Hearing device comprising an anti-feedback power down detector

Abstract

The application relates to a hearing device comprising a) a forward path between an input transducer for converting an input sound to an electric input signal and an output transducer for converting an electric output signal to an output sound, the forward path comprising a signal processing unit for applying a level and/or frequency dependent gain to the electric input signal or a signal originating therefrom and for providing a processed signal, and feeding the processed signal or a signal originating therefrom to the output transducer, an acoustic feedback path being defined from said output transducer to said input transducer; b) a configurable anti-feedback system comprising a feedback estimation unit for providing an estimate of said acoustic feedback path; c) a number of detectors, each providing a detector signal for characterizing a signal of the forward path. The object of the present application is to save power in a hearing device. The problem is solved in that the hearing device further comprises an activation control unit configured to control the anti-feedback system based on said detector signals, and to bring the anti-feedback system into one of at least two predefined modes based on said detector signals, said at least two predefined modes comprising an ON-mode and an OFF-mode. The invention may e.g. be used in hearing aids, headsets, ear phones, active ear protection systems, or similar portable devices, where a need for feedback cancellation and low power consumption is important.

Claims

1. A hearing device comprising a forward path between an input transducer for converting an input sound to an electric input signal and an output transducer for converting an electric output signal to an output sound, the forward path comprising a signal processing unit for applying a level and/or frequency dependent gain to the electric input signal or a signal originating therefrom and for providing a processed signal, and feeding the processed signal or a signal originating therefrom to the output transducer, an acoustic feedback path being defined from said output transducer to said input transducer; a configurable anti-feedback system comprising a feedback estimation unit for providing an estimate of said acoustic feedback path, wherein the feedback estimation unit comprises an update part implementing an adaptive algorithm and a variable filter part for filtering an input signal according to variable filter coefficients determined by said adaptive algorithm; a number of detectors, each providing a detector signal for characterizing a signal of the forward path; wherein the hearing device further comprises an activation control unit configured to control the anti-feedback system based on said detector signals, and to bring the anti-feedback system into one of at least two predefined modes based on said detector signals, said at least two predefined modes comprising an ON-mode and an OFF-mode, and to allow operation of the anti-feedback system in a number of different ON-modes, including a maximum power consumption ON-mode, and wherein the update part of the feedback estimation unit is configured to update said filter coefficients of the variable filter part with a configurable update frequency.

2. A hearing device according to claim 1 wherein the anti-feedback system is operated in a number of frequency bands.

3. A hearing device according to claim 1 wherein at least one of said detector signals is frequency dependent.

4. A hearing device according to claim 1 wherein the activation control unit is configured to control the anti-feedback system based on a predefined criterion comprising said detector signals.

5. A hearing device according to claim 2 configured to allow the activation control unit to bring the anti-feedback system in a first band into a first one of said modes of operation, and to bring the anti-feedback system in a second band into a second one of said modes of operation.

6. A hearing device according to claim 1 wherein the anti-feedback system is configured to be operated only in a limited number of frequency bands.

7. A hearing device according to claim 1 wherein the number of detectors comprises a level detector for estimating a current level of a signal of the forward path.

8. A hearing device according to claim 1 wherein the number of detectors comprises an auto-correlation detector for providing a measure of the current auto-correlation of a signal of the forward path.

9. A hearing device according to claim 8 wherein the activation control unit is configured to bring the anti-feedback system in an ON-mode, if said measure of the current auto-correlation fulfils a predefined auto-correlation-criterion for the ON-mode AND if the current level of a signal of the forward path fulfils a predefined level-criterion for the ON-mode.

10. A hearing device according to claim 8 wherein the activation control unit is configured to bring the anti-feedback system in an OFF-mode, if said measure of the current auto-correlation fulfils a predefined auto-correlation-criterion for the OFF-mode AND if the current level of a signal of the forward path fulfils a predefined level-criterion for the OFF-mode.

11. A hearing device according to claim 1 comprising first and second input transducers, and corresponding first and second configurable feedback cancellation systems comprising first and second feedback estimation units for estimating first and second acoustic feedback paths from said output transducer to said first and second input transducers, respectively.

12. A hearing device according to claim 1 wherein—in a mode of operation of the anti-feedback system other than the maximum power ON-mode—the update frequency of the update part is scaled down by a predefined factor X compared to a maximum update frequency.

13. A hearing device according to claim 1 wherein the activation control unit is configured—in a specific ON-mode of operation—to control the update parts of the first and second feedback estimation units to alternatingly update the filter coefficients of the respective variable filter parts.

14. A hearing device according to claim 1 wherein the hearing device comprises a hearing aid, a headset, an earphone, an ear protection device or a combination thereof.

15. A hearing device according to claim 1 wherein a switching time period for activation of an ON-mode is smaller than the switching time period for activation of an OFF-mode.

16. A hearing device according to claim 1 wherein the number of detectors are configured to provide that a switching time period for activation of an ON-mode is smaller 100 ms.

17. A binaural hearing aid system comprising first and second hearing device according to claim 1, wherein the binaural hearing system is adapted to establish a communication link between the first and second hearing devices and to provide that information can be exchanged or forwarded from one to the other.

18. A binaural hearing aid system according to claim 17 configured to provide that a detector signal defining a measure of a first property of a signal of the forward path of a given hearing device is transmitted to the other hearing device for comparison with a measure of the first property of a signal of the forward path of the other hearing device, wherein the comparison is used to influence a whether or not a change of mode of operation of the anti-feedback system of the hearing aid in question should be initiated.

