Digital Polyphonic Keyboard Instrument

by Michael J. Bauer

Electronic music technology has always been a source of fascination to me, from the analog synthesizers of the late 1960s (Moog, ARP, EMS, etc) to the sophisticated digital synthesizers, audio processors and studio workstations of today.

In the early 1970s, there was feverish activity in the field of analog sound synthesis. Several electronics hobbyist magazines of the era published do-it-yourself projects for the construction of analog synthesizers, inspired by commercial instruments such the "Mini Moog", "ARP Odyssey", "EMS VCS-3", etc. These instruments all suffered severe limitations for use in live performance, which also made studio composition a tedious task. They were monophonic (producing only one note at a time) and the pitch of the analog tone generators drifted annoyingly. It could take several minutes to change the "patch" to produce a different sound.

In early 1972, I was part way through building my own analog synthesizer, a blend of published circuit designs and my own, when I hit on the idea of a digital instrument having a bank of eight (or more) identical "programmable digital oscillators" (PDOs), each capable of generating any pitch (over a range of several octaves), and controlled by digital logic which would scan a keyboard contact matrix to determine which keys were pressed, and assign each active note to one of the PDOs. The keyboard logic would also output the binary keycodes of the notes in progress. When converted to an analog voltage level (using an exponential DAC), the keycode would be able to control parameters in the audio post-processing circuitry (following the AF output from the digital oscillator). For example, the keycode control voltage could be used to make the resonant frequency of a voltage-controlled filter (VCF) track the pitch of the note generated by the PDO.

A hybrid polyphonic instrument incorporating the best advantages of both the digital and analog worlds was realised. As far as I know, mine was the first digital polyphonic instrument design ever published (Electronics Australia, April 1976). Unfortunately, the instrument was not destined to be a commercial success. Apart from my lack of entrepreneurial skills, there were other factors leading to its swift obsolescence. The keyboard logic was implemented entirely from small-medium scale TTL devices. It was based on a kind of state-machine architecture. A little invention called the "micro-processor" made my TTL design obsolete. In the same year as my E.A. article was published, Motorola introduced the M6800. I bought a Motorola 6800 'D1' evaluation kit, and so began my career in embedded electronics design.


For historical interest, the original design of my "Digital Polyphonic Keyboard" is posted here...

Electronics Australia article (April, 1976): [Page1] [Page2] [Page3]

Keyboard control logic and tone bank modules (close-up photo)

Overall system block diagram

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