Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS
  • H04R3/00—Circuits for transducers
  • H04R3/005—Circuits for transducers for combining the signals of two or more microphones
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS
  • H04R25/00—Electric hearing aids
  • H04R25/40—Arrangements for obtaining a desired directivity characteristic
  • H04R25/405—Arrangements for obtaining a desired directivity characteristic by combining a plurality of transducers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS
  • H04R25/00—Electric hearing aids
  • H04R25/40—Arrangements for obtaining a desired directivity characteristic
  • H04R25/407—Circuits for combining signals of a plurality of transducers

Definitions

• the invention relates to a method and an apparatus for picking up sound.
• a hearing aid In a hearing aid, sound is picked up, amplified and at the end transformed to sound again. In most cases omnidirectional microphones are used for picking up sound. However, in case of omnidirectional microphones the problem occurs that ambient noise is picked up in the same way. It is known to enhance the quality of signal transmission by processing a signal picked up by the hearing aid. For example, it is known to split the signal into a certain number of frequency bands and to amplify preferably those frequency ranges in which the useful information (for example speech) is contained and to suppress those frequency ranges in which usually ambient noise is contained. Such signal processing is very effective if the frequency of ambient noise is different from the typical frequencies of speech.
• US-A 5,214,709 teaches that usually pressure gradient microphones are used to pick up the sound at two points with a certain distance to obtain a directional recording pattern.
• the largest disadvantage of the simple small directional microphones is that they measure air velocity, not sound pressure, therefore their frequency response for the sound pressure has a +6dB/octave slope. This means that their pressure sensitivity in the range of low frequencies is much lower than at high frequencies. If inverse filtering is applied the own microphone noise is also amplified on the low frequencies and the signal to noise ratio remains as bad as it was before the filtering.
• the second problem is that if the directional microphone is realized with two omnidirectional pressure microphones, their matching is critical and their frequency characteristic depends very much on the incoming sound direction.
• the inverse filtering is not recommended and can have a negative effect. Because of the mentioned reasons omnidirectional pressure microphones with linear frequency response and a good signal to microphone noise ratio on whole frequency range are mostly used for peaceful and silent environments. When the noise level is high, the directionality is introduced, and since the signal level is high, the signal to microphone noise ratio is not important.
• US-A 5,214,907 describes a hearing aid which can be continuously regulated between an omnidirectional characteristic and a unidirectional characteristic.
• the special advantage of this solution is that at least in the omnidirectional mode a linear frequency response can be obtained.
• the method of the invention is characterized by the steps of claim 1.
• a directional signal can be obtained which has a high quality and which in its behaviour is essentially independent of the frequency of the input signals.
• a cardioid, hyper-cardioid or other directional characteristic can be obtained.
• a typical distance between the first and second microphone is in the range of 1 cm or less. This is small compared to the typical wavelength of sound which is in the range of several centimeters up to 15 meters.
• the principal difference of the invention to prior art is that it is a sort of feedback solution, that is not the signal of a microphone is delayed but a composite signal which contains already delayed information. It has been found that in that way no dependency on frequency is observed.
• two subtractors are provided, each of which is connected with a microphone to feed a positive input to the subtractor, and wherein the output of each subtractor is delayed for a predetermined time and sent as negative input to the other subtractor.
• the output of the first subtractor represents a first directional signal and the output of the second subtractor represents a second directional signal.
• the maximum gain of the first signal is obtained when the source of sound is situated on the prolongation of the connecting line between the two microphones.
• the maximum gain of the other signal is obtained when the source of sound is on the same line in the other direction.
• the above method relates primarily to the discrimination of the direction of sound. Based upon this method it is possible to analyse the signals obtained to further enhance the quality for a person wearing a hearing aid for example.
• One possible signal processing is to mix the first signal and the second signal. If for example both signals have the form of a cardioid with the maximum in opposite direction, a signal with a hyper-cardioid pattern can be obtained by mixing these two signals in a predetermined relation. It can be shown that a hyper-cardioid pattern has advantages compared to a cardioid pattern in the field of hearing aids especially in noisy situations. Furthermore, it is possible to split the first signal and the second signal into sets of signals in different frequency ranges.
• the present invention relates further to an apparatus for picking up sound with at least two essentially omnidirectional microphones, each of which is connected with an input port of a subtractor, a delaying unit with an input port connected with an output port of a first subtractor for delaying the output signal for a predetermined time.
• an output port of the delaying unit is connected with a negative input port of a second subtractor.
• three microphones are provided wherein the signals of the second and the third microphone are mixed in an adder, with an output port of which being connected to the second subtractor. This allows shifting the direction of maximum gain within a given angle.
• three microphones and three discrimination units are provided wherein the first microphone is connected to an input port of the second and the third discrimination unit, the second microphone is connected to an input port of the first and the third discrimination unit, and the third microphone is connected to an input port of the first and the second discrimination unit.
• three sets of output signals are obtained so that there are six signals whose direction of maximum gain is different from each other. By mixing these output signals these directions may be shifted to any predetermined direction.
• more than three microphones are provided which are arranged at the corners of a polygone or polyeder and wherein a set of several discrimination units is provided, each of which is connected to a pair of microphone.
• a set of several discrimination units is provided, each of which is connected to a pair of microphone.
• the microphones are arranged at the corners of a polyeder the directions in threedimensional space may be discriminated. At least four microphones have to be arranged on the corners of a tetraeder.
• a very strong directional pattern like shotgun microphones with a length of 50 cm or more with a characteristic like a long telephoto lens in photography may be obtained if at least three microphones are provided which are arranged on a straight line and wherein a first and a second microphone is connected with the input ports of a first discrimination unit, and the second and the third microphone is connected to the input ports of a second discrimination unit and wherein a third discrimination unit is provided, the input ports of which are connected to an output port of the first and the second discrimination unit and wherein a fourth discrimination unit is provided, the input ports of which are connected to the other output ports of the first and the second discrimination unit.
• Fig. 1 a block diagram of an embodiment of the invention
• Fig. 2 a circuit diagram of the essential part of the invention
