AU7544198A

AU7544198A - A method for electronically beam forming acoustical signals and acoustical sensor apparatus

Info

Publication number: AU7544198A
Application number: AU75441/98A
Authority: AU
Inventors: Werner Hottinger; Joseph Maisano
Original assignee: Phonak AG
Current assignee: Sonova Holding AG
Priority date: 1997-08-20
Filing date: 1998-06-08
Publication date: 1999-03-08
Anticipated expiration: 2018-06-08
Also published as: EP0820210A2; JP2001516196A; CA2301216A1; AU746584B2; IL134435A0; RU2185710C2; ATE213581T1; NZ502883A; IL134435A; EP1005783B1; WO1999009786A1; TR200000457T2; DE69803933T2; DK1005783T3; KR20010023076A; CA2301216C; EP1005783A1; EP0820210A3; CN1267445A; DE69803933D1

Abstract

A predetermined characteristic of amplification in dependency of the direction (θ) from which acoustical signals are received at two spaced apart acoustical/electrical transducers (1, 2) is formed in that repetitively a mutual delay signal (A10) is determined from the output signals of the transducers and according to the reception delay at the transducers, one (S1) of the output signals is filtered, thereby the filtering transfer characteristic is controlled in dependency of the mutual delay signal (A12). The output signal of the filtering (14) is exploited as electrical reception signal (Sr). <IMAGE>

Description

WO99/09786 PCT/IB98/00889 - 1 A method for electronically beam forming acoustical signals and acoustical sensor apparatus The present invention is generically directed on a technique for so-called "beam forming" on acoustical signals. 5 The use of directional acoustical/electrical transducers and especially of such microphones is one of the most efficient ways for improving signal to noise ratio in audio systems. It is known to realise directional microphones by using an array of microphone cells and time delaying and superimposing the 10 output signals of such cells following up the known "delay and sum" technique. With two omnidirectional microphone cells this known principle is shown in fig. 1. Two omnidirectional microphones, 1 and 2, are provided with a mutual distance p. The output signal of one 15 of the microphones according to signal Al is time delayed by the time amount T, the time delayed signal according to A,' is superimposed at a superimposing unit 3 to the undelayed output signal A 2 of microphone 2. At the output of the superimposing unit 3 there results the output signal Ar with an amplification 20 versus impinging angle 0 characteristic, as shown in fig. 2 for one frequency o considered. Thereby, it is customary to select as delay time T as the quotient of distance p and velocity of sound c. With this arrangement there results, as shown in fig. 2, a first order cardoid characteristic. It may be shown that 25 the amplitude of the resulting signal A is proportional to the sine of the signal frequency o and to the distance p. The maximum gain in target direction (1800) occurs at the frequency f, = c/(4p) . For a distance p of 12 mm, fr becomes approx. 7 kHz.

WO99/09786 PCT/IB98/00889 - 2 By staggering more than one of the fig. 1 double-cell arrange ments and superimposing the resulting signals A, of the more than one double-cell arrangements, higher order cardoid charac teristics may be realised. 5 In fig. 3 a known arrangement to realise second order cardoid characteristics according to fig. 4 is shown. Thereby, a nar rower beam can be achieved. The higher the order of the direc tional microphone arrangement, the higher becomes the directiv ity index and the gain at fr' but the higher will also be the 10 roll-off for low and high frequencies and the number of un wanted side-lobes. With respect to the definition of the direc tivity index please refer to speech communication 20 (1996), 229 - 240, "Microphone array systems for hands-free telecommu nications", Garry W. Elko. 15 In fig. 5 there is shown the gain versus frequency characteris tic of the first and second cardoid characteristics for an im pinging angle 0 = 1800. Therefrom, high and low frequency roll offs are clearly evident. Such techniques for beam forming are well-known and have been 20 realised using analogue signal processing, as e.g. shown in the US-A-2 237 298, US-A-4 544 927, US-A-4 703 506, US-A-5 506 908 or using digital signal processing, both in time or in fre quency domain, as shown in the EP-A-0 381 498 (time domain) or in the US-A-5 581 620 (frequency domain). 25 Beam formings realised with any of these principles has the following drawbacks: a) The resulting signal is dampened at low frequencies, which results in a bad signal to noise ratio.

WO99/09786 PCT/IB98/00889 - 3 b) The directivity index is very sensitive to matching of the individual microphone cells, especially at low frequencies. c) The distance p between the microphone cells should be large (> 12 mm) for audio range. 5 d) The frequency band with a high gain in target direction is rather small, as may clearly be seen from fig. 5. e) The directivity largely depends upon the number of micro phone cells and thus on the complexity of the overall ar rangement. 10 f) As one aims for a high directivity by increasing the number of cells, more unwanted side-lobes are introduced. Several techniques have been proposes to overcome some of these drawbacks: In the WO 95/20305 (E. Lindemann) an adaptive noise reduction 15 system for use in binaural hearing aid is proposed. It detects the power of the received signals to separate the desired from unwanted signals. There is proposed a "broad side" microphone-cell array, i.e. target direction is perpendicular to the axis from one micro 20 phone to the other, in contrary to the arrangement according e.g. to Fig. 1 and the principles of the present invention, which is "in line". The disclosed apparatus is bulky (>> 5 cm), so that it may not be implemented for one ear hearing aid. 25 Two equal beam lobes are generated in target and in opposing directions.

