Method for directional signal processing in an acoustic system

Abstract

A method for directional signal processing for an acoustic system, wherein first and second input transducers generate a first and a second input signal from an ambient sound. First and second intermediate signals are generated from the first and second input signal, wherein a preliminary superposition parameter is obtained for a first superposition of the first intermediate signal and the second intermediate signal in such a way that for the first superposition an attenuation in a first target direction has a maximum. A superposition parameter is formed from the preliminary superposition parameter so that a second superposition of the first and second intermediate signals, formed in the first target direction with the superposition parameter, has a pre-specified first value for a gain that is greater than zero. An output signal of the acoustic system is formed from the second superposition.

Claims

1. A method for directional signal processing in an acoustic system, the method comprising: generating a first input signal from an ambient sound with a first input transducer of the acoustic system; generating a second input signal from the ambient sound with a second input transducer of the acoustic system; using the first input signal and the second input signal to generate a first intermediate signal and a second intermediate signal, respectively; obtaining a preliminary superposition parameter for a first superposition of the first intermediate signal and the second intermediate signal to form the first superposition with an attenuation having a maximum in a first target direction; forming with the preliminary superposition parameter a superposition parameter such that a second superposition of the first intermediate signal and the second intermediate signal, formed in the first target direction using the superposition parameter, has a prespecified first value for a gain that is greater than zero; and forming an output signal of the acoustic system using the second superposition.

2. The method according to claim 1, which comprises generating the first intermediate signal as a cardioid signal and generating the second intermediate signal as an anti-cardioid signal.

3. The method according to claim 1, which comprises forming the superposition parameter so that, for the second superposition, the gain in the first target direction has a global minimum with the prespecified first value.

4. The method according to claim 3, wherein: a real part of the superposition parameter is formed from a linear function of the preliminary superposition parameter, dependent on the first value of the gain in the first target direction, with the function merging into the preliminary superposition parameter when the first value approaches zero; and/or an imaginary part of the superposition parameter is linearly dependent on the real part, and approaches zero when the first value approaches zero.

5. The method according to claim 1, which comprises forming the superposition parameter in such a way that for the second superposition the gain in the first target direction has the prespecified first value and the gain in a second target direction has a prespecified second value.

6. The method according to claim 5, wherein the second value is less than the first value.

7. The method according to claim 5, wherein the second value is equal to zero.

8. The method according to claim 5, wherein the real part and the imaginary part of the superposition parameter are described by a circle in a complex plane with the preliminary superposition parameter as origin and a radius, which has a square that is quadratically dependent on the first value of the gain and quadratically dependent on the preliminary superposition parameter.

9. The method according to claim 5, which comprises forming the superposition parameter in such a way that a signal resulting from the second superposition has a maximum directionality index.

10. The method according to claim 9, which comprises, in a given environment of the preliminary superposition parameter around a critical value, carrying out a regularization of the superposition parameter such that: a value of the superposition parameter to be applied is specified for the critical value of the preliminary superposition parameter; and for values from the specified environment around the critical value, the preliminary superposition parameter is continuously mapped to the superposition parameter.

11. The method according to claim 5, wherein: the superposition parameter is formed so that for the second superposition the gain in the first target direction has the prespecified first value and has a prespecified second value in a second target direction; and the gain in the second target direction has a global minimum with the prespecified second value.

12. An acoustic system, comprising: a first input transducer for generating a first input signal from an ambient sound; a second input transducer for generating a second input signal from the ambient sound; and a control unit configured for carrying out the method according to claim 1.

13. The acoustic system according to claim 12, comprising a hearing aid having said first input transducer and said second input transducer.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a plan view of a hearing aid in an environment with a dominant useful signal source and an interference signal source;

(2) FIG. 2 shows a plan view of a suppression of the interference signal source by the hearing aid according to FIG. 1 by means of adaptive directional microphones;

(3) FIG. 3 shows a block diagram of a method for directional noise suppression for the hearing aid according to FIG. 1;

(4) FIG. 4 shows a plan view of a directional characteristic for the directional noise suppression according to FIG. 3 under the secondary condition of a finite, globally minimum gain in a given direction;

(5) FIG. 5 shows a plan view of a directional characteristic of a directional noise suppression in a given direction under the secondary condition of a maximum directionality index; and

(6) FIG. 6 shows a plan view of a directional characteristic of a directional noise suppression with a given noise suppression in a given direction and a given minimum gain.

