PATENT ABSTRACT
In a hearing aid ( 40 ), a direct-digital H-bridge output driver stage ( 1 ) driven by a sigma-delta modulator ( 2 ) is configured to operate in a power-saving three-level output mode or a power-consuming two-level output mode. The three-level output mode of the H-bridge output driver stage ( 1 ) has low power consumption but suffers the disadvantage of emitting capacitive noise potentially interfering with the reception of radio signals in a radio receiver ( 17 ) in the hearing aid ( 40 ). By providing a novel method of selecting the two-level output mode whenever the radio receiver ( 17 ) is receiving signals, and selecting the three-level output mode whenever the radio receiver ( 17 ) is idle, this capacitive interference does not disturb the radio receiver ( 17 ) in the hearing aid ( 40 ). The invention provides a method and a hearing aid.

PATENT DESCRIPTION
RELATED APPLICATIONS 
     The present application is a continuation-in-part of application PCT/EP2011052890, filed on 28 Feb. 2011, in Europe, and published as WO 2012116721 A1. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This application relates to hearing aids. More specifically, it relates to a method for driving a digital output stage of a hearing aid. It also relates to a hearing aid configured for employing the method. 
     In this context, a hearing aid is defined as a small, battery-powered device, comprising a microphone, an audio processor and an acoustic output transducer, configured to be worn in or behind the ear by a hearing-impaired person. By fitting the hearing aid according to a prescription calculated from a measurement of a hearing loss of the user, the hearing aid may amplify certain frequency bands in order to compensate the hearing loss in those frequency bands. In order to provide an accurate and flexible amplification, most modern hearing aids are of the digital variety. 
     Contemporary digital hearing aids incorporate a digital signal processor for processing audio signals from the microphone into electrical signals suitable for driving the acoustic output transducer according to the prescription. In order to save space and improve efficiency, some digital hearing aid processors use a digital output signal to drive the acoustic output transducer directly without performing a digital-to-analog conversion of the output signal. If the digital signal is delivered to the acoustic output transducer directly as a digital bit stream with a sufficiently high frequency, the coil of the acoustic output transducer performs the duty as a low-pass filter, allowing only frequencies below e.g. 15-20 kHz to be reproduced by the acoustic output transducer. The digital output signal is preferably a pulse width modulated signal, a sigma-delta modulated signal, or a combination thereof. 
     The most recent generations of hearing aids also incorporate a tiny radio receiver for the purpose of receiving radio signals intended for the hearing aid circuitry. Typical uses of such a radio receiver are remote controlling volume and program settings from a wireless remote control carried around by the hearing aid user, streaming of audio signals from an external source such as a television set, a compact disc player or a mobile telephone, wireless programming of the hearing aid by a hearing aid fitter according to a prescription, thus eliminating the need for cumbersome wires and fault-prone electrical contacts between the fitting equipment and the hearing aid, or synchronization signals from another hearing aid. The radio receivers employed for this purpose must be physically small, have modest power requirements, and perform reliably within the intended range of the transmitter used. 
     An H-bridge is an electronic circuit for controlling inductive loads such as electric motors or loudspeakers. It operates by controlling the direction of a flow of current through a load connected between the output terminals of the H-bridge by opening and closing a set of electronic switches present in the H-bridge. The switches may preferably be embodied as semiconductor switching elements such as BJT transistors or MOSFET transistors. This operating principle permits a direct digital drive output stage to be employed in order to enable a suitably conditioned digital signal to drive a loudspeaker directly, thus eliminating the need for a dedicated digital-to-analog converter and at the same time reducing the power requirements for the output stage. 
     A sigma-delta modulator is an electronic circuit for converting a signal into a bit stream. The signal to be converted may be digital or analog, and the sigma-delta modulator is typically used in applications where a signal of a high resolution is to be converted into a signal of a lower resolution. In this context, a sigma-delta modulator is used for driving the H-bridge output stage in the hearing aid. 
     The diaphragm of a loudspeaker has a resting or neutral position assumed whenever no current flows through the loudspeaker coil and two extreme positions assumed whenever the maximal allowable current flows in either direction through the loudspeaker. By applying a sufficiently fast-changing bit stream from an H-bridge represented by positive and negative voltage impulses to the loudspeaker terminals, any position between the two extreme diaphragm positions of the loudspeaker may be attained. The higher the number of positive impulses in the bit stream is, the more the loudspeaker diaphragm will move towards the first extreme position, and the higher the number of negative impulses in the bit stream is, the more the loudspeaker diaphragm will move towards the second extreme position. Due to the low-pass filtering effect of the loudspeaker coil, no audible switching noise will emanate from the loudspeaker when driven in this way, provided the switching period of the bit stream is well above the reproduction frequency limit of the loudspeaker. Thus, a digital bit stream may control a loudspeaker directly. 
