Method and device for separating acoustic signals

• US 7,327,852 B2
• Filed: 01/31/2005
• Issued: 02/05/2008
• Est. Priority Date: 02/06/2004
• Status: Active Grant

• Current Assignee: Analog Devices International Unlimited Company (Analog Devices, Inc.)
• Original Assignee: Dietmar Ruwisch
• Inventors: Ruwisch, Dietmar
• Primary Examiner(s): LE, HUYEN D
• Application Number:
  US10/557,754
• Publication Number:
  US 20070003074A1
• Time in Patent Office: 1,100 Days
• Field of Search: 381/66, 381/71.1, 381/71.11, 381/91, 381/92, 381/94.2, 381/94.7, 381/94.9, 381/122, 381/356, 379/406.08, 379/406.12, 704/233, 704/231
• US Class Current: 381/356
• CPC Class Codes: G10L 2021/02165 Two microphones, one receiving mainly the noise signal and the other one mainly the speech signal
• CPC Class Codes: G10L 2021/02166 Microphone arrays; Beamforming
• CPC Class Codes: G10L 21/02 Speech enhancement, e.g. noise reduction or echo cancellation reducing echo effects in line transmission systems H04B3/20; echo suppression in hands-free telephones H04M9/08

Abstract

In a method of separating acoustic signals from a plurality of sound sources comprising the following steps: disposing two microphones (MIK1, MIK2) at a predefined distance (d) from one another; picking up the acoustic signals with both microphones (MIK1, MIK2) and generating associated microphone signals (m1, m2); and separating the acoustic signal of one of the sound sources (SI) from the acoustic signals of the other sound sources (S2) on the basis of the microphone output signals (m1, m2), the proposed separation step comprises the following steps: applying a Fourier transform to the microphone output signals in order to determine their frequency spectra (M1, M2); determining the phase difference between the two microphone output signals (m1, m2) for every frequency component of their frequency spectra (M1 , M2); determining the angle of incidence of every acoustic signal allocated to a frequency of the frequency spectra (M1, M2) on the basis of the relative phase angle and the frequency; generating a signal spectrum (5) of a signal to be output by correlating one of the two frequency spectra (M1, M2) with a filter function which is selected so that acoustic signals from an area around a preferred angle of incidence are amplified relative to acoustic signals from outside this area; and applying an inverse Fourier transform to the resultant signal spectrum.

Claims

1. Method of separating acoustic signals from a plurality of sound sources (S1, S2), comprising the following steps:

disposing two microphones (MIK1, MIK2) at a predefined distance (d) from one another;

picking up the acoustic signals with both microphones (MIK1, MIK2) and generating associated microphone signals (m1, m2); and

separating the acoustic signal of one of the sound sources (S1) from the acoustic signals of the other sound sources (S2) on the basis of the microphone signals (m1, m2),

in which the separation step comprises the following steps;

applying a Fourier transform to the microphone signals in order to determine their frequency spectra (M1, M2);

determining the phase difference (φ

) between the two microphone signals (m1, m2) for every frequency component of their frequency spectra (M1, M2);

determining the angle of incidence (θ

) of every acoustic signal allocated to a frequency of the frequency spectra (M1, M2) on the basis of the phase difference (φ

) and the frequency;

generating a signal spectrum (S) of a signal to be output by correlating one of the two frequency spectra (M1, M2) with a filter function (F<sub>θ

0</sub>) which is selected so that acoustic signals from an area (γ

_3db) around a preferred angle of incidence (θ

₀) are amplified relative to acoustic signals from outside this area (γ

_3db); and

applying an inverse Fourier transform to the resultant signal spectrum, characterised in that the filter function (F<sub>θ

0</sub>) is dependent on the angle of incidence θ and

has a maximum at the preferred angle of incidence (θ

₀) when the angle of incidence θ

is varied, and the correlation of the filter function (F<sub>θ

0</sub>) with one of the two frequency spectra comprises multiplying the same.

2. Method as claimed in claim 1, characterised in that the filter function (F_θ

₀) is expressed as follows;

F<sub>θ

0</sub>(f,T)=Z(θ

−

θ

₀)+DΔ

²_fZ(θ

−

θ

₀)

in which

f is the respective frequency

T is the instant at which the frequency spectra (M1, M2) are determined

Z(θ

−

θ

₀) is an allocation function with a maximum at θ

₀

D≧

0 is a diffusion constant and

Δ

² is a discrete diffusion operator.

3. Method as claimed in claim 2, characterised in that the allocation function (Z) is expressed as follows:

$Z<br><br>\left(\mathrm{\vartheta <br><br>}-{\mathrm{\vartheta <br><br>}}_0\right)={\left(\frac{1+\cos <br><br>\left(\mathrm{\vartheta <br><br>}-{\mathrm{\vartheta <br><br>}}_0\right)}{2}\right)}^n$ $\mathrm{where}$ $n><br><br>0.$

4. Method as claimed in claim 1, characterised in that the angle of incidence θ

is determined by the equation

θ

arc cos(x(f,T))

with

x(f,T)φ

,c/2π

fd

where

φ

is the phase difference between the two microphone signal components (m1, m2)

c is the acoustic velocity

f is the frequency of the acoustic signal component and

d is the predefined distance of the two microphones (MIK1, MIK2).

5. Method as claimed in claim 4, characterised in that it additionally incorporates the following step:

limiting the value of x(f,T) to the interval [−

1,1].

6. Method as claimed in claim 5, characterised in that it additionally incorporates the following step:

reducing signal components whose value of x(f,T) lay outside of the interval [−

1,1] prior to limitation.

7. Device for implementing the method as claimed in claim 1, comprising:

two microphones (MIK1, MIK2);

a sampling and Fourier transform unit (20) connected to the microphones for discretizing and digitising the microphone signals (m1, m2) and applying a Fourier transform to them;

a calculating unit (30) connected to the sampling and Fourier transform unit (20) for calculating the angle of incidence (θ

) of every acoustic signal component; and

at least one signal generator (40) connected to the calculating unit (30) for outputting the separated acoustic signal, at least one signal generator (40) having means for multiplying one of the Fourier transformed frequency spectra (M1, M2) by a filter function (F<sub>θ

0</sub>) which is dependent on θ and

has a maximum at a preferred angle of incidence (θ

₀) when θ

is varied.

8. Device as claimed in claim 7, characterised in that the distance (d) between the microphones satisfies the equation:

d<

c/4f_A

where c is the acoustic velocity and f_A is the sampling frequency of the stereo sampling and Fourier transform unit (20).

9. Device as claimed in claim 7, characterised in that the device has a signal generator (40) for every sound source (S1, S2) to be separated.