19. A binaural hearing aid system according to claim 18 wherein the property comprises auto-correlation.

20. A hearing device according to claim 1 configured so that the configurable update frequency has a maximum value in the maximum power consumption ON-mode of the anti-feedback system, and in different modes of operation of the anti-feedback system other than the maximum power consumption ON-mode, the update frequency of the update part is scaled down with different factors compared to said maximum value of the configurable update frequency.

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) FIGS. 1A, 1B and 1C shows three embodiments of a hearing device according to the present disclosure,

(3) FIG. 2 illustrates an embodiment of a hearing device according to the present disclosure,

(4) FIG. 3A, 3B, 3C and FIG. 3D schematically shows four exemplary criteria according to the present disclosure for controlling an anti-feedback system based on detector signals from two detectors,

(5) FIG. 4A shows a block diagram of an embodiment of a power down/power on controller for an anti-feedback system of a hearing device according to the present disclosure, and

(6) FIG. 4B shows a power down/power on detector for an anti-feedback system of a hearing device according to the present disclosure,

(7) FIG. 5 shows an exemplary Power ON timing of the anti-feedback system according to the present disclosure when the anti-feedback system is brought from an OFF-mode to an ON-mode of operation,

(8) FIG. 6A, FIG. 6B and FIG. 6C shows three exemplary criteria according to the present disclosure for controlling an anti-feedback system based on detector signals from an auto-correlation detector and a level detector, and

(9) FIG. 7 shows an embodiment of a binaural hearing aid system comprising first and second hearing devices.

(10) 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.

(11) 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

(12) 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.

(13) 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.

(14) FIG. 1A, 1B, 1C schematically shows three embodiments of a hearing device according to the present disclosure.

(15) FIG. 1A schematically shows exemplary basic functions of a hearing device (HD) comprising a forward or signal path from an input transducer (IT) to an output transducer (OT). The input transducer (IT) comprises a microphone for converting an input sound (Acoustic input in FIG. 1A, 1B, 1C) to an analogue electric input signal and an analogue-to-digital (AD) converter to digitize the analogue electric input signal from the microphone with a predefined sampling rate, e.g. 20 kHz, and provide a digitized electric input signal to the forward path. The output transducer (OT) comprises a digital-to-analogue (DA) converter to convert a digital signal to an analogue electric output signal and a loudspeaker presenting the analogue electric output signal to a user as an output sound (Acoustic output). The forward path comprises a signal processing unit (SPU) for applying a level and/or frequency dependent gain to the signal from the input transducer (or a signal derived therefrom) and providing an enhanced signal to the output transducer. An ‘external’ or ‘acoustic’ feedback path (FBP) from output to input transducer of the hearing device is indicated. The external feedback path leaks a part of the output sound from the output transducer (Acoustic output) to the input transducer (as indicated by the bold arrow from the output transducer to the input transducer. The input sound (Acoustic input) presented at the input transducer (IT) comprises this leaked ‘feedback signal’ in combination with any sound from the environment (as indicated by the bold arrow beneath the acoustic feedback path). The hearing device (HD) further comprises an anti-feedback system (FBCS) comprising a feedback estimation unit (FBE) for estimating the acoustic feedback path (FPB) from the output transducer to the input transducer and providing a signal fbp representative thereof. The anti-feedback system (FBCS) further comprises a summation (subtraction) unit (‘+’) for subtracting the signal fbp representative of the current acoustic feedback path from the (digitized) electric input signal and providing a feedback corrected signal (error signal err), which is fed to the signal processing unit (SPU), to the feedback estimation unit (FBE). The hearing device (HD) further comprises a battery (BAT) for providing current to the functional blocks of the hearing device (cf. signals pwr) and a power down detector and an activation control unit (PDD-ACU). The power down detector part (PDD) comprises a number of detectors, each providing a detector signal for characterizing a signal of the forward path (here signal err). Alternatively or additionally, the hearing device (HD) may comprise a number of detectors (unit DETi, i=1, 2, . . . , N.sub.D, where N.sub.D is the number of detectors) providing respective detector signals det (bold arrow from the DETi unit to the PDD-ACC-unit). In an embodiment, one or more of the detectors are external to the hearing device, and the hearing device is configured to receive control signals from such external detectors (e.g. via appropriate wireless transceivers, see e.g. FIG. 7). A number of detectors that may be used to provide input to the power down detector and activation control unit (PDD-ACU) are disclosed in US20140321682A1. The activation control part (ACU) is configured to control the anti-feedback system (FBCS) based on the detector signals (cf. power control signal pct), and to bring the anti-feedback system into one of at least two predefined modes based on the detector signals. The at least two predefined modes comprises an ON-mode and an OFF-mode. The ON-mode comprises a normal power consumption mode. The OFF-mode comprises a minimum power consumption mode. The processing of the hearing device may be performed fully or partially in the time domain.

(16) FIG. 1B shows an embodiment of a hearing device (HD) as shown in FIG. 1A, but additionally comprising a time to time frequency conversion unit (TF) (e.g. an analysis filter bank) located in the forward path between the summation unit (‘+’) and the signal processing unit (SPU) and a time-frequency domain to time domain conversion unit (FT) (e.g. a synthesis filter bank) located in the forward path between the signal processing unit (SPU) and the output transducer (OT). Thereby the signal processing of the forward path between the conversion units (TF, FT) can be performed in a number of frequency bands. In particular, the input signal err to and the processed signal ref from the signal processing unit (SPU) are provided in a number of frequency bands (e.g. 16 or 32 or 64). The time to time frequency conversion unit (TF) converts processed band split signal ref to a time domain signal out, which is fed to output transducer (OT) for presentation to a user as an acoustic signal (Acoustic output). In the embodiment of FIG. 1B, the feedback estimation unit (FBE) provides the acoustic feedback path estimate signal fbp in the time domain and is configured to base the estimate on the processed frequency domain signal ref. In an embodiment, the acoustic feedback path estimate signal fbp is additionally based on the time domain signal out (cf. dashed line to the feedback estimation unit (FBE), and FIG. 1C). In the embodiments of FIGS. 1B and 1C, the input and output transducers (IT and OT, respectively) are assumed to contain possible analogue to digital (AD) and digital to analogue (DA) converters.