WO99/09786 PCT/IB98/00889 - 4 In such a hearing aid a connection between the left and right ear system must be present, making the apparatus for hearing aid unhandy. Furthermore, as described by the same author in "Two microphone non-linear frequency domain beam former for 5 hearing aid noise reduction" 1995, IEEE ASSP Workshop on Appli cations of Signal Processing to Audio and Acoustics, October 15 - 18, Mohonk, New Paltz, New York, such beam forming is effi cient only up to about 2 kHz and leads to distortions of the desired signals. LO The US-A-4 653 102 proposes the use of two directional micro phones aimed in target direction and of a third microphone aimed in opposite direction. The signal of the third microphone supposedly only containing noise is used to shape the response of the two primary microphones. This technique obviously has L5 the drawback within reverberating rooms, where the desired sig nal is reflected on walls, floor, ceiling and furniture and is therefore considered as noise by the system. This technique is further unhandy as making use of at least three microphones. Attention is further drawn to US-A 5 400 409 and 5 539 859. 20 As an example of known beam-forming techniques, the US-5 539 859 proposes a technique wherein reception characteristic is logged in on that direction wherefrom the highest energy im pinges on a pair of microphones and considered in the sound en vironment. Principally, all sound impinging from directions 25 other than from highest energy direction is considered as noise and its reception is cancelled. Thereby, an analogue to digital conversion and subsequent time to frequency domain conversion is performed on the output sig nals of two microphones. Exploiting the knowledge of the fixed WO99/09786 PCT/IB98/00889 - 5 mutual distance between the two microphones, wherefrom phase difference of the impinging signal spectra is dependent, there is determined the mutual phasing and thus impinging direction of highest energy sound signals, i.e. direction of highest en 5 ergy sound source within the acoustical surrounding. Signals impinging from that direction are amplified by means of in phase shifting and adding similarly to an auto correlation technique, whereby signals from other impinging angles are can celled as noise. 10 By such a technique the energy distribution in the sound envi ronment traps the selectivity of reception, and it is not pos sible to freely select or preselect a maximum reception charac teristic, e.g. in direction wherefrom sound is desired to be selectively received, irrespective of its relative energy. One 15 field whereat such selectivity irrespective of energy distribu tion within the sound surrounding would clearly be advantageous is hearing aid technique. It is an object of the present invention to provide a method for electronically forming a predetermined characteristic of 20 amplification in dependency of direction from which acoustical signals are received at at least two spaced apart acousti cal/electrical transducers and a respective acoustical sensor apparatus, with which only a small number of microphones or mi crophone cells has to be used and which is thus enabling small 25 and compact directional transducer or microphone realisation. Thereby, the preferred apparatus according to the present in vention is a hearing aid apparatus, and especially a one ear hearing aid apparatus.

