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Chapter 14 EVOC 20 PolySynth 151
An envelope follower is coupled to each filter band. The envelope follower of each
band tracks (follows) any volume changes in the portion of the audio source allowed to
pass by the associated bandpass filter. In this way, the envelope follower of each band
generates dynamic control signals.
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 via VCAs—Voltage
Controlled Amplifiers. This allows the volume changes of the bands—and thus the
changes of the original sound—in the analysis filter bank to be imposed on the
matching bands in the synthesis filter bank.
The more bands a vocoder offers, the more precisely the original sound’s character will
be re-modeled.
How Does a Filter Bank Work?
If you removed all circuits responsible for transferring the sonic characteristics from the
analysis to the synthesis signal from a vocoder, and dispensed with the detection of
voiced or unvoiced signals, you’d be left with two filter banks—the analysis and
synthesis filters. To use these musically, you would need to ensure that you could
control the output level of each bandpass filter. With this level of control, you can apply
unique and dramatic changes to the frequency spectrum.
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