Abstract:
A programmable hearing aid including means for receiving and transmitting data wirelessly from and to a portable module being in proximity to said hearing aid. Said portable module has means for transmitting audio signals, fitting data or special instructions to the hearing aid processor and including means for receiving data transmitted from said hearing aid, including data representing a monitoring of real-time signal processing parameters in the hearing aid. A preferred embodiment of the hearing aid/portable module combination utilizes Miller-coded direct sequence spread-spectrum radio signal transmitters and receivers for transmitting and receiving data between the heading aid and the portable module. This enables remote controlling or monitoring of, transmitting audio to, or programming of a hearing aid without the need for external connectors.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is a continuation-in-part of application No. PCT/DK2005/000026, filed on Jan. 17, 2005, in Denmark and published as WO-A1-2006/074655, the contents of which are incorporated hereinto by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to hearing aids and to methods of operating hearing aids. The invention, more specifically relates to hearing aid systems comprising hearing aids, wireless transceivers and remote controls.  
         [0004]     2. The Prior Art  
         [0005]     Hearing aids capable of being operated by remote controls are known. Remote controls have been used primarily for selecting among different listening programmes stored in the hearing aids and for individual adjustment of the output levels of the hearing aids. The data bandwidth of the communications channels available in existing remote control systems for use in hearing aids is comparatively small and mainly used for simple commands like “adjust output level up one notch” or “change to program  2 ”, these command types taking up but a small number of bits of information. Existing wireless communications channels for the remote control of hearing aids in use today are usually one-way channels, i.e. it is not possible to transmit information from the hearing aid via the communications channel.  
         [0006]     Recent developments in hearing aid signal processors encompass a multitude of different parameters and settings stored in non-volatile memory circuits in the hearing aid, each setting having a specific relation to the performance of the hearing aid, e.g. gain and compression levels in different frequency bands. The values of these parameters and settings will usually be decided and stored in the hearing aid during a fitting session with the user and a fitter. The effect of changing one or more parameters in the hearing aid may, to some extent, be monitored by the fitter through simulation in computer software, and, in some systems, monitored by reading out the parameters from the hearing aid in real-time, as described in the following.  
         [0007]     An industry standard programming interface is the NOAH-Link® interface, manufactured by Madsen Electronics, Taastrup, Denmark. This programming interface comprises a transponder unit worn in a string around a hearing aid user&#39;s neck and connected, during use, to one or two hearing aids via cables and connectors. The transponder unit is capable of transmitting or receiving digital programming signals from a personal computer equipped with a similar transponder and running suitable software for the purpose of programming the hearing aid.  
         [0008]     The transponders in the programming device and the personal computer preferably utilize the industry standard Bluetooth® wireless networking interface for communication, and the personal computer runs a version of the industry standard Compass® hearing aid fitting software. During use, a fitter of hearing aids may use this programming interface to program a prescription frequency response into the hearing aids of the user as decided, based on a hearing test, and according to the user&#39;s preferences. Data regarding the condition, programming, type, and serial number etc. of the hearing aids to be programmed may also be read out by the system for display in the computer. Although the link between the transponder and the personal computer is wireless, the system requires galvanic connection between the transponder of the programming device and the hearing aid circuitry.  
         [0009]     However, connector sockets in hearing aids are complicated in design and manufacture, a potential source of error, and add significantly to the bulk of the hearing aids. The fitting of hearing aids with cables is a significant complication for the fitter.  
         [0010]     U.S. Pat. No. 5,615,229 describes a magnetically coupled short-range communication system for transmitting audio signals between a magnetic transmission element and a magnetic receiving element in a hearing aid. The audio signals are transmitted as a time variant modulated, pulse coded data stream. This is a simplex system, and the magnetic receiving element in the hearing aid appears to be power-intensive, thus putting a great strain on the hearing aid battery.  
         [0011]     WO 98 48526 devises a magnetic-induction time-multiplexed two-way short-range communications system for transmission and reception of signals between a telephone base unit and a portable headset in close proximity to said base unit. It has duplex capabilities and an adequate bandwidth, but the size of the receiver and transmitter in this system prohibits its use in hearing aid systems.  
         [0012]     US 2004/0037442 describes a wireless binaural hearing aid system utilizing direct sequence spread spectrum technology to synchronize operation between individual hearing prostheses. This system enables two hearing aids to communicate wirelessly with each other for the purpose of synchronizing the sampling of the sounds picked up by the hearing aid microphones. A remote control is not involved in this system.  
         [0013]     U.S. Pat. No. 5,390,254 discloses a hearing aid adapted for control by hand-held radio-controlled volume and tone controls and utilizing a radio link to enable enhanced real-time signal processing of the incoming sound via a remote processor. The wireless system utilized in this hearing aid is essentially based on analog processing, and although such a system could be made to function in practice it would be very cumbersome to use due to the size and power consumption of the components involved. However, no practical suggestions as to how such a wireless system might be implemented in practice are devised in U.S. Pat. No. 5,390,254, and no reference to any supporting literature in this respect are made.  
