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Kenton MIDI Retrofits

With analogue technology enjoying a renaissance, there's a growing demand for MIDI updates to old equipment. Vic Lennard talks to Kenton Electronics about the Ins and Outs of retrofitting.


TODAY, WE'RE USED to regarding many pre-MIDI, analogue synthesisers as "classic" instruments. Monophonic synths such as the ARP Odyssey and the evergreen Minimoog certainly deserve such a tag, and some of the later polyphonic synths like Roland's Jupiter 8 and Sequential Circuit's Prophet 5 are often viewed in the same way.

Sadly, classic qualities have rarely stood in the way of progress, and the appearance of MIDI sounded the death knell for many an analogue synth's development. Some manufacturers incorporated MIDI in existing synths, but even this failed to insure them against the arrival of FM synthesis. These days, wonderful but outdated instruments often change hands for a small fraction of their original asking price. Yet they remain firm favourites with musicians working in all areas of music - as demonstrated by the popularity of MT's own retrospective features on instruments like the Yamaha CS80, Sequential Circuits Pro One and the Roland Bassline.

Happily, there are methods of admitting non-MIDI synths to the world of MIDI. The first is to use a MIDI/CV interface; this sits between the MIDI controller and the synth, and converts incoming MIDI information into Control Voltage (CV) and Gate signals which the synth can understand. This has various disadvantages but the principal ones are that MIDI note information is all that can usually be interpreted; no velocity, pitchbend or patch changes. Neither will a MIDI/CV unit provide a MIDI Out from a non-MIDI synth.

The second option is to delve into the workings of the synth and incorporate a MIDI interface within the synth's circuitry.


THE HEART OF a MIDI instrument is usually a software program held in ROM. This controls the operation of the device, and to alter it means rewriting the software. This is usually either impractical due to lack of information, or likely to lead to response delays due to the extra software routines. The alternative is to create a circuit which operates between the instrument's software and the outside world. This circuit will also contain software, and can alter most aspects of performance. It also has the advantage of being easily updated if necessary. The inclusion of another processor will mean that the extra facilities that MIDI is adding will not slow down the working of the synth.

Modifications to mono synths generally provide MIDI In and Thru ports, and response to MIDI information for notes and velocity, modulation wheel (MIDI controller #1), foot controller (#4), sustain pedal (#64), pitchbend and channel aftertouch. Most polyphonic synth retrofits also recognise program changes and incorporate a MIDI Out capable of transmitting notes and program changes. Drum machine modifications give MIDI In, Out and Thru sockets, MIDI clock and start/stop/continue commands and receive/transmit notes. Most drum machine modifications also respond to note velocity.

Each company which handles retrofitting does so in a different way - we're dealing with a computer program running from a ROM and the commands within that program are proprietary. Getting specific, let's look at what Kenton Electronics' line of retrofits offer.

Kenton Electronics are one of the companies currently specialising in MIDI retrofits. Kenton's John Price ventured into retrofitting when he wanted to bring MIDI to his Oberheim OB-X. The only company prepared to carry out such a modification at the time wanted £500 for an interface incorporating only notes and running in Omni On mode. Having previously designed his own interface for the Oberheim and a Hammond organ, Price chose to do the job himself. After many months spent designing circuitry and writing software, he decided to offer this service to others, and officially opened Kenton Electronics in 1986.

The main point of a Kenton conversion is to offer the relevant MIDI sockets on a non-MIDI synth. Incoming MIDI data arrives at the retrofit board which converts it into a form that can be handled by the synth. Physically, the computer-designed circuit boards plug in between the keyboard and the sound source of the synth.

Consider an incoming MIDI note on. It arrives at a Universal Asynchronous Receiver Transmitter (UART) which immediately informs a Z80 microprocessor that there's data to be processed. This is termed 'under interrupt' because momentarily the processor deals with this particular piece of information, stopping all other activities. The lack of buffering ensures the fastest possible response time and is helped by the fact that all instructions for the Z80 processor are written in machine code with very few subroutines. In the case of an analogue synth, the note value will be converted into a CV and will enter the synth's circuits at a relevant point. For a digital synth, the note value will be held in a latch system and then read by the synth as though it had come from its own keyboard.

A second circuit board is dedicated to the analogue side: pitchbend, modulation, aftertouch and note velocity are converted into a form which can be used by the synth via a Digital to Analogue converter (DAC). A sample and hold circuit then converts this into a control voltage which will either be sent directly to the synth or via a voltage controlled amplifier (VCA). For example, pitchbend information might be summed with the master tune; modulation information would take a similar path as both this and pitchbend affect the pitch of the oscillators. MIDI note velocity will also be output as a voltage which will either be sent to the VCA, to change the level of the note, or to the filter, to change the timbre.

MIDI aftertouch can be treated in various ways. By default, it will be summed into the MIDI modulation but can also be sent along the path to the synth's VCA or VCF. The routing of aftertouch on some synths can be rather special - for instance the Memorymoog has rear sockets for two foot controllers which can be wired into the analogue board. In this way aftertouch can be assigned to pitchbend, VCA for volume, LFO for modulation, VCF for filter or the pitch of oscillator 2.


