Controlling Vintage Synths Via MIDI

Feature | Exploration

Published in SOS March 1995
Bookmark and Share

Vintage analogue synths are more sought after than ever, but their pre-MIDI control systems mean that most need a MIDI to CV converter to fit into today's studios. Kenton Electronics' TOM CARPENTER explains how pre-MIDI instruments operate and what you need to bear in mind when looking for a converter.

 

Modern music is now dominated by the unbeatable sounds of the original analogue synthesizer. 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 on the second-hand market. Many of these people want to integrate their new purchases into a modern music setup, and are thus then faced with the problem of converting the synth to play via MIDI.

Most of the older monosynths were built before MIDI was developed, but many of them do have another method of control -- known as 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 over MIDI, but to many a musician this is uncharted territory. Working for Kenton Electronics, I am forever explaining what customers need to get their old gear up and running, why they need it, how to use it, and so on. To many it's a complete mystery, so this article aims to explain the concepts involved.

CV & GATE

In order to control one of these old synthesizers from a modern MIDI instrument or sequencer, you need a MIDI to CV (MIDI-CV) Converter unit. This transforms the MIDI signal into the Control voltages and Gate pulses needed by old equipment. These two go hand in hand, with a Control voltage telling the synth which note to play, and the Gate pulse telling it when to start and stop the note. However, just to complicate matters, analogue synths are as non-standard as digital interfaces -- there is not simply one type of CV and Gate, as you'll see.

There are many factors to consider when buying a converter, and some of them are not immediately apparent if you're new to analogue synths and MIDI converters. To help you avoid buying a unit that limits you, or spending too much on a unit that has features you'll never use, here are some basic points to look out for.

MIDI-CV converters provide two different types of outputs to control analogue synths: CVs and Gates.

CV OUTPUTS (V/OCT, HZ/V)
The Control Voltage is the 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 octaves are 1V apart (with each semitone being represented by 0.084V). For example, if middle C (C3) is represented by 1V, C2 will represented by 0V, C4 will be represented by 2V, and C5 will be represented by 3V. Synths using this system include:

• Roland SH101
• Sequential Circuits Pro 1
• ARP Odyssey
• Oberheim OB1

Some synths, most notably those made by Korg and Yamaha, use a different pitch scaling system, a linear method called Hertz per volt (Hz/V). Starting from middle C again, this means that for the next octave up, the voltage would be doubled, or halved for the octave below. For example, C3 is represented by 1V, so C2 would be represented by 0.5V, C4 would be represented by 2V, and C5 would be represented by 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. Synths which need the Hz/V output include:

• Korg MS10
• Korg MS20
• Yamaha CS5
• Yamaha CS15

Note that the Korg Mono/Poly is an exception to the rule. Although all other Korg synths use Hz/V scaling, this one actually uses V/oct scaling. As I said, analogue synths are non-standard, to say the least!

CVs can also be used to control functions such as filter cutoff or volume control. This depends on what control inputs your synth has. Most monosynths have at least a Filter input -- the Pro 1, for example. 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. To allow you to access these inputs, some converters provide additional CV outputs, often called Auxiliaries, but also labelled Level on the old Groove converters, and Vel (velocity) on some others, including Philip Rees models. Plugging the Aux CV into the external control input of the synth (the Filter input, for example), allows you to control cut-off frequency over MIDI.

The Aux CVs are not controlled by MIDI note numbers. A MIDI-CV converter should allow you to set which MIDI controller (Modulation Wheel, for example -- or even velocity, aftertouch, or pitch bend), will control the synth's extra input. The higher the voltage range of the auxiliary, the more control you have, and the wider the range of synths you can interface.

• GATES
The Gate signal, as mentioned earlier, is a voltage that tells the synth when to play a note and when to stop playing it. 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 voltage to work. Most synths, such as Roland models, will work on low Gate voltages (as low as 5V, as offered on most cheaper, single-channel converters). But some other synths require a much higher voltage, perhaps higher than 10V, to gate correctly. If your synth requires a higher Gate voltage, it will not play the notes if your MIDI-CV converter only gives out, say 5V.

