Theremin

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Theremin

What is it?

With the name Theremin, is indicated a really particular instrument, invented by the russian Mr. Lev Sergeyevich Termen on 1919. This instrument is also known as Thereminvox and Aetherphone. Basically it is a radio transmitter on Long Wave range (30kHz - 300kHz) with two antennas and some control.

How does it work?

The principle of work is based on heterodyne. Basically, there are two oscillators working at a relatively high frequency (around 160 kHz). This frequency is enough high to be not heard by the human ear. Those oscillators work on exactly the same frequency, one of them connected to a vertical antenna, the other one not. The output of those oscillators are sent to a mixer circuit that detects each difference in frequency between them. If an oscillator changes its own frequency, to the output of the mixer, there will be the difference between the two oscillators which normally falls down into audio range.

Theremin is made a manner that if you put your hand close to its vertical antenna, the frequency of an oscillator is changed and the sound of the difference between the two oscillators will be produced through a speaker. The closer your hand is to the antenna, the higher the pitch and vice versa. The further away, the lower the pitch because of the smaller difference in variations of capacitance between your hand and the vertical antenna.

Practical and Technical Example

Pitch

Suppose that in rest condition, both oscillators are working on 160 kHz. At that times, you put close to the antenna your hand, and the oscillator connected to this aerial, changes its own frequency to 160.44 kHz. Normally mixer stage generates two frequencies: the SUM [160khHz + 160.44 kHz] and the DIFFERENCE [160.44kHz - 160kHz]. This is called: HETERODYNE. At the output of mixer, circuitry lets go out only the difference due a low pass filter circuit, so that to our loudspeaker is sent a frequency of 440 Hz, which is our standard A natural pitch. Moving your hand toward the antenna, that difference increases so that you go higher in pitch and vice versa, producing all the scales' notes exactly as in the violin or any other fretless string instrument.

Lev Termin imitated the movement of an orchestra's Director. Generally speaking: when hands are low, less louder is the sound from the orchestra and vice versa. He had applied this basic concept to his instrument. But how is it obtained ?

Volume

There is a third oscillator in the Theremin, connected to a second antenna. To the output of this oscillator is applied a Band Pass Filter, and the output of this filter is connected to a VCA [Voltage Controlled Amplifier]. It's just an amplifier to which volume is controlled by a voltage [DC]. The frequency on which this oscillator works is exactly the frequency of the filter. Being turned each other, voltage is detected to the output of the filter, and volume is to the maximum. When you move your hand very closest to the second antenna, variation of capacitance untunes the oscillator from the filter so far that not any voltage is detected to the filter output such that no audio can be heard in the loudspeaker. The more you move your hand away from the antenna, the more oscillator's frequency comes back in tune and the higher the voltage to the filter's output is. Thus, the louder the audio is from the loudspeaker.

Practice

Studying the Theremin requires practice as much as any other instrument, especially fretless ones (Violin, Viola, ect.) More than almost any other instrument, it requires a very keen ear and it can create a very magical sound atmosphere when played in just the right way. One who plays Theremin is called a Thereminist

External References

You can watch some video here in order to understand and better appreciate the Theremin
Debussy's Clair de Lune on solo Theremin
"The Swan" by Saint-Saëns
The Legend of Zelda
Don't worry be happy
1st Lesson