Patent Publication Number: US-9837971-B2

Title: Method and system for excursion protection of a speaker

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/482,327, filed May 4, 2011, entitled SYSTEM AND METHOD FOR LOUDSPEAKER EXCURSION PROTECTION, naming Chenchi Luo et al as inventors, which is hereby fully incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     The disclosures herein relate in general to digital signal processing, and in particular to a method and system for excursion protection of a speaker. 
     Many portable electronic devices are relatively small and inexpensive. Accordingly: (a) such devices may have speakers that are relatively small and inexpensive; and (b) drive units of the speakers may have relatively low power handling capacity and relatively low sensitivity, which increases risk that a powerful amplifier might push them to power handling and mechanical limits in an attempt to reach higher sound volumes. Causes of speaker failure include: (a) over-excursion (e.g., excessive backward and/or forward movement) of the speaker&#39;s diaphragm; and (b) overheating of the speaker&#39;s voice coil. For example, if the speaker receives an input voltage signal whose level is relatively high and whose frequency is relatively low, then the speaker&#39;s voice coil may exit its safe gap and thereby damage the speaker. In some cases, a sensor can directly monitor excursion of the speaker&#39;s diaphragm, but the sensor&#39;s size and expense may be impractical for many portable electronic devices. 
     In a conventional dynamic range compression (“DRC”) technique, the input voltage signal is received by a dynamic range compressor. In one example: (a) if the input voltage signal&#39;s amplitude exceeds a threshold&#39;s limit, then the signal is dynamically compressed by the dynamic range compressor; and (b) otherwise, the signal is unmodified by the dynamic range compressor. However, the input voltage signal&#39;s amplitude is nonlinearly related to excursion of the speaker&#39;s diaphragm, so that: (a) DRC may unnecessarily compress the signal (in a manner that distorts sound and/or reduces perceived loudness of the speaker), despite peak excursion of the speaker already being within a safe operating range; and/or (b) over-excursion of the speaker may still occur, despite the input voltage signal&#39;s amplitude being within the threshold&#39;s limit. Accordingly, a different technique would be useful for keeping such excursion within a safe operating range, in order to protect the speaker. 
     SUMMARY 
     For protecting a speaker, an input signal is received, and an excursion of the speaker that would be caused by the input signal is predicted. In response to the predicted excursion exceeding a threshold, a targeted excursion of the speaker is determined by compressing the predicted excursion. The targeted excursion is translated into an output signal, which is output to the speaker. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an information handling system of the illustrative embodiments. 
         FIG. 2  is a side sectional view, in elevation, of a speaker of  FIG. 1 . 
         FIG. 3  is a data flow diagram of operations for protecting the speaker of  FIG. 1 . 
         FIG. 4  is a graph of peak excursion (x-axis) in response to voltage signals (y-axis), according to a first simulation of operation with a control device of  FIG. 1  enabled. 
         FIG. 5  is a graph of peak excursion (x-axis) in response to voltage signals (y-axis), according to a second simulation of operation with the control device of  FIG. 1  disabled. 
         FIG. 6  is a graph of an example frequency response of a transfer function. 
         FIG. 7  is a graph of an example frequency response of a highpass filter. 
         FIG. 8  is a graph of a gain mapping characteristic of an excursion compression operation of the information handling system of  FIG. 1 . 
         FIG. 9  is a graph of excursion when playing an example clip of music without filtering. 
         FIG. 10  is a graph of excursion when playing the example clip of music with conventional highpass filtering protection. 
         FIG. 11  is a graph of excursion when playing the example clip of music with the control device of  FIG. 1  enabled for excursion protection according to the illustrative embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an information handling system, indicated generally at  100 , of the illustrative embodiments. In response to electrical signals from a control device  102 , a speaker  104  outputs audio signals, so that a human user  106  is thereby enabled to hear such audio signals. In the example of  FIG. 1 : (a) the speaker  104  is a micro-loudspeaker; and (b) the control device  102  and the speaker  104  are components of a portable handheld electronics device (not shown in  FIG. 1 ), such as a mobile smartphone, whose various components are housed integrally with one another. 
     The control device  102  includes various electronic circuitry components for performing the control device  102  operations, such as: (a) a multimedia interface digital signal processor (“DSP”)  108 , which is a computational resource for executing and otherwise processing instructions, and for performing additional operations (e.g., communicating information) in response thereto; (b) an amplifier (“AMP”)  110  for receiving electrical signals from the DSP  108 , and for outputting amplified versions of those signals (“output voltage signals”) to the speaker  104  under control of the DSP  108 ; (c) a computer-readable medium  112  (e.g., a nonvolatile memory device) for storing information; and (d) various other electronic circuitry (not shown in  FIG. 1 ) for performing other operations of the control device  102 . 
     The DSP  108  executes various processes and performs operations (e.g., processing and communicating information) in response thereto. For example, the DSP  108  receives: (a) input voltage signals (e.g., from an audio decoder of the portable handheld electronics device); (b) instructions of computer-readable software programs that are stored on the computer-readable medium  112 ; and (c) optionally, the output voltage signals from the amplifier  110 , so that the DSP  108  controls the output voltage signals in a feedback loop. Accordingly, the DSP  108  executes such programs and performs its operations in response to such input voltage signals, such instructions, and optionally in response to the output voltage signals. For executing such programs, the DSP  108  processes data, which are stored in memory of the DSP  108  and/or in the computer-readable medium  112 . 
       FIG. 2  is a side sectional view, in elevation, of the speaker  104 . As shown in  FIG. 2 , a voice coil is attached to a diaphragm, which is mounted on a fixed frame via a suspension. A permanent magnet generates a concentrated magnetic field in a region of the voice coil&#39;s gap. Such magnetic field is conducted to such region via a magnetic circuit. Rear-side ventilation occurs through holes in a rear enclosure of the fixed frame. 
     According to laws of electrodynamics, in response to the concentrated magnetic field, an electromotive force (“EMF”) f c  is generated by an electrical current passing through the voice coil. Such voice-coil force f c  varies in response to an amount of such electrical current, which varies in response to the output voltage signals from the amplifier  110 . Such voice-coil force f c  causes a displacement x d  (which is excursion) of the diaphragm, thereby generating a sound wave output of audio signals. 
       FIG. 3  is a data flow diagram of the control device  102  operations for protecting the speaker  104 . As shown in  FIG. 3 , in response to a current sampling interval n&#39;s input voltage v c [n] signal (e.g., from an audio decoder of the portable handheld electronics device), the current sampling interval n&#39;s targeted peak excursion x d   c [n] of the speaker  104 , a previous sampling interval n−1&#39;s targeted peak excursion x d   c [n−1] of the speaker  104 , the previous sampling interval n−1&#39;s driving output voltage v* c [n−1] signal, and a nonlinear voltage-to-excursion model  302 , the control device  102  predicts a next sampling interval n+1&#39;s estimated peak excursion x d [n+1] of the speaker  104  that would be caused by such v c [n], x d   c [n], x d   c [n−1], v* c [n−1] and {dot over (x)} d   c [n−1] (“voltage-to-excursion operation”). 
     In response to x d [n+1], the control device  102  selectively performs an excursion compression  304  operation to determine and specify the next sampling interval n+1&#39;s targeted peak excursion x d   c [n+1] of the speaker  104 , so that: (a) in response to x d [n+1] exceeding a programmable displacement threshold&#39;s safe peak excursion limit, the control device  102  specifies x d   c [n+1] by compressing x d [n+1] at a programmable compression ratio (e.g., instead of hard clipping the excursion at the safe peak excursion limit); and (b) otherwise, the control device  102  specifies x d   c [n+1]=x d [n+1]. 
     In response to a nonlinear excursion-to-voltage model  306  (which is an inverse of the nonlinear voltage-to-excursion model  302 ), the control device  102  translates x d   c [n+1] into the current sampling interval n&#39;s driving output voltage v* c [n] signal (“excursion-to-voltage operation”), which the control device  102  outputs from the amplifier  110  (under control of the DSP  108 ) to substantially cause x d   c [n+1] at the speaker  104 . Accordingly, the speaker  104 : (a) receives the v* c [n] signal from the amplifier  110 ; and (b) outputs audio signals in response thereto. In that real-time manner, the control device  102 : (a) directly protects the speaker  104 ; (b) makes fewer modifications to the driving output voltage v* c [n] signal (in comparison to the input voltage v c [n] signal); and (c) causes less perceived distortion of sound and/or perceived loudness of the speaker  104 . 
     As shown in  FIG. 3 , the control device  102  includes registers  308  for storing values of: (a) x c   c [n+1] from the excursion compression  304  operation; and (b) v* c [n] from the excursion-to-voltage operation. For example: (a) if the current sampling interval n=i+1, then the stored value of x d   c [i+1] is x d   c [n], the stored value of x d   c [i] is x d   c [n−1], and the stored value of v* c [i] is v* c [n−1]; and (b) if the current sampling interval n=i+2, then the stored value of x d   c [i+2] is x d   c [n], the stored value of x d   c [i+1] is x d   c [n−1], and the stored value of v* c [i+1] is v* c [n−1]. 
       FIG. 4  is a graph of the speaker  104  peak excursion (x-axis) in response to voltage signals (y-axis) from the amplifier  110 , according to a first simulation of operation with the control device  102  enabled. As shown in  FIG. 4 , with the control device  102  enabled, the scatter plot is compressed horizontally (x-axis) in response to an example of the programmable displacement threshold&#39;s safe peak excursion limit=±0.45 mm. 
       FIG. 5  is a graph of the speaker  104  peak excursion (x-axis) in response to voltage signals (y-axis) from the amplifier  110 , according to a second simulation of operation with the control device  102  disabled. As shown in  FIG. 5 , with the control device  102  disabled, even if such operation had implemented a conventional DRC technique: (a) the scatter plot would be compressed vertically (y-axis) in response to a voltage threshold&#39;s example limit of v* c [n]=±3 volts; (b) regions of overcompression would exist, where the voltage signals would be unnecessarily compressed (in a manner that distorts sound and/or reduces perceived loudness of the speaker  104 ), despite peak excursion of the speaker  104  already being within a safe operating range (within an example of the programmable displacement threshold&#39;s safe peak excursion limit=±0.5 mm); and (c) regions of protection failure would exist, where over-excursion of the speaker  104  may still occur (beyond the example of the programmable displacement threshold&#39;s safe peak excursion limit=±0.5 mm), despite the voltage signal&#39;s amplitude being within the voltage threshold&#39;s example limit of v* c [n]=±3 volts. Accordingly, in a comparison of  FIG. 4  and  FIG. 5 , a more effective (e.g., precise and timely) is achieved with the control device  102  enabled ( FIG. 4 ). 
     Referring again to  FIG. 2 , a continuous-time nonlinear model for electrical behavior of the speaker  104  is:
 
