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Zeta Mirror 6

MIDI Guitar Controller

Dave Lockwood gets down to some serious MIDI guitar histrionics...

There is no denying that the history of guitar synthesizers and MIDI guitar controllers is littered with heroic failures! Yet despite the variety of different systems, they can all be seen to fall into one of two broad groups: those which use fret scanning, and those which rely on pitch analysis. In the former category one can include devices such as the legendary SynthAxe, the ill-fated Stepp DG1, and the Yamaha G10 ultrasonic system. By reading pitch from the detection of left hand positioning, these offer imperceptible delay and reliable pitch tracking, but the systems have the disadvantage that they often feel rather alien to the player, and cannot also be used as a normal guitar.

Pitch extraction systems are much simpler, and can even be 'bolted on' as an addition to a regular guitar, giving both familiarity and the versatility of offering the regular guitar sound at the same time. The control signal is derived from analysis of the frequency of string vibration, usually requiring two complete cycles (some systems only use one and a half) before the pitch can be reliably determined. The time taken for this analysis is therefore pitch-dependent, with the result that high notes on the top E string will respond almost instantly, whilst the open bottom E will be taking some 30 milliseconds to respond (even in the fastest of systems). This tends to have a destructive effect on any fast playing, and rhythmic or 'feel' parts become virtually impossible. Pitch bending, however, is usually tracked very well, as the actual pitch of the string is analysed, as opposed to being simulated by mechanical detection. This results in a far more natural feel to bends.

The optimum compromise, it would appear, is a system that uses a 'real' guitar, but which employs left hand positional detection to determine pitch (for minimum delay), and normal pitch extraction techniques to follow bending (delay in the pitch bend information does not seem to compromise technique at all). This is precisely what California-based company Zeta Systems offer in their Mirror 6 MIDI guitar controller.


The Zeta system is based around a very pleasant, if slightly unconventional, guitar. Information from the host of on-board detectors is fed into a 1U rack-mount unit, where all the processing and programming takes place, which also outputs the actual MIDI signal. The rear panel includes the interface for the heavy-duty multicore that connects to the guitar; MIDI In, Out, Thru; 1/4" jacks for Guitar Direct Out, Synth In (L+R), guitar effects loop insert point; and a main stereo output pair, which can carry synth, guitar or both.

The guitar has a radically shaped, sculpted hardwood body with a bolt-on maple neck and 24-fret ebony fingerboard. One of the most striking visual features is the 'hollow' headstock design. I am told this is incorporated in order to reduce unwanted headstock resonance, which may indeed be so, but it certainly also results in a fashionable radical appearance and a reversal of the normal positioning of the in-line machine heads (high quality sealed Gotoh units) which are also rotated 90 degrees from the normal orientation by this configuration. A hex wrench-operated string lock is placed just behind the nut, operating in conjunction with the Floyd Rose licensed, Kahler-built, tremolo bridge unit. This is a highly efficient bridge, based on the original Floyd design, using two knife-edge pivots. Strings are locked at the saddles, with the essential fine-tuners located vertically, as normal.

Apart from the hex pickup used for pitch bend and velocity detection, there are just two conventional pickups: a humbucker in the bridge position, and a neck position single coil. Pickup type varies with the precise model chosen — the Standard has passive EMG Select units, whilst the Deluxe model has active EMGs, with just basic three-way switching provided (no coil-tap, phase reverse, etc). There are four rotary controls: a master volume, synth/guitar blend, guitar tone control, and an assignable MIDI 'soft control'. In addition to the pickup selector switch, output can be switched to guitar, synth, or both, and a synth Mode switch selects Neck mode (for hammering and two-handed tapping), Lead mode for fast linear playing, or Rhythm mode for slower parts.

The combination of the flattish fingerboard, wide nickel silver alloy frets, and comparatively short 24.625" scale length gives the Zeta guitar a pleasant 'soft' feel in the left hand. It is certainly a comfortable instrument to play, even with the supplied strings, which I felt were a rather heavier gauge than necessary. Pickup positioning is slightly compromised by the extension of the fingerboard — the single-coil unit has had to be slanted away from its optimum placement, which robs it of much of its traditional sonic characteristic. The humbucker is particularly 'fruity' however. Assessing the Mirror 6 purely as a conventional guitar, I do miss the extra combinations that would have been made possible by the inclusion of a middle pickup.