(7) Identical and equivalent elements and dimensions are identified with the same reference signs throughout the figures.

DETAILED DESCRIPTION OF THE INVENTION

(8) Referring now to the figures of the drawing in detail and first, in particular, to FIG. 1 thereof, there is shown a schematic plan view of an acoustic system 1, which in the exemplary case is designed as a hearing aid 2. The hearing aid 2 has a first input transducer M1 and a second input transducer M2, which in this case are provided by microphones and are configured to generate a first input signal E1 and a second input signal E2 from an ambient sound 4. The input signals E1 and E2 are each fed to a control unit 6 for carrying out a method for directional signal processing yet to be described. The control unit 6 is implemented in this case on a signal processing device 8 of the hearing aid 2. In a manner yet to be described, an output signal 10 is generated by the signal processing device 8 based on the two input signals E1, E2 and is converted by an output transducer 12 of the hearing aid 2 into an output acoustic signal (not shown). The output transducer 12 in this case is provided by a loudspeaker.

(9) Based on the first input signal E1 and the second input signal E2, a first intermediate signal Z1 is now generated (dashed line) by means of a time-delayed superposition. The first intermediate signal Z1 is generated as a cardioid signal 16, the directional characteristic 18 of which is ideally rotationally symmetrical about a connecting line 20 through the first input transducer M1 and the second input transducer M2 (in the drawing plane of FIG. 1 only one axis of symmetry with respect to the connecting line 20 can be seen). Likewise, from the first input signal E1 and the second input signal E2 a second intermediate signal Z2 is generated by a further, time-delayed superposition (dotted line). The second intermediate signal Z2 is generated here as an anti-cardioid signal 22, the directional characteristic 24 of which is also rotationally symmetrical about the connecting line 20. In addition, the first intermediate signal Z1 and the second intermediate signal Z2 are ideally mirror-symmetrical to each other with respect to a plane of symmetry 26 of the first input transducer M1 and the second input transducer M2 (in FIG. 1, a section of the symmetry plane 26 with the drawing plane is shown).

(10) According to its directional characteristic 18, the first intermediate signal Z1 has a maximum sensitivity in a maximum direction 28 and a minimum sensitivity in a minimum direction 30 opposite to the maximum direction 28. In the minimum direction 30, the first intermediate signal Z1 ideally undergoes total attenuation. The maximum direction 28 and the minimum direction 30 run along the connecting line 20. According to its directional characteristic 24, the second intermediate signal Z2 has a maximum sensitivity in a maximum direction 32 and a minimum sensitivity in a minimum direction 34. The maximum direction 32 of the second intermediate signal Z2 coincides with the minimum direction 30 of the first intermediate signal Z1, the minimum direction 34 of the second intermediate signal Z2 coinciding with the maximum direction 28 of the first intermediate signal Z1.

(11) The hearing aid 2 is designed in such a way that, if worn by a user as intended, the connection line 20 is aligned along the frontal direction of the user. A common situation when using the hearing aid 2 is that the user is in conversation with another person. Accordingly, he directs his view and thus his frontal direction to the interlocutor, whereby, due to the spatial associations just described, the maximum direction 28 of the first intermediate signal Z1 is also aligned to the interlocutor as the dominant useful signal source 36 (here schematically indicated by a loudspeaker symbol). If an interference signal 38 from an interference signal source 40 now occurs in the ambient sound, the said interference signal 38 is suppressed by means of adaptive directional microphones. Usually, from the first intermediate signal Z1 and the second intermediate signal Z2, a first superposition U1 of the form
U1=Z1−a1.Math.Z2
is formed with a first superposition parameter a1 by minimizing the signal energy of the first superposition U1. Assuming that the maximum direction 28 of the first intermediate signal Z1 according to FIG. 1 is oriented toward the interlocutor as a useful signal source 36, and that the second intermediate signal Z2 in its minimum direction 34, which is also oriented toward the other interlocutor, undergoes total attenuation in the ideal case, if the signal energy is minimized as stated, the contribution of the interlocutor due to their assumed suppression by the second intermediate signal Z2 is not affected. The minimization of the signal energy thus only affects the interference signal 38 of the interference signal source 40.