     2. The Prior Art 
     Digital radio receivers, such as the kind disclosed in WO-A1-09/062500, are especially useful, as they require very little power while maintaining a comparatively high selectivity in the reception. Other types of radio receivers may be employed, but the limited power available in a hearing aid puts a severe restriction on the selectivity, and, as a consequence, the obtainable range and reliability of the radio receiver. A remote control transmitter for use with a hearing aid has a desirable range of approximately one meter while an internal transmitter in another hearing aid has a desirable range of roughly thirty centimeters. The remote control transmitter is capable of issuing various commands to the hearing aid such as program selection and volume control, and also of performing streaming of a digitally represented audio signal to the hearing aid, thus being highly dependent on the existence of a reliable transmission link from the transmitter to the receiver. A pair of hearing aids having a set of transmitters and receivers may have the capability to exchange central parameters relating to the signal processing in the hearing aids apart from program selections and volume settings. This capability is also dependent on the presence of a reliable transmission link between the two hearing aids. 
     From EP-B1-1716723 is known a digital output stage for a hearing aid, said output stage comprising a sigma-delta converter and an H-bridge for driving an acoustic output transducer for a hearing aid. The output stage is denoted a three-level output stage because it is capable of delivering a bit stream consisting of three individual signal levels to the acoustic output transducer. In the following, these levels are denoted “+1”, “−1” and “0”, where “+1” equals the maximum positive voltage across the acoustic output transducer, “−1” equals the maximum negative voltage across the acoustic output transducer, and “0” equals no voltage. This utilizes the fact that a positive voltage pulse makes the diaphragm of the acoustic output transducer move in one direction, and a negative voltage pulse makes the diaphragm of the acoustic output transducer move in the other direction. By delivering a clocked bit stream consisting of “+1”-levels and “−1”-levels interspersed with “0”-levels as voltage pulses to the acoustic output transducer, any position deviation within the confinements of the mechanical suspension of the acoustic output transducer diaphragm may thus be obtained, as the loudspeaker coil acts as an integrator of the voltage pulses. The digital output stage of the prior art generates the “0”-level by applying a “+1”-level and a “−1”-level simultaneously to both terminals of the acoustic output transducer. 
     This way of generating the “0”-level for the acoustic output transducer has the advantages of being very easy to implement, as no extra components are needed to provide the “0”-level, and to save power, as the “0”-level uses no extra current and the provision of three separate levels effectively doubles the possible voltage swing across the acoustic output transducer. However, it also has some inherent drawbacks, which will be explained in greater detail in the following. 
     The “+1”-levels and “−1”-levels both generate differential voltages over the wires and terminals of the acoustic output transducer. This is not the case with the “0”-level. With the “0”-level, both wires carry the same voltage simultaneously, and since this is a voltage rapidly switching between the “+1”-level and “−1”-level it radiates more common mode signal energy to its immediate surroundings. This radiation results in increased crosstalk to nearby circuitry such as telecoils or wireless transmission receiver coils typically present in the hearing aid. Since this crosstalk has frequencies above 1 MHz, it does not possess a problem to a nearby telecoil, which may usually be found in a hearing aid, since a telecoil is configured to convey frequencies below 8-10 kHz. A wireless receiver coil, however, inevitably suffers a very considerable reduction in its signal-to-noise ratio from the capacitive interference signal induced by this crosstalk phenomenon, often to a degree where reliable signal reception becomes impossible. 
     This capacitive interference emanates mainly from electrically exposed parts of the output circuit, primarily the wires connecting the output pads of the electronic circuit chip of the hearing aid to the input terminals of the acoustic output transducer. It is not possible to shorten these wires further for mechanical reasons, but some reduction in the capacitive coupling between these wires and sensitive electronic circuits in the vicinity may be achieved by twisting the wires and keeping them physically close together. 
     The voltage pulses from the H-bridge output stage of the hearing aid are essentially presented to the output transducer as a square wave signal having a frequency of 1-2 MHz, and the resulting switching noise components from the “0”-levels generated in this manner may thus disturb the operation of electronic circuits sensitive to capacitive interference in this frequency range, such as a radio receiver. In cases where the afflicted electronic equipment incorporates a wireless remote control receiver in the hearing aid the problems caused by electromagnetic interference are exceptionally severe, as the effective operating range of the wireless remote control is limited considerably by the capacitive interference emanating from the output stage, excluding the remote control signals from proper reception. 
     WO-A1-03/047309 discloses a digital output driver circuit for driving a loudspeaker for a mobile device such as a hearing aid or a mobile phone. The digital driver circuit comprises an input, a modulator and a three-level H-bridge and is integrated into the loudspeaker enclosure in order to shield the driver circuit from electromagnetic interference and to keep the wires connecting the driver output to the loudspeaker short. The driver circuit further comprises a feedback circuit connected to the loudspeaker for regulating the supply voltage for the driver circuit. 
     An output driver integrated into a loudspeaker, such as described by the teachings of WO-A1-03/047309, is not interchangeable with dynamic standard loudspeakers of the kind used in hearing aids. If, for example, a hearing aid housing and circuitry may be adapted for use with a range of different loudspeakers having different impedance values, e.g. for treating different degrees of hearing loss, a loudspeaker having an integrated output driver would not be well suited for this configuration. Hearing aids configured for being used with receiver-in-the-ear (RITE) loudspeakers would also be impractical to implement using this method. In cases where this type of flexibility is desired, long wires between the output stage terminals of the hearing aid circuit and the terminals of the loudspeaker of the hearing aid are unavoidable. An extra set of long wires for the signal from the loudspeaker to the feedback circuit would also be required by the prior art output driver, which would further increase the capacitive interference noise. 
     The invention, in a second aspect, provides a method of driving an output stage for a hearing aid, said hearing aid having at least one input transducer, an analog-to-digital converter, a digital signal processor, a sigma-delta modulator, a first quantizing block, a second quantizing block, a decoder, an H-bridge output converter, an acoustic output transducer, a timer, a controller and a radio receiver, the radio receiver having an idle mode of operation and a listening mode of operation, said method comprising the steps of generating a driving signal in the sigma-delta modulator based on an output signal from the digital signal processor, processing, in the first quantizing block, using the sigma-delta modulator output signal to generate a first bit stream adapted for defining two discrete levels, processing, in the second quantizing block, using the sigma-delta modulator output signal to generate a second bit stream adapted for defining three discrete levels, the controller using the timer to execute a control sequence for enabling the decoder to select one bit stream among the first and the second bit streams and control the operating mode of the radio receiver, the decoder selecting the first bit stream whenever the radio receiver is in the listening mode, the decoder selecting the second bit stream whenever the radio receiver is in the idle mode, and providing a drive signal for the H-bridge output converter based on the selected bit stream. 
     This method of driving an output stage of the H-bridge variety for a hearing aid achieves that the power efficiency of an output stage operating with three levels is maintained as closely as possible while minimizing the problems caused by the interference also associated with a three-level output stage. 
     By taking the operating mode of the radio receiver into account when selecting the operating mode of the sigma-delta modulator, the H-bridge output converter is driven in a three-level mode whenever the radio receiver is in the idle mode, i.e. when it is not receiving any signals. In this case, power consumption is reduced by driving the H-bridge output converter in a three-level mode. Whenever the radio receiver is in the listening mode, the H-bridge output converter is driven in a two-level mode. In this case, the power consumption is increased somewhat, but the interference associated with driving the H-bridge output converter in the three-level mode is reduced. 
     In a preferred embodiment, the controller enables the radio receiver to enter the listening mode periodically, e.g. twenty times per second, in turn causing the H-bridge output converter to operate in the two-level mode for the duration the radio receiver is in the listening mode. The duration of the listening mode period may be relatively short, e.g. ten milliseconds, unless the radio receiver detects a radio signal within the listening mode period. Otherwise, the radio receiver may reenter the idle mode, in turn causing the H-bridge output converter to operate in the three-level mode again. However, if the radio receiver detects the presence of a radio signal within the listening mode period, reentrance by the radio receiver to the idle mode is suppressed until no radio signal has been detected for the duration of a predetermined period, e.g. a tenth of a second. Then the radio receiver reenters the idle mode, thus forcing the H-bridge output converter to operate in the three-level mode again. 
     The invention, in a second aspect, provides a hearing aid having at least one input transducer, an analog-to-digital converter, a digital signal processor, a sigma-delta modulator, a first quantizing block, a second quantizing block, a decoder, an H-bridge output converter, an acoustic output transducer, a timer, a controller and a radio receiver, the radio receiver having an idle mode of operation and a listening mode of operation, the sigma-delta modulator being adapted for generating a driving signal based on an output signal from the digital signal processor, the first quantizing block being adapted for generating a first bit stream and the second quantizing block being adapted for generating a second bit stream based on the sigma-delta modulator output signal, the first bit stream incorporating two discrete levels and the second bit stream incorporating three discrete levels, the controller being adapted for enabling the decoder to select one bit stream among the first and the second bit streams and for controlling the operating mode of the radio receiver, wherein said controller is configured to make the decoder select the first bit stream whenever the radio receiver is in the listening mode, and make the decoder select the second bit stream whenever the radio receiver is in the idle mode. 
     Additional features will appear from the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained in greater detail with respect to the drawings, where 
         FIG. 1  is a schematic of an H-bridge output stage for a hearing aid according to an embodiment of the invention, 
         FIG. 2  is a table showing possible states of the H-bridge output stage of the hearing aid according to an embodiment of the invention, 
         FIG. 3  is a flowchart of an algorithm for controlling the operating modes according to an embodiment of the invention, 
         FIG. 4  is a graph illustrating the operating sequence of the output stage and the radio receiver of the hearing aid according to an embodiment of the invention, and 
         FIG. 5  is a schematic of a hearing aid having an H-bridge output stage according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The schematic in  FIG. 1  shows an output stage  1  for use with a hearing aid according to the invention. The output stage comprises a sigma-delta modulator  2 , a first comparator  8  constituting a first quantizer, a second quantizer  13  comprising a second comparator  9  and a third comparator  10 , a decoder  11 , an H-bridge  12 , a controller  16 , a control wire  14 , a controlled switch  15 , a radio receiver  17 , an antenna  18  and an acoustic output transducer  19 . The sigma-delta modulator  2  comprises a difference node  3 , a first summing node  4 , a second summing node  5 , a first unit delay block  6  and a second unit delay block  7 . The H-bridge comprises a first transistor  20 , a second transistor  21 , a third transistor  22 , and a fourth transistor  23 . Also shown in  FIG. 1  is an output terminal from a digital signal processor DSP of the hearing aid. 