Some synths -- Moog, Korg, and Yamaha, for example -- use an S-TRIG (Short Trigger) instead of a 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 its S-TRIG output (0V), and to turn off the note, the output will be open circuit (literally like opening and closing a switch).

RULES AND EXCEPTIONS

Unless you know the background of your analogue synth quite well, it will not always be clear what type of CV and Gate signals are required to play it. For instance, the Korg MS20 requires an S-TRIG signal, but the input is labelled TRIG. Another example is the Yamaha CS5, a synth whose pitch input is marked CV (which would normally indicate a V/oct signal), but which actually requires a Hz/V signal. The best way to check is either to ask someone who knows before buying a converter, or if you already own the converter, try all types of output until the synth works correctly. If you do connect your synth to the wrong outputs, it shouldn't do any harm.

A further point to watch for is that some synths use stereo jacks for their 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 jack is wired tip or ring in or out is hard to say, as Moogs are very non-standard and this varies from synth to synth! Octave, who made the Cat and Kitten synths, also use stereo jacks. CV and Gate outputs are on one stereo jack, and the inputs are on another stereo jack. It's best to call the supplier of your MIDI-CV converter for more information, if you think you have such a synth, as space is limited here.

PROBLEMATIC SYNTHS

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. CVs can be labelled CV In, OSC In, Keyboard In, VCO In, Key Volt, and so on. Gates can also be labelled in a variety of ways, including Gate In, S-TRIG, V-TRIG (voltage trigger, same as gate), and Trig In. There are some synths which cannot be connected to a MIDI-CV converter via CV, Gate or Aux Outputs (as they do not have them); these include:

JEN SX1000: the keyboard of this synth is matrix-scanned by a decoder which generates notes directly.

OSC OSCAR: pre-MIDI versions are only suitable for internal MIDI retrofits.

EDP WASP: this synth has its own special digital interface and needs a MIDI-CV converter with a dedicated Wasp mode.

YAMAHA CS01

TEISCO 100P, SH1000 and most other preset synths or monophonic string machines.

 

The following machines cannot be played from a converter, but can be fitted with the necessary CV/GATE sockets to allow them to be used with a converter:

ROLAND TB303

OCTAVE CAT

ROLAND MC202: this does have CV and Gate inputs, but they have limitations which can cause problems when in use with a MIDI-CV converter.

MOOG PRODIGY: early models lacked CV and Gate sockets.

ARP ODYSSEY early models lacked CV and Gate sockets.

KORG MINI 700S

EMS VCS3

ROLAND SH3A

Note, however, that playback from the TB303 and MC202 can be synchronised to MIDI via a converter that also has a Sync 24 clock output.

EXTRA FUNCTIONS

Some MIDI-CV converters provide extra functions to allow you to maximise the expressive potential of your synth. Most monosynths have 'portamento' or glide controls, which allow notes to glide up and down as you play the keyboard. This is a great effect, popular in dance music, but because of the way most monosynths are designed, when they are played from a MIDI-CV converter the portamento facility no longer works. To ensure that you can continue to use portamento when playing your synths over MIDI, look for a converter with portamento built into its CV channels.

Many monosynths (nearly all, in fact) 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. The Kenton Pro4 converter is currently alone in providing a solution to this problem, featuring four independent, MIDI-programmable LFOs, sync-able to MIDI clock and with a choice of nine waveshapes. These LFOs can modulate either the CV outputs to provide vibrato effects, or the Aux outputs, which will provide filter sweeps if plugged into filter inputs, tremolo if plugged into a VCA (volume) input, or pulse width modulation if plugged into a pulse width input. These LFO sweeps will be independent of the synth's own internal LFO.

Some MIDI-CV converters also provide 'clock pulse' and 'Sync 24' outputs for synchronising pre-MIDI sequencers, arpeggiators, and drum machines. Sync 24 is a synchronising standard developed by Roland in the '80s to allow their drum machines and sequencers to play in time together. It can be used to synchronise instruments such as the TB303 Bassline, MC202 Microcomposer, and TR606 or TR808 drum machines to MIDI clock, and uses a 5-pin DIN socket, similar to, though not compatible with, a MIDI socket.

