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Vocoder

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This article is about the voice encoder. For the dictation machine, see voice recorder.
Early 1970s vocoder, custom-built for electronic music band Kraftwerk

A vocoder (/ˈvoʊkoʊdər/, a portmanteau of voice and encoder) is a category of speech coding that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption or voice transformation.

The vocoder was invented in 1938 by Homer Dudley at Bell Labs as a means of synthesizing human speech.[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.

By encrypting the control signals, voice transmission can be secured against interception. Its primary use in this fashion is for secure radio communication. The advantage of this method of encryption is that none of the original signal is sent, only envelopes of the bandpass filters. The receiving unit needs to be set up in the same filter configuration to re-synthesize a version of the original signal spectrum.

The vocoder has also been used extensively as an electronic musical instrument. The decoder portion of the vocoder, called a voder, can be used independently for speech synthesis.

Theory

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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 created or modified by the mouth in different fashions.

The vocoder examines speech by measuring how its spectral characteristics change over time. This results in a series of signals representing these frequencies at any particular time as the user speaks. In simple terms, the signal is split 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 gives the instantaneous representation of the spectral energy content. To recreate speech, the vocoder simply reverses the process, processing a broadband noise source by passing it through a stage that filters the frequency content based on the originally recorded series of numbers.

Specifically, in the encoder, the input is passed through a multiband filter, then the output of each band is measured using an envelope follower, and the signals from the envelope followers are transmitted to the decoder. The decoder applies these as control signals to corresponding amplifiers of the output filter channels.

Information about the instantaneous frequency of the original 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. Since the parameters change slowly compared to the original speech waveform, the bandwidth required to transmit speech can be reduced. This allows more speech channels to utilize a given communication channel, such as a radio channel or a submarine cable.

Analog vocoders typically analyze an incoming signal by splitting the signal into multiple tuned frequency bands or ranges. To reconstruct the signal, a carrier signal is sent through a series of these tuned bandpass filters. In the example of a typical robot voice the carrier is noise or a sawtooth waveform. There are usually between 8 and 20 bands.

The amplitude of the modulator for each of the individual analysis bands generates a voltage that is used to control amplifiers for each of the corresponding carrier bands. The result is that frequency components of the modulating signal are mapped onto the carrier signal as discrete amplitude changes in each of the frequency bands.

Often there is an unvoiced band or sibilance channel. This is for frequencies that are outside the analysis bands for typical speech but are still important in speech. Examples are words that start with the letters s, f, ch or any other sibilant sound. Using this band produces recognizable speech, although somewhat mechanical sounding. Vocoders often include a second system for generating unvoiced sounds, using a noise generator instead of the fundamental frequency. This is mixed with the carrier output to increase clarity.

In the channel vocoder algorithm, among the two components of an analytic signal, considering only the amplitude component and simply ignoring the phase component tends to result in an unclear voice; on methods for rectifying this, see phase vocoder.

History

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Schematic circuit of Dudley's Vocoder
(based on: Dudley 1940, p. 508, Fig.7[2])
SIGSALY (1943–1946) speech encipherment system
HY-2 Vocoder (designed in 1961), was the last generation of channel vocoder in the US.[3][4]

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.[7]

To demonstrate the speech synthesis ability of its decoder section, the voder (voice operating demonstrator)[8] was introduced to the public at the AT&T building at the 1939–1940 New York World's Fair.[9] The voder consisted of an electronic oscillator – a sound source of pitched tone – and noise generator for hiss, a 10-band resonator filters with variable-gain amplifiers 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.[10] 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.[9][media 1]

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 1,200 bit/s. In 1953, KY-9 THESEUS[11] 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.[12]

Later work in this field has since used digital speech coding. The most widely used speech coding technique is linear predictive coding (LPC).[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.[14]

Applications

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  • 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.[15]
  • Musical and other artistic effects[16]

Modern implementations

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See also: Speech codec and Audio codec

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 resolution, 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.[citation needed]

Several vocoder systems are used in NSA encryption systems:

Modern vocoders that are used in communication equipment and in voice storage devices today are based on the following algorithms:

Vocoders are also currently used in psychophysics, linguistics, computational neuroscience and cochlear implant research.