• Fig. 3 a schematical view of a double membran microphone
• FIGs. 4a and 4b circuit diagrams of two variants of a further embodiment of the invention
• FIG. 5 a circuit diagram of yet another embodiment of the invention
• Fig. 6 a detailed circuit diagram of another embodiment
• Fig. 7 a block diagram of a further embodiment of the invention.
• Figs. 8, 9 and 10 typical directional patterns obtained by methods according to the invention.
• Fig. 1 shows that sound is picked up by two omnidirectional microphones la, lb.
• the first microphone la produces an electrical signal f(t) and the second microphone lb produces an electrical signal r(t).
• signals f(t) und r(t) are identical with the exception of a phase difference resulting from the different time of the sound approaching the microphones la, lb.
• the signals of the microphones la, lb fulfill the following equation :
• d represents the distance between the microphones la and lb, c sound velocity and ⁇ the angle between the direction 3 of sound approaching and the connection line 2 between the microphones la and lb.
• Block 4 represents a discrimination unit to which signals f(t) and r(t) are sent.
• the outputs of the discrimination circuit 4 are designated F(t) and R(t).
• the amplitude of F(t) and R(t) depends on angle ⁇ wherein a cardioid pattern is obtained for example. That means that the amplitude A of signals F and R corresponds to equation 2:
• Signals F(t) and R(t) are processed further in the processing unit 5, the output of which is designated with FF(t) and RR(t).
• the discrimination unit 4 is explained further.
• the first signal f(t) is sent into a first subtractor 6a, the output of which is delayed in a delaying unit 7a for a predetermined time T 0 .
• Signal r(t) is sent to a second subtractor 6b, the output of which is sent to a second delaying unit 7b, which in the same way delays the signal for a time T 0 .
• the output of the first delaying unit 7a is sent as a negative input to the second subtractor 6b, and the output of the second delaying unit 7b is sent as a negative input to the first subtractor 6a.
• the output signals F(t) and R(t) of the circuit of Fig. 2 are obtained as outputs of the first and the second subtractors 6a, 6b respectively.
• the following equations 3, 4 represent the circuit of Fig. 2 mathematically:
• a system according Fig. 2 simulates an ideal double membrane microphone as shown in Fig. 3.
• a cylindrical housing 8 is closed by a first membrane 9a and a second membrane 9b.
• the distance d between membranes 9a and 9b is chosen according equation (5):
• signal F(t) can be obtained from first membrane 9a and signal R(t) can be obtained from membrane 9b. It has to be noted that the similarity between the double membrane microphone and the circuit of Fig. 2 applies only to the ideal case. In reality results differ considerably due to friction, membrane mass and other effects.
• circuit of Fig. 2 only corresponds to a double membrane microphone when the delay T 0 is equal for the delaying units 7a and 7b. It is an advantage of the circuit of Fig. 2 that it is possible to have different delays T 0a and T 0b in the delaying units 7a and 7b respectively to obtain different output functions F(t) and R(t).
• the direction in which the maximum gain is obtained is defined by the connecting line between microphones la and lb.
• the embodiments of Fig. 4a and 4b make it possible to shift the direction in which the maximum gain is obtained without moving microphones.
• Fig. 4a as well as in Fig. 4b three microphones la, lb, lc are arranged at the corners of a triangle.
• signals of microphones lb and lc are mixed in an adder 10.
• the output of the adder 10 is obtained according to the following equation (6):
• Fig. 4b there are three discrimination units 4a, 4b and 4c, each of which is connected to a single pair out of three microphones la, lb, lc. Since microphones la, lb, lc are arranged at the corners of an equilateral triangle, the maximum of the output functions of discrimination unit 4c is obtained in directions 1 and 7 indicated by clock 11. Maximum gain of discrimination unit 4a is obtained for directions 9 and 3 and the maximum gain of discrimination unit 4a is obtained for directions 11 and 5.
• the arrangement of Fig. 4b produces a set of six output signals which are excellent for recording sound with high discrimination of the direction of sound. For example, in a _ .
• concert hall it is possible to pick up sound with only one small arrangement of three microphones contained in the housing of one conventional microphone with the possibility of recording on six channels giving an excellent surround impression.
• the directions mentioned above can be changed in a continuous way similar to embodiment shown in Fig. 4a for example by mixing output function F from discrimination unit 4c with output function F from discrimination unit 4a. In this way the maximum gain can be directed to any direction between 1 and 3 on clock 11.
• the directions of the maximum gain can not only be changed within a plane but also in three dimensional space.
• the above embodiments have a directional pattern of first order. With an embodiment of Fig. 5 it is possible to obtain a directional pattern of higher order.
• a first discrimination unit 4a processes signals of the first and the second microphone la, lb respectively.
• a second discrimination unit 4b processes signals of the second and the third microphones lb and lc respectively.
• Front signal F x of the first discrimination unit 4a and front signal F 2 of the second discrimination unit 4b is sent into a third discrimination unit 4c.
• Rear signal Ri of the first discrimination unit 4a and rear signal R 2 of the second discrimination unit 4b are sent to a fourth discrimination unit 4d.
• All discrimination units 4a, 4b, 4c and 4d of Fig. 5 are essentially identical. From third discrimination unit 4c a signal FF is obtained which represents a front signal of second order. In the same way a signal RR is obtained from the fourth discrimination unit 4d which represents a rear signal of second order. These signals show a more distinctive directional pattern than signals F and R of the circuit of Fig. 2.
• Fig. 6 a detailed circuit of the invention is shown in which the method of the invention is realized as an essentially analogue circuit.
• Microphones la, lb are small electret pressure microphones as used in hearing aids.
• After amplification signals are led to the subtractors 6 consisting of inverters and adders.
• Delaying units 7a, 7b are realised by followers and switches driven by signals Q and Q ⁇ obtained from a clock generator 12.
• Low pass filters and mixing units for the signals F and R are contained in block 13.
• Fig. 7 shows a block diagram in which a set of a certain number of microphones la, lb, lc, ... lz are arranged at the corners of a polygone or a threedimensional polyeder for example.
• a n-dimensional discrimination unit 14 After digitization in an A/D-converter 19 a n-dimensional discrimination unit 14 produces a set of signals. If the discrimination unit 14 consists of one discrimination unit of the type of Fig. 2 for each pair of signals, a set of n (n - 1) directional signals for n microphones la, lb, lc, ... lz are obtained.
• signals are analysed and eventually feedback information 16 is given back to discrimination unit 14 for controlling signal processing. Further signals of discrimination unit 14 are sent to a mixing unit 18 which is also controlled by analysing unit 15.
• the number of output signals 17 can be chosen according to the necessary channels for recording the signal.
• T 0 is chosen according the equation (7):
• k 1 the double membrane microphone of Fig. 3 is simulated so that a cardioid pattern (line 20) is obtained.
• Fig. 9 shows the directional pattern for a signal processing according the following equation (8) :
• the present invention allows picking up sound with a directional sensitivity without frequency response or directional pattern being dependent on frequency of sound. Furthermore, it is easy to vary the directional pattern from cardioid to hyper-cardioid, bi-directional and even to omnidirectional pattern without moving parts mechanically.

