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Vocoding. History of Vocoding. Vocoder = abbreviation of “voice encoding” Initially intended for transmitting speech signals over phone lines which couldn’t be “tapped.” Speech signal was analyzed and only the “cryptic” cryptic results of the analysis were sent over the lines
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History of Vocoding • Vocoder = abbreviation of “voice encoding” • Initially intended for transmitting speech signals over phone lines which couldn’t be “tapped.” • Speech signal was analyzed and only the “cryptic” cryptic results of the analysis were sent over the lines • Results were received on the other end and were used to synthetically rebuild the original voice signal. • Eventually became more popular in experimental and popular music as a means of modulating the singer and created synthetic sounding voices (“robot voice,” specifically.)
How does it work • Vocoding is a 2-part process. • Analysis – input signal passes through a bank of bandpass filters. Sound is analyzed and broken into segments according to the number of bandpass filters in the filterbank • If 20 filters, the sound is divided into 20 segments. If 10 filters, sound is divided into 10 segments, etc. • Synthesis – receives input signal from the analysis filter bank, allows for modifications to be made to the original signal, and then outputs the final modulated result
Analysis • Input signal passes through the filter bank of bandpass filters. • An envelope follower is attached to each filter. This tracks changes in amplitude of the signal that is allowed to pass through the filter • The envelope follower creates dynamic control signals, which are then sent to the synthesis filter bank.
Synthesis • Receives control signals (original signal) from the analysis portion of the process. • Synthesis filter bank contains identical bandpass filters as the analysis portion. • Levels of corresponding filters can now be adjusted or modifiedto modulate the original signal.
Processing Speech • Speech is made up of vowel sounds (voiced) and consonant sounds (unvoiced). • When talking about the vocoding process, we always refer to these two types of sounds: • Voiced • Unvoiced • We talk about these as two different types of sounds, because they are produced through different means.
Processing Speech • Vowels – produced by oscillation of vocal chords. • Consonants – produced by blocking and restricting air flow with lips, tongue, palette, throat and larynx. • SO, a vocoder must also be able to produce different sounds for the voiced and unvoiced parts of a signal. • Why? • Because the vocoder was originally intended to transmit and produce speech signals. In order to do this, the vocoder must be able to reproduce all kinds of voice sounds.
Processing Speech • How does a vocoder produce voiced and unvoiced sounds? • With an unvoiced/voiced detector. • Detects voiced and unvoiced portions of a sound and substitutes corresponding portions of synthesis signal with noise. This is a mixture of noise+synth wit hthe original signal (or blend). • When U/V detector detects voiced sounds, it passes the signal to the synthesis section, where normal synthesis is carried out.
Side Chain • Vocoding is a process that makes use of side chaining. • Side chaining is when you have an input signal that is not heard directly, but has a direct influence on your output signal. • In vocoding (with a DAW), an audio file is side-chained to the vocoder, meaning the audio file will not actually be heard, but when you play your MIDI keyboard, you will hear the processed voice with vocoding, but not the original signal. This will be more clear in the Logic/Ableton demos.