Linear prediction-based

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Further information: Linear predictive coding

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

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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.[24][25][26]

Artistic effects

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Uses in music

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Channel vocoder setting as a musical application; the Dutch captions are "Band-pass filters" and "Level meters"

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.

History

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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.[27] Later he was instrumental in the founding of the Studio for Electronic Music of WDR in Cologne, in 1951.[28]

Siemens Synthesizer (c. 1959) at Siemens Studio for Electronic Music was one of the first attempts to use a vocoder (rear) to create music

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.[29][30][media 2]

In 1968, Robert Moog developed one of the first solid-state musical vocoders for the electronic music studio of the University at Buffalo.[31]

In 1968, Bruce Haack built a prototype vocoder, named Farad after Michael Faraday.[32] 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.[33][media 3]

In 1970, Wendy Carlos and Robert Moog built another musical vocoder, a ten-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 ten-band vocoder was fairly intelligible but relied on specially articulated speech.

In 1972, Isao Tomita's first electronic music album Electric Samurai: Switched on Rock was an early attempt at applying speech synthesis technique through a vocoder[citation needed][clarification needed] to electronic rock. The album featured electronic renditions of contemporary rock and pop songs, while utilizing synthesized voices in place of human voices. In 1974, he utilized synthesized voices in his popular classical music album Snowflakes are Dancing, which became a worldwide success and helped to popularize electronic music.

In 1973, the British band Emerson, Lake and Palmer used a vocoder on their album Brain Salad Surgery, for the song "Karn Evil 9: 3rd Impression".

The 1975 song "The Raven" from the album Tales of Mystery and Imagination by The Alan Parsons Project features Alan Parsons performing vocals through an EMI vocoder. According to the album's liner notes, "The Raven" was the first rock song to feature a digital vocoder.

Pink Floyd used a vocoder on three of their albums, first on their 1977 Animals for the songs "Sheep" and "Pigs (Three Different Ones)", then in 1987 on A Momentary Lapse of Reason on "A New Machine Part 1" and "A New Machine Part 2", and finally on 1994's The Division Bell, on "Keep Talking".

The Electric Light Orchestra was among the first to use the vocoder in a commercial context, with their 1977 album Out of the Blue. The band extensively uses it on the album, including on the hits "Sweet Talkin' Woman" and "Mr. Blue Sky".[34] On following albums, the band made sporadic use of it, notably on their hits "The Diary of Horace Wimp" and "Confusion" from their 1979 album Discovery, the tracks "Prologue", "Yours Truly, 2095", and "Epilogue" on their 1981 album Time,[35] and "Calling America" from their 1986 album Balance of Power.

In the late 1970s, French duo Space Art used a vocoder during the recording of their second album, Trip in the Centre Head.[36]

Phil Collins used a vocoder to provide a vocal effect for his 1981 international hit single "In the Air Tonight".[37]

Vocoders have appeared on pop recordings from time to time, most often simply as a special effect rather than a featured aspect of the work. However, many experimental electronic artists of the new-age music genre often utilize vocoder in a more comprehensive manner in specific works, such as Jean-Michel Jarre (on Zoolook, 1984) and Mike Oldfield (on QE2, 1980 and Five Miles Out, 1982).

Vocoder module and use by Mike Oldfield can be clearly seen on his Live At Montreux 1981 DVD (Track "Sheba").

There are also some artists who have made vocoders an essential part of their music, overall or during an extended phase. Examples include the German synthpop group Kraftwerk, the Japanese new wave group Polysics, 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.[38] 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.