         [0014]     EP 1 445 982 A1 describes an apparatus and method for mutual wireless communication between one or two hearing aids and a remote control unit for the purpose of controlling program selection and adjusting output volume. The communication is controlled by assigning different priorities to the hearing aids and the remote control unit and making each unit transmit in its own time slot according to the assigned priority. Apparently, no means to communicate from the remote control unit by other means than those provided for communication to the hearing aids, are provided.  
         [0015]     EP 1 460 769 A1 discloses an electronic module and a mobile transceiver comprising several receivers for receiving electrical or electromagnetic signals carrying audio signals and a radio transmitter for transmitting radio signals carrying audio signals. The mobile transceiver comprises a prioritising module and a transmitter for transmitting audio received by one of the receivers to a hearing aid comprising a receiver. The actual transmission scheme used by the mobile transceiver is not disclosed, and no means for transmitting signals from the hearing aid to the mobile transceiver is disclosed.  
       SUMMARY OF THE INVENTION  
       [0016]     It is an object of the invention to provide a hearing aid system with wireless communication between one or two hearing aids and a portable device that has sufficient bandwidth for transmitting digital audio to the hearing aids.  
         [0017]     It is a further object of the invention to provide a hearing aid system that has a capability for conveying information from the hearing aids to other external equipment.  
         [0018]     It is still a further object of the invention to provide a hearing aid system with wireless communication between a hearing aid and an external unit that operates with a very low power consumption.  
         [0019]     It is another object of the invention to provide a hearing aid system with wireless communication between two hearing aids at high capacity yet at low power consumption.  
         [0020]     It is another object of the invention to provide a broadband, bidirectional, wireless, digital communications channel to be used for communicating between a remote control and one or two hearing aids during programming.  
         [0021]     It is an additional object of the invention to provide a hearing aid with the capability of wireless transmission at high capacity yet operating at very low power consumption.  
         [0022]     According to the invention, in a first aspect, this object is fulfilled by a hearing aid system comprising a portable module having a first transceiver for transmitting and receiving electromagnetic signals, a Miller encoder for generating data for transmission, a Miller decoder for decoding received signals and means for producing output data based on the decoded signals, at least one hearing aid having a second transceiver for transmitting and receiving electromagnetic signals, a Miller decoder for decoding received signals, means for storing programming information derived from the decoded signals, means for producing an output signal based on the decoded signals, and a Miller encoder for generating data for transmission, the first and the second transceiver being adapted for transmitting and receiving Miller-encoded signals modulated according to a direct sequence spread spectrum (DSSS) scheme.  
         [0023]     This hearing aid system uses a digital wireless transmitter circuit. Such a transmitter circuit is preferably physically small in size, small enough to fit into a completely-in-the-canal (CIC) hearing aid. The power consumption of such a transmitter, when used in a hearing aid, has to be very low. The maximum power consumption of a transmitter of this kind is comparable with that of a standard hearing aid output transducer.  
         [0024]     A system of this kind should have a spatial range of at least 1 meter, a high reliability, preferably with error-correction, being adequate for avoiding deadlock situations or loss of information due to simultaneous transmission or interference from similar systems nearby, a bandwidth wide enough for transmitting (compressed) audio signals and other real-time signals between a hearing aid and a portable device, and an acceptably low power consumption, especially with respect to the transceiver in the hearing aid.  
         [0025]     Using the transmitter circuit, a wireless, digital communications channel is made available from one or more hearing aids to a portable module, all incorporating an embodiment of the transmitter circuit for one or more of the following purposes: transferring audio signals from the hearing aid to the portable module for the purpose of monitoring the signal processing in the hearing aid, transferring real-time parameters from the hearing aid to the portable module for the purpose of logging, or transferring digital real-time signal processing parameters from the hearing aid to the portable module for the purpose of monitoring the signal processor in the hearing aid during use. The portable module may then relay the digital signals from the hearing aids to e.g. a computer or similar means for picking up the relayed signals for analysis and further processing.  
         [0026]     Such a transmitter may preferably be manufactured as an embeddable, monolithic, electronic module for building into a hearing aid acting as a host system for the transmitter. Spread spectrum transmitter of this kind have several benefits over similar devices known in the art. They may be made physically very small, thus fitting well within the confined space of a behind-the-ear or an in-the-ear hearing aid housing, they have a noise-like frequency spectrum footprint, thus causing little or no interference problems, and they consume very little power, making this transmission technology very well suited for hearing aid applications where power consumption and battery life are at a premium.  
         [0027]     A spread-spectrum transmitter is characterised by the fact that it transmits signals, not on a single carrier frequency but instead a range of frequencies. A frequency-hopping spread-spectrum transmitter transmits on a set of discrete frequencies within this range, and a direct-sequence spread-spectrum transmitter transmits on practically every frequency within the range, having a noise-like frequency spectrum footprint. This allows for excellent immunity to noise, and thus makes the power requirements for a desired transmission range significantly smaller.  
