Patent Publication Number: US-2018041850-A1

Title: Hearing device and a method for receiving wireless audio streaming

Description:
RELATED APPLICATION DATA 
     This application is a continuation of U.S. patent application Ser. No. 13/899,329 filed on May 21, 2013, pending, which claims priority to, and the benefit of, Danish Patent Application No. PA 2013 70264, filed on May 15, 2013, and European Patent Application No. 13167810.4, filed on May 15, 2013. The entire disclosures of both of the above applications are expressly incorporated by reference herein. 
    
    
     FIELD 
     The present application relates to a hearing device configured for receiving audio streaming and an associated method, in particular a hearing device and a method for receiving an audio stream from an audio system transmitting using a plurality of transmitters. 
     BACKGROUND 
     Wireless communication to and from hearing devices has been increasing in continuation of the developments within wireless communication technology. In a crowded environment where audio is distributed to the crowd, for example in an airport or in a movie theatre, it is known to stream audio via a telecoil solution having a limited bandwidth with limited possibilities of separating different audio streams. The limited transmission range of known transmitters is a limiting factor for a user moving in a larger area, e.g. an airport. Further, the limited battery power of hearing devices compared to e.g. a smartphone, sets a limit to the possibilities of wireless communication. 
     SUMMARY 
     There is a need for a hearing device that enables reception of audio streams and a method that enables the hearing device of a moving user to continue the receiving of the audio stream when the user moves from a transmission area of one transmitter to a transmission area of a second transmitter. 
     Despite the known solutions there is still a need for a method of efficiently switching between receiving from one transmitter to receiving from a second transmitter with a minimum or at least a reduced power consumption of the hearing device. 
     Accordingly, a hearing device is provided, the hearing device comprising an antenna; a receiver unit coupled to the antenna and configured to wirelessly receive audio packages and pilot packages of audio streams; a processing unit coupled to the wireless receiver unit for receiving data via the antenna; and a receiver coupled to an output of the processing unit for conversion of an output signal into an output audio signal. The processing unit is configured to receive audio packages of a primary audio stream from a primary transmitter unit with a primary transmitter unit address via the wireless receiver unit and to evaluate a first parameter of the primary audio stream. If the first parameter fulfills a search criterion, the processing unit is configured to search for pilot packages from available transmitter units; determine and select an optimum transmitter unit based on one or more one pilot package parameters of pilot packages from available transmitter units; and receive audio packages of an audio stream from the optimum transmitter unit according to one or more pilot packages from the optimum transmitter unit. 
     Also disclosed is a method for receiving wireless audio streams in a hearing device. The method comprises receiving audio packages of a primary audio stream from a primary transmitter unit having a primary transmitter unit address, and evaluating a first parameter of the primary audio stream. If the first parameter fulfils a search criterion, the method comprises searching for pilot packages from available transmitter units; determining and selecting an optimum transmitter unit, e.g. based on one or more one pilot package parameters of pilot packages from available transmitter units; and receiving audio packages of an audio stream from the optimum transmitter unit according to one or more pilot packages from the optimum transmitter unit. 
     It is an advantage that simple switching between different transmitter units is provided substantially without affecting the quality of the audio stream. 
     It is an important advantage that transmission of data from the hearing device to the first and/or second transmitter is limited or not required (no handshaking) for switching between wireless audio streams, which provides a power efficient receipt of wireless data. 
     The hearing device may comprise a processing unit configured to compensate for hearing loss or disability of the hearing device user. 
     According to some embodiments, an easy and efficient method of a hearing device switching between wireless transmitters in response to fading signals or signal quality is provided. 
     A method for receiving wireless audio streams in a hearing device includes: receiving audio packages of a primary audio stream from a primary transmitter unit having a primary transmitter unit address; evaluating a first parameter of the primary audio stream; and if the first parameter fulfills a search criterion: searching for pilot packages from available transmitter units; determining an optimum transmitter unit based on one or more pilot package parameters of the pilot packages from the available transmitter units; and receiving audio packages of an audio stream from the optimum transmitter unit. 
     Optionally, the act of determining the optimum transmitter unit comprises: evaluating at least one of the one or more of the pilot packages from the available transmitter units; and if the at least one of the one or more pilot package parameters fulfills a switch criterion, determining a secondary transmitter unit from the available transmitter units as the optimum transmitter unit. 
     Optionally, the primary transmitter unit is selected as the optimum transmitter unit if the at least one of the one or more pilot package parameters does not fulfill the switch criterion. 
     Optionally, the switch criterion is fulfilled if the pilot package parameter of one of the available transmitter units is larger than a pilot package parameter of the primary transmitter unit. 
     Optionally, the act of determining the optimum transmitter unit comprises: evaluating at least one of the one or more pilot package parameters from the available transmitter units; and selecting the optimum transmitter unit based on the act of evaluating the at least one of the one or more pilot package parameters. 
     Optionally, at least one of the one or more pilot packages comprises a transmitter unit address of one of the transmitter units, a time offset, and a channel identifier of a following audio package, and wherein the audio packages of the audio stream from the optimum transmitter unit is received based on the time offset. 
     Optionally, at least one of the one or more pilot packages comprises an audio stream identifier, and wherein the act of determining the optimum transmitter unit is based on the audio stream identifier. 
     Optionally, the first parameter is a parameter indicative of signal strength of the primary audio stream. 
     Optionally, one of the one or more pilot package parameters is indicative of a pilot package signal strength. 
     Optionally, the search criterion is fulfilled if the first parameter is below or above a search threshold value. 
     Optionally, the act of searching for the pilot packages is performed in time intervals in which audio packages from the primary transmitter unit is not being received. 
     Optionally, the act of searching for the pilot packages comprises receiving data on one or more dedicated pilot channels. 
     Optionally, the optimum transmitter unit is determined based on best available signal strength. 
     A hearing device includes: an antenna; a receiver unit coupled to the antenna; a processing unit coupled to the receiver unit for receiving data via the antenna; a receiver coupled to an output of the processing unit for conversion of an output signal into an output audio signal; wherein the processing unit is configured to: receive audio packages of a primary audio stream transmitted wirelessly from a primary transmitter unit to the receiver unit, the primary transmitter unit having a primary transmitter unit address; evaluate a first parameter of the primary audio stream; wherein, if the first parameter fulfills a search criterion, the processing unit is configured to: search for pilot packages transmitted wirelessly from available transmitter units to the receiver unit; determine an optimum transmitter unit based on one or more one pilot package parameters of the pilot packages from the available transmitter units; and receive audio packages of an audio stream from the optimum transmitter unit. 
     Optionally, the hearing device further includes a microphone for conversion of an acoustic audio signal into an input audio signal. 
     Optionally, the processing unit is configured to compensate for hearing loss or hearing disability of a user of the hearing device. 
     Other and further aspects and features will be evident from reading the following detailed description of the embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate the design and utility of embodiments, in which similar elements are referred to by common reference numerals. These drawings are not necessarily drawn to scale. In order to better appreciate how the above-recited and other advantages and objects are obtained, a more particular description of the embodiments will be rendered, which are illustrated in the accompanying drawings. These drawings depict only exemplary embodiments and are not therefore to be considered limiting to the scope of the claims. 
         FIG. 1  schematically illustrates an exemplary hearing device, 
         FIG. 2  schematically illustrates an exemplary hearing device, 
         FIG. 3  schematically illustrates use of an exemplary hearing device, 
         FIG. 4  schematically illustrates an electromagnetic signal, 
         FIG. 5  schematically illustrates receiving in a hearing device an electromagnetic signal from one of a plurality of transmitter units, 
         FIG. 6  schematically illustrates receiving in a hearing device an electromagnetic signal from one of a plurality of transmitter units, 
         FIG. 7  is a flow diagram of an exemplary method according to some embodiments, 
         FIG. 8  is a flow diagram of an exemplary method according to some embodiments, 
         FIG. 9  (a) to (d) illustrate exemplary audio blocks and 
         FIG. 10  (a) and (b) illustrate an exemplary audio package and an exemplary pilot package. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments are described hereinafter with reference to the figures. It should be noted that the figures are not necessarily drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the invention or as a limitation on the scope of the invention. The claimed invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described. 
     The present disclosure relates to a hearing device and a method enabling receipt of audio stream(s) representing audio track(s) from an audio system and capable of automatic switching, i.e. switching without user input, between receiving a primary audio stream representing a first audio track from a primary transmitter unit to receiving a secondary audio stream representing the first audio track from a secondary transmitter unit. 
     In an audio system, one or more transmitter units are configured for broadcasting or transmitting audio stream(s), e.g. to one or more hearing devices. A transmitter unit may transmit one or more audio streams. A transmitter unit is identified by a transmitter unit address. 
     An audio track is an audible signal. Exemplary audible signals are an airport call, a movie sound track, speech or a piece of music. 
     An audio stream is an electromagnetic signal representative of an audio track. An audio stream is represented by a number of audio blocks each comprising one or more audio packages. An audio package comprises a finite part of an audio stream. 
     An audio block comprises one or more audio packages (see also  FIG. 9( a )  to (d) and corresponding description). An audio block may comprise one or more primary audio packages and/or one or more secondary audio packages. A secondary audio package may constitute a copy of a primary audio package allowing a hearing device to recover lost packages. Further, an audio block may comprise first audio packages and/or second audio packages for example constituting audio stream data for a left or a right ear, respectively. An audio block may comprise a first primary audio package and/or a second primary audio package. Further, a first secondary audio package and/or a second secondary audio package may be included in an audio block. Sending the same audio data in two or more audio packages provides improved quality of sound at the receiver 
     An audio stream comprises a number of pilot packages. A pilot package comprises information about a subsequent such as the following or next audio block. 
     An audio frame (see also  FIG. 4  and corresponding description) comprises an audio block with one or more audio packages, and optionally one or more pilot packages including a primary and/or a secondary pilot package. An audio frame may comprise a tertiary pilot package to further improve handover between transmitter units. An audio block may have a length between 0.240 ms and 8.0 ms, such as in the range from 0.380 ms to 6.8 ms, such as in the range from 2.0 ms to 3.5 ms, e.g. 2.85 ms. 
     An audio package (see also  FIG. 10 ( a )  and corresponding description) may comprise an audio stream identifier, which may comprise one or more elements such as audio track identification, transmitter unit address, audio group identification etc. Audio track identification may enable selection of audio streams representative of the same audio track, and audio group identification may identify a certain group of audio tracks or audio streams e.g. an audio group may be audio tracks in the same or similar language, e.g. English, Danish or Chinese. 
     An audio package may comprise transmitter unit address of the transmitter unit transmitting the audio package, thus enabling the hearing device to sort out audio packages sent from other transmitter units. 
     A pilot package (see also  FIG. 10 ( b )  and corresponding description) may comprise information enabling receipt of a subsequent such as the following audio block from the transmitter unit sending the audio stream in question. A pilot package comprises one or more time offsets indicative of timing between packages in the audio stream. Further, a pilot package may comprise a channel identifier. A channel identifier indicates the channel (RF) of audio package(s) in a subsequent such as the following or next audio block. Even further, a pilot package may comprise transmitter unit address and/or audio stream identifier. A pilot package may comprise network identifier or other data indicative of a group of transmitter units, thereby enabling a hearing device to select or limit searching for pilot packages of a group of transmitter units. A pilot package from a transmitter unit enables a hearing device to receive a subsequent such as the following audio block from the transmitter unit in question by configuring the hearing device in accordance with information comprised within the received pilot package. A primary pilot package in an audio block may comprise transmitter unit address, a primary time offset, and channel identifier of the first primary audio package of a subsequent such as the following audio block. A secondary pilot package in an audio block may comprise transmitter unit address, a secondary time offset, and channel identifier of the first primary audio package of a subsequent such as the following audio block. 
     A time offset comprised within the pilot package may indicate the time of transmittal of a subsequent such as the following audio block. The time offset may e.g. be the time from start of transmission of the pilot package to the time of start of transmission of a subsequent such as the following audio block and/or to the time of start of transmission of an audio package in a subsequent such as the following audio block. 
     The channel identifier or RF channel identifier comprised within the pilot package may be indicative of the channel where the following audio block is transmitted. A channel is indicative of the frequency or frequency range on which an (first) audio package of the following audio block is sent. 
     The transmitter unit address comprised within the pilot package may be transmitter unit address of the transmitter unit from where the pilot package was transmitted, thus enabling the hearing device to identify the transmitter unit from where it may receive following audio packages corresponding to the pilot package in question. 
     An audio frame may have a time duration or period length T. Period length T may be defined as being from start of transmission of an audio block to the start of transmission of the following audio block. T may be defined as being the time from start of transmission of the first primary audio package of the first audio block to the start of transmission of the first primary audio package of the second audio block. The period length T may be fixed e.g. be a time of between 2 ms and 20 ms, such as between 5 ms and 12 ms or such as between 6 ms and 10.2 ms, or between 6 ms and 6.5 ms, or between  7 . 2  ms and 7.7 ms, or between 9.8 ms and 10.2 ms. A long period length T may be beneficial for providing increased possibility of power saving in a hearing device, as a long period length T provides for a long duration where the receiver may be switched off. However, a long T leads to an increased latency, meaning that a long T is not sufficient if the audio needs to be synchronized with e.g. a movie. Hence a short T is beneficial if the audio is to be synchronized with another input, or is to be happening as close to real time as possible. 
     The audio system may comprise a first transmitter unit and optionally a second transmitter unit or optionally any number of a plurality of transmitter units. 
     Transmitting and receiving the wireless audio stream may be achieved by using wireless technology, thus transmitting/receiving audio blocks at one or more frequencies, e.g. in the range from 2.4 GHz to 2.5 GHz, in the range from 800 MHz to 1 GHz, in the range from 3.6 GHz to 3.7 GHz, and/or in the range from 4.9 GHz to 5.9 GHz. 
     A pilot package is sent and/or received on a pilot channel. Pilot packages may be transmitted and/or received on pilot channels selected from one or more available channels. The pilot channel for a pilot package may be selected from a subset of available channels, thus further reducing load on the hearing device during search for available transmitter units, by allowing the hearing device to only need to listen for pilot packages of available transmitter units at known pilot channel(s). The channel(s) for pilot packages may be at one or more frequencies, e.g. in the range from 2.4 GHz to 2.5 GHz, in the range from 800 MHz to 1 GHz, in the range from 3.6 GHz to 3.7 GHz, and/or in the range from 4.9 GHz to 5.9 GHz. 