(17) The feedback estimation units (FBE) of the embodiments of a hearing device shown in FIGS. 1A and 1B may comprise an adaptive filter, which is controlled by a prediction error algorithm, e.g. an LMS (Least Means Squared) algorithm, in order to predict and cancel the part of the input transducer (here microphone) signal that is caused by feedback. FIG. 1C illustrates an example of this. FIG. 1C shows an embodiment of a hearing device (HD) as shown in FIG. 1B, but where the feedback estimation unit (FBE in FIGS. 1A and 1B) comprises an adaptive filter. The adaptive filter in FIG. 1C comprises a variable filter part (Filter in FIG. 1C) and an adaptive prediction error algorithm part (Update in FIG. 1C). The feedback estimation unit (adaptive filter Update, Filter) is (here) aimed at providing a good estimate of the ‘external’ feedback path from the output transducer (OT) to the input transducer (IT). The prediction error algorithm (of the Update unit) uses a reference signal (ref) together with a signal of the forward path originating from the microphone signal (here feedback corrected signal err from the combination unit (+)) to find the setting (filter coefficients) of the adaptive filter (when applied to the Filter) that minimizes the prediction error when the reference signal (ref) is applied to the adaptive filter (input to Filter part). The update of filter coefficients (acoustic feedback path estimate) as determined in the Update part of the adaptive filter is controlled by power control signal pct from the activation control unit (ACU). In the embodiment of FIG. 1C, the calculation of filter coefficients in the Update part of the adaptive filter is performed in the frequency domain based on signals err and ref and transferred to the variable filter part (Filter). The variable filter part is configured to filter time domain signal out and provide acoustic feedback path estimate signal fbp in the time domain. Alternatively, the variable filter part (Filter) may likewise work in the frequency domain. In this case, the subtraction unit ‘+’ is located between the time to time frequency conversion unit (TF) and the signal processing unit (SPU), and the signal ref is used instead of signal out as an input to the variable filter part (Filter).

(18) The signal processing unit (SPU in FIG. 1A, 1B, 1C) is e.g. adapted to adjust the electric input signal to an impaired hearing of the user.

(19) To provide an improved de-correlation between the output and input signal, it may be desirable to add a probe signal to the output signal. This probe signal can be used as the reference signal to the algorithm part of the adaptive filter, and/or it may be mixed with the ordinary output of the hearing aid to form the reference signal. Alternatively, a (small) frequency or phase shift may be introduced in a signal of the forward path.

(20) FIG. 2 shows an embodiment of a hearing device according to the present disclosure. The embodiment of a hearing device shown in FIG. 2 is a further specified embodiment of the hearing device embodiments illustrated in FIG. 1A, 1B, 1C as regards the power down detector and control unit (PDD-ACU in FIG. 1A, 1B, 1C). The hearing device (HD) is e.g. embodied in a hearing aid configured to provide a level and/or frequency dependent gain to an input audio signal from the environment of a user of the hearing aid to thereby compensate for a hearing impairment of the user. Such compensation is e.g. implemented by processing algorithms of a forward path of the hearing aid (between one or more input transducer(s) (here two microphones denoted F and R) and output transducer(s) (here a loudspeaker denoted OT)), cf. blocks DIR (providing spatial filtering to reduce noise) and G(f) (providing a gain to compensate for hearing impairment). The forward path further comprises a time to frequency conversion unit (AFB, here an analysis filter bank) and a frequency to time domain conversion unit (S-FB, here a synthesis filter bank). The front (F) and rear (R) microphones (e.g. relating to a) locations of the microphones in a hearing aid, when a user is wearing the hearing aid, and b) to a look direction defined by a user's nose) convert sound from the environment to time variant electric signals I.sub.F(n) and I.sub.R(n), respectively, n being a time index. Each of the microphone paths comprises a subtraction unit (‘+’) for subtraction of a signal representative of an estimate of the respective acoustic feedback paths from the output transducer (OT) to the respective front (F) and rear (R) microphones. The acoustic feedback path estimates are provided by respective feedback estimation units (FBE.sub.F, FBE.sub.R) for the front and rear microphones. The feedback estimation units (FBE.sub.F, FBE.sub.R) and the subtraction units (‘+) together form part of a respective front and rear anti-feedback systems. The feedback corrected (time domain) signals (err.sub.F, err.sub.R) provided by subtraction units (‘+’) of the front and rear microphone paths are fed to respective analysis filter banks (AFB) to provide the signals in a time frequency representation in the form of electric input signals I.sub.F(k,m) and I.sub.R(k,m), respectively, k being a frequency index and m being a time index, respectively. The filter banks (AFB) provide the signals in a number N.sub.FB of frequency bands. The time-frequency domain signals are fed to a beamformer unit DIR providing a beamformed (and possibly further noise reduced) signal IN(k,m). A gain unit G(f) is configured to apply a gain profile (intended to compensate a user's hearing impairment) to the beamformed input signal IN(k,m) and to provide a processed signal Y(k,m) (in the time-frequency domain). A synthesis filter bank (S-FB) converts the processed signal Y(k,m) from a time-frequency domain signal to a time domain signal out, which is fed to the output transducer (OT) for conversion to a sound for being presented to a user of the hearing device (HD). The time domain output signal out is fed to the respective feedback estimation units (FBE.sub.F, FBE.sub.R) for the front and rear microphones. The feedback estimation units (FBE.sub.F, FBE.sub.R) may alternatively be implemented fully or partially in the frequency domain (by branching off from or inserting signals in the frequency domain part of the forward path and/or by introducing relevant time <-> frequency converters).