         [0028]     A Miller-coding spread-spectrum transceiver has an almost rectangular frequency spectrum distribution footprint as opposed to a regular spread-spectrum transceiver having a frequency spectrum distribution footprint having more rounded ends. This ensures that the frequencies at the ends of the utilized frequency range of the transceiver have a power level that is comparable to the frequencies near the center of the utilized frequency range. A Miller-coding transceiver may be easily implemented in current silicon-chip technology.  
         [0029]     Signals representing e.g. programming data, remote control signals, real-time audio signals, condition readout requests or identity requests may be transmitted from the portable module to the hearing aid, and signals representing e.g. acknowledge signals, condition readouts, real-time signal processing readouts or identity signals may be transmitted from the hearing aid to the portable module.  
         [0030]     According to a preferred embodiment of the hearing aid system the first transceiver comprises a master section comprising an output stage, a frequency reference crystal, and an oscillator controlled by said frequency reference crystal, said master section being electrically detachable from the transceiver circuitry.  
         [0031]     This preferred embodiment enables signals of arbitrary origin to be transmitted from the portable module to one or more hearing aids. This feature may, for instance, be used for controlling the hearing aid with the portable module, programming the hearing aid via the portable module, transferring digital audio signals to the hearing aid from the portable module, transferring programming data to the hearing aid from the portable module, or transmitting data wirelessly from an external source, such as a personal computer or similar appliance, wirelessly to the hearing aid via the portable module.  
         [0032]     The transmitter/receiver combination present in the hearing aid and the portable module renders the hearing aid system capable of mutual, bidirectional communication between the hearing aid and external equipment. This transmitter/receiver combination may preferably be integrated into a single, monolithic unit embeddable into a hearing aid or a portable module. In this application, this combination is hereinafter referred to as a transceiver.  
         [0033]     The transceiver may be put into one of three states or modes of operation, denoted the “sleep” mode, the “receive” mode, and the “transmit” mode, respectively. In the “sleep” mode, the transceiver is idle, i.e. doing nothing but waiting for a signal from the host system ordering it to change its state. In this state, the transceiver circuitry draws very little current from its host system. In the “receive” mode, the receiver of the transceiver is activated for a predetermined period and “listening” for transmissions from another transceiver. Whenever a transmission is detected, the receiver decodes the message as it is received and presents the decoded, received message to the host system as a binary bit stream. In the “transmit” mode, the transmitter of the transceiver is activated by the host system whenever a message is ready for transmission.  
         [0034]     The message to be transmitted, which will be presented by the host system as a binary bit stream, is fed to the signal input of the transceiver and transmitted by the transmitter of the transceiver for the purpose of being received by a receiver located within the transmission range and being capable of recognizing the transmitted message. In a preferred embodiment, the transmitter of the transceiver has an effective range of approximately 1 meter.  
         [0035]     The “receive” mode may be initiated by e.g. a watchdog timer preprogrammed with a predetermined listening period and interval, or triggered by the conclusion of a transmission. If a message—or a part of a message—is received during a “reception” period, the receiver is left open until an acknowledge signal from the host is sent back to the first transceiver, or until a predetermined time period has elapsed. During this period, a message transmitted by a nearby second transceiver may be picked up, detected and decoded by the receiver of the first transceiver and transferred to the hearing aid processor as a binary bit stream. If, however, no message is sent during the predetermined time period, the transceiver reenters the “sleep” mode until another “receive” mode signal is produced by the hearing aid processor.  
         [0036]     Transmission of messages from the hearing aid may be initiated by transmitting a dedicated transmission request message from the transceiver of the portable module during a “reception” period. When the hearing aid receives the dedicated transmission request, the hearing aid processor prepares a message for transmission and transmits it using the “transmit” mode of the transceiver in the hearing aid immediately after the end of the “reception” period. The transmitted messages may comprise, but are not limited to, acknowledge messages, identification messages, parameter readout messages, signal processing status messages, logging messages, and audio streaming block messages. These messages may then be picked up and relayed by the portable module to e.g. a personal computer, a fitting system or an associated remote control unit.  
         [0037]     According to a preferred embodiment of the hearing aid system, the transmitter comprises a master section comprising an output stage, a frequency reference crystal, and an oscillator controlled by said reference crystal, said master section being electrically detachable from the transmitter circuitry. In this embodiment, the transmitter is preferably placed in the portable module.  
         [0038]     The transmitter also comprises a slave section comprising a selectable output stage. The transceiver uses the phase-locked loop for locking its reception frequency onto the transmitting frequency of the oscillator in the transceiver acting as master and for monitoring this frequency after a master transmission has terminated, said reception frequency being used as a transmission frequency at which the transceiver acting as slave sends an acknowledge signal following a transmission from the transceiver acting as master. In this way the transceiver acting as slave does not need a reference crystal oscillator. Since a crystal reference takes up space and consumes power, dispensing with a crystal is a substantial advantage if the transceiver is to be built into, and used in, even the smallest hearing aids, such as a completely-in-the-canal hearing aid.  
         [0039]     The transmitters may initially be in “sleep” mode, and the “reception” mode may be activated at regular intervals in the two hearing aids, respectively, by a watchdog timer constituting part of the hearing aid processor, said hearing aid processor acting as the host system to the transceiver. The “transmit” mode is activated by the hearing aid processor immediately following a reception, and data is then transmitted from the hearing aid to the portable module dependent of the contents of the received and decoded message. The hearing aid processor is capable of transmitting settings, real-time parameters or audio from the hearing aid via the portable module to the computer. If none of these data is required, the hearing aid processor transmits a short acknowledge signal.  
         [0040]     The invention, in a second aspect, provides a method of operating a hearing aid system, comprising the steps of: selecting a hearing aid having input means for receiving input data; receiving input data in the hearing aid; decoding the input data; and Miller encoding output data for transmission, characterised by the steps of: transmitting from the hearing aid electromagnetic signals based on the output data and modulated according to a DSSS scheme; receiving the electromagnetic signals modulated according to a DSSS scheme in a portable module; demodulating and Miller decoding the electromagnetic signals, and; producing output data in the portable module based on the Miller decoded signals.  
         [0041]     This enables the hearing aid to be operated from e.g. a remote control associated with the portable module and having means for recalling stored programs in the hearing aid, adjusting the volume in the hearing aid, or transmitting audio signals to the hearing aid. The audio signals may, for instance, originate from a telecoil loop system, and the telecoil be disposed in the portable module instead of being placed in the hearing aid. Given that the transceiver circuitry takes up less space than the average telecoil, a telecoil functionality may be built into even completely-in-the-canal hearing aids where space considerations until now have been a prohibitive factor.  
         [0042]     The invention, in a third aspect, provides a method of programming a hearing aid comprising the steps of determining a hearing loss to be compensated by a hearing aid; selecting a hearing aid adapted for compensating a hearing loss according to program settings stored in the hearing aid and for receiving and transmitting electromagnetic signals modulated according to a DSSS scheme; using a computer to generate program settings for the hearing aid suitable for compensating the hearing loss; transmitting the program settings from the computer to an portable module; transmitting the program settings from the portable module to the hearing aid by electromagnetic transmission modulated according to a DSSS scheme; receiving the electromagnetic transmission in the hearing aid; decoding and storing the received program settings in the hearing aid; transmitting from the hearing aid electromagnetic signals based on data from the hearing aid modulated according to a DSSS scheme; receiving and decoding in the portable module electromagnetic signals modulated according to a DSSS scheme in order to produce a decoded output; and transmitting data based on the decoded output from the portable module to the computer.  
         [0043]     In this way, hearing aids may be programmed without being galvanically connected to any external hardware, thus eliminating the need for wires and connectors—and thus the problems of wear and corrosion associated with this type of connection.  
         [0044]     Further features and advantages of the hearing aid system according to the invention will become evident from the dependent claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]     The invention will now be described in further detail in conjunction with several embodiments and the accompanying drawings, in which:  
         [0046]      FIG. 1  shows a preferred embodiment of a hearing aid and a portable module,  
         [0047]      FIG. 2  is a block schematic showing a direct sequence spread spectrum transceiver for use in a hearing aid system according to the invention,  
         [0048]      FIG. 3  is a is a graph showing the frequency spectrum of a phase modulated spread spectrum (PM) transceiver,  
         [0049]      FIG. 4  is a graph showing the frequency spectrum of a spread spectrum (FSK) transceiver,  
         [0050]      FIG. 5  is a graph showing the frequency spectrum of a squared Miller-coded direct sequence spread spectrum transceiver (FSK-DSSS) according to the invention,  
         [0051]      FIG. 6  is a timing diagram showing the communication between a master and a slave transceiver,  
         [0052]      FIG. 7  is a prior art hearing aid system with a wireless programming device used in conjunction with two hearing aids and a computer, and  
         [0053]      FIG. 8  is a preferred embodiment with a portable module used as a link device between two hearing aids and a computer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0054]      FIG. 1  shows a hearing aid  1  placed in proximity of a portable module  7  according to an embodiment of the invention. The hearing aid  1  comprises a hearing aid processor  2  connected to a microphone  4  and a first transceiver  6 . The hearing aid processor  2  is further connected to an output transducer  3 . The first transceiver  6  is connected to a first antenna  5 . The portable module  7  comprises a second processor  8  connected to a second transceiver  9 , an auxiliary interface  10 , a second microphone  11 , an input/output interface  12 , a telecoil  13  and a second antenna  14 .  
         [0055]     The second processor  8  in the portable module  7  is capable of communicating wirelessly with the hearing aid  1  via the second transceiver  9 , and capable of communicating wirelessly with a computer or the like (not shown) via the auxiliary interface  10 , which may also be wireless.  