     Pilot package(s) may be transmitted/received in the same frequency range as audio blocks. Alternatively or in combination, pilot packages may be transmitted in a different frequency range than audio blocks and/or by using a different modulation scheme. Pilot package(s) may be transmitted/received within an audio block, i.e. a pilot package may be interleaved within an audio block. 
     Transmission and/or receipt of pilot packages may be performed using a frequency hopping scheme, i.e. the pilot channel may be selected according to a frequency hopping scheme, for example between three or more channels. The frequency hopping scheme may be random or pseudo-random or predetermined. 
     The first parameter of the primary audio stream may be a parameter indicative of signal strength and/or signal quality of the primary audio stream or one or more audio blocks or audio packages of the primary audio stream. The first parameter of the primary audio stream may be a parameter indicative of or including signal strength or signal quality of pilot packages of the primary audio stream. The first parameter may be Received Signal Strength Indication (RSSI) of the primary audio stream or of one or more selected audio and/or pilot packages of the primary audio stream. 
     In exemplary method(s), the first parameter may be or comprise Received Channel Power Indicator (RCPI). 
     In exemplary method(s), the search criterion may be fulfilled if the first parameter is below or above a search threshold value, for example if the RSSI of the primary audio stream is below or above a search threshold value, e.g. a first threshold value, depending on whether a high or low RSSI, respectively, indicates high signal quality. 
     The search criterion may comprise a combination of one or more logical expressions evaluating whether one or more parameters is above, below and/or equal to one or more search threshold values, respectively. 
     A pilot package parameter (p) is a parameter indicative of one or more characteristics of a pilot package, such as signal strength. A pilot package parameter may be a function of one or more characteristics of pilot packages from respective transmitter units. The pilot package parameter may be based on RSSI of one or more pilot package. 
     The first pilot package parameter from each available transmitter unit may be a function of the signal strength (RSSI) of one or more pilot packages from the respective transmitter unit. The first pilot package parameter may comprise an averaging function, for example based on RSSI values for pilot packages. 
     The first pilot package parameter may be a function of pilot package characteristics for pilot packages from two or more audio frames. Accordingly, searching for pilot packages from available transmitter units and determining and selecting an optimum transmitter unit may be performed, while the hearing device receives and processes audio blocks from the primary transmitter unit. Including characteristics from a plurality of pilot packages may ensure an improved switching scheme since random packet losses or errors may be taken into account, e.g. in order to avoid unnecessary switching between transmitter units. 
     Receiving audio packages of a primary audio stream comprises receiving audio packages from a primary transmitter unit (not shown) with a transmitter unit address. 
     Evaluation of the first parameter may comprise evaluation of one or more different characteristics such as signal strength, signal quality and/or rate of errors. 
     Searching for pilot packages may be initiated if the first parameter fulfills a search criterion. The processing unit may return to receiving audio packages/evaluation of the first parameter, if the first parameter does not fulfill the search criterion. The search criterion may be such as whether or not the signal strength of the received audio packages from the first transmitter is satisfactory, e.g. if signal quality is below a given threshold, if the error rate is above a certain threshold and/or if a predetermined timeout has passed. Incorporating a predetermined timeout in the search criterion enables setting a maximum time between searches, e.g. to ensure that the search is initiated on a regular basis, e.g. at least every 10 seconds. Alternatively or additionally, a minimum time between consecutive searches may also be incorporated in the search criterion, in order to reduce power consumption. 
     The pilot packages may be received from available transmitter units and evaluated according to one or more parameters including but not limited to signal strength, signal quality, audio stream identifier, and/or rate of errors. 
     Determining and selecting an optimum transmitter unit may comprise evaluating at least one pilot package parameter, including a first pilot package parameter, for one or more pilot packages from available transmitter units. Evaluating at least one pilot package parameter may comprise comparing a first pilot package parameter of pilot packages from the primary transmitter unit with a first pilot package parameter of pilot packages from available secondary transmitter units. Further, if the at least one pilot package parameter fulfils a switch criterion, the method may comprise selecting a secondary transmitter unit from the available transmitter units as the optimum transmitter unit. 
     The method may comprise selecting the primary transmitter unit as the optimum transmitter unit if the at least one pilot package parameter does not fulfil the switch criterion, i.e. the hearing device continues to receive audio packages from the primary transmitter unit if a better transmitter unit is not found. 
     The switch criterion may be fulfilled if the first pilot package parameter of an available secondary transmitter unit is larger than the first pilot package parameter of the primary transmitter unit. The secondary transmitter unit with the best first pilot package parameter, e.g. largest RSSI, may then be selected as the optimum transmitter unit. 
     In the method, selecting the optimum transmitter unit may be based on the evaluation of at least one pilot package parameter of available transmitter units including the primary transmitter unit if available. 
