Patent Publication Number: US-6704422-B1

Title: Method for controlling the directionality of the sound receiving characteristic of a hearing aid a hearing aid for carrying out the method

Description:
This is a Continuation-in-Part of application Ser. No. 09/445,485 filed Dec. 7, 1992, which is a 371 of PCT/EP99/04375, filed Jun. 24, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for controlling the directionality of the sound receiving characteristic of a hearing aid comprising spaced apart first and second sound receiving microphone means, a signal processor for processing signals supplied by said microphone means and an output transducer for emission of sound signals in response to output signals from the signal processor, said method comprising the steps of changing over said sound receiving characteristic between an omnidirectional characteristic and a directional characteristic and, when operating the hearing aid with said directional characteristic, combining the signals supplied by said first and second microphone means into an overall combined signal, which is supplied to the signal processor, an adjustable time or phase delay being imposed on at least one signal. 
     Hearing aids having a directional sound receiving characteristic are useful to improve speech perception in noisy environments, where human speech may be received simultaneously from different directions, as is the case e.g. in the noise environment frequently referred to as cocktail party noise. 
     With a directional sound receiving characteristic, e.g. in the shape of a cardioid or super cardioid characteristic, the speech perception in a hearing aid is improved by reduced reception of sound coming from the back of the user, while maintaining the level of sound coming from the area in front of the user. 
     On the other hand, in environments with only a low noise level or no significant speech signals the hearing aid user will normal prefer an omnidirectional or spherical sound receiving characteristic offering the same perception of sound irrespective of the direction, from which it arrives. 
     As will be further explained in the following a prior art hearing aid of the kind defined above, offering the possibility of changing the sound receiving characteristic between an omnidirectional characteristic and a directional characteristic of varying shape has been disclosed in U.S. Pat. No. 5,757,933. 
     With this prior art hearing aid operating with an omnidirectional characteristic only the signal from the first microphone facing the area in front of the user is supplied to the signal processor. By manual operation of a switch a signal derived from the second microphone facing the rear of the user and subjected to inversion followed by adjustable phase delay and adjustable attenuation is combined via a summing node with the signal derived from the first microphone. 
     When the sound receiving characteristic in a hearing aid of this type is changed or changes from the omnidirectional to a directional shape, the arrival time of the sound changes during the transition. This change of phase or time delay may become confusing in a binaural hearing aid system using a pair of separate hearing aids operating with independent and automatic change of the sound receiving characteristic. When phase or arrival times change differently in the two hearing aids this will degrade or deteriorate the user&#39;s ability to locate the various sound sources in the surrounding space and the advantage of a binaural hearing aid system will be degraded. 
     Furthermore, the phase and time relationship in a hearing aid degrades the quality of the sound perceived by the user. It may sound like the result of a Doppler-effect. 
     At the same time, in hearing aids of this type also the amplitude characteristic will change during transition between the omnidirectional and a directional characteristic, e.g. from a flat response to a response in which the amplitudes of higher frequencies will be increased. This increase may be in the area of 6 dB/octave. This results in the serious problem, that hearing aids of this type can not be perfectly fitted with an optimum transfer characteristic for both the omnidirectional and the directional characteristic. 
     SUMMARY OF THE INVENTION 
     On this background, it is the object of the present invention to provide a method of the kind defined, in which the deficiencies of the prior art hearing aid are remedied by effecting a smooth change-over between the omnidirectional characteristic and any directional characteristic substantially without changing the phase relationship or time delay and the amplitude characteristic of the signals. The change-over between the omnidirectional characteristic and a directional characteristic and vice versa may be controllable or even automatic. 
     According to the invention this object is achieved by a method of the kind defined, which is characterized in that said change-over of the sound receiving characteristic from the omnidirectional characteristic to the directional characteristic and vice versa is effected by controlled attenuation and time or phase delay of signals deriverd from both of the signals (X front , X back ) from the first and second microphone means before forming said overall combined signal (Y), using an adjustable attenuation control parameter (omni) and a delay (T), whereby said overall combined signal (Y) is determined by 
       Y=X   front *(1−omni* e   −jωT )+ X   back *(omni− e   −jωT ), 
     to change over the hearing aid between said omnidirectional characteristic and any desired form of said directional characteristic as a smooth change-over substantially without affecting phase relationship, time delay and amplitude characteristic of the hearing aid. 
     For carrying out the method as defined the invention further relates to a hearing aid with controllable directionality of its sound receiving characteristic, comprising spaced apart first and second sound receiving microphone means, a signal processor for processing signals supplied by said microphone means and an output transducer for emission of sound signals in response to output signals from the signal processor, and further comprising change-over control means for change over of the sound receiving characteristic between an omnidirectional characteristic and a directional characteristic and combining means for combination of the signal from the first and second microphone means to provide an overall combined signal supplied to the signal processor, when operating the hearing aid with said directional characteristic, and adjustable time or phase delay means being provided for producing a phase-delayed modification of at least one signal. 
     According to the invention this hearing aid is characterized in that said change-over control means comprises controllable attenuation means and controllable time or phase delay means acting on signals derived from the signals from both of the first and second microphone means, respectively, said attenuation and phase delay means being controlled for forming said overall combined signal (Y) using an adjustable attenuation control parameter (omni) and a delay (T), whereby said overall combined signal (Y) is determined by 
     
