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Shadow Guitar To MIDI Converter

Article from Sound On Sound, September 1986

The 'guitar synthesizer' appears to be going through a new lease of life at present, with many manufacturers releasing guitar-based synth controlling devices that make good use of MIDI's powers of expression. Ian Gilby tries out one such model: the Shadow GTM-6.

Ian Gilby tries out the first of a new generation of 'guitar synthesizer'.

There must be thousands of synth players out there who, like me, started out on guitar but were sidetracked somewhere along the line by the attraction of synthesized sounds and consequently took up playing keyboards. Yet most of them I bet would still prefer to be playing guitar if only they had access to the same range of sounds as their keyboard-playing counterparts. Well that is certainly now a possibility with the introduction of the Shadow GTM-6 Guitar-To-MIDI Converter reviewed here.


In terms of previous products. Shadow are probably best known for their range of pickups and transducers that fit onto acoustic guitars enabling them to be amplified. Interestingly enough, the GTM-6 uses a version of Shadow's popular piezo pickup design which incorporates six small piezo crystals that form part of the string saddles of a special bridge assembly. This is intended as a replacement for the existing bridge on whatever guitar you wish to use with the GTM-6, provided it conforms to one of the two types presently available: either a Gibson or a Fender style bridge (which cover most guitars actually). Fitting of the new bridge, need I say, is best carried out by someone experienced in such matters.

The Shadow system I reviewed had the pickup already fitted to Shadow's own guitar - a white, Fender Strat copy whose fingerboard suffered from one or two 'dead-spots', as it happened, which caused a sustained note fretted at that particular spot to 'choke' and cut off abruptly. 'That won't help the pitch-tracking,' I thought...

What the pickup does in simplistic terms is transmit whatever notes are played on the guitar, down a stereo cable to the input of the 2U high, rackmounting GTM-6 module. This is the 'brain' of the Shadow system. It contains the circuitry that converts the pitch of the note(s) played on the guitar into voltages suitable for re-transmission in the form of MIDI data to control any MIDI synthesizer, sampler or expander connected to the GTM-6's rear panel MIDI outputs.

Which basically means that you can 'play' whatever sounds that synth or sampler can produce, from the guitar instead of the keyboard.

The problem is, it isn't that simple! Pitch-to-voltage conversion is a very, very difficult process to achieve properly, its degree of success relying totally on the accuracy with which the original note's pitch is detected in the first place. And the guitar is a notoriously complex generator of pitch information. For a start, the pitch doesn't change in discrete steps as on an electronic keyboard, and the numerous ways in which a guitar string can be bent and plucked cause variations in the harmonic structure which complicates matters for the poor old converter. If any guitar synthesizer or guitar-to-MIDI device is to find favour with guitarists, it must first and foremost be capable of tracking pitch very accurately in order to translate every nuance of expression a guitarist puts into his playing.


To help the Shadow system fulfill such exacting demands, it offers the user control of several important parameters that can affect the accuracy of the pitch-tracking. The first is 'string sensitivity'.

Six small trim pots on the GTM-6's front panel control threshold detectors and allow you to individually set how hard each string must be hit before the unit will recognise it as a valid 'trigger' thus causing a sound to be played on the connected synth. These pots enable the system to be altered to suit variations in personal playing styles and/or the type of strings used on the guitar. If the sensitivity is set too low, for example, no sound will be heard when a note is plucked gently; if it is set too high, then simply lifting a finger off a string may well cause a note to sound. To help optimise the sensitivity, Shadow have included a function that uses the front panel display as a simple level meter. Plucking a string continually at your intended playing strength causes two horizontal lines to appear in the lower, middle or upper part of the display. You just adjust the trim pot accordingly until the lines always appear in the middle position.

If you have the GTM-6 connected to a synth that doesn't respond to velocity information, then once the string sensitivity has been set, how hard you play will have no effect on the synthesized sound you hear - you'll merely be triggering the synthesizer. However, the 'Dynamics' control knob on the GTM-6 is provided for when you do use a velocity-sensitive synth and it determines how much the synth will respond to your playing. If you have velocity set up to control the synth's voice envelope, say, then the harder you pluck the louder the synth sound becomes. I found using velocity added far more scope for expression in my playing.


The second important parameter affecting pitch-tracking accuracy concerns how the GTM-6 responds to string bending. You are offered a choice of three bend modes on the unit: 'Trigger', 'Quantize' and 'Bend'.

'Trigger' mode has been set so that when you pluck a note and bend it, you hear the first note followed by the second one a semitone higher which triggers only when the bent string reaches that pitch. 'Quantize' works in a similar way but does not initiate a new attack to the second note. I found both of these modes hard to come to terms with - they had a disconcerting effect on the way I played and made fast runs sound decidedly mechanical. My main concern was that neither mode responded to or indeed recognised finger vibrato, so I tried the third mode, 'Bend'.

According to the manual this mode (quote) "follows even the slightest bending of your left hand accurately". That may well be true, but it doesn't quite tell the whole truth. What the GTM-6 failed to do was 'translate' the pitch accurately, instead it approximated the note to the nearest recognised pitch level (known as 'quantization' in digital terminology). This would have been acceptable had the GTM-6's pitch resolution been greater so that the steps were not discernible to the ear. Unfortunately, they were and I for one was continually put off by them.