     The output terminal of the digital signal processor DSP is connected to the input of the sigma-delta converter  2 . The output terminal of the digital signal processor DSP is connected to a first input of the difference node  3  of the sigma-delta converter  2 , and a feedback loop from the output of the sigma-delta converter  2  is connected to a second input of the difference node  3 . The output of the difference node  3  is connected to a first input of the first summing node  4 , and the output of the first unit delay block  6  is connected to a second input of the first summing node  4 . The output of the first summing node  4  is split between an input of the first unit delay block  6  and a first input of the second summing node  5 . An output of the second unit delay block  7  is connected to a second input of the second summing node  5 , and the output of the second summing node  5  is split between an input of the second unit delay block  7 , the feedback loop feeding the difference node  3 , and the positive inputs of the first comparator  8 , the second comparator  9  and the third comparator  10 , respectively. 
     The output of the sigma-delta modulator  2  is connected to the positive input terminals of the first comparator  8 , the second comparator  9 , and the third comparator  10 , respectively. The negative input terminal of the first comparator  8  is connected to logical LOW, the negative input terminal of the second comparator  9  is connected to the logical level X, and the negative input terminal of the third comparator  10  is connected to the logical level Y. The output of the first quantizer  8  is connected to a first input of the decoder  11 , and the outputs of the second quantizer  13  are connected to a second and a third input of the decoder  11 . Based on the output signal from the sigma-delta modulator  2 , the first quantizer  8  is capable of generating two different quantization levels and the second quantizer  13  is capable of generating three different quantization levels. 
     A first output of the decoder  11  is connected to the first transistor  20  of the H-bridge  12 , a second output of the decoder  11  is connected to the second transistor  21  of the H-bridge  12 , a third output of the decoder  11  is connected to the third transistor  22  of the H-bridge  12 , and a fourth output of the decoder  11  is connected to the fourth transistor  23  of the H-bridge  12 . The source terminals of the first transistor  20  and the third transistor  22  are connected to V ss . The drain terminal of the first transistor  20  and the source terminal of the second transistor  21  are connected to a first terminal of the acoustic output transducer  19 . The drain terminal of the third transistor  22  and the source terminal of the fourth transistor  23  are connected to a second terminal of the acoustic output transducer  19 , and the drain terminals of the second transistor  21  and the fourth transistor  23  are connected to V dd . 
     The control wire  14  of the controller  16  is connected to the control input of the controlled switch  15  and to a control input of the decoder  11 , respectively. The controlled switch  15  connects an output of the radio receiver  17  to an input of the controller  16 , disabling this connection whenever the controlled switch  15  is open. A signaling wire connects the radio receiver  17  to the controller  16  for providing data based on radio signals picked up by the antenna  18  and demodulated by the radio receiver  17  to the controller  16 . 
     When in use, the digital signal processor DSP provides a bit stream representing an audio signal to the input of the sigma-delta modulator  2 . The bit stream is conditioned by the sigma-delta modulator  2  in order to suit the inputs of the first comparator  8 , the second comparator  9  and the third comparator  10 , respectively. The first comparator  8  acts as a first two-level quantizer on the output signal from the sigma-delta modulator  2 , and the second comparator  9  and the third comparator  10  in combination act as a second three-level quantizer  13  on the output signal from the sigma-delta modulator  2 . 
     The first comparator  8  outputs a logical LOW level whenever the level of the output signal from the sigma-delta modulator  2  is below a first, predetermined limit and a logical HIGH level whenever the signal is above said first, predetermined limit. The second comparator  9  outputs a logical LOW level whenever the input signal is below the limit X and a logical HIGH level whenever the input signal is above the limit X. The third comparator  10  outputs a logical LOW level whenever the input signal is below the limit Y and a logical HIGH level whenever the input signal is above the limit Y. 
     Together, the second comparator  9  and the third comparator  10  may thus generate four possible levels for the decoder  11 . However, only three of these levels are utilized in the decoder  11 , as the condition where the output of the second comparator  9  is logical HIGH and the output of the third comparator  10  is logical LOW is treated equally to the condition where the output of the second comparator  9  is logical LOW and the output of the third comparator  10  is logical HIGH. The three conditions may be interpreted by the decoder  11  as e.g. the symbol “−1” for input levels resulting in both comparator outputs being logical LOW, the symbol “0” for input levels resulting in the two comparator outputs being mutually different, i.e. one comparator output is logical LOW while the other comparator output is logical HIGH, and the symbol “+1” for input levels resulting in both comparator outputs being logical HIGH. In this way, the first quantizer  8  effectively generates two discrete levels from the input signal from the sigma-delta modulator  2 , and the second quantizer  13  effectively generates three discrete levels from the input signal from the sigma-delta modulator  2 . 
     The decoder  11  is capable of selecting either the two-level output from the first quantizer  8  or the three-level output from the second quantizer  13  as the input signal to be decoded. The decoder  11 , together with the H-bridge  12 , is capable of driving the loudspeaker  19  in a two-level mode of operation whenever the output signal from the first quantizer  8  is selected as the input signal, and in a three-level mode of operation whenever the output signal from the second quantizer  13  is selected as the input signal. 
