Beginner's Guide to MIDI-CV Converters

Go to...
INTRODUCTION
SOME BASIC POINTS TO LOOK OUT FOR
AUXILIARY CV's - More control
UNDERSTANDING MIDI-CV SPECIFICATIONS
APPLICATIONS OF THE PRO-4`S SPECIAL FEATURES



INTRODUCTION

Modern music is now dominated by the sound of the original analogue synthesizer. Revived in recent years from the 70`s and 80`s, their unbeatable sounds can be heard in all manner of music. Even though this revival has been happening for a few years now, the bubble has not yet burst and people are still rushing out to buy them. They then rush home and pause for a moment thinking, `How the hell can I play this from MIDI?

Most of these older mono-synths were built before MIDI was developed, but many of them do have another method of control, this is called Control Voltage (CV) and GATE. But what are CV and GATE? Most musicians have heard of them, and perhaps know they need some sort of converter to run these old synths from MIDI, but to many musician this is uncharted territory.

In order to control one of these old synthesizers from a modern MIDI instrument or sequencer, the device needed is a MIDI to CV (MIDI-CV) CONVERTER unit. This converts the MIDI signal into the CV and GATE voltages needed by the old equipment.
But it is not all as simple as just CV's and GATEs, analogue synths are as non standard as digital interfaces. There is no just one type of CV and GATE as you will see.

Note;
MIDI-CV converters are not generally the appropriate solution for the control of polyphonic synths. Although a few early poly-synths do have CV and GATE inputs fitted, this only enables you to use one of the voices, so you effectively get a monophonic synth when using it from a converter. The way to control poly-synths using MIDI is to get an internal retrofit.

There are many factors to consider when buying a converter that are not immediately apparent to someone new to analogue synths and MIDI converters. So to help you make the correct choice of MIDI to CV converter, and not regret the few pounds saved by buying a unit that limits you or even does not work, or spending too much on a unit that has features you will never use, then the following text should help make all a bit more clearer.


Go to...
INTRODUCTION
SOME BASIC POINTS TO LOOK OUT FOR
AUXILIARY CV's - More control
UNDERSTANDING MIDI-CV SPECIFICATIONS
APPLICATIONS OF THE PRO-4`S SPECIAL FEATURES



SOME BASIC POINTS TO LOOK OUT FOR

MIDI-CV converters can have up to four different types of outputs used to control analogue synths, usually labelled CV, GATE, S-TRIG and AUX.

Below is a description of what they do (or don't do in some cases).

Pitch - CV outputs (V/oct, Hz/V)

The CV is a voltage that tells the synth what note to play. Most synths use the 1 Volts per Octave (V/oct) pitch scaling system to control the pitch. This means, that each octave is 1V (V=volts) apart (or 0.084V per semitone). For example, middle C (C3) is represented by the voltage 1V,C2 will be 0V, C4 will be 2V, and C5 will be 3V. Synths using this system include Roland SH101, Sequential Circuits Pro 1, ARP Odyssey, Oberheim OB 1.

Some other synths, most notably Korg and Yamaha, use a different pitch scaling system. This is a linear method called Hertz per volt (Hz/V). This means that for the next octave the voltage is doubled, and halved for the octave below. For example, C3 is represented by 1V, C2 will be 0.5V, C4 will be 2V, and C5 will be 4V.

If you use a Hz/V synth with a V/oct pitch output (or vice-versa), the synth will play out of tune as the octave scaling will be wrong.

Some synths that need the Hz/V output - Korg MS10 & MS20, Yamaha CS5 & CS15.

Note;
The Korg Monopoly is an exception. Although all Korg synths use Hz/V scaling, this synth actually uses V/oct scaling. As I mentioned, analogue synths are non standard to say the least.

Gate - (Or S-TRIG)

The GATE signal is a voltage that tells the synth when to play the note. The GATE voltage will usually be a positive voltage when the note is on, and 0V when off.
Different models of synths need different levels of GATE voltages to work. Most synths, such as Roland, will work on low GATE voltages, (as low as 5V as offered on most cheaper, single channel converters). But many other synths require a much higher voltage, maybe above 10V, to gate correctly. If your synth requires a higher GATE voltage, the synth will not play the note if the converter only gives out, say 5V.