Certain bending techniques used commonly by most rock guitarists also caused the unit problems. For example, striking two strings simultaneously but only bending one, whilst in 'Bend' mode, caused the GTM-6 to revert to its 'Trigger' mode resulting in the bent string suddenly leaping up a semitone. Not exactly what you need for Chuck Berry licks!!

There is a way to overcome this bend problem and that is to use six different synthesizers (with MIDI Thru sockets, mind you) and assign one to each of the guitar controller's strings, though that can be a very expensive solution. I tried it on four strings only using a Casio CZ-101 operating in Mono mode and it did improve matters considerably. The reason this is possible is because the GTM-6 lets you assign a separate MIDI channel to any string in Program mode, by pressing the 'Channel' button and either playing the string in question or typing its number (1-6) into the unit using the front panel keypad, and then typing the required channel number (1-16). If you use a CZ-101 for this, however, don't forget that in Mono mode the four voices are always assigned to adjacent MIDI channels eg. 1/2/3/4 or 3/4/5/6 etc.


Shadow have seen fit to include some very worthwhile additional features on the GTM-6 which increase the versatility of the unit. Each string, for example, can be individually transposed up or down three octaves in semitone steps so that the bottom E and A strings could easily be dropped an octave to allow two-part synthesized bass/melody harmonisations. Likewise you can keep your guitar on standard (EADGBE) tuning but transpose the pitches being sent to the synth to form a new tuning. With this in mind, it would have been useful if the GTM-6 could have stored and recalled any programmed string transpositions so that you could change key on the synth quickly, but this is not possible unfortunately.

The 'Tune' function on the GTM-6 is used, in the first instance, to synchronise the tuning of the guitar's strings with the GTM-6's pitch detection circuits. This is vital if good pitch-tracking is to be obtained because the GTM-6 requires your guitar's tuning as a reference in order to work out what pitch it should raise a semitone bend to on a particular string, for instance. A side benefit of the 'Tune' function is that the GTM-6 can also be used as an electronic tuner with A (440HZ) being the reference.

Built into the Shadow GTM-6 Guitar-To-MIDI Converter is a 'sequencer' of sorts which can, if you wish, be assigned its own MIDI channel and thus its own synthesizer if a spare one is available. It records whatever you play in real-time up to a maximum capacity of 1000 notes and continually loops what you have played. In reality this means that a phrase or chord pattern lasting around 90 seconds can be stored and recalled even if the power has been switched off.

It works much like a 'freeze' facility on a digital delay and uses a supplied footpedal unit to trigger the record and replay functions. What I found annoying about it though was the fact that it appeared that you had to fill up all 90 seconds of memory space otherwise there would be a silent gap where the unfilled memory occurred. What was useful was that you could also play over the top of what you had recorded in the sequencer using either the guitar synth sound or the straight guitar (or both), but within limitations.

I say that, because you are restricted to whatever number of voices the attached synth has. If the sequence plays six-note chords, and your synth is only eight-note polyphonic, then you can only play a maximum of two additional notes over the sequence. Unless you use a separate synth for playback of the sequence that is.

Infinite sustain, something most rock guitarists crave for it seems, is a piece of cake to achieve with the GTM-6 using its special 'Hold' function and footpedal. You simply press down the 'Hold' pedal after you have struck the note or chord you wish to sustain and keep it pressed as long as you want it to sound. Whilst doing this, you are free to play over it using the natural sound of the guitar (which has its own audio output on the GTM-6 rear panel).

That about covers the features apart from mentioning that it is also possible to select program changes remotely via MIDI on the GTM-6 and to chain them together so that you can change the sounds on up to three synthesizers by stepping on the 'Chain' footpedal and advancing the selection. Very useful.


You have to have your wits about you when setting up and playing Shadow's GTM-6, for it is so easy to instantly dismiss it as 'hopeless' when it misses half the notes you've played, only to realise that you have been trying to play very fast runs using a synthesized string sound that has a very slow attack to it! You really do need to choose an appropriate synth patch to match what you are playing.

Having said that, the biggest source of grief with the GTM-6 (apart from the poorly-written user's manual which assumes the guitarist is already familiar with MIDI) is the very noticeable time delay that exists between plucking a string and hearing the synthesized note. I'm not exactly Speedy Gonzales on the fingerboard, yet the delay really bugged me. It actually made me slow down whatever I was playing, causing me to lose time - and that was even with the natural guitar sound mixed in. Playing whilst hearing only the synth guitar on its own was intolerable I'm afraid to say. It was as if the guitar had been put through an ADT unit and the original signal removed...

The GTM-6 rack module is priced at £899 and the pickup at £140 or so, which I reckon is over the top. For that sort of money there should at least be some sort of sound generation facility built in to the GTM-6 so you don't have to go out and buy a synth. That asking price also limits the potential market severely. I shouldn't think there are that many guitarists who own two or more guitars, and if my only guitar were a 1958 Les Paul I certainly wouldn't consider putting a Shadow replacement pickup/bridge assembly on it. Not in a million years!

Oh well, it looks like I'm stuck with keyboards... pity.

(Contact Details)

Also featuring gear in this article

Previous Article in this issue

Stars & Chains

Next article in this issue

Fairlight Series III

Publisher: Sound On Sound - SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

The current copyright owner/s of this content may differ from the originally published copyright notice.
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Sound On Sound - Sep 1986

Donated by: Gavin Livingstone

Gear in this article:

Guitar Synthesizer > Shadow > GTM-6

Review by Ian Gilby

Previous article in this issue:

> Stars & Chains

Next article in this issue:

> Fairlight Series III

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