Field of the Invention
The invention relates to a method and a hearing device with improved feedback recognition and feedback suppression by the use of a directional microphone.
A frequent problem with hearing devices is the feedback between the output of the hearing device and the input, which becomes noticeable as an interfering whistling. FIG. 1 shows the principle of acoustic feedback. A hearing device 1 has a microphone 2, which receives an acoustic wanted signal 10, converts the signal into an electrical microphone signal 11 and outputs it to a signal processing unit 3. In the signal processing unit 3, the microphone signal 11 is inter alia prepared and amplified and output to a receiver 4 as an electrical receiver signal 12. In the receiver 4, the electrical receiver signal 12 is again converted into an acoustic output signal 13 and output to an eardrum 7 of a hearing device wearer.
The problem now consists in part of the acoustic output signal 13 reaching the input of the hearing device 1 by way of an acoustic feedback path 14, where it is superimposed with the wanted signal 10 and received by the microphone 2 as a sum signal. Therefore interfering feedback whistling results with an unfavorable phasing and amplitude of the fed-back output signal. In particular, with an open hearing device supply, the attenuation of the acoustic feedback is minimal, as a result of which the problem is intensified.
Adaptive systems for feedback suppression have been available as a solution for some time. To this end, the acoustic feedback path 14 in the hearing device 1 is digitally recreated. The recreation takes place for instance by an adaptive compensation filter 5, which is fed by the receiver signal 12. After a filtering in the compensation filter 5, a filtered compensation signal 15 is subtracted from the microphone signal 11. Ideally the effect of the acoustic feedback path 14 is cancelled and a feedback-free input signal 16 is produced for the signal processing unit 3.
A regulation and/or adaptation of filter coefficients of the adaptive compensation filter 5 is needed for effective feedback suppression. To this end, the microphone signal 11 is evaluated and checked for possible feedbacks with the aid of a detection unit 6. By regulating and/or adapting the filter coefficients, artifacts may however also develop, since additional signal components are generated or a feedback whistling occurs in the case of an adaptive compensation filter 5 which is not adjusted optimally. European patent EP 1 033 063 B1, corresponding to U.S. Pat. Nos. 6,104,822 and 6,072,884, discloses a hearing device with feedback suppression, with two adaptive compensation filters which operate in parallel being used in order to improve the feedback suppression.
The wanted signal 10 represents the greatest problem for optimal feedback suppression because it represents an interference signal from the perspective of a system for feedback suppression. What makes matters worse is that as a result of the amplification of the wanted signal 10 by the signal processing unit 3, the feedback signal 14 is highly correlated with the wanted signal 10, as a result of which the detection unit 6 is only able to make a distinction between feedback 14 and the wanted signal 10 with difficulty.
A correct adjustment of the adaptation speed of the compensation filter 5 is consequently of huge significance. If the adaptation is too slow, the feedback whistling occurs for some time until the suppression activates. If the adaptation is too quick, so-called “musical” artifacts (musical noise) occur, since the compensation filter 5 also attempts to compensate for the wanted signal. The detection unit 6 which always selects the optimal adaptation speed is therefore needed for feedback recognition. Therefore the behavior of the detection unit 6 is mainly responsible for a smooth function of the feedback suppression.
Directional microphone systems are among the interference noise suppression methods which have been established for years and these are subsequently shown to improve speech intelligibility in hearing situations in which the wanted signal and the noise signals are received from different directions. In modern hearing devices the directional effect is produced by differential processing of two or more adjacent microphones with omnidirectional characteristic.
FIG. 2 shows a simplified block diagram of a directional microphone system in the first arrangement with two microphones 21, 22 at a distance of around 10 to 15 mm. For sound signals arriving from the front V this causes an external delay of T2 between the first and the second microphone 21, 22, which corresponds for example to the distance from the microphones 21, 22 to one another. The signal R2 from the second microphone 22 is delayed by time T1 in a delay unit 23, inverted in the inverter 24 and added in the first adder 25 to the signal R1 from the first microphone 21. The sum yields the directional microphone signal RA that can be fed via a signal processing function to a receiver for example. The directional sensitivity essentially results from a subtraction of the second microphone signal R2, which was delayed by time T2, from the first signal R1. Thus after appropriate equalization, sound signals from the front V are not attenuated, whereas sound signals from the rear S, for example, are canceled out.
Adaptive directional microphones are microphones which are able to adjust to different ambient situations during current operation. Generally, the objective pursued here is the receiving and conveying of wanted sound output by a wanted sound source as effectively as possible, while the interfering sound originating from one or several interference sound sources is to be attenuated as effectively as possible in the output signal output by the adaptive directional microphone. International patent disclosure WO 00/19770 A1, corresponding to U.S. Pat. No. 6,751,325, discloses a hearing device with an adaptive directional microphone, with which the directional amplification/attenuation can be varied in accordance with the result of a signal analysis.