Infrared Optical Theremin
Technical Disclosure

In response to several Internet inquiries, I have provided the following Technical Disclosure for the Infrared Optical Theremin. (This is a summarized version of the original Disclosure.)

Disclosure of Invention

1. Date of Disclosure: 03-28-96

2. Title of Invention: Optical Theremin

3. Inventor: Arthur Harrison

4. Historic Information

The term "Theremin" has historically been applied to musical instruments with a similar purpose of the invention, in existence since their initial conception circa 1918 by Leon Theremin. The applicable patent number for Theremin's invention is 1,661,058 (February, 1928).

5. Unique Properties of Invention

Whereas the qualities afforded in prior implementations of the Theremin are manifested by utilization of radio frequency phenomena, the new invention achieves these qualities by utilization of optical phenomena. Accordingly, benefits of optical technology applied to Theremin design include the following:

  1. No susceptibility to radio frequency interference from other devices.
  2. No emission of radio frequency energy which may interfere with other devices.
  3. Much less tendency to vary in quality of performance over time and with temperature variations.
  4. No requirement for periodic calibration.
  5. No parametric variation, e.g., pitch and volume shifts, with change of operator body position otherwise caused by variable stray capacitance.
  6. No interaction between pitch and volume control otherwise caused by mutual coupling between hands and antennas.
  7. Improved volume and pitch articulation resulting from sharply defined detection spaces.
  8. Consistent timbre over entire pitch range afforded by the precise synthesis as opposed to the heterodyne derivation of waveforms.
  9. Increased compactness lending enhanced portability.

Whereas the Theremin has been well established in the music performance community, certain attributes of the existing design, namely, timbre quality and ergonomic factors, have become accepted and desirable by virtue of familiarity. To that effect, the invention has been designed to emulate certain features of the original instrument, although utilizing entirely different principles of physics.

6. Purpose of Invention

This disclosure addresses the invention from the perspective of its application as an optical Theremin, a musical instrument operated by moving each hand in the proximity of respective areas on the instrument, one area for the purpose of controlling the frequency of the electric voltage waveform, and the other area for the purpose of controlling the amplitude of the electric voltage waveform, present at the instrument's electrical output terminals.

The basis of the invention, a photoelectric detection scheme employing various techniques for isolation from extraneous sources of light radiation, lends itself to other applications which require the non-tactile control of processes or the detection of objects. Examples include:

  1. Proximity detectors
  2. Position limit mechanisms
  3. Machine safety interlocks
  4. Vehicular anti-collision mechanisms
  5. Automatic door opening mechanisms
  6. Man-machine interfaces for physically disabled persons
  7. Fluid level controls
  8. Reflectivity measuring devices

7. Theory of Operation

Each aforementioned area of the instrument is comprised of a semiconductor infrared energy emitter, also referred to as "light emitting diode," adjacent to a corresponding semiconductor infrared energy detector, also referred to as "phototransistor." The emitter/detector pairs are coplanar and separated approximately 18 inches apart to facilitate the nominal horizontal distance between the operator's hands. Furthermore, each emitter/detector pair is oriented so that the beam of infrared light is emitted upward from the surface of the instrument toward the hands and reflected from the hands downward into the upward facing apertures of the detectors.

Each emitter and detector is equipped with a suitable lens to facilitate the projection and reception of the infrared light in an area immediately above each emitter/detector pair. One pair facilitates sensing the hand's position for the control of pitch, the other for volume. Alternative physical arrangements of the emitter/detector pairs, for example, having them mounted at a right angle with respect to each other, may be facilitated to obtain other forms of the instrument.

8. Circuit Description

The constituents of the electronic circuitry in the invention are represented in the following figure, "Block Diagram of the Optical Theremin." Note that the interconnecting lines and arrows in the figure are representative of the signal process sequence, and not representative of specific electrical connections.

The overall operating principle is as follows:

The clock (A) produces two output voltage waveforms, output phase 1 which is a square wave with a period of 6.25 microseconds and fifty percent duty factor, and output phase 2 which is identical to output phase 1, except shifted 180 degrees in time so that the logical binary values of each phase are always opposed; that is, when one phase has a binary logic value of "1", the other has a binary logic value of "0."

Phase 1 of the clock is connected to each of two light emitting diode drivers (B) which cause current to flow through both the pitch and volume light emitting diodes (C), thus illuminating them whenever the logic value of phase 1 is "1". During this interval of time, infrared light of a wavelength of 890 nanometers projects upward from each light emitting diode and is reflected by the respective hand downward to each phototransistor (D). A voltage proportional to the sum of the intensity of reflected light plus ambient light present from extraneous sources such as artificial room light and sunlight is then present at the output of each phototransistor.

For each emitter/detector pair, the amount of reflected light varies with the distance of the hand. Accordingly, the value of a phototransistor output voltage increases as the hand distance decreases. When the logic value of clock phase 1 is "0", both light emitting diodes are off, and the output voltage of each phototransistor is proportional only to the ambient light present from extraneous sources such as artificial room lighting and sunlight.

There are two primary provisions in the invention for eliminating the effects of ambient light variations. They are embodied in 1) the phototransistor servos, and 2) the synchronous demodulators.

The current which flows through a phototransistor collector-emitter circuit is primarily related to: 1) the amount of light striking the phototransistor's base-emitter junction, and 2) the amount of current flowing into the phototransistor's base. The phototransistor servos (E) establish a fixed direct-current value through each phototransistor which keeps the average emitter currents fixed. Since the servos only respond to relatively long-term averages of the phototransistor outputs, the changes in output voltage resulting from the reflected modulated light is unaffected by the servos. By maintaining fixed average direct-current values via servo operation, the phototransistors' output amplitude response variations to reflected modulated light variations remains independent of ambient light effects.