(21) The lower part of FIG. 2 exemplifies an embodiment of a power down detector (PDD) and activation control unit (ACU) (denoted PDD-ACU in FIG. 1A, 1B, 1C) according to the present disclosure.

(22) The power down detector (PDD) comprises two detectors, an autocorrelation detector and a broadband level detector, each providing a detector signal for characterizing a signal of the forward path (here feedback corrected input signal I.sub.F(k,m) from the front microphone (F)). Other signals could be chosen as inputs to the detectors, e.g. the input signal I.sub.R(k,m) from the rear microphone (R) or the beamformed signal IN(k,m) or one of the time-domain input signals from one of the microphones, etc.). The input signals to the detectors may be equal or different. The input signal I.sub.F(k,m) from the front microphone (F) is split into two branches, one for each of the detectors.

(23) The left branch represents the autocorrelation detector and provides a measure ACM of autocorrelation in the input signal I.sub.F(k,m). The left branch comprises an ABS-unit (ABS) for providing absolute values input signal |I.sub.F(k,m)| of the generally complex values of each of the time-frequency units of the input signal I.sub.F(k,m). In the embodiment of FIG. 2, the input signal I.sub.F(k,m) is represented by the same number N.sub.FB of bands in the auto correlation detector as in the forward path. This need not be the case. A smaller number of bands N.sub.AC may e.g. be considered in the auto correlation detector than in the forward path. In an embodiment, a broadband autocorrelation measure ACM may be determined and used to determine a resulting power control input. An autocorrelation unit (AC) provides the autocorrelation measure ACM, e.g. based on a calculation of the autocorrelation function or a simplified (preferably less power consuming) measure, e.g. based on spectral flatness SFM of the input signal, cf. description below related to FIG. 4A, 4B. In an embodiment, the auto-correlation measure is determined in the same (or in at least some of the) frequency bands wherein the acoustic feedback path(s) is/are estimated (in respective feedback estimation unit(s)).

(24) The right branch represents a broadband level detector and provides a broadband level BB-LVL of the input signal I.sub.F(k,m). The right branch comprises a band sum unit (BS) for providing an accumulated broadband signal I.sub.F(m) in the time domain and an ABS-unit (ABS) for providing an absolute value of the input signal |I.sub.F(m)|. A level estimator (LE) provides an estimate the level of the signal |I.sub.F(m)|, representing the broadband level BB-LVL of the input signal I.sub.F(k,m).

(25) The autocorrelation measure ACM and the broadband level BB-LVL are fed to a logic unit LGC for applying a logic criterion (or one or more logic criteria, e.g. involving probabilistic or binary values of the detector signals) to the detector signals ACM and BB-LVL to provide a resulting detector signal PDC, which is fed to the activation control unit (ACU).

(26) In the embodiment of FIG. 2, the autocorrelation measure ACM is provided on a frequency band level (k,m). This has the advantage that the power control can be considered on a sub-band level. Alternatively, the autocorrelation measure ACM can be provided as a single broadband value. The detector values may be binary or continuous numbers (on a sub-band level or broadband level).

(27) The activation control unit (ACU) comprises (e.g. predetermined) criteria for selecting one of a number of modes of operation of the anti-feedback system(s) based on the resulting detector signal PDC.

(28) The respective feedback estimation units (FBE.sub.F, FBE.sub.R) for the front and rear microphones are controlled by the power control signal pct from the activation control unit (ACU), cf. bottom part of FIG. 2. The activation control unit (ACU) provides a (such as one or more) power mode control signal(s) pct, here indicated as a band level signal by bold arrow representing pct (as opposed to a thin line arrows, e.g. BB-LVL). The number of bands may be equal to or smaller than the number of bands N.sub.FB of the input signal I.sub.F(k,m). This allows the feedback estimation units (FBE.sub.F, FBE.sub.R) to be controlled on a corresponding band level, e.g. to put the feedback estimation units (FBE.sub.F, FBE.sub.R) in different modes of operation (e.g. ON and OFF) depending on the values of the resulting detector signal PDC (and the criteria of the activation control unit (ACU)) in the frequency bands in question.

(29) FIG. 3 shows four exemplary criteria (FIG. 3A, 3B, 3C and FIG. 3D) according to the present disclosure for controlling an anti-feedback system based on detector signals from two detectors.

(30) FIG. 3A illustrates a first exemplary possible modes of operation (ST.sub.xy) of the anti-feedback system (AFB), each specific mode being defined by an AND combination of two sub-ranges of values of two detector signals DS1 and DS2. Each of the detector signals DS1 and DS2 take on values between a minimum and a maximum value [DS1.sub.min; DS1.sub.max] and [DS2.sub.min; DS2.sub.max], respectively. The range of valid values [DS1.sub.min; DS1.sub.max] of the first detector signals DS1 is divided into (N1+1) sub-ranges. Likewise, the range of valid values [DS2.sub.min; DS2.sub.max] of the second detector signals DS2 is sub-divided into (N2+1) sub-ranges. Thereby, a multitude (N1+1)(N2+1) of distinct (AND) combinations of sub-ranges of the two detector signals DS1 and DS2 are defined (e.g. mode ST.sub.22 corresponds to combination (DS.sub.11≦DS1≦DS.sub.12) AND (DS.sub.21≦DS2≦DS.sub.22). In an embodiment, each combination of sub-ranges is associated with a different mode of operation (ST.sub.xy) of the anti-feedback system (as indicated in FIG. 3A). In an embodiment, a number (more than 1) of the different combinations of sub-ranges are associated with the same different mode of operation (ST.sub.xy) of the anti-feedback system.

