Abstract:
An audio system comprises an audio driver configured to receive a target audio signal and a feedback signal and to generate an adjusted audio signal responsive to the target audio signal and the feedback signal. A loudspeaker is configured to convert the adjusted audio signal into acoustical sound. A test signal generator is configured to generate a test signal having a higher frequency than the target audio signal. The test signal causes a test current to flow through the loudspeaker. A current sensing circuit is configured to measure the test current flowing through the loudspeaker and to generate a current sense signal indicative of the test current. A feedback circuit is configured generates the feedback signal responsive to the current sense signal.

Description:
BACKGROUND 
     1. Field of Technology 
     Embodiments disclosed herein relate to audio systems, and more specifically to an audio system for reducing audio distortion of a loudspeaker. 
     2. Description of the Related Arts 
     A loudspeaker is a device that receives an electrical signal and converts the electrical signal to audible sound. Loudspeakers can include a voice coil that is inside of a magnet and is also attached to a diaphragm (e.g., a cone). When an electrical signal is applied to the voice coil, the coil generates a magnetic field that causes the voice coil and its attached diaphragm to move. The movement of the diaphragm pushes the surrounding air and generates sound waves. 
     For better sound fidelity, the sound waves produced by a loudspeaker should be proportional to the electrical signal applied to the loudspeaker. However, in a real loudspeaker, the movement of the diaphragm is not exactly proportional to the applied electrical signal, and this deviation leads to loss of acoustical fidelity. The loss of acoustical fidelity is especially pronounced with small loudspeakers, such as those found in mobile phones, tablet computers, laptops, and other portable devices. 
     There are several causes of the deviation between the electrical signal and the movement of the diaphragm. First, the coil and its associated parasitics are reactive and the magnetic field created by the coil varies depending on the frequency of the applied electrical signal. This results in a non-flat frequency response of the coil. Second, the effect of the magnetic field of the magnet on the coil is not constant as the position of the coil changes inside the magnet. As the coil moves backward and forward in response to the applied electrical signal, its position relative to the magnet changes. This changes the amount by which the magnetic field of the coil and the magnetic field of the magnet interact, resulting in movement of the diaphragm the extent of which is dependent upon the current position of the coil. Third, the springiness of the suspension supporting the diaphragm is not constant, and varies depending on how far it the diaphragm is displaced from its nominal position. All of these factors lead to increased distortion in the sound produced by a loudspeaker. 
     SUMMARY OF THE INVENTION 
     Embodiments disclosed herein describe an audio system that measures a test current through the loudspeaker as a way to measure the capacitance of the loudspeaker. The test current is used as feedback to generate a feedback signal that represents an actual displacement of the loudspeaker diaphragm. The feedback signal can then be used in a feedback loop to adjust a target audio signal, resulting in increased audio fidelity. 
     In one embodiment, the audio system comprises an audio driver configured to receive a target audio signal and a feedback signal and to generate an adjusted audio signal responsive to the target audio signal and the feedback signal. A loudspeaker is configured to convert the adjusted audio signal into acoustical sound. A test signal generator is configured to generate a test signal having a higher frequency than the target audio signal. The test signal also causes a test current to flow through the loudspeaker. A current sensing circuit is configured to measure the test current flowing through the loudspeaker and to generate a current sense signal indicative of the test current. A feedback circuit configured to generate the feedback signal responsive to the current sense signal. For example, the feedback circuit may be a look up table or a non-linear circuit that generates the feedback signal so that it represents an actual displacement of the loudspeaker. 
     In one embodiment, a method of operation in an audio system is disclosed. The method comprises generating an adjusted audio signal responsive to a target audio signal and a feedback signal; converting the adjusted audio signal into acoustical sound with a loudspeaker; generating a test signal having a higher frequency than the target audio signal, the test signal causing a test current to flow through the loudspeaker; measuring the test current flowing through the loudspeaker; generating a current sense signal indicative of the test current; and generating the feedback signal responsive to the current sense signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the embodiments disclosed herein can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
         FIG. 1  is a physical diagram of a loudspeaker, according to one embodiment. 