       
           Y=X   front *(1−omni* e   −jωT )+ X   back *(omni− e   −jωT ), 
       
     
     to change over the hearing aid between said omnidirectional characteristic and any desired form of said directional characteristic as a smooth change-over substantially without affecting phase relationship, time delay and amplitude characteristic of the hearing aid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the invention will be further explained with reference to the accompanying drawings, in which 
     FIG. 1 is a schematic block diagram of the prior art hearing aid of U.S. Pat. No. 5,757,933, 
     FIGS. 2-5 are graphic representations illustrating variation of the sound receiving characteristic of the hearing aid in FIG. 1 between the omnidirectional characteristic and different directional shapes and concurrent variation of amplitude characteristics of the front and back microphones used therein, 
     FIG. 6 shows a schematic arrangement of the front end of a first embodiment of a hearing aid according to the present invention, 
     FIGS. 7 to  10  are graphic representation corresponding to the representations in FIGS. 2 to  5  with respect the hearing aid shown in FIG. 6, 
     FIG. 11 shows a schematic arrangement of a second embodiment, 
     FIG. 12 shows a similar schematic arrangement of a third embodiment, 
     FIG. 13 schematically shows a further improvement of the arrangement shown in FIG. 6, and 
     FIG. 14 shows a still further development of a hearing aid embodying the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the prior art hearing aid in FIG. 1 two non-directional microphone circuits including a front microphone MICF and a back microphone MICB. Whereas the output signal from the front microphone MICF is supplied directly to the hearing aid signal processor via a summing node SN, the signal from the back microphone is supplied to the summing node SN via an inverter, an adjustable phase delay circuit and an attenuator with adjustable gain only by closure of a manually operated switch SW, whereby the sound receiving characteristic of the hearing is changed from the omnidirectional characteristic of front microphone MICF to a directional characteristic of varying shape. 
     The combined signal Y formed at the summing node SN with switch SW closed and supplied to the signal processor will thus be related to the signals X front  and X back  from front and back microphones MICF and MICB, respectively, by the relation 
     