A MIDI RETROFIT has to be easy and intuitive to use but must also offer enough facilities to make its cost worthwhile. Additionally, you don't want a treasured instrument turned into something that looks like a botched brain operation. A Kenton Electronics retrofit looks as though it's part of the original synth design with only the MIDI sockets being visible along with a small red push button.

Pressing this button twice accesses Program mode. From here you can alter many of the retrofit facilities; changes are saved in memory. Once Program mode has been entered, each key on the keyboard has a specific purpose - for instance, pressing a key in the range between C1 (note #36) and D#2 (note #51) will select the receive MIDI channel (1-16 respectively). Other facilities include the MIDI transmit channel for polyphonic synths and in transmit mode you can set the fixed velocity for key on and key off independently. You can set up the synth to receive or ignore MIDI patch changes, mod wheel, pitchbend and aftertouch. The information which is to be sent to the amplifier and filter of the synth can also be chosen from MIDI note velocity, MIDI controller #4 (foot controller), aftertouch (instead of modulation) or nothing. When all settings are complete, C6 (note #96) acts as an "enter" key; the synth then returns to normal mode.

Pressing the red button once puts you into Patch Change mode. Each key then selects a program change after which you're returned to normal mode to carry on playing. If you press once and hold for a moment you enter Transpose mode where middle C plays continuously until you press another key on the keyboard which will then become the new middle C. Again, you are immediately returned to play mode.

The red button is on the rear of the synth which could make it difficult to access under certain circumstances. To alleviate this problem the button can be operated remotely by using MIDI controller #95 for Program mode and controller #94 for Transpose mode. This was important on the older Kenton boards which were in Omni On mode when first turned on and had to have the MIDI receive channel reset each time. In fact, there was a desk accessory on the Atari ST which let you set this channel. Now EEPROMs (Electrically Eraseable PROMs) are used to store all settings - no batteries to run out.


IN MUCH THE same way as every picture tells a different story, so does each synth. The main differences are sorting out where to patch the information created by the retrofit.

The OB-X went through at least eight ROM revisions, several of which are incompatible with each other. Pitchbend has to be injected at a point where it can be summed with other like information and this is where the master tune and pitchbend wheel meet. Modulation has to be found a similar place to enter the synth, and in this case mod information via the analogue board VCA is mixed with the wiper of the mod wheel.

Yamaha synths like the PF15, a DX7-sounding piano with an 88-note weighted keyboard, use a system where the synth information is encoded digitally. For instance, an eight-bit code is used for velocity as opposed to the seven bits used by MIDI. The output of this chip has to be read, converted into MIDI and any additional MIDI data incorporated.

On the PPG Wave, the internal and MIDI pitchbend data has to be summed, which necessitates restricting the range, otherwise the sum of the two elements can go beyond the top of the range acceptable by the PPG and cause the synth to crash. To be able to access glide on the Minimoog you can't use the control voltage input, while the Minimoog's gate has around a 30 millisecond delay due to the presence of a large capacitor. Korg's Poly 6/61 has its own micro-controller which means that the signal lines are being used for control information as well as notes.

A lot of the quirks are found through painstakingly monitoring various points on the circuit. Retrofitting is certainly a job that requires patience.


THE KENTON MODIFICATION is available for all the classic synths made by Roland, Sequential Circuits, Moog, ARP, Oberheim, Korg and Yamaha as well as the Rhodes Chroma and Linn drum machines. Recent additions include many organs such as tonewheel Hammonds, Yamaha B-, C-, D- and E-series, also FE, FS and FX ranges and Technics G7. In fact, Kenton will give a quote on modifying any non-MIDI device. Costs are around £150-£180 with certain obscure instruments being a little more expensive to modify.

How well do they work? I had the good fortune to play a Minimoog from a DX7II keyboard and it was an unexpected delight. The experience of having MIDI note velocity controlling the level of the volume via the VCA and the aftertouch operating the filter, while using the glide facility on the Minimoog via the Yamaha keyboard exceeds the vocabulary of the digitally-dominated hi-tech musician.

Finally, it's worth mentioning that, while Kenton are happy to work for you and I, they've also been of service to a wide variety of pro acts: did you read the MT story on Marillion's Steve Hogarth and his MIDI gloves? Notes, pitchbend, modulation, aftertouch, program changes and sustain information all generated from a pair of gloves - and all designed and built by Kenton Electronics.

Kenton Electronics can be contacted at: (Contact Details).

Previous Article in this issue

Ensoniq EPS16 Plus

Next article in this issue

Alesis SR16

Music Technology - Copyright: Music Maker Publications (UK), Future Publishing.


Music Technology - Feb 1991

Feature by Vic Lennard

Previous article in this issue:

> Ensoniq EPS16 Plus

Next article in this issue:

> Alesis SR16

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