POST BURNING SITE

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The theremin uses the heterodyne principle to generate an audio signal. The instrument's pitch circuitry includes two radio frequency oscillators set below 500kc to minimize radio interference. One oscillator operates at a fixed frequency.

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The frequency of the other oscillator is controlled by the performer's distance from the pitch control antenna.

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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.

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Some versions functioned with a change in capacitance between the performer and the instrument in the order of 0.01 picofarads to produce the full span of frequency shift.

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The difference between the frequencies of the two oscillators at each moment allows the creation of a difference tone in the audio frequency range, resulting in audio signals that are amplified and sent to a loudspeaker.

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To control volume, the performer's other hand acts as the grounded plate of another variable capacitor.

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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.

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When a hand approaches the antenna, the natural frequency of that circuit is further lowered, which further detunes the connected oscillator and lowers its resonant plate current.

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The RF plate current of the oscillator is picked up by another winding and used to power the filament of another diode-connected triode, which thus acts as a variable conductance element changing the output amplitude.

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The harmonic timbre of the output, not being a pure tone, was an important feature of the theremin.[13] Theremin's original design included audio frequency series/parallel LC equalization elements as well as a 3-winding variable-saturation transformer to control or induce harmonics in the audio output.[5]

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Modern circuit designs often simplify this circuit and avoid the complexity of two heterodyne oscillators by having a single pitch oscillator, akin to the original theremin's volume circuit

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This approach is usually less stable and cannot generate the low frequencies that a heterodyne oscillator can. Better designs (e.g. Moog, Theremax) may use two pairs of heterodyne oscillators, for both pitch and volume.[14]

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Important in theremin articulation is the use of the volume control antenna.

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Unlike touched instruments, where simply halting play or damping a resonator silences the instrument, the thereminist must "play the rests, as well as the notes", as Clara Rockmore observed.

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Although volume technique is less developed than pitch technique, some thereminists have worked to extend it, especially Pamelia Kurstin with her "walking bass" technique[16] and Rupert Chappelle.