A Synthaxe ad, from 1985.
A guitar synthesizer device, developed in the UK in the 1980s by Bill Aitken, Mike Dixon, and Tony Sedivy, with some funding from Richard Branson. The Synthaxe does not itself actually include a synthesizer; rather, it is a controller which outputs MIDI, to control an external synthesizer. The Synthaxe has no conventional pickups, and produces no sound by itself.
The Sythaxe design started by discarding the idea of performing pitch to voltage conversion on conventional guitar strings. Rather, the stings were made to be the "row" component of a switching matrix, with segmented frets forming the "column" component. Given the limits of 1980s technology, this was a far more accurate (and faster) way of detecting the notes being played, while still retaining somewhat of a conventional guitar-string feel for the player. It also simplified the process of creating alternate tunings and scales, since the actual vibration of the string contributed nothing to the result. Force sensors detected string bending. Having accomplished this, the designers then decided to separate the function of strumming the string from the neck. Accordingly, on the guitar body, a second set of strings was mounted, and since their only function was to detect strumming, there was no need for them to be made from conventional guitar-string material, so a thick silicone rubber was used. Going further down this route, the designers decided that there was no need for the strumming strings to be in line with the neck; accordingly, the neck was angled upwards at an angle that the designers felt was more ergonomic.
A number of other controls were also mounted on the guitar body. A set of six buttons hinged in a manner like piano keys were the most prominent, along with several other buttons and rocker switches, and a force sensor that was positioned to be pressed on by the heel of the right hand, somewhat in the manner that the guitarist would use the heel of the hand on a conventional guitar's bridge to mute the strings. A "whammy bar" resembled that device on a conventional guitar, but it had no physical coupling with the strings; rather, it produced a control signal.
The Synthaxe's control system provided a huge number of options for how to configure the guitar's various apparatus to produce MIDI note and control messages. Conventionally, plucking one of the strumming strings caused a MIDI Note On message to be sent for the corresponding neck string, with the note value determined by which fret the string was touching, and the velocity determined by the force applied to the strumming string. Since the guitar had no sustain as such, the Note Off would normally be either sent after a fixed interval, or when the performer released the string. However, a huge number of variations were possible. A "touch guitar" mode allowed for playing notes using the neck strings alone, without any strumming, in the manner of a Chapman Stick. Another allowed for the six piano-like keys to be the note initiators, one for each string, and the note held for as long as the key was held down, even if the guitarist took his hand off the neck entirely. The neck string bending sensors could be mapped to alternate functions, as could the whammy bar, which normally sent MIDI Pitch Wheel messages but could easily be remapped to a continuous controller message, or to aftertouch. This also applied to the string force sensors, which could be mapped to polyphonic aftertouch, or to other functions, and the keys and other controls in the guitar body were also re-mappable. Switching to alternate tunings and scales was trivially easy, since the neck strings were not "tuned" and their tension had no effect on the sound. (Accordingly, most performers strung the neck entirely with "high E" strings of .010-.013 diameter, and left then tensioned fairly slack, for a very light touch.)
The Synthaxe guitar itself was accompanied by two other components. A control console handled the setup functions, and contained the MIDI outputs, of which there were eight; different setups could be created to use different outputs, for rapidly switching from one synth to another. A rather large and bulky pedalboard contained foot switches for stepping through setups and activating alternate tunings; additional optional pedals were available that duplicated some of the controls on the console. It was necessary to have the console and the pedalboard in order for the guitar to operate; the latter was somewhat of a bone of contention for some performers who did not use it, but it had to be present because it contained the system's power supply. Other dislikes by owners were the odd spacing of the frets on the neck (which were all evenly spaced, unlike a real guitar), the weight (the guitar was built around a steel frame)
Nonetheless, the Synthaxe made a splash in the market, with its radical shape and fiberglass body (available in four metal-sheen colors, and gained the favor of some high-profile guitarists, including Al DiMeola, Lee Ritonaur,, and Chuck Hammer of David Bowie's band. The most notable user was Allan Holdsworth who bought two, became a renown performer on the instrument, and continued to use them until his death in 2017. The list price of £10,000 made the Synthaxe an article for serious and//or wealthy musicians only; only about 100 were made before the company set up to manufacture them folded in 1988. (At least a few of the final production units were given to employees, in lieu of paychecks.) Nonetheless, the Synthaxe became an icon of the 1980s. Perhaps the most radical user is Roy "Futureman" Wooten, who bought one of Lee Ritonaur's units, and heavily modified it to use to play drum synthesizers. He continues to use his "drumitaur" as one of his primary instruments today.