The combination of hardware and materials used results in a particularly heavy instrument, which feels strangely inert — in spite of the fact that it sustains forever, you gain the impression that nothing at all is vibrating except the string itself. This is part of the central dilemma of the MIDI guitar: the more complex the resonance, the more character the guitar sound will have, and the more difficult will be the job of detecting precise information to be translated into MIDI data. Whatever the compromises on the guitar side, it is the MIDI performance of this system that matters most, and in that area the Zeta is by some distance the best conventional guitar-based system I have yet encountered.


The essence of the Zeta system is that it recognises that no single set of parameters can ever be sufficient to control all types of synth voice, and every playing style. Consequently the Zeta control unit offers 100 presets, allowing the user to store complete setups of parameters and all their values, optimised for different situations. Four presets are stored in ROM (termed 'Shadow Presets'), offering four different basic 'styles', and using these as a starting point considerably speeds up preset creation.

The editing procedure, using the LCD screen on the control module, has been made as simple as possible. Presets are selected via the standard increment/decrement arrangement of the Value switches, or the Parameter buttons. Activating the Edit switch brings forth a display of the parameters and values that make up the selected Preset. Pressing 'Parameter' in this mode changes the parameter being viewed, whilst the Value buttons now alter the data assigned to those parameters. Edits can be abandoned by pressing 'Edit' twice more, which restores the Preset to its original state; if the alterations have been successful, the new Preset can be named and stored to another location.

A clever facility allows editing from the guitar itself. With Edit selected, and the guitar in Neck mode, the fretboard itself becomes the means of selecting parameters for editing, whilst the on-board 'soft-knob' performs the Value selection. This is an excellent feature, well thought-out and executed, which has the effect of making the edit process seem more closely related to the guitar.

Zeta Systems have recognised that a parameter-based editing system is not the fastest and most intuitive when you need to alter something quickly, and have therefore provided two dedicated switches and a pair of dedicated pots for certain fundamental parameters. The 'Bend' button simply enables or disables the bend function, irrespective of the setting of the many individual bend parameters, whilst 'Velocity' switches velocity response on or off. The two rotary controls govern the Attack and Release characteristic of the detector circuitry (not the audio envelope), ie. how hard a string has to be played to initiate a MIDI note, and how quiet the decay of that string must become before the note will be deemed to have stopped. This is immensely useful to temporarily adjust the system characteristics to a slightly different playing style, or to help you out with a part that you can't actually play cleanly enough!

Settings from the front panel controls override those of the Presets, but are lost when the Preset number is changed, unless stored. This is achieved using the normal Store process — a Preset always includes the current settings of the four dedicated controls. In Edit mode, they too have duplicate functions, automatically calling up their own parameter without the need to step through the whole parameter listing — very clever, and a great time and frustration saver. Presets can be saved and loaded as System Exclusive data, either individually or as a complete bank.


Parameters available to adjust the Zeta system to individual playing styles include Pick Style, which controls the gain of the hexaphonic pickup. Three settings are possible: Hard, Medium, and Soft. Although my right-hand technique is very light, in practice I found the Soft setting produced too much spurious data to make its extra sensitivity worthwhile, and preferred the Medium setting for general playing, and the Hard setting for bass lines and percussion parts.

The Attack setting (determining the string amplitude at which a MIDI Note-On will be sent) is complemented by the Re-Attack parameter, which is a function dedicated to combatting that scourge of the MIDI guitar — spurious retriggering. This is caused when the string vibration after the initial pick is sufficient to exceed the trigger threshold a second time, causing the note to be re-attacked. Systems without a separate re-attack parameter leave you juggling a single threshold between insufficient sensitivity and unwanted re-triggering. The final detector-envelope parameter is Release, which determines the amplitude at which a note is deemed to have ceased.

However 'clean' your technique may be, the basic laws of physics tend to dictate that imparting energy to a string by picking it will also impart some energy to its neighbours (to which it is mechanically coupled by the bridge). The Isolation parameter is a clever software implementation which seeks to suppress strings around the one with the greatest amplitude (which is assumed to be the one you want — the effect is automatically reduced when it detects chords). Like most of the editing parameters, it has a wide enough range to be overdone, but when set correctly it is highly effective in cleaning up the MIDI output.

A closely related problem is dealt with by the Open Sensitivity parameter. It is inevitable that in plucking a string, the pick will come to rest against another. On an ordinary guitar, the low-level noise this generates is lost beneath the sound of the wanted note, but in a MIDI system this can be a major source of unwanted triggering. Similarly, when the damping effect of left-hand pressure ceases as you move to another string, there can be sufficient energy remaining in the string to exceed the Note-On threshold for the open note. The Open Sensitivity parameter provides a separate threshold just for the open strings, so that they can be suitably desensitised (or disabled, if necessary), without compromising normal playing. I found this a vital and highly effective parameter, and was quite happy to trade off having to pick open strings very hard against the dramatic tidying-up effect this function can offer.

