My first significant exposure to the theremin was around 1974 when a college friend, persuing a degree in music, showed me the unit he had purchased. It was a "Maestro," one of a few commercial types available at the time. Some years later, during his visit up north, he brought the same theremin to my house and kindly allowed me to "dissect" it. At the time, I only had a vague idea about how they worked, and some of my notions were confirmed when I saw the constituents of a heterodyne system: transistorized Colpitts oscillators and a voltage controlled amplifier IC (the only IC in the set). I began thinking about designing theremins at that time, although some more years would pass before I ever actually built them.
My first idea was to make a sonar system with the types of ultrasonic transducers used for intrusion detectors. I did a little research into commercially available sonar ranging modules (used for machine position sensors) and determined that purchasing such modules would probably be cheaper than designing them. I also saw, by looking at the specifications for these units, some possible shortcomings to the ultrasonic approach, although I may someday do some experiments which prove otherwise. The details of this subject are too lengthy to relate for the purpose of this installment, so perhaps I'll return to ultrasonics some other time. (Author's note: Later experiments showed that the wavelengths for ultrasonic systems are too short for obtaining a convenient gestural interface.)
In 1994, I was designing a circuit for an infrared remote control when I noticed how nicely the amplitude of a signal received from an infrared light emitting diode (LED) correlated with distance. At once, I thought about the theremin and how infrared techniques may be applied to make one. I configured an infrared emitter next to a detector with both devices aimed upward. This resulted in a reflective sensor in which the detector output voltage would rise as an object was brought near them. If that object happened to be a hand, the basis for a different type of theremin was apparent.
Traditional theremins utilize the principle of heterodyning where the presence of the hand (or virtually any object) changes the resonant frequency of a tuned circuit in a radio frequency oscillator. The sound output is the "beat" frequency which results when that oscillator is mixed with a fixed frequency oscillator. According to a historical account of the invention, Theremin and his contemporaries noticed the continuous tones which emitted from mis-aligned A.M. radios, a phenomenon also resulting from the heterodyne process, which Theremin ingeniously put to work in his invention. To my knowledge, every successive implementation of the instrument used the heterodyne principle.
The infrared system which I devised has little in common with the heterodyne technique. The emitter-detector pair, for instance, does not in itself produce an audio tone. To achieve this, the detector output is converted into an audible waveform via a voltage controlled oscillator (VCO). In this respect, the infrared design is really a "synthesized" theremin. In addition to the VCO, several other elements in the infrared design are used to eliminate variables from ambient light. If Theremin had access to circuits such as VCOs in the '20s, his invention may have been designed differently. However, it was not until thirty years later that transistors spurred the development of such relatively sophisticated techniques. It is a credit to Theremin that he was able to achieve his results with only simple vacuum tube circuits.
In future installments, I'll compare these varied techniques and their performance.
Arthur Harrison
23 February 1996
©1996 by Arthur Harrison