How the EVOC 20 Polysynth Works

EVOC 20 PolySynth “listens” to an incoming audio signal—typically of a spoken or sung performance—and imposes the sonic characteristics and level changes of this signal onto the integrated synthesizer.

When you play notes and chords with your MIDI keyboard, the internal synthesizer will “sing” at the pitches of incoming MIDI notes, but with the articulations—level changes, vowel and consonant sounds—of the incoming audio signal.

This results in the classic “singing robot” or “synthetic voice” sounds that vocoders are mainly known for.

EVOC 20 PolySynth offers more than vocoding, however. You can use it as a synthesizer, or for more subtle effects processing—such as the creation of (somewhat) natural sounding vocal harmonies from a solo voice performance. If your musical tastes are more extreme, feel free to try processing other audio material, such as drum or instrument loops.

What Is a Vocoder?

The word vocoder is an abbreviation for VOice enCODER. A vocoder analyzes and transfers the sonic character of the audio signal arriving at its analysis input to the synthesizer’s sound generators. The result of this process is heard at the output of the vocoder.

The classic vocoder sound uses speech as the analysis signal and a synthesizer sound as the synthesis signal. This sound was popularized in the late 1970s and early 1980s. You’ll probably know it from tracks such as “O Superman” by Laurie Anderson, “Funky Town” by Lipps Inc., and numerous Kraftwerk pieces—such as “Autobahn,” “Europe Endless,” “The Robots,” and “Computer World.”

In addition to these “singing robot” sounds, vocoding has also been used in many films—such as with the Cylons in Battlestar Galactica, and most famously, with the voice of Darth Vader from the Star Wars saga. Also see A Brief Vocoder History.

Vocoding, as a process, is not strictly limited to vocal performances. You could use a drum loop as the analysis signal to shape a string ensemble sound arriving at the synthesis input.

How Does a Vocoder Work?

The speech analyzer and synthesizer features of a vocoder are actually two bandpass filter banks. Bandpass filters allow a frequency band—a slice in the overall frequency spectrum—to pass through unchanged, and cut the frequencies that fall outside the band’s range.

In the EVOC 20 plug-ins, these filter banks are named the Analysis and Synthesis sections. Each filter bank has a matching number of corresponding bands—if the analysis filter bank has five bands (1, 2, 3, 4, and 5), there will be a corresponding set of five bands in the synthesis filter bank. Band 1 in the analysis bank is matched to band 1 in the synthesis bank, band 2 to band 2, and so on.

The audio signal arriving at the analysis input passes through the analysis filter bank, where it is divided into bands.

An envelope follower is coupled to each filter band. The envelope follower of each band tracks, or follows, any volume changes in the audio source—or, more specifically, the portion of the audio that has been allowed to pass by the associated bandpass filter. In this way, the envelope follower of each band generates dynamic control signals.

Figure. Vocoder signal flow.

These control signals are then sent to the synthesis filter bank—where they control the levels of the corresponding synthesis filter bands. This is done with voltage-controlled amplifiers (VCAs) in analog vocoders. Volume changes to the bands in the analysis filter bank are imposed on the matching bands in the synthesis filter bank. These filter level changes are heard as a synthetic reproduction of the original input signal—or a mix of the two filter bank signals.

The more bands a vocoder offers, the more precisely the original sound’s character will be reproduced by the synthesis filter bank. The EVOC 20 PolySynth provides up to 20 bands per bank. See EVOC20 Block Diagram for a detailed image of the EVOC20 Polysynth signal path.

Setting Up Your EVOC 20 PolySynth Host Application

To use of the EVOC 20 PolySynth, you need to insert it into the Instrument slot of an instrument channel strip. You also need to provide an audio signal as the analysis audio source, via a sidechain.

To set up the EVOC 20 PolySynth in your host application
  1. Insert the EVOC 20 PolySynth into the Instrument slot of an instrument channel strip.

  2. Choose an input source in the Side Chain menu in the plug-in header of the EVOC 20 PolySynth. This can be an audio track, live input, or bus, depending on the host application.

    The EVOC 20 PolySynth is now ready to accept incoming MIDI data and has been assigned to an input, audio track, or bus—via a side chain.

  3. If applicable to your host application and needs, mute the audio track serving as the side chain input, start playback, and play your MIDI keyboard.

  4. Adjust the volume levels of the EVOC 20 PolySynth and the Side Chain source (if not muted) to taste.

  5. Experiment with the knobs, sliders, and other controls. Have fun, and feel free to insert other effect plug-ins to further enhance the sound.