WO99/09786 PCT/IB98/00889 -6 It is a further object to provide such method and apparatus with good frequency response in the audio band, i.e. between approx. 0,1 and 10 kHz. Still a further object of the present invention is to provide 5 such method and apparatus which allow high signal to noise ra tio realisation without unwanted side-lobes and with easily variable beam form, e.g. for acoustical zooming. These and other objects are realised by the inventive method, which comprises the steps of repetitively determining from sig 10 nals dependent from the acoustical signals a respective mutual delay signal according to reception delay at the at least two transducers; subjecting a signal dependent from the output sig nal of at least one of the at least two transducers to filter ing with a filtering transfer characteristic; and of control 15 ling the filtering transfer characteristic in dependency of the mutual delay signal; further exploiting a signal dependent from the output signal of the filtering as electrical reception sig nal. To fulfil the above mentioned objects the inventive acoustical 20 sensor apparatus comprises at least two acoustical/electrical transducers, arranged at a predetermined mutual distance in target direction, a time delay detection unit, which has at least two inputs and an output, the inputs thereof being re spectively operationally connected to the outputs of the two 25 transducers, whereby the time delay detection unit generates an output signal in dependency of the time delay of acoustical signals, impinging on the at least two spaced apart transduc ers, preferably a time domain to frequency domain converter unit generating the output signal of said time delay detection WO99/09786 PCT/IB98/00889 - 7 unit in frequency domain; a weighing unit with a predetermined weighing characteristic and with an input and with an output, whereby the input thereof is operationally connected to the output of the time delay detection unit and preferably receiv 5 ing the signal at said output of said time delay detection unit in frequency domain mode; with a filter unit with a controlla ble transfer characteristic, which has at least one input, a control input and an output and whereat the input is operation ally connected to at least one of the outputs of the at least 10 two transducers, preferably via at least one time domain to frequency domain converter, the control input is operationally connected to the output of the weighing unit, the filter unit generating an output signal in dependency of its input signal and of its transfer characteristic which is controlled by the 15 signal - preferably a spectral signal - which is applied to the control input of the filter unit, this weighing - preferably spectral weighing - result signal being dependent from the out put signal of the time delay detection unit and the weighing characteristic of the weighing unit. 20 Other objects, advantages and specific embodiments of the pres ent invention shall be exemplified with the help of further figures. The figures show: Fig. 1: A functional block diagram of a two-cell directional microphone arrangement according to the prior art 25 principle of "delay and sum"; Fig. 2: the first order cardoid amplification characteristic of prior art arrangement according to fig. 1; Fig. 3: departing from the prior art arrangement of fig. 1 a further arrangement following up the technique of WO99/09786 PCT/IB98/00889 - 8 "delay and sum" for realising second order character istic; Fig. 4: the second order amplification characteristic as real ised by the prior art arrangement according to fig. 3; 5 Fig. 5: in dependency of frequency the amplification charac teristic of the arrangement according to fig. 1 or 3 at maximum amplification impinging angle of acoustical signals; Fig. 6: a simplified functional block diagram of an inventive 10 apparatus operating according to the inventive method and further showing the sequence of process signals; Fig. 7: in a representation according to fig. 6 a first pre ferred realisation form of an inventive apparatus op erating according to the inventive method; 15 Fig. 8: in an inventive apparatus operating according to the inventive method according to fig. 6 a further pre ferred form of realisation of a time delay detection unit; Fig. 9: a polar diagram of signals as realised by the embodi 20 ment of fig. 8 for explaining operation of a compara tor unit as provided in the fig. 8 embodiment; Fig. 10: the course of comparison results in dependency of im pinging angle of an acoustical signal and as realised by the embodiment according to fig. 8; WO99/09786 PCT/IB98/00889 -9 Fig. 11: a preferred form of realising superimposing result signal dependency from impinging angle of an acousti cal signal at an embodiment according to fig. 8; Fig. 12: in a representation according to fig. 10 the course of 5 comparison results as realised with a preferred em bodiment resulting in the fig. 11 dependency; Fig. 13: in polar diagrammatic representation the dependency of superimposing result signals from impinging angle of acoustical signals and from frequency as realised by 10 the embodiment according to fig. 8; Fig. 14: a preferred realisation form of the fig. 8 embodiment, additionally counteracting frequency dependency as shown in fig. 13; Fig. 15: in a representation according to fig. 13 therdepend 15 ency of the superimposing result signal with normali sation as realised by the embodiment of fig. 14 with a first preferred normalisation frequency function; Fig. 16: a representation according to fig. 15, realised with a second preferred normalisation frequency function at 20 the embodiment of fig. 14; Fig. 17: a first (rigid line) and second (dashed line) pre ferred realisation form of amplitude filter character istic at the embodiment of fig. 6 or 7; Fig. 18a: the effect of the amplitude filter amplitude versus 25 amplitude transfer characteristic according to fig. 17 (rigid line) on the output signal of the delay detec- WO99/09786 PCT/IB98/00889 - 10 tion unit as provided in the embodiment of fig. 6 or 7; Fig. 18b: the representation of the output signal of a time de lay detection unit passed through the amplitude filter 5 with a transfer characteristic according to fig. 17 (rigid line) and as realisable by the embodiment of fig. 6 or 7; Fig. 19: the spectrum of an acoustical signal converted into electrical and input to a controllable frequency fil 10 ter as provided by the present invention according to fig. 6; Fig. 20: the electrical reception result signal realised by am plitude filter characteristic according to fig. 17 (rigid line) and reception signal as exemplified in 15 fig. 19 at the inventive embodiment according to fig. 6; Fig. 21: the resulting dependency of amplification from imping ing angle of an acoustical signal as realised by the fig. 17 amplitude filter characteristic (rigid and 20 dashed lines); Fig. 22: the amplification versus impinging angle characteris tic as realised by the fig. 6 or fig. 8, 14 embodi ments of the invention, making use of an amplitude filter characteristic with a maximum to minimum spec 25 tral amplitude transfer behaviour; Fig. 23: in a simplified signal/functional block diagram a fur ther preferred embodiment of the invention; WO99/09786 PCT/IB98/00889 - 11 Fig. 24: in a signal flow, functional block representation, a further mode of realisation of the time delay detec tion unit as shown in fig. 6 and Fig. 25: in a signal flow, functional block representation, a 5 further mode of realisation of the time delay detec tion unit following the technique as shown in fig. 8 or fig. 14. In fig. 6 there is shown in form of a functional block diagram, together with principle signal processing diagrams, the princi 10 ple of the inventive method and apparatus. At least two acoustical/electrical transducers 1 and 2, as es pecially of microphones or microphone cells, are provided with a predetermined mutual distance p along axis a. Acoustical sig nals IN are received by the transducers 1 and 2 as they impinge 15 from different spatial directions 0. The acoustical signals IN have frequency spectra which vary in time. Output signals of transducer 1, S 1 (t, o) and of transducer 2, S 2 (t,o), are formed as electrical signals at the output of the transducers 1 and 2. Due to the mutual distance p of the two transducers 1 and 2 20 which is preferably smaller than 5 cm, preferably between 0,5 and 1,5 cm, especially for the inventive sensor being a one ear hearing aid apparatus - and as shown with the two respective pointer diagrams below the functional block diagram of fig. 6, the acoustical signals IN impinge on the transducers 1 and 2 25 with a time delay dt, which may be expressed by the phase dif ference Ap, at each spectral frequency o according to (1) Ap = o.dt., where WO99/09786 PCT/IB98/00889 - 12 p (2) dt, - cos 0.. c C If the source of acoustical signal IN is a point source, then the time delay dt, becomes equal for all spectral components at the different o. The output signals S, and S 2 of the transduc 5 ers 1 and 2 are operationally connected to the respective in puts of a time delay detection unit 10, which generates an out put signal A, 0 according to the spectral distribution of time delays dt,, which are, as was explained, a function of the im pinging angle 0 at which the respective frequency components 10 impinge on the transducers 1 and 2 and thus in fact of 03. Purely as example a possible spectrum of output signal A 0 is also shown in fig. 6. This spectrum varies in time according to the time variation of impinging acoustical signal IN. The out put signal A0 of time delay detection unit 10 is input to a 15 weighing unit 12. As the spectrum of dt. with respective spec tral amplitudes of A, is input to the weighing unit 12 with the preselected weighing transfer characteristic W, there re sults at a certain moment in time, as an output signal A 12 , a spectral signal W(), as also shown as an example in fig. 6. 20 A 12 results from respectively weighing the spectral amplitudes of A0 according to the characteristic W. As A, 0 indicates ac cording to dt. from which direction 0, each frequency component of the acoustical signal IN impinges, its specific weighing by means of function W is nothing else than predetermining which 25 impinging directions 0, shall be amplified or attenuated. Thus, the weighing unit 12 determines with its characteristic W the beam shape. The output signal A, 2 is applied to a filter unit 14 with a controllable transfer filter characteristic. There, each spec- WO99/09786 PCT/IB98/00889 - 13 tral line of the time varying spectrum of the output signal