(12) For the situation shown in FIG. 1, where the interference signal source 40 is arranged at a right angle with respect to the frontal direction (and thus the direction of the useful signal source 36 or the maximum direction 28 of the first intermediate signal Z1 according to FIG. 1), for the first superposition U1, as shown by FIG. 2, this leads to a complete attenuation in a first target direction 42 aligned with the interference signal source 40. However, there are situations in which a total attenuation of the interference signal 38 of the interference signal source 40 in an output signal resulting from the described adaptive directional microphone is undesirable, for example for a pedestrian on the road where the ability to hear other road users in a timely manner is important for safety, but also in conversation situations with several interlocutors, where it may be advantageous to at least perceive the interjections of another interlocutor on whom the user is not currently concentrating (and thus not focusing his/her gaze), so that he/she can turn to them for attentive listening.

(13) In order to achieve this, a method for directional signal processing is performed in the hearing aid 2 according to FIG. 1, which will be explained using FIG. 3 by means of a corresponding block diagram. As already shown in FIG. 1, the first input transducer M1 and the second input transducer M2 of the hearing aid 2 generate the first input signal E1 and the second input signal E2 respectively from the ambient sound 4. From the first input signal E1 and the second input signal E2, the first intermediate signal Z1 and the second intermediate signal Z2 are generated by a time-delayed superposition 44, which here is shown only schematically. The first intermediate signal Z1 is generated as the cardioid signal 16, the second intermediate signal Z2 as the anti-cardioid signal 22 according to FIG. 1.

(14) To obtain information about the first target direction 42 of the interference signal source 40 according to FIG. 2 (and thus implicitly to determine the first target direction 42), the first superposition U1 is formed according to FIG. 2 from the first intermediate signal Z1 and the second intermediate signal Z2 by means of an adaptive directional microphone 46. The first superposition U1=Z1−a.sub.0.Math.Z2 provides for the present method a preliminary superposition parameter a.sub.0 (which corresponds to the first superposition parameter a1 of the first superposition U1 according to FIG. 2), wherein the first target direction 42 of the interference signal source 40 according to FIG. 2 is also determined implicitly via the relationship with the first intermediate signal Z1 and the second intermediate signal Z2 using the preliminary superposition parameter a.sub.0.

(15) For said first target direction 42 a first value g1>0 of a gain is now specified, which should comprise a signal yet to be generated from the two intermediate signals Z1, Z2. In the first superposition U1, in the first target direction 42 as shown in FIG. 2, the gain is assumed to be zero (complete attenuation). Using an adaptive directional microphone 48, a second superposition U2=Z1−a.Math.Z2 with the generally complex superposition parameter a is then formed from the first intermediate signal Z1 and the second intermediate signal Z2, specifically under the mentioned secondary condition of a gain of g1 in the first target direction 42, which is specified by the preliminary superposition parameter a.sub.0 of the first superposition U1. From the second superposition U2, if necessary by means of further signal processing steps schematically symbolized by a signal processing block 49, such as frequency band-dependent amplification and/or compression, the output signal 10 is generated, which according to FIG. 1 is converted into an output sound signal by the output transducer 12.

(16) From the knowledge that the first and the second intermediate signal Z1 or Z2 are given by the cardioid signal 16 or the anti-cardioid signal 22 according to FIG. 1, a transfer function G (ω, φ) of the second superposition U2 can be determined with respect to a sound signal incident from an angle φ (with respect to the frontal direction) (depending on the propagation time difference T between the two input transducers M1 and M2). This transfer function can be represented as

(17) $G\left(\omega ,\varphi \right)=2.Math.\sin \frac{\omega T(1+\cos \left(\varphi \right)}{2}-a.Math.\sin \frac{\omega T(1-\cos \left(\varphi \right)}{2}.Math.$
Under the approximation ωT<<1, which is valid especially for low frequencies and propagation time differences T (for hearing aids, T is in the region of 10.sup.−5 s, the approximation is thus valid for large parts of the audible spectrum), the above formula can be approximated to
G(ω,ϕ)=|ωT(1+cos ϕ)−a.Math.ωT(1−cos ϕ)|  (i)

(18) In the frontal direction (i.e. for φ=0), G(ω, φ=0)=2ωT=2kd (with the distance d between the two input transducers M1 and M2 and the wave number k) is thus independent of a. It can now be shown that for a real superposition parameter a=a.sub.0∈ the transfer function given in equation (i) becomes zero at an angle φ.sub.0, for which the following applies:

(19) $\begin{array}{cc}\begin{array}{ccc}{a}_0=\frac{1+\cos {\varphi }_0}{1-\cos {\varphi }_0}& \mathrm{or}& \cos {\varphi }_0=\frac{a_0-1}{a_0+1}\end{array}& \left(\mathrm{ii}\right)\end{array}$

(20) For a generally complex superposition parameter a=aRe+i.Math.aIm, the requirement of a gain of g1 in the first target direction φ.sub.0 can be implemented by means of the transfer function (i) by appropriately equating the transfer function to g1. Due to the additional degree of freedom provided by the imaginary part aIm, the superposition parameter a is not yet fully defined by the first value g1 for the gain in the first target direction given by a.sub.0. It can be shown that in the complex plane for aRe, aIm the permissible real and imaginary parts for a given first value g1, and a given first target direction 42 (and thus given φ.sub.0 or a.sub.0), form a circle around a.sub.0 with radius g1(a.sub.0+1):
(aRe−a.sub.0).sup.2+aIm.sup.2=g1.sup.2.Math.(a.sub.0+1).sup.2  (iii)

(21) Several special cases can now arise, or be implemented accordingly for the second superposition U2.

(22) As a secondary condition, it can be required that the gain in the first target direction 42 should additionally form a global minimum, which, however—unlike in the case shown in FIG. 2—should now no longer assume the value 0 but the first value g1>0. This is shown in FIG. 4.

(23) By minimizing the squared magnitude of the transfer function according to equation (i) (with complex a), it is possible to determine the general angle φ for which the transfer function becomes a minimum. Inserting the so determined value of cos φ into the squared equation (i) first supplies the condition

(24) $\begin{array}{cc}\cos {\left(\varphi \right)}_{\min }=-\frac{1-aR{e}^2-{\mathrm{aIm}}^2}{{\left(1+aRe\right)}^2+{\mathrm{aIm}}^2}& \left(\mathrm{iv}\right)\end{array}$
and equating the transfer function with the required first value g1 for the (globally minimum) gain in the first target direction 42 supplies the dependency
aIm=±(aRe+1).Math.√{square root over (ε)}  (v)
with ε=g1.sup.2/(1−g1.sup.2). Inserting the intermediate result given in equation (v) into equation (iv) and representing the minimum angle min via the corresponding real-valued superposition parameter a.sub.0 according to equation (iii) (for which at φ.sub.min the gain would disappear, i.e. φ.sub.min=φ.sub.0 and corresponding substitution from equation (iii)) produces

(25) $\begin{array}{cc}aRe=\frac{a_0-\epsilon }{1+\epsilon },& \left(\mathrm{vi}\right)\end{array}$
so that the superposition parameter a=aRe+i.Math.aIm, which forms the basis of the second superposition shown in FIG. 4, is obtained from the relationships given in equations (v) and (vi).

(26) Another possibility is illustrated by FIG. 5: here, it is not required that the gain in the second target direction should be a minimum, but that the resulting second superposition U2=Z1−a.Math.Z2 should have a maximum directionality index (DI).

(27) The DI can be determined from the squared magnitude of the transfer function in the maximum direction (i.e. in the maximum direction 28 of the first intermediate signal Z1 according to FIG. 1), normalized over the integral of the squared magnitude of the transfer function across all spatial directions. The DI is usually defined by the logarithm to base ten of the specified variables:

(28) $\begin{array}{cc}\mathrm{DI}\left(\omega ,\varphi ,\theta \right)=10.Math.{\log }_{10}\left(\frac{{.Math.G\left(\omega ,\varphi =0,\theta =\pi /2\right).Math.}^2}{\int d\Omega {.Math.G\left(\omega ,\Omega \right).Math.}^2}\right)& \left(\mathrm{vii}\right)\end{array}$
where the integration in the denominator takes place over the normalized unit sphere, so that for an omnidirectional signal DI=0 is obtained. It can be shown that the DI according to equation (vi) can be represented as a function of the superposition parameter a=aRe+i.Math.aIm as
DI=−10.Math.log.sub.10(aRe.sup.2−aRe+1+aIm.sup.2)+10.Math.log.sub.10(3).  (viii)