       
           Y=X   front   −X   back *omni* e   −jωT , 
       
     
     where the adjustable parameter omni represents the adjustable gain of the attenuator, whereas T represents the adjustable time delay corresponding to the difference in arrival time for sound signals received by the front and back microphones MICF and MICB, respectively. 
     The graphic representations in FIGS. 2 and 3 illustrate the variation of the sound receiving characteristic of the hearing aid in FIG. 1 from the omnidirectional shape ND and various directional shapes D 1  to D 10  ranging from weak cardioid to super cardioid form for values of the adjustable parameter omni ranging from 0 to 1, measured at 1 kHz and 100 Hz, respectively, whereas the graphic representations in FIGS. 4 and 5 show the variation in the amplitude characteristics of the signals received from the areas in front and back of the hearing aid, respectively, for correspondingly varying values of the parameter omni. 
     As will appear from these representations the change-over between the omnidirectional characteristic and the various shapes of directional characteristic results in this prior art hearing aid not only in the desired gradual reduction in gain or amplitude response for the signals received from the area behind the user, but is accompanied also by a significant change in gain or amplitude response for the signals received from the area in front of the user. In consequence thereof an adjustment or fitting of the hearing to compensate for a users specific hearing impairment for listening in quiet surroundings, where use of the omnidirectional characteristic is preferred, will not provide an optimum compensation, when a change over is made to a directional characteristic, e.g. for use of the hearing aid in a more noisy sound environment, such as a party. 
     FIG. 6, shows, in principle, the front end of a first embodiment of a hearing aid according to the inventions including a change-over controller for controlling change of the directionality of sound receiving characteristic of the hearing aid from the omnidirectional characteristic to a directional characteristic and vice versa. This change may be effected as a switch-over or as a gradual and smooth change-over. 
     The front end of the hearing aid comprises at least two microphone circuits, i.a. a front microphone Fmic and a back microphone Bmic and possibly optional preprocessing circuits for the electrical output signals from the microphones. The distance between the two microphones may be as small as 1 mm or as wide as a few cm. 
     The front end further contains at least two controllable amplifiers or attenuators  1  and  2 , at least one time or phase delay device  3  and at least three combining circuits  4 ,  5  and  6 . It is to be understood that the combining circuits may contain positive as well as negative input terminals, so as to form adding or subtraction operations or combinations thereof. 
     In the structure, the back microphone Bmic is connected to the controllable amplifier or attenuator  1  and to a first adding circuit  4 . 
     The front microphone Fmic is connected directly to the controllable amplifier or attenuator  2  and to a second adding circuit  6 . The output of the controllable amplifier or attenuator  2  is further connected directly to a second input of the first adding circuit  4 , whereas the output of the controllable amplifier  1  is directly connected to a positive input of a subtraction circuit  5 . 
     Between the output of the first adding circuit  4  and the negative input of the subtraction circuit  5  a preferable controllable delay device  3  is included. 
     In the following description the adding and subtracting circuits will generally be referred to as combining circuits. 
     In operation, sounds from the environment of the hearing aid is picked up both by the front microphone Fmic and the back microphone Bmic. The distance between the two microphones may be as small as 1 mm and as wide as a few cm. 
     The output signal of the front microphone Fmic is supplied to the combining circuit  6 . The output signal of the back microphone Bmic is supplied to the controllable attenuator or controllable amplifier  1 , the gain of which may be controllably changed from zero to one, i.e. from no amplification to full amplification. This change-over may be effected as a switch-over or as a controlled gradual change. This means that any amplification between zero and one may be controllably achieved. 
     The output signal, if any, of the front microphone Fmic is also supplied to a controllable attenuator or amplifier  2 , the amplification of which may controllably be changed from zero to one, i.e. from no amplification to full amplification. Also in this case the change-over may be effected as a switch-over or as a gradual controlled change. This means that any amplification between zero and one may be achieved. 
     The output signal, if any, of the controllable attenuator or amplifier  2  is supplied to a second input of the combining circuit  4 . The output signal, if any, of combining circuit  4  is supplied to the controllable delay device  3 , the delay of which may be controlled from as small as 1 μs up to 1000 μs or more. 
