Modcan VCDO 58A/B


The Modcan VCDO 58A/B is a voltage-controlled digital oscillator for use in analog sound synthesis. Introduced in September 2006, it features a combination of synthesis techniques not found in previous analog synthesis modules and consequently has caused some curiosity about how it works. This document will cover some of the high-level architectural details of this unique device to promote a better understanding of its capabilities and applications.


VCDO Hardware Block Diagram

The heart of the VCDO hardware is a DSP microcontroller. This part integrates the processor, memory and peripherals required to sample the control voltages and compute the waveforms. Control voltages are buffered, scaled and summed with the front panel knob settings in the control voltage signal conditioning section. The DAC and output amplifier convert the digital waveform to an audio frequency voltage.

Theory of Operation

VCDO Signal Processing

The VCDO signal processing consists of the following stages:

Firmware Implementation

Although it is beyond the scope of this description to dig deeply into the firmware design, the VCDO shares many architectural details with my earlier Digital VCO project. The VCDO uses a much more sophisticated processor, higher sample rates, wider data words and overall higher quality analog signal processing.

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Eric Brombaugh

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