Vocoder

[1] 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.

The receiving unit needs to be set up in the same filter configuration to re-synthesize a version of the original signal spectrum.

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 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, measured by an envelope follower, represents the spectral energy distribution across time.

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 analyzed envelope amplitude signals from the modulator.

The digital encoding process involves a periodic analysis of each of the modulator's multiband set of filter envelope amplitudes.

The decoder applies the pulse code modulations as control signals to 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.

The development of a vocoder was started in 1928 by Bell Labs engineer Homer Dudley,[5] who was granted patents for it on March 21, 1939,[6] and Nov 16, 1937.

[13] Another speech coding technique, adaptive differential pulse-code modulation (ADPCM), was developed by P. Cummiskey, Nikil S. Jayant and James L. Flanagan at Bell Labs in 1973.

Since the late 1970s, most non-musical vocoders have been implemented using linear prediction, whereby the target signal's spectral envelope (formant) is estimated by an all-pole IIR filter.

This is in contrast with vocoders realized using fixed-width filter banks, where the location of spectral peaks is constrained by the available fixed frequency bands.

This restriction is the primary reason that LP coding is almost always used in tandem with other methods in high-compression voice coders.

features a vocoder ("Pretty young thing/You make me sing"), courtesy of session musician Michael Boddicker.

[34] The 1980 version of the Doctor Who theme, as arranged and recorded by Peter Howell, has a section of the main melody generated by a Roland SVC-350 vocoder.