7.3.10 Experiments with Filtering: Vocoders and Pitch Glides

 

Vocoders were introduced in Section 7.1.8. The implementation of a vocoder is sketched in Algorithm 7.6 and diagrammed in Figure 7.49. The MATLAB and C++ exercises associated with this section encourage you to try your hand at the implementation.

[equation class=”algorithm” caption=”Algorithm 7.6 Sketch of an implementation of a vocoder”]

algorithm vocoder

/*

Input:

c, an array of audio samples constituting the carrier signal

m, n array of audio samples constituting the modulator signal

Output:

v, the carrier wave modulated with the modulator wave */

{

Initialize v with 0s

Divide the carrier into octave-separated frequency bands with bandpass filters

Divide the modulator into the same octave-separated frequency bands with bandpass filters for each band

use the modulator as an amplitude envelope for the carrier

}

[/equation]

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Another interesting programming exercise is implementation of a pitch glide. A Risset pitch glide is an audio illusion that sounds like a constantly rising pitch. It is the aural equivalent of the visual image of a stripe on a barber pole that seems to be rising constantly. Implementing the pitch glide is suggested as an exercise for this section.