Abstract:
A method for driving a condenser microphone is provided. The condenser microphone comprises a membrane and an electrode constituting a capacity. A polarization voltage is applied between the membrane and the electrode. According to the method, an electrical signal generated by the condenser microphone based on a received acoustic signal causing a deflection of the membrane is detected, and the polarization voltage is varied in response to the detected electrical signal.

Description:
[0001]    The present invention relates to a method for driving a condenser microphone, a control circuit for a condenser microphone, a condenser microphone, a mobile device, and a headset. 
       BACKGROUND OF THE INVENTION 
       [0002]    A condenser microphone, which is also called capacitor microphone or electrostatic microphone, is an acoustic to electric transducer or sensor that converts sound into an electrical signal. Condenser microphones are used in a wide variety of applications, for example telephones, mobile phones, studio microphones and headsets. 
         [0003]    The condenser microphone comprises a moveable membrane and an electrode or two electrodes. The membrane is arranged in parallel and spaced apart from the electrode or between the two electrodes. The arrangement of membrane and electrode(s) is called capsule. The membrane as well as the electrode are electrically conducting. Thus, a capacity is constituted. The value of the capacity depends on the area of the membrane and the electrode, and a distance between the electrode and the membrane. Intruding sound makes the membrane swing and thus the distance between the membrane and the electrode is changed. There are two operating modes for evaluating the change of capacity: The direct current (DC) biased mode and the radio frequency (RF) or high frequency (HF) mode. With the DC-biased mode the membrane and the electrode are biased with a fixed charge and a voltage maintained across the membrane and the electrode changes with the vibrations of the membrane. The RF or HF mode uses a comparatively low RF voltage generated by a low noise oscillator, at a frequency of several MHz, for example 8 MHz. The membrane and the electrode are part of a resonant circuit that modulates the frequency of the oscillator signal. Demodulation yields a low-noise audio frequency signal with a very low sound impedance. 
         [0004]    However, due to the small distance between the membrane and the electrode, a dynamic range of the condenser microphone is limited and distortions are present when the membrane is largely deflected or touches the electrode. Furthermore, as microphones in general are sensitive to wind noise or acoustic pressure of high value and low frequency, also condenser microphones are sensitive to wind noise. 
         [0005]    Therefore, there is a need for an improvement in operating a condenser microphone which makes the condenser microphone more robust against wind noise, increases the dynamic range of the condenser microphone, and reduces distortions. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention, this object is achieved by a method for driving a condenser microphone as defined in claim  1 , a control circuit for a condenser microphone as defined in claim  8 , a condenser microphone as defined in claim  10 , a mobile device as defined in claim  11 , a headset as defined in claim  13 , and a studio microphone as defined in claim  14 . The depending claims define preferred and advantageous embodiments of the present invention. 
         [0007]    According to an aspect of the present invention a method for driving a condenser microphone is provided. The condenser microphone comprises a membrane and an electrode constituting a capacity. A polarization voltage is applied between the membrane and the electrode. According to the method an electrical signal generated by the condenser microphone is detected. The electrical signal is based on a received acoustic signal which causes a deflection of the membrane. Furthermore, according to the method, the polarization voltage is varied in response to the detected electrical signal. For example, the polarization voltage may be varied such that it causes a mechanical force on the membrane, and the mechanical force counteracts a current deflection of the membrane. Thus, the dynamic range of the condenser microphone may be extended. 
         [0008]    According to another embodiment, the membrane is arranged in a minimal deflected position when no acoustic signal is acting on the membrane. Varying the polarization voltage includes applying a voltage which causes a mechanical force on the membrane which urges the membrane to the minimal deflected position. This keeps the membrane in the minimal deflected position, the so-called middle position, and avoids a distortion as the membrane is operated near the middle position. The minimal deflected position may comprise a non-deflected position when no acoustic signal is acting on the membrane. 
         [0009]    According to another embodiment varying the polarization voltage comprises applying a voltage on the membrane that causes a mechanical force on the membrane which urges the membrane away from the electrode when the electrical signal indicates that a current deflection of the membrane in the direction of the electrode is larger than a predetermined threshold. Thus, when the membrane is in danger to come into contact with the electrode, the membrane is kept away from the electrode by the electrically induced mechanical force. This may be useful when strong wind noise is applied to the condenser microphone. 
         [0010]    According to another embodiment, the polarization voltage comprises a direct current voltage and varying the polarization voltage comprises adjusting a voltage level of the direct current voltage. Thus, the condenser microphone may be operated in the above-described DC-biased mode. Furthermore, the condenser microphone may be operated in the above-described radio frequency (RF) or high frequency (HF) mode. In this case, originally no direct current polarization voltage is needed for sound extraction from the capsule, so a direct current voltage across the membrane and the electrode(s) is added to the radio frequency or high frequency voltage to create the electrically induced force on the membrane. Thus, the condenser microphone may be operated in each of the above-described operating modes, as applicable, and may utilize the above-described advantageous method. 
