Spectravox (2019 Moogfest)

Moog riff on the analog vocoder

Spectravox is an analog electronic instrument that explores and extends the topology of the analog Vocoder, a device used to analyze and resynthesize speech or other audio signals.

This is a limited-edition, DIY synthesizer for the Moogfest 2019 Engineer Workshop. Approximately 100 were built, but Moog is gauging interest and collecting feedback from engineers in considering making this a commercial product.

The Spectravox is a riff on the Vocoder. It features a Program Input for 10-band analysis of human vocalizations or other audio inputs, a built-in Carrier signal, including a VCO or external signal and a noise source.

Built-in 1V/oct sawtooth-core VCO with selectable Saw or Pulse waveform outputs, normalled to the Carrier input.

White Noise source with dedicated output

Carrier Mix control fades between the VCO (or Carrier Input) and Noise source for processing by the Synthesis Filters

10 bands of globally Voltage Controlled Filters with variable Resonance. Band 1 is low pass, bands 2-9 are bandpass, and band 10 is high pass. Spectral Shift control and input sweep all filters’ frequencies up or down. Audio-rate spectral modulation, anyone?

Modulation Source is a triangle-core LFO (1V/oct) with rates from very slow (0.06Hz) to audio rate (500Hz) with a Mod Amount control for sweeping all 10 filter bands up and down automatically.

10 VCAs, one for each of the 10 filter bands for use with Program Analyzer Outputs or a fixed value for use as a filter bank. (Vocode On/Off). 10 Sliders attenuate CV applied to each VCA.

Hiss function allows routing noise to bands 9 and 10 for plosive/sibilance simulation. Buzz function routes normal Carrier Mix to bands 9 and 10.

A master VCA follows the sum of the 10 bands of filter. It can be ON for Droning or Vocoding applications, or be controlled by the internal EG. An External VCA control input is provided and summed with the voltage generated by the VCA mode switch.

A basic, percussive EG (Variable Decay) can be triggered by the Program Signal Trigger Pulse or external Gate or Trigger pulse.