Hearing device comprising a loop gain limiter

Abstract

A hearing device comprises an input transducer providing an input gain G.sub.I, a signal processor comprising a compressor for determining a frequency and level dependent desired compressor gain G.sub.P to compensate for a hearing impairment of the user, and to provide a resulting compressor gain G′.sub.P, and an output transducer for providing output stimuli perceivable as sound for the user based on a processed signal, the output transducer providing an output gain, G.sub.O. A resulting forward path gain G′ is defined in a logarithmic representation as G.sub.I+G′.sub.P+G.sub.O. The hearing device further comprises a loop gain estimator for continuously estimating a current loop gain ΔL(n), configured to provide a loop gain estimate within a predefined number of feedback loop delays after a feedback buildup has started, and a loop gain controller for dynamically controlling said resulting forward path gain G′ in dependence of said estimate of said current loop gain ΔL(n). A resulting loop gain, LG′, is determined as a sum of the resulting forward path gain G′ and a feedback gain H when given in a logarithmic representation. The loop gain controller is configured to provide that the resulting loop gain is limited to stay below a predefined value.

Claims

1. A hearing device configured to be worn by a user at or in an ear, the hearing device comprising a forward path comprising an input transducer for providing an electric input signal representing sound in the environment of the hearing device, the input transducer providing an input gain G.sub.I, a signal processor for processing said electric input signal or a signal based thereon and providing a processed signal, the signal processor comprising a compressor for determining a frequency and level dependent desired compressor gain G.sub.P to compensate for a hearing impairment of the user, and to provide a resulting compressor gain G′.sub.P, an output transducer for providing output stimuli perceivable as sound for the user based on said processed signal, the output transducer providing an output gain, G.sub.O, a resulting forward path gain G′ being defined in a logarithmic representation as G.sub.I+G′.sub.P+G.sub.O, a loop gain limiter comprising a loop gain estimator for continuously estimating a current loop gain ΔL(n), configured to provide a loop gain estimate within a predefined number of feedback loop delays after a feedback buildup has started, wherein the loop gain estimate is calculated as the current level of a signal of the forward path at time index n minus the level of the same signal one feedback loop earlier, a loop gain controller for dynamically controlling said resulting forward path gain G′ in dependence of said estimate of said current loop gain ΔL(n), an acoustic feedback path from the output transducer to the input transducer, the feedback path exhibiting a feedback gain H, wherein a resulting loop gain, LG′, is determined as a sum of the resulting forward path gain G′ and the feedback gain H when given in a logarithmic representation, and wherein the loop gain controller is configured to provide that the resulting loop gain is limited to stay below a predefined value.

2. A hearing device according to claim 1 wherein the loop gain controller is configured to decrease said resulting forward path gain G′ in case said estimate of said current loop gain ΔL(n) is larger than or equal to a maximum loop gain value LGmax.

3. A hearing device according to claim 1 configured to estimate the current loop gain ΔL(n) in a number of frequency bands K, where K is larger than one.

4. A hearing device according to claim 1 wherein the maximum value LGmax of loop gain is smaller than or equal to 3 dB.

5. A hearing device according to claim 1 wherein the loop gain estimator is configured to estimate said current loop gain ΔL(n) based only on information about the signal level.

6. A hearing device according to claim 1 wherein the loop gain estimator is configured to estimate said current loop gain ΔL(n) as ΔL(n)=L(n)−L(n−n.sub.D), where L(n) is the signal level in dB of a signal of the forward path at the time index n, and L(n−n.sub.D) is the signal level of the same signal one feedback loop earlier, where nD is defined by a loop delay D of said feedback loop.

7. A hearing device according to claim 1 wherein the loop gain estimator is configured to estimate a current loop gain ΔL(n) within less than three feedback loops after a feedback buildup has started.

8. A hearing device according to claim 1 wherein the loop gain estimator comprises a level estimator for estimating a current level of the electric input signal or another signal of the forward path of the hearing device.

9. A hearing device according to claim 8, wherein the level estimator is configured to operate in a number of frequency bands K, where K is larger than one.

10. A hearing device according to claim 1 wherein the loop gain controller is configured to determine said resulting gain G′ according to the following expression
G′(n)=G(n)−ΔG(n) where ΔG(n) is the gain reduction at a given point in time n, wherein the gain reduction is larger than or equal to 0 dB.

11. A hearing device according to claim 10 wherein the loop gain controller is configured to determine said resulting gain G′ according to the following expression $G^{\prime }\left(n\right)=G-\max \left(\frac{\Delta L\left(n\right)-\mathrm{LG}\max }{a},0\right)$ where the parameter α is used to control the degree of loop gain limitation, and LGmax is a maximum acceptable value of loop gain before gain reduction is initiated.

12. A hearing device according to claim 1 configured to smooth the resulting forward path gain G′ over time to provide a smoothed resulting gain G*.

13. A hearing device according to claim 12 comprising a smoothing unit for smoothing the resulting forward path gain G′ over time according to the following expression
G*(n)=β.Math.G′(n)+(1−β).Math.G*(n−1) where β is a positive parameter.

