A pre-synthesizer electronic instrument, originally created by Maurice Martenot (prononced "mar-ten-oh") in 1928. (The French name translates literally as "Martenot waves".) The original concept of the instrument was that it would allow the performer to play continuously-variable pitches, via a thimble-like ring tied into a loop of string that ran across rollers at the left and right ends of the performance board. The performer inserted his or her right index finger into the ring, and then moved it left and right to control pitch. For control of dynamics, a small drawer opened towards the performer from the left end of the performance board; this contained a spring-loaded glass bar that was pressed down by the left hand to increase volume. Playing was always a two-handed operation as the instrument produced no sound until the glass bar was pressed.

The original circuit, rendered with vacuum tubes (as the transistor had not been invented at the time) consisted of a resonant oscillator circuit which produced a sine wave. Martenot devised it based on radio circuits that he had worked with during World War I. The sliding ring controlled the pitch by turning a multi-plate variable capacitor, similar to that found in radio tuners of the day; the capacitor was part of the oscillator circuit and the change in its value changed the resonant frequency of the circuit, producing changes in the output pitch. The original circuit produced an impure sine wave. The instrument is monophonic.

Because some performers found the ring control difficult to use, Martenot added a keyboard at some point after the original version. The keyboard had a unique feature: the performer could shift the entire keybed assembly left and right a small distance by applying a sideways force to the keys; doing so would shift the pitch of the keyed note. The combination of the ring and keyboard encourages performers to play with a rather wide and fairly quick vibrato, similar to the vibrato that a singer employs. Because of this, the Martenot is noted for vocal-like sounds. Inclusion of a spring reverb added to the ethereal sound that became associated with the instrument.

Later versions incorporated additional oscillator circuits to produce other waveforms: approximations of several waveforms that would later become staples of analog synthesizers, including triangle, square, and pulse waveforms. Controls were added into the left-hand drawer that allowed the individual circuits to be turned on and off, blended, and modified to an extent. This considerably extended the timbral range of the instrument. A further timbral modification capability consisted of the unique loudspeaker array used with the instrument. In addition to a conventional loudspeaker, several types with mechanically resonant elements were available, and the Martenot could be switched from one to another during performance. One type used a metal resonator cone similar to that of a Dobro guitar; another (known as the "palm" for its unique shape) contained an array of resonating strings, somewhat in the manner of a sitar.

Among the instrument's early advocates was classical composer Olivier Messiaen, who wrote significant parts for it in several of his works. Martenot himself frequently performed with the instrument, including several orchestra stints through 1940. Martenot set up a company to produce instruments to order.  It was popular with film and television producers in the '50s and '60s and became associated in particular with science fiction movies and shows.  But it fell out of popularity after 1970, and was produced sporadically until several years after Martenot's death in 1980. More recently, Radiohead's Jonny Greenwood has taken an interest in the Ondes Martenot; he has used one on several Radiohead albums, and he also commissioned Analogue Solutions to build a device called the French Connection. This is a master keyboard which incorporates the Martenot's sliding ring control, and its left-and-right keybed movement feature for pitch control; the device outputs control voltage outputs and is meant to be used with an analog synthesizer. 

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