     The decision about which output to use as an input of the decoder  11  is determined by the state of the control wire  14  of the controller  16 . The control wire  14  may be in an asserted state or in an unasserted state, respectively. Whenever the control wire  14  is in the asserted state, the decoder  11  uses the output signal from the two-level output of the first quantizer  8  as its input signal. Asserting the control wire  14  also closes the switch  15 , thereby enabling the radio receiver  17  to receive radio signals via the antenna  18 . Whenever the radio receiver  17  is enabled to receive radio signals, information about the presence of a radio signal is conveyed to the controller  16  through a separate wire (not shown). Whenever the control wire  14  is in the unasserted state, the decoder  11  uses the output signal from the three-level output of the second quantizer  13  as its input signal. Unasserting the control wire  14  also opens the switch  15 , thereby disabling the radio receiver  17  from receiving radio signals. 
     Whenever the decoder  11  receives a “−1”-symbol for decoding, it turns on the second transistor  21  and the third transistor  22 , respectively, of the H-bridge  12 . The second transistor  21  connects the upper terminal of the acoustic output transducer  19  to the positive voltage V dd , and the third transistor  22  connects the lower terminal of the acoustic output transducer to the negative voltage V ss , and the loudspeaker membrane moves inwards. 
     Whenever the decoder  11  receives a “+1”-symbol for decoding, it turns on the first transistor  20  and the fourth transistor  23 , respectively, of the H-bridge  12 . The first transistor  20  connects the upper terminal of the acoustic output transducer  19  to the negative voltage V ss , and the fourth transistor  23  connects the lower terminal of the acoustic output transducer to the positive voltage V dd , and the loudspeaker membrane moves outwards. 
     Whenever the decoder  11  receives a “0”-symbol for decoding, it turns on the second transistor  21  and the fourth transistor  23 , respectively, of the H-bridge  12 . Both the second transistor  21  and the third transistor  22  then connect the upper terminal and the lower terminal of the acoustic output transducer  19  to the negative voltage V ss , and the loudspeaker membrane moves towards its resting position. 
     The controller  16  coordinates the quantization resolution of the output signal from the sigma-delta modulator  2  with the operation of the radio receiver  17  in such a way that the radio receiver  17  is disabled whenever the decoder  11  is using the three-level input for controlling the H-bridge  12 , and in such a way that the radio receiver  17  is enabled whenever the decoder  11  is using the two-level input for controlling the H-bridge  12 . 
     The table shown in  FIG. 2  illustrates the possible states of the connecting wires of an acoustic output transducer similar to the acoustic output transducer  19  in  FIG. 1  when connected to the H-bridge output stage of the hearing aid according to an embodiment of the invention. Beside the table is sketched an acoustic output transducer having connecting terminals A and B. In the configuration of a preferred embodiment of the hearing aid according to the invention, a sigma-delta converter together with a first quantizer, a second quantizer and a decoder may generate either two or three different output symbols intended for the H-bridge output stage of the hearing aid. 
     When the symbol “−1” is generated, the H-bridge output stage connects the terminal A of the acoustic output transducer to a negative voltage, preferably the negative battery voltage, denoted V dd , and the terminal B of the acoustic output transducer to a positive voltage, preferably the positive battery voltage, denoted V ss . This induces an electromotive force in the transducer coil of the acoustic output transducer in the direction from terminal B to terminal A, and a transducer membrane mechanically connected to the transducer coil will thus move in one direction, say, inwards. 
     When the symbol “+1” is generated, the H-bridge output stage connects the terminal A of the acoustic output transducer to the positive battery voltage V ss , and the terminal B of the acoustic output transducer to the negative battery voltage V dd . This induces an electromotive force in the transducer coil of the acoustic output transducer in the opposite direction, i.e. from terminal A to terminal B, and the transducer membrane will thus move in the opposite direction, say, outwards. 
     When the symbol “0” is generated, the H-bridge output stage connects both the terminal A and the terminal B of the acoustic output transducer to the negative battery voltage V dd . No electromotive force is induced in the transducer coil of the acoustic output transducer in this case, and the transducer membrane will thus move towards its resting position. 
     When the H-bridge is put into two-level mode, the symbol “0” is not generated. The switching between two-level mode and three-level mode is beneficially performed in the decoder. By changing the quantization resolution of the output signal from the sigma-delta modulator from two levels to three levels, or vice versa, in the decoder, the feedback history of the sigma-delta modulator is preserved in its entirety. As shown in  FIG. 1 , this may be performed by the decoder having both the two-level and the three-level quantization resolution available at all times, and selecting the appropriate quantization resolution for driving the output for the acoustic output transducer of the hearing aid as necessary. The fact that the feedback history of the sigma-delta modulator is preserved in its entirety implies that switching between the two-level mode and the three-level mode of the sigma-delta modulator is performed seamlessly with regard to the output signal to the acoustic output transducer without any audible artifacts. 
     An easy way of providing both a two-level modulation and a three-level modulation of the bit stream could be to employ two separate sigma-delta modulators. If a two-level sigma-delta modulator in parallel with a three-level sigma-delta modulator were used instead of a single sigma-delta modulator having both two-level and three-level capability, the feedback history of the sigma-delta modulator would be lost every time a transition from the two-level mode to the three-level mode, or vice versa, were made. This configuration would inevitably introduce undesirable, spurious transients into the output signal. By introducing a single sigma-delta modulator capable of selectively producing both a two-level and a three-level modulation of the output bit stream, the feedback history of the output stage is preserved when switching between different quantizing resolutions. 
     In  FIG. 3  is shown a flowchart illustrating a preferred control algorithm for a radio receiver and an H-bridge output stage of the hearing aid according to the invention. The timing values used by the algorithm in  FIG. 3  are calculated and detected by an external subroutine, and are thus not shown. Only the timing flags are passed implicitly to the algorithm shown in  FIG. 3  based on the timing values encountered by the system. The algorithm, initiating in step  301 , continues immediately to step  302 , where the radio receiver is put into an idle mode. The algorithm sets the H-bridge output stage in a three-level mode in step  303  and enters a loop in step  304 . In step  304 , the algorithm determines if fifty milliseconds have elapsed since the radio receiver was last put into the idle mode. If this is not the case, the algorithm loops back into step  304  until the fifty milliseconds have elapsed, and continues to step  305 , where the radio receiver is put into a listening mode. The algorithm then continues unconditionally to step  306 , where the H-bridge output stage is put into a two-level mode. 
     The algorithm continues in step  307 , where an indicator in the radio receiver informs the algorithm if a radio signal is present. If this is not the case, the algorithm branches out into a test, carried out in step  308 , to determine if ten milliseconds have elapsed since the radio receiver were put into the listening mode without detecting a signal. If ten milliseconds have not yet elapsed, the algorithm loops back into step  307  in order to test if a radio signal has been picked up yet by the radio receiver. Otherwise, if ten milliseconds have elapsed without the radio receiver detecting the presence of a radio signal, the algorithm loops back into step  302 , where the radio receiver is put back into the idle mode, and continues unconditionally into step  303 , where the H-bridge is put back into the three-level mode and the procedure of the algorithm is repeated indefinitely. 
     If, however, a radio signal is indeed detected by the radio receiver while the algorithm is processing step  307 , the algorithm instead continues into step  309 , where a subroutine (not shown) is called for carrying out the process of decoding the data bits received by the radio receiver of the hearing aid. The algorithm continues into step  310 , where a test is carried out in order to determine if one hundred milliseconds have elapsed since a signal was detected by the radio receiver. If this is not the case, the algorithm loops back into step  309  and continues the process of decoding the data bits received by the radio receiver. Otherwise, the algorithm continues into step  311 , where a test is carried out in order to determine if a radio signal is still present. If this is the case, the algorithm loops back into step  309  and continues the decoding process. If this is not the case, the algorithm instead loops back into step  302 , where the radio receiver is put back into the idle mode, and continues to step  303 , where the H-bridge is put back into the three-level mode. 
     The essence of the functionality of the algorithm shown in  FIG. 3  is as follows: The radio receiver of the hearing aid is put into the idle mode and the H-bridge output stage of the hearing aid is put into the three-level mode for fifty milliseconds. Then the radio receiver listens for the presence of a radio signal while the H-bridge output stage is put into the two-level mode in order to minimize interference. If no signal has been detected by the radio receiver for a period of ten milliseconds, the radio receiver is put back into the idle mode and the H-bridge output stage is put back into the three-level mode in order to conserve power. However, if the radio receiver of the hearing aid detects the presence of a radio signal, reception and decoding of the received radio signal is commenced. Every 0.1 seconds a test is performed in order to determine if a radio signal is still present. If this is the case, the reception and decoding of the received radio signal continues. If a radio signal is no longer deemed to be present, the radio receiver is once again put back into the idle mode and the H-bridge output stage is put back into the three-level mode in order to conserve power. 
       FIG. 4  shows an exemplified set of graphs illustrating the interoperational characteristics between an output stage and a radio receiver in a hearing aid according to the invention. The upper graph in  FIG. 4  illustrates the state of the control wire  14  of the controller  16  as shown in  FIG. 1 , the middle graph in  FIG. 4  shows the output signal of the H-bridge  12  seen across the input terminals of the acoustic output transducer  19  in  FIG. 1 , and the lower graph in  FIG. 4  shows the activity of the receiver  17  in  FIG. 1  when controlled by the controllable switch  15  controlled by the control wire  14  of the controller  16  in  FIG. 1 . All three graphs are assumed to be synchronous. 
     The upper graph in  FIG. 4  illustrates that the control wire  14  of  FIG. 1  is asserted for short periods of time, thus enabling the radio receiver  17  in  FIG. 1  and forcing the H-bridge output stage to operate in the two-level mode. Whenever the control wire is unasserted, the radio receiver is disabled and the H-bridge output stage is operated in the three-level mode. This is illustrated by the middle graph in  FIG. 4 , where an arbitrary output signal from the H-bridge output stage is exhibiting three-level operation when the control wire is unasserted and two-level operation when the control wire is asserted. The lower graph in  FIG. 4  illustrates the operation of the receiver  17  in  FIG. 1 . 
     The operation of the output stage of the hearing aid according to the invention, as illustrated by the graphs in  FIG. 4 , will now be explained in further detail with reference to the elements shown in  FIG. 1 . Below the lower graph in  FIG. 4  is suggested a timeline with eight time instants, labeled from T 1  to T 8 . At the instant 0, the control wire  14  is unasserted, the radio receiver  17  is inactive, and the H-bridge output stage  1  is operating in the three-level output mode, delivering the three-level digital output signal directly to the acoustic output transducer  19  of  FIG. 1 . 
     At the instant T 1 , the control wire  14  is asserted, and the H-bridge output stage  1  changes its operation from the three-level output mode to the two-level output mode. At the same time, the radio receiver  17  is activated. This condition persists until the instant T 2 , approximately ten milliseconds later, where the control wire  14  is unasserted, the radio receiver  17  is inactivated, and the H-bridge output stage  1  is set to change its operation back into the three-level output mode. From the instant T 2  until the instant T 3 , approximately fifty milliseconds later, the control wire  14  is unasserted, leaving the H-bridge in the three-level output mode and the radio receiver  17  inactive. In this case, a radio signal R 0 , superimposed onto the lower graph of  FIG. 4  in a dotted line, incidentally occurs between the instant T 2  and the instant T 3 . Because the radio receiver  17  is in its inactive mode, the radio signal R o  is not picked up by the radio receiver  17  of the hearing aid. 
     At the instant T 3 , the radio receiver  17  is activated again by asserting the control wire  14 , and the H-bridge output stage  1  changes its operation from the three-level output mode to the two-level output mode. Since no radio signal is detected by the radio receiver  17  between the instant T 3  and the instant T 4 , the control wire  14  is unasserted at the instant T 4 , approximately ten milliseconds later, when the radio receiver  17  is deactivated again, and the H-bridge output stage  1  has its operation changed back into the three-level output mode. 
     Between the instant T 4  and the instant T 5 , another radio signal R 1 , superimposed onto the lower graph of  FIG. 4  in a thin, solid line, occurs, but since it is still present at T 5 , it is detected by the radio receiver  17 . The detection of the radio signal R 1  by the radio receiver  17  makes the controller  16  keep the control wire  14  asserted, thus keeping the radio receiver  17  active and the H-bridge output stage  1  operating the two-level output mode. Within the time period between the instant T 5  and the instant T 6 , a third radio signal R 2 , superimposed onto the lower graph of  FIG. 4  in a thin, solid line, is detected and decoded by the radio receiver  17 . The radio receiver  17  keeps a reception flag asserted during reception of the radio signal R 2 , and thus prevents the return of the radio receiver  17  to its inactive state. This, in turn, also delays the return of the H-bridge output stage  1  to the two-level output mode. 
     When the radio signal R 2  ceases, a timing function delays the unassertion of the control wire  14  for a predetermined period of time. As no other radio signal is detected before the instant T 6 , the control wire  14  is unasserted again at T 6 . Hereby the radio receiver  17  is inactivated, and the H-bridge output stage  1  changes its operation back to the three-level output mode. At the instant T 7 , after approximately another fifty milliseconds, the radio receiver  17  is activated again by asserting the control wire  14 , and the H-bridge output stage  1  changes its operation from the three-level output mode to the two-level output mode. The control wire  14  is unasserted again at the instant T 8 , approximately ten milliseconds later, whereby the radio receiver  17  is deactivated again, and the H-bridge output stage  1  changes its operation back into the three-level output mode. 
     In order to demonstrate the operating principles of the H-bridge output stage according to an embodiment of the invention, the three bursts of radio transmission illustrated by the lower graph in  FIG. 4  are shown as being rather short. This is done to illustrate, in as brief a way as possible, the fact that the radio receiver  17  is only capable of receiving radio signals when it is activated by the controller  16  of the hearing aid, and that the radio receiver  17  has the ability to delay a pending inactivation whenever a radio signal is encountered. In a practical example, radio transmissions intended for the hearing aid will be significantly longer, preferably spanning a considerably longer period of time than the sixty milliseconds shown elapsing between two activations of the radio receiver in the example. 
     In  FIG. 5  is shown a schematic of a hearing aid  40  incorporating an H-bridge output stage according to an embodiment of the invention. The hearing aid  40  comprises an acoustic input transducer  30 , an analog-to-digital converter  31 , a digital signal processor  32 , a sigma-delta modulator  2 , a first quantizer block  8 , a second quantizer block  13 , a decoder  11 , an H-bridge  12 , a controller  16 , a control wire  14 , a controllable switch  15 , a timer  33 , an acoustic output transducer  19 , and a radio receiver  17  having an antenna  18 . In  FIG. 5  is also shown a radio transmitter  34  having an antenna  35 . The sigma-delta converter  2 , the decoder  11 , the controller  16 , the H-bridge  12 , the acoustic output transducer  19  and the radio receiver  17  are considered to be similar to the corresponding parts of the system shown in  FIG. 1 . 