Coldplay have used a vocoder in some of their songs. For example, in "Major Minus" and "Hurts Like Heaven", both from the album Mylo Xyloto (2011), Chris Martin's vocals are mostly vocoder-processed. "Midnight", from Ghost Stories (2014), also features Martin singing through a vocoder.[39] The hidden track "X Marks the Spot" from A Head Full of Dreams was also recorded through a vocoder.

Noisecore band Atari Teenage Riot have used vocoders in variety of their songs and live performances such as Live at the Brixton Academy (2002) alongside other digital audio technology both old and new.

The Red Hot Chili Peppers song "By the Way" uses a vocoder effect on Anthony Kiedis' vocals.

Among the most consistent users of the vocoder in emulating the human voice are Daft Punk, who have used this instrument from their first album Homework (1997) to their latest work Random Access Memories (2013) and consider the convergence of technological and human voice "the identity of their musical project".[40] 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.

Ye (Kanye West) used a vocoder on the outro of his song "Runaway" (2010).[41]

Producer Zedd, American country singer Maren Morris and American musical duo Grey made a song titled "The Middle" which featured a vocoder and reached the top ten of the charts in 2018.[42]

Voice effects in other arts

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See also: Robotic voice effects

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,[media 4] linear prediction vocoders, speech synthesis,[media 5][media 6] ring modulation and comb filter.

Example of vocoder
Demonstration of the "robotic voice" effect found in film and television.
Problems playing this file? See media help.

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.[38] 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.

See also

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Notes

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  1. ^ 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.