     The output signal, if any, of delay device  3  is supplied to the negative input of combining circuit  5 , the output of which is supplied to the second input of the combining circuit  6 . 
     Thereby, the output signal of the front microphone Fmic may be attenuated in attenuator or controllable amplifier  2  before it is added to the undelayed output signal of the back microphone Bmic in the combining circuit  4 , the output signal of which is then delayed in delay device  3  before being supplied to the combining circuit  5 . The controllable delay of delay device  3  will usually have the same value as the acoustical delay between the arrival times of sounds at the front microphone Fmic and at the back microphone Bmic. Preferably this delay is also adjustable and/or controllable. 
     Additionally, the output signal of the attenuator or controllable amplifier  1  is supplied to the positive input of the combining circuit  5 . In this combining circuit the delayed output signal of delay device  3  is subtracted from the attenuated output signal of amplifier or attenuator  1 . The output signal of the combining circuit  5  is supplied, as a processed signal to the combining circuit  6 . The output signal of the combining circuit  6  is then used as an input signal for further processing in the remaining components of the hearing aid such as the signal processor, which need not to be described here. 
     The remaining parts of the hearing aid may as known in the art, comprise more than one signal processing channel having, and with such a structure either a common change-over controller or a separate controller for each channel may be provided. 
     As further known in the art, the output signals of both microphones Fmic and Bmic may advantageously be converted into a digital representation before being supplied to the change-over controller with its components  1  to  6 . 
     The function of the circuit in FIG. 6 is as follows: 
     For the directional mode of operation the signal transfer of the controllable attenuators  1  and  2  is set at zero, i.e. no signal is transferred. 
     The output signal of the front microphone Fmic is directly supplied to the second adding circuit  6 . The output signal of the back microphone Bmic is supplied via the first adding circuit  4  and delay device  3  to the negative input of the subtraction circuit  5 , where the signal changes its polarity. The output signal of the subtraction circuit S is then supplied to a second input of the second adding circuit  6 . Thus, the delayed signal from the back microphone Bmic is subtracted from the undelayed output signal of the front microphone Fmic. 
     The directional front characteristic may then be created by adjusting the delay T of the delay device to be the same as the acoustical delay A between the back microphone Bmic and the front microphone Fmic. With this delay the signals, that are first received at the back microphone Bmic and are later received at the front microphone Fmic, are then suppressed in the adding circuit  6 , where the delayed signal of the back microphone is subtracted from the output signal of the front microphone. 
     This mode of operation results in an output signal from adding circuit  6 , which is the result of the subtraction of the delayed output signal of the back microphone Bmic from the output signal of the front microphone Fmic, thus cancelling sound coming directly from the back of the user. 
     By adjusting T&lt;A, sound coming partly from the side of the user is cancelled, the direction of the cancelling effect is controlled by the ratio of T/A. 
     For the omnidirectional mode of operation both attenuators  1  and  2  are set for a full signal transfer. 
     The output signals from the front microphone Fmic and the back microphone Bmic are supplied to the first adding circuit  4 , where they are combined and supplied via delay device  3  to the subtraction circuit  5 , where the combined and delayed signal is subtracted from the output signal of the back microphone Bmic. 
     The output signal of the subtraction circuit  5  is then supplied to the second adding circuit  6 , where it is combined with the undelayed output signal of the front microphone Fmic. The addition of these signals creates the omnidirectional characteristic. This mode of operation results in an output signal from the adding circuit  6 , which is generated by the addition of the signals from the front and back microphones from which the delayed front and back microphone signals are subtracted. 
     The sound signals received at the two microphones differ with respect to their arrival time at the respective microphones from a source, the distance of which is different for the two respective microphones. 
     This difference is the acoustical delay A and the relationship between the sound signals X front  and X back  received at the front and back microphones, respectively, may be generally expressed as 
     