         [0011]    According to a further embodiment, an output signal is generated in response to the electrical signal and the polarization voltage. When the polarization voltage is varied, the electrical signal does not linearly represent the acoustic signal any more. Based on the polarization voltage this non-linearity may be compensated and a compensated output signal may be generated. 
         [0012]    According to another aspect of the present invention, a control circuit for a condenser microphone is provided. The condenser microphone comprises a membrane and an electrode constituting a capacity. The control circuit comprises a polarization voltage supply unit for applying a variable polarization voltage between the membrane and the electrode. The control circuit comprises furthermore a control unit adapted to detect an electrical signal which is generated by the condenser microphone based on a received acoustic signal. The received acoustic signal causes a deflection of the membrane. The control unit is furthermore adapted to control the polarization voltage supply unit to vary the polarization voltage in response to the detected electrical signal. 
         [0013]    The control circuit may be adapted to perform the above-described method and comprises therefore the above-described advantages. 
         [0014]    According to another aspect of the present invention, a condenser microphone is provided. The condenser microphone comprises a membrane, an electrode arranged spaced apart from the membrane, and the above-described control circuit. The membrane and the electrode constitute a capacity. The condenser microphone comprises the same advantages as the above-described method. 
         [0015]    According to another aspect of the present invention, a mobile device is provided which comprises a condenser microphone as defined above. The mobile device may comprise a mobile telephone, a personal digital assistant, a mobile navigation system, a mobile computer or a mobile music player. 
         [0016]    Finally, according to another aspect, a headset comprising the condenser microphone as described above is provided. 
         [0017]    Although specific features described in the above summary and the following detailed description are described in connection with specific embodiments, it is to be understood that the features of the embodiments can be combined with each other unless specifically noted otherwise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention will now be described in more detail with reference to the accompanying drawings. 
           [0019]      FIG. 1  shows a block diagram of a condenser microphone according to an embodiment of the present invention. 
           [0020]      FIG. 2  shows a flow chart of a method for driving a condenser microphone according to an embodiment of the present invention. 
           [0021]      FIG. 3  shows a mobile device comprising a condenser microphone according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    In the following, exemplary embodiments of the present invention will be described in more detail. It has to be understood that the following description is given only for the purpose of illustrating the principles of the invention and is not to be taken in a limiting sense. Rather, the scope of the invention is defined only by the appended claims and not intended to be limited by the exemplary embodiments hereinafter. 
         [0023]    It is to be understood that the features of the various exemplary embodiments described herein may be combined with each other unless specifically noted otherwise. Same reference signs in the various instances of the drawings refer to similar or identical components. 
         [0024]      FIG. 1  schematically shows a block diagram of a condenser microphone  100 . The condenser microphone comprises a membrane  101  and an electrode  102 . The membrane  101  and the electrode  102  are arranged in parallel and spaced apart from each other such that the membrane  101  may swing or oscillate when acoustic noise  103  is applied to the membrane  101 . The electrode  102  is rigid such that it is essentially not swinging or oscillating due to the acoustic noise  103 . The membrane  101  and the electrode  102  are electrically conducting elements and arranged electrically insulated from each other. The distance between the membrane  101  and the electrode  102  defines a capacity. 
         [0025]    The condenser microphone  100  comprises furthermore a polarization voltage supply unit  104  generating a polarization voltage U Pol . The polarization voltage supply unit  104  applies the polarization voltage U Pol  over a resistor  105  to the capacity constituted by the membrane  101  and the electrode  102 . As described above in the background of the invention, due to the acoustic noise  103  the capacity of the arrangement of the membrane  101  and the electrode  102  is varied and a corresponding electrical signal U Sig  is generated either in the direct current operating mode (DC) or the radio frequency operating mode (RF). 
         [0026]    The condenser microphone  100  comprises furthermore a control unit  106  which is connected to the electrical signal U Sig  and to the polarization voltage supply unit  104 . Via the connection  107  between the control unit  106  and the polarization voltage supply unit  104  the polarization voltage supply unit  104  can be controlled via a control signal from the control unit  106 .  FIG. 2  shows the control loop for controlling the polarization voltage supply unit  104 . In step  201  the control unit  106  detects the electrical output signal U Sig  of the condenser microphone  100  and in response to the detected signal U Sig  the polarization voltage supply unit  104  is varied in step  202 . In the direct current operating mode (DC) a voltage level of the direct current polarization voltage of the polarization voltage supply unit  104  is adjusted. In the radio frequency or high frequency operating mode (RF or HF) a direct current voltage is added to the oscillating voltage of the polarization voltage supply unit  104 . 