 v   c ( t )= R   eb   i   c ( t )+φ 0   {dot over (x)}   d ( t ),  (1)
 
where v c (t) is a voltage input across terminals of the voice coil, R eb  is a blocked electrical resistance, i c (t) is a voice coil current, φ 0  is a transduction coefficient at an equilibrium state x d (t)=0, x d (i) is a diaphragm excursion, and {dot over (x)} d (t) is a diaphragm velocity.
 
     Mechanical dynamics of the speaker  104  can be modeled as a single-degree-of-freedom mechanical oscillator by:
 
 m   d   {umlaut over (x)}   d ( t )+ c   d   {dot over (x)}   d ( t )+ k   d   x   d ( t )= f   c ( t ),  (2)
 
where m d  is a mass of the diaphragm, c d  is a mechanical resistance due to diaphragm suspension, k d  is a mechanical stiffness due to diaphragm suspension, {umlaut over (x)} d (t) is a diaphragm acceleration, and f c (t) is an EMF exerted on the voice coil. At the equilibrium state where x d (t)=0,
 
 f   c ( t )=φ 0   i   c ( t ).  (3)
 
     By combining the electrical and mechanical loudspeaker models of equations (1), (2) and (3), an s-domain transfer function of excursion versus voltage input at the equilibrium state is 
     
       
         
           
             
               
                 
                   
                     
                       
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     A z-domain transfer function can be obtained by applying either a bilinear transformation or an impulse invariance method to equation (4). 
     However, in order to yield a more precise model, several additional nonlinear factors are considered. For example, mechanical nonlinearities are caused by variations of the transduction coefficient φ and suspension stiffness k d , as parabola-like functions in relation to the excursion x d (t). Accordingly, a more precise expression for f c (t) is
 
 f   c ( t )=φ( x   d ( t )) i   c ( t )− k   1 ( x   d ( t )) x   d ( t ),  (5)
 
where k 1 (x d (t)) is a variation of the suspension stiffness k d  as a function of excursion, which is expressed as
 
 k   1 ( x   d ( t ))= k   d ( x   d ( t ))− k   d (0),  (6)
 
     Similarly, electrical nonlinearities are caused by variations of R eb  in relation to temperature T, as expressed by
 
 R   eb ( T )= R   eb ( T   0 )(1+α( T−T   0 )),  (7)
 
where α is a temperature coefficient (α copper =0.004K −1 ), and T 0  is an ambient temperature. Accordingly, equation (1) is rewritten as
 
 v   c ( t )= R   eb ( t ) i   c ( t )+φ( x   d ( t )) {dot over (x)}   d ( t ),  (8)
 