Some other synths, like Moog, Korg, and Yamaha, use S-TRIG (Short Trigger) instead of GATE. This signal still tells the note when to play, but it is a different type of signal (electrically). To tell the note to play, the converter will provide a short circuit at it's S-TRIG output (0V), and to turn off the note the output will be open circuit (literally like opening and closing a switch).

A point to watch for; unless you know the synth, it will not always be clear what type of CV and GATE signals are required to play the synth.
For instance, the Korg MS20 requires an S-TRIG signal, but the input is labelled TRIG. Another example is the Yamaha CS5. The pitch input is marked CV, but requires a Hz/V signal. The best way to check is either ask someone who knows, before buying the converter, or if you already own the converter, just try all types of output till the synth works correctly. If you do plug your synth to the wrong outputs, it shouldn't do any harm.

A further point to watch for! Some synths use stereo jacks for the CV and GATE connections. Moog, for instance, use a stereo jack for CV In/Out, and a stereo jack for S-TRIG In/Out. Whether the tip or the ring is in or out is hard to say as Moogs are very non-standard. It varies from synth to synth. Best to call analogue solutions for more information as space is limited here.
Octave who made the Cat and Kitten synths use stereo jacks. CV and GATE outputs are on one stereo jack, and the inputs are on another stereo jack.

As regards the converter itself, outputs will be marked clearly and correctly.

Note;
an output marked CV will be the V/oct standard, and an output marked Hz/V will be the Hz/V standard.

Here's a general guide of what synths hook up to what converter outputs;

Make Synth inputs MIDI-CV outputs
 
Roland CV CV
  Gate Gate
ARP CV CV
  Gate Gate
  Trig Trig
Korg VCO In Hz/V
  Trig S-TRIG
Moog CV (or KYBD or OSC) CV
  S-TRIG S-TRIG
Yamaha CV (or KEY VOLT) Hz/V
  Trig S-TRIG
SCI CV CV
  Gate Gate


Go to...
INTRODUCTION
SOME BASIC POINTS TO LOOK OUT FOR
AUXILIARY CV's - More control
UNDERSTANDING MIDI-CV SPECIFICATIONS
APPLICATIONS OF THE PRO-4`S SPECIAL FEATURES



AUXILIARY CV's - More control

CV's can also be used to control functions such as filter cut-off or volume control. This depends on what control inputs your synth has. Most mono-synths have at least a Filter input, e.g. the Pro 1. Some synths, such as the Minimoog, also have VCA inputs (volume). Synths such as the Korg MS20 and ARP 2600 have even more inputs to control effects such as Pulse Width. Some converters provide additional CV outputs, usually called Auxiliaries (AUX`s), most notably on the Kenton converters (available directly from analogue solutions, call for discounted prices - number at end of text). They are labelled LEVEL on the old Groove converters, and VEL (velocity) on some others like Philip Rees. By plugging the AUX CV into the external control input of the synth, e.g. Filter input, the cut-off frequency can be controlled over MIDI.

The AUX CV's are not controlled by MIDI note numbers. The converter should allow you to set which MIDI controller, e.g. Modulation Wheel, (or even velocity, after-touch, or pitch bend), will control the level of the AUX voltage to control the synth`s extra input. The higher the voltage range of the auxiliary, the more control, and of a wider range of synths you can interface.

Only synths that have the appropriate inputs can be controlled from a MIDI-CV converter.

Basically, the synth at least needs some sort of CV and GATE inputs;

CV's can be labelled CV In, OSC In, Keyboard In, VCO In, Key Volt, etc.

GATEs (and S-TRIG) can be labelled GATE In, S-TRIG, V-TRIG (voltage trigger, same as gate), Trig In, etc.

Any additional inputs may be utilised, like Filter, VCF fcM, VCF, PORTA (portamento), Loudness, VCO, PWM, etc.

