Abstract:
In a digital hearing aid device with an output amplifier having a sigma-delta modulator, the output transducer has a high current consumption even when no output signal perceivable as an acoustic output signal is generated. A linear digital filtering in connection with the sigma-delta modulation reduces the number of the high-frequency edges in the (typically) pulse-density-modulated output signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention concerns a hearing aid device of the type having an input transducer for acquisition of an input signal and conversion into an electrical signal, an A/D converter for conversion of the electrical signal into a digital signal, a signal processing unit for processing and amplification of the digital signal, a sigma-delta modulator for generation of at least one output bit stream, an output stage for generation of an electrical output signal and an output transducer for conversion of the electrical output signal into an output signal perceivable by a user.  
         [0003]     2. Description of the Prior Art  
         [0004]     In modern digital audio apparatuses, a component known as a sigma-delta modulator is used for conversion of digital signals into analog signals to activate a speaker or earphone. These sigma-delta converters transform the digital signal representation into a bit stream, which directly represents the acoustic output signal. Since the individual output bits of this output signal are output with a high rate, analog filtering typically must ensue for limitation to the required audio frequency range in order to keep the higher-frequency interference signals away from the speaker.  
         [0005]     The speaker used in hearing aid devices, which speaker is typically called as earpieces and normally operates according the magnetic principle. Hearing device earpieces inherently exhibit a strong low-pass characteristic. In hearing aid devices with a sigma-delta modulator, the analog filtering of the output signal can be omitted. Due to the high system clock frequency of a sigma-delta modulator, its energy consumption is, however, quite high, which is disadvantageous for use in hearing aid devices. The argument against the selection of a lower (and thus more advantageous in terms of energy) system clock frequency is that the system noise would increase with such a lower frequency.  
         [0006]     A hearing device is known from United States Patent Application Publication No. 2003/0081803 A1 in which a sigma-delta modulator generates an output bit stream with the three states +1, 0, −1. This bit stream is supplied to an output stage in the form of an H-bridge that delivers an output signal for direct activation of the earpiece. A circuit that initially, periodically converts the sigma-delta-modulated data stream from each value different from 0 to the 0-state is located between the sigma-delta modulator and the H-bridge. Overall energy is thereby taken from the output signal, so the system noise is also reduced. Disadvantages of this technique are that the non-linearities are generated as well as signal deformation.  
         [0007]     A hearing aid device with a microphone, a transfer characteristic component for signal processing, and an output amplifier (which is essentially formed of a sigma-delta converter, a clock pulse generator and a low-pass filter) is known from EP 0 793 897 B1.  
         [0008]     A sigma-delta modulator to which an FIR filter is connected is known from EP 0 815 651 B1.  
       SUMMARY OF THE INVENTION  
       [0009]     An object of the present invention is to provide a hearing aid device with an output amplifier that has a sigma-delta modulator via which the energy consumption of the hearing aid device as well as the system noise is reduced.  
         [0010]     This object is achieved in an hearing aid device in accordance with the invention having an input transducer for acquisition of an input signal and conversion into an electrical signal, an A/D converter for conversion of the electrical signal into a digital signal, a signal processing unit for processing and amplification of the digital signal, a sigma-delta modulator for generation of at least one output bit stream, an output stage for generation of an electrical output signal; an output transducer for conversion of the electrical output signal into an output signal that can be perceived by a user, and a linear digital filter connected between the sigma-delta modulator and the output stage, such that three different voltage states can be generated at the output of the linear digital filter and at the output of the output stage.  
         [0011]     The linear digital filter according to the invention is a linear system in the mathematical sense that converts an input sequence into an output sequence. The linear digital filter used in connection with the invention is also frequency-selective, such that specific frequency components are passed through and other frequency components are suppressed. The “Return to Zero” circuit known from the cited publication US 2003/0081803 A1 is neither linear nor frequency-selective. The circuit used therein, moreover, is not a digital filter.  
         [0012]     The invention offers the advantage that the energy consumption of the total system can be reduced by the linear digital filter. In particular the number of the high-frequency edges in the typical pulse-density-modulated output signal is reduced. The system noise also can be reduced at least in a specific frequency range by the frequency-selectivity of the filter. Moreover, interference signals caused by the sigma-delta modulator can be frequency-selectively reduced by the linear digital filter.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  shows the signal path in a hearing aid device with a linear digital filter according to the invention.  
