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Chapter 8 EVOC 20 PolySynth 141
Set the number of lter bank bands
m To set the number of frequency bands the EVOC 20 PolySynth’s lter bank uses, drag the Bands
eld vertically.
The greater the number of frequency bands, the more precisely the sound can be reshaped. As
the number of bands is reduced, the source signal’s frequency range is divided up into fewer
bands, and the resulting sound is formed with less precision by the synthesis engine. You may
nd that a good compromise between sonic precision—allowing incoming signals such as
speech and vocals to remain intelligible—and resource usage is around 10 to 15 bands.
EVOC 20 PolySynth (U/V) detection parameters
Human speech consists of a series of voiced sounds—tonal sounds or formants—and unvoiced
sounds. The main distinction between voiced and unvoiced sounds is that voiced sounds
are produced by an oscillation of the vocal cords, whereas unvoiced sounds are produced by
blocking and restricting the air ow with lips, tongue, palate, throat, and larynx.
If speech containing voiced and unvoiced sounds is used as a vocoder’s analysis signal but the
synthesis engine doesn’t dierentiate between voiced and unvoiced sounds, the result sounds
rather weak. To avoid this problem, the synthesis section of the vocoder must produce dierent
sounds for the voiced and unvoiced parts of the signal.
The EVOC 20 PolySynth includes an Unvoiced/Voiced detector for this specic purpose. This
unit detects the unvoiced portions of the sound in the analysis signal and then substitutes
the corresponding portions in the synthesis signal with noise, with a mixture of noise and
synthesizer signal, or with the original signal. If the U/V detector detects voiced parts, it passes
this information to the Synthesis section, which uses the normal synthesis signal for these
portions.
A formant is a peak in the frequency spectrum of a sound. In the context of human voices,
formants are the key component that enables humans to distinguish between dierent vowel
sounds—based purely on the frequency of the sounds. Formants in human speech and singing
are produced by the vocal tract, with most vowel sounds containing four or more formants.