S

1 (t,o) is amplified or attenuated according to the controlling spectrum W, A,

,

,. Thus, unit 14 is a filter unit for input signal S at which the transfer characteristic is varied, as 5 controlled by A 12 . In dependency of the kind of filter unit 14 the weighing unit 12, generally spoken, calculates adjustment of filter characteristic determining coefficients as a function of A 10 . Thus, along the channels 10 and 12 there is predetermined by 10 the weighing transfer function W which spatial directions 0 shall be "aimed" at. At the filter unit 14 this beam shaping information is applied to the electrical analogon S, of the acoustical signal IN, thus resulting in an output signal Sr(t,o) representing the shaped reception signal. 15 By adjusting the weighing transfer function W by applying a control signal C, to a control input C 12 , the beam form can be adjusted and thus acoustical zooming is realised. As shown in dashed line, it may be advantageous to subject both transducer output signals to a controlled filtering at unit 14. 20 In fig. 7 there is shown a first preferred form of realisation of the inventive principle according to fig. 6. Thereby, the output signals S, and S, are first converted from analogue to digital form in respective analogue/digital converters 16 and 17. The digital output signals of the respective converters 16 25 and 17 are input to respective complex time domain/frequency domain converters 18 and 19. The output spectra Sl(t,u) and S 2 (t,o) of converters 18, 19 are input to the spectral time delay detection unit 10'. Unit 10' WO99/09786 PCT/IB98/00889 - 14 computes according to formula (1) the phase difference spectrum A(p divided by the respective frequency ) to result in an out put signal spectrum Ao' according to the time delay dt,, as was explained in connection with fig. 6. The output signal of the 5 time delay detection unit 10', Ao', is further treated, as was explained in connection with fig. 6, by the weighing filter unit 12 and the controllable filter unit 14. In the following table there is exemplified how the unit 10' operates. Out of the spectral phase distribution Apn of signal S, and p2n of sig 10 nal S2 the time delay dt. is calculated for each spectral line within an interesting spectral band. 0) 02 13 S1 (w) A 11 A12 A13 Ain 21l 912 1_ _3 n S2 (w) A 21 A2 2

A

23 A2n 9 2 1 T22 23 P2n [dto ~ 911-9p21/J~ 1 12 (22/(2 1 P3 (23/(3 IDn 2Nn/ n So as to extract the phase information 9 out of the two signals 15 S 1 and S2 the time domain to frequency domain conversion units 18 and 19 perform complex (real and imaginary) operation. A second preferred realisation form of the present invention, and especially as concerns realisation of the time delay detec tion unit 10, shall be explained with the help of fig. 8 and 9. 20 The output signal of one of the transducers, as shown e.g. of transducer 1, S

,

(t,)) is fed to a time delay unit 20, wherein, in a first form of this realisation, signal S, is time delayed by a predetermined frequency independent time delay Tr.

