Abstract:
Inductive transmission to a hearing apparatus and in particular to a hearing device is to be improved. To this end, it is proposed to equip the hearing apparatus with two or three orthogonal coils. The coil signals are added with different signs and that signal which exhibits the highest level is forwarded from the resulting signals for further processing. It is thus possible to ensure an optimum reception quality in all alignments of the hearing apparatus.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of German application No. 10 2006 029 717.2 EP filed Jun. 28, 2006, which is incorporated by reference herein in its entirety. 
     FIELD OF INVENTION 
     The present invention relates to a hearing apparatus having two coils for inductive transmission. In particular, the present invention relates to a hearing device but also to other hearing apparatuses such as headsets, earphones and suchlike. 
     BACKGROUND OF THE INVENTION 
     Analog inductive transmission with a transmit and receive coil in the baseband is predominantly used for the purpose of wireless data transmission to hearing devices for telephoning or for inductive reception in buildings, e.g. churches. Here a large transmit coil is generally used in the floor or in the telephone and a correspondingly aligned receive coil is used in the hearing device. 
     In principle, in the case of inductive audio transmission by means of transmit and receive coils, the problem consists in that a significantly reduced signal quality can be expected if the coils are not aligned optimally in relation to one another. To this end, the left side of  FIG. 1  shows an optimal alignment of a transmit coil SS and a receive coil SE. The two coils SS and SE are arranged coaxially here so that a transmission with the highest degree of efficiency results, since the receive coil SE lies parallel to the magnetic field lines. 
     If on the other hand the receive coil SE is not parallel to the magnetic field lines, the degree of efficiency of the inductive transmission drops accordingly. The most minimal receive signal can then be expected if the axis of the receive coil SE is perpendicular to the magnetic field lines. This case is shown on the right in  FIG. 1 . 
     The problem of inadequate alignment of the transmit and receive coils is intensified in that further to the development of hearing devices with digital, inductive receivers, the telephone coil is increasingly housed in a hearing device remote controller. Such a remote controller can and is essentially more easily moved than a hearing device. It will thus also frequently assume an inadequate alignment in respect of the magnetic field of the transmit coil. 
     Methods are known from the publication DE 201 14 461, with which the problem of reduced signal quality in the case of poor alignment can be counteracted by using a number of coils. A number of coils are positioned orthogonal to one another and search out the receive signal, which supplies the strongest useful signal, from the receive signals. If necessary, the input signals are weighted differently. However optimum reception is not always achieved by this means, irrespective of the alignment of the transmit and receive coils. 
     The publication DE 601 09 268 T2 discloses an apparatus for controlling an antenna. This comprises a triaxial magnetic bearing sensor having two magnetic bearing sensors, each of which detects two components of the geomagnetic vector in the directions of two of the three axes of a right-hand coordinate system. Each of the two magnetic bearing sensors measures voltages which are excited in two coils arranged orthogonal to one another. 
     Furthermore, the publication DE 38 54 051 T2 discloses an antenna structure for generating a uniform field. The antennas have coils, which are arranged according to orthogonal x-, y- and z-axes. 
     SUMMARY OF INVENTION 
     An object of the present invention thus consists in improving the reception quality during inductive transmission. 
     As a solution, a hearing apparatus having a first coil for inductive transmission and a second coil for inductive transmission is provided according to the invention, with the two coils being arranged orthogonal to one another, a third coil for inductive transmission being arranged orthogonal to the first and second coil, an evaluation circuit being connected to all three coils, in the evaluation circuit all the coil signals with the same sign being summed for a first output signal and each two of the three coil signals with the same sign and the third coil signal with the other sign respectively being summed for a second, third and fourth output signal, with the third coil signal for the second output signal being assigned to the first coil, for the third output signal to the second coil and for the fourth output signal to the third coil, and that output signal of the four output signals which exhibits the highest level being selected by the evaluation circuit for further processing. 
     The system according to the invention thus enables a reception which is optimal in all spatial directions, irrespective of the alignment of the transmit coils to the receive coil system. An optimum audio reception of this type is then attained with a single telephone coil system, if the direction of the transmit magnetic field runs parallel to the coil axis. With the multiple coil system according to the publication DE 201 14 461 U1, this optimum audio reception is only achieved for the alignment of the transmit field in the axis of one of the receive coils. 
     The evaluation circuit preferably cross-fades smoothly between the sum signal and the differential signal as an output signal and/or between the output signals in the case of three orthogonal coils. This enables the changeover between the coil output signals to be less disruptive or not perceived. 
     Furthermore, a changeover from the sum signal to the differential signal or from one of the output signals to another one in the case of three orthogonal coils can be carried out on the basis of a hysteresis in the evaluation circuit. This avoids frequent toggling in the case of minimal signal changes. 
