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168 Chapter 13 Vocoder—Basics
How Does a Vocoder Work?
The speech analyzer and synthesizer referred to above are actually two filter banks of
bandpass filters. Bandpass filters allow a frequency band (a slice) in the overall
frequency spectrum to pass through unchanged, and cut the frequencies which fall
outside of the band’s range.
In Logic’S EVOC 20 plug-ins, these filter banks are named the Analysis and Synthesis
sections. These filter banks have 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 up to 20 bands.
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
Analysis
Filter Bank
Env.
Follower
VCA
Envelope Follower 1—5
Control Signal 1—5
1—5
Audio
Output
Band 1—5
Synthesis
Filter Bank
Band 1—5
U/V
Detection
Synthesis
Audio Source