(29) The argument of the logarithm remains the same for aRe, aIm, which describe circles in the complex plane around the point (0.5, 0). For said point aRe=0.5, aIm=0, the DI has its maximum. In this case, the associated second superposition U2 forms a directional characteristic in the form of a hypercardioid. From equation (viii) and equation (iii) it is thus apparent that for a.sub.0≠0 and a superposition parameter a according to equation (iii), the DI according to equation (viii) is maximized by a real-valued superposition parameter a=aRe which is to be determined according to equation (iii), i.e.
aRe=a.sub.0±g1.Math.(a.sub.0+1)  (ix)

(30) This results in the desired gain with the first value g1 in the first target direction 42, while in general in a second target direction 50 a total attenuation (i.e. a gain with a second value g2=0) occurs.

(31) The plus sign in equation (ix) applies for a.sub.0<0.5, the minus sign for a.sub.0>0.5. In order to avoid discontinuities for aRe in the case of a moving noise source 40 and thus variable a.sub.0 in the environment of a.sub.0=0.5, a regularization can be carried out in a manner yet to be described (not shown in detail), which for a range a.sub.0≤0.5−d1 and a range a.sub.0≥0.5+d2, with d1, d2>0, preferably d1=d2 and particularly preferably d1, d2<<1, initially provides the value for aRe according to equation (ix). In the range 0.5−d1<a.sub.0<0.5 and 0.5<a.sub.0<0.5+d2, a non-vanishing imaginary part aIm #0 can be applied such that the real part aRe resulting from equation (iii) runs along the maximum gradient of the DI according to equation (ix). It can be shown that this is the case when for a.sub.0=0.5 the value aRe=(1−3.Math.g1.sup.2)/2 is passed through.

(32) A further possibility is illustrated in FIG. 6. There, the superposition parameter a for the second superposition U2 is determined in such a way that a gain with the value g1 occurs in the first target direction 42. Furthermore, a second value of g2<g1 is specified as the global minimum for the gain, which should not be undershot in any direction. In particular, there is a second target direction 50, in which the gain, i.e. the gain factor, assumes exactly the second value g2. In this case, the additional degree of freedom of the imaginary part aIm in the superposition parameter a is used to specify, in addition to a predefined first value g1 of a gain in a first direction, the second value g2 which the gain must not fall below in any direction.

(33) In order to determine the superposition parameter a, the relationship between the real and imaginary part given in equation (v) for a minimum, finite gain must be inserted into the general equation (iii) for a specification of the first value g1, however, in this case the parameter ε=g2.sup.2/(1−g2.sup.2) must be used to allow for the second value g2 of the gain, which is now intended to form the global minimum (in equation (v), the global minimum was given by the first value g1, which now determines the gain in the first target direction).

(34) This results in a quadratic equation for the real part aRe of the superposition parameter a, the positive or negative solution of which is chosen depending on the value of a.sub.0 (a.sub.0>0.5 or a.sub.0<0.5) and thus depending on the first target direction 42 in which the first value g1 of the gain is defined. In an immediate environment of a.sub.0=0.5, a regularization of the type already described in FIG. 5 can be carried out in order to avoid discontinuities. The imaginary part aIm can then be determined using equation (v) (with ε=g2.sup.2/(1−g2.sup.2)).

(35) Although the invention has been illustrated and described in greater detail by means of the preferred exemplary embodiment, the invention is not restricted by the examples disclosed and other variations can be derived therefrom by the person skilled in the art without departing from the scope of protection of the invention.

(36) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 1 acoustic system 2 hearing aid 4 ambient sound 6 control unit 8 signal processing device 10 output signal 12 output transducer 16 cardioid signal 18 directional characteristic (of the first intermediate signal) 20 connecting line 22 anti-cardioid signal 24 directional characteristic (of the second intermediate signal) 26 plane of symmetry 28 maximum direction (of the first intermediate signal) 30 minimum direction (of the first intermediate signal) 32 maximum direction (of the second intermediate signal) 34 minimum direction (of the second intermediate signal) 36 useful signal source 38 interference signal 40 interference signal source 42 first target direction 44 time-delayed superposition 46 adaptive directional microphone 18 adaptive directional microphone 50 second target direction a superposition parameter a.sub.0 preliminary superposition parameter E1, E2 first, second input signal g1, g2 first, second value (of the gain) M1, M2 first, second input transducer U1, U2 first, second superposition Z1, Z2 first, second intermediate signal