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/hybrid 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 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 usually took a long time 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 "voice modules", each capable of generating a sound at any pitch over a range of several octaves. The voice modules would be 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 voice modules. The essential part of each voice module was a "programmable digital oscillator" (PDO) which later became known as a "digitally controlled oscillator" (DCO) in digital synthesizer terminology.

The keyboard logic could also output binary key-codes corresponing to the notes in progress. When converted to an analog voltage level (using an exponential DAC), the key-code would be able to control sound parameters in the audio post-processing circuitry following the audio output from the digital oscillator. For example, the key-code 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 DCO.

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 to be 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 article was published, Motorola introduced the M6800 'D1' evaluation kit. I acquired one of these, and so began my career in embedded electronics.

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

Photo Gallery

<!> Attention: collectors of vintage electronic music apparatus...

I would be grateful to hear from anyone, preferably resident in Australia, who may be keen to take possession of the prototype instrument (pictured above) with a view to preserving it for posterity.  Contact details can be found on the autrhor's home page, here: