WO2008003344A2

WO2008003344A2 - Method for operating a wirelesss audio signal receiver unit and system for providing hearing assistance to a user

Info

Publication number: WO2008003344A2
Application number: PCT/EP2006/006634
Authority: WO
Inventors: François Callias; Evert Dijkstra; François Marquis
Original assignee: Phonak Ag
Priority date: 2006-07-06
Filing date: 2006-07-06
Publication date: 2008-01-10
Also published as: DE602006017902D1; EP2044806A2; DK2044806T3; ATE486469T1; EP2044806B1; WO2008003344A3

Abstract

The invention relates to a method for operating a receiver unit (12) for receiving audio signals from a remote transmission unit (10) via a wireless audio link (56), comprising: connecting an audio signal output (41) of the receiver unit (12) to an audio signal input (42) of a hearing instrument (14A, 14B) comprising means (52) located at a user's ear or in the user's ear canal for stimulating the user's hearing according to the audio signals from the receiver unit (12) and a microphone arrangement (44); measuring by means (34, 38) included in the receiver unit (12) the impedance of the audio signal input (42) of the hearing instrument (14A, 14B); and adjusting the impedance of the audio signal output (41) of the receiver unit (12) according to the measured impedance of the audio signal input (42) of the hearing instrument (14A, 14B). The invention also relates to such a receiver unit and to a system for providing hearing assistance to a user, comprising such a receiver unit.

Description

Method for operating a wireless audio signal receiver unit and system for providing hearing assistance to a user

The present invention relates to a method for operating a receiver unit for receiving audio signals from a remote transmission unit via a wireless audio link, wherein an audio signal output of the receiver unit is connected to an audio signal input of a hearing instrument comprising means located at a user's ear or in the user's ear canal for stimulating the user's hearing according to the audio signals from the receiver unit. The invention also relates to a system for providing hearing assistance to a user, comprising a remote transmission unit, a receiver unit for receiving audio signals from the transmission unit via a wireless audio link, a hearing instrument, means for connecting an audio signal output of the receiver unit to an audio signal input of the hearing instrument, wherein the hearing instrument comprises means located at a user's ear or in the user's ear canal for stimulating the user's hearing according to the audio signals from the receiver unit.

Usually in such systems the wireless audio link is an FM radio link. The benefit of such systems is that sound captured by a remote microphone at the transmission unit can be presented at a high sound pressure level to the hearing of the user wearing the receiver unit at his ear(s). In particular, the microphone of the hearing instrument can be supplemented or replaced by the remote microphone which produces audio signals which are transmitted wirelessly to the FM receiver and thus to the hearing instrument. In particular, FM systems have been standard equipment for children with hearing loss in educational settings for many years. Their merit lies in the fact that a microphone placed a few inches from the mouth of a person speaking receives speech at a much higher level than one placed several feet away. This increase in speech level corresponds to an increase in signal-to-noise ratio (SNR) due to the direct wireless connection to the listener's amplification system. The resulting improvements of signal level and SNR in the listener's ear are recognized as the primary benefits of FM radio systems, as hearing-impaired individuals are at a significant disadvantage when processing signals with a poor acoustical SNR.

Most FM systems in use today provide two or three different operating modes. The choices are to get the sound from: (1) the hearing instrument microphone alone, (2) the FM microphone alone, or (3) a combination of FM and hearing instrument microphones together. Usually, most of the time the FM system is used in mode (3), i.e. the FM plus hearing instrument combination (often labeled "FM+M" or "FM+ENV" mode). This operating mode allows the listener to perceive the speaker's voice from the remote microphone with a good SNR while the integrated hearing instrument microphone allows to listener to also hear environmental sounds. This allows the user/listener to hear and monitor his own voice, as well as voices of other people or environmental noise, as long as the loudness balance between the FM signal and the signal coming from the hearing instrument microphone is properly adjusted. The so-called "FM advantage" measures the relative loudness of signals when both the FM signal and the hearing instrument microphone are active at the same time. As defined by the ASHA (American Speech-Language-Hearing Association 2002), FM advantage compares the levels of the FM signal and the local microphone signal when the speaker and the user of an FM system are spaced by a distance of two meters. In this example, the voice of the speaker will travel 30 cm to the input of the FM microphone at a level of approximately 80 dB-SPL, whereas only about 65 dB-SPL will remain of this original signal after traveling the 2 m distance to the microphone in the hearing instrument. The ASHA guidelines recommend that the FM signal should have a level 10 dB higher than the level of the hearing instrument's microphone signal at the output of the user's hearing instrument.