Another immutable fact of guitar-playing life is that the actions of fretting a note with the left-hand and picking it with the right do not occur simultaneously. In practice, the former always precedes the latter, albeit by a fraction, however fast you are playing. Effectively, notes are often sounded first by a hammer-on and then picked shortly afterwards, and whilst on a regular guitar the two sounds are indistinguishable, double-triggering will result via MIDI. For MIDI sound modules that implement Legato mode (where the pitch of a note can be changed without re-triggering the attack phase of the envelope) this is not too destructive, although it is wasteful of memory if you are recording into a sequencer. When using modules that do not implement Legato mode, it is extremely beneficial to be able to suppress the re-triggering of a note for a short period after it has been initially triggered. The Zeta system provides a 'window' effect to exclude secondary triggering, and this has a dramatic effect on the quality of the MIDI data, without compromising overall system response.

Legato mode on/off can be stored as one of the parameters of each Preset, as can the choice of four different velocity curves covering Linear, Compressed Linear, Logarithmic, and Exponential responses. Yet more sophisticated control parameters include the Touch function, which prevents open notes from sounding if touched anywhere along the string's length, and the intriguingly named 'Chukka' function. This simulates the guitaristic technique of strumming across strings which are damped but not fretted, producing a scraping effect. The Chukka parameter threshold is actually the number of strings that need to be touched simultaneously to activate the function. Chukka works by sending Note-On events closely followed by Note-Offs for the open strings, and naturally this requires a synth voice with fast attack and release settings in order to work properly.


Good string-bending performance is obviously vital to creating a MIDI guitar controller that can be played with a useful amount of expression, and it is an area in which MIDI is less than ideally equipped. The Zeta system attempts to overcome the inherent limitation of channel-based pitch bend messages with some clever features, optimising the bend mode according to what you play.

In Poly mode (all notes on a single MIDI channel), as every MIDI guitarist will be painfully aware, there can only be a single set of pitch bend messages, so that when more than one string is bent, either all notes are bent by the same amount or, more often, the system automatically disables the pitch bend parameter. The Mirror 6 offers two mutually exclusive bend systems: one tracks the actual pitch of the strings via the hex pickup (using the normal pitch-to-MIDI technique of pitch analysis), whilst the other uses what Zeta insists on calling the 'whammy bar' as a source of MIDI controller information (the data is extracted from bridge movement by means of a 'Hall effect' proximity detector). The really clever bit is that you can specify that when more than a given number of strings are sounding, the system will automatically switch from string-based (pitch extraction) bending to whammy-based bending. This means that, unlike some systems, you are never left with no method of generating bend information.

For seriously guitar-like bend performance, Mode 4 (multi-mono, Legato on, with a separate channel for each string) is a necessity — unfortunately, some older synth modules do not have this capability, and it is not always possible to allocate six channels to the guitar. I have always found pitch extraction systems to have a far more natural feel to their string bend performance than other methods (such as the mechanical displacement sensors of Yamaha's G10). The Zeta is no exception to this; indeed, the combination of virtually instantaneous note response (within one millisecond) and the feel of pitch-extracted bending is something guaranteed to make the guitarist feel comfortable and, for once, genuinely in control of the system. Pre-bending (bending the string before picking it) would appear to present a potential problem, but in practice nothing particularly unpleasant occurs besides a little delay. The Zeta can be a delight to play and, given a MIDI voice of suitable characteristics, great fluidity and expression is possible, way beyond anything achievable from a keyboard.

Further expressive possibilities are offered by other sources of controller information, such as a breath controller input, a pedal source, and (Deluxe model only) an accelerometer (a means of detecting motion, so you can generate vibrato over MIDI by shaking the guitar, just as you can with a real guitar!). There is also the 'soft-knob' assignable controller on the front of the guitar, and each of these control sources can be mapped to any MIDI Continuous Controller destination, and mapped somewhere else in a different Preset (there is no automatic check on duplicate assignments however, so that is the first place to start looking if odd things are happening!). Global Control (an important aid to reducing unnecessary data in Mode 4), where a single set of controller information is transmitted, but applied to all voices, is available for devices that are able to implement it.

