Senior Design Expo 2019

Frequency Stabilization and External Control
Module for the Buchla 100 Analog Music
Synthesizer
William Mauro
Advisor: Prof. David Vallancourt
Analog oscillators have long been plagued by
frequency drift due to temperature effects and
component parameters which vary with age,
especially in the case of discrete designs. Such
oscillators find common use as a sound source in
electronic music synthesizers like the historic
Buchla 100 modular system, however the
inherent susceptibility of analog oscillators to
frequency drift necessitates constant retuning
which interferes with studio recordings and live
performance alike. Frequency stabilization
techniques are routinely employed to synchronize
oscillators in communication systems but as of
yet have seldom been adapted for use in musical
circuits.
The Frequency Stabilization and External
Control Module offers a novel pitch stabilization
solution for existing electronic music
synthesizers while also providing facilities for
external control by a special purpose keyboard
and the MIDI standard.[1] Eight identical
stabilization channels are offered to the user
which may be engaged simultaneously in order to
allow for polyphonic control. The module
operates by constantly measuring the frequency
of a musical tone oscillator in the Buchla 100
synthesizer and continually adjusting its tuning
control voltage to maintain a constant output
frequency. Frequency measurement is
accomplished using an embedded system based
on the high-performance STMicroelectronics
Arm Cortex-M7 32-bit RISC core for extremely
precise readings to within 0.1 Hz.[2]
Measurements are rapidly obtained by counting
the number of clock cycles during a single period
of the input signal from the analog oscillator. The
measurement is compared with a desired
frequency selected by the user and the resultant
difference dictates the amount of control voltage
offset necessary to keep the analog oscillator
locked in tune. In order to produce the tuning
control voltage output, the microprocessor is
connected over an SPI bus to an AD5360 16-bit
digital-to-analog (DAC) converter which
provides extraordinary accuracy theoretically to
within 1mV.
The Frequency Stabilization and External
Control Module provides several modes by which
the user is able to choose the stable pitch
frequency for the analog oscillator. “Keyboard”
mode allows the user to select the pitch frequency
in a conventional manner by actuating piano keys
on a special purpose-built keyboard in either
monophonic (single-note), polyphonic (multiplenote),
or split configurations. Additionally, the
tone interval between notes on the keyboard can
be varied using a potentiometer mounted on the
panel in order to allow for playback of
non­western musical scales. A bar strip of
indicator lights displays the channel
configuration of the depressed keys.
Furthermore, an implemented joystick controller
mounted next to the keyboard may be used to
alter several tone characteristics. The “MIDI” and
“MIDI CC” modes allow the analog oscillators of
the Buchla 100 to stabilize at frequencies dictated
by MIDI note or continuous controller (CC)
information from a computer or external device.
MIDI data is received on the UART channels of
the STM32 embedded microcontroller. The MIDI
standard is widely adopted in modern electronic
musical instruments and computer interfaces,
allowing for an extensive variety of control
options.
References:
[1] The Complete MIDI 1.0 Detailed
Specification. [Online].
Available:
https://www.midi.org/specifications­old/item/the
-midi-1-0-specification.
[2] STMicroelectronics, "32-bit Arm®
Cortex®- M7 400MHz MCUs," STM32H7
Datasheet, Jun. 2017 [Revised Jul. 2018]
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