When following the ASHA guidelines (or any similar recommendation), the relative gain, i.e. the ratio of the gain applied to the audio signals produced by the FM microphone and the gain applied to the audio signals produced by the hearing instrument microphone, has to be set to a fixed value in order to achieve e.g. the recommended FM advantage of 1OdB under the above- mentioned specific conditions. Accordingly, heretofore - depending on the type of hearing instrument used - the audio output of the FM receiver has been adjusted in such a way that the desired FM advantage is either fixed or programmable by a professional, so that during use of the system the FM advantage - and hence the gain ratio - is constant in the FM+M mode of the FM receiver.

WO 02/23948 Al relates to an example of such an FM receiver which not only receives audio signals from a remote microphone transmitter but in addition may communicate with remote devices such as a remote control or a programming unit via wireless link for data transmission.

EP 1 638 367 A2 relates to another example of an FM receiver for receiving audio signals from a remote microphone transmitter, wherein the FM receiver upon receipt of a polling signal from the remote microphone transmitter is capable of transmitting status information regarding the FM receiver to the remote microphone transmitter.

A further example of an FM receiver for receiving audio signals from a remote microphone transmitter is known from EP 0 671 818 Bl, wherein the FM receiver is equipped with a squelch function by which the audio signal in the receiver is muted if there is excessive noise due to a large distance between the transmission unit and the receiver unit exceeding the reach of the FM link.

WO 97/21325 Al relates to a hearing system comprising a remote unit with a microphone and an FM transmitter and an FM receiver connected to a hearing aid equipped with a microphone. The hearing aid can be operated in three modes, i.e. "hearing aid only", "FM only" or "FM+M". In the FM+M mode the maximum loudness of the hearing aid microphone audio signal is reduced by a fixed value between 1 and 10 dB below the maximum loudness of the FM microphone audio signal, for example by 4dB. Both the FM microphone and the hearing aid microphone may be provided with an automatic gain control (AGC) unit.

WO 02/30153 Al relates to a hearing system comprising an FM receiver connected to a digital hearing aid, with the FM receiver comprising a digital output interface in order to increase the flexibility in signal treatment compared to the usual audio input parallel to the hearing aid microphone, whereby the signal level can easily be individually adjusted to fit the microphone input and, if needed, different frequency characteristics can be applied.

Depending on the type of hearing instrument, there are generally two alternatives of how the audio output of the receiver unit is connected to the audio input of the hearing instrument: On the one hand, there are hearing instruments having an audio input which is parallel to the microphone of the hearing instrument and hence has a relatively low input impedance. On the other hand, there are hearing instruments having an audio input which is separate from the microphone of the hearing instrument and which has a relatively high input impedance. In the first case, the microphone of the hearing instrument can be muted by setting the output impedance of the receiver unit to a relatively low value ("FM only" mode), while in the "FM+M" mode the output impedance of the receiver unit is set to a relatively high value in order to allow mixing of the audio output signals of the receiver unit and the hearing instrument microphone signals at comparable levels. The appropriate switching of the output impedance of the receiver unit usually is provided by a manually operable switch at the receiver unit. In the first case, i.e. in the case of a hearing instrument having a low impedance audio input, one practical problem is that the achieved audio signal levels are often not identical in the "FM only" mode and in the "FM+M" mode. This is caused by tolerances of the audio input impedance of the hearing instrument due to variations of the impedance of the microphone of the hearing instrument and by the fact that the audio output impedance of the receiver unit is fixed and also has tolerances. Practically, a spread of the hearing instrument input impedance as large as from 2 kOhm to 11 kOhm has been measured. Usually the desired FM advantage, which theoretically could be predetermined by setting the gain applied to the audio signals in the receiver unit and/or the audio output impedance of the receiver unit accordingly, in practice is achieved only for a hearing instrument having a microphone which has exactly the impedance value (e.g. 3.9 kOhm) assumed when setting the gain and/or audio output impedance. In other words, in practice the desired FM advantage usually will not be achieved due to the practical variations of the audio input impedance of the hearing instrument.

hi the second case, i.e. in the case of a hearing instrument having a high impedance audio input, switching between the "FM only" mode and the "FM+M" mode is done within the hearing instrument. In this case, the output impedance of the receiver unit should be set to the low value in order to achieve the desired FM-advantage. If the receiver unit is used at the high output impedance setting, the desired FM-advantage will not be achieved.

A further problem occurring with FM systems results from the fact that the receiver unit has to be mechanically and electrically connected to the hearing instrument, usually via a so- called "audio shoe". It may happen that there is no electrical connection between the audio output of the receiver unit and the audio input of the hearing instrument, hi this case the wireless audio link will not be working, which, however, may not be recognized by the user, in particular if the user is a child.

It is an object of the invention to provide for a method for operating a receiver unit for receiving audio signals from a remote transmission unit via a wireless audio link, which receiver unit is connected to an audio signal input of a hearing instrument, wherein variations of the actually provided audio signal level due to variations of the input impedance of the hearing instrument should be reduced. It is a further object to provide for such a receiver unit.

These objects are achieved by a method as defined in claim 1 and a receiver unit as defined in claim 28, respectively. The invention is beneficial in that, by measuring the impedance of the audio signal input of the hearing instrument by means included in the receiver unit and by adjusting the impedance of the audio signal output of the receiver unit according to the measured impedance of the audio signal input of the hearing instrument, the impedance of the audio signal output of the receiver unit can be automatically adapted to the actual impedance of the audio signal input of the hearing instrument, so that the desired audio signal level can be automatically achieved regardless of the practical variations of the impedance of the audio signal input of the hearing instrument. In particular, the receiver unit is enabled to automatically detect to which kind of audio input (either high impedance input or low impedance input) the receiver unit has been connected in order to automatically set the output impedance accordingly, so that specifically in the case in which the receiver unit connected to a high impedance audio signal input automatically the appropriate output impedance is set without the need for operation of a corresponding switch by the user. In case of connection to a low impedance audio input, the practical variations of the impedance of the hearing instrument microphone can be automatically compensated for, so that the audio signal level in the "FM only" and in the "FM+M" mode can be balanced automatically. In addition, by measuring the impedance of the audio signal input of the hearing instrument the receiver unit is enabled to automatically detect if there is no connection between the receiver unit and the hearing instrument, so that, for example, a corresponding alarm signal can be issued. Similarly, also the case in which there is a short-circuit connection between the receiver unit and the hearing instrument can be detected automatically.

Preferred embodiments of the invention are defined in the dependent claims.

In the following an example of the invention will be illustrated by reference to the attached drawings, wherein

Fig. 1 is a block diagram of a wireless hearing assistance system comprising a receiver unit according to the invention, wherein two alternative ways of connecting the receiver unit to the hearing instrument are shown;

Fig. 2 is a schematic example of how the receiver unit may be provided with a circuit for measuring the impedance of the audio signal input of the hearing instrument and for adjusting accordingly the impedance of the audio signal output of the receiver unit; and Fig. 3 shows an example of how the measured audio input impedance of the hearing instrument may be classified.

Fig. 1 shows a block diagram of an example of a system for providing hearing assistance to a user which comprises a remote transmission unit 10, a receiver unit 12 and two alternative examples of a hearing instrument 14A and 14B, respectively. The transmission unit 10 comprises a microphone arrangement 16 (which may consist of at least two spaced apart microphones for achieving acoustic beam forming capability), a central unit 18 for processing the audio signals captured by the microphone arrangement 16 and for controlling the transmission unit 10, a transmitter/modulator 20, an FM antenna 22, an inductive antenna 24, a control panel 26 and a display 28.

The receiver unit 12 comprises an FM antenna 30, a receiver/demodulator 32, a central unit 34, an amplifier 36, a measurement/adjustment unit 38, an inductive antenna 40 and an audio signal output 41.

The hearing instrument 14A comprises an audio input 42, a microphone arrangement 44 (which usually comprises at least two spaced-apart microphones for achieving acoustic beam forming capability) connected in parallel to the audio input 42, a pre-amplifier 46, a central unit 48, a power amplifier 50 and an output transducer for stimulating the user's hearing, which usually will be a loudspeaker. In the hearing instrument 14A the audio input 42 has a relatively low impedance.

The hearing instrument 14B differs from the hearing instrument 14A essentially in that the audio input 42 has a relatively high impedance and thereby is essentially separated from the microphone arrangement 44. The signals supplied to the audio input 42 are amplified by a pre-amplifier 46A, while the audio signals captured by the microphone arrangement 44 are amplified by a pre-amplifier 46B, with the respective amplified signals being combined prior to being supplied to the central unit 48.

The values of the impedance of the audio input 42 of the hearing instrument 14B may range from 20 kOhm to 100 kOhm, whereas typical values for the impedance of the audio input 42 of the hearing instrument 14A are from 2 kOhm to 15 kOhm, in which case the impedance is determined by the impedance of the microphone arrangement 44. The audio signal output 41 of the receiver unit 12 usually is electrically connected to the audio input 42 via an interface 54 which usually also serves to mechanically connect the receiver unit 12 to the hearing instrument 14A, 14B. Such interface usually is a so-called "audio shoe". The hearing instrument 14A, 14B may be of any type, e.g. behind the ear (BTE), in the ear (ITE) or completely in the channel (CIC).

The transmission unit 10 may be for use by another person, for example, a teacher in a classroom, or it may be for use by the user of the hearing instrument 14A, 14B. In the latter case, the user, for example, may put the transmission unit 10 on a table in front of him, he may hold it in his hand or he may wear it somewhere at his body. In addition to the microphone arrangement 16, the transmission unit 10 may be adapted for receiving audio signals from a remote source, for example, from a mobile phone via a "Bluetooth" link (not shown in Fig.1).

In normal operation of the system, the audio signals captured by the microphone arrangement 16 are processed in the central unit 18 and then are modulated in the transmitter 20 for being transmitted via the antenna 22 over a wireless audio link 56 to the antenna 30. Usually the audio link 56 is a narrow band FM link. The signals received at the antenna 30 are demodulated in the demodulator 32, and the demodulated audio signals are processed in the central unit 34 prior to being amplified in the amplifier 36. The audio signals then pass through the unit 38 to the audio output 41 and from there via the audio input 42 and the pre- amplifier 46 / 46A to the central unit 48 for being processed there. The processed audio signals are amplified in the power amplifier 50 and then are reproduced by the output transducer 52 as sound stimulating the user's hearing.

Usually the gain provided to the audio signals in the receiver unit 12 by the amplifier 36 will be constant. However, according to a modified embodiment, the amplifier 36 may be a variable gain amplifier which is controlled by the central unit 34 according to control commands sent from the transmission unit 10, for example, via the FM link 56. Such control commands may be generated manually by operating the control panel 26 accordingly or they may be generated according to an auditory scene analysis performed by the central unit 18 based on the audio signals captured by the microphone arrangement 16. Such a variable gain system is described in the pending European patent application 06 002 886.7.

One problem encountered by such wireless audio systems is the fact that the level at which the audio signals captured by the remote microphone arrangement 16 will be finally reproduced by the hearing instrument 14 A, 14B - and in particular also the level relative to the audio signal level of the hearing instrument microphone arrangement 44 - will not only depend on the gain applied in the receiver unit 12 by the amplifier 36 (which could be set accordingly during fitting of the receiver unit 12 or even during operation of a variable gain receiver unit 12) but also on the impedance of the audio input 42 of the hearing instrument 14A, 14B, which, however, may considerably differ for the specific type of hearing instruments 14A, 14B. In particular, the audio input impedance will be largely different depending on whether a hearing instrument 14A with a high impedance audio input 42 or a hearing instrument 14B with a low impedance audio input 42 is connected to the receiver unit 12.

In conventional receiver units the first problem (model and tolerance dependent variation of the audio input impedance, which is particularly significant for the type of low audio input impedance hearing instruments 14B) is not addressed, while the second problem (use of a high audio input impedance hearing instrument 14A or a low audio input impedance hearing instrument 14B) is addressed by providing a switch in the unit 38 by which the output impedance of the receiver unit 12 can be varied between a relatively low value which is used for connection to a high audio input impedance hearing instrument 14B and a relatively high output impedance which is used for connection with a low audio input impedance hearing instrument 14A in the "FM+M" mode (in which the user should hear both the audio signals from the receiver unit 12 and from the microphone arrangement 44). The low value of the output impedance in this case is used for muting the microphone arrangement 44 of the hearing instrument 14A in the "FM only" mode so that the user can hear only the audio signals from the receiver unit 12).

