Noise reduction by using spectral decomposition and non-linear transformation

• US 5,133,013 A
• Filed: 09/15/1989
• Issued: 07/21/1992
• Est. Priority Date: 01/18/1988
• Status: Expired due to Fees

• Current Assignee: British Telecommunications PLC (BT Group PLC)
• Original Assignee: British Telecommunications PLC (BT Group PLC)
• Inventors: Munday, Edward
• Primary Examiner(s): Kemeny, Emanuel S.
• Assistant Examiner(s): Doerrler, Michelle
• Application Number:
  US07/401,455
• Time in Patent Office: 1,040 Days
• Field of Search: 381/41-47, 381/71, 381/73.1, 381/94
• US Class Current: 704/226
• CPC Class Codes: G10L 21/0208 Noise filtering
• CPC Class Codes: G10L 21/0232 Processing in the frequency domain
• CPC Class Codes: H03G 7/007 of digital or coded signals

Abstract

A noise reduction system for enhancing noisy speech signals by performing a spectral decomposition on the signal, passing each spectral component through a non-linear stage which progressively attenuates lower intensity spectral components (uncorrelated noise) but passes higher intensity spectral components (correlated speech) relatively unattenuated, and reconstituting the signal. Frames of noisy signal are transformed into the frequency domain by an FFT (Fast-Fourier Transform) device, with windowing. Each transformed frame is then processed to effect a non-linear transfer characteristic, which is linear above a soft "knee" region, and rolls off below, and transformed back to a reconstituted time-domain signal with reduced noise by an IFFT (Inverse Fast Fourier Transform) device (with overlapping). A level control matches the signal to the characteristic. In further embodiments, the characteristic may vary between frequency bands, and may be matched to speech formants by tracking formants using an LSP (Linear Spectral Pairs) technique.

Claims

1. A noise reduction apparatus comprising:

first conversion means for converting a time-varying input signal into output signals representing the magnitude of spectral components of said input signals;

processing means for applying to said output signals a transfer characteristic which attenuates the low magnitude spectral components of said output signals relative to the high magnitude spectral components of said output signals, said transfer characteristic being substantially linear for said high magnitude spectral components and non-linear for said low magnitude spectral components, said non-linear region having an average slope not exceeding 10 at detectable signal levels on a plot having identical logarithmic axes for said output signals and said transfer characteristic, and

second conversion means for converting the output of said processing means into a time-varying signal.

2. A noise reduction apparatus as in claim 1 wherein said transfer characteristic comprises a plurality of transfer characteristics which attenuate the low magnitude spectral components of said output signals relative to the high magnitude spectral components of said output signals, said transfer characteristics being substantially linear for said high magnitude components and non-linear for said low magnitude components, different said transfer characteristics being assigned to various portions of the frequency spectrum of said output signals.

3. An apparatus as in claim 2 in which the frequency assignment of said different transfer characteristics is predetermined.

4. A noise reduction apparatus comprising:

first conversion mmeans for converting a time-varying input signal into output signals representing the magnitude of the spectral components of said input signal;

processing means for applying to said output signals a plurality of transfer characteristics which attenuate the low magnitude spectral components of said output signals relative to the high magnitude spectral components of said output signals, said transfer characteristics being substantially linear for said high magnitude components and non-linear for said low magnitude components, different said transfer characteristics being assigned to various portions of the frequency spectrum of said output signals, said processing means further comprising means for delivering a time-average spectral distribution of said output signals, and means for periodically determining the frequency assignment of said different transfer characteristics in accordance with said time-averaged spectral distribution; and

second conversion means for converting the output of said processing means into a time-varing signal.

5. A noise reduction apparatus comprising:

first conversion means for converting a time-varying input signal into output signals representing the magnitude of the spectral components of said input signal;

processing means for applying to said output signals a plurality of transfer characteristics which attenuate the low magnitude spectral components of said output signals relative to the high magnitude spectral components of said output signals, said transfer characteristics being substantially linear for said high magnitude components and non-linear for said low magnitude components, different transfer characteristics being assigned to various portions of the frequency spectrum of said output signals, said processing means further comprising means for detecting the spectral position of components of said output signals, and means for varying the frequency assignment of said different transfer characteristics in accordance with the spectral position of components of said output signals, and

second conversion means for converting the output of said processing means into a time-varying signal.

6. An apparatus as in claim 5, in which said detection means employs a Line Spectral Pair analysis method to detect the spectral position of components of said output signals.

7. A noise reduction apparatus comprising:

first conversion means for converting a time-varyinig input signal into output signals representing the magnitude of the spectral components of said input signals;

processing means for applying to said output signals at least one transfer characteristic which attenuates the low magnitude spectral components of said output signals relative to the high magnitude spectral components of said output signals, said transfer characteristic being substantially linear for said high magnitude components and non-linear for said low magnitude components;

level adjusting means for maintaining the spectral components of said output signals within a predetermined relationship with said transfer characteristic, said level adjusting means being operatively coupled to said processing means, and

second conversion means for converting the output of said processing means into a time-varying signal.

8. Apparatus as in claim 7, in which said level adjusting means is an automatic gain control circuit responsive to the average level of said time varying input signal.

9. Apparatus as in claim 7, in which said level adjusting means maintains the noise spectrum of said output signals in the non-linear region of said transfer characteristic.

10. A method of reducing noise in a time-varying signal comprising the steps of:

(a) converting the time-varying signal into a plurality of signals representing the magnitude of spectral components of the time-varying signal;

(b) processing signals of different spectral components so that low magnitude spectral components are attenuated relative to high magnitude spectral components while leaving the relationship between the high magnitude spectral components, said processing differs between different spectral components;

(c) converting the processed signals into a time-varying signal having an attenuated noise content; and

(d) adjusting the level of at lest some of the spectral components of the time-varying signal to maintain these components within a predetermined relationship.

11. A noise reduction apparatus comprising:

first conversion means for converting a time-varying input signal into output signals representing the magnitude of the spectral components of said input signal;

processing means for applying to said output signals at least one transfer characteristic which attenuates the low magnitude spectral components of said output signals relative to the high magnitude spectral components of said output signals, said transfer characteristic being substantially linear for said high magnitude components and non-linear for said low magnitude components;

level adjusting means for maintaining the spectral components of said output signals within a predetermined relationship with said transfer characteristic, said level adjusting means operatively coupled to said apparatus to adjust said input signal, said output signals or said at least one transfer characteristic, and

second conversion means for converting the output of said processing means into a time-varying signal.