Many analogue synths do not have the necessary CV/Gate sockets to enable them to be controlled by a converter, but analogue solutions can fit the necessary sockets to do so, see Basic control input modifications (CV, Gate, VCF control inputs for use with MIDI-CV converters) and Advanced control input/output modifications (modular synth upgrade, trigger inputs/outputs, separate audio outputs, etc. for increased creativity).

Go to...
INTRODUCTION
SOME BASIC POINTS TO LOOK OUT FOR
AUXILIARY CV's - More control
UNDERSTANDING MIDI-CV SPECIFICATIONS
APPLICATIONS OF THE PRO-4`S SPECIAL FEATURES



UNDERSTANDING MIDI-CV SPECIFICATIONS

Read the MIDI-CV converter's specification carefully. Make sure it has the necessary outputs and control parameters needed to control your synth correctly and to it's fullest potential. Also, make sure the converter will not limit you should you expand and buy more or different analogue synths and pre-MIDI equipment. To give you an idea of what should be available on a MIDI-CV converter, I`ll explain to you some of the parameters found on some converters.

Pitch outputs;

CV - (V/oct)

On the PRO-2, this has a range of -1V to +4V (5 octaves), with a pitch bend range of +/-1V (+/- 1 octave).

On the PRO-4, this has a range of -1V to +6V (7 octaves), with a pitch bend range of +/-2V (+/- 2 octaves). It also has transpose (+/- 24 semitone range).

This will control the pitch of all types of V/oct synths, including Roland, Sequential Circuits, Oberheim, Octave, ARP, Moog.

Hz/V (usually optional);

On the PRO-2, this has a range of 0.25V to 14V, (5.5 octaves) with a pitch bend range of +/- 1 octave.

On the PRO-4, this has a range of 0.0625V to approx 13V, (7.5 octaves) with a pitch bend range of +/- 2 octave. It also has transpose (+/- 24 semitone range).

This will control the pitch of all types of Hz/V synths including Yamaha and Korg.


GATE outputs;

This gives +15V for on, and 0V for off.

By providing a +15V GATE output, the PRO-2/4 can control just about all synths that use a GATE input.


S-TRIG & TRIG outputs;

The PRO-4 has 7 triggering options selectable in the software to be sure there is nothing the PRO-4 can't interface with. The PRO-2 has three triggering options (GATE, S-TRIG and TRIG, the more common types).

The two main types of trigger worth mentioning are;

S-Trig - This provides the different type of GATE signal required to control synths such as Yamaha, Korg, and Moog.
Trigger - Used to trigger ARP envelopes or analogue sequencers.

There is no dedicated trigger output on the PRO-2 like there is on the PRO-4, but one of the AUX outputs on the PRO-2 can be configured to be a trigger instead of an auxiliary.


AUX outputs;

These have a wide range of -12.8V to +12.7V on the PRO-4. This will take full use of the synths external control range.

Synths such as Korgs need this additional, negative voltage range to make full use of external control.

The AUX outputs have a range of 0V to +14V on the PRO-2. Still a high range making good use of the synth`s external control range.

The PRO-2 has 2 AUX`s per channel, and the PRO-4 has 8 that can be assigned to be used with any of its channels.


Go to...
INTRODUCTION
SOME BASIC POINTS TO LOOK OUT FOR
AUXILIARY CV's - More control
UNDERSTANDING MIDI-CV SPECIFICATIONS
APPLICATIONS OF THE PRO-4`S SPECIAL FEATURES



APPLICATIONS OF THE PRO-4`S SPECIAL FEATURES

The PRO-4 has many features designed to give you the sort of control over an analogue synth that you take for granted with modern keyboards. Here's a brief overview of some of the features.


Portamento;

Most mono-synths have portamento or glide controls allowing the notes to glide up and down as you play the keyboard. This is a great effect, popular in dance music. But, due to the way most mono synths are designed, when they are played from a MIDI-CV converter the portamento no longer works.

To get around this problem, each of the 4 CV channels on the PRO-4 has a built in portamento so you can still enjoy this effect when playing your synths from MIDI. The portamento time can be set for each of the channels, and also you have a choice of fixed rate or fixed time.
Fixed Time means the time taken to glide between semitones remains constant irrespective of how many semitones the destination note is.
Fixed Rate means the time taken to glide between the first note and the destination note remains constant.