However, with such conventional receiver units, in practice often a problem arises in the case in which the receiver unit is connected to a low audio input impedance hearing instrument 14A, since in this case the levels of the audio signals from the receiver unit 12 are often not identical in the switch positions "FM-only" and "FM+M" due to model and tolerance dependent variations of the impedance of the microphone arrangement 44.

Fig. 2 shows a schematic example of how the unit 38 may be designed in order to avoid this problem. In the representation of Fig. 2, the amplifier 36 is represented by a current source 36 and the impedance of the audio input 42 is represented by an impedance 142. The audio output 41 of the receiver unit 12 comprises an audio signal pin 41 A and a ground pin 4 IB. In practice there is always a capacitor 58 in series to the impedance 142, the value of which will depend on the hearing instrument model.

The measurement/adjustment unit 38 comprises a switch Ml for setting the output impedance to a high value when the receiver unit 12 is in a stand-by or OFF-mode, a lower resistance resistor RO which may have, for example, a value of 100 Ohm, a higher resistance resistor Rl which, for example, may have a value of 1 kOhm, a variable resistance resistor R2, a switch M3 for bypassing the variable resistor R2, a switch M2 for switching between the "FM-only" and "FM+M" mode, and an amplitude detector 60.

The open position of the switch M2 sets the "FM+M" mode, while the closed position sets the "FM only" mode. In the "FM only" mode the output impedance of the receiver unit 12 is determined by the resistor RO, while in the "FM+M" mode the output resistance is primarily determined by the resistor R2. In the "FM only " mode the resistor RO is connected in parallel to the input impedance 142, while in the "FM+M" mode a serial connection of the resistors R2, Rl and RO is connected in parallel to the input impedance 142.

The unit 38 has two functions: (1) the input impedance of the audio input 42, i.e. the value of the load impedance 142, is to be measured and (2) the output impedance of the receiver unit 12 is to be adjusted according to the determined value of the input impedance by adjusting the variable resistor R2 accordingly. To this end, a signal indicative of the input impedance is supplied to the central unit 34 which, in turn, acts on the variable resistor R2 to adjust the output impedance and which may generate a status signal indicative of the type of audio input to which the receiver unit is connected, as will be discussed in more detail below.

For performing a measurement of the input impedance, the switch Ml is closed, the switch M3 is opened and the switch M2 is opened by the central unit 34, i.e. the output impedance is set to that of the "FM+M" mode. The central unit 34 will cause the output signal of the demodulator 32 to be muted. In view of the serial capacitance 58 the measurement will be carried out with an AC signal, for example, a simple sine wave signal at a frequency, for example between IkHz and 10kHz. The measurement frequency preferably is programmable, since there is some uncertainty of the value of the capacitive load 58 which depends on the hearing instrument model. A frequency of 10 kHz usually will be attenuated by the hearing instrument 14A, 14B by more than 40 dB due to the usual pass-band of 100 Hz to 6 kHz and therefore will not be perceived at all by the user of the hearing instrument. In view of the fact that the microphone arrangement 44 of the hearing instrument 14A, 14B will be fully operating during the impedance measurement, the test signal is used at a relatively high level corresponding, for example, to a sound pressure level of at least 85 dB or 90 dB at the microphone. The measurement typically will have a duration of less than 200 msec.

The principle of the impedance measurement is to vary the value of the variable resistor R2 while measuring the voltage levels U_OUTL on the low output impedance line (corresponding to the output impedance in the closed position of the switch M2, i.e. "FM only" mode) and U_OUTH on the high output impedance line (open position of the switch M2, i.e. "FM+M" mode). These two voltage levels are compared in the amplitude detector 60, the output signal of which is provided to the central unit 34. The amplitude detector 60 may be implemented, for example, as an A/D-converter followed by a logic or a digital signal processor, or it may be implemented as peak level detectors followed by a decision logic. If it is detected that the levels U_OUTL and U_OUTH are equal, this means that the signal output level is balanced for both positions of the switch M2 (i.e. for both the "FM-only" mode and the "FM+M" mode), so that the respective value of the variable resistor R2 should be used as the output impedance in the "FM+M" mode.