WO99/09786 PCT/IB98/00889 - 15 Looking back on fig. 1, signal S, accords thus with signal A,. The output signal of time delay unit 20 thus accords with sig nal A,' of fig. 1. The time delay signal according to A,' is superimposed to the 5 output signal S 2 (t,0) from transducer 2 at a superimposing unit 23 according to unit 3 of fig. 1, thus resulting in an output signal according to A,(t,co) of fig. 1. As is known, and as was explained in connection with fig. 1, the output signal A,(t,0) depends from the impinging acoustical signal direction 0 ac 10 cording to the first order cardoid beam of fig. 2, which car doid function nevertheless varies with frequency o. The output signal A, of superimposing unit 23 and e.g. the output signal

S

2 (t,0) from transducer 2 are input to a ratio unit 25, as a comparator unit. 15 For understanding the functioning of ratio unit 25, attention is drawn to fig. 9. In fig. 9 the cardoid attenuation charac teristic of output signal Ar at a specific spectral frequency

W

i is shown. At a specifically considered impinging angle 0 o the output signal A, of superimposing unit 23 is Ar o (w 1 ) with an 20 amplitude value as indicated in fig. 9. Simultaneously, at that frequency o, considered and at that impinging angle 0. consid ered, the amplitude of signal S2 is A 2 o 1 ) as also shown in fig. 9. It must be emphasised that as the amplitude A 20 varies, the amplitude of A, 0 varies proportionally. Thus, the ratio of 25 Ar.o to A 20 according to fig. 9 is indicative of the impinging angle 0 o . In the division unit 25 of fig. 8 there is formed for each spectral component amplitude the ratio of Ar to A,, where from there results a signal spectrum at the output of division unit 25 with a ratio spectrum. The spectrum of A, 0 according to WO99/09786 PCT/IB98/00889 - 16 fig. 6 thus becomes the spectrum of an amplitude ratio which nevertheless is indicative of the impinging angle 0, at which each frequency component of the spectrum of the acoustical sig nal impinges with respect to the axis a of the two transducers 5 (see fig. 6). In fig. 8 the dotted line block indicates the de lay detection unit 10 according to fig. 6. Further signal proc essing is performed as was explained by means of fig. 6, i.e. via weighing unit 12 and controllable filter unit 14. In this embodiment it is possible to perform a time domain to 10 frequency domain conversion at the output side of comparator unit 12. Thus, the output ratio signal of unit 25 is a measure for the time delay dt. and is input to the weighing unit 12. In fig. 10 there is shown the course of the ratio Ar to A, as a 15 function of 0 at a specific frequency o,. This amplitude ratio is shown for T at unit 20 of fig. 8 being selected to be S= p/c, wherein p is the distance of the transducers 1 and 2 and c is 20 the velocity of sound. When selecting T to be p/c and as may be seen from the cardoid beam function of fig. 2 signal attenuation or dampening for 0 near 00 becomes very high. Thus, in this area of impinging angle 0 any kind of noise in A 2 25 according to S 2 of fig. 8 would falsify comparison result WO99/09786 PCT/IB98/00889 - 17 formed at unit 25. This problem can be eliminated by choosing a delay T which is different, thereby preferably larger than p/c. In fig. 11 the resulting cardoid diagram is shown for r = 1.2 p/c, whereas fig. 12 shows in analogy to fig. 10 the course of 5 the amplitude of A, divided by the amplitude A. Further, it must be noted that the cardoid function as shown in the figures 2, 9 and 11 is only valid for one specific fre quency considered. In fact, considering different frequencies, the cardoid function varies as shown in fig. 13, wherein the 10 amplitude A, of the output signal of superimposing unit 23 ac cording to fig. 8 is shown for p = 12 mm, a delay T of 42 mi croseconds and for frequencies of 0.5, 1, 2, 4 and 7.2 kHz. From this polar diagram the frequency dependency of the cardoid amplification function is clearly evident. Although such de 15 pendency may be neglected in a first approximation, in a pre ferred form of realising the inventive method principally as shown in fig. 8, such dependency is taken into account. Thus, a preferred realisation form of the fig. 8 technique is shown in fig. 14. Here, the same reference numbers are used as in the 20 figures 7 or 8. The outputs of the transducers 1 and 2 are con verted into digital form by respective analogue to digital con verters 16, 17 and the resulting digital signal of transducer 1 is time delayed by a time delay r', which is larger than p/c. The output signal S 2 of transducer 2 is further converted into 25 frequency domain by a linear (not complex) time to frequency domain conversion unit 18', whereas the output signal A, of su perimposing unit 23 is converted to frequency domain at a lin ear time to frequency domain conversion unit 19'. The frequency dependent polar diagram according to fig. 13 is taken into ac 30 count by a normaliser unit 30 which is in fact a filter. In a WO99/09786 PCT/IB98/00889 - 18 first embodiment the transfer characteristic of the filter is selected proportional to 1/o. This results in a frequency de pendency of the pole diagram as is shown in fig. 15 for the same distance and frequency values as shown in fig. 13. 5 As may be seen, a good matching is achieved for small angles 0 and frequencies up to about 4 kHz. At 4 kHz the deviation is about 10 %, at 0 = 180. A further, even improved normalisation function or filter char acteristic at unit 30 of fig. 14 is achieved when the filter 10 characteristic is selected as a function l/sin(w). The result is shown in fig. 16. The characteristics match well from 0.5 to 4 kHz. A further advantage of this normalisation technique is improved sensitivity in backwards direction. This improved sen sitivity may be exploited for adaptive beam forming, that is 15 for selectively eliminating noise sources from the re4r side. It is evident for the skilled artisan, that such normalisation may be performed also in signal path 1 to 23 and/or 2 to 23. In this embodiment of fig. 14 it is highly advantageous that only one-dimensional TFC's 18', 19' have to be used and not 20 complex TFC's as in the embodiment of fig. 7. Fig. 24 shows in block diagram form, that the signal A,, (dt,.) may also be generated as the output signal of a comparator unit 60 to which on one hand the output signal of an omnidirectional transducer 61, having equal amplification of its acousti 25 cal/electrical reception characteristic substantially irrespec tive of the impinging angle 0 and the output signal of a direc tional transducer 62 with selected, beam shaped reception char acteristic are led to.