Low Frequency Oscillators (LFOs);

Most mono-synths (well, nearly all) have only one LFO. This means you cannot have the filter cut-off and pulse width modulating at different speeds with different wave shapes. They will all sweep up and down together.
To provide even more flexibility with the PRO-4, it includes 4 independent, MIDI programmable LFOs, synchable to MIDI clock and with a choice of 9 wave shapes. These LFOs can modulate either the CV outputs to provide vibrato effects, or the AUX outputs, which if plugged into filter inputs will provide filter sweeps, if plugged into a VCA (volume) input will provide tremolo, or if plugged into a pulse width input will give you pulse width modulation. All these LFO sweeps will of be independent of the synths own internal LFO.

The PRO-4`s LFOs can be synchronised to MIDI clock. This means the LFO will rise up and fall in time with your music.


TR606/808/CR78/8000 drum machine ports and EDP Wasp port;

The TR606 and TR808 drum machine can be played from MIDI (without the need of a full retrofit) via the PRO-4, or the PRO-2 (optional on the PRO-2). This allows you to play the sounds from a MIDI keyboard, rather than just synchronising them from SYNC 24.

The EDP Wasp can also be played from the PRO-4, and PRO-2 using the special Wasp port (optional on the PRO-2). This synth cannot be played using CV and GATE as it has its own special digital interface.


Clock Pulse (Arpeggio) and SYNC 24 clock outputs;

The clock pulse (arpeggio) outputs can be plugged to the arpeggio or clock inputs of mono-synths` sequencers & arpeggiators, allowing these to be played in time to MIDI clock.

On the PRO-4, the clock pulse output can be inverted, providing an even wider range of units to be connected.

The clock pulse will send out pulses at the same rate as MIDI clock, that is at 24 pulses per quarter note (24ppqn). The rate can be changed as the output has a variable divide ratio. This means that your sequence does not just have to play in time, but you could have it playing twice as fast, half the speed, or even triplets, the choice is yours.

Below is a table of values you can set the divide ratio to to obtain a clock pulse at various musical time intervals:

Note type; Divide ratio;
 
Crotchets 24
Crotchet triplets 16
Quavers 12
Quaver triplets 8
Semiquavers 6
Semiquaver triplets 4
Demisemiquavers 3
Demisemiquaver triplets 2

The SYNC 24 is a synchronising standard developed by Roland in the 80`s to allow their drum machines and sequencers to play in time together. The SYNC 24 output can be used to synchronised instruments such as the TB303, MC202, TR606 or TR808 to MIDI clock. It uses a 5 pin DIN socket (similar to a MIDI socket, although MIDI leads are generally not compatible. A 5 pin DIN lead available from Hi-Fi shops must be used), providing the Roland instrument with a 24ppqn clock pulse, and a start/stop signal voltage (5v for start, 0v for stop).

Some drum machines, for instance some old Korg machines, require a clock rate of 48ppqn. This clock rate is very uncommon so at present no converters support this clock rate. 24ppqn output can be turned into 48ppqn by using a divide by two circuit, but that's another story!


Polyphonic Mode (PRO-4 only);

This mode allows you to use the four CV channels polyphonically. This lets you play 4 separate analogue mono synths as if they were one 4 voice poly synth. Some four 4 voice poly synths such as the Oberheim OB-4 can be fitted with CV/Gate/Filter inputs allowing it to be played polyphonically from the PRO-4.

There is a trick to utilise the PRO-2`s two channels to give two note polyphony. Set one channel's note priority to HIGH, and the other to LOW, then set both channels to the same MIDI channel.

I hope this text has helped explain a few of the mysteries of MIDI-CV converters. Interfacing with analogue synths may seem daunting at first, but watch out for a few of the points I have mentioned, and you should be okay. Good luck!



(published in Sound On Sound, May 1995).

Written by Tom Carpenter, (c) October 1994 a n a l o g u e s o l u t i o n s *

 


Concussor Modular Synthesizer from Analogue Solutions
Tel:+44 1384 35 36 94 Fax:+44 1384 35 36 94
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