         [0056]     The first antenna  5  and the first transceiver  6  of the hearing aid  1  enables reception of digital data signals representing messages concerning e.g. program or volume control changes while the hearing aid  1  is in use. The available bandwidth of the receiver of the first transceiver  6  is sufficiently wide to convey digitally represented audio signals to the hearing aid processor  2  of the hearing aid  1  for the purpose of acoustic reproduction by the output transducer  3 .  
         [0057]     The second processor  8  of the portable module  7  is capable of generating digital data signals for transmission to the hearing aid  1  regarding e.g. program changes or volume control information. The second transceiver  9  and the second antenna  14  transmit digital data signals to the hearing aid  1 . The audio signals may originate from the auxiliary interface  10 , the microphone  11 , or the telecoil  13 . External audio signals may be input to the portable module  7  via the auxiliary interface  10 , either wireless or by an external audio source (not shown) connected to the auxiliary interface  10 .  
         [0058]      FIG. 2  shows a spread-spectrum digital transceiver  39  according to an embodiment of the invention for use in the hearing aid  1  and the portable module  7  shown in  FIG. 1 . For simplicity, similar transceiver circuits  39  may be used in both the portable module  7  and the hearing aid  1 . The transceiver  39  comprises two main branches for receiving and transmitting signals, respectively. The transceiver  39  is capable of entering either a reception mode or a transmission mode. An input antenna  72  is provided for reception of wireless signals and an output antenna  70  is provided for the transmission of wireless signals. The input antenna  72  is connected to the input of a low noise input amplifier  41  and the output antenna  70  is connected to the output of a power output amplifier pair  68 ,  69 .  
         [0059]     The receiving branch of the transceiver  39  comprises an amplifier and shaper section  41 ,  42 ,  43 ,  44 ,  45 ,  46 , a demodulation and limiting section  47 ,  48 ,  49 ,  50 ,  51 ,  52 ,  53 , and a digital input section  54 ,  55 ,  56 . The amplifier and shaper section comprises a low noise input amplifier  41 , a first preamplifier  42 , a first band pass filter  43 , a second preamplifier  44 , a second band pass filter  45  and a first limiter  46 . The demodulating and limiting section comprise an FM demodulator  47 , a first low pass filter  48 , a second limiter  49 , a phase comparator  50 , a second low pass filter  51 , a third limiter  52  and a first multiplexer  53 . The digital input section comprises a clock data recovery block  54 , a Miller decoder  55  and a first correlator  56 . The output of the digital input section  54 ,  55 ,  56  is connected to the input of a CPU interface  61 .  
         [0060]     The transmitting branch comprises a digital output section  62 ,  63 ,  64 , an oscillator and phase-lock section  57 ,  58 ,  59 ,  60 ,  65 , a crystal-controlled master oscillator section  66 ,  67 , and a power amplifier output section  68 ,  69 ,  70 . The digital output section comprises a correlator  62 , a Miller encoder  63  and a voltage controlled oscillator (VCO) waveform interface block  64 . The output of the CPU interface  61  is connected to the input of the correlator  62 . The oscillator and phase-lock section comprises a voltage controlled oscillator (VCO)  60 , a third low pass filter  59 , a charge pump  58 , a second multiplexer  65  and a phase/frequency detector  57 . The crystal-controlled master oscillator section comprises a master oscillator  66  and a frequency-controlling crystal reference  67 . The power amplifier output section comprises the master power amplifier (MA)  68 , the slave power amplifier (SL)  69  and the second antenna  70 .  
         [0061]     When the transceiver  39  is in reception mode, a wireless spread-spectrum signal may be picked up by the antenna  72  and presented to the input of the low noise amplifier  41 . The signal is amplified by the low noise amplifier  41  and the amplified signal is then presented to the input of the first preamplifier  42  for further amplification and impedance-matching. The signal from the first preamplifier  42  is band-limited by the first band-pass filter  43 , further amplified by the second preamplifier  44 , and further band-limited by the second band-pass filter  45 . The amplified, band limited signal is then limited by the first limiter  46  before being presented to the demodulating and limiting section  47 ,  48 ,  49 ,  50 ,  51 ,  52 ,  53 .  
         [0062]     The signal from the limiter  46  acts as the input signal to the FM demodulator  47 , the phase comparator  50  and the second multiplexer  65 , respectively. In the embodiment shown, the transceiver  39  is capable of transmitting, receiving and processing both Miller-coded FM signals and BPSK signals, and thus two different demodulator means are provided for. Received, Miller-coded FM-signals are demodulated by the FM demodulator  47 , filtered by the first low-pass filter  48 , and limited by the second limiter  49  before being presented to the first multiplexer  53 . Received BPSK signals, on the other hand, are demodulated by the phase comparator  50 , filtered by the second low-pass filter  51 , and limited by the third limiter  52  before being presented to the input of the first multiplexer  53  for conversion into a digital bit stream.  
         [0063]     When the signal leaves the multiplexer  53 , it is considered to be a digital signal or bit stream. This digital bit stream enters the clock data recovery block  54  in the digital input section of the transceiver  39  for preconditioning, and the preconditioned bit stream is output to the Miller decoder  55  for decoding. The Miller-decoded bit stream is then despread in the first correlator  56 , and the decoded, despread bit stream is fed to the CPU interface  61  for the purpose of being interpreted as digital information by a CPU (not shown) connected to the CPU interface  61 .  