(31) FIG. 3B illustrates a second exemplary possible modes of operation (ST.sub.xy) of the anti-feedback system (AFB), each specific mode being defined by an OR combination of two sub-ranges of values of two detector signals DS1 and DS2. As in FIG. 3A, each of the detector signals DS1 and DS2 take on values between a minimum and a maximum value [DS1.sub.min; DS1.sub.max] and [DS2.sub.min; DS2.sub.max], respectively. Each or the range of valid values [DS1.sub.min; DS1.sub.max] and [DS2.sub.min; DS2.sub.max] of the first and second detector signals DS1 and DS2, respectively, are divided into (N+1) sub-ranges. Thereby, a multitude (N+1) of distinct (OR) combinations of sub-ranges of the two detector signals DS1 and DS2 are defined (e.g. mode ST.sub.2 corresponds to combination [(DS1.sub.1≦DS1≦DS1.sub.2) AND (DS2≦DS2.sub.2)] OR [(DS2.sub.1≦DS2≦DS2.sub.2) AND (DS1≦DS1.sub.2]).

(32) FIG. 3C schematically illustrates criteria for bringing the anti-feedback system into each of four states (ST1, ST2, ST3, ST4) defined by a combination of two detector signals (DS1, DS2), each detector signal lying within minimum (DS1.sub.min, DS2.sub.min) and maximum values (DS1.sub.max, DS2.sub.max). Two sub-ranges are defined for each detector signal by an intermediate (threshold) value (DS1.sub.th, DS2.sub.th) lying between the minimum and maximum values. FIG. 3C corresponds to FIG. 3A with N1=N2=1.
ST1: (DS1.sub.min≦DS1≦DS1.sub.th) AND (DS2.sub.min≦DS2≦DS2.sub.th)
ST2: (DS1.sub.th<DS1≦DS1.sub.max) AND (DS2.sub.th<DS2≦DS2.sub.max)
ST3: (DS1.sub.th<DS1≦DS1.sub.max) AND (DS2.sub.min≦DS2≦DS2.sub.th)
ST4: (DS1.sub.min≦DS1≦DS1.sub.th) AND (DS2.sub.th<DS2≦DS2.sub.max)

(33) The four states may e.g. correspond to three different ON-states and an OFF state of the feedback cancellation system.

(34) FIG. 3D schematically illustrates criteria for bringing the anti-feedback system into each of three states (ST1, ST2, ST34) defined by a combination of two detector signals (DS1, DS2), each detector signal lying within minimum (DS1.sub.min, DS2.sub.min) and maximum values (DS1.sub.max, DS2.sub.max). Three sub-ranges are defined for each detector signal by two intermediate (threshold) values (DS1.sub.th1, DS1.sub.th2, DS2.sub.th1, DS2.sub.th2) lying between the minimum and maximum values. FIG. 3D corresponds to FIG. 3B with N=2.
ST1: (DS1.sub.min≦DS1≦DS1.sub.th1) AND (DS2.sub.min≦DS2≦DS2.sub.th1)
ST2: (DS1.sub.th2<DS1≦DS1.sub.max) OR (DS2.sub.th2<DS2≦DS2.sub.max)
ST34: [(DS1.sub.th1<DS1≦DS1.sub.th2) OR (DS2.sub.th1≦DS2<DS2.sub.ths)] AND [(DS1≦DS1.sub.th2) AND (DS2≦DS2.sub.th2)]

(35) The three states may e.g. correspond to two different ON-states and an OFF state of the feedback cancellation system.

(36) FIG. 4A shows a block diagram of an embodiment of a power down/power on controller, and FIG. 4B shows a power down/power on detector for an anti-feedback system of a hearing device according to the present disclosure.

(37) FIG. 4A illustrates a power down detector (PDD) and activation control unit (ACU) coupled to a feedback estimation unit (FBE) to control a mode of operation regarding a level of power consumption of the feedback estimation unit (FBE), as e.g. discussed in connection with FIGS. 1A, 1B, 1C and 2. The power down detector (PDD) is configured to decide, based on a number of detector signals DS1, DS2, . . . , DSN.sub.D, whether the anti-feedback system should be scaled down or up and sends a corresponding control signal to the activation control unit (ACU). The activation control unit (ACU) effectuates the scaling down or up (mode control) by sending a number of control signals pct to the feedback estimation unit (FBE). The activation control unit (ACU) may receive other input signals to govern the determination of the power control signal(s) pct. In FIG. 4A, an input signal GLIM for limiting the allowable gain changes during a power down and/or a power up of the anti-feedback system (cf. e.g. FIG. 5 and corresponding discussion below). Other limiting parameters may be fed to the activation control unit (ACU) to implement particular modes of operation or to define transitions (e.g. timing) from one mode of operation to another.