References

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  1. ^ US 135416A, "System for the artificial production of vocal or other sounds", issued 1937-04-07 
  2. ^ Dudley, Homer (October 1940). "The Carrier Nature of Speech". Bell System Technical Journal. XIX (4).
  3. ^ "HY-2". Cryptomuseum.com. Retrieved 2019-07-31.
  4. ^ "HY-2 Vocoder". Crypto Machines.
  5. ^ Mills, Mara (2012). "Media and Prosthesis: the Vocoder, the Artificial Larynx, and the History of Signal Processing". Qui Parle. 21 (1): 107–149. doi:10.5250/quiparle.21.1.0107. S2CID 143012886.
  6. ^ US application 2151091, Dudley, Homer W., "Signal Transmission", published May 21, 1939, assigned to Bell Telephone Laboratories, Inc.  (filed October 30, 1935)
  7. ^ US application 2098956, Dudley, Homer W., "Signaling system", published 1937-11-16, assigned to Bell Telephone Laboratories, Inc. 
  8. ^ US apprication 2121142, Dudley, Homer, "Signal Transmission", published 1938-06-21, assigned to Bell Telephone Laboratories, Inc. 
  9. ^ a b "The 'Voder' & 'Vocoder' Homer Dudley, USA, 1940". 120 Years of Electronic Music (120years.net). 2013-09-21. The Vocoder (Voice Operated reCorDER) and Voder (Voice Operation DEmonstratoR) developed by the research physicist Homer Dudley, ... The Voder was first unveiled in 1939 at the New York World Fair (where it was demonstrated at hourly intervals) and later in 1940 in San Francisco. There were twenty trained operators known as the 'girls' who handled the machine much like a musical instrument such as a piano or an organ, ... This was done by manipulating fourteen keys with the fingers, a bar with the left wrist and a foot pedal with the right foot.
  10. ^ "The Voder (1939)". Talking Heads: Simulacra. Haskins Laboratories. Based on James L. Flanagan (1965). "Speech Synthesis". Speech Analysis, Synthesis and Perception. Springer-Verlag. pp. 172–173. (See schematic diagram of the Voder synthesizer.)
  11. ^ "KY-9". Cryptomuseum.com. Retrieved 2019-07-31.
  12. ^ "Campbell.qxd" (PDF). Retrieved 2019-07-31.
  13. ^ Gupta, Shipra (May 2016). "Application of MFCC in Text Independent Speaker Recognition" (PDF). International Journal of Advanced Research in Computer Science and Software Engineering. 6 (5): 805–810 (806). ISSN 2277-128X. S2CID 212485331. Archived from the original (PDF) on 2019-10-18. Retrieved 18 October 2019. LPC methods are the most widely used in speech coding
  14. ^ Cummiskey, P.; Jayant, Nikil S.; Flanagan, James L. (1973). "Adaptive quantization in differential PCM coding of speech". The Bell System Technical Journal. 52 (7): 1105–1118. doi:10.1002/j.1538-7305.1973.tb02007.x.
  15. ^ Aguiar, Daniel E.; Taylor, N. Ellen; Li, Jing; Gazanfari, Daniel K.; Talavage, Thomas M.; Laflen, J. Brandon; Neuberger, Heidi; Svirsky, Mario A. (2016-03-01). "Information theoretic evaluation of a noiseband-based cochlear implant simulator". Hearing Research. 333: 185–193. doi:10.1016/j.heares.2015.09.008. ISSN 0378-5955.
  16. ^ Ernst Rothauser. Dissertation and patents on vocoder technology.
  17. ^ "Voice Age" (licensing). VoiceAge Corporation.
  18. ^ "MELPe – FAQ". Compandent Inc.
  19. ^ "IMBE and AMBE". Digital Voice Systems, Inc. Archived from the original on 2017-07-07. Retrieved 2008-11-08.
  20. ^ "SPR Vocoders". DSP Innovations Inc. Archived from the original on 2016-04-09. Retrieved 2008-11-08.
  21. ^ "RALCWI Vocoder IC's". CML Microcircuits. CML Microsystems Plc. Archived from the original on 2018-03-15. Retrieved 2013-05-17.
  22. ^ "TWELP Vocoder". DSP Innovations Inc.
  23. ^ "Noise Rubust Vocoders". Raytheon BBN Technologies. Archived from the original on 2014-04-02.
  24. ^ Kleijn, W.B.; Haagen, J. (1995). "A speech coder based on decomposition of characteristic waveforms". 1995 International Conference on Acoustics, Speech, and Signal Processing. Vol. 1. (AT&T Bell Labs., Murray Hill, NJ). pp. 508–511. doi:10.1109/ICASSP.1995.479640. ISBN 978-0-7803-2431-2. S2CID 9105323.
  25. ^ Kleijn, W.B.; Shoham, Y.; Sen, D.; Hagen, R. (1996). "A low-complexity waveform interpolation coder". 1996 IEEE International Conference on Acoustics, Speech, and Signal Processing Conference Proceedings. International Conference on Acoustics, Speech, and Signal Processing. Vol. 1. (AT&T Bell Labs., Murray Hill, NJ). pp. 212–215. doi:10.1109/ICASSP.1996.540328. ISBN 978-0-7803-3192-1. S2CID 44346744.