         FIG. 2  is an electrical model of a loudspeaker  10  from  FIG. 1 , according to one embodiment. 
         FIG. 3  is a simplified version of the electrical model from  FIG. 2  at high frequencies, according to one embodiment 
         FIG. 4  is a block diagram of an audio system with reduced audio distortion, according to one embodiment. 
         FIG. 5  is a circuit diagram of an audio system with reduced audio distortion, according to one embodiment. 
         FIG. 6  illustrates signal waveforms of the audio system, according to one embodiment. 
         FIG. 7  is a circuit diagram of an audio system with reduced audio distortion, according to another embodiment. 
         FIG. 8  is a circuit diagram of an audio system with reduced audio distortion, according to yet another embodiment. 
         FIG. 9  is a physical diagram of a loudspeaker, according to another embodiment. 
         FIG. 10  is simplified electrical model of the loudspeaker from  FIG. 9  at high frequencies, according to another embodiment. 
         FIG. 11  is a circuit diagram of an audio system with reduced audio distortion, according to a further embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The Figures (FIG.) and the following description relate to various embodiments by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles discussed herein. 
     Reference will now be made in detail to several embodiments, examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict various embodiments for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein. 
     Embodiments disclosed herein describe an audio system that measures a test current through the loudspeaker as a proxy for the capacitance of the loudspeaker. The test current is used as feedback to generate a feedback signal that represents an actual displacement of the loudspeaker diaphragm. The feedback signal can then be used in a feedback loop to adjust a target audio signal, resulting in a displacement of the speaker that more accurately matches the target audio signal, which increases audio fidelity. 
       FIG. 1  is a physical diagram of a loudspeaker  10 , according to one embodiment. Loudspeaker  10  includes a magnet  12 , a coil  14 , and a diaphragm  16  attached to the coil  14 . When an electrical signal is applied to the coil  14 , it causes the coil  14  to generate a magnetic field that interacts with the magnetic field of the magnet  12 . The coil  14  and the diaphragm  16  move back and forth to produce sound waves. If the coil  14  is closer to the center of the magnet  12 , the interaction between the magnetic fields is stronger. If the coil  14  is further from the center of the magnet  12 , the interaction is weaker. This changing magnetic field results in a non-constant force that creates acoustical distortion. 
     The coil  14  also generates an electric field  18  that interacts with the magnet  12 . The electric field  18  changes depending on the position of the coil  14  relative to the magnet  12 . Similar to the magnetic field, if the coil is in the center of the magnet  12 , the electrical field  18  interaction between the coil  14  and the magnet  12  is stronger. If the coil  14  moves away from the magnet  12 , the electric field  18  is reduced. 
       FIG. 2  is an electrical model of a loudspeaker  10  from  FIG. 1 , according to one embodiment. Resistor R 1  and inductor L 1  model the moving coil  14  inside the loudspeaker  10 . Capacitor C 2 , inductor L 2  and resistor R 2  model the combined inertia of air, springiness of the diaphragm  16 , and induced electromotive force (EMF) caused by the movement of the coil  14 . The loudspeaker  10  also includes two speaker terminals through which electrical audio signals can be provided to the speaker. 
     Capacitor C 1  represents a self-capacitance of the loudspeaker  10  caused by the electric field  18  inside the loudspeaker  10 . C 1  varies with the movement of the coil  14 . When a positive voltage is applied to the coil  14 , it moves away from the magnet  12 , reducing the interaction of the electric field  18  with the magnet  12  and also reducing the capacitance of capacitor C 1 . When a negative voltage is applied to the coil  14 , it moves towards the magnet  12 , increasing the interaction of the electric field  18  with the magnet  12  and also increasing the capacitance of capacitor C 1 . Thus, the value of C 1  depends on the position of the coil  14  and diaphragm  16  and is directly linked to the acoustical sound generated by the loudspeaker  10 . In some embodiments, C 1  varies between 10 pF and 100 pF. 