     Equations (2), (5) and (8) complete the continuous-time nonlinear model of the speaker  104 . 
     For digital processing, the control device  102  implements a discrete-time nonlinear model of the speaker  104 , which is discussed hereinbelow. From equation (2), a transfer function of mechanical receptance x m (s) is 
     
       
         
           
             
               
                 
                   
                     
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     Using the impulse-invariance method, a z-domain transfer function for X m (s) is 
                         H     X   m       ⁡     (   z   )       =           x   d     ⁡     (   z   )           f   c     ⁡     (   z   )         =         b   1     ⁢     z     -   1           1   +       a   1     ⁢     z     -   1         +       a   2     ⁢     z     -   2                 ,           (   10   )               
where a 1 , a 2 , b 1  are functions of m d , c d , k d  and a sampling frequency F s . Accordingly, a discrete-time diaphragm excursion x d [n] is expressed as
 
 x   d   [n]=b   1   f   c   [n− 1 ]−a   1   x   d   [n− 1 ]−a   2   x   d   [n− 2]  (11)
 
where a discrete-time EMF f c [n] exerted on the voice coil is determined from equations (8) and (5) as:
 
     
       
         
           
             
               
                 
                   
                     
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     A diaphragm velocity {dot over (x)} d [n] is computed by differentiating x d [n] according to a first order IIR filter, which is expressed as
 
 {dot over (x)}   d   [n]= 2 F   s ( x   d   [n]−x   d   [n− 1])− a   dt   {dot over (x)}   d   [n− 1],  (13)
 
where F s  is a sampling frequency, and 0&lt;a dt ≦1 is a differentiator coefficient for ensuring stability.
 
     Equations (11), (12) and (13) complete the discrete-time nonlinear model of the speaker  104 . The control device  102  implements the discrete-time nonlinear model of the speaker  104  by performing the voltage-to-excursion operation in accordance with equations (11), (12) and (13). In such implementation by the control device  102 , the diaphragm excursion x d [n+1] is a function of: v c [n]; x d   c [n]; x d   c [n−1]; v* c [n−1]; and the current sampling interval n&#39;s diaphragm velocity {dot over (x)} d   c [n]. The control device  102  computes {dot over (x)} d   c [n] in accordance with equation (13), so that {dot over (x)} d   c [n] is a function of: x d   c [n]; x d   c [n−1]; and {dot over (x)} d   c [n−1]. 
     In view of point-wise nonlinearity of the transduction coefficient φ and suspension stiffness k d  in relation to the excursion x d (t), the voltage-to-excursion relationship is inverted for performance of the excursion-to-voltage operation by the control device  102 . For example, according to equation (11), 
     
       
         
           
             
               
                 
                   
                     
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     Moreover, according to equations (8) and (5), 
     
       
         
           
             
               
                 
                   
                     