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• Health & Medical Sciences (AREA)
• General Health & Medical Sciences (AREA)
• Otolaryngology (AREA)
• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Circuit For Audible Band Transducer (AREA)
• Obtaining Desirable Characteristics In Audible-Bandwidth Transducers (AREA)
• Signal Processing Not Specific To The Method Of Recording And Reproducing (AREA)
• Stereophonic Arrangements (AREA)
• Holo Graphy (AREA)

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• 1999
  • 1999-10-07 EP EP99890319A patent/EP1091615B1/en not_active Expired - Lifetime
  • 1999-10-07 DE DE69904822T patent/DE69904822T2/de not_active Expired - Lifetime
  • 1999-10-07 AT AT99890319T patent/ATE230917T1/de not_active IP Right Cessation
• 2000
  • 2000-09-23 WO PCT/EP2000/009319 patent/WO2001026415A1/en not_active Ceased
  • 2000-09-23 CA CA002386584A patent/CA2386584A1/en not_active Abandoned
  • 2000-09-23 AU AU72893/00A patent/AU7289300A/en not_active Abandoned
  • 2000-09-23 US US10/110,073 patent/US7020290B1/en not_active Expired - Fee Related
  • 2000-09-23 JP JP2001528423A patent/JP4428901B2/ja not_active Expired - Fee Related

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