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Chapter 8 EVOC 20 PolySynth 135
Vocoder basics
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 may be
familiar with tracks such as “O Superman” by Laurie Anderson, “Funkytown” 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 lms—such as
with the Cylons in Battlestar Galactica, and most famously, with the voice of Darth Vader from the
Star Wars saga. See Vocoder history on page 152.
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.
The speech analyzer and synthesizer features of a vocoder are two bandpass lter banks.
Bandpass lters allow a frequency band—a slice in the overall frequency spectrum—to pass
through unchanged. Frequencies that fall outside the band are cut.
In the EVOC 20 plug-ins, these lter banks are named the analysis and synthesis banks. Each lter
bank has a matching number of corresponding bands—if the analysis lter bank has ve bands
(1, 2, 3, 4, and 5), there will be a corresponding set of ve bands in the synthesis lter 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 lter bank, where it is
divided into bands.
An envelope follower is coupled to each lter band. The envelope follower of each band tracks,
or follows, volume changes in the audio source—or, more specically, the portion of the audio
that has been allowed to pass by the associated bandpass lter. In this way, the envelope
follower of each band generates dynamic control signals.
These control signals are then sent to the synthesis lter bank—where they control the levels of
the corresponding synthesis lter bands. This is done with voltage-controlled ampliers (VCAs)
in analog vocoders. Volume changes to the bands in the analysis lter bank are imposed on the
matching bands in the synthesis lter bank. These lter level changes are heard as a synthetic
reproduction of the original input signal—or a mix of the two lter bank signals.
Analysis
audio source
Synthesis
audio source
Analysis
filter bank
band 1–5
Synthesis
filter bank
band 1–5
Envelope
follower
1–5
VCA
1–5
Control signal 1–5
Audio output
U/V
detection