         [0014]      FIG. 2  shows a first embodiment of a linear digital filter used in connection with the invention.  
         [0015]      FIG. 3  shows a second embodiment of a linear digital filter used in connection with the invention.  
         [0016]      FIG. 4  shows a third embodiment of a linear digital filter used in connection with the invention.  
         [0017]      FIG. 5  shows a linear digital filter according to the invention that includes both a first filter and second filter. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]      FIG. 1  shows the signal path of a hearing aid device between an input transducer and an output transducer. An input signal is acquired by the input transducer and converted into an electrical signal. At least one microphone  1  that acquires an acoustic input signal typically serves as the input transducer. Modern hearing aid devices frequently have a microphone system with a number of microphones in order to achieve a reception dependent on the incident direction of acoustic signals (a directional characteristic). The input transducer alternatively can be fashioned as a telephone coil or an antenna for acquisition of electromagnetic input signals. In a digital hearing aid device, the input signals converted into electrical input signals by the input transducer (the microphone  1  in the exemplary embodiment) are initially converted into a digital signal by an A/D converter  2 , and this digital signal is supplied to a signal processing unit  3  for further processing and amplification. The further processing and amplification normally ensues dependent on the signal frequency, to compensate the individual hearing loss of a hearing aid device user. The signal filterings typical in hearing aid devices thus occur in the signal processing unit  3 . In digital hearing aid devices, the conversion of the digital output signal of the signal processing unit  3  into a signal that can be supplied to the output transducer typically ensues via a sigma-delta modulator  4  that normally emits a pulse-density-modulated signal. In a digital hearing aid device, the output signal is conventionally initially supplied to an output stage  6  and from this directly to an output transducer fashioned as an earpiece  7 . Low-pass filtering of the output signal supplied to the earpiece  7  is normally not required since the earpiece  7  already exhibits a strong low-pass characteristic anyway. Nevertheless, it is possible that an analog low-pass filter for suppression of high-frequency signal portions is connected upstream from an output transducer  7 , in particular when an earpiece (typically used) is not used as an output transducer. Namely, other types of output transducers in hearing aid devices are known, for example for generation of mechanical oscillations that directly excite specific parts of the ear (such as, for example, the ossicles) to oscillations or that directly stimulate nerve cells of the ear. Normally, however, digital filter means have not been used between the sigma-delta modulator  4  and the output stage  6  so far. In contrast to a, linear digital filter is provided in this segment of the signal path of the hearing aid device according to the invention. This serves to reduce the number of high-frequency edges in the typically pulse-density-modulated output signal of the sigma-delta modulator  4 .  
         [0019]     The input signal in the filter  5  is a single bit stream. A higher-order encoding of the output signals can be used as an output signal over both earpiece feed lines. In particular three different states, for example “1,0” (1st state), “0,0” (2nd state), “0,1” (3rd state), are realized by two output signal lines of the filter  5 .  
         [0020]      FIG. 2  shows a first and very simple embodiment of the linear digital filter  5  that is designated as a filter unit  51 . At its input, the filter unit  51  receives a 1-bit data stream that is directly supplied to the first input of an adder  512  as well as to the second input of the adder  512  after a delay produced by a delay element  511 . In the simplest case, a signal delay by one clock pulse ensues in the delay element  511 , but a delay of a higher number of clock pulses (generally by “n” clock pulses) can also ensue.  
         [0021]     The output signal of the filter unit  51  can have the numerical values 0, 1 or 2. It is accordingly a 2-bit signal. The output stage  6  for impedance conversion can thereby be selected such that, upon application of a “2” (thus the voltage states “1, 0” at both output signal lines), coil current flows through the exciter coil of the earpiece  7  in one direction, upon application of a “1” (thus the voltage states “0, 1” at both output signal lines) coil current flows through the exciter coil in the opposite direction, and upon application of a “0” (thus the voltage states “0, 0” at both output signal lines) the exciter coil is not excited. Given this approach, the low-current effect caused by the filter can also be easily illustrated. Namely, if no signal is present at the input transducer (for example at the microphone  1  according to  FIG. 1 ), the sigma-delta modulator  4  supplies an output signal with a 1-bit output which changes between 0 and 1 with the clock frequency with which the sigma-delta modulator  4  is operated. This in turn causes a high current consumption of the earpiece  7 , although its membrane experiences nearly no deflection in this state. It is different in the invention, where in this state a “0” is always present at the input of the output stage  6  and the coil of the earpiece  7  is thereby not excited. Thus no current consumption by the earpiece  7  occurs.  