(38) The exemplary concept implemented in FIG. 2 is based on monitoring the broadband level (BB-LVL) and auto-correlation (AC) of one of the microphone signals (here ‘front’ microphone signal I.sub.F, cf. also FIG. 2). A block diagram of an exemplary implementation of the concept is shown in FIG. 4B. Instead of the broadband level (BB-LVL, e.g. based on input signal |I.sub.F(m)|), a level estimate (SB-LVL) on a per frequency sub-band basis may be provided (e.g. based on input signal |I.sub.F(k,m)|).

(39) The input signal (e.g. |I.sub.F(k,m)| or |I.sub.F(k,m)|.sup.2) to the auto correlation detector (AC) is a time-frequency domain signal, e.g. based on an output signal of an analysis filter bank, i.e. a signal composed of M/2 bands, e.g. ranging from 0 to f.sub.s/2 Hz, e.g. 10 kHz (M being e.g. the number of frequency bins or bands of a fast Fourier transformation (FFT), e.g. a 512 point FFT, or the number of frequency bands in a filter bank, e.g. providing 64 bands, f.sub.s being a sampling frequency, e.g. 20 kHz). Based on this band split signal, or a subset of the bands (e.g. the bands where the anti-feedback system is active, e.g. above a lower AFB-threshold frequency f.sub.AFB,th, e.g. above 1.5 kHz), the auto-correlation AC unit provides an auto-correlation measure ACM. In an embodiment, auto-correlation is estimated using a Spectral Flatness Measure (SFM) given by the geometric mean divided by the arithmetic mean, i.e.

(40) $\begin{array}{cc}\mathrm{SFM}=\frac{\sqrt[N]{\underset{n=0}{\overset{N-1}{.Math.}}{S}_{\mathrm{xx}}\left[n\right]}}{\frac{1}{N}\underset{n=0}{\overset{N-1}{.Math.}}{S}_{\mathrm{xx}}\left[n\right]}& \mathrm{Equation}\left(1\right)\end{array}$

(41) where S.sub.xx is the power spectrum of the signal x. By taking log.sub.2 to the expression in Eq. (1), an expression of the Spectral Flatness Measure (SFM) more suited for implementation in a logarithmic environment can (in a second embodiment) be achieved:

(42) $\begin{array}{cc}\mathrm{SFM}={\log }_2\left(\frac{\sqrt[N]{\underset{n=0}{\overset{N-1}{.Math.}}{S}_{\mathrm{xx}}\left[n\right]}}{\frac{1}{N}\underset{n=0}{\overset{N-1}{.Math.}}{S}_{\mathrm{xx}}\left[n\right]}\right)\mathrm{SFM}=\frac{1}{N}\underset{n=0}{\overset{N-1}{.Math.}}{\log }_2\left(S_{\mathrm{xx}}\left[n\right]\right)-{\log }_2\left(\frac{1}{N}\underset{n=0}{\overset{N-1}{.Math.}}{S}_{\mathrm{xx}}\left[n\right]\right)& \mathrm{Equation}\left(2\right)\end{array}$

(43) The SFM AC measure of Eq. 2 is close to 1 when S.sub.xx is flat (white noise) and close to 0 when S.sub.xx is peaky (pure tone). To obtain an AC measure that is linearly proportional to the amount of autocorrelation, an inversion of the SFM measure is performed (e.g. in that ACM=−SFM of eq. 2). The dynamic range of the SFM is primarily influenced by the choice of filterbank, which provides the power spectrum. Any other appropriate AC measure can be used as appropriate for the practical application (e.g. adapted to the specific hardware and/or software configuration, power constraints, etc.). In an embodiment, the AC-measure is based on a broadband (e.g. time-domain) calculation of autocorrelation.

(44) Both the broadband level and the AC measure are e.g. passed through a level estimator (LE) with a relatively fast attack time (low.fwdarw.high level and high.fwdarw.low SFM transition) and a relatively slow release time (high.fwdarw.low level and low.fwdarw.high SFM transition). The level estimators are introduced in order to limit the number of transitions from one mode to the other (e.g. OFF to ON and vice versa). By having a slow release, the power is kept ON for a minimum period directly related to the release time constant. The same function is e.g. provided by hysteresis blocks (HYST). The outputs of the respective hysteresis blocks (HYST) represent (stable, ‘low-pass filtered’) values of broadband level (BB-LVL) and AC measure (ACM), which are logically combined according to a predetermined criterion or criteria in block LGC. The criteria may be based on logic operations, e.g. comprising Boolean operators (AND, OR, XOR, etc. or negations thereof). Furthermore, a Timer (block TIM in FIG. 4B) has been inserted to assure that the power is kept ON for a minimum period, which can be determined by parameter in the Timer. Thereby the number of (unnecessary) ON-OFF and OFF-ON transitions is reduced. One of the reasons for limiting the number of transitions from OFF to on and vice versa, is that is takes some time to scale up the anti-feedback system. Furthermore, a gain reduction is preferably introduced, while scaling up the AFB system in order to prevent the risk of howl (cf. GLIM in FIG. 4A). An exemplary course of events during a power ON (activation) of the AFB-system is illustrated in FIG. 5.

(45) FIG. 5 shows an exemplary Power ON timing of the anti-feedback system according to the present disclosure when the anti-feedback system is brought from an OFF-mode to an ON-mode of operation.

(46) When a howl or near howl occur it takes t.sub.DET s before it is detected. At that time a gain reduction is introduced and the AFB system is powered ON/scaled up, which takes t.sub.ON s. Once the AFB system is running it takes t.sub.CONV s further for the system to converge to the acoustic feedback path—during this period, the gain is gradually increased until no gain reduction resides. Other courses of the timing of the AFB-power ON (after a partial power down) may be envisioned, and implemented depending of the specific application. The slopes of the gain changes during power on of the AFB-system are preferably configurable, to minimize artifacts as indicated by dashed lines in FIG. 5 (cf. changes of Gain around Time=Detection and during convergence of the AFB-algorithm (between Time=AFB ON and AFB Converged) in FIG. 5).

