate), or control modules such as LFOs and envelope generators transmitting CV data:

• CV (Control Voltage) indicates which note (event) to play: a different voltage for each key pressed; those voltages are typically connected to one or more oscillators, thus producing the different pitches required. Note that such a method implies that the synthesizer is monophonic. CV can also control parameters such as rate, depth or duration of a control modules.
• Gate (sometimes called Trigger) indicates when a note should start, a pulse that is used to trigger an event, typically an ADSR envelope. In the case of triggering a drum machine, a clock signal or LFO square wave could be employed to signal the next beat (or rest).

CV

While the concept of CV (Control Voltage) was fairly standard on analog synthesizers, the implementation was not. For pitch control via CV, there are two prominent implementations:

• Volts per octave. This standard was popularized by (if not created by) Bob Moog in the 1960s; it was widely adopted for control interfacing. One volt represents one octave, so the pitch produced by a voltage of 3 V would be one octave lower than that produced by a voltage of 4 V. Notable followers of this standard include Roland, Moog, Sequential Circuits, Oberheim and ARP. This "standard" also required control modules to carry the source voltage (B+, 5v) on the ring of a TRS jack, with the processed voltage returning on the tip. However, many other manufacturers have used different implementations with voltages including -5 to 5v, 0 to 5v, 0 to 10v with the B+ possibly on the tip. This makes interoperability of modules problematic.
• Hertz per volt. This method (used by most but not all Korg and Yamaha synthesizers) represented an octave of pitch by doubling voltage, so the pitch represented by 2 V would be one octave lower than that represented by 4 V, and one higher than that represented by 1 V.

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):

Note	A1	A2	A3	B3	C3	D3	E3	A4	A5
Volts per octave scheme, V	1.000	2.000	3.000	3.167	3.250	3.417	3.583	4.000	5.000
Frequency, Hz	55	110	220	247	261	294	330	440	880
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. Commercial solutions are available to get round this problem, most notably the Korg MS-02 CV/trigger interface.

On synthesizers, this signal is usually labelled as "CV", "VCO In", "Keyboard In", "OSC" or "Keyboard Voltage".

CV, as applied to control of other parameters usually follows a pattern of minimum to maximum voltage. For example the Moog modular synthesizers also used the 0-5v control voltage for all other parameters. They were represented on the front panel of many synthesizers as knobs, but often a patch bay allowed the input or output of the related CV to synchronize multiple modules together. So, for example, the pitch voltage from a keyboard could also be used to control the rate of an LFO, which could be applied to the volume of the oscillator output, creating a tremolo that became faster as pitch rose. Modules that can be controlled by CV include VCF, VCA, high and low frequency oscillators, ring modulators, sample and hold circuits and noise injection.

Gate

Gate (Trigger) also has two implementations:

• V-Trigger ("voltage trigger", sometimes called "positive trigger"). This method involves keeping normally low voltage (around 0 V) on trigger and producing a fixed positive voltage to indicate a note is on. The amount of voltage required differs from synthesizer to synthesizer, but generally it is from 2 to 10 V. V-Trigger is used by Roland and Sequential Circuits synthesizers, among others.
• S-Trigger ("short circuit trigger", sometimes called "negative trigger"). This one involves keeping voltage high normally, shorting the trigger circuit whenever the note should play. S-Trigger is used by Moog, Korg and Yamaha synthesizers, among others.

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".

Modern usage

Since the publishing of the MIDI standard in 1983, usage of CV/Gate to control synthesizers 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 analog synthesizers and various other equipment. 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 a 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 synthesizers, the exposure of synthesizer parameters via CV/Gate provided a way to achieve some of the flexibility of modular synthesizers. Some synthesizers could also generate CV/Gate signals and be used to control other synthesizers.

One of the main advantages of CV/Gate over MIDI is in the resolution. The fundamental MIDI Control message uses 7 bits or 128 possible steps for resolution. Thirty two Controls per Channel allow MSB and LSB together to specify 14 bits or 16,384 possible steps total resolution. Control Voltage is analogue and by extension infinitely variable. There is less likelihood of hearing the zipper effect or noticeable steps in resolution over large parameter sweeps. Human hearing is especially sensitive to pitch changes, and for this reason MIDI Pitch Bend uses 14 bits fundamentally. Beyond the 512 directly defined 14-bit Controls, MIDI also defines tens of thousands of 14-bit RPN's and NRPN's, but there is no method described for going beyond 14 bits.

A major difference between CV/Gate and MIDI is that in many analogue synthesizers no distinction is made between voltages that represent control and voltages that represent audio. This means audio signals can be used to modify control voltages and vice versa. In MIDI they are separate worlds and there is no easy way to have audio signals modify control parameters.

Some software synthesizers emulate control voltages to allow their virtual modules to be controlled as early analog synthesizers were. For example, Propellerheads Reason allows myriad 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).

In 2009, Mark of the Unicorn released a virtual instrument plug-in, Volta, allowing Mac-based audio workstations with Audio Units support to control some hardware devices. CV control is based on the audio interface line level outputs, and as such only supports a limited number of synthesizers.

In recent years, many guitar effects processors have been designed with CV input. Implementations vary widely and are NOT compatible with one another so it's critical to understand how a manufacturer is producing the CV before attempting to use multiple processors in a system. Moog has facilitated this by producing two interfaces designed to receive and transmit CV in a system, the MP-201 (which includes MIDI) and the CP-251. Examples of effects allowing the use of CV include delays (Electroharmonix DMB and DMTT, Toneczar Echoczar, Line6, Strymon and many others), tremolo (Goatkeeper), Flange (Foxrox Paradox), envelope generators/lowpass filters/ring modulators (Big Briar, WMD) and even distortion (WMD).

See also

• DIN sync
• DCB
• MIDI
• Open Sound Control

External links

• Gates and Triggers tutorial at Synthesizers.com
• Analogue Solutions' Beginner's guide to MIDI-CV conversion — a detailed article on all aspects of MIDI-CV conversion;