       FIG. 3  is a simplified version of the electrical model from  FIG. 2  at high frequencies, according to one embodiment. At high frequencies outside of the audio frequency range, such as 10 MHz, C 2  is assumed to be a short circuit and so C 2 , L 2 , and R 2  can all be removed from the circuit model. Resistor Rs represents the high frequency resistance of the loudspeaker  10  and corresponds to resistor R 1  from  FIG. 2 . Inductor Ls represents the high frequency inductance of the loudspeaker  10  and corresponds to inductor L 1  from  FIG. 2 . Capacitor Cs represents the self-capacitance of the loudspeaker  10  and corresponds to capacitor C 1  from  FIG. 2 . 
     Embodiments of the present disclosure use the capacitance Cs of the coil  14  as a proxy for the displacement of the diaphragm  16 . The capacitance Cs can be measured and used as feedback to adjust the level of the electrical signal provided to the loudspeaker  10 , thereby compensating for deviations between the electrical signal and the displacement of the coil  14  and diaphragm  16 . As a result, the loudspeaker  10  has reduced distortion and better frequency response. 
       FIG. 4  is a block diagram of an audio system with reduced audio distortion, according to one embodiment. The audio system includes an audio driver  410  that receives a target audio signal  402  at its positive input and a feedback signal  408  at its negative input. In one embodiment, the target audio signal  402  is in an audible frequency range between 20 to 20,000 Hz and represents sound that is to be produced by the loudspeaker  10 . The audio driver compares the target audio signal  402  with the feedback signal  408  to generate an adjusted audio signal  404 . In one embodiment, the audio driver  410  may be an audio amplifier or include an amplification stage. 
     The compensation circuit  406  is coupled to an output of the audio driver  410  and a terminal  430  of the loudspeaker  10 . The compensation circuit  406  passes the adjusted audio signal  404  onto the loudspeaker  10 , which converts the adjusted audio signal  404  into acoustical sound. The capacitance of the capacitor Cs varies as the adjusted audio signal  404  is converted to acoustical sound by the loudspeaker  10 . The compensation circuit  406  also includes a test signal generator (not shown) that injects a high frequency test current into the capacitor Cs. A current level of the high frequency test current is measured and used as an indication of the instantaneous value of capacitor Cs. The measured current is converted to a voltage proportionate to the displacement of the diaphragm  16 , which is sent as the feedback signal  408  to the audio driver  410 . The loop gain of the audio driver  410  causes the target audio  402  and feedback signal  408  to eventually converge on one another. Since the feedback signal  408  can be an accurate representation of the actual acoustical sound produced by the loudspeaker  10 , this ensures that the generated acoustical sound is similar to the target audio signal  402 , thereby increasing the fidelity of sound produced by the loudspeaker  10 . 
     The bottom terminal  432  of the loudspeaker  432  is coupled to ground to provide a discharge path for signals input to the loudspeaker via the top terminal  430 . In other embodiments, the compensation circuit  406  can also be coupled to the bottom terminal  432  of the loudspeaker  12  or a power supply input of the audio driver  410 , as will be explained herein. In other embodiments, the audio driver  410  can be a differential driver instead of a single ended driver. 
       FIG. 5  is a circuit diagram of an audio system with reduced audio distortion, according to one embodiment. The compensation circuit  406  includes a test signal generator  506  that generates an alternating current (AC) test signal  508 . The test signal  508  oscillates at a higher frequency than the audio frequency range of the target audio signal  402 . For example, the test signal  508  can have a frequency of 10 MHz, which is well above the 20 hz-20 khz range of the target audio signal  402 . In one embodiment, the test signal  508  can have a substantially fixed voltage amplitude and a substantially fixed frequency. However, the current of the test signal  508  may vary as the loudspeaker  10  produces acoustical sound. 