         [0064]     When the transceiver  39  is in transmission mode, digital information prepared by the CPU (not shown) is processed by the CPU interface  61  and enters the second correlator  62  as a digital bit stream. In the second correlator  62 , the bit stream is spread, and the spread bit stream leaves the second correlator  62  and enters the Miller encoder  63 . In the Miller encoder  63 , the bit stream is converted into a spread-spectrum, Miller-encoded bit stream which is fed to the input of the VCO waveform interface block  64  for providing a control voltage for modulating the VCO  60  based on the bit stream from the Miller encoder  63 .  
         [0065]     The VCO  60  forms, together with the third low pass filter  59 , the charge pump  58  and the phase/frequency detector  57 , a phase-locked loop which serves two purposes. It locks the frequency of the receiving branch of the transceiver  39  to the carrier frequency of the transmitter for proper reception of wireless signals, and it determines the transmission frequency of the transmitting branch of the transceiver  39 . The output of the VCO  60  is fed to the master power amplifier  68  and the slave power amplifier  69  in the power amplifier output section for final amplification before being transmitted wirelessly by the second antenna  70 .  
         [0066]     The transmitting branch in the transceiver  39  is capable of using one of two different modulation schemes for transmission, squared Miller-coded frequency modulation (MFM) or binary phase shift keying (BPSK). The two types of modulation are used according to the bandwidth demand by the type of information to be sent, and are selected accordingly by the CPU (not shown) in the portable module or the hearing aid, respectively. BPSK modulation is used for information with a modest bandwidth demand such as program change information, volume change information, and identification messages. MFM is used for information requiring a higher bandwidth such as streaming audio, programming information, or real-time parameter readout from the hearing aid.  
         [0067]     In order to keep down costs of manufacture and maintain simplicity, the hearing aid system according to the invention utilizes similar transceivers  39  for both the master transceiver  9  placed in the portable module  7  and the slave transceiver  6  placed in the hearing aid  1  as shown in  FIG. 1 , but not all blocks in the transceiver  39  are used in both master and slave. When the portable module  7 , hereinafter denoted the master, transmits a message, the message is coded and modulated into a wireless signal using one of the two available modulation schemes as described previously, the crystal reference  67  and the master oscillator  66  being used as a frequency reference together with the second multiplexer  65  to control the phase-locked loop section  57 ,  58 ,  59 ,  60  of the transceiver  39  for transmission using the master power amplifier  68  and the second antenna  70 .  
         [0068]     In order to conserve power, the transceiver  39  in the hearing aid, hereinafter denoted the slave, does not rely on a local reference crystal  67  or local master oscillator  66  for frequency control, but instead uses the VCO  60  as a local oscillator to generate the transmitter carrier frequency and lock onto a received carrier frequency while switching off the respective local oscillator  66 ,  67 . This is decided at the time of manufacture, where the master oscillator  66  and the master output amplifier  68  are disconnected electronically from the rest of the transceiver circuitry, and no crystal reference  67  is provided to the unit. The slave transceiver  39  spends the majority of its operative life in “sleep” mode as discussed earlier, where no transmission or reception by the slave transceiver  39  can take place. At regular intervals, the slave transceiver  39  is put in “reception” mode for a predetermined period by a watchdog circuit or by similar means in order to listen for transmissions from a master transceiver  39 .  
         [0069]     When a message is received and decoded by the slave transceiver while it is in “receive” mode, the received signal itself is demodulated and decoded in the way described previously. When the demodulated and decoded message is recognized by the CPU in the slave system, any required actions contained in the message are carried out and an acknowledge message is prepared by the CPU.  
         [0070]     During preparation, the phase-locked loop  65 ,  57 ,  58 ,  59 ,  60  is still locked onto the frequency used at reception of the transmission from the master. When the transmission is terminated, the phase-lock  57 ,  58 ,  59 ,  60  is opened, thereby enabling the VCO  60  to run free at approximately the same frequency. This frequency is now used by the slave transceiver  39  for the transmission of the acknowledge message. This eliminates the need for a bulky and power-consuming crystal reference  67  in the slave. The slave power amplifier  69  then transmits the acknowledge message via the second antenna  70 . When the acknowledge message has been successfully transmitted, the slave transceiver  39  returns to the “sleep” mode.  
         [0071]     As stated previously, the power consumption in the “sleep” mode is very modest, in “reception” mode power consumption is typically about ten times that consumed in “sleep” mode, and in “transmission” mode the power consumption is about twice that in “reception” mode. The transmissions from the slave are usually of relatively short duration and thus do not put any excessive strain on the hearing aid battery supplying the slave transceiver  39 .  
