A vocoder (Template:Pron-en, a portmanteau of the words voice and encoder) is an analysis/synthesis system, mostly used for speech. In the encoder, the input is passed through a multiband filter, each band is passed through an envelope follower, and the control signals from the envelope followers are communicated to the decoder. The decoder applies these (amplitude) control signals to corresponding filters in the (re)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 encrypted and then transmitted. The advantage of this method of "encryption" is that no 'signal' is sent, but rather envelopes of the bandpass filters. The receiving unit needs to be set up in the same channel configuration to resynthesize a version of the original signal spectrum. The vocoder as both hardware and software has also been used extensively as an electronic musical instrument.
Whereas the vocoder analyzes speech, transforms it into electronically transmitted information, and recreates it, the voder (from Voice Operating Demonstrator) generates synthesized speech by means of a console with fifteen touch-sensitive keys and a foot pedal, basically consisting of the "second half" of the vocoder, but with manual filter controls, needing a highly trained operator.
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 numbers representing these modified 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. Thus, 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, processing a broadband noise source by passing it through a stage that filters the frequency content based on the originally recorded series of numbers. Information about the instantaneous frequency (as distinct from spectral characteristic) of the original voice signal is discarded; it wasn't important to preserve this for the purposes of the vocoder's original use as an encryption aid, and it is this "dehumanizing" quality of the vocoding process that has made it useful in creating special voice effects in popular music and audio entertainment.
Most analog vocoder systems use a number of frequency 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 the output of 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.
The first experiments with a vocoder were conducted in 1928 by Bell Labs engineer Homer Dudley, who was granted a patent for it on March 21, 1939. The Vocoder was introduced to the public at the AT&T building at the 1939-1940 New York World's Fair. Dudley's vocoder was used in the SIGSALY system, which was built by Bell Labs engineers (Alan Turing was briefly involved) in 1943. The SIGSALY system was used for encrypted high-level communications during World War II. Later work in this field has been conducted by James Flanagan.
Linear prediction-based vocodersEdit
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 spectral peaks can generally only be determined to be within the scope of a given frequency band. 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.
Modern vocoder implementationsEdit
Even with the need to record several frequencies, and the additional unvoiced sounds, the compression of the vocoder system is impressive. Standard speech-recording systems capture frequencies from about 500 Hz to 3400 Hz, where most of the frequencies used in speech lie, typically use a sampling rate of 8 kHz (slightly greater than the Nyquist rate.) The sampling resolution is typically at least 12 or more bits per sample resolution (16 is standard), for a final data rate in the range of 96-128 kbit/s. However a good vocoder can provide a reasonably good simulation of voice with as little as 2.4 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 kbit/s and 6.4 kbit/s. Many voice systems use even 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
(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.)
Vocoders are also currently used in developing psychophysics, linguistics, computational neuroscience and cochlear implant research.
Modern vocoders that are used in communication equipment and in voice storage devices today are based on the following algorithms:
- Algebraic code-excited linear predictive codecs (ACELP 4.7 kbit/s – 24 kbit/s)
- Mixed-excitation vocoders (MELPe 2400, 1200 and 600 bit/s)
- Multi-band excitation vocoders (AMBE 2000 bit/s – 9600 bit/s)
- Sinusoidal-Pulsed Representation vocoders (SPR 300 bit/s – 4800 bit/s)
- Tri-Wave Excited Linear Predictive vocoders (TWELP 2400 – 3600 bit/s)
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.
In 1969, electronic music pioneer Bruce Haack built one of the first truly musical vocoders. He named it 'Farad' after 19th-century English chemist / physicist Michael Faraday and unlike its successors and predecessors, 'Farad' was programmed by touch and proximity relays. This invention was first used on Haack's album The Electronic Record for Children (1969), a DIY home pressing found mostly in libraries and elementary schools. In 1970 Wendy Carlos and Robert Moog followed with 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. "Timesteps" was originally 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.Template:Citation needed
Bruce Haack's Electric Lucifer (1970) was the first rock album to include the vocoder and was followed several years later by Kraftwerk's Autobahn. Another of the early songs to feature a vocoder was "The Raven" on the 1976 album Tales of Mystery and Imagination by progressive rock band The Alan Parsons Project; the vocoder also was used on later albums such as I Robot. Following Alan Parsons' example, vocoders began to appear in pop music in the late 1970s, for example, on disco recordings. Jeff Lynne of Electric Light Orchestra used the vocoder in several albums such as Time (featuring the Roland VP-330 Plus MkI). ELO songs such as "Mr. Blue Sky" and "Sweet Talkin' Woman" both from Out of the Blue (1977) use the vocoder extensively. Featured on the album are the EMS Vocoder 2000W MkI, and the EMS Vocoder (-System) 2000 (W or B, MkI or II).
Template:Listen Giorgio Moroder made extensive use of the vocoder on the 1975 album Einzelganger and on the 1977 album From Here to Eternity. Another example is Pink Floyd's album Animals, where the band put the sound of a barking dog through the device. Vocoders are often used to create the sound of a robot talking, as in the Styx song "Mr. Roboto". It was also used for the introduction to the Main Street Electrical Parade at Disneyland.