WO99/09786 PCT/IB98/00889 - 19 According to fig. 25 time delaying T may also be performed by one of the transducers itself. Thereby, in the embodiment of fig. 25 as well as of fig. 8 T may be selected to be zero. 5 With the help of fig. 23 a further preferred embodiment, espe cially of realising the time delay detection unit 10 of fig. 6 shall be explained. The output signals of the transducers 1 and 2 are first converted by respective analogue to digital con verters 16 and 17 and then by respective time domain to fre 10 quency domain converters 18, 19 finally into frequency domain. One signal, as an example S 2 , of the converted output signals of the transducers, which, after time to frequency domain con version may be represented as a spectrum of S 2 1 pointers, is converted to its conjugate complex pointers at a conversion 15 unit 50. At the output of this unit 50, the conjugatecomplex pointers S* 2 , are generated. This spectrum S 2 * and the pointer spectrum S, are multiplied to form the scalar product spectrum

S

3 in a multiplication unit 52. As may be shown, the pointers

S

3 , of spectrum S 3 have a phase angle with respect to the real 20 axis, which is Ap. Thus, the imaginary part of the pointers S31 of S 3 become (3) Im (S3.) = S 3 . sin (ARp) with (4) Ap, = o (p/c) cos(9,) WO99/09786 PCT/IB98/00889 - 20 According to fig. 23, a conversion unit 53 forms the imaginary part of the pointers S 3 . and a further unit 54 forms the ampli tudes S 3 I of these pointers. For small values of Ay. the sinus in (3) may be approximated by 5 Ay itself, so that there results from (3) (3) ' Im (S 3 ,) = Si S* 2 c 0 (p/c) cos (0,) Thus, and as performed by unit 55, dividing the imaginary parts Im (S3) of the pointers S 3 , of spectrum S 3 by the respective values of the scalar product according to IS., there results 10 an output signal which accords with A(p. As was already ex plained with the help of fig. 7, Ay is further divided in unit 56 by the respective pointer frequency ). The resulting signal is Ato according to fig. 6 or A 0 ' according to fig. 7. All the units 50, 52, 53, 54, 55 and 56 are preferably realised 15 in one calculator unit. Let's turn back to the generic block diagram of fig. 6 having described different possibilities of realising the delay detec tor unit 10. By means of the figs 17 to 22 we will further explain with a 20 specific example the effect of amplitude filter unit 12 and of controllable filter unit 14. In fig. 17 there are shown examples of two weighing signal characteristics of unit 12. According characteristic I each dt,. spectral line amplitude of signal Ao (see fig. 6) is attenu 25 ated to zero, if such amplitude is below or above predetermined WO99/09786 PCT/IB98/00889 - 21 values dtmin, dtms, and is set to be "one" if such spectral component amplitude is between these two values. Such selection of weighing function W results in an output sig nal spectrum A 12 , as shown in the figures 18a and 18b. 5 The figures 18a and 18b are self-explanatory for the skilled artisan. Fig. 19 shows a spectrum example of signal S,. At the control lable filter unit 14 all spectral lines of S (Fig. b) are am plified by the value 1 according to A 12 or are nullified ac 10 cording to zero values of A 12 . This results, according to fig. 20, in a spectrum Sr as an output signal spectrum of controlla ble filter unit 14 of fig. 6. If the weighing function I of fig. 17 is applied to the technique according to fig. 7 there results a beam form as shown in fig. 21 in strong lines. If an 15 amplitude filter characteristic is applied as shown by II in fig. 17, there results the characteristic as shown in fig. 21 in dashed line. Fig. 22 shows the resulting beam if in analogy to fig. 17 and with an eye on figures 8 and 9 all ratio values which exceed 20 (Ar/A 2 )max are discarded. This is realised by the amplitude filter characteristic as also indicated in Fig. 22. In fig. 22 the ratio Ar/A 2 is denoted with r(o). It is clear for the skilled artisan that only examples of the invention were described with the help of the figures. For in 25 stance more than two transducers or microphones arranged in linear, planar or spatial array form can be used. Additionally, directional microphones may be used instead of the omnidirec- WO99/09786 PCT/IB98/00889 - 22 tional. Beam forming following the inventive principle can also be made by the combination of the functions of two or more mi crophones. As is perfectly clear to the skilled artisan also the delay detector can be realised in many other ways. Further, 5 normalisation, which was explained with the help of normaliser unit 30 in fig. 14, may clearly be done by providing time do main to frequency domain conversion just after the analogue to digital converters 16 and 17 and providing frequency-specific arrays or tables of time delays T. 10