     Receiving audio packages of an audio stream from the optimum transmitter unit may be based on the time offset of a pilot package. Accordingly, the processing unit may be configured to control the receiver unit according to the time offset in a pilot package. 
     Determining and selecting an optimum transmitter unit may be based on the audio stream identifier. Thereby is prevented that a transmitter unit sending a different audio stream is selected. Transmitter units not sending the desired audio stream or not belonging to the desired group of transmitter units may be filtered out in the receiver unit during searching for pilot packages in order to reduce processing in the processing unit. 
     In methods employing a switch criterion, the method may return to receiving audio packages from the primary transmitter unit if the switch criterion is not fulfilled. The switch criterion may be such as whether or not the pilot package parameters for secondary transmitter unit pilot packages are larger than pilot package parameter of the primary transmitter unit pilot packages. 
     If the switch criterion is fulfilled and/or a secondary transmitter unit is selected as the optimum transmitter unit, the processing unit configures the wireless receiver unit for receiving audio packages from the secondary transmitter unit according to information in pilot packages (time offset and RF channel identifier) received from the secondary transmitter unit. 
     The processing unit may be configured to turn off the wireless receiver unit in one or more time intervals of an audio frame, e.g if the first parameter does not fulfil the search criterion. The wireless receiver unit may be configured to receive audio packages of audio streams at frequencies in the range from 2.4 GHz to 2.5 GHz and/or in the range from 800 MHz to 1 GHz. 
       FIG. 1  schematically illustrates an exemplary hearing device. The hearing device  2  comprises a loudspeaker or receiver  4 , a processing unit  6 , an antenna  8  and a wireless receiver unit  10 . The wireless receiver unit  10  is coupled to the antenna  8  and configured to receive audio packages of an audio stream and pilot packages of a plurality of audio streams. The processing unit  6  is configured to compensate for hearing loss or hearing disability of the hearing device user and is coupled to the wireless receiver unit  10  for receiving data via the antenna  8  and/or for exchanging control signals between the receiver unit  10  and the processing unit  6 . The loudspeaker  4  is coupled to an output port of the processing unit for conversion of an output signal into an output audio signal. The processing unit  6  is configured to receive audio packages of a first audio stream via the wireless receiver unit  10 , and evaluate a first parameter of the first audio stream. In the hearing device  2 , RSSI (Received Signal Strength Indicator) is the first parameter, and the receiver unit  10  calculates RSSI and feeds RSSI to the processing unit  6 . Other parameters indicative of signal quality or combinations thereof may be used as a first parameter. If the first parameter fulfills a search criterion, e.g. if RSSI falls below a search threshold value, i.e. the signal quality is dropping below an acceptable level (high RSSI indicates good signal quality), the processing unit  6  searches for pilot packages from available transmitter units by controlling the receiver unit  10  accordingly. After searching, e.g. for a predetermined time period or number of audio frames, or until a number of pilot packages are received, the processing unit  6  determines and selects an optimum transmitter unit, e.g. based on one or more one pilot package parameters including a first pilot package parameter (p 1,1 , . . . P 1,N ) of pilot packages from N available transmitter units. 
     Upon selection of an optimum transmitter unit, the processing unit  6  controls the wireless receiver unit  10  to receive audio packages of an audio stream from the optimum transmitter unit according to at least the latest pilot package from the optimum transmitter unit. The processing unit  6  is optionally configured to compensate the received audio stream for hearing loss or disability of the hearing device user. 
       FIG. 2  schematically illustrates an exemplary hearing device. The hearing device  2 ′ comprises the same features as the hearing device  2  in  FIG. 1 . Additionally, the hearing device  2 ′ comprises a microphone  12 . The microphone  12  is coupled to the processing unit  6  for conversion of an acoustic audio signal into an electronic representation of the acoustic audio signal  14  that is sent to the processing unit  6 . The processing unit  6  is configured to compensate the received audio stream and/or the electronic representation for hearing loss or disability of the hearing device user. 
       FIG. 3  schematically illustrates use of an exemplary hearing device for receiving audible information from an audio system comprising a plurality of transmitter units  51 ,  52 ,  53 ,  54 ,  55 . The transmitter units  51 ,  52 ,  53 ,  54 ,  55  are distributed in an area. 
     A user wearing a hearing device (not shown), is located in a first position  50  in the area and receives an audio stream from a first transmitter unit  51  acting as the primary transmitter. When the user moves to a second position  50 ′ or e.g. the user turns his/her head or the receiving conditions otherwise change, the audio stream signal transmitted from the first transmitter unit  51  may become weak, e.g. due to the limited transmission range of the transmitter unit or limited penetration properties of the wireless signal. The hearing device detects that the signal quality is decreasing (search criterion fulfilled) and performs a search for available transmitter units, which is facilitated by receiving pilot packages of available transmitter units of the plurality of transmitter units  51 ,  52 ,  53 ,  54 ,  55 . As illustrated, the hearing device identifies the fourth transmitter unit  54  as the optimum transmitter unit transmitting the same audio track by evaluating at least one pilot package parameter, e.g. RSSI, of received pilot packages, and selects the fourth transmitter unit  54  as the optimum transmitter unit and starts receiving audio packages therefrom according to one or more pilot packages from the fourth transmitter unit  54 . 