14. A hearing device according to claim 1 wherein the loop gain estimator is configured to provide a corrected current loop gain estimate ΔL′ (n)=ΔL(n)+ΔG(n−1), wherein ΔG(n−1)=G(n−1)−G′(n−1) is the gain reduction one loop delay prior to the current time n, and wherein the loop gain controller is configured to determine said resulting gain G′(n) based on the corrected current loop gain estimate ΔL′(n).

15. A hearing device according to claim 14 wherein the loop gain estimator is configured to multiply the gain reduction ΔG(n−1) one loop delay prior to the current time n with a leaking factor γ, where γ is smaller than 1.

16. A hearing device according to claim 1 wherein the signal processor comprises a combination unit configured to apply said resulting processor gain G′.sub.P to said electric input signal or to a signal originating therefrom.

17. A hearing device according to claim 1 wherein the loop gain controller for dynamically controlling said resulting forward path gain G′ is configured to apply a gain reduction ΔG, only if the estimated loop gain is within a given range.

18. A hearing device according to claim 1 being constituted by or comprising a hearing aid.

19. A method of operating a hearing device configured to be worn by a user at or in an ear, the method comprising providing an electric input signal representing sound in the environment of the hearing device, thereby providing an input gain G.sub.I, processing said electric input signal, or a signal based thereon, and providing a processed signal, thereby determining a frequency and level dependent desired compressor gain G.sub.P to compensate for a hearing impairment of the user, and a resulting compressor gain G′.sub.P, providing output stimuli perceivable as sound for the user based on said processed signal, thereby providing an output gain, G.sub.O, a resulting forward path gain G′ being defined in a logarithmic representation as G.sub.I+G′.sub.P+G.sub.O, continuously estimating a current loop gain ΔL(n), configured to provide a loop gain estimate within a predefined number of feedback loop delays after a feedback buildup has started, wherein the loop gain estimate is calculated as the current level of a signal of the forward path at time index n minus the level of the same signal one feedback loop earlier, dynamically controlling said resulting forward path gain G′ in dependence of said estimate of said current loop gain ΔL(n), limiting a resulting loop gain, LG′, defined as a sum of the resulting forward path gain G′ and the feedback gain H when given in a logarithmic representation, to stay below a predefined value, and where H is the feedback gain exhibited by the feedback path from an output transducer to an input transducer of the hearing device.

20. A non-transitory computer readable medium storing a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of claim 19.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The aspects of the disclosure may be best understood from the following detailed description taken in conjunction with the accompanying figures. The figures are schematic and simplified for clarity, and they just show details to improve the understanding of the claims, while other details are left out. Throughout, the same reference numerals are used for identical or corresponding parts. The individual features of each aspect may each be combined with any or all features of the other aspects. These and other aspects, features and/or technical effect will be apparent from and elucidated with reference to the illustrations described hereinafter in which:

(2) FIG. 1A illustrates a hearing device comprising a loop gain limiter according to a first embodiment of the present disclosure;

(3) FIG. 1B shows a hearing device comprising a loop gain limiter according to a second embodiment of the present disclosure; and

(4) FIG. 1C shows a hearing device comprising a loop gain limiter according to a third embodiment of the present disclosure,

(5) FIG. 2 shows the feedback loop of a hearing device comprising an electric forward path from input to output transducer, and an acoustic (and/or mechanical) feedback loop from output to input transducer,

(6) FIG. 3 shows a schematic example of the signal level limitation for a signal with dynamic level over time,

(7) FIG. 4A illustrates average signal level increase [dB] with loop gain limitation, where loop gain LG=10 dB, maximum loop gain LGmax=0 dB, and the limitation parameter a=1;

(8) FIG. 4B illustrates average signal level increase [dB] with loop gain limitation, where LG=20 dB, LGmax=5 dB, and the limitation parameter a=1; and

(9) FIG. 4C illustrates average signal level increase [dB] with loop gain limitation, where LG=10 dB, LGmax=0 dB, and the limitation parameter a=2,

(10) FIG. 5 schematically illustrates an activation range for the gain limitation, wherein a rising signal level can be limited, whereas a falling signal level remains unaffected,

(11) FIG. 6 shows the applied gain limitation in the example shown in FIG. 4A,

(12) FIG. 7 shows a strategy for applying the gain limitation only within a certain range, and

(13) FIG. 8 shows an embodiment of a hearing device (HD) according to the present disclosure.

(14) The figures are schematic and simplified for clarity, and they just show details which are essential to the understanding of the disclosure, while other details are left out. Throughout, the same reference signs are used for identical or corresponding parts.

(15) Further scope of applicability of the present disclosure will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the disclosure, are given by way of illustration only. Other embodiments may become apparent to those skilled in the art from the following detailed description.

DETAILED DESCRIPTION OF EMBODIMENTS

(16) The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. Several aspects of the apparatus and methods are described by various blocks, functional units, modules, components, circuits, steps, processes, algorithms, etc. (collectively referred to as “elements”). Depending upon particular application, design constraints or other reasons, these elements may be implemented using electronic hardware, computer program, or any combination thereof.

(17) The electronic hardware may include microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate arrays (FPGAs), programmable logic devices (PLDs), gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. Computer program shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise.

(18) The present application relates to the field of hearing devices, e.g. hearing aids, in particular to feedback control using a loop gain limiter.

