Human speech consists of a series of voiced sounds—tonal sounds or formants—and unvoiced (U/V) 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 flow with lips, tongue, palate, throat, and larynx.
If speech containing voiced and unvoiced sounds is used as a vocoder analysis signal but the synthesis engine doesn’t differentiate between voiced and unvoiced sounds, the result sounds rather weak. To avoid this problem, the synthesis section of the vocoder must produce different sounds for the voiced and unvoiced parts of the signal.
The EVOC 20 PolySynth includes an Unvoiced/Voiced detector for this specific 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 different 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.
Sensitivity knob: Determine how responsive U/V detection is. Use higher settings to recognize more of the individual, unvoiced portions of the input signal. When high settings are used, the increased sensitivity to unvoiced signals can lead to the U/V sound source being used on the majority of the input signal, including voiced signals. Sonically, this results in a sound that resembles a radio signal that is breaking up and contains a lot of static, or noise. The U/V sound source is determined by the Mode pop-up menu.
Mode pop-up menu: Choose the sound source used to replace the unvoiced content in the input signal.
Noise: Uses noise alone for the unvoiced portions of the sound.
Noise + Synth: Uses noise and the synthesizer for the unvoiced portions of the sound.
Blend: Uses the analysis signal after it has passed through a highpass filter for the unvoiced portions of the sound. The Sensitivity parameter has no effect when this setting is used.
U/V Level knob: Set the volume of the signal used to replace the unvoiced content in the input signal.
Important: Take care with the Level knob, particularly when a high Sensitivity value is used, to avoid internally overloading the EVOC 20 PolySynth.