Vocoder

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A vocoder (name derived from voice coder, formerly also called voder) is a speech analyser and synthesizer. It was originally developed as a speech coder for telecommunications applications in the 1930s, the idea being to code speech for transmission. Its primary use in this fashion is for secure radio communication, where voice has to be digitized, encrypted and then transmitted on a narrow, voice-bandwidth channel. The vocoder has also been used extensively as an electronic musical instrument. As an instrument, it is primarily used with guitars and synthesizers and produces a sound that can be described as a "talking guitar" or "talking keyboard". Vocoders are also often used to create the sound of a robot talking.

The vocoder is related to, but essentially different from, the computer algorithm known as the "phase vocoder".

How a vocoder works

Vocoder theory

The human voice consists of sounds generated by the opening and closing of the glottis by the vocal cords, which produces a periodic waveform with many harmonics. This basic sound is then filtered by the nose and throat (a complicated resonant piping system) to produce differences 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 not modified by the mouth in the same fashion.

The vocoder examines speech by finding this basic carrier wave, which is at the fundamental frequency, and measuring how its spectral characteristics are changed over time by recording someone speaking. This results in a series of numbers representing these modified frequencies at any particular time as the user speaks. In doing so, the vocoder dramatically reduces the amount of information needed to store speech, from a complete recording to a series of numbers. To recreate speech, the vocoder simply reverses the process, creating the fundamental frequency in an oscillator, then passing it through a stage that filters the frequency content based on the originally recorded series of numbers.

Early vocoders

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

An example of an early vocoder was the Sonovox, which was used in a number of songs from the 1940s to the 1960s, and is used to create the voice of The Reluctant Dragon, Casey Junior the train in Dumbo, the instruments in the Rusty in Orchestraville recordings and the piano in Sparky's Magic Piano

Linear prediction-based vocoders

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. In linear prediction coding, the all-pole filter replaces the bandpass filterbank of its predecessor and is used at the encoder to whiten the signal (i.e., flatten the spectrum) and again at the decoder to re-apply the spectral shape of the target speech signal. In contrast with vocoders realized using bandpass filterbanks, the location of the linear predictor's spectral peaks is entirely determined by the target signal and need not be harmonic, i.e., a whole-number multiple of the fundamental frequency.

Modern vocoder implementations

Even with the need to record several frequencies, and the additional unvoiced sounds, the compression of the vocoder system is impressive. Standard systems to record speech record a frequency from about 500 Hz to 3400 Hz, where most of the frequencies used in speech lie, which requires 64kbit/s of bandwidth (due to Nyquist frequency). However a vocoder can provide a reasonably good simulation with as little as 2400 bit/s of bandwidth, a 26× improvement.

Several vocoder systems are used in NSA encryption systems:

  • LPC-10, FIPS Pub 137, 2400 bit/s, which uses linear predictive coding
  • Code Excited Linear Prediction, (CELP), 2400 and 4800 bit/s, Federal Standard 1016, used in STU-III
  • Continuously Variable Slope Delta-modulation (CVSD), 16 Kbit/s, used in wideband encryptors such as the KY-57.
  • Mixed Excitation Linear Prediction (MELP), MIL STD 3005, 2400 bit/s, used in the Future Narrowband Digital Terminal FNBDT, NSA's 21st century secure telephone.
  • Adaptive Differential Pulse Code Modulation (ADPCM), former ITU-T G.721, 32Kbit/s used in STE secure telephone

(ADPCM is not a proper vocoder but rather a waveform codec. ITU has gathered G.721 along with some other ADPCM codecs into G.726.)

Musical applications

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 Walter Carlos and Robert Moog developed one of the first truly musical vocoders. A 10-band device inspired by the vocoder designs of Homer Dudley, it was originally called a spectrum encoder-decoder, and later referred to simply as a vocoder. The carrier signal came from a Moog modular synthesizer, and the modulator from a microphone input. The output of the 10-band vocoder was fairly intelligible, but relied on specially articulated 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 called Timesteps, which featured the vocoder in two main sections. Originally, Timesteps was intended as merely an introduction to vocoders for the "timid listener", but Kubrick chose to include the piece on the soundtrack, much to the surprise of Walter Carlos.

In the late 1970s vocoder appeared also on some pop music recordings. Especially some disco recordings and some more experimental artists (so called new age genre) utilized vocoder sometimes. Vocoder has since then appeared on pop recordings occasionally. Artists that have made vocoder an essential part of their works include a German group Kraftwerk which plays on technological themes, Roger Troutman (and his band Zapp) and Herbie Hancock (during his late 1970s disco period) who both used vocoder vocals extensively on afro-grooves.

Linear prediction coding is also used as a musical effect (generally for cross-synthesis of musical timbres), but is not as popular as bandpass filterbank vocoders, to which the musical use of the word vocoder refers.

Examples of vocoders in music

The vocoder is a popular instrument used by many bands. Listed below are examples of songs that utilize the vocoder. This is list is in no sense intended to be comprehensive.

1960s

1970s

1980s

1990s

2000s

Television and Film Applications

Vocoders have also been used in television and film, usually for robots or talking computers.

See also

External links

de:Vocoder es:Vocoder fr:Vocoder nl:Vocoder pl:Vocoder