(19) The loop gain limiter is used to limit the maximum loop gain for each feedback loop, when the feedback is building up, i.e. loop gain ≥0 dB.

(20) This limitation can help the feedback control system, e.g. a feedback cancellation system, using adaptive filters, to better handle the up-building feedback/howl.

(21) It has no (negative) effect on the feedback control system when the loop gain is not critical.

(22) When the loop gain is positive, the signal level increases for each loop. The loop gain limiter may e.g. limit the (average) slope of this increase.

(23) FIG. 1A illustrates a hearing device comprising a loop gain limiter (cf. dashed enclosure denoted ‘LGL’) according to a first embodiment of the present disclosure. The loop gain limiter (LGL) comprises a loop gain estimator (LGE) and a loop gain controller (LGC). The hearing device (HD), e.g. a hearing aid, may e.g. be configured to be worn by a user at or in an ear or to be partially implanted in the head of the user. The hearing device comprises a forward path comprising an input transducer (IT) for providing an electric input signal (IN) representing sound in the environment of the hearing device (ExtAcln). The forward path further comprises a signal processor (SPU) for processing the electric input signal (IN), or a signal based thereon (e.g. a processed version thereof) and providing a processed signal (OUT). The signal processor (SPU) comprises a compressor for determining a frequency and level dependent desired compressor gain G.sub.P to compensate for a hearing impairment of the user (taking into account possible gain (amplification or attenuation) provided by the input and output transducers). The signal processor is configured to provide a resulting compressor gain G′.sub.P. The resulting compressor gain G′.sub.P may be larger than or equal to, or smaller than the desired compressor gain G.sub.P. The forward path further comprises an output transducer (OT) for providing output stimuli (AcOut) perceivable as sound for the user based on said processed signal (OUT). A fraction of the acoustic output (AcOut) from the output transducer (OT) may leak to, and be picked up by, the input transducer (IT) (cf. feedback signal (AcFB) in FIG. 1), be amplified by the forward path, output via the output transducer (OT), etc., and under specific conditions (including that loop gain is larger than 1) result in the build-up of oscillations (which may lead to feedback howl). If the input transducer (IT) provides an input gain G.sub.I, and the output transducer provides an output gain, G.sub.O, a total desired forward path gain G can be defined as G.sub.I+G.sub.P+G.sub.O (in a logarithmic representation; or G.sub.I.Math.G.sub.P.Math.G.sub.O in a linear representation).

(24) The hearing device (HD), here the loop gain limiter (LGL), further comprises a loop gain estimator (LGE) for estimating a current loop gain ΔL(n), wherein the loop gain estimate is calculated as the current level of a signal of the forward path at time index n minus the level of the same signal one feedback loop earlier. The feedback loop is represented by the electric forward path of the hearing device from the input transducer (IT) to the output transducer (OT) and an acoustic feedback path from the output transducer (OT) to the input transducer (IT).

(25) The feedback path exhibits a feedback gain H. Hence, an unmodified loop gain, LG, would be determined as a sum of the desired forward path gain G and the feedback gain H (in a logarithmic representation), cf. e.g. FIG. 2.

(26) The loop gain may be determined for any signal of the forward path (e.g. the electric input signal (IN), the processed output signal (OUT), or any signal tapped therebetween (IN′)).

(27) The hearing device (HD), here the loop gain limiter (LGL), further comprises a loop gain controller (LGC) for dynamically controlling (e.g. reducing) the resulting forward path gain G′.sub.FP in dependence of the estimate of the current loop gain ΔL(n). The loop gain controller provides a control signal (ΔG(n)) to the signal processor (SPU) for modifying the gain of the signal processor from the desired processor gain G.sub.P to the resulting processor gain G′.sub.P to thereby provide a resulting loop gain LG′ for the hearing device. The loop gain controller (LGC) may be configured to decrease said resulting forward path gain G′ in case the estimate of the current loop gain ΔL(n) is larger than or equal to a maximum loop gain value LGmax. (e.g. set as a predefined criterion for the hearing aid in question, e.g. defined by a given hearing aid style (open or closed fitting)). Thereby the resulting loop gain LG′ is reduced compared to the original loop gain LG (without the loop gain limiter).

(28) FIG. 1B shows a hearing device comprising a loop gain limiter (LGL) according to a second embodiment of the present disclosure. The embodiment of FIG. 1B is similar to the embodiment pf FIG. 1A, but an embodiment of the signal processor SPU (enclosed in dashed rectangular outline in FIG. 1B) is described in further detail in the following. The electric input signal IN from the input transducer (IT), e.g. a microphone, is fed to the signal processor (SPU), which provides a processed signal OUT which is fed to an output transducer (OT), e.g. a loudspeaker. The signal processor (SPU) of FIG. 1B comprises a level detector (LD) for estimating a level L of the current electric input signal IN (or a signal derived therefrom, e.g. a feedback corrected input signal). The signal processor (SPU) further comprises a compressor (CG) for determining a desired processor gain G.sub.P from the estimated level L of the electric input signal IN (or a signal derived therefrom). The loop gain estimator (LGE) is configured to estimating a current loop gain ΔL(n) based on the level estimates L(n) and L(n−1) of the level detector (LD) wherein the loop gain ΔL(n) is determined by a change in level L of a signal of the forward path (here the electric input signal IN) from one loop delay D earlier (n−1) to the current time n, where n is a time index.