     A combiner circuit  510  is coupled to the output of the audio driver  410  and a terminal  430  of the loudspeaker  10 . The combiner circuit  510  combines the test signal  508  with the adjusted audio signal  404  to generate a combined signal  502  that is provided to the loudspeaker  10 . Combiner circuit  510  may include an inductor L 3  and a capacitor C 3 . Inductor L 3  is selected to pass audio frequencies but to block the frequency of the test signal  508 . L 3  prevents the current of the test signal  508  from flowing through output of the audio driver  410 . Capacitor C 3  is selected to block audio frequencies but to pass the frequency of the test signal  508 . Capacitor C 3  prevents the adjusted audio signal  404  from affecting current measurement of the test signal  508 . 
     The combined signal  502 , which includes both an adjusted audio signal portion and a test signal portion, is provided to the top terminal  430  of the loudspeaker  10 . The adjusted audio signal portion causes the coil  14  of the loudspeaker  10  to move back and forth, thereby producing acoustical sound that is audible to a listener. The test signal portion of the combined signal  502  generates a test current through the capacitance Cs but does not cause the loudspeaker to produce acoustical sound. Substantially all of the test current for the test signal portion flows through the capacitor Cs and not inductor Ls. This is because the test signal portion operates at a high frequency, and inductor Ls is an open circuit at high frequencies. 
     The capacitance Cs changes over time as the coil  14  moves back and forth to produce acoustical sound. Because Cs changes and the test current of test signal  508  flows through Cs, the current level of the test signal  508  is dependent on Cs and changes as the value of Cs changes. Thus, when the coil  14  moves further from the magnet, the capacitance Cs decreases and so does the current level of the test signal  508 . As the coil  14  moves towards the magnet, the capacitance Cs increases and so does the current level of the test signal  508 . 
     Current measuring circuit  520  is coupled between the test signal generator  506  and the signal combiner  510 . Current measuring circuit  520  measures the current level of the test signal  508  (which can have a fixed voltage amplitude and varying current) and generates a current sense signal  512  indicating the measured current level of the test signal  508 . The current measuring circuit  520  may include, for example, a series resistor that is coupled between the test voltage generator  506  and the signal combiner  510 , as well as a differential amplifier to amplify a voltage difference across the resistor. 
     Amplitude detector  514  receives the current sense signal  512  and detects the amplitude of the current sense signal  512 . The amplitude detector  514  then generates a current amplitude signal  516  that represents the time varying amplitude of the current sense signal  512 . As the current level of the test signal  508  is tied to the capacitance Cs of the loudspeaker  10 , the instantaneous level of the current amplitude signal  516  also represents the instantaneous capacitance Cs of the loudspeaker  10 . In one embodiment, the amplitude detector  514  includes a diode D 1  and a capacitor C 4  coupled to the output of the diode D 1 . Diode D 1  acts as a half-wave rectifier and capacitor C 4  smoothes the half-wave rectified signal to generate the current amplitude signal  516 . 
     The feedback circuit  518  is coupled to the output of the amplitude detector  514  and receives the current amplitude signal  516 . The feedback circuit  518  converts the current amplitude signal  516  into a feedback signal  408  that represents the extent of displacement of the diaphragm  16 . In one embodiment, the feedback circuit  518  includes a look up table that maps values for the current amplitude signal  516  to displacement values representing the extent of displacement of the diaphragm  16 . The displacement values are then converted into voltages that are output as the feedback signal  408 . In one embodiment, the mapping between the current amplitude signal  516  and the diaphragm  16  displacement may be determined in advance through actual measurements of the diaphragm  16  displacement and current amplitude signal  516 , which are then stored into the look up table. 
     In other embodiments, the feedback circuit  518  can be a non-linear circuit that converts the current amplitude signal  516  into a feedback signal  408  that represents an approximate extent of the diaphragm  16  displacement. 