       FIG. 4  schematically illustrates a part of an exemplary audio stream as an electromagnetic signal  100  transmitted from any of the transmitter units  51 ,  52 ,  53 ,  54 ,  55  as illustrated in  FIG. 3 . 
     The audio stream/electromagnetic signal  100  represents an audio track and is divided into audio frames  104 ,  114 ,  124 . Each audio frame comprises an audio block  101 ,  111 ,  121 , a primary pilot package  102 ,  112 ,  122 , and optionally a secondary pilot package  103 ,  113 ,  123 . The audio blocks  101 ,  111 ,  121  comprise at least a first primary audio package (not shown) and may in addition comprise any number of audio packages. The number of audio packages does not need to be the same for all audio blocks. 
     The first audio frame  104  starts by a first primary audio package of the first audio block  101  being transmitted. After transmitting the first primary audio block  101 , a first primary pilot package  102  is transmitted. Optionally, a first secondary pilot package  103  is transmitted in the first audio frame. The first secondary pilot package  103  may be transmitted at an arbitrary time during the first audio frame, preferably between transmission of the first primary pilot package and transmission of the second audio block  111 . A primary pilot package may be transmitted within the audio block of the respective audio frame. 
     The first primary pilot package  102  comprises information as to how (channel identifier) and when (time offset) to receive the first primary audio package of the second audio block  111 . The first primary pilot package  102  comprises a first primary timeoffset  106  indicating the time from transmittal of the first primary pilot package  102  to transmittal of the first primary audio package of the second audio block  111  of the second audio frame  114 . 
     The first secondary pilot package  103  comprises, as the first primary pilot package, information as to how (channel identifier) and when (time offset) to receive the first primary audio package of the second audio block  111 . Among a number of information the first secondary pilot package  103  comprises a first secondary timeout  108  indicating the timeout from transmittal of the first secondary pilot package  103  before transmittal of the first primary audio package of the second audio block  111  of the second audio frame  114 . 
     In an exemplary electromagnetic signal, the duration or period length of an audio frame  104 ,  114 ,  124  is constant, and may e.g. be a time of between 2 ms and 20 ms, such as between 5 ms and 12 ms or such as between 6 ms and 10.2 ms, or between 6 ms and 6.5 ms, or between 7.2 ms and 7.7 ms, or between 9.8 ms and 10.2 ms. 
     The primary pilot packages  102 ,  112 ,  122  may be transmitted at a time within the audio frame resulting in that the primary time offset  106 ,  116 ,  126  is fixed and equal for all audio frames  104 ,  114 ,  124 . The secondary pilot packages  103 ,  113 ,  123  may be transmitted at a time chosen randomly or pseudo randomly, resulting in a random or pseudo random secondary time offset  108 ,  118 ,  128 . It is to be understood that the time offsets of pilot packages within the same audio frame indicates to the same transmittal time of the first primary audio package of the following audio block. 
       FIG. 5  schematically illustrates receiving in a hearing device an audio stream/electromagnetic signal from one or more of a plurality of available transmitter units.  FIG. 5  illustrates three transmitter units  51 ,  52 ,  53  each transmitting an electromagnetic signal in accordance with the description of the electromagnetic signal  100  of  FIG. 4 . 
     A hearing device, e.g. hearing device  2 ,  2 ′, receives audio packages of audio blocks  101 ′ from the second transmitter unit  52  acting as primary transmitter unit in receive period  212 . If an evaluation of the received audio packages in audio block  101 ′ yields a demand for an alternative audio source, i.e. if the search criterion is fulfilled, the hearing device  2 ,  2 ′ searches for pilot packages sent from available transmitters  51 ,  52 ,  53  in one or more search periods  214  followed by an evaluation of the pilot package parameter(s). In the illustrated example, the evaluation shows that the second transmitter unit  52  despite the low signal quality still provides the best available signal quality and the hearing device continues to receive audio blocks from the second transmitter unit acting as primary transmitter unit, i.e. a new transmitter unit is not selected. The receive periods  212  of the hearing device may be extended such that primary pilot packages from the primary transmitter are received in order to evaluate the signal quality (first parameter) based on primary pilot packages  102 ′,  112 ′ and  122 ′ from the primary transmitter unit. 
     The search period  214  may vary in time and/or in duration, and may be conducted when the hearing device  2 ,  2 ′ is not receiving audio packages  212 . In the example of  FIG. 5 , the search during search periods  214  leads to reception of pilot packages from two alternative transmitter units  51 ,  53 .. 