(29) The feedback loop delay D (or loop delay) is in the present context taken to mean the time required for a signal to travel through the loop consisting of the (electric) Forward Path of the hearing device and the (acoustic) Feedback Path from output transducer to input transducer of the haring device (as illustrated in FIG. 2). The loop delay is taken to include the processing delay d of the (electric) forward path of the hearing device from input to output and the delay d′ of the acoustic feedback path from the transducer to the input of the hearing device, in other words, loop delay D=d+d′. At least an estimate of the feedback loop delay is assumed to be known, e.g. measured or estimated in advance of the use of the hearing device, and e.g. stored in a memory or otherwise built into the system. In an embodiment, the hearing device is configured to measure or estimate the loop delay during use (e.g. automatically, e.g. during power-on, or initiated by a user via a user interface). In an embodiment, the hearing device is configured to provide one value of loop magnitude (and possibly loop phase) for each time index m, or for each time period corresponding to a current feedback loop delay (D), i.e. at times m′=p.Math.D, where p=0, 1, 2, . . . , Other periodic or non-periodic schemes or algorithms may be used for providing the values of loop gain.

(30) The hearing device (HD), here the loop gain limiter (LGL), further comprises a loop gain controller (LGC) for dynamically controlling (e.g. reducing) the resulting forward path gain G′ in dependence of the estimate of the current loop gain ΔL(n). In the embodiment of FIG. 1B, the loop gain controller (LGC) further receives one or more inputs from one or more detectors, e.g. an SNR estimator, a correlation detector, a feedback detector, a transient detector, etc. The aim of the detector input(s) (DET) is to make the gain control more robust (to avoid unnecessary gain changes). A resulting (processor) gain G′.sub.P is provided by the gain modification unit (CG′) based on a desired (processor) gain G.sub.P (according to a hearing impairment of the user) and on the loop gain control signal (ΔG(n)). The resulting (processor) gain G′.sub.P may be smaller than the desired gain G.sub.P, if the estimated current loop gain ΔL(n) is larger than a desired maximum threshold value (LGmax), or equal to the desired gain G.sub.P, if the estimated current loop gain ΔL(n) is smaller than or equal to the desired maximum threshold value (LGmax). An algorithm for determining a resulting gain G′ in dependence of the current loop gain ΔL(n) is described below (cf. e.g. eq. (1)). The hearing device (HD) further comprises a smoothing unit (SM) for smoothing the resulting gain G′.sub.P over time, and providing a smoothed resulting gain <G′.sub.P>. An algorithm for providing such smoothing is described below. The forward path of the hearing device (HD) further comprises a combination unit (‘X’) for applying the resulting (e.g. G′.sub.P or the corresponding smoothed processor gain <G′.sub.P>, respectively) to the electric input signal (IN), thereby providing the processed output signal (OUT), which is fed to the output transducer (OT) for presentation to the user.

(31) FIG. 1C shows a hearing device comprising a loop gain limiter (LGL) according to a third embodiment of the present disclosure. The embodiment of FIG. 1C is similar to the embodiment of FIG. 1A, but embodiments of the input (IT) and output (OT) transducers (enclosed in dashed rectangular outline in FIG. 1C) are described in further detail in the following. The embodiment of FIG. 1C further comprises a feedback control system comprising a feedback estimation unit (FBE) and a combination unit (‘+’). The input transducer (IT) comprises a microphone (MIC) for picking up a sound (ExtAcln (and AcFB)) from the environment and providing an analogue electric input signal, and an analogue to digital converter (AD) for converting the analogue electric input signal to a (digital) electric input signal (IN), which is fed to the processor (SPU), in particular to the combination unit (‘+’), here a subtraction unit of the feedback control system. The output transducer (OT) comprises a digital to analogue converter (DA) for converting a signal (OUT) from the processor (SPU) (here from the signal processor (PRO)) to an analogue signal, which is fed to loudspeaker (SPK) for conversion to an acoustic signal (AcOut). The feedback estimation unit (FBE) of the feedback control system of the embodiment of a hearing device (HD) of FIG. 1C is configured to estimate feedback path (FBP) from the output transducer (OT) to the input transducer (IT) and to provide a signal FBP.sub.est representative of such estimate. The feedback path estimate is subtracted from the (digitized) input signal (IN) in subtraction unit (‘+’) to thereby provide a feedback corrected input signal IN′. The feedback corrected input signal IN′ is fed to the signal processor (PRO) and to the loop gain estimator (LGE) as discussed in connection with FIGS. 1A and 1B. The feedback corrected input signal IN′ may further be feed to the feedback estimation unit (FBE), e.g. in case the feedback estimate FBP.sub.est is provided by an adaptive filter. In such case, the feedback corrected input signal IN′ is fed to an adaptive algorithm for determining updated filter coefficients of a variable filter, the adaptive algorithm and the variable filter together constituting the adaptive filter. In the embodiment of FIG. 1C, the loop gain estimator (LGE) thus works on a feedback corrected input signal (which may still contain uncompensated feedback components).

(32) The embodiments of a hearing device (HD) shown in FIG. 1A-1C may comprise one or more filter banks allowing that some or all signal processing of the forward path may be conducted in the frequency domain. Alternatively, some or all signal processing of the forward path may be conducted in the time domain.

