Electrical audio signal processing systems and devices – One-way audio signal program distribution – Public address system
Patent
1983-12-19
1987-12-01
Kemeny, E. S. Matt
Electrical audio signal processing systems and devices
One-way audio signal program distribution
Public address system
G10L 500
Patent
active
047109599
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to speech technology and, in particular, digital encoding techniques and methods for synthesizing speech.
Attention is directed to an article by one of the inventors herein, E. M. Hofstetter, and P. E. Blankenship et al., entitled "Vocoder Implementations on the Lincoln Digital Voice Terminal" Proc. of EASCON 1975, Washington, D. C. (Sept. 1975), in which various methods of compressing speech bandwidth are described. Attention is also directed to an article by Hofstetter et al. entitled "Microprocessor Realization of a Linear Predictive Vocoder" Lincoln Laboratory Technical Note 1976-37 (Sept. 1976), in which a dedicated microprocessor for linear predictive coding of speech is described. Both of these articles are incorporated herein by reference.
The principal method of transmitting speech electronically up until the present has been via an analog signal proportional to speech pressure on a transducer such as a microphone. Although electronic devices for bandwidth compression have been known since 1939 and many algorithms for digitally encoding speech have been proposed since the 1960's only with the expotentially decreasing cost of digital electronic technologies of the past fifteen years has a low-cost, low-power, compact, reliable vocoder implementation been foreseeable.
Of the various methods for encoding speech, one preferred method is linear predictive coding (LPC). For a seminal description of this technique see J. D. Markel, A. H. Gray, Jr. Linear Prediction of Speech (Springer-Verlag, N.T. 1967). Essentially, LPC seeks to model the vocal tract as a time varying linear all-pole filter by using very short, weighted segments of speech to form autocorrelation coefficients. From the coefficients, the critical frequency poles of the filter are estimated using recursion analysis.
In addition to modeling the vocal tract as a filter, a voice encoder must also determine the pitch period and voicing state of the vocal cords. One method of doing this is the Gold Method, described by M. L. Malpass in an article entitled "The Gold Pitch Detector in a Real Time Environment" Proc. of EASCON 1975 (Sept. 1975), also incorporated herein by reference. See also, generally B. Gold, "Description of a Computer Program for Pitch Detection", Fourth International Congress on Acoustics, Copenhagen, Aug. 21-28, 1962 and B. Gold, "Note on Buzz-Hiss Detection", J. Acoust. Soc. Amer. 36, 1659-1661 (1964).
For communication processing purposes, the encoding techniques described above must also be performed in the opposite direction in order to synthesize speech.
There exists a need for voice encoders and synthesizers (hereinafter "vocoders") in many communication and related areas. Bandwidth compression is one obvious advantage. Digital speech signals can also be coupled to encryption devices to insure private, secure communications of government defense, industrial and financial data. Moreover, data entry by vocal systems, private or not, represents a significant improvement over key punching in many applications. Additionally, voice authentication and vocal control of automated processes will also depend upon high quality vocoders. Likewise, vocoders may find significant use in entertainment, educational, and business applications.
Thus, there exists a need for high quality vocoders, preferably vocoders which are low cost and manufacturable from stock electronic components, such as standard signal processing chips.
SUMMARY OF THE INVENTION
We have developed a very compact, flexible, fully digital, full duplex 2.4 kilobit per second, linear predictive coding vocoder using only commercially available devices. A total of 16 integrated circuits and 4 discrete component carriers are used occupying 18 square inches and dissipating 5.5 watts of power. In one preferred embodiment, the design is a distributed signal processing architecture based on three Nippon Electric Company Signal Processing Interface (SPI) .mu.PD7720 16-bit, 250 ns cycle time signal processing single-chip microcomputers a
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Feldman Joel A.
Hofstetter Edward M.
Engellenner Thomas J.
Kemeny E. S. Matt
Maslow James E.
Massachusetts Institute of Technology
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