Editing Theremin (section)

== Operating principles ==
[[File:Block diagram Theremin.svg|thumb|upright=1.5|Block diagram of a theremin. Volume control in blue, pitch control in yellow and audio output in red.]]
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 their hands in the proximity of two metal antennas. While commonly called antennas, they are not used as [[radio antennae]] for receiving or broadcasting radio waves, but rather act as plates of [[capacitor]]s. 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.

The theremin uses the [[heterodyne]] principle to generate an audio signal. The instrument's pitch circuitry includes two [[radio frequency]] [[Electronic oscillator|oscillator]]s set below 500 [[Hertz|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 has significant [[body capacitance]], and thus can be treated as the [[Ground (electricity)|grounded]] plate of a [[variable capacitor]] in an [[LC circuit|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 linearise 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 two or three 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 [[Farad|picofarads]] produces a full octave of frequency shift.<ref>{{cite web|url=http://www.channelroadamps.com/articles/theremin/|title=Channel Road Amplification: Vacuum Tube Theremin|website=channelroadamps.com}}</ref>

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.

In the earliest theremins, the [[radio frequency]] 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.<ref>{{cite web |title=RCA Theremin circuit diagram |url=http://www.pavekmuseum.org/theremin/theop.html |url-status=dead |archive-url=https://web.archive.org/web/20220417112911/http://www.pavekmuseum.org/theremin/theop.html |archive-date=2022-04-17 |website=Pavek Museum of Broadcasting}}</ref> The harmonic [[timbre]] of the output, not being a pure tone, was an important feature of the theremin.<ref>{{cite web|title=How is the Electro-Theremin different from the traditional theremin?|url=http://www.electrotheremin.com/etfaq.htm }}</ref> Theremin's original design included audio frequency series/parallel LC formant filters as well as a 3-winding variable-saturation transformer to control or induce harmonics in the audio output.<ref name="patent" />

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. 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.<ref>{{cite news|last=Vennard |first=Martin |url=https://www.bbc.co.uk/news/magazine-17340257 |title=Leon Theremin: The man and the music machine |publisher=BBC News|date=1929-03-12 |access-date=2012-03-13}}</ref>