(33) FIG. 2 shows the feedback loop of a hearing device comprising an electric forward path from input to output transducer, and an acoustic (and/or mechanical) feedback loop from output to input transducer.

(34) Knowledge (e.g. an estimate or a measurement) of the length of one loop delay is assumed to be available.

(35) The loop delay D is defined as the time required for a signal travelling (once) through the acoustic loop, as illustrated in FIG. 2. The acoustic loop consists of the forward path (of the hearing device), and the (acoustic) feedback path. The loop delay D is taken to include the processing delay d of the (electric) forward path (Forward Path) of the hearing device from input transducer to output transducer and the delay d′ of the acoustic feedback path (Feedback Path) from the output transducer to the input transducer of the hearing device, LoopDelay D=d+d′.

(36) Typically, the acoustic part d′ of the loop delay is much less than the electric (processing) part d of the loop delay, d′<<d (in particular when the forward path comprises processing of signals in frequency sub-bands). The loop delay D may be approximated by the processing delay d of the forward path of the hearing device (D≈d). The electric (processing) part d of the loop delay may e.g. be in the range between 2 ms and 10 ms, e.g. in the range between 5 ms and 8 ms, e.g. around 7 ms. The loop delay may be relatively constant over time (and e.g. determined in advance of operation of the hearing device) or be different at different points in time, e.g. depending on the currently applied algorithms in the signal processing unit (e.g. dynamically determined (estimated) during use). The hearing device (HD) may e.g. comprise a memory unit wherein typical loop delays in different modes of operation of the hearing device are stored. In an embodiment, the hearing device is configured to measure a loop delay comprising a sum of a delay d of the forward path and a delay d′ of the feedback path. A predefined (or otherwise determined) test-signal may e.g. be inserted in the forward path, and its round trip travel time measured (or estimated), e.g. by identification of the test signal when it arrives in the forward path after a single propagation (or a known number of propagations) of the loop. The test signal may be configured to included significant content at frequencies where feedback is likely to occur (e.g. in a range between 1 and 4 kHz).

(37) Loop Gain Estimation

(38) The first part of the concept according to the present disclosure comprises an estimation of the actual loop gain ΔL (in dB) for each feedback loop (one feedback loop.Math.the signal travels once around the acoustic loop including forward path (of the hearing aid) and acoustic feedback path, cf. FIG. 2). The gain contribution of the feedback path is unknown, but the effect of the total loop gain can be observed during a feedback build-up. When the instrument becomes unstable, i.e. feedback is building up, the signal level increases after each loop by a level amount that corresponds to the loop gain. Hence, by assuming that feedback is currently building up and by assuming a certain loop delay D, we can estimate the loop gain based on the signal level difference from one loop to the next. This can be done by determining
ΔL(n)=L(n)−L(n−n.sub.D),

(39) where L(n) is the electric input (e.g. microphone) signal level (in dB) at the time index n, and L(n−n.sub.D) is the signal level of the same signal one feedback loop early (in other words one loop delay D earlier, where n.sub.D is defined by the loop delay D). The level may be sampled by a frequency of 1/D or any other sampling frequency f.sub.s, preferably configured to provide that the loop delay D can be represented by a number p of sampling time units 1/f.sub.s, e.g. D=p/f.sub.s (or n.sub.D=p), where p is an integer. The loop gain may alternatively be determined based on a smoothed or filtered version of the input signal level L.

(40) The main feature of the proposed loop gain estimator is its speed. As it may be based only on level information from the forward path and the assumption about the current loop delay. The loop delay may be predefined or estimated during wearing time. By explicitly excluding more advanced information from e.g. a correlation detector, the resulting loop gain estimate may be of worse quality, but it can be calculated within the shortest possible time. In fact, in the situation of a sudden strong feedback build-up, the proposed estimator is able to provide a loop gain estimate within the time corresponding to 1 feedback loop.

(41) The underlying loop gain LG is given by
LG=G+H,

(42) where G is the desired forward path gain, whereas H is the feedback path gain in a logarithmic representation, where levels are given relative to a common reference level. LG=G.Math.H in a linear representation. Typically 0<H<1 (attenuation) in a linear representation, i.e. corresponding to H<0 in a logarithmic representation.

(43) The Basic Loop Gain Limiting Processing

(44) The value of this loop gain estimate ΔL (in dB) is then used to control the applied forward path gain G′ (in dB) for the feedback loop as

(45) $\begin{array}{cc}{G}^{\prime }\left(n\right)=G\left(n\right)-\max \left(\frac{\Delta L\left(n\right)-LG\max }{a},0\right)& \left(1\right)\end{array}$

(46) Where G′(n) is the resulting gain and G(n) is the desired compressor gain at time instance n. The parameter a is used to control the degree of loop gain limitation, the default value is a=1. The larger the value of a, the less loop gain limitation is provided. Equation (1) is an exempla of an expression for the resulting gain at a present time n which depends on the loop gain estimate ΔL(n) at time n. Other expressions may be envisioned, e.g. a dependence not only on ΔL(n) but also on ΔL(n−1), ΔL(n−2), ΔL(n−3), . . . ).