In the following, an example of a measurement sequence is given.

The measurement may start with a connection integrity check for which the variable resistor R2 is set to its highest value, for example, 1.2 MOhm. If it is found by the amplitude detector 60 that U_OUTH is equal to or larger than U_OUTL, it is decided that no connection to an audio input of a hearing instrument exists, whereupon the measurement is terminated and a corresponding status signal indicating "no connection" is issued.

If it is found that U_OUTH is less than U_OUTL, it is checked whether the audio input is a high impedance audio input by setting the variable resistor R2 to, for example, 150 kOhm. If it is found that U_OUTH is equal to or larger than U_OUTL, it is decided that the receiver unit 12 is connected to a high impedance (i.e. separate) audio input, whereupon the measurement is terminated and a corresponding status signal indicating "connection to high impedance audio input" is issued.

If it is found that UOUTH is less than U_OUTL, it is checked whether the receiver unit 12 is connected to a low impedance audio input, i.e. to a microphone arrangement 44 of the hearing instrument 14A, by setting the variable resistor R2 to a lower value, for example, 127 kOhm.

If it is found that UQUTH is equal to or larger than UQ_UTL, it is decided that the receiver unit 12 is connected to a low impedance audio input, whereupon the measurement is terminated and a corresponding status signal "connection to low impedance audio input" is generated.

If it is detected that U_OUTH is less than U_OUTL, the value of the variable resistor R2 is further reduced, for example, to 108 kOhm, and the steps described above for the value of 127 kOhm are repeated, and so on. The value of the variable resistor R2 may be gradually reduced in, for example, 14 logarithmic steps downward to a value of R2 of 15 kOhm.

If even for the lowest value of R2 it is found that U_OUTH is less than U_OUTL, it is decided that there is a short circuit between the pins 41A and 41B, whereupon the measurement is terminated and a corresponding status signal indicating "short circuit connection" is issued.

If the value of R2 at which U_OUTH has been found to be equal to or larger than U_OUT_L was between 127 kOhm and 15 kOhm, the respective value of R2 is set by the central unit 34 for operating the receiver unit 12 in the "FM+M" mode.

If it has been found that the receiver unit 12 is connected to a high impedance audio input, switch M2 is set by the central unit 34 to the closed position, i.e. the output impedance is set to the low value determined by the resistor RO.

Fig. 3 gives a practical example of how the measured audio input impedance of the hearing instrument may be classified, with the actual impedance R_LOAD of the audio input, i.e. the value of the impedance 142, being shown together with the corresponding setting of the resistance of the variable resistor R2, i.e. the setting of the resistance of the resistor R2 for which for a given impedance RJLOAD of the audio input U_OUTH equals U_OUTL- For such condition, R2 equals (Rl/R0)*R_LOAD, i.e. in the example of Figs. 2 and 3 R2 = 10*R_LOAD.

According to Fig. 3, for values of R LOAD less than 1.5 kOhm (R2 less than 15 kOhm) the connection status is evaluated as "short circuit connection", for values of R LOAD from 1.5 kOhm to less than 15 kOhm (R2 from 15 kOhm to less than 150 kOhm) the connection status is evaluated as "low impedance audio input connection", for values of R LO AD from 15 kOhm to less than 120 kOhm (R2 from 150 kOhm to less than 1.2M0hm) the connection status is evaluated as "high impedance audio input connection", and for values of R_L0AD equal to or greater than 120 kOhm (R2 equal to or greater than 1.2 MOhm) the connection status is evaluated as "no connection". The inductive antenna 40 of the receiver unit 12 is provided for establishing a bidirectional data link to an external device, for example, the remote transmission unit 10 in order to transmit control commands from the remote transmission unit 10 via the inductive antenna 24 to the central unit 34 of the receiver unit 12 and to transmit the status signal indicative of the audio output connection status of the receiver unit 12 from the receiver 12 to the remote transmission unit 10. The received status signal may be converted to corresponding signal to be displayed on the display 28, for example, to an alarm signal indicating "no connection" or "short circuit connection".