The two distinct voltages present at the phototransistor outputs, one proportional to the ambient light and the other proportional to the reflected modulated light plus ambient light, are alternately presented to the synchronous demodulators.

Each synchronous demodulator contains two electronic switches which conduct the two voltage values to respective storage capacitors. The switches are controlled by output phase 1 and output phase 2 of the clock. A logic value "1" at output phase 1 closes the switch associated with the interval during which both reflected and ambient light are present. A logic value "1" at output phase 2 closes the switch associated with the interval during which only ambient light is present.

Each storage capacitor thus obtains an average charge value related to the respective light conditions.

In each the pitch and volume circuits, difference amplifiers (G) subtract these two charge values and produce a voltage which is thus primarily related to the intensity of the reflected modulated light in exclusion of the ambient.

The fastest fluctuations of ambient light, occurring at frequencies related to the 60-hertz mains voltage, have little effect on these demodulated voltages which are derived from samples occurring every 6.25 microseconds, a rate which exceeds the ambient light fluctuation period of 8.33 milliseconds by a factor of over one-thousand. The output of the difference amplifier associated with the pitch circuit is connected to the input of voltage controlled oscillator (H).

Voltage variation at the output of said difference amplifier ranges from approximately -50 millivolts to +6.5 volts, corresponding with hand distances ranging from greater than 13 inches to two inches, respectively, from the pitch emitter/detector pair. Within these bounds, the voltage controlled oscillator responds with a zero-volt output for inputs less than zero volts and a continuously variable audio frequency output, directly proportional to input voltage, ranging from 13.75 hertz for zero volts to 1760 hertz for 6 volts or greater.

Since the amount of reflected energy entering the phototransistor aperture diminishes approximately four times for each doubling of total light path distance, the frequency change per unit distance is not linearly incremental. Thus, larger frequency changes are obtained for hand motion closer to the emitter/detector pair than for hand motions further away. The musical pitch scale is likewise not linearly incremental for frequency change per interval; each successive increase of one octave requires a doubling of frequency. Thus, within a scale, larger frequency changes are required to obtain intervals high in pitch than for equivalent intervals which are lower in pitch. In the invention, the two aforementioned relationships form a composite transfer function in which pitch varies approximately linearly with hand distance.

Traditional Theremins utilize radio frequency inductance-capacitance tuned oscillators which shift their output frequency in accordance with variations in capacitance between the user's hand and an antenna. The transfer function which describes the Colpitts oscillator frequency versus capacitance is non-linear, and, as in the invention, the composite transfer functions of capacitance versus frequency combined with the inherent non-linearity of the musical pitch scale provide an overall approximate linearity. Thus, a user familiar with traditional Theremins will make the transition to the invention with little difficulty.

The audio frequency voltage waveform present at the output of the voltage controlled oscillator is approximately sinusoidal in shape. Since it is desirable for the instrument to provide a final waveform which is highly sinusoidal, that is, a waveform which contains only the fundamental frequency component of an audible tone, a further provision of the invention is the voltage controlled filter (J). This filter, which has a single-pole, low pass response, sufficiently reduces the undesired harmonic content of the waveform from the voltage controlled oscillator to produce the desired final audible waveform in the output transducer.

The voltage controlled filter's corner frequency, that is, the frequency at which the filter provides 3 decibels of amplitude reduction, varies in correspondence with the same control voltage applied to the voltage controlled oscillator. Thus, the filter's characteristic is continuously adapted to the instantaneous voltage controlled oscillator frequency.

Timbre variations, that is, the inclusion of harmonics in the output waveform, may be implemented by altering applicable circuit component values in the voltage controlled oscillator and voltage controlled filter, or by utilizing alternative triangular or square waveforms inherent in the voltage controlled oscillator. The instrument may alternately be designed with two or more voltage controlled oscillators, electively set at intervals, to produce a polyphonic waveform output. The frequency range of the audible output may be reduced or increased by altering applicable circuit component values in the voltage controlled oscillator.

The output from the voltage controlled filter is connected to the input of the voltage controlled amplifier (K). The voltage controlled amplifier controls the amplitude of the waveform from the voltage controlled filter in accordance with the value of the voltage present at the output of the volume circuit difference amplifier. An increase of voltage causes a proportional increase of voltage controlled amplifier gain, thus, as the hand moves toward the volume emitter/detector pair, a corresponding increase results in the amplitude of the waveform at the output of the voltage controlled amplifier.

As in the pitch circuit, the amount of reflected energy entering the volume phototransistor aperture diminishes approximately four times for each doubling of total light path distance. Accordingly, the voltage change per unit distance at the output of the volume circuit synchronous demodulator is not linearly incremental. Since, however, the amplitude response of the ear is also non-linear, the two relationships tend to cancel each other, resulting in an average waveform amplitude response of 4.9 decibel per inch over the instrument's amplitude range of 49 decibels.

The output of the voltage controlled amplifier is connected to the input of the power amplifier (L) which provides sufficient gain and an appropriate impedance for the output transducer. The power supply (M) provides direct current to the other blocks.

Disclosure ©1996 by Arthur Harrison

Note: Component-level schematics of the prototype system used for evaluating the concepts described in this disclosure are not available for public dissemination.