(47) Equation (1) implies that if the loop gain per feedback loop, ΔL, is below or equal to the maximum allowed loop gain value LGmax, G′=G, i.e., no forward gain reduction for reducing loop gain.

(48) On the other hand, if the loop gain estimate ΔL is bigger than the maximum allowed loop gain value LGmax, a reduction is applied to the desired forward gain G to form the applied forward path gain G′. For example, if ΔL=20 dB, and LGmax=6 dB, we obtain G′=G−14 dB (for a=1), i.e., a reduction of 14 dB.

(49) This limitation of G to G′ implies that the actual loop gain ΔL in the next feedback loop will be exactly equal to the maximum allowed loop gain value, i.e., ΔL=LGmax=6 dB, hereby in the second loop, we allow G′=G, i.e., without any reduction.

(50) If the underlying loop gain LG remains the same, i.e., both the G and H remain constant, and the parameter a=1, the gain pattern for G′ will be, G′=G−max(ΔL(n)−LGmax,0), G, G′=G−max(ΔL(n)−LGmax,0), G, . . . as illustrated in FIG. 3.

(51) FIG. 3 shows a schematic example of the signal level limitation for a signal with dynamic level over time. FIG. 3 shows the signal level L after each loop (x-axis is number of loops (Loop #), y-axis is signal level L in [dB] (Signal Level [dB]). LG=6 dB, LGmax=0 dB, and the limitation parameter a=1. The straight line indicated by square open symbols □ represents development of signal levels without a gain limiter. The step step-like graph indicated by open circular symbols ◯ represents development of signal levels provided by an embodiment of a gain limiter according to the present disclosure.

(52) Furthermore, the limited loop gain provides a steady-state average signal level increase ΔL.sub.avg which can be computed as

(53) $\Delta L_{\mathrm{avg}}=\frac{a}{a+1}.Math.\mathrm{LG}+\frac{1}{a+1}.Math.\mathrm{LG}\max$

(54) Which is derived for LG≥LGmax in the following:

(55) $L\left(n+1\right)=L\left(n\right)+\left(LG-\frac{\Delta L\left(n\right)-LG\max }{a}\right)L\left(n+1\right)-L\left(n\right)=LG-\frac{\Delta L\left(n\right)-LG\max }{a}\Delta L\left(n+1\right)=LG-\frac{\Delta L\left(n\right)-LG\max }{a}$

(56) Now considering n.fwdarw.∞ (steady-state),

(57) $\Delta L_{\mathrm{a\nu g}}=LG-\frac{\Delta L_{\mathrm{a\nu g}}-LG\max }{a}$$\Delta L_{\mathrm{a\nu g}}+\frac{\Delta L_{a\nu g}}{a}=LG+\frac{LG\max }{a}$$\Delta L_{\mathrm{a\nu g}}=\frac{a}{a+1}.Math.\mathrm{LG}+\frac{1}{a+1}.Math.\mathrm{LG}\max$

(58) A few examples of the average signal level increase ΔL.sub.avg are illustrated in FIG. 4A, 4B, 4C.

(59) FIG. 4A shows average signal level increase [dB] with loop gain limitation, where loop gain LG=10 dB, maximum loop gain LGmax=0 dB, and the limitation parameter a=1. FIG. 4B illustrates average signal level increase [dB] with loop gain limitation, where LG=20 dB, LGmax=5 dB, and the limitation parameter a=1. FIG. 4C illustrates average signal level increase [dB] with loop gain limitation, where LG=10 dB, LGmax=0 dB, and the limitation parameter a=2. The legend used in FIG. 4A-4C is the same used in FIG. 3: The straight line indicated by square open symbols □ represents development of signal levels without a gain limiter. The step-like graph indicated by open circular symbols ◯ represents development of signal levels provided by an embodiment of a gain limiter according to the present disclosure. Further, the straight dashed line indicates an average level increase (in dB) per loop. The (unmodified) signal levels vary between 0 dB and 100 dB over 10 loops (reflecting that howl can build up very fast, here over 50-100 ms).

(60) From FIG. 4A, 4B, 4C we observe that with a loop gain limiter according to the present disclosure, a smaller (average) increase is allowed for each feedback loop. Hence, it will limit the severity of feedback build-up. By varying parameters LGmax and a, the deviation from linearity of the resulting gain G′ (and thus a potential need for smoothing thereof, cf. FIG. 1C) can be influenced.

(61) Signal Level Limiting Effect

(62) FIG. 5 schematically illustrates an activation range for the gain limitation, wherein a rising signal level can be limited, whereas a falling signal level remains unaffected. The raising signal level can be limited (cf. indication ‘Areas with limitation’), whereas the falling signal level is unaffected. The same notation as used in FIG. 3 is applied for the respective graphs regarding the symbols indicating calculation points for the respective graphs.

(63) Gain Smoothing

(64) The applied gain G′ (n) determined based on (1) can jump in its value. In the example shown in FIG. 4A, the applied gain for each feedback loop is illustrated in FIG. 6.

(65) FIG. 6 shows the applied gain limitation in the example shown in FIG. 4A as a function of loop number (Loop #). The parameter x may represent a desired processor gain G.sub.P (or total desired compressor gain G). The gain reduction ΔG per loop is in the example of FIG. 6 set to 10 dB (e.g. represented by the second term of Eq. (1) above:

(66) $\Delta G=\max \left(\frac{\Delta L\left(n\right)-LG\max }{a},0\right)$

(67) This may be provided in case ΔL(n)=10 dB, LGmax=0 dB and a=1.

