The Harmonic Tone Generator: One of the First Analog Voltage-Controlled Synthesizers

[Photo of JWB & HTG circuit boards] [Photo of HTG in a rack] [Photo of HTG in a rack] [Photo of HTG in a rack]

             JB and circuit boards              HTG in rack                      Harmonic Tone Generator                      Harmonic Envelope Controls

James Beauchamp invented the Harmonic Tone Generator (HTG) in 1964 under the direction of Lejaren Hiller in the Experimental Music Studio at the University of Illinois at Urbana-Champaign. The instrument synthesized six exact harmonics with variable fundamental frequency from 0 to 2000 Hz. The amplitudes of the six harmonics, the fundamental frequency, and the phase of the second harmonic were programmed by voltage control. The fundamental frequency (pitch) was controlled by an external keyboard or generators to provide vibrato and other effects. Control of amplitude was provided by special envelope generators or external generators or even by microphone or prerecorded sounds.

The harmonics were derived by generating pairs of ultrasonic frequencies which were nonlinearly mixed to produce audio difference frequencies. That is to say, one set of frequencies, 50 KHz, 100 KHz, ..., 300 KHz, was fixed. Another set, 50-52 KHz, 100-104 KHz, ..., 300-312 KHz, was variable. When 50 and 50-52 KHz, etc., were mixed, the sine tones 0-2 KHz, ... were derived. Harmonics were generated by full-wave rectification (even harmonics) and square wave chopping (odd harmonics), followed by band pass filtering to separate the harmonics.

The envelope generators consisted of variable delays and attack/decay circuits. In response to a trigger signal from the keyboard, after a programmed delay, the envelope generator would either rise and then go into an immediate decay while the key is depressed or it would rise (slowly) and decay after the key is released. Having the upper harmonics delayed with respect to the lower ones gave an interesting effect.

Because the amplitude controls were "bipolar" (i.e., either positive or negative controls were effective), the instrument could serve as a multi-frequency "ring modulator", which was especially useful when the controls were derived from a voice or musical instrument. The frequency control was also bipolar and was capable of producing rich sound spectra when the control was taken from a sine generator operating at frequencies ranging from 20 Hz through several hundred Hz. This FM effect was very popular for producing interesting sounds for electronic music compositions.

During the spring of 1964 Arthur Maddox and Beauchamp produced a series of three improvizations for Harmonic Tone Generator alone and five improvizations for HTG and acoustic instruments. Click here to hear these improvizations.

A paper on the Harmonic Tone Generator was given at the 1964 fall meeting of the Audio Engineering Society. It was published in the Journal of Audio Engineering Society in October, 1966 as "Additive Synthesis of Harmonic Musical Tones".

Several electronic music compositions utilized the Harmonic Tone Generator as their main source of electronic sounds. Among them are:

Herbert Brun, "Futility, 1964"(1964)

Kenneth Gaburo, "Antiphonics III", "Lemon Drops"(1965), "Hydrogen Jukebox", and "For Harry"(1966)

Lejaren Hiller, "Machine Music"(1964) and "A Triptych for Hieronymus"(1966)

Salvatore Martirano, "Underworld"(1965)

In January, 2015 Scott Schwartz, director of the Sousa Archives on the University of Illinois Urbana-Champaign campus, announced that the museum had acquired the original analog Harmonic Tone Generator for its collection. Meanwhile, Mark Smart, an electronics professional at the Dept. of Electrical and Computer Engineering at the UIUC, created a digital simulation of the HTG. The original HTG and its simulation have been displayed at the museum since September, 2015. The simulation version is operable by visitors to the museum.

(1) James W. Beauchamp, "The Harmonic Tone Generator, A Voltage-Controlled Device for Additive Synthesis of Audio Harmonic Spectra", Audio Engineering Society Preprint No. 323 (1964).
(2) James W. Beauchamp, "Electronic Instrumentation for the Synthesis, Control, and Analysis of Harmonic Musical Tones", Doctoral Thesis, University of Illinois at Urbana-Champaign, 1965.
(3) J. W. Beauchamp, "Additive Synthesis of Harmonic Musical Tones", J. Audio Eng Soc 14(4), 332-342 (1966).

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