Buchla harmonic generators

[Dan Slater]

Some info about the Buchla harmonic generators:

These units provided 10 coherently related sinewaves (harmonic numbers 1 - 10).
There were at least 3 different versions of this unit that all used the same
basic concept.

The first SFTMC (San Francisco Tape Music Center) version was fairly complex.
The harmonics were synthesized by waveshapers operating on the triangle wave
output of the fundamental VCO. The second harmonic was formed by full wave
rectification of the fundamental, followed by a sine shaper circuit. The fourth
harmonic was formed by full wave rectification of the second harmonic triangle
wave followed by a sine shaper. Other harmonics were formed in a similar manner
although the diode function generators (waveshapers) were more complex. The 3rd
harmonic was generated by a diode function generator that clipped the input
triangle wave converting it into a trapazoidal wave. By subtracting this
waveform (with 2x scaling) from the fundamental triangle wave, a 3rd harmonic
triangle wave was formed. The waveshaper for the 7th harmonic used a similar
concept and was particularly complex and used a number of diode breakpoints.
Each of the 10 harmonic channels had an audio output, a AM modulation input, an
AM modulation level potentiometer, harmonic amplitude potentiometer and a phase
adjust potentiometer / switch allowing I believe a 360 degree phase adjust
range for each harmonic. I have never seen this unit in person or the
schematics. Probably only one or two were ever built.

A simpler version of this unit was later released by CBS in 1970 as the model
148 harmonic generator. The 148 included 10 sinewave harmonic outputs, a
frequency CV in and an FM modulation input. I have used this unit. The spectral
purity is not spectacular, particularily the 7th harmonic, but the system
definitely works, particularily if you occasionally tweek up the spectral
purity.

This was followed later by a 200 series version, a second generation version of
the 100 series 148 harmonic generator module. This module is a wide band FM
controllable oscillator that provides a fundamental and 9 sine wave harmonics.
The harmonic synthesizer circuitry is the same as that in the 148 harmonic
generator. Additional circuitry was added at the input and output. The harmonic
generator frequency is controlled by a simple control voltage processor that
provides bipolar summing frequency control inputs. The harmonic generator
outputs are sent to a relatively simple 10 input, 3 output audio mixer that
produces all, even and odd harmonic mix outputs.

 In essence this module is a 10 band Fourier synthesizer with fixed phase
relationships. The harmonics are produced by waveshaping the fundamental. Like
the 148, this module is double width and provides the first 10 partials as
separate outputs. It also includes a 10 channel slide pot mixer providing a mix
of all partials, an even partial mix and an odd partial mix. The sum, even and
odd partial mix outputs are simultaneously available. The oscillator includes
variable CV and wide band audio FM inputs. The module is fairly complex,
requiring 3 separate circuit cards.

Dan Slater