Vocoder

A vocoder (name derived from voice coder, formerly also called voder) is a speech analyser and synthesizer . It wasoriginally developed as a speech coder for telecommunications applications in the 1930s , the idea being to code speech for transmission. Although not used in this fashion, the vocoder has been used extensively as an electronic musical instrument .

The human voice consists of sounds generated by the opening and closing of the glottis by the vocal cords , whichproduces a periodic waveform. This basic sound is then modified by the nose and throat to produce differences in pitch in acontrolled way, creating the wide variety of sounds used in speech. There are another set of sounds, known as the unvoiced and plosive sounds, which are notmodified by the mouth in the same fashion.

The vocoder examines speech by finding this basic frequency, the fundamental frequency, and measuring how it ischanged over time by recording someone speaking. This results in a series of numbers representing these modified frequencies atany particular time as the user speaks. In doing so, the vocoder dramatically reduces the amount of information needed to storespeech, from a complete recording to a series of numbers. To recreate speech, the vocoder simply reverses the process, creatingthe fundamental frequency in an oscillator , then passingit into a modifier that changes the frequency based on the originally recorded series of numbers.

Of course, the actual qualities of speech cannot be reproduced this easily. In addition to a single fundamental frequency, thevocal system adds in a number of resonant frequencies that add character and quality to the voice, known as the formant . Without capturing these additional qualities, the vocoder will never sound"real".

In order to address this, most vocoder systems use what are effectively a number of vocoders, all tuned to differentfrequencies (using band-pass filters). The various values of these filters arestored not as the raw numbers, which are all based on the original fundamental frequency, but as a series of modifications tothat fundamental needed to modify it into the signal seen in that filter. During playback these settings are sent back into thefilters and then added together, modified with the knowledge that speech typically varies between these frequencies in a fairlylinear way. The result is recognizable speech, although somewhat "mechanical" sounding. Vocoders also often include a secondsystem for generating unvoiced sounds, using a noise generator instead of the fundamental frequency.

Even with the need to record several frequencies, and the additional unvoiced sounds, the compression of the vocoder system isimpressive. Standard systems to record speech record a frequency from about 500Hz to 8kHz, where most of the frequencies used inspeech lie, which requires 64kbit/s of bandwidth (due to Nyquistfrequency ). However a vocoder can provide a reasonably good simulation with about 3kbit/s of bandwidth, a 20ximprovement.

For musical applications, a source of musical sounds is used as the oscillator,instead of extracting the fundamental frequency. For instance, one could use the sound of a guitar as the input to the filter bank, a technique that became popular in the 1970s .

In 1970, electronic music pioneers Wendy Carlos and Robert Moog developed one of the first truly musical vocoders. A 10-band deviceinspired by the vocoder designs of Homer Dudley, it was originally called a spectrum encoder-decoder, and later referred tosimply as a vocoder. The carrier signal came from Carlos' Moog modular synthesizer , and the modulator from a microphone input. The output of the 10-band vocoder was fairly intelligible, but relied on especiallyarticulated speech .

Carlos' and Moog's vocoder was featured in several recordings, including the soundtrack to Stanley Kubrick 's A Clockwork Orange , in which the vocoder sang the vocal part of Beethoven 's Ninth Symphony. Also featured in the soundtrack was a piece calledTimesteps, which featured the vocoder in two main sections. Originally, Timesteps was intended as merely an introduction tovocoders for the "timid listener", but Kubrick choose to include the piece on the soundtrack, much to the surprise of WendyCarlos.