(47) Ideally, the anti-feedback system should be in an ON-mode of operation, and continuously updating the filter coefficients of an adaptive filter for estimating the acoustic feedback path (thereby updating the ‘estimate of the acoustic feedback path’) in situations where the acoustics around the hearing device are dynamic. ‘Dynamic acoustics’ is in the present context taken to mean that the acoustic feedback path is changing rapidly with time (e.g. within ms or a few s, as opposed to when it is static, e.g. over tens of seconds or more). Since the anti-feedback system itself cannot be used for detection of dynamic acoustics, indicators or detectors of such dynamic acoustics must be found in other parts of the hearing device. Below possible indicators are proposed: Level changes; a high level may indicate howl or a risk of howl. Gain changes; a low gain may indicate howl or a risk of howl. Autocorrelation (AC); a relatively high autocorrelation may indicate (near) howling Noise reduction gain or wind noise gain; when the noise reduction gain is high, it is an indication of the presence of noise i.e. little autocorrelation, and hence low risk of howl, and vice versa. Front/rear signal level differences; due to the physical distance between the two microphones, level differences can be detected in cases with dynamic acoustics. Binaural autocorrelation detection; different levels at the two hearing devices of a binaural hearing system may indicate a dynamic acoustic situation.

(48) Three exemplary criteria according to the present disclosure for controlling an anti-feedback system based on detector signals from an auto-correlation detector and a level detector are shown in FIG. 6A, FIG. 6B and FIG. 6C. The activation control unit (AFB-CONT in FIG. 2) is configured to control the anti-feedback system based on a predefined criterion comprising a number of detector signals. In the embodiment of a hearing device shown in FIG. 2, where the activation control unit bases its control on two detector signals (N.sub.D=2), auto-correlation and broad-band level estimates of a signal of the forward path, three exemplary different criteria are graphically illustrated in FIGS. 6A, 6B and 6C, respectively.

(49) FIG. 6A illustrates the following criteria for bringing the anti-feedback system into an ON-state and an OFF-state, respectively:
ON: (AC.sub.th<AC≦AC.sub.max) AND (LVL.sub.th<BB-LVL≦LVL.sub.max)
OFF: (AC.sub.min≦AC≦AC.sub.th) AND (LVL.sub.min≦BB-LVL≦LVL.sub.th)

(50) In the OFF-state, the feedback cancellation system is turned into a low-power mode, where it does not estimate the acoustic feedback path and thus does not cancel the acoustic feedback path. The aim of the OFF-state is to save power by not activating the feedback estimation unit. In the ON-state, the feedback cancellation system is on and a feedback estimate is repeatedly determined with a predefined or dynamically determined adaptation rate.

(51) FIG. 6B illustrates a first criterion for bringing the anti-feedback system into an ON-state (comprising an ON-FAST and an ON-SLOW mode) and an OFF-state, respectively:
ON-FAST: (AC.sub.th<AC≦AC.sub.max) AND (LVL.sub.th<BB-LVL≦LVL.sub.max)
ON-SLOW: [(AC.sub.th<AC≦AC.sub.max) AND (LVL.sub.min≦BB-LVL<LVL.sub.th)] OR [(AC.sub.min≦AC≦AC.sub.th) AND (LVL.sub.th<BB-LVL≦LVL.sub.max)]
OFF: (AC.sub.min≦AC≦AC.sub.th) AND (LVL.sub.min≦BB-LVL≦LVL.sub.th)

(52) The ON-FAST and ON-SLOW modes represent a mode, where the feedback cancellation system is on and a feedback estimate is repeatedly determined with a predefined or dynamically determined relatively high or relatively low update frequency f.sub.upd, respectively. The ON-FAST mode consumes more power than the ON-SLOW mode due to the more frequent update frequency of the ON-FAST mode.

(53) FIG. 6C illustrates a second criterion for bringing the anti-feedback system into an ON-state (comprising an ON-FAST and an ON-SLOW mode) and an OFF-state, respectively:
OFF: (AC.sub.min≦AC≦AC.sub.th1) AND (LVL.sub.min≦BB-LVL≦LVL.sub.th1)
ON-SLOW: [(AC.sub.th1<AC≦AC.sub.th2) OR (LVL.sub.th1≦BB-LVL<LVL.sub.th2)] AND [(AC≦AC.sub.th2) AND (BB-LVL≦LVL.sub.th2)]
ON-FAST: (AC.sub.th2≦AC≦AC.sub.max) OR (LVL.sub.th2≦BB-LVL≦LVL.sub.max)

(54) Other criteria based on auto-correlation and broad-band level estimates may be used. Similarly, other (or additional) detector signals may be used to control the anti-feedback system. In an embodiment, an output signal of a feedback detector for detecting whether tonal elements in a signal of the forward path at a given point in time comprises frequency elements that are due to feedback from the output transducer to the input transducer, is used to control the anti-feedback system, e.g. together with other sensor signals. In an embodiment, the detector signals from the broad-band (or sub-band) level detector and the auto-correlation detector are combined with a detector signal from a feedback detector for detecting whether or not tonal elements (e.g. in a given frequency band) present in a signal of the forward path at a given point in time are due to feedback from the output transducer to the input transducer.