Claims

1. A method for electronically forming a predetermined char acteristic of amplification in dependency of direction (0) from 5 which acoustical signals (IN) are received at at least two spaced apart acoustical/electrical transducers (1, 2), compris ing, at least within a predetermined frequency band, the steps of: - repetitively determining from signals (S I, S 2 ) dependent from 10 said acoustical signals a respective mutual delay signal (dt.) according to reception delay at said at least two transducers; - subjecting a signal dependent from the output signal (S) of at least one (1) of said at least two transducers (1, 2) to 15 filtering with a filtering transfer characteristic (14); - controlling said filtering transfer characteristic (14)in de pendency of said mutual delay signal (A 12 ); - exploiting a signal dependent from the output signal of said filtering (14) as electrical reception signal (Sr). 20

2. The method of claim 1, further comprising the step of de termining said mutual delay signal (dt.) as a spectral signal.

3. The method of claim 1 or 2, further comprising the step of performing said repetitive determining from said signals (S 1 , S 2 ) converted into frequency domain. WO99/09786 PCT/IB98/00889 - 24

4. The method of one of claims 1 to 3, comprising the step of subjecting said signal dependent from the output signal (S,) to said filtering converted into frequency domain.

5. The method of claim 4, further comprising the step of re 5 converting said signal exploited into time domain.

6. The method of one of claims 1 to 5, further comprising the step of performing said determining by monitoring phase differ ence of spectral components of said signals and dividing said phase difference monitored by the frequency of said respective 10 spectral components.

7. The method of one of claims 1 to 5, thereby performing said determining by - providing one of said at least two transducers with an at least approximately omnidirectional acoustical to electrical 15 reception characteristic; - providing one of said at least two transducers with a direc tional, beam-shaped acoustical to electrical reception char acteristic; - comparing signals dependent from the output signals of said 20 at least two transducers and exploiting the result signal of said comparing as said mutual delay signal.

8. The method of one claims 1 to 5, further comprising the step of performing said determining by - superimposing signals dependent from the output signals of 25 said at least two transducers and WO99/09786 PCT/IB98/00889 - 25 - comparing the result signal of said superimposing and at least one of said dependent signals.

9. The method of claim 8, further comprising the step of de laying one of said dependent signals before said superimposing 5 by a predetermined or frequency dependent time amount.

10. The method of one of claims 1 to 5, further comprising the step of performing said determining by - converting signals dependent from the output signals of said at least two transducers into frequency domain; 10 - forming the conjugate complex pointers of one of said signals converted; - multiplying the pointers of the other of said signals con verted with said conjugate complex pointers to get multipli cation result pointers; 15 - forming the amplitudes of said multiplication result point ers; - forming the imaginary pointers components of said multipli cation result pointers; - forming the ratio of said imaginary pointer components and 20 said amplitudes multiplied by the respective frequency, the result signal of said ratio forming being said respective control delay signal in spectral representation.

11. The method of claim 9, further comprising the step of per forming said delaying with a time delay which is different 25 from, preferably larger than the quotient of the mutual dis- WO99/09786 PCT/IB98/00889 - 26 tance (p) of said at least two transducers (1, 2) and velocity (c) of sound.

12. The method of one of claims 7 to 9 or 11, further compris ing the step of normalising at least one of the comparing re 5 sult signal, of the superimposing result signal and of at least one of said dependent signals with a frequency dependent nor malising function (30).

13. The method of one of claims 7 to 9, 11 or 12, further com prising the step of performing said comparing by spectrally 10 forming the ratio of amplitudes of the superimposing result signal and of at least one of said dependent signals.

14. The method of one of claim 1 to 13, further comprising the step of controlling said transfer characteristic by subjecting said delay spectral signal to weighing and controlling said 15 transfer characteristic by the result of said weighing.