The clock pulse (arpeggio) outputs of a MIDI-CV converter can be plugged into the arpeggio or clock inputs of monosynths' sequencers and arpeggiators, allowing these to be played in time to MIDI clock. The clock pulse will send out pulses at the same rate as MIDI clock -- 24 pulses per quarter note (ppqn), although this can often be varied on the converter. This means that you could have your sequencer or arpeggiator playing twice as fast, at half-speed, or with a triplet feel.

I hope this article 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 OK. Good luck!

 

PRE-MIDI POLYS

MIDI-CV converters are not generally the appropriate solution for the control of polyphonic synths. Although a few early polysynths do have CV and Gate inputs fitted, this only enables you to use one of the voices, so your polysynth effectively becomes a monophonic synth when you're using it from a converter. The way to control polysynths using MIDI is through an internal retrofit.

 

CONNECTING YOUR SYNTH TO A CONVERTER

Here's a general guide to which synths hook up to which MIDI-CV converter outputs:

SYNTH
MANUFACTURER

SYNTH
INPUTS

CONVERTER
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 (SEQUENTIAL)

CV

CV

 

Gate

Gate

NB: An output marked CV will be to the V/oct standard, and an output marked Hz/V will be to the Hz/V standard.

 

BOX OF TRICKS

The Kenton Pro4 has a unique mode which allows you to use its four CV channels polyphonically. This lets you play four separate analogue monosynths as if they were one four-voice polysynth. Some four-voice poly synths, such as the Oberheim OB4, can be fitted with CV/Gate/Filter inputs, allowing them to be played polyphonically from the Pro4.

There's also a trick to utilise the Pro2's two channels to give two-note polyphony. Set one channel's note priority to HIGH, and the other to LOW, then set both CV channels to the same MIDI channel.

 

CONTACTS

The following companies currently make or are about to launch MIDI-CV converters:

• CP Technology, Red Lion Yard, Market Place, Blanford, Dorset DT11 7EB. Tel: 0202 885592. Phone or write for an information pack.

• Kenton Electronics, 12 Tolworth Rise South, Surbiton, Surrey KT5 9NN. Tel: 081 337 0333. Kenton also install internal MIDI retrofits to pre-MIDI instruments.

• Philip Rees Modern Music Technology, Unit 2, Clarendon Court, Park Street, Charlbury, Oxford OX7 3PT. Tel: 0608 811215.

• Technical Leisure, Unit 10, 98 Goldstone Villas, Hove, East Sussex BN3 3RU. Tel: 0273 822342/708486.

A few converters may also be available on the second-hand market. Names to look out for include the Vesta Kozo MD1, Groove Electronics, Cision and possibly dBM. If you're lucky, you might even come across the classic four-channel Roland MPU101 converter.

 

Home | Search | News | Current Issue | Tablet Mag | Articles | Forum | Blog | Subscribe | Shop | Readers Ads

Advertise | Information | Privacy Policy | Support | Login Help

 

Email: Contact SOS

Telephone: +44 (0)1954 789888

Fax: +44 (0)1954 789895

Registered Office: Media House, Trafalgar Way, Bar Hill, Cambridge, CB23 8SQ, United Kingdom.

Sound On Sound Ltd is registered in England and Wales.

Company number: 3015516 VAT number: GB 638 5307 26

         

All contents copyright © SOS Publications Group and/or its licensors, 1985-2014. All rights reserved.
The contents of this article are subject to worldwide copyright protection and reproduction in whole or part, whether mechanical or electronic, is expressly forbidden without the prior written consent of the Publishers. Great care has been taken to ensure accuracy in the preparation of this article but neither Sound On Sound Limited nor the publishers can be held responsible for its contents. The views expressed are those of the contributors and not necessarily those of the publishers.

Web site designed & maintained by PB Associates | SOS | Relative Media