     In accordance with a preferred embodiment, the hearing apparatus comprises a remote controller and a hearing device, with the orthogonal coils and the evaluation circuit being incorporated in the remote controller and the selected output signal being transmitted to the hearing device. The hearing device remote controller can thus be moved freely without the quality of the signal being adversely affected by an unfavorable alignment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described in more detail with reference to the appended drawings, in which: 
         FIG. 1  shows two different arrangements of transmit and receive coils; 
         FIG. 2  shows two orthogonal receive coils in a transmission field; 
         FIG. 3  shows an inventive evaluation circuit according to a first embodiment and 
         FIG. 4  shows an inventive evaluation circuit according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The exemplary embodiments described in more detail below represent preferred embodiments of the present invention. 
     The invention is based on the idea of accommodating two or three telephone coils, which are aligned orthogonal to one another, in the receive device and of combining the received signals of the coil to form an optimum overall signal.  FIG. 2  illustrates this fundamental idea on the basis of two orthogonal receive coils S 1  and S 2 , which are located in the magnetic field M of a transmitter. The angle of the directional vector of the magnetic field M is marked as phi. If the directional vector of the magnetic field, as illustrated in the example in  FIG. 2 , points from the direction of the first quadrant, the magnetic field will result in a positive signal in coil S 1  and coil S 2 . The amplitude ratio of the signals is influenced by the angle phi, which determines the respective components along the plotted axis. For phi=45°, the same amplitude (and same phase) can be expected at both coils. For phi=90°, the signal at coil S 1  completely disappears. For phi&gt;90°, the sign of the signal, which is injected into coil S 1 , is reversed. The phase difference between the signals at coil S 1  and coil S 2  always amounts to 0° or 180°, since a transmission in the baseband is present. 
     Utilizing the knowledge obtained in conjunction with  FIG. 2 , an evaluation circuit is proposed in accordance with a first embodiment of the present invention for signal processing, as shown in  FIG. 3 . The signals of the two coils S 1 , S 2  are alternately added up in an adding unit Ad and subtracted in a subtraction unit SU. Accordingly, an addition signal ad and a differential signal su result. Depending on the direction of the magnetic field, the coil signals are superimposed constructively in-phase and destructively out of phase. If the direction of the magnetic field of the transmitter lies in one of the coil axes, addition signal ad and differential and/or subtraction signal su are dominated by the corresponding coil by virtue of amplitudes received in a significantly different manner. 
     Once the angle of the field moves out of the coil axis, the coil signals are added together in correct phase sequence in one of the addition and differential signals ad, su formed and thus result in an optimum signal with amplified amplitude compared with a single coil signal. The other of the two addition and differential signals ad, su formed exhibits a reduced amplitude compared with the strongest coil signal due to superimposition in phase opposition. 
     Both signals, the addition signal ad and the differential signal su, are fed to a level meter PM. A level comparison is then used to forward either the sum signal and/or addition signal ad or the differential and/or subtraction signal su to a signal processing circuit (e.g. inductive, digital RF transmitter) (not shown). A switch ST controlled by the level meter PM is used to this end. A changed alignment of the receive coil by the user will in some instances then result in a change in the selected signal. 
     For practical considerations, in order for instance to avoid an excessively frequent and rapid changeover, the changeover can be slowed down in a smooth manner by fading in/fading out and by means of a hysteresis. To this end, the switch ST is provided with a corresponding electronic system. 
     The evaluation principle illustrated for the two-dimensional according to  FIG. 3  can be extended according to  FIG. 4  to three dimensions. To this end, three coils S 1 , S 2  and S 3  are positioned orthogonal to one another in the hearing device and are added in a computing unit RE with all four possible sign combinations. In principle eight sign combinations are conceivable, however two of these combinations only represent a phase-displaced signal pair in each instance. A first output signal as 1 =S 1 +S 2 +S 3 , a second output signal as 2 =S 1 +S 2 −S 3 , a third output signal as 3 =S 1 −S 2 +S 3  and a fourth output signal as 4 =S 1 −S 2 −S 3  result from the computing unit RE. The signal with the highest level is then selected from the four output signals as 1 , as 2 , as 3  und as 4  in a level evaluator PA and output. 
     A preferred use of the evaluation circuits illustrated according to  FIGS. 3 and 4  for processing telephone coil signals lies in their use for an inductive radio frequency hearing device remote controller. Here the telephone coil signal is injected into the remote controller and forwarded to the hearing device by means of digital RF transmission. The variable position of the remote controller during use means that scarcely one of the coils of the receive coil system is aligned parallel to the transmit magnetic field. Nevertheless optimum reception is always achieved with the system according to the invention. The proposed system according to the invention can, in principle, also be used for magnetic radio transmission in the non-baseband.