Generally, the measurement of the audio input impedance and the respective adjustment of the audio output impedance by the receiver unit 12 may be initiated by an external command, for example, received via the inductive link 57, or it may be initiated automatically upon startup of the receiver unit 12. For example, the receiver unit 12 may be designed such that the connection integrity check (in which the resistor R2 is set to the highest value) may be performed only upon request via the inductive link 57, while the audio impedance calibration, i.e. the measurement of the audio input impedance in order to adjust the audio output impedance accordingly, may be performed on request via the inductive link 57 or it may be performed automatically upon start-up of the receiver unit 12. However, the latter only makes sense if the receiver unit 12 is connected to a low impedance audio input.

The inductive link may be, for example, a 41 kHz link.

The remote device connected via the inductive link 57 to the receiver unit 12, rather than being part of the remote transmission unit 10, also could be a separate remote control or remote programming unit for the receiver unit 12.

Claims

1. A method for operating a receiver unit (12) for receiving audio signals from a remote transmission unit (10) via a wireless audio link (56), comprising:

connecting an audio signal output (41) of the receiver unit (12) to an audio signal input (42) of a hearing instrument (14A, 14B) comprising means (52) located at a user's ear or in the user's ear canal for stimulating the user's hearing according to the audio signals from the receiver unit (12) and a microphone arrangement (44);

measuring by means (34, 38) included in the receiver unit (12) the impedance of the audio signal input (42) of the hearing instrument (14A, 14B); and

adjusting the impedance of the audio signal output (41) of the receiver unit (12) according to the measured impedance of the audio signal input (42) of the hearing instrument (14A, 14B).

2. The method of claim 1, wherein the output impedance of the receiver unit (12) is adjusted such that a predetermined audio signal output level is achieved.

3. The method of claim 2, wherein the output impedance of the receiver unit (12) is set to a pre-determined first value if connection to a high impedance audio input (42) separate from the microphone arrangement (44) of the hearing instrument (14B) has been detected by measuring an audio input impedance less than a first threshold but equal to or higher than a second threshold.

4. The method of claim 3, wherein the output impedance of the receiver unit (12) is set to said pre-determined first value by setting a first switch (M2) to a first position.

5. The method of claim 4, wherein by setting said first switch (M2) to said first position a non-variable first resistor (RO) is connected in parallel to the audio input (42).

6. The method of claim 5, wherein the output impedance of the receiver unit (12) is set to a value selected from a plurality of pre-determined values according to the measured audio input impedance if connection to a low impedance audio input (42) parallel to the microphone arrangement (44) of the hearing instrument (14A) has been detected by measuring an audio input impedance equal to or higher than a third threshold but less than said second threshold.

7. The method of claim 6, wherein the output impedance of the receiver unit (12) is set to said selected value by setting said first switch (M2) to a second position and by adjusting a variable resistor (R2) according to the measured audio input impedance.

8. The method of claim 7, wherein by setting said first switch (M2) to said second position serial connection of said variable resistor (R2), of said non-variable first resistor (RO) and of said non- variable second resistor (Rl) is connected in parallel to the audio input (42).

9. The method of claim 8, wherein the audio input impedance is measured with the first switch (M2) being set to said second position.

10. The method of claim 9, wherein the audio input impedance is measured by varying the resistance of the variable resistor (R2) and comparing the voltage on the first non- variable resistor (RO) and the voltage on the serial connection of second non-variable resistor (Rl) and the variable resistor (R2).

11. The method of claim 10, wherein the resistance of the variable resistor (R2) is varied in logarithmic steps.

12. The method of one of claims 10 and 11, wherein the value of the resistance of the variable resistor (R2) for which a pre-determined condition for the voltage on the first non- variable resistor (RO) and the voltage on the second non- variable resistor (Rl) and the variable resistor (R2) is fulfilled is taken as representative of the audio input impedance.

13. The method of one of claims 10 to 12, wherein the resistance of the variable resistor (R2) is set to the value for which a pre-determined condition for the voltage on the first non- variable resistor (RO) and the voltage on the second non- variable resistor (Rl) and the variable resistor (R2) is fulfilled in order to set the output impedance of the receiver unit (12) to said selected value.

