Music Sound

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, as in the Styx song "Mr. Roboto".

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

Contents 1 How a vocoder works 1.1 Vocoder theory 1.2 Early vocoders 1.3 Linear prediction-based vocoders 1.4 Modern vocoder implementations 2 Musical applications 3 Television and film applications 4 References 4.1 Cited references 5 External links

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 Casey Junior the train in Dumbo and The Reluctant Dragon, the instruments in the Rusty in Orchestraville recordings and the piano in Sparky's Magic Piano

Also in 1964 physicist John Larry Kelly, Jr created one of the most famous moments in the history of Bell Labs by using an IBM 704 computer to synthesize speech. Kelly's voice recorder synthesizer (vocoder) recreated the song Daisy Bell, with musical accompaniment from Max Mathews. Arthur C. Clarke of 2001: A Space Odyssey fame was coincidentally visiting friend and colleague John Pierce at the Bell Labs Murray Hill facility at the time of this remarkable speech synthesis demonstration and was so impressed that he used it in the climactic scene of his novel and screenplay for 2001: A Space Odyssey,^[1] where the HAL 9000 computer sings the same song as he is being put to sleep by astronaut Dave Bowman.^[2]

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 basic 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 carrier, 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 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. Later improved vocoders use a high-pass filter to let some sibilance through from the microphone; this ruins the device for its original speech-coding application, but it makes the "talking synthesizer" effect much more intelligible.

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 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 Wendy Carlos.

In the late 1970s, vocoder began to appear in pop music, for example on disco recordings. Vocoder has appeared on pop recordings from time to time ever since, but in most of cases vocoder works just as a some kind of special effect in pop music. However, many experimental electronic artists and representators of "new age" genre often utilize vocoder in a more comprehensive manner. Jean Michel Jarre (album Zoolook, 1984) and Mike Oldfield (album Five Miles Out, 1982) are good examples. There are also some artists who have made vocoder an essential part of their music. Those include the famous German group, Kraftwerk, jazz/fusion keyboardist Herbie Hancock during his late 1970s disco period, the funk band Zapp, and more recently, avant-garde-pop group Trans Am. The song "O Superman" by avant-garde musician, Laurie Anderson, is a popular recording released in 1981 that incorporates the vocoder. In 2005, artist Imogen Heap's track Hide and Seek used the vocoder exclusively, with zero other instrumental support.

This use of the vocoder should not be confused with the talk box guitar effect invented by Doug Forbes and popularized by Peter Frampton. In the talk box effect, amplified sound is actually fed via a tube into the performer's mouth and is then shaped by the performer's lip, tongue, and mouth movements before being picked up by a microphone. The song "Livin' on a Prayer" by Bon Jovi is a more recent recording with a talk box effect. In contrast, the vocoder effect is produced entirely electronically.

The vocoder should also not be confused with the Antares Auto-Tune Pitch Correcting Plug-In, which can also be used to achieve a robotic sounding vocal effect.

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, and the musical use of the word vocoder refers exclusively to the latter type of device.

Television and film applications

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

In Transformers, the vocal effects of Soundwave were created with vocoders.
The Cylons from Battlestar Galactica used a Roland Vocoder to create their monotone voice.
A vocoder was used by Wendy Carlos for the soundtrack to Stanley Kubrick's A Clockwork Orange.
In the film "Sgt. Pepper's Lonely Heart Club Band", the robotic singing of the Computerettes in the song "Mean Mr. Mustard" was achieved by using a vocoder.
A vocoder was used in the soundtrack for the movie Donnie Darko to create tension and mystery.
The voices of the Daleks in Doctor Who are created using a ring modulator, not a vocoder.

References

Cited references

External links

• Vocod'o'rama, an Analogue Vocoder Fan Site.
• Vocoders at Vintage Synth Explorer
• GPL implementation of a vocoder, as a LADSPA plugin
• Various links on compression of human speech