(55) FIG. 7 shows a binaural hearing system (e.g. a binaural hearing aid system) according to the present disclosure. FIG. 7 shows an embodiment of a binaural hearing aid system comprising first and second hearing devices (HD1, HD2) adapted for being located at or in left and right ears of a user.

(56) The hearing devices HD1 and HD2, which in various embodiments may be equivalent to the hearing devices described in connection with FIG. 1A, 1B, 1C, each comprise a time to time-frequency conversion unit (TF) for converting time domain input signals INm and INw to time-frequency input signals IFB allowing processing in the respective signal processing units (SPU) and feedback estimation units (FBE) in a number of frequency channels FB.sub.1, FB.sub.2, . . . , FB.sub.N (as indicated in the drawing by bold arrows representing signals IFB, err, ref, and fbp). Each hearing device comprises an input transducer (IT) comprising a microphone providing an analogue electric input signal, and an analogue to digital conversion unit (AD) providing digitized input microphone signal INm. Each hearing device further comprises a wireless transceiver comprising antenna (ANT) and transceiver circuitry (Rx/Tx) providing digitized input wireless signal INw. The time-frequency conversion unit (TF) is configured to select one of the input signals INm or INw (or a mixture of them) and provide it as band split signals IFB. The hearing devices HD1, HD2 each further comprise a time-frequency to time conversion unit (FT) for converting processed output signals ref to respective time domain signals OUT, which are fed to respective digital to analogue transformation units (DA) and on to the output transducer (OT), here a loudspeaker. As described in connection with FIGS. 1A, 1B, and 1C, each hearing device (HD1, HD2) further comprises an anti-feedback system comprising a feedback estimation unit (FBE) for estimating an acoustic feedback path from the output transducer (OT) to the input transducer (IT) and providing a signal fbp representative thereof. The anti-feedback system further comprises a summation (subtraction) unit (‘+’) for subtracting the signal fbp representative of the current acoustic feedback path from the (digitized) electric input signal (IFB) and providing a feedback corrected signal (error signal err), which is fed to the signal processing unit (SPU), and to the feedback estimation unit (FBE). The hearing devices (HD1, HD2) each further comprises a battery (BAT) for providing current to the functional blocks of the hearing device (cf. signals pwr), including to the anti-feedback system, and a power down detector and activation control unit (PDD-ACU) for controlling the modes of operation of the anti-feedback system to economize on its power consumption without sacrificing on solving its primary task: to avoid or minimize howl. The hearing devices (or alternatively the power down detector part (PDD)), each comprises one or more detectors (DET1), each providing a detector signal det1 for characterizing or analysing or receiving a) the microphone signal INm and b) the wirelessly received signal INw. In an embodiment, detector signals from one or more detectors external to the hearing devices (HD1, HD2), e.g. from a smartphone, may be received via antenna and wireless transceivers (ANT, Rx/Tx), e.g. represented by signal INw.

(57) The hearing devices (HD1, HD2) of FIG. 7 are further adapted for exchanging information between them via a wireless communication link, e.g. a specific inter-aural (IA) wireless link (IA-WLS). The inter-aural link may e.g. be based on inductive (near-field) communication, or alternatively on radiated field (far-field) communication. The two hearing devices are adapted to allow the exchange of status signals, e.g. including the transmission of detector signals generated or received by a hearing device at a particular ear to the hearing device at the other ear. To establish the inter-aural link, each hearing device comprises antenna and transceiver circuitry (here indicated by block IA-Rx/Tx). The detector signals det1, XD1 from the local (det1) and the opposite (XD1) device, respectively, are e.g. used together to influence a decision regarding activation a particular mode of operation of the anti-feedback system (e.g. an ON-mode or an OFF-mode) in the local device (e.g. HD1). In an embodiment, the hearing assistance system further comprises an auxiliary device for transmitting an audio signal and/or a detector signal to the hearing devices.

(58) The activation control part (ACU) is configured to control the anti-feedback system (including the feedback estimation unit FBE) based on the detector signals det1, XD1 (cf. power control signal pct), and to bring the anti-feedback system into one of at least two predefined modes based on the detector signals. The at least two predefined modes comprises an ON-mode and an OFF-mode. The ON-mode comprises a normal power consumption mode. The OFF-mode comprises a minimum power consumption mode. A main part of the processing of the hearing devices is performed in the time-frequency domain (cf. bold arrows on signals IFB, err, ref, fbp), but may alternatively be performed partially in the time domain and the time frequency domain. In an embodiment, the feedback estimation is performed partially in the time domain and partially in the time-frequency domain.

(59) In an embodiment, the detector unit DET1 comprises an autocorrelation detector providing a measure of a current signal of the forward path, here a time-domain digitized microphone input signal INm. A current value of the autocorrelation measure of a given hearing device (HD1) is transmitted to the other hearing device (HD2) for comparison with a corresponding value generated in the other hearing device (HD2). In a situation, where the measures of an autocorrelation are indicative of the autocorrelation being larger than a threshold value in one of the hearing devices (HD1, HD2), but not the other, the source of the current autocorrelation is associated with a howl (or build-up of a howl) in that one of the hearing devices (e.g. HD2) having an autocorrelation larger than the threshold value. Consequently, an activation of an ON-mode of operation of the anti-feedback system in the relevant hearing device (HD2) may preferably be initiated. The autocorrelation measurements may e.g. be further compared with other detector signals (locally generated and/or received from the other hearing device), e.g. input level estimates, requested gain values, etc., to further improve the confidence of the decision on activation (or not) of a particular mode of operation of the anti-feedback system.

(60) 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.

(61) 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 elements 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.

(62) 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.

(63) 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.

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