  26. ^ Gottesman, O.; Gersho, A. (2001). "Enhanced waveform interpolative coding at low bit-rate". IEEE Transactions on Speech and Audio Processing. 9 (November 2001). (Dept. of Electr. & Comput. Eng., California Univ., Santa Barbara, CA): 786–798. doi:10.1109/89.966082. S2CID 17949435.
  27. ^ Meyer-Eppler, Werner (1949), Elektronische Klangerzeugung: Elektronische Musik und synthetische Sprache, Bonn: Ferdinand Dümmlers
  28. ^ Diesterhöft, Sonja (2003), "Meyer-Eppler und der Vocoder", Seminars Klanganalyse und -synthese (in German), Fachgebiet Kommunikationswissenschaft, Institut für Sprache und Kommunikation, Technische Universität Berlin, archived from the original on 2008-03-05
  29. ^ "Das Siemens-Studio für elektronische Musik von Alexander Schaaf und Helmut Klein" (in German). Deutsches Museum. Archived from the original on 2013-09-30.
  30. ^ Holmes, Thom (2012). "Early Synthesizers and Experimenters". Electronic and Experimental Music: Technology, Music, and Culture (4th ed.). Routledge. pp. 190–192. ISBN 978-1-136-46895-7. (See also excerpt of pp. 157–160 from the 3rd edition in 2008 (ISBN 978-0-415-95781-6))
  31. ^ Bode, Harald (October 1984). "History of Electronic Sound Modification" (PDF). Journal of the Audio Engineering Society. 32 (10): 730–739.
  32. ^ BRUCE HAACK – FARAD: THE ELECTRIC VOICE (Media notes). Bruce Haack. Stones Throw Records LLC. 2010.{{cite AV media notes}}: CS1 maint: others in cite AV media (notes) (link)
  33. ^ "Bruce Haack's Biography 1965–1974". Bruce Haack Publishing.
  34. ^ Out of the Blue (booklet). Electric Light Orchestra. Epic, Legacy. 2007.{{cite AV media notes}}: CS1 maint: others in cite AV media (notes) (link)
  35. ^ "In 1981, Electric Light Orchestra Took Us to the Future". National Review. 2021-08-08. Retrieved 2021-10-19.
  36. ^ Richard, Philippe (29 November 2016). "Musique. Space Art, pionniers de l'electro à la française" [Music. Space Art, pioneers of French electro]. Ouest France (in French). Retrieved 28 April 2021.
  37. ^ Flans, Robyn (5 January 2005). "Classic Tracks: Phil Collins' "In the Air Tonight"". Mix Online. Retrieved 25 February 2015.
  38. ^ a b Tompkins, Dave (2010–2011). How to Wreck a Nice Beach: The Vocoder from World War II to Hip-Hop, The Machine Speaks. Melville House. ISBN 978-1-61219-093-8.
  39. ^ "Midnight is amazing! But it sounds like Chris's voice has autotune in some parts. I thought Coldplay doesn't use autotune?". Coldplay "Oracle". 5 March 2014. Archived from the original on 2 May 2014. Retrieved 25 March 2014.
  40. ^ "Daft Punk: "La musique actuelle manque d'ambition"" (interview). Le Figaro. May 3, 2013.
  41. ^ Estelle Caswell (2016-09-01). "Kanye deconstructed: the human voice as the ultimate instrument". Retrieved 2023-09-26.
  42. ^ "Zedd And Griff Drop Bubbly Future Pop Jam "Inside Out"". EDM.com. Jason Heffler. 23 October 2020. Retrieved 23 October 2021.
Multimedia references
  1. ^ One Of The First Vocoder Machine [sic] (Motion picture). c. 1939. A demonstration of the voder (not the vocoder).
  2. ^ Siemens Electronic Music Studio in Deutsches Museum (multi part) (Video).  Details of the Siemens Electronic Music Studio, exhibited at the Deutsches Museum.
  3. ^ Bruce Haack (1970). Electric to Me Turn – from "The Electric Lucifer" (Phonograph). Columbia Records.   A sample of earlier Vocoder.
  4. ^ T-Pain (2005). I'm Sprung (CD Single/Download). Jive Records.  (A sample of Auto-Tune effect (a.k.a. T-Pain effect).)
  5. ^ Earlier Computer Speech Synthesis (Audio). AT&T Bell Labs. c. 1961.  (A sample of earlier computer-based speech synthesis and song synthesis, by John Larry Kelly, Jr. and Louis Gerstman at Bell Labs, using IBM 704 computer. The demo song "Daisy Bell", musical accompanied by Max Mathews, impressed Arthur C. Clarke and later he used it in the climactic scene of the screenplay for his novel 2001: A Space Odyssey.)
  6. ^ TI Speak & Spell (Video). Texas Instruments. c. 1980.  (A sample of speech synthesis.
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