       FIG. 6  schematically illustrates receiving in a hearing device an electromagnetic signal from one of a plurality of available transmitter units.  FIG. 6  illustrates a similar transmission setup as described in  FIG. 5 . No search is initiated after receipt of the first audio block  101 ′ and optionally first primary pilot package  102 ′ during receive period  212  since the RSSI is high and the search criterion not fulfilled. Accordingly, the hearing device  2 ,  2 ′ continues receiving the second audio block  111 ′ and optionally the second primary pilot package from the second transmitter unit  52  acting as primary transmitter unit. The hearing device then determines that the search criterion is fulfilled and searches  214 ′ for available transmitter units by searching for pilot packages on one or more pilot channels. Upon receipt of the pilot package  113 ″, the hearing device selects the third transmitter unit  53  as the optimum transmitter unit and starts receiving during receive period  212 ′ the audio package  121 ″ from the third transmitter unit  53  according to information included in the pilot package  113 ″, such as time offset and channel identifier. The reception of pilot package  113 ″ during search period  214 ′ leads to a switch of transmitter unit, thus the following reception  212 ′ of audio package is configured according to parameters of the pilot package  113 ″ received during search  214 ′ and audio package  121 ″ is received from the third transmitter unit  53 . A reason for not switching according to an earlier received pilot package, e.g. pilot package  113 ″, may be such as a too weak signal and/or an incorrect audio track. 
       FIG. 7  is a flow diagram of an exemplary method according to some embodiments. The method  400  illustrated is designed to be performed while the user of e.g. a hearing device continuously receives an audio stream, hence do not interrupt the receiving and playback of an audio stream. The method  400  comprises receiving  401 , e.g. via a wireless receiver unit, audio packages of a primary audio stream from a primary transmitter unit having a primary transmitter unit address. The method comprises evaluating  402  at least a first parameter of the audio stream being received from the primary transmitter. The parameter evaluation  402  may comprise different elements such as signal strength, signal quality and/or error rate. If the first parameter fulfils a search criterion  404 , the method comprises searching  406  for pilot packages from available transmitter units. Searching for pilot packages may comprise searching for pilot packages with predetermined properties, e.g. a transmitter unit address and/or audio stream identifier, network identifier or other data indicative of a group of transmitter units. Searching for pilot packages may comprise searching for pilot packages on one or a set of pilot channels. Pilot packages may be received on channels different from the audio channels used for receipt of audio blocks/packages. The search criterion may comprise one or more logical expressions, e.g. whether or not the first parameter is below or above a search threshold and/or if a predetermined timeout has passed. The first parameter may be indicative of or comprise error rate. If the search criterion is not met, the method  400  proceeds with receiving  401  audio packages from the primary transmitter unit. Receiving audio packages from the primary transmitter and searching for pilot packages may be performed in parallel or in a multiplexing scheme during on or more audio frames. 
     Upon searching for pilot packages from available transmitter units, the method  400  proceeds to determining and selecting  414  an optimum transmitter unit based on one or more one pilot package parameters of pilot packages from available transmitter units. In the method  400 , determining and selecting  414  comprise evaluating the received pilot packages found during searching  406  by evaluating  408  at least one pilot package parameter, including a first pilot package parameter, for one or more pilot packages from each available transmitter unit. If a switch criterion is fulfilled  410 , i.e. if a secondary transmitter unit different from the primary transmitter unit provides a better signal or higher signal quality, the method  400  proceeds to selecting the secondary transmitter unit as new transmitter unit, i.e. switching to a new transmitter unit. The switch criterion may be fulfilled if a pilot package parameter of the primary transmitter unit is less than a pilot package parameter of the secondary transmitter unit having the largest pilot package parameter. The switch criterion may be fulfilled if a pilot package parameter of the primary transmitter unit is larger than a pilot package parameter of the secondary transmitter unit having the lowest pilot package parameter. 
     If the switch criterion is not met, the method  400  or hearing device returns to receiving audio packages from the primary transmitter. If the switch criterion  410  is met, a secondary transmitter unit is selected  412  as the optimum transmitter unit, and audio packages are received  401  from the secondary transmitter unit according to a time offset and a channel identifier included in the pilot package from the secondary transmitter unit. 
       FIG. 8  is a flow diagram of an exemplary method  400 ′ according to some embodiments. The method comprises determining the optimum transmitter unit by selecting the transmitter unit with the best pilot package parameter (e.g. max or min) from the received pilot packages. 