     The audio driver  410  receives the feedback signal  408  and compares the feedback signal  408  to the target audio signal  402  to adjust a level of the adjusted audio signal  404 . The loop gain of the audio driver  410  causes the target audio signal  402  and feedback signal  408  to eventually converge onto one another, thereby ensuring that the acoustical output of the loudspeaker  10  matches that of the target audio signal  402 . 
       FIG. 6  illustrates signal waveforms of the audio system from  FIG. 5 , according to one embodiment. Signal waveforms are shown for the adjusted audio signal  404 , the test signal  508 , the current sense signal  512 , and the current amplitude signal  516 . The adjusted audio signal  404  is a time-varying voltage signal that causes the voice coil  14  to move back and forth to produce acoustical sound. The movement of the coil  14  creates variations in the capacitance Cs of the loudspeaker  10 . The test signal  508  has a substantially constant frequency and voltage amplitude. However, the current level of the test signal  508 , represented by the current sense signal  512 , changes as the capacitance Cs changes. The changing current of the test signal  508  is captured in the voltage level of the current sense signal  512 . Finally, the current amplitude signal  516  is the time varying amplitude of the current sense signal  512  and is indicative of the changing current amplitude of the test signal  508  and tracks the changing capacitance Cs of the loudspeaker  10 . 
       FIG. 7  is a circuit diagram of an audio system with reduced audio distortion, according to another embodiment. The audio system of  FIG. 7  is similar to the audio system of  FIG. 6 , except that the current detector circuit  520  is now coupled to the other terminal  432  of the loudspeaker  10 . Current detector circuit  520  still detects a level of a test current flowing through the capacitor Cs but performs the measurement in a slightly different manner. 
     Specifically, current detector circuit  520  detects a current of the combined signal  502 . The current of the combined signal  502  includes both audio frequency components of the adjusted audio signal  404 , as well a high frequency component of the test signal  508 . To separate the audio frequency components from the high frequency component of the test signal  508 , current detector circuit  520  includes a series capacitor C 5 . Capacitor C 5  acts as a high pass filter that filters out the audio frequency components of the detected current but passes the frequency components of the test signal  506 . As a result, current sense signal  512  indicates a current level of the test signal  508  but not the adjusted audio signal  404 . In other embodiments, capacitor C 5  may be placed between the current detector circuit  520  and the loudspeaker  10  to filter out the audio frequency components before detecting the current level of the test signal  508 . 
       FIG. 8  is a circuit diagram of an audio system with reduced audio distortion, according to yet another embodiment. The audio system of  FIG. 8  is similar to the audio system of  FIG. 7 , except that test signal generator  506  is now coupled to a power supply input of the audio driver  410  and indirectly causes a high frequency test current to flow through the speaker  10  by varying the power supply input to the audio driver  410 . 
     As shown, the audio driver  410  is powered by a DC supply  802 , such as a battery or other power source. The test signal generator  506  generates a test signal  508  which is combined with the DC supply  802  via capacitor C 6  to generate an adjusted power supply voltage  804 . The adjusted power supply voltage  804  has both a DC component from the DC supply voltage  802  and an AC component from the test signal generator  506 . The AC component of the power supply signal  804  varies the output of the audio driver  410  and causes the adjusted audio signal  404  to have a high frequency AC component that matches the frequency of the test signal  508 . 
     The high frequency AC component of the adjusted audio signal  404  causes a high frequency test current to flow through capacitor Cs of the loudspeaker  10 . The current detection circuit  520  measures a current level of the test current. The level of this test current is reflected in the current sense signal  512 , amplitude detected by the amplitude detector circuit  514  to generate a current amplitude signal  516 , and then used by the feedback circuit  518  to generate the feedback signal  408 . The embodiment of  FIG. 8  may be simpler to implement than the previous embodiments of  FIG. 5  and  FIG. 7  due to the lack of a combiner circuit  510  and its associated discrete components. 