(68) In this case, the applied gain is a square function, other shapes of the applied gain can also occur, and they can have big jumps in its value. This jumping gain value is typically not good for sound quality, and we would like to avoid that. This can be done using a smoothing filter, e.g. a first order IIR filter, so that the smoothed gain G*(n) is computed as
G*(n)=β.Math.G′(n)+(1−β).Math.G*(n−1)

(69) where β is the filter coefficient, and β is positive and close to 0, e.g. <0.2.

(70) Activation Range (Vs. Transient Noise Reduction)

(71) FIG. 7 illustrates a strategy for applying the gain limitation only within a certain range. Preferably, the gain limitation should only be activated within a certain range. For very low loop gain (here exemplified as <+6 dB), there is no need for reducing forward gain, because a feedback control system can handle the feedback situation. For very high loop gains (expected to be due to transient input signals) no gain reduction is needed for reducing feedback. For very high loop gains (ΔL, here ≥+24 dB), e.g., when it is beyond what is physical possible for a given system setup, the estimated loop gain does not represent the feedback situation, but it could be caused by the transient/onsets of the input signal. In this case, no gain limitation should be applied unless a transient protection is desired. Loop gains beyond what is physically possible, can e.g. be beyond +20 dB, when the hearing aid gain is fitted so that the loop gain is 0 dB with no obstacles close to hearing aid, and we know that variations in feedback path, e.g., when a phone is placed close to the hearing aid, can increase the loop gain to maximum +20 dB.

(72) In a related field of transient noise reduction, the signal level difference is also computed, and based on that a gain limitation is applied to suppress transients. However, the main differences to this loop gain limiter concept are twofold.

(73) First, the level difference estimate in the loop gain limiter concept has to be the loop gain estimation, and it is not the case for the transient noise reduction. In other words, the time frame for computing the level difference can be chosen more freely for transient noise reduction, whereas for the loop gain limiter the time frame has to be chosen according to loop delay (equal to the sum of an acoustic feedback delay from the output transducer to the input transducer and an electric forward (processing) path delay of the hearing device from the input transducer to the output transducer).

(74) Second, FIG. 7 can be used for the loop gain limitation concept, whereas it does not apply for the transient noise reduction. The level difference can be as large as 50-80 dB for the transient noise reduction, whereas it would very likely to be a false detection for loop gain limiter.

(75) The applied gain G′ determined by equation (1) does only take into account of the current loop gain estimate ΔL(n) and the loop gain threshold LGmax. The concept of loop gain limiter can be improved by also taking account the gain reduction already applied.

(76) More specifically, if the estimate ΔL(n) is equal to the LGmax, according to equation (1), G′=G, i.e., no gain reduction should be applied. However, if ΔL(n)=LGmax because G′ was already reduced from G, allowing G′=G would lead to ΔL(n)>LGmax in the next loop, as described earlier. The applied gain G′ and loop gain ΔL(n) will jump forth and back as a consequence. By taking into account the latest gain reduction ΔG(n−1)=G−G′(n−1), one can improve the gain reduction G′ to avoid gain (and loop gain) jumps. The latest gain reduction loop gain estimate ΔG(n−1) can be added to loop gain estimate ΔL(n) to obtain a corrected loop gain estimate, and we then compute the gain reduction G′ based on that. In the previous example, the second loop estimation would actually show that the corrected loop gain is still critical, and again the applied gain G′ should be reduced. This can avoid the gain and loop gain jumps.

(77) However, when using ΔG(n−1) to improve G′(n), we should consider to use a leaking factor on ΔG(n−1) to avoid a constant gain reduction or even oscillations. A step input signal can e.g. cause a constant attenuation if we compensate ΔL(n) with ΔG(n−1), and this can be avoided by using a leak factor on ΔG(n−1).

(78) FIG. 8 shows an embodiment of a hearing device (HD) according to the present disclosure. The hearing device (HD), e.g. a hearing aid, is of a particular style (sometimes termed receiver-in-the ear, or RITE, style) comprising a BTE-part (BTE) adapted for being located at or behind an ear of a user, and an ITE-part (ITE) adapted for being located in or at an ear canal of the user's ear and comprising a receiver (loudspeaker). The BTE-part and the ITE-part are connected (e.g. electrically connected) by a connecting element (IC) and internal wiring in the ITE- and BTE-parts (cf. e.g. wiring Wx in the BTE-part). The connecting element may alternatively be fully or partially constituted by a wireless link between the BTE- and ITE-parts.