       FIG. 9  (a) to (d) illustrate four different exemplary audio blocks  200 ,  200 ′,  200 ″,  200 ″&#39; sent from transmitter units.  FIG. 9( a )  illustrates an audio block  200  comprising a primary audio package  202 , e.g. a mono audio signal.  FIG. 9( b )  illustrates an audio block  200 ′ of an exemplary mono signal comprising a primary audio package  202  and a secondary audio package  204 . The secondary audio package  204  may be a copy of the primary audio package  202 , to enhance chances of a successful reception of data in a hearing device.  FIG. 9( c )  illustrates an audio block  200 ″ of an exemplary stereo signal. The audio block  200 ″ in  FIG. 9( c )  comprises a first primary audio package  206  and a second primary audio package  208  for right and left ear, respectively. The audio block  200 ″ illustrated in  FIG. 9( c )  further comprises a first secondary audio package  210  and a second secondary audio package  212 . The first secondary audio package  210  may be a copy of the first primary audio package  206 , and the second secondary audio package  212  may be a copy of the second primary audio package  208 .  FIG. 9( d )  illustrates an audio block  200 ″&#39; of an exemplary stereo signal. The audio block  200 ″&#39; illustrated in  FIG. 9 d    comprises a first tertiary audio package  214  and a second tertiary audio package  216 . The first tertiary audio package  214  may be a copy of the first secondary right audio package  210  and/or the first primary audio package  206 . The second tertiary audio package  216  may be a copy of the second secondary audio package  212  and/or the second primary audio package  208 . A “copy of” means that at least the audio data of the audio packages are identical. 
       FIG. 10( a )  illustrates an exemplary audio package comprising transmitter unit address, optional audio stream identifier, and audio stream data. Transmitter unit address may be indicative of the transmitter unit from which the audio package was transmitted. Audio stream identifier may comprise data indicative of the audio stream, e.g. audio track identification, audio group identification etc. Audio stream data comprises a finite part of the audible information forming the audio stream. 
       FIG. 10( b )  illustrates an exemplary primary and/or secondary pilot package comprising transmitter unit address, channel identifier, time offset, optional audio stream identifier, and optional network identifier. Transmitter unit address may be indicative of the transmitter unit from which the pilot package was transmitted. Channel identifier may be indicative of the channel where the next audio block of the audio stream is transmitted, i.e. the channel identifier may be a frequency or a number indicating a frequency. Time offset may be indicative of when the next audio block is transmitted. Audio stream identifier may comprise data indicative of the audio stream, e.g. audio track identification, audio group identification etc. Network identifier may be indicative of a group of transmitter units, e.g. a network identifier may comprise a subnet address. The information comprised within a pilot package may enable the hearing device to receive a subsequent such as the following or next audio block from the transmitter unit transmitting the audio stream in question by configuring the hearing device in accordance with the information. 
     Although particular embodiments have been shown and described, it will be understood that they are not intended to limit the claimed inventions, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the claimed inventions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The claimed inventions are intended to cover alternatives, modifications, and equivalents. 
     LIST OF REFERENCES 
       2 ,  2 ′ hearing device 
       4  receiver/loudspeaker 
       6  processing unit 
       8  antenna 
       10  wireless receiver unit 
       12  microphone 
       14  converted audio signal 
       50  first position for a user of a hearing device 
       50 ′ second position for a user of a hearing device 
       51  first transmitter unit 
       52  second transmitter unit 
       53  third transmitter unit 
       54  fourth transmitter unit 
       55  fifth transmitter unit 
       100  electromagnetic signal 
       101 ,  101 ′,  101 ″ first audio block (A 1 ) 
       102 ,  102 ′,  102 ″ first primary pilot package (P 1,1 ) 
       103 ,  103 ′,  103 ″ first secondary pilot package (P 1,2 ) 
       104  first audio frame 
       106  first primary time offset (Δt 1,1 ) 
       108  first secondary time offset (Δt 1,2 ) 
       111 ,  111 ′,  111 ″ second audio block (A 2 ) 
       112 ,  112 ′,  112 ″ second primary pilot package (P 2,1 ) 
       113 ,  113 ′,  113 ″ second secondary pilot package (P 2,2 ) 
       114  second audio frame 
       116  second primary time offset (Δt 2,1 ) 
       118  second secondary time offset (Δt 2,2 ) 
       121  third audio block (A 3 ) 
       122  third primary pilot package (P 3,1 ) 
       123  third secondary pilot package (P 3, 2 ) 
       124  third audio frame 
       126  third primary time offset (Δt 3, 1 ) 
       128  third secondary time offset (Δt 3, 2 ) 
       212 ,  212 ′ reception of audio blocks 
       214 ,  214 ′ search and receive pilot packages 
       200 ,  200 ′,  200 ″,  200 ′″ audio block 
       202  primary audio package 
       204  secondary audio package 
       206  first primary audio package 
       208  second primary audio package 
       210  first secondary audio package 
       213  second secondary audio package 
       215  first tertiary audio package 
       216  second tertiary audio package 
       400 ,  400 ′ a method for operating a hearing device 
       401  receiving audio stream from selected transmitter unit 
       402  evaluating first parameter 
       404  determining if search criterion is fulfilled 
       406  searching for pilot packages 
       408  evaluating pilot packages 
       410  determining if switch criterion is fulfilled 
       412  selecting new transmitter unit 
       414  determining and selecting optimum transmitter unit