       FIG. 9  is a physical diagram of a loudspeaker  10 , according to another embodiment. The physical diagram of  FIG. 9  is similar to that of  FIG. 1 , but now includes a printed circuit board (PCB) ground plane  902 . The PCB ground plane  902  may be, for example, for a PCB that the loudspeaker  10  is mounted to. In other embodiments, the PCB ground plane  902  may be replaced with another grounded object that is adjacent to the loudspeaker  10 . The coil  14  also has an electric field  904  that interacts with the ground plane  902  of the PCB. The strength of the electric field  904  changes as the coil  14  and diaphragm  16  move back and forth to produce acoustical sound. 
       FIG. 10  is simplified electrical model of the loudspeaker  10  from  FIG. 9  at high frequencies, according to one embodiment. The loudspeaker model from  FIG. 10  is similar to the loudspeaker model from  FIG. 3 , but now the model includes a capacitor Cg in place of capacitor Cs. Capacitor Cg is connected to ground and represents the electric field  904  between the coil  14  and the PCB ground plane  902 . The capacitance of capacitor Cg also changes as the coil  14  and diaphragm  16  move back and forth to produce acoustical sound. 
       FIG. 11  is a circuit diagram of an audio system with reduced audio distortion, according to a further embodiment. At a functional level, the audio system of  FIG. 11  uses capacitance Cg as a proxy for the displacement of the diaphragm  16 . The audio system measures a current through the capacitance Cg and uses the current to generate feedback signal  408  for adjusting the level of the adjusted audio signal  404 , thereby compensating for deviations between the target audio signal  402  and the actual displacement of the diaphragm  16 . 
     At a circuit level, the audio system of  FIG. 11  is similar to the audio system of the  FIG. 5  but now includes a differential audio driver  1110  that outputs a differential adjusted audio signal  1104 . Signal combiner  1112  is also different and now includes two inductors L 3  and L 4  coupled between the outputs of the audio driver  1110  and the loudspeaker  10 . Inductors L 3  and L 4  are chokes that block the test signal  506  from flowing back through the outputs of the audio driver  1110 . 
     Signal combiner  510  combines test signal  508  with the differential adjusted audio signal  1104  to generate a differential combined signal  1102 . The adjusted audio signal portion of the combined signal  1102  is converted to acoustical sound by the loudspeaker  10 . Capacitor Cg changes as the loudspeaker  10  produces acoustical sound. The test signal  506  is blocked by inductor L 4  and L 3 , and so the only discharge path available to the test signal  506  is through capacitor Cg. The current sensing circuit  520  measures the current level of the test signal  506 , which represents the amount of test current flowing through capacitor Cg. Current sensing circuit  520  then generates current sensing signal  512  to indicate a current level of the test signal  506 . 
     Amplitude detector  514  detects an amplitude of the current sense signal  512  and generates a current amplitude signal  516 . Feedback circuit  518  receives the current amplitude signal  516  and uses the current amplitude signal  516  to generate a feedback signal  408 . In one embodiment, feedback circuit  518  uses a look up table that maps levels of the current amplitude signal  516  to displacement values that are used to generate the feedback signal  408 . The look up table for the feedback circuit  518  in  FIG. 11  may have different values than the look up table for the feedback circuit  518  in  FIG. 5 . 
     Audio driver  1110  receives the target audio signal  402  and the feedback signal  408  and generates the differential adjusted audio signal  1104  by comparing its two input signals. The resulting adjusted audio signal  1104  compensates for deviations between the target audio signal  402  and the actual movement of the loudspeaker diaphragm  16 . As a result, the displacement of the speaker diaphragm  16  matches that of the target audio signal  402  to increase the audio fidelity of the audio system. 
     Upon reading this disclosure, those of skill in the art will appreciate still additional alternative designs for reducing audio distortion in an audio system. Thus, while particular embodiments and applications have been illustrated and described, it is to be understood that the embodiments discussed herein are not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the spirit and scope of the disclosure.