(79) In the embodiment of a hearing device in FIG. 8, the BTE part comprises two input units comprising respective input transducers (e.g. microphones) (M.sub.BTE1, M.sub.BTE2), each for providing an electric input audio signal representative of an input sound signal (S.sub.BTE) (originating from a sound field S around the hearing device). The input unit further comprises two wireless receivers (WLR.sub.1, WLR.sub.2) (or transceivers) for providing respective directly received auxiliary audio and/or control input signals (and/or allowing transmission of audio and/or control signals to other devices, e.g. a remote control or processing device or a telephone). The hearing device (HD) comprises a substrate (SUB) whereon a number of electronic components are mounted, including a memory (MEM) e.g. storing different hearing aid programs (e.g. parameter settings defining such programs, or parameters of algorithms, e.g. optimized parameters of a neural network) and/or hearing aid configurations, e.g. input source combinations (M.sub.BTE1, M.sub.BTE2, M.sub.ITE, WLR.sub.1, WLR.sub.2), e.g. optimized for a number of different listening situations. In a specific mode of operation, two or more of the electric input signals (e.g. from the microphones) are combined to provide a beamformed signal provided by applying appropriate complex weights to (at least some of) the respective signals. The memory (MEM) may e.g. comprise different sets of parameters for a loop gain limiter according to the present disclosure.

(80) The substrate (SUB) further comprises a configurable signal processor (DSP, e.g. a digital signal processor), e.g. including a processor for applying a frequency and level dependent gain, e.g. providing beamforming, noise reduction, filter bank functionality, and other digital functionality of a hearing device, e.g. implementing a loop gain estimator and a feedback control unit, according to the present disclosure (as e.g. discussed in connection with FIG. 1A-1C). The configurable signal processor (DSP) is adapted to access the memory (MEM). The configurable signal processor (DSP) is further configured to process one or more of the electric input audio signals and/or one or more of the directly received auxiliary audio input signals, based on a currently selected (activated) hearing aid program/parameter setting (e.g. either automatically selected, e.g. based on one or more sensors, or selected based on inputs from a user interface). The mentioned functional units (as well as other components) may be partitioned in circuits and components according to the application in question (e.g. with a view to size, power consumption, analogue vs. digital processing, acceptable latency, etc.), e.g. integrated in one or more integrated circuits, or as a combination of one or more integrated circuits and one or more separate electronic components (e.g. inductor, capacitor, etc.). The configurable signal processor (DSP) provides a processed audio signal, which is intended to be presented to a user. The substrate further comprises a front-end IC (FE) for interfacing the configurable signal processor (DSP) to the input and output transducers, etc., and typically comprising interfaces between analogue and digital signals (e.g. interfaces to microphones and/or loudspeaker(s)). The input and output transducers may be individual separate components, or integrated (e.g. MEMS-based) with other electronic circuitry.

(81) The hearing device (HD) further comprises an output unit (e.g. an output transducer) providing stimuli perceivable by the user as sound based on a processed audio signal from the processor or a signal derived therefrom. In the embodiment of a hearing device in FIG. 8, the ITE part comprises the output unit in the form of a loudspeaker (also termed a ‘receiver’) (SPK) for converting an electric signal to an acoustic (air borne) signal, which (when the hearing device is mounted at an ear of the user) is directed towards the ear drum (Ear drum), where sound signal (S.sub.ED) is provided. The ITE-part further comprises a guiding element, e.g. a dome, (DO) for guiding and positioning the ITE-part in the ear canal (Ear canal) of the user. The ITE-part further comprises a further input transducer, e.g. a microphone (M.sub.ITE), for providing an electric input audio signal representative of an input sound signal (S.sub.ITE) at the ear canal. In other embodiments, the output transducer may comprise a vibrator of a bone conduction hearing aid.

(82) The electric input signals (from input transducers M.sub.BTE1, M.sub.BTE2, M.sub.ITE) may be processed in the time domain or in the (time-) frequency domain (or partly in the time domain and partly in the frequency domain as considered advantageous for the application in question).

(83) The embodiments of a hearing device (HD) exemplified in FIGS. 1A-1C and 8 are portable devices comprising a battery (BAT), e.g. a rechargeable battery, e.g. based on Li-Ion battery technology, e.g. for energizing electronic components of the BTE- and possibly ITE-parts. In an embodiment, the hearing device, e.g. a hearing aid, is adapted to provide a frequency dependent gain and/or a level dependent compression and/or a transposition (with or without frequency compression) of one or more frequency ranges to one or more other frequency ranges, e.g. to compensate for a hearing impairment of a user. The BTE-part may e.g. comprise a connector (e.g. a DAI or USB connector) for connecting a ‘shoe’ with added functionality (e.g. an FM-shoe or an extra battery, etc.), or a programming device, or a charger, etc., to the hearing device (HD).

(84) It is intended that the structural features of the devices described above, either in the detailed description and/or in the claims, may be combined with steps of the method, when appropriately substituted by a corresponding process.

(85) As used, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well (i.e. to have the meaning “at least one”), unless expressly stated otherwise. It will be further understood that the terms “includes,” “comprises,” “including,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element but an intervening element may also be present, unless expressly stated otherwise. Furthermore, “connected” or “coupled” as used herein may include wirelessly connected or coupled. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. The steps of any disclosed method is not limited to the exact order stated herein, unless expressly stated otherwise.

(86) It should be appreciated that reference throughout this specification to “one embodiment” or “an embodiment” or “an aspect” or features included as “may” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Furthermore, the particular features, structures or characteristics may be combined as suitable in one or more embodiments of the disclosure. The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects.

(87) The claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language of the claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” Unless specifically stated otherwise, the term “some” refers to one or more.

(88) Accordingly, the scope should be judged in terms of the claims that follow.