         [0072]     When the master receives the signal from the slave, the reception follows the same principles as described previously. The transceivers  39  in both the master and the slave are capable of mutual communication using one of the two different modulation schemes selectable by the CPU in either unit based on the type of communication desired and the bandwidth required. The types of communication to be exchanged between the master and the slave may incorporate, but is not limited to, identity handshakes, short instructions, acknowledge signals, programming information, settings, digitally represented real-time audio signals, real-time readout of signal processing parameters, and the like.  
         [0073]     When transmitting real-time digital audio, usually some kind of digital compression of the signal is used. The digital representation of the audio signal is collected in a buffer (not shown) of adequate capacity, and the master transceiver  39  then fetches the digital data representing the audio signal in data packets of a size suitable for transmission using the interface  61 . The slave transceiver  39  has a similar buffer (not shown) for collecting the received data packets for decoding and decompression of the data packets. Such a buffer configuration ensures sufficient bandwidth overhead for the purpose of transmitting audio without dropouts or data loss, given that the transceivers are within range of one another. Means for handling retransmission of incompletely received or otherwise erroneously transmitted data packets may be provided in the CPU&#39;s in both the master and in the slave.  
         [0074]      FIG. 3  is a frequency graph showing the power distribution of a spread spectrum signal. The main carrier frequency is shown in  FIG. 3  as a vertical line extending above an area containing the involved frequencies. The spectrum shown in  FIG. 3  has a certain power near the main carrier frequency and tapers out at the ends of the frequency spectrum of the transmitter. Spread spectrum transmission presents several advantages over transmission technologies utilizing fixed frequencies. It is relatively immune to interference from other signals, it has a noise-like frequency spectrum footprint reducing the risk of the transmission disturbing other forms of communication, and the individual frequencies used may be transmitted using a lot less power than fixed-frequency systems because the expected frequencies are known in advance.  
         [0075]     A more preferred spread spectrum technique is to use frequency shift keying (FSK). It utilizes two carriers for transmission, and it has a frequency spectrum resembling the frequency spectrum shown in  FIG. 4 . The FSK power spectrum has a more rectangular shape than the spread spectrum technique shown in  FIG. 3 . The two carrier frequencies, carrier  1  and carrier  2 , may be 20 dB lower in power than the carrier of the PM spread spectrum modulation technique shown in  FIG. 3 , and thus the total bandwidth of the spread spectrum transmitter may be utilized more efficiently and the effective transmission range per Watt may be larger.  
         [0076]     In this application, Miller coding is to be understood as a preferred method of encoding serial, digital data such as data for the purpose of wireless transmission. The bit period, i.e. the duration of one bit, “1” or “0”, respectively, has to be determined in advance. The information is encoded into the digital bit stream as the spacings between signal transitions without regard to polarity. Allowed spacings between transitions in Miller coding are 1, 1.5, and 2 bit periods. An input of “1” gives a transition at the end of a bit period, i.e. one bit period, an input of “0” gives a transition in the middle of a bit period, i.e. 1.5 bit periods, unless a transition took place at the start of the same bit period, in that case nothing is done, i.e. two bit periods. A “0” following a “1” thus never produces a transition during a bit period. A history of the last bit received is used in the decoding, and thus the last bit received is stored in a convenient manner.  
         [0077]     Decoding starts upon reception of a two bit period spacing corresponding to the bit combination “01”. A one bit period spacing corresponds to the bit “0” if the last bit was “0”, and the bit “1” if the last bit was “1”. A 1.5 bit period corresponds to “1” if the last bit was “0”, and the bit combination “00” if the last bit was “1”.  
         [0078]     An even more preferred transmission technique is to use Miller-coding together with FSK direct sequence spread spectrum (FSK-DSSS), and its frequency spectrum is shown in  FIG. 5 . Such a modulation scheme does not utilize a carrier frequency as such, but is primarily defined by its bandwidth and its code sequence. The advantages of the Miller-coded FSK-DSSS technique are the same as those mentioned for FSK-DSSS, but Miller-coded FSK-DSSS transmission is even more efficient. Thus it constitutes an almost ideal choice for a digital transmission system where low power consumption, immunity to noise and interference, and long range per Watt are essential requirements.  
         [0079]      FIG. 6  is a timing diagram showing the relative timings involved during a communication between a master transceiver and one or two slave transceivers. Three timelines show the master transmission timing denoted Master Tx, slave listening timing denoted Slave listen, and slave transmission timing denoted Slave Tx. The timings are denoted T 1 : master transmission period, T 2 : timing gap period between two independent master transmissions, allowing the master to listen for signals from the slave, T 3 : slave wakeup and listening period, T 4 : the time period elapsed between the starting times of two consecutive slave listening periods, T 5 : the slave transmission period, and T 6 : the time elapsed between the start of a master transmitting and the end of the slave transmitting an acknowledge signal.  
         [0080]     Note that T 5  is divided into two parts, denoted R and L, respectively, each allowing a transmission from a respective slave unit. This is a way of allowing the slave units in both a right hearing aid and a left hearing aid sufficient time to respond to the messages from the master. In practice, this is done by adding a delay period to the response time of one of the slave units—in this case the left—and making use of that delay period dependent on the reading of a dedicated bit in the hearing aid EPROM memory that codes the hearing aid as a right or a left hearing aid.  