The sort of adventurous guitarist who might be interested in this system would certainly want something to do with his feet, and Zeta offers a five-way footswitch controller (FS64). This gives remote control of Bypass (of the MIDI system), Hold mode, Preset selection, Transpose, and a user-definable 'Soft' switch. The software allows these switches considerable flexibility of operation. For example, the Hold mode can either sustain notes indefinitely, or only whilst the pedal is held (like a standard sustain pedal), whilst the switching characteristic can be set to momentary or latching types, according to the application. The Transpose function simply shifts the whole MIDI note output by the preprogrammed number of semitones, whilst Split mode allows some very clever stuff — like a capo that you can play below! Alternate tunings are catered for, although it is also necessary to retune the guitar itself if you are going to use pitch bend.


The Zeta Mirror 6 system undoubtedly works, but like all such systems, it works best on its own terms — the MIDI system must have a number of specific capabilities for some of the advantage not to be lost. The Mirror 6 philosophy appears to be to detect everything possible from the guitar and then apply a powerful logic in screening out information that is not going to be useful to the MIDI data stream. If you set it up properly, the system works very well, but if you get it wrong it can be unplayable [isn't that also true of any standard electric guitar, if badly set up? - Ed.]. Getting it right, however, is not a one-off operation, for different synth voices can require radically different setups, hence the 100 memory locations. The beauty of the Zeta system is that it will let you do it. If you are prepared to spend enough time setting it up, you can always find something that you can work with.

Although the Zeta has wired frets, it nevertheless appears to use standard fretwire — I believe the frets are seated on conductive contacts in the fret slots. Although this must make re-fretting a particularly tricky job, it is comforting to know that it is possible at all. The system is certainly dependent on the strings and frets being kept clean — performance appears to decline rapidly if this is not strictly maintained. Frequent string replacement is probably the best answer, but if you can afford the Zeta, you can probably afford to put new strings on it!


The MIDI guitar market remains both small and underdeveloped at present, and a high-end product like the Mirror 6 will do nothing to change that. Anyone willing to pay this sort of price will do so in recognition that the system is unique — nothing else will do quite the same job, and if you want one then you have got to accept a share of the development costs.

The ultimate challenge in the MIDI guitar market, I believe, is to make a mass-market affordable controller, with no delay, with about the same data output as a mother keyboard, and which is still effective in Poly mode. Until then, those who just want a 'substitute keyboard' are left with a number of overkill solutions — if you disable string-based bending, abandon multi-mono and the guitaristic effects (such as Chukka and AmpTrak), the Zeta performs superbly, but you are wasting half of what you have paid for!

The Zeta can by no means be said to be the perfect solution, but I have little hesitation in concluding that the Mirror 6 is the best MIDI guitar controller I have so far encountered (I have yet to have an extended look at a SynthAxe). Using the unit to its full capability, ie. Mode 4 with AmpTrak in operation, and a sympathetic voice module, you have the feeling that you really are in control of what goes out over MIDI, and that some of the many intricate things that guitarists instinctively do all the time are actually being analysed and translated into worthwhile data. A unique experience, offered by a unique system.


Zeta Mirror 6 DeLuxe £3,299.
Zeta Mirror 6 Standard (no tremolo, passive pickups) £2,999.
AmpTrak System £299.
FS64 Footswitch £169.
Case £115.

Harbourtown Distribution, (Contact Details).


The visible AmpTrak hardware consists simply of a multipin connector which plugs into the back of the control module, terminated at the other end of its short cable with six 1/4" jacks. Where the voice module employed not only runs in multi-mono mode, but also has an individual output for each voice (ie. one per string), the synth outputs can be brought back into the control module (via the multipin) ready to be processed by on-board VCAs, which are controlled by the actual amplitude of the guitar strings. The processed output appears at the Left/Right and Mix outputs on the rear panel.

The result of AmpTrak processing is a far more natural feel to the guitar when triggering MIDI sounds. Techniques such as palm muting, which are impossible to simulate over MIDI, suddenly become feasible as the synth voice tracks the dynamics of the string exactly. For AmpTrak to work at its best, it helps if the synth voices have been edited to remove their natural envelope entirely, thus allowing the VCAs to do all the envelope shaping. It still works on most voices if you don't do this, but the effect is less spectacular.

AmpTrak is a worthwhile addition to the Zeta system for those interested in using MIDI guitar in live performance — the element of instinctive control that it adds allows you to feel in command to a much greater extent.

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


Sound On Sound - Jun 1991

Donated by: Rob Hodder

Gear in this article:

Guitar Synthesizer > Zeta > Mirror 6

Review by Dave Lockwood

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