         [0027]    By varying the polarization voltage a mechanical force between the membrane  101  and the electrode  102  may be generated or varied. The mechanical force may provide an attraction between the membrane  101  and the electrode  102 , for example by applying a different polarity between the membrane  101  and the electrode  102 , or a repulsion, for example by applying the same polarity to the membrane  101  and the electrode  102 . 
         [0028]    As soon as the polarization voltage is varied, the detected signal U Sig  is no longer linear with respect to the received acoustic noise  103 . The unlinearity induced by the change of the polarization voltage is predictable and can be compensated in later filtering stages. Therefore, as shown in  FIG. 1 , the condenser microphone  100  may comprise a correction unit  108  coupled to the detected signal U Sig  and the connection  107  providing the control signal controlling the polarization voltage. The correction unit  108  contains knowledge about how the control signal affects the detected signal U Sig , so a reverse transformation may be conducted and a corrected output signal U Cor  may be generated and output by the correction unit  108 . 
         [0029]    The mechanical force may be used to control a membrane deflection in the following ways: 
         [0030]    First, the mechanical force may be used to keep the membrane  101  as close to a centered position as possible independent of sound pressure. Therefore, a wider dynamic range of the condenser microphone may be achieved. The maximum sound pressure level (SPL) before the membrane hits or touches the electrode may be increased with the counterforce from the electric feedback of the control unit  106 . 
         [0031]    In the following some exemplary figures of improvements for a condenser microphone are given. However, these exemplary figures are not to be taken in a limiting sense. For example, a measurement microphone usually may provide a dynamic range from the noise floor at 14 dB (A) to 134 dB as maximum SPL, resulting in a dynamic range of 120 dB. As preliminary calculations indicate, this dynamic range may be increased by 10 dB by the above-described counterforce from the feedback from the control unit  106 . Furthermore, when the condenser microphone  100  comprises two electrodes  102  sandwiching the membrane  101  between the two electrodes  102 , the dynamic range may be increased by more than 40 dB. However, the increased dynamic range cannot only be used to increase the maximum sound pressure level, but may also reduce noise floor by allowing microphone constructions which are normally prohibited by saturation at very low sound pressure levels. For example, a small condenser microphone may have a noise floor at 30 dB (A) and a maximum sound pressure level of 120 dB, giving a range of 90 dB. This range may be increased by approximately 16 dB with the proposed feedback method for a condenser microphone with a single electrode  102 . 
         [0032]    Furthermore, distortion from non-flat movements of the membrane  101  may by eliminated or reduced. In condenser microphones the membrane is fixed along its outer circular edge. For small sound pressure level the membrane moves like a piston, but for large excursions or deflections the membrane will form a bent shape, giving a non-linear transduction from sound pressure to output voltage resulting in a distortion or non-linearity. If the membrane is kept in the middle even for higher sound pressure levels, distortions due to bent-shaped deflections of the membrane are eliminated or reduced. The dynamic range increase and the distortion reduction may be used to increase performance in measurement systems, in high quality audio recordings. Furthermore, the same method may be used to improve performance of very small condenser microphone units allowing to build smaller condenser microphones without reducing performance. 
         [0033]    Second, the mechanical force fed back from the control unit  106  may serve as a wind saturation protection. In windy conditions, the membrane  101  sometimes reaches the electrode  102  causing a non-linear output which is very difficult to eliminate by later filtering techniques. By controlling the polarization voltage U Pol  such that a mechanical force keeps the membrane  101  away from the electrode  102  prohibits such large deflections caused by wind. When the voltage swing of the output signal U Sig  indicates that the membrane  101  is close to the electrode  102 , a counterforce is applied by changing the polarization voltage U Pd . 
         [0034]    The above-described condenser microphone  100  may be used for example in a headset or, as shown in  FIG. 3 , in a mobile device  301 . 
         [0035]    While exemplary embodiments have been described above, various modifications may be implemented in other embodiments. For example, as already indicated above, the condenser microphone  100  may comprise two electrodes  102  which are arranged in parallel and enclose the membrane  101  in between the electrodes  102 . One pole of the polarization voltage supply unit  104  is connected to both electrodes  102  and the other pole of the polarization voltage supply unit  104  is connected via the resistor  105  to the membrane  101 . 
         [0036]    Finally, it is to be understood that all the embodiments described above are considered to be comprised by the present invention as it is defined by the appended claims.