vocoder

The development of a vocoder was started in 1928 by Bell Labs engineer Homer Dudley, who was granted patents for it on March 21, 1939, and Nov 16, 1937. To demonstrate the speech synthesis ability of its decoder section, the voder (voice operating demonstrator) was introduced to the public at the AT&T building at the 1939–1940 New York World's Fair. The voder consisted of an electronic oscillator – a sound source of pitched tone – and a noise generator for hiss, 10 bands of resonator filters, each controlled by a variable-gain amplifier as a vocal tract, and the manual controllers including a set of pressure-sensitive keys for filter control, and a foot pedal for pitch control of tone. The filters controlled by keys convert the tone and the hiss into vowels, consonants, and inflections. This was a complex machine to operate, but a skilled operator could produce recognizable speech. Dudley's vocoder was used in the SIGSALY system, which was built by Bell Labs engineers in 1943. SIGSALY was used for encrypted voice communications during World War II. The KO-6 voice coder was released in 1949 in limited quantities; it was a close approximation to the SIGSALY at 1200 bit/s. In 1953, KY-9 THESEUS 1650 bit/s voice coder used solid-state logic to reduce the weight to 565 pounds (256 kg) from SIGSALY's 55 short tons (50,000 kg), and in 1961 the HY-2 voice coder, a 16-channel 2400 bit/s system, weighed 100 pounds (45 kg) and was the last implementation of a channel vocoder in a secure speech system. Later work in this field has since used digital speech coding. The most widely used speech coding technique is linear predictive coding (LPC). 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.

Terminal equipment for systems based on digital mobile radio (DMR). Digital voice scrambling and encryption Cochlear implants: noise and tone vocoding is used to simulate the effects of cochlear implants. Musical and other artistic effects

Even with the need to record several frequencies and additional unvoiced sounds, the compression of vocoder systems is impressive. Standard speech-recording systems capture frequencies from about 500 to 3,400 Hz, where most of the frequencies used in speech lie, typically using a sampling rate of 8 kHz (slightly greater than the Nyquist rate). The sampling resolution is typically 8 or more bits per sample, for a data rate in the range of 64 kbit/s, but a good vocoder can provide a reasonably good simulation of voice with as little as 5 kbit/s of data. Toll quality voice coders, such as ITU G.729, are used in many telephone networks. G.729 in particular has a final data rate of 8 kbit/s with superb voice quality. G.723 achieves slightly worse quality at data rates of 5.3 and 6.4 kbit/s. Many voice vocoder systems use lower data rates, but below 5 kbit/s voice quality begins to drop rapidly. 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 wide band 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, 32 kbit/s used in STE secure telephone Modern vocoders that are used in communication equipment and in voice storage devices today are based on the following algorithms: Algebraic code-excited linear prediction (ACELP 4.7–24 kbit/s) Mixed-excitation linear prediction (MELPe 2400, 1200 and 600 bit/s) Multi-band excitation (AMBE 2000 bit/s – 9600 bit/s) Sinusoidal-Pulsed Representation (SPR 600 bit/s – 4800 bit/s) Robust Advanced Low-complexity Waveform Interpolation (RALCWI 2050, 2400 and 2750 bit/s) Tri-Wave Excited Linear Prediction (TWELP 300–9600 bit/s) Noise Robust Vocoder (NRV 300 and 800 bit/s) Vocoders are also currently used in psychophysics, linguistics, computational neuroscience and cochlear implant research.

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 filter bank 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. One advantage of this type of filtering is that the location of the linear predictor's spectral peaks is entirely determined by the target signal and can be as precise as allowed by the time period to be filtered. 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. LP filtering also has disadvantages in that signals with a large number of constituent frequencies may exceed the number of frequencies that can be represented by the linear prediction filter. This restriction is the primary reason that LP coding is almost always used in tandem with other methods in high-compression voice coders.

Waveform-interpolative (WI) vocoder was developed at AT&T Bell Laboratories around 1995 by W.B. Kleijn, and subsequently, a low-complexity version was developed by AT&T for the DoD secure vocoder competition. Notable enhancements to the WI coder were made at the University of California, Santa Barbara. AT&T holds the core patents related to WI and other institutes hold additional patents.

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 synthesizer as the input to the filter bank, a technique that became popular in the 1970s.

Werner Meyer-Eppler, a German scientist with a special interest in electronic voice synthesis, published a thesis in 1948 on electronic music and speech synthesis from the viewpoint of sound synthesis. Later he was instrumental in the founding of the Studio for Electronic Music of WDR in Cologne, in 1951. One of the first attempts to use a vocoder in creating music was the Siemens Synthesizer at the Siemens Studio for Electronic Music, developed between 1956 and 1959. In 1968, Robert Moog developed one of the first solid-state musical vocoders for the electronic music studio of the University at Buffalo. In 1968, Bruce Haack built a prototype vocoder, named Farad after Michael Faraday. It was first featured on "The Electronic Record For Children" released in 1969 and then on his rock album The Electric Lucifer released in 1970. Vocoder effects have been used by musicians in both electronic music and as a special effect, along with more traditional instruments. In 1969, Sly and the Family Stone used it in "Sex Machine", a song on the album Stand!. Other artists have made vocoders an essential part of their music, overall or during an extended phase. Examples include the German synthpop group Kraftwerk, US funk band Midnight Star ("Freak-A-Zoid"), Stevie Wonder ("Send One Your Love", "A Seed's a Star") and jazz/fusion keyboardist Herbie Hancock during his late 1970s period. In 1982 Neil Young used a Sennheiser Vocoder VSM201 on six of the nine tracks on Trans. The chorus and bridge of Michael Jackson's "P.Y.T. (Pretty Young Thing)" features a vocoder ("Pretty young thing/You make me sing"), courtesy of session musician Michael Boddicker. Among the most consistent users of the vocoder in emulating the human voice was Daft Punk, who used this instrument from their first album, Homework (1997), to their final one, Random Access Memories (2013); the duo described the convergence of technological and human voice "the identity of their musical project". For instance, the lyrics of "Around the World" (1997) are integrally vocoder-processed, "Get Lucky" (2013) features a mix of natural and processed human voices, and "Instant Crush" (2013) features Julian Casablancas singing into a vocoder.

Robot voices became a recurring element in popular music during the 20th century. Apart from vocoders, several other methods of producing variations on this effect include: the Sonovox, Talk box, Auto-Tune, linear prediction vocoders, speech synthesis, ring modulation and comb filter. Vocoders are used in television production, filmmaking and games, usually for robots or talking computers. The robot voices of the Cylons in Battlestar Galactica were created with an EMS Vocoder 2000. 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. A similar Roland VP-330 vocoder was used to create the voice of Soundwave, a character from the Transformers series.