Vocoder

A vocoder (, a portmanteau of voice and encoder) is a category of speech coding that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption or voice transformation. The vocoder was invented in 1937 by Homer Dudley at Bell Labs as a means of synthesizing human speech. This work was developed into the channel vocoder which was used as a voice codec for telecommunications for speech coding to conserve bandwidth in transmission. By encrypting the control signals, voice transmission can be secured against interception. Its primary use in this fashion is for secure radio communication. The advantage of this method of encryption is that none of the original signal is sent, only envelopes of the bandpass filters. The receiving unit needs to be set up in the same filter configuration to re-synthesize a version of the original signal spectrum. The vocoder has also been used extensively as an electronic musical instrument. The decoder portion of the vocoder, called a voder, can be used independently for speech synthesis.

Theory The human voice consists of sounds generated by the periodic opening and closing of the glottis by the vocal cords, which produces an acoustic waveform with many harmonics. This initial sound is then filtered by movements in the nose, mouth and throat (a complicated resonant piping system known as the vocal tract) to produce fluctuations in harmonic content (formants) in a controlled way, creating the wide variety of sounds used in speech. There is another set of sounds, known as the unvoiced and plosive sounds, which are created or modified by a variety of sound-generating disruptions of airflow occurring in the vocal tract. The vocoder analyzes speech by measuring how its spectral energy distribution characteristics fluctuate across time. This analysis results in a set of temporally parallel envelope signals, each representing the individual frequency band amplitudes of the user's speech. Put another way, the voice signal is divided into a number of frequency bands (the larger this number, the more accurate the analysis) and the level of signal present at each frequency band, occurring simultaneously, is measured by an envelope follower, representing the spectral energy distribution across time. This set of envelope amplitude signals is called the "modulator". To recreate speech, the vocoder reverses the analysis process, variably filtering an initial broadband noise (referred to alternately as the "source" or "carrier"), by passing it through a set of band-pass filters, whose individual envelope amplitude levels are controlled, in real time, by the set of envelope amplitude signals from the modulator. The digital encoding process involves a periodic analysis of each of the modulator's multiband set of envelope amplitudes. This analysis results in a set of digital pulse code modulation stream readings. Then the pulse code modulation stream outputs of each band are transmitted to a decoder. The decoder applies the pulse code modulations as control signals to the corresponding amplifiers of the output filter channels. Information about the fundamental frequency of the initial voice signal (as distinct from its spectral characteristic) is discarded; it was not important to preserve this for the vocoder's original use as an encryption aid. It is this dehumanizing aspect of the vocoding process that has made it useful in creating special voice effects in popular music and audio entertainment. Instead of a point-by-point recreation of the waveform, the vocoder process sends only the parameters of the vocal model over the communication link. Since the parameters change slowly compared to the original speech waveform, the bandwidth required to transmit speech can be reduced. This allows more speech channels to utilize a given communication channel, such as a radio channel or a submarine cable. Analog vocoders typically analyze an incoming signal by splitting the signal into multiple tuned frequency bands or ranges. To reconstruct the signal, a carrier signal is sent through a series of these tuned band-pass filters. In the example of a typical robot voice the carrier is noise or a sawtooth waveform. There are usually between 8 and 20 bands. The amplitude of the modulator for each of the individual analysis bands generates a voltage that is used to control amplifiers for each of the corresponding carrier bands. The result is that frequency components of the modulating signal are mapped onto the carrier signal as discrete amplitude changes in each of the frequency bands. Often there is an unvoiced band or sibilance channel. This is for frequencies that are outside the analysis bands for typical speech but are still important in speech. Examples are words that start with the letters s, f, ch or any other sibilant sound. Using this band produces recognizable speech, although somewhat mechanical sounding. Vocoders often include a second system for generating unvoiced sounds, using a noise generator instead of the fundamental frequency. This is mixed with the carrier output to increase clarity. In the channel vocoder algorithm, among the two components of an analytic signal, considering only the amplitude component and simply ignoring the phase component tends to result in an unclear voice; on methods for rectifying this, see phase vocoder.

History

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