APPARATUS FOR REDUCING MULTIPATH DISTORTION OF SIGNALS UTILIZING CEPSTRUM TECHNIQUE

• US 3,662,108 A
• Filed: 06/08/1970
• Issued: 05/09/1972
• Est. Priority Date: 06/08/1970
• Status: Expired due to Term

• Current Assignee: James Loton Flanagan
• Original Assignee: James Loton Flanagan
• Inventors: Flanagan, James Loton
• Primary Examiner(s): Claffy, Kathleen H.
• Assistant Examiner(s): Leaheey, Jon Bradford
• Application Number:
  US05/044,269
• Time in Patent Office: 701 Days
• Field of Search: 179/1SA,1J,1P,15.55R 324/77E,77F,77B 325/305,65,304,40,56
• US Class Current: 704/228
• CPC Class Codes: G10K 2210/3025 Determination of spectrum characteristics, e.g. FFT
• CPC Class Codes: G10K 2210/505 Echo cancellation, e.g. multipath-, ghost- or reverberation-cancellation
• CPC Class Codes: G10L 2021/02082 the noise being echo, reverberation of the speech
• CPC Class Codes: G10L 2021/02166 Microphone arrays; Beamforming
• CPC Class Codes: H03H 17/0216 Quefrency domain filters
• CPC Class Codes: H03H 17/0286 Combinations of filter structures
• CPC Class Codes: Y10S 367/901 Noise or unwanted signal reduction in nonseismic receiving system

Abstract

Distortion of an acoustic signal caused by multipath transmission is minimized by utilizing a cepstrum technique. Signals received at a plurality of microphones are processed to obtain signal representations of their complex cepstrums. The cepstrum signals are then summed. In summing, the portions of the cepstrum signals representative of the undistorted acoustic signal cohere while the portions of the cepstrum signals representative of the multipath transmitted signals do not. Selective clipping of the summed cepstrum signal eliminates the distortion components. Inverse transformation of the clipped summed cepstrum signal yields a replica of the original acoustic signal.

Claims

1. Signal processing apparatus which comprises means for receiving signals at each of a plurality of spatially separated locations, said received signals being emitted from a source and being characterized by distorted components and undistorted components, means in circuit relationship with said receiving means for generating signals representative of the cepstrum of each of said received signals, processing means supplied with said cepstrum signals for eliminating the distortion components therefrom, and synthesizer means responsive to said processed cepstrum signals for generating a replica of the signal emanating from said source.

2. Apparatus as defined in claim 1 wherein said cepstrum signal generating means includes a plurality of cepstrum analyzers in a one-to-one relationship with said receiving locations.

3. Apparatus as defined in claim 1 wherein said synthesizer means includes means for inverse cepstrum transforming said processed cepstrum signals.

4. Apparatus as defined in claim 1 wherein said processing means includes means for selectively algebraically combining said cepstrum signals, and means for suppressing selected portions of said combined cepstrum signals.

5. Apparatus as defined in claim 4 wherein said combining means is an added and said suppressing means is a center clipper.

6. Apparatus for reducing multipath distortion in an acoustic signal which comprises, a plurality of transducers located in spatially separate relation to one another receiving signals emanating from a source, means in a one-to-one relationship with said transducers for generating signals representative of the complex cepstrum of each of the signals generated by said transducers, means for selectively combining said complex cepstrum signals, means for processing said combined signals to suppress selected portions thereof, and synthesizer means responsive to said processed signals for generating a replica of the signal emanating from said source.

7. Apparatus as defined in claim 6 wherein said combining means includes means for algebraically summing said complex cepstrum signals.

8. Apparatus as defined in claim 7 wherein said processing means includes a center clipper having a predetermined characteristic so that distorted components of said combined complex cepstrum signals are suppressed and undistorted components are not suppressed.

9. Apparatus as defined in claim 8 wherein said synthesizing means includes means for inverse cepstrum transforming said processed signal.

10. Apparatus as defined in claim 9 wherein said inverse cepstrum transformation means includes means supplied with sAid processed signal for generating a plurality of first signals each representative of the logarithm of the amplitude component of the Fourier transform of a segment of said replica signal and for generating a plurality of second signals each representative of the corresponding phase component of the Fourier transform of a segment of said replica signal, means supplied with said first signals for generating a corresponding plurality of signals each representative of the amplitude spectrum of a segment of said replica signal, means supplied with said second signals for generating signals representative of the time derivative of each of said second signals, a plurality of oscillator means responsive in a one-to-one relationship to said differentiated second signals for generating a plurality of phase modulated cosine signals each representative of the phase spectrum of a segment of said replica signal, a plurality of multiplier means in a one-to-one relationship with said oscillator means, each of said multipliers being supplied with the phase spectrum signal and the amplitude spectrum signal for a segment of said replica signal, and means for summing the individual signals developed by said plurality of multiplier means, thereby yielding said replica signal.

11. A method for minimizing multipath distortion in an acoustic signal comprising the steps of analyzing signals emanating from a source and received at a plurality of receiving stations to obtain representations of the individual cepstrums of said received signals, said cepstrums being characterized by distortion components and nondistortion components, summing said cepstrum representations, processing said summed cepstrum representations selectively to eliminate said distortion components therefrom, and further processing of said previously processed representations by inverse cepstrum transforming thereby to synthesize a replica of said source signal.