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    The theremin (originally known as the etherphone, thereminophone or thereminvox) is an electronic musical instrument controlled without physical contact by the thereminist (performer). It is named after the Westernized name of its Soviet inventor, Léon Theremin, who patented the device in 1928.

    The instrument's controlling section usually consists of two metal antennas that sense the relative position of the thereminist's hands and control oscillators for frequency with one hand, and amplitude (volume) with the other. The electric signals from the theremin are amplified and sent to a loudspeaker.

    Operating principles

    The theremin is distinguished among musical instruments in that it is played without physical contact. The thereminist stands in front of the instrument and moves his or her hands in the proximity of two metal antennas. The distance from one antenna determines frequency (pitch), and the distance from the other controls amplitude (volume). Higher notes are played by moving the hand closer to the pitch antenna. Louder notes are played by moving the hand away from the volume antenna.

    Most frequently, the right hand controls the pitch and the left controls the volume, although some performers reverse this arrangement. Some low-cost theremins use a conventional, knob operated volume control and have only the pitch antenna. While commonly called antennas, they are not used for receiving or broadcasting radio waves, but act as plates of capacitors.

    The theremin uses the heterodyne principle to generate an audio signal. The instrument's pitch circuitry includes two radio frequency oscillators set below 500 kHz to minimize radio interference. One oscillator operates at a fixed frequency. The frequency of the other oscillator is almost identical, and is controlled by the performer's distance from the pitch control antenna.

    The performer's hand acts as the grounded plate (the performer's body being the connection to ground) of a variable capacitor in an L-C (inductance-capacitance) circuit, which is part of the oscillator and determines its frequency. In the simplest designs, the antenna is directly coupled to the tuned circuit of the oscillator and the 'pitch field' that is the change of note with distance, is highly nonlinear, as the capacitance change with distance is far greater near the antenna. In such systems, when the antenna is removed, the oscillator moves up in frequency.

    To partly linearize the pitch field, the antenna may be wired in series with an inductor to form a series tuned circuit, resonating with the parallel combination of the antenna's intrinsic capacitance and the capacitance of the player's hand in proximity to the antenna. This series tuned circuit is then connected in parallel with the parallel tuned circuit of the variable pitch oscillator. With the antenna circuit disconnected, the oscillator is tuned to a frequency slightly higher than the stand alone resonant frequency of the antenna circuit. At that frequency, the antenna and its linearisation coil present an inductive impedance; and when connected, behaves as an inductor in parallel with the oscillator. Thus, connecting the antenna and linearising coil raises the oscillation frequency. Close to the resonant frequency of the antenna circuit, the effective inductance is small, and the effect on the oscillator is greatest; farther from it, the effective inductance is larger, and fractional change on the oscillator is reduced.

    When the hand is distant from the antenna, the resonant frequency of the antenna series circuit is at its highest; i.e., it is closest to the free running frequency of the oscillator, and small changes in antenna capacitance have greatest effect. Under this condition, the effective inductance in the tank circuit is at its minimum and the oscillation frequency is at its maximum. The steepening rate of change of shunt impedance with hand position compensates for the reduced influence of the hand being further away. With careful tuning, a near linear region of pitch field can be created over the central 2 or 3 octaves of operation. Using optimized pitch field linearisation, circuits can be made where a change in capacitance between the performer and the instrument in the order of 0.01 picofarads produces a full octave of frequency shift.

    The mixer produces the audio-range difference between the frequencies of the two oscillators at each moment, which is the tone that is then wave shaped and amplified and sent to a loudspeaker.

    To control volume, the performer's other hand acts as the grounded plate of another variable capacitor. As in the tone circuit, the distance between the performer's hand and the volume control antenna determines the capacitance and hence natural resonant frequency of an LC circuit inductively coupled to another fixed LC oscillator circuit operating at a slightly higher resonant frequency. When a hand approaches the antenna, the natural frequency of that circuit is lowered by the extra capacitance, which detunes the oscillator and lowers its resonant plate current.
    Type: Analog
    Synthesis: Oscillator(s), Subtractive
    Oscillators: 3
    Waveforms: Sine, Square
    Osc Modulation: Glide / Portamento, Input, Optical
    Envelopes: 1
    Envelope Notes::
    + Envelope follower
    Filters: 2
    Types: Band Pass, Low Pass
    Filter Modulation: Envelope, Input
    Polyphony & Tuning
    Polyphony: 1
    Timbrality: 1
    Tuning: Standard
    Modes: Mono
    Case: Desktop
    Controls: Knobs, Switches, Theremin
    Display Type: VU Meter
    Audio Output Connections: Mono Out
    Year Released: 1920
    Design Notes:

    Designed by Lev Sergeyevich Termen, aka Léon Theremin

    The theremin was used in movie soundtracks such as Miklós Rózsa's Spellbound, The Lost Weekend, and Bernard Herrmann's The Day the Earth Stood Still. It has also been used in theme songs for television shows such as the ITV drama Midsomer Murders. This has led to its association with eerie situations. Theremins are also used in concert music (especially avant-garde and 20th- and 21st-century new music) and in popular music genres such as rock.
    Manuals & Documents
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    References & Sources
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