15. The method of claim 14, further comprising the step of ad justing said predetermined characteristic of amplification by adjusting a weighing characteristic for said weighing.

16. An acoustical sensor apparatus comprising 20 - at least two acoustical/electrical transducers (1, 2) at a predetermined mutual distance (p); - a time delay detection unit (10) with at least two inputs and an output, the inputs thereof being respectively operation ally connected to the outputs of said at least two transduc 25 ers (1, 2), said time delay detection unit (10) generating an output signal (A, 0 ) in dependency of the time delay of acous- WO99/09786 PCT/IB98/00889 - 27 tical signals (IN) impinging on said at least two transducers (1, 2); - a weighing unit (12) with a predetermined weighing character istic and with an input and with an output, the input thereof 5 being operationally connected to the output of said time de lay detection unit (10); - a filter unit (14) with a controllable transfer characteris tic and with at least one input, a characteristic control in put and an output, the input thereof being operationally con .0 nected to the output of at least one of said at least two transducers, the control input thereof being operationally connected to the output of said weighing unit (12), the fil ter unit (14) generating an output signal (Sr) in dependency of its input signal and said characteristic controlled by the L5 signal (A 12 ) applied at said control input, being dependent from said output signal of said delay detection unit con verted by said weighing characteristic of said weighing unit.

17. The apparatus of claim 16, said control input of said fil ter unit (14) receiving said output signal of said delay detec 20 tion unit in frequency domain mode.

18. The apparatus of one of claims 16 or 17, further compris ing time domain to frequency converters interconnected between said at least two transducers and said time delay detection unit (10), said detection unit being a spectral time delay de 25 tection unit.

19. The apparatus of one of claims 16 to 18, wherein one of said at least two transducers is a transducer with at least ap proximately an omnidirectional acoustical/electrical reception WO99/09786 PCT/IB98/00889 - 28 characteristic on said time delay detection unit comprises a comparator unit, the inputs thereof being operationally con nected to the outputs of said at least two transducers, the output of said comparator unit being operationally connected to 5 the output of said time delay detection unit.

20. The apparatus of one of claims 16 to 18, said time delay detection unit comprising a superimposing unit, the inputs thereof being operationally connected to the outputs of said at least two transducers, the output thereof being operationally 10 connected to the output of said time delay detection unit.

21. The apparatus of claim 20, wherein said time delay detec tion unit comprises a time delay unit (20) with an input opera tionally connected to the output of one of said transducers (1) and with an output operationally connected to one input of said 15 superimposing unit(23), the second input of said superimposing unit being operationally connected to the output of the second of said at least two transducers (2).

22. The apparatus of claim 21, wherein the output of said su perimposing unit being operationally connected to one input of 20 a comparator unit (25), the second input thereof being opera tionally connected to said output of said second transducer (2); the output of said comparator unit being the output of said spectral delay detector unit. 25

23. The apparatus of one of claims 21 or 22, wherein said time delay unit (20) performs signal delaying by an amount which is different than given by the mutual distance (p) of said trans- WO99/09786 PCT/IB98/00889 - 29 ducers (1, 2) divided by velocity of sound (c), which is pref erably larger.

24. The apparatus of one of the claims 16 to 23, wherein said time delay detection unit comprises a normalising filter unit 5 (30) with predetermined transfer characteristic, which is pro vided at its input and/or output.

25. The apparatus of one of claims 20 to 24, wherein said com parator unit is a ratio forming unit of amplitudes of respec tive frequency components applied to its inputs. 10

26. The apparatus of claim 16, wherein said time delay detec tion unit (10) is a spectral time delay detection unit and performs spectral phase difference measurement (Aqw) and divi sion of spectral phase difference by the respective frequency (o. 15

27. The apparatus of claim 26, wherein said time delay detec tion unit comprises a calculator unit with two inputs opera tionally connected to the outputs of said at least two trans ducers which - forms the conjugate complex pointers of a signal at one of 20 its inputs; - multiplies said conjugate complex pointers with the respec tive pointers of the signal applied to its second input; - divides the imaginary part of multiplying result pointers by the amplitude of said multiplying result pointers; WO99/09786 PCT/IB98/00889 - 30 - further divides the dividing result pointers by their respec tive frequency and emits as an output signal at its output the result pointers of said further dividing.

28. The apparatus of claim 16, further comprising a time to 5 frequency domain conversion unit (18') interconnected between the output of said second transducer and said filter unit (14).

29. The apparatus of claim 16, further comprising a time to frequency domain conversion unit (19') interconnected between the output of said superimposing unit (23) and control input of 10 said frequency filter unit (14).

30. The apparatus of one of claims 16 to 29, being a hearing aid apparatus, the mutual distance of said at least two trans ducers being 4 cm at most, preferably 0,5 cm to 1,5 cm.

31. The apparatus of one of claims 16 to 30, said weighing 15 unit comprising a control input to adjust weighing characteris tic of said weighing unit.