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       FIG. 6  is a graph of an example frequency response of the transfer function in equation (4).  FIG. 6  shows a linearized micro-loudspeaker excursion-to-voltage frequency response with m d =0.014 g, c d =0.039 Ns/m, k d =284 N/m, φ 0 =0.3 N/A and R eb =7.5Ω. The lowpass characteristic suggests that large excursions are caused by low frequency components of the input voltage v c [n] signal. 
       FIG. 7  is a graph of an example frequency response of a highpass filter.  FIG. 7  shows the frequency response of a corresponding digital Butterworth highpass filter with f cutoff =200 Hz and F s =16 kHz. Although the graph of  FIG. 6  indicates that a highpass filtering of the input signal might protect the speaker  104  against excessive excursion (by reducing gain in low frequency signal components), the graph of  FIG. 7  indicates that such highpass filtering could impair low frequency content more than necessary when protecting against only a any over-excursions. 
       FIG. 8  is a graph of a gain mapping characteristic of the excursion compression  304  operation.  FIG. 8  shows an input-output relationship for the excursion compression  304  operation, which the control device  102  performs at a programmable compression ratio (e.g., instead of hard clipping the excursion at the safe peak excursion limit) to directly protect against over-excursion. For example, by suitably programming the compression ratio (e.g., if the compression ratio is less than 5), the control device  102  is adaptable to perform the excursion compression  304  operation with relatively smooth compression (e.g., soft knees) and less perceived distortion.  FIG. 8  shows the compression ratio programmed to implement piecewise linear curves for the excursion compression  304  operation in the illustrative embodiments, but the compression ratio is programmable to implement smoother nonlinear curves for the excursion compression  304  operation in alternative embodiments. By comparison, if the compression ratio=∞, then the excursion compression  304  operation equates to hard clipping. 
       FIG. 9  is a graph of excursion when playing an example clip of music without filtering.  FIG. 10  is a graph of excursion when playing the example clip of music with conventional highpass filtering protection, in a case where the highpass filter&#39;s frequency response is the same as shown in  FIG. 7 .  FIG. 11  is a graph of excursion when playing the example clip of music with the control device  102  enabled for excursion protection according to the illustrative embodiments, in a case where the gain mapping curve has a threshold of 0.9x max  and a compression ratio of 2. In the example of  FIGS. 9-11 , the sampling frequency is F s =16 kHz, and the programmable displacement threshold&#39;s safe peak excursion limit is x max =±0.5 mm. 
     As shown in  FIG. 9 , without protection, the excursions occasionally exceed an example safe peak excursion limit=±0.5 mm. As shown in the example of  FIG. 10 , conventional highpass filtering protection causes distortion relative to the graph of  FIG. 9  (e.g., such filtering reduces excursion, but suppresses a bass range of sounds, and degrades the input voltage signal-to-noise ratio to −0.93 dB). In a comparison of  FIG. 10  and  FIG. 11 , relatively little distortion is caused by the control device  102  enabled for excursion protection according to the illustrative embodiments (e.g., the input voltage signal-to-noise ratio is enhanced to 18.97 dB). 
     As shown in  FIG. 11 , with the control device  102  enabled for excursion protection according to the illustrative embodiments, the control device  102  modifies the driving output voltage v* c [n] signal (in comparison to the input voltage v c [n] signal), but only if the control device  102  (in accordance with the discrete-time nonlinear model of the speaker  104 ) predicts that the unmodified input voltage v c [n] signal would otherwise cause over-excursion of the speaker  104 . Accordingly, in a comparison of  FIG. 10  and  FIG. 11 , the control device  102  makes fewer modifications to the driving output voltage v* c [n] signal (in comparison to the input voltage v c [n] signal), distortions are relatively few in comparison to the example of  FIG. 10 , and such modifications have a smaller average magnitude. 
     In the illustrative embodiments, a computer program product is an article of manufacture that has: (a) a computer-readable medium; and (b) a computer-readable program that is stored on such medium. Such program is processable by an instruction execution apparatus (e.g., system or device) for causing the apparatus to perform various operations discussed hereinabove (e.g., discussed in connection with a block diagram). For example, in response to processing (e.g., executing) such program&#39;s instructions, the apparatus (e.g., programmable information handling system) performs various operations discussed hereinabove. Accordingly, such operations are computer-implemented. 
     Such program (e.g., software, firmware, and/or microcode) is written in one or more programming languages, such as: the DSP  108  native assembly language; a procedural programming language (e.g., C); an object-oriented programming language (e.g., Java, Smalltalk, and C++); and/or any suitable combination thereof. In a first example, the computer-readable medium is a computer-readable storage medium. In a second example, the computer-readable medium is a computer-readable signal medium. 
     A computer-readable storage medium includes any system, device and/or other non-transitory tangible apparatus (e.g., electronic, magnetic, optical, electromagnetic, infrared, semiconductor, and/or any suitable combination thereof) that is suitable for storing a program, so that such program is processable by an instruction execution apparatus for causing the apparatus to perform various operations discussed hereinabove. Examples of a computer-readable storage medium include, but are not limited to: an electrical connection having one or more wires; a portable computer diskette; a hard disk; a random access memory (“RAM”); a read-only memory (“ROM”); an erasable programmable read-only memory (“EPROM” or flash memory); an optical fiber; a portable compact disc read-only memory (“CD-ROM”); an optical storage device; a magnetic storage device; and/or any suitable combination thereof. 
     A computer-readable signal medium includes any computer-readable medium (other than a computer-readable storage medium) that is suitable for communicating (e.g., propagating or transmitting) a program, so that such program is processable by an instruction execution apparatus for causing the apparatus to perform various operations discussed hereinabove. In one example, a computer-readable signal medium includes a data signal having computer-readable program code embodied therein (e.g., in baseband or as part of a carrier wave), which is communicated (e.g., electronically, electromagnetically, and/or optically) via wireline, wireless, optical fiber cable, and/or any suitable combination thereof. 
     Although illustrative embodiments have been shown and described by way of example, a wide range of alternative embodiments is possible within the scope of the foregoing disclosure.