         [0081]     Note that T 1  may be of variable length according to the type of message sent. T 2  is always greater than T 5  in order to allow for the master to receive and decode an acknowledge from both of the slaves. T 4  minus T 3  is equal to the “hibernate” period when the transceiver in the slave is deactivated, and is always smaller than T 6  in order to ensure that a listening period in the slave overlaps a full transmission period from the master.  
         [0082]     When a transmission from the master is initiated, it sends out a series of start sequences at regular intervals for the duration of the period T 1 . The master then pauses for the duration of T 2  in order to be able to receive a response from a slave. The slaves listen at regular intervals T 3  initiated periodically at intervals T 4 . Whenever a slave recognizes part of a start sequence from a master when listening, the slave prepares to decode the start sequence in order to verify that it is in fact the particular unit addressed by the master. If this is the case, the slave prepares an acknowledge response and waits until the end of T 1  before it transmits the acknowledge response during T 5 . The master receives and decodes the acknowledge response sent by the slave during T 2 , and, if the slave transmission is approved, the master transmits data to the slave.  
         [0083]     The start sequence is usually only used initially to establish communication between a master and a slave for the first time or in case communication is lost due to a transmission error. In case of a first time communication between a master and a slave, unique identification tags, device status, and the like, are exchanged in order for the master and slave to be able to recognize each other more easily and securely during subsequent transmissions. In cases where two hearing aids are employed for binaural alleviation of a hearing loss, the master transmits a start sequence to be picked up by both the left and the right hearing aid.  
         [0084]     During manufacture, each hearing aid is equipped with a bit indicating if it is intended for use in a right ear or a left ear. A hearing aid for the right ear has its slave transmitter set up as described earlier, but a hearing aid for the left ear, on the other hand, has its transmitter set up to await the expiry of a built-in delay equivalent to the duration of a transmission from a slave, before transmitting, in order to avoid transmission collisions with the acknowledge transmission from the hearing aid for the right ear.  
         [0085]     A prior art hearing aid system is shown in  FIG. 7 , where a programming device  30  is connected to two hearing aids  1 R and  1 L via cables  15 R and  15 L. The programming device  30  is communicating wirelessly with a computer  31  through a wireless communications channel  100  for the purpose of programming the hearing aids with prescribed frequency responses, respectively, in order to alleviate a user&#39;s hearing loss.  
         [0086]     During use, the hearing aids  1 R and  1 L are connected to the programming device  30  via the cables  15 R and  15 L. The programming device  30  communicates with the computer  31  via the communications channel  100  in order to convey programming information to the hearing aids  1 R,  1 L. The programming device  30  may receive information regarding the programming from the hearing aids  1 R,  1 L via the communications channel  100 , for instance the locations of the various hearing programmes available to the user, initial sound levels for the individual programs, use of telecoil etc.  
         [0087]      FIG. 8  shows an embodiment of the hearing aid system of the invention, comprising a portable module  7  having a transceiver (not shown), a computer  31 , and a right and a left hearing aid  1 R and  1 L also having transceivers (not shown). The portable module  7  communicates with the computer  31 , running hearing aid fitting software, via a first communications link  100 , and with hearing aids  1 R and  1 L via a second and a third communications link  103  and  104 , respectively. All three communications links  100 ,  103 ,  104 , are bidirectional, wireless communications links.  
         [0088]     During fitting of one hearing aid or a pair of hearing aids, the fitter prepares a prescriptional fitting with the aid of the hearing aid fitting software running on the computer  31 . The prescriptional fitting data are transmitted to the portable module  7  via the first communications link  100 . The portable module  7  transmits the received prescriptional fitting data to the hearing aids  1 R and  1 L via the second and third communications links  103  and  104 , respectively. This preferred embodiment of the hearing aid system of the invention leaves out the wireless programming device  30  of the prior art entirely, having the functionality required for programming the hearing aids  1 R,  1 L built into the portable module  7 . This preferred embodiment of the invention enables programming a prescriptional fitting into one or a pair of hearing aids without the need for any electrical wires or connectors connected between the hearing aids and the programming device.  
         [0089]     A suitable transmission frequency for the hearing aid system according to the invention is about 12 MHz. The bandwidth of the signal makes it possible to execute transmissions with a data rate of up to around 100 kbit/s upstream and 10 kbit/s downstream, thus rendering the system capable of real-time transmission of (compressed) audio signals upstream or continuously variable parameters upstream or downstream. Direct communication between the hearing aids is also possible at a bit rate of up to 100 kbit/s.  
         [0090]     The DSSS coded signals possess an inherently high immunity to noise and interference, and if e.g. eight different spreading codes are used for the DSSS, up to eight similar systems may be used simultaneously within the reliable range of the system of about 1 m. Alternative embodiments may also utilize other frequency bands for transmission, enabling larger bandwidths and thus higher data throughput rates to be used.