       
           X   back   =X   front   *e   −jωA , 
       
     
     where e −jωA  is the acoustical delay for the actual direction to the sound source. 
     The combined signal Y from adding circuit  6  is 
     
       
           Y=X   front *(1 −omni*e   −jωT )+ X   back *(omni−e −jωT ) 
       
     
     where omni is an adjustable parameter controlling attenuators  1  and  2  and having preferably a value in the range from 0 to 1, i.e. the lower limit omni=0 means no signal transfer through attenuators  1  and  2 , whereas the upper limit omni=1 means maximum signal transfer through attenuators  1  and  2 . 
     Although the invention is not limited thereto the parameter omni should preferably be substantially the same for both attenuators  1  and  2 . 
     If the full directional mode of operation is chosen with omni=0, then the combined signal Y becomes 
     
       
           Y=X   front *(1 −e   −jω(A+T) ) 
       
     
     If the delay T is selected equal to the delay A directly from the back microphone to the front microphone in the directional mode of operation, then the part of the sound signal X coming directly from the back of the user is suppressed to the maximum extent and a directional characteristic known as a cardioid characteristic is achieved. 
     The signal process described so far is preferably performed as a digital process in the time or frequency domain. If processing in the frequency domain is employed, it is advantageous to use microphone circuits, which are capable of generating a delayed microphone output signal in combination with a non-delayed microphone output signal. Such microphone circuits are described in applicants&#39; U.S. Pat. No. 6,339,647. 
     FIGS. 7 to  10  are graphic representations of sound receiving characteristics and amplitude response of a hearing aid embodying the front end part shown in FIG.  6  and corresponding to the representations in FIGS. 2 to  5  and using the same reference designations as in these figures, As will appear from FIGS. 7 and 8 the part of the sound receiving characteristic representing the area in front of the user is unaffected by the change over between the omnidirectional characteristic ND and the various directional shapes D 1  to D 10  and as illustrated by FIG. 9 the amplitude response of signals received from the area in front of the user is unaffected by the change over and remains the same irrespective of the change of the sound receiving characteristic to suppress sounds coming from the area behind the user. Thereby, the adjustment or fitting of the hearing aid to compensate for the user&#39;s hearing impairment in quiet surroundings, where the omnidirectional characteristic is used, will provide optimum listening performance also when the hearing aid is used in a more noisy environment using a directional shape of the sound receiving characteristic. 
     The circuit in FIG. 11 is similar to the circuit in FIG.  6  and includes a change-over controller with components  1  to  6 . Similar components have been assigned the same reference numerals. 
     Additionally, signal processing units  7  and  8  are placed at the outputs of the at least two microphones, i.e. the front microphone Fmic and the back microphone Bmic. The processed output signals of the two signal processing units  7  and  8  are then supplied to the change-over controller with components  1  to  6 . The signal processing units  7  and  8  may perform an equalizing function on the two output signals of the two microphones and/or may contain various filters, e.g. band pass filters. With the use of band pass filters the microphone signals may be split up into several bands, each equipped with its own change-over controller. The respective output signals from the adding circuits  6  in the various bands or channels may then be combined into a composite combined signal to be further processed in the remaining stages of the hearing aid. 
     FIG. 12 shows a similar circuit diagram of a third embodiment, so that for the same components the same reference numerals are used. In this circuit the time delay for the output signals of the two microphones Fmic and Bmic is effected in separate delay units  3   a  and  3   b  representing the delay device  3 . Otherwise, the function is similar to the function of the circuits of FIGS. 6 and 11. Furthermore, a control unit  9  is shown, which may control the attenuation of the controllable attenuators  1  and  2  as well as the delays of delay units  3   a  and  3   b . This embodiment of the invention is of special advantage in combination with microphone input circuits, which are capable of supplying a delayed microphone signal together with an undelayed microphone signal for a hearing aid. Such a circuit has been disclosed and described in applicants&#39; copending International Patent Application PCT/EP99/00767. 
     As has been stated previously, it is of great importance that, in the change-over controller, the amplitude response as well as the time and phase of the audio signals are not changed when their directivity changes. 
     FIG. 13 schematically shows a further improvement of the front end circuit of a hearing aid including a change-over controller as described so far with reference to FIG.  6 . Similar components have been designated with the same reference numerals as before. 
     Because of the technique used in combining the output signals of the two microphones Fmic and Bmic, the resulting amplitude response of the output signals of the adding means  6  will—of course—in the relevant frequency range—rise with 6 dB per octave compared to the amplitude response of a single microphone. 
     This behaviour may be observed in all systems, in which a delayed version of the output signal from the back microphone is subtracted from the undelayed output signal from the front microphone, while achieving a directional effect. 
     However, in most cases, it is desirable to compensate for this change in the amplitude response by adding a filter at the output of the front end of the hearing aid, i.e. at the adding circuit  6 . Such a filter, of course, means a reduction of 6 dB per octave in the relevant frequency range. The drawback of such a solution is that more circuit components, time and power would be required, all of which are of very crucial importance in modern hearing aid technology. 
     However, the change-over controller of the present invention could also be adapted to, perform this compensation filtering. Therefore there will be no need to add a filter at the output of the adding circuit  6 . 
     For this purpose an additional subtraction circuit  10  is arranged between the adding circuit  4  and the delay device  3 , an the output signal of the adding circuit  6  is directly supplied to the negative input of adding means  10  in a feedback loop. 
     This new arrangement has already the desired effect. 
     It may be preferable to include a controllable amplifier or attenuator  11  into the feedback loop. 
     Thus, the output signal of the change-over controller is fed back from the adding circuit  6  via the controllable attenuator  11  to the negative input of subtraction circuit  10 . Thus, the output signal of attenuator  11  is subtracted in the subtraction circuit  10  from the output signal of adding circuit  4 . 
     The resulting output signal.of subtraction circuit  10  is supplied to the delay device  3  and hence to the negative input of the subtraction circuit  5 , the positive input of which is connected to the output of the controllable attenuator  1 . 
     In principle, in the embodiments in FIGS. 6 and 11 to  13  subtraction circuit  5  and adding circuit  6  could also be combined into a single combining circuit, provided this has, in every respect, the same properties as the two separate adding means  5  and  6 . 
     Ideally, the gain factor of attenuator  11  should be one or unity for the filtering being able to perform the 6 dB per octave fall at very low frequencies. However, this would probably result in a loop gain of unity so that the circuit might become unstable. Therefore, it is preferred to have the gain of the amplifier or attenuator  11  set to a little less than one or unity. 
     In FIG. 14 a further development of a hearing aid embodying the invention is shown, in which the controllable attenuation and phase delay operations, to which the signals from the front and back microphones Fmic and Bmic are subjected before forming the overall combined signal as represented by the relationship stated in the foregoing, i.e. 
     
