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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.1 How a vocoder works
The human voice consists of sounds generated by the opening and closing of the glottis by the vocal cords, which produces a periodic waveform. This basic sound is then modified by the nose and throat to produce differences in pitch in a controlled way, creating the wide variety of sounds used in speech. There are 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 frequency, the fundamental frequency, and measuring how it is 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 into a modifier that changes the frequency based on the originally recorded series of numbers.
Of course, the actual qualities of speech cannot be reproduced this easily. In addition to a single fundamental frequency, the vocal system adds in a number of resonant frequencies that add character and quality to the voice, known as the formant. Without capturing these additional qualities, the vocoder will never sound "real".
In order to address this, most vocoder systems use what are effectively a number of vocoders, 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.
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 500Hz to 8kHz, where most of the frequencies used in speech lie, which requires 64kbit/s of bandwidth (due to Nyquist frequencyThe Nyquist rate named after the Nyquist-Shannon sampling theorem, is immediately below the minimum theoretical sampling rate that will fully describe a given band-limited signal, enabling its faithful reconstruction from the samples. If the signal's larg). However a vocoder can provide a reasonably good simulation with as little as 2400 bit/s of bandwidth, a 26x improvement.
2 Vocoder alogrithms
Several vocoder algorithms are used in NSA encryption systemsThe National Security Agency took over responsibility for all U. Government encryption systems when it was formed in 1952. The technical details of most NSA-approved systems are still classified, but much more about its early systems has become known and:
- LPC-10, FIPSFIPS could mean Federal Information Processing Standard, publicly announced standards developed by the U. Federal government. FIPS, a computer program for splitting harddisk partitions. Pub 137, 2400 bit/s, which uses linear predictive codingLinear predictive coding (LPC) is a tool used mostly in audio signal processing and speech processing for representing the spectral envelope of a digital signal of speech in compressed form, using the information of a linear predictive model. It is one of
- Code Excited Linear Prediction, (CELP), 2400 and 4800 bit/s, Federal Standard 1016, used in STU-IIISTU-III is a family of secure telephones introduced in 1987 by the NSA for use by the United States Government, its allies and its contractors. STU-III desk units look like typical office telephones, plug into a standard telephone wall jack and can make c
- Continuously Variable Slope Delta-modulation (CVSD), 16 Kbit/s, used in wideband encryptors such as the KY-57The Speech Security Equipment (VINSON), TSEC/KY-57 is a portable, tactical cryptographic device in the VINSON family, designed to provide voice encryption for a range of military communication devices such as radio or telephone. The KY-57 can accept signa.
- Adaptive Differential Pulse Code Modulation (ADPCM), ITU-TITU-T is the telecom standardization organization of the International Telecommunication Union (ITU). It was previously known as CCITT or Comite Consultatif International Telephonique et Telegraphique (Consultative Committee for International Telegraphy a G.721, 32Kbit/s used in STE secure telephone
- 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.
