What is MIDI

MIDI is a digital protocol which allows synthesisers and computers to talk to each other. Until recently all MIDI devices had 5 pin DIN sockets for this purpose, lately however MIDI is increasingly being carried over USB. It’s still the same MIDI protocol, but it’s hitching a ride on the USB cable, instead of having a separate one for itself. USB also carries MIDI much faster than it used to go over MIDI cables.

What is CV

CV means control voltage. Before MIDI came along, there were various different means by which synthesisers could “talk” to each other and to the early sequencers. One of the most common was the CV and Gate method. There were however many different varieties of this adopted by different manufacturers but all still using two control signals; one to define the pitch of the note and the other to say whether the note should be playing or not. The differences were in the voltages used to acheive the same result.

The signal defining the pitch of the note was usually labelled in one of the following ways:
Keyboard In
Key Volt In
Oscillator In

There were two main standards defining what voltage would play what note Volt per Octave (V/Oct) and Hertz per Volt (Hz/V).

Volts per Octave
In the Volt per Octave standard, the pitch would rise by one octave for every one volt increase in the control voltage, so typically zero volts would play the bottom C on the instrument, one volt would play the C one octave above that and so on. However not all instruments played a C when zero volts was applied. Most notably the Minimoog would play an F.
This system was used by Moog, Roland, ARP and indeed most manufacturers other than Yamaha and Korg.

Hertz per Volt
In the Hertz per Volt standard the frequency of the note is directly related to the voltage. A pitch of a note goes up one octave when its frequency doubles, meaning that the voltage will have to double for every octave rise. Depending on the footage (octave) selected, nominally one volt gives 1000Hz, two volts 2000Hz and so on. In terms of notes, bottom C would be 0.25 volts, the next C up would be 0.5 volts, then 1V, 2V, 4V, 8V for the following octaves. This system was used mainly by Yamaha and Korg.

Other systems
There were a few other systems around.
1) 1.2 Volt per Octave system, very similar to the V/Oct system detailed above, but where the voltage rises by 0.1 volts for every semitone rise in pitch. To my knowledge only used by EML and Buchla.
2) 0.32 Volts per Octave again as above. Used by EMS (Synthi). Note that Kenton make a special lead to connect Kenton converters to EMS synths. This lead contains all the necessary components to interface correctly with EMS synths. Kenton EMS lead.
3) DCB was not an analogue system at all but a digital system and a forerunner to MIDI. Used by Roland.

What is Gate or S-Trig

Gate or trig is another control voltage which defines when the note should play and is usually labelled in one of the following ways:
Gate In
V-Trig In
S-Trig In
Trig In

There were two main systems, Gate or V-Trig (meaning voltage trigger) and S-Trig (meaning short circuit trigger)

Gate or V-Trig
In the Gate or V-Trig system, zero volts means the note is off, a voltage present means the note is on. Different voltages were used by some manufactuers, but the default setting on Kenton converters will drive most of them. There is also a 15V setting on Kenton converters for synths that require more voltage.

In a true S-Trig system, you shouldn’t have to apply a voltage to the input at all (although you can). Open circuit means the note is off, and a short circuit (connecting the two terminals together) will turn the note on. Kenton converters have an S-Trig setting, but also have settings for S-Trig with pullup. This is needed by Yamaha (and possibly other) synths which do not necessarily turn the note off without a voltage applied.
It is effectively the reverse of the Gate system above.

An input labelled only Trig is usually an S-Trig input, but could possibly be a gate input. If you find that notes play when they should be off, and turn off when they should be on, then try the other setting.

Other inputs and their control

There are several other inputs you may find on your synth, some are more common than others. Most common are for control of the filter cutoff frequency and for clock.

Filter Cutoff
This input will usually be labelled in one of the following ways:
Filter Cutoff
Filter In

An analogue voltage, usually between zero and 10 volts, will change the cutoff frequency of the low pass filter. This usually adds on to the setting on the front panel of the synth, so if this is already at maximum, you won’t hear any effect.

Control this using an Aux (auxiliary) output on a Kenton converter

This input will usually be labelled in one of the following ways:
Arpeggio Clock (or Arp clock)

This is for synchronising the internal arpeggiator or internal sequencer. It will not clock the LFO on any synths that I am aware of.

Control this from the Clock output socket on a Kenton converter (or an Aux switched to clock on some models)

Kenton have written a separate article entirely about MIDI clock.

This input will usually be labelled in one of the following ways:

An analogue voltage, usually between zero and 10 volts, will change the volume of the synth.

Other inputs
Modular systems or semi-modular synths often have other inputs for control of features such as resonance, ring modulation etc.

Control these using the Aux outputs of Kenton converters.