14. The method of one of the preceding claims, wherein the measured impedance of the audio signal input (42) of the hearing instrument (14A, 14B) is classified into categories in order to generate a status signal representative of the connection status of the audio signal output (41) of the receiver unit (12).

15. The method of claim 14, wherein the status signal is indicative of the type of audio input (42) to which the receiver unit (12) is connected.

16. The method of one of claims 14 and 15, wherein the status signal is indicative of the quality of the connection of the audio signal output (41) to the audio input (42) of the hearing instrument (14A, 14B).

17. The method of one of claims 14 to 16, wherein the categories of the status signal include: "no connection" if the measured audio input impedance is equal to or higher than a first threshold, "connection to a high impedance audio input separate from the microphone arrangement of the hearing instrument" if the measured audio input impedance is equal to or higher than a second threshold but less than said first threshold, "connection to a low impedance audio input parallel to the microphone arrangement of the hearing instrument" if the measured audio input impedance is equal to or higher than a third threshold but less than said second threshold, and "short circuit connection" if the measured audio input impedance is less than said third threshold.

18. The method of claim 17, wherein an alarm signal is generated if the measured impedance of the audio signal input (42) of the hearing instrument (14A, 14B) has been classified into the categories "no connection" or "short circuit connection".

19. The method of one of the preceding claims, wherein the impedance of the audio signal input (42) of the hearing instrument (14A, 14B) is measured with alternating current.

20. The method of claim 19, wherein the impedance of the audio signal input (42) of the hearing instrument (14A, 14B) is measured at at least two different frequencies.

21. The method of one of the preceding claims, wherein the impedance of the audio signal input (42) of the hearing instrument (14A, 14B) is measured at an audio signal level corresponding to a microphone sound pressure level of at least 85 dB.

22. The method of one of the preceding claims, wherein during the measurement of the impedance of the audio signal input (42) of the hearing instrument (14A, 14B) the microphone arrangement (44) of the hearing instrument (14A, 14B) is fully working.

23. The method of one of claims 14 to 18, wherein the measurement of the impedance of the audio signal input (42) of the hearing instrument (14A, 14B) is initiated by a command received via an inductive link (57) from a remote device (10).

24. The method of claim 23, wherein the status signal is transmitted via the inductive link (57) from the receiver unit (12) to the remote device (10).

25. The method of one of the preceding claims, wherein the measurement of the impedance of the audio signal input (42) of the hearing instrument (14A, 14B) is automatically initiated when the receiver unit (12) is turned on.

26. The method of one of the preceding claims, wherein the audio signals received at the receiver unit (12) are muted during measuring the impedance of the audio signal input (42) of the hearing instrument (14A, 14B).

27. The method of one of the preceding claims, wherein the wireless audio link (56) is a Radio Frequency link, such as a Frequency Modulation link.

28. A receiver unit (12) for receiving audio signals from a remote transmission unit (10) via a wireless audio link (56), comprising an audio signal output (41) adapted for being connected to an audio signal input (42) of a hearing instrument (14A, 14B) comprising means (52) to be located at a user's ear or in the user's ear canal for stimulating the user's hearing according to the audio signals from the receiver unit (12), means for measuring the impedance of the audio signal input (42) of the hearing instrument (14A, 14B) and means (12, 34, 38) for adjusting the impedance of the audio signal output (41) of the receiver unit (12) according to the measured impedance of the audio signal input (42) of the hearing instrument (14A, 14B).

29. A system for providing hearing assistance to a user, comprising the receiver unit (12) of claim 28, a remote transmission unit (10), a hearing instrument (14A, 14B), and means (54) for connecting the audio signal output (41) of the receiver unit (12) to an audio signal input (42) of the hearing instrument (14A, 14B), wherein the hearing instrument (14A, 14B) comprises means (52) located at a user's ear or in the user's ear canal for stimulating the user's hearing according to the audio signals from the receiver unit (12).

30. The system of claim 29, wherein the transmission unit (10) comprises a microphone arrangement (16) for capturing the audio signals which are to be transmitted to the receiver unit (12).

31. The method of one of claims 29 and 30, wherein the hearing instrument (14A, 14B) comprises a microphone arrangement (44) for capturing audio signals and means for mixing that audio signals with the audio signals provided at the audio signal input (42) in order to provide the mixed audio signals to the stimulating means (52).