     When in use, the microphone  30  of the hearing aid  40  picks up acoustic signals and converts them into electrical signals and feeds the electrical signals to an input of the analog-to-digital converter  31 . The digital output signal from the analog-to-digital converter  31  is used as the input for the digital signal processor  32 , where the main part of the signal processing, e.g. filtering, compression, prescription gain calculation etc. takes place. The output signal from the digital signal processor  32  is a digital signal, which is fed to the input of the sigma-delta modulator  2 . 
     The output signal from the sigma-delta modulator  2 , which may be considered to be a digital bit stream, is split into two branches, one branch going to the first quantizing block  8 , and the second branch going to the second quantizing block  13 . The output signals from the first and second quantization blocks  8 ,  13 , are presented as input signals to the decoder  11 . The decoder  11  generates a set of control signals for the H-bridge  12 . The output terminals of the H-bridge  12  are connected to the input terminals of the acoustic output transducer  19 , and the H-bridge  12  generates a digital output signal for the acoustic output transducer  19 . 
     The output signal from the first quantization block  8  is a two-level bit stream intended for driving the H-bridge  12  in a two-level mode via the decoder  11 . The output signal from the second quantization block  13  is a three-level bit stream intended for driving the H-bridge  12  in a three-level mode via the decoder  11 . The decoder  11  is thus capable of selecting either the output signal from the first quantization block  8  or the output signal from the second quantization block  13  as the input signal for generating the set of control signals for the H-bridge  12 . 
     When the two-level output signal from the first quantization block  8  is used, the decoder  11  is said to be operating in a two-level mode, and when the three-level output signal from the second quantization block  13  is used, the decoder  11  is said to be operating in a three-level mode. The radio receiver  17  is capable of operating in an idle mode, wherein radio signal reception is suppressed, and in an active mode, wherein radio signal reception is enabled. 
     The controller  16  determines which mode the decoder  11  is supposed to be using in a given situation in order to generate the set of control signals for the H-bridge  12 . For this purpose, the controller  16  utilizes information from the timer  33  and the radio receiver  17 , respectively, to determine what the mode of operation for the decoder  11  should be. The timer  33  generates a timing sequence similar to the timing sequence shown in  FIG. 4 . This timing sequence is used by the controller  16  to control the operation of the decoder  11  and the radio receiver  17  of the hearing aid  40 . During a first phase of the timing sequence, the timer  33  sends a signal to the controller  16  at regular intervals in order to make it change the operation of the radio receiver  17  from the idle mode to the active mode and force the decoder  11  to select the two-level bit stream from the first quantizer block  8  for the H-bridge  12  in order for it to operate in the two-level mode. 
     When the controller  16  determines that the radio receiver  17  should change its mode of operation from the idle mode to the active mode based on the signal from the timer  33 , the controller  16  asserts the control wire  14  in order to engage the controlled switch  15  for activating the radio receiver  17 . Simultaneously, the controller  16  forces the decoder  11 , via the control wire  14 , to select the two-level bit stream originating from the first quantizing block  8  for controlling the H-bridge  12 . The radio receiver  17  is now in the active mode, and the H-bridge  12  is producing a two-level bit stream for the acoustic output transducer  19 . 
     Unless the radio transmitter  34  transmits a radio signal which is picked up by the radio receiver  17  while it is in the active mode, the controller  16  waits for a signal from the timer  33  and unasserts the control wire  14  upon detecting the signal from the timer  33 , thus disengaging the controlled switch  15 , in turn forcing the radio receiver  17  back into the idle mode, and makes the decoder  11  select the three-level bit stream from the second quantizing block  13  for controlling the H-bridge  12 . If, however, the radio transmitter  34  transmits a radio signal, and this radio signal is detected by the radio receiver  17 , a signal is sent from the radio receiver  17  to the controller  16 , informing the controller  16  to postpone signals from the timer  33  until the radio receiver  17  informs the controller  16  that it has finished receiving and decoding the radio signal. 
     The timer  33  now enters a second phase in the timing sequence, wherein the controller  16  regularly checks the status of the radio receiver  17  in order to determine that the radio receiver  17  is still receiving and decoding a radio signal. If this is the case, the controller maintains status quo, i.e. it keeps the H-bridge  12  operating in the two-level mode and keeps the radio receiver  17  in the active mode. When the radio transmitter  34  ends a transmission, the radio receiver  17  stops detecting a radio signal, and thus ends the decoding process. Upon terminating the decoding process, the radio receiver  17  sends a signal to the controller  16  in order to convey the information that reception of the radio signal has ended. Upon getting this piece of information, the controller  16  then waits for a signal from the timer  33  before deactivating the radio receiver  17  and forcing the H-bridge  12  into the three-level mode, producing a three-level bit stream to the acoustic output transducer  19 . 
     In a preferred embodiment, the first phase of the timing sequence of the timer  33 , as described in the foregoing, is considerably shorter than the second phase. This relationship between the two phases of the timing sequence is preferred because it allows the H-bridge  12  to operate for as long as possible in the power-saving three-level mode of operation during the first phase of the timing sequence, and prevents premature reentrance of the H-bridge  12  into the three-level mode of operation while the radio receiver  17  receives and decodes a radio signal, thus reducing the risk of the reception of the radio signal being corrupted by capacitive interference from the H-bridge  12 .