         [0022]     It is noted that the three logical count values “0”, “1”, “2” only represent three different output states of the linear digital filter  5 . Naturally, these could be designated otherwise, for example 0, 0.5, 1 or −1, 0, +1. These three output states are converted in the output stage  6  such that the positive input voltage of the earpiece  7 , the negative input voltage of the earpiece  7  or no voltage is applied via the exciter coil of the earpiece  7 .  
         [0023]     In a further embodiment of the invention, the filter is a filter unit  52 A with a delay element  521  and a change-over switch  522 . An input bit stream in the filter unit  52 A is directly supplied to a first input of the change-over switch  522  and, on the other hand, supplied to a second input of the change-over switch  522  through a delay element  521 . The delay in the delay element  521  generally ensues by “m” clock pulses, whereby m is a natural number. The change-over switch  522  switches between both inputs with the clock frequency T, whereby T is a multiple of the clock frequency with which the sigma-delta modulator is operated. The filter unit  52 A serves for conversion of an input bit stream into an output bit stream, in that a specific frequency is suppressed dependent on the delay due to the delay element  521 . A notch filter is accordingly realized by the filter unit  42 A. It can be shown that the filter  52 A, like the filter  51 , is a linear filter.  
         [0024]     Given the use of the filter  52 A in the signal path of a hearing aid device according to  FIG. 1 , two similar filters  52 A and  52 B are advantageously connected in parallel, whereby a filter unit  52  results. The filter unit  52  thereby converts a two-bit input signal into a two-bit output signal. The filter unit  52  can thus be directly connected to a filter  51  according to  FIG. 2 . Moreover, it is possible to connect a number of filters  52  directly in series, one after the other. By the selection of different signal delays, a number of notches (in particular a number of closely adjoining notches) can then be generated. It is thus possible to suppress frequency ranges in the output signal.  
         [0025]      FIG. 4  shows a further embodiment of a digital filter according to the invention. The filter unit  53 A has a change-over switch  51 , a delay element  532  and an adder  533 . An input bit stream into the filter unit  53 A is supplied to the output of the change-over switch  531 . The first output of the change-over switch  531  is directly supplied to the second input of the adder  533  with the first input of the adder  533  and the second output of the change-over switch  531  through the delay element  532 . This filter unit  53 A also converts an input bit stream into an output bit stream and, dependent on the signal delay in the delay element  532 , generates a notch at a specific signal frequency.  
         [0026]     Just as in the filter  52  according to  FIG. 3 , here two similar filters  53 A and  53 B complement one another to form a filter  53 , since it converts a two-bit input stream into a two-bit output stream. The filter  53  can also be directly connected to a filter  51  according to  FIG. 2  and, if applicable, multiple filters  53  can be connected in series.  
         [0027]     The exemplary embodiment according to  FIG. 5  shows a section of the signal path of a hearing aid device between a sigma-delta modulator  4  and an output stage  6  between which filter means  51  and  52  according to  FIGS. 2 and 3  are present. A one-bit output signal of the sigma-delta modulator  4  forms the input signal in the filter unit  51 . The two-bit output signal arising from this serves as an input signal to a first filter unit  52 . A further filter unit  52  is in turn connected downstream from this. Its output signal is in turn supplied to the output stage  6 . The first filter unit  52  is clocked at twice the clock frequency of the sigma-delta modulator, and the second filter unit  52  is clocked at four times the clock frequency of the sigma-delta modulator. In the exemplary embodiment, this is achieved by the clock pulse generated by an oscillator  8  being halved in each of dividers  9  and  10 .  
         [0028]     By means of the filter units  51  and  52 , multiple notches are generated that serve for suppression of interference signals that, for example, are caused by the sigma-delta modulator  4 . The filter in particular serves for reduction of electromagnetic interference radiation that is emitted via the earpiece coil. Furthermore, the reduction of the number of high-frequency edges in the typical pulse-density-modulated output signal of the filter units  51  and  52  leads to a reduced current consumption of the output transducer.  
         [0029]     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.