Theremin - Operating Principles

Operating Principles

The theremin is rare among musical instruments in that it is played without physical contact. The musician 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). 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 frequency, but act as plates in a capacitor.

The theremin uses the heterodyne principle to generate an audio signal. The instrument's pitch circuitry includes two radio frequency oscillators. One oscillator operates at a fixed frequency. The frequency of the other oscillator 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. (Although the capacitance between the performer and the instrument is on the order of picofarads or even hundreds of femtofarads, the circuit design gives a useful frequency shift.) 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.

To control volume, the performer's other hand acts as the grounded plate of another variable capacitor. In this case, the capacitor detunes another oscillator; that detuning is processed to change the attenuation in the amplifier circuit. The distance between the performer's hand and the volume control antenna determines the capacitance, which regulates the theremin's volume.

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.