Vocoders have appeared on pop recordings from time to time ever since, 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 Five Miles Out, 1982). 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, Stevie Wonder ["Send One Your Love", "A Seed's a Star"] and jazz/fusion keyboardist Herbie Hancock during his late 1970s period.
Other voice effectsEdit
"Robot voices" became a recurring element in popular music during the late 20th century. Several methods of producing variations on this effect are: the Talk box (Sonovox), Auto-Tune, linear prediction vocoders, ring modulation, speech synthesis and comb filter.
Television, film and game applicationsEdit
Vocoders have been used in television, film and games usually for robots or talking computers.
- The Cylons from Battlestar Galactica used the EMS Vocoder 5000 and a ring-modulator to create their duo-tone voice effects. The 1980 version of the Doctor Who theme has a section generated by a Roland SVC-350 Vocoder. It is first obvious about 15 seconds into the theme.
Analogue vocoder modelsEdit
- Analog-Lab X-32
- Bode Model 7702
- Doepfer Modular Vocoder subsystem A-129
- Electro-Harmonix Vocoder
- Elektronika (Электроника) EM 26
- EMS Vocoder 1000
- EMS Vocoder 2000
- EMS Vocoder 3000
- EMS Vocoder 5000
- Farad - Bruce Haack Custom Model
- FAT PCP-330 Procoder
- Korg VC-10
- Korg DVP-1 (Curtis Chip Filters)
- Kraftwerk Custom Model (Above Photo)
- Krok (Крок) 2401Vocoder (Вокодер)
- MAM Vocoder VF11
- Moog Modular Vocoder
- Moog Vocoder [Bode]
- Next! VX-11 Vocoder
- PAiA 6710 Vocoder
- Roland SVC-350 (10 Band)
- Roland VP-330 Vocoder Plus (10 Band)
- Sennheiser VSM 201
- Synton Syntovox 202
- Synton Syntovox 216
- Synton Syntovox 221
- Synton Syntovox 222
Hardware DSP vocoder modelsEdit
- Access Virus C Series/Virus TI Series [32-band]
- Akai Professional Mini AK Virtual Analog Synth [40-band]
- Alesis Akira
- Alesis Ion [40-band]
- Alesis Metavox
- Alesis Micron [40-band]
- Behringer 2024 DSP Virtualizer Pro
- Clavia Nord Modular
- Clavia Nord Modular G2
- DigiTech Talker
- Digitech S100/S200
- Digitech StudioQuad 4
- Electrix Warp Factory
- Electro Harmonix IRON LUNG Vocoder
- Electro Harmonix V256 Vocoder
- Electro Harmonix Voice Box Vocoder and Harmonizer
- Ensoniq FIZMO
- Eventide SP2016
- Eventide H3000
- Eventide H7600
- Eventide H8000
- Eventide Orvillle
- Korg MicroKORG
- Korg MicroKORG XL
- Korg MS2000 [16-band]
- Korg MS2000B
- Korg RADIAS
- Korg RADIAS-R
- Korg R3
- Novation A-station Analog Modeling Synthesizer Vocoder
- Novation K-Station KS4 / KS5 / KS Rack [12-band]
- Novation Nova [40-band]
- Quasimidi Sirius
- Red Sound Vocoda
- Red Sound Darkstar
- Roland Juno-Stage [10-band]
- Roland SP-808 [10-band]
- Roland JP-8080 [12-band]
- Roland VP-550
- Roland VP-770
- Symbolic Sound Kyma/Pacarana
- TC Helicon VoiceTone Synth HardTune & Vocoder Pedal
- Waldorf Q
- Zoom Studio 1201
Software vocoder modelsEdit
- Arboretum Systems Ionizer
- Arturia Vocoder
- Crysonic CryCoder
- Eiosis ELS Vocoder
- Fruity Vocoder
- Opcode Fusion Vocode
- Native Instruments Vokator
- Propellerheads Reason BV-512 [4 to 512-band]
- Prosoniq OrangeVocoder
- VirSyn MATRIX Vocoder
- ↑ Wendy Carlos about vocoders
- ↑ Homer Dudley. Signal Transmission US Patent No.2151019, May 21, 1939. (Filed Oct. 30, 1935)
- ↑ Voice Age
- ↑ Compandent
- ↑ Digital Voice Systems Inc.
- ↑ DSP Innovations Inc.
- ↑ TWELP
- ↑ http://www.emsrehberg.de/Vocoder_3000/Vocoder__5000/vocoder__5000.html
- ↑ http://www.bigbluewave.co.uk/ems_vocoder_5000.htm
- ↑ http://monopole.ph.qmw.ac.uk/~thomas/synthdiy/EMS%20VOCODER%205000.htm
- ↑ http://filters.muziq.be/model/roland/svc350
- "How Vocoders Work" from PAIA
- Penn State - basic voder/vocoder operation with simple diagrams
- Description, photographs, and diagram for the vocoder at obsolete.com
- Vokator Tutorial. Description of a modern Vocoder.
- Electro-Harmonix Voice Box Vocoder + Harmonizer
- GPL implementation of a vocoder, as a LADSPA plugin
- O'Reilly Article on Vocoders
- The only Russian rackmount Vocoder, Крок Вокодер, in the Soviet Synthesizer Museumde:Vocoder