       
           Y=X   front *(1−omni* e   −jωT )+ X   back *(omni− e   −jωT ) 
       
     
     are implemented by a different circuit structure 
     In this case, the change-over means comprises a first adding circuit  12  connected with the front and back microphones Fmic and Bmic and a first subtraction circuit  13  having a positive input connected with the front microphone Fmic and a negative input connected with the back microphone Bmic. First and second phase delay devices  14  and  15  are connected with the first subtraction and adding circuits  13  and  12 , respectively. A second adding circuit  16  is connected with the first subtraction circuit  13  and the first phase delay device  14  and a second subtracting circuit  17  has its negative input connected with the first adding circuit  12  and its positive input connected with second phase delay device  15 . A first controllable attenuator  18  acts on the signal from the second adding circuit  16  for attenuation of this signal by a factor (1−omni)/2 and a second controllable attenuator  19  acts on the signal from the second subtraction circuit  17  for attenuation of this signal by a factor (1+omni)/2, whereas a third adding circuit  20  is connected with the first and second attenuators  18  and  19  for addition of the signals therefrom to provide the overall combined signal to be supplied to the signal processor. 
     The microphones used in the described embodiments are preferably omnidirectional microphones. 
     When two microphones are used in the omnidirectional mode of operation, both microphones generate an electrical noise signal N. These two noise signals have a similar power: 
     
       
         | N   back   |=|N   front |, 
       
     
     where N back  is the noise signal from the back microphone Bmic, and N front  is the noise signal from the front microphone Fmic. 
     The noise signals N are random signals. Therefore, the resulting signal amplitude is less than twice the single amplitude. Thus, a 3 dB-noise reduction results. The total noise signal can be calculated as: 
     
       
         | N|   2   =|N   front | 2 *|1−omni* e   −jωT | 2   *+|N   back | 2 *|1−omni* e   −jωT | 2   →|N|=|N   front *2 0.5   |1−omni*   e   −jωT | 
       
     
     It has been shown that with the new front end of a hearing aid comprising a change-over controller in accordance with the invention a great variety of directional characteristics patterns may be controllably realized.