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How Vocoders Work

by Craig Anderton

A block diagram of the PAiA Vocoder is shown below — note that it has two inputs.  Usually, one input is for an Instrument such as keyboard or guitar, while the other is for a Microphone.  Talking into the Microphone impresses vocal effects on to whatever is plugged into the Instrument input via a fairly complex process, as described below.

Vocoder Diagram

Different human speech sounds are associated with different parts of the frequency spectrum.  An “S” sound, for example, contains lots of high frequencies.  Plosive sounds, such as “P”, “B” , on the other hand, contain lots of low frequency energy.  One section of the vocoder uses band-pass filters to split the Microphone signal into eight frequency bands, each covering a specific part of the audio spectrum, somewhat like a graphic equalizer.  When you speak an “S” into the Microphone, the higher frequency filters fed by the Mic will produce an output but there will be no output from the lower frequency filters.  Similarly, speaking a plosive into the Microphone will give an output from the low frequency filters, while little (if any) signal will pass through the higher frequency filters.  Vowel sounds produce outputs from the various midrange filters.  These outputs go through individual envelope followers to provide eight Control Voltages (CVs) that track the energy in the part of the spectrum covered by the filter.

Filters are also used to split the Instrument signal into eight different bands, which are tuned to the same frequencies as the Mic channel filters.  However, these filters are followed by Voltage Controlled Amplifiers (VCAs) so that the filter outputs may be amplified or attenuated by a Control Voltage increase or decrease.  The Control Voltages used are the ones produced by the Mic filters and envelope followers.

Now consider what happens when you play a note into the Instrument input while speaking into the Mic input.  If an output occurs from the lowest frequency Mic filter, then its envelope follower controls the VCA of the lowest frequency Instrument filter, and passes the corresponding frequencies from the Instrument.  If an output occurs from the Mic’s highest frequency filter, then that output controls the VCA of the highest Instrument filter, and passes any Instrument signals present at that frequency.  The midrange sections act similarly.

As you speak, the various Mic filters produce output signals which correspond to the energies present in your voice.  Since these signals control the VCAs, which in turn control the set of equivalent filters connected to the Instrument, you superimpose a replica of the voice’s energy patterns on to the sound of the Instrument plugged into the Instrument input.  Of course, you are not limited to just a Mic for the modulation input; in fact, percussive Instruments and program material can produce very interesting results.

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