Abstract:
An audio driver with reduced rub and buzz distortion that includes a digital processing module. A digital to audio converter (DAC) operable to receive a digital audio signal supplied by the digital processing module. One or more analog driver stages operable to receive an analog audio signal supplied by the DAC. A peak amplitude compressor.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. provisional application Ser. No. 61/290,001, filed Dec. 23, 2009, which is hereby incorporated by reference for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to audio noise reduction, and more specifically to the prevention of rub and buzz distortion. 
       BACKGROUND OF THE INVENTION 
       [0003]    Rub and buzz is a particular type of sound distortion which originates in mechanical faults in loudspeakers (as used herein, speaker and loudspeaker may be used interchangeably). Also referred to as “triggered distortion” in the literature, rub and buzz is an impulsive and unwanted form of non-linear distortion, which is both annoying to the listener and may even damage the loudspeaker. Furthermore, rub and buzz is problematic because it is not easily measurable using normal techniques such as total harmonic distortion (THD) analysis or root-mean-square (RMS) fast Fourier transform (FFT) testing, and therefore often requires human intervention to detect. Rub and buzz distortion is typically caused by mechanical or structural defects in speakers, for example, a misaligned voice coil that rubs due to a bent surround or a loose spider. 
         [0004]      FIG. 1  is a diagram of a typical prior art audio output system that includes digital processing module  102 , which can have digital filters, digital equalizers and other digital circuitry used to process audio signals.  FIG. 1  further includes digital to analog converter (DAC)  104 , analog output stages  106  which typically comprise an amplifier and output driver which drives speaker  108 . However, no circuitry is available to address the problem of rub and buzz sound distortion. 
       SUMMARY OF INVENTION 
       [0005]    One of the difficulties in detecting rub and buzz distortion is that, contrary to other types of speaker nonlinearities, it is often manifested as high order harmonics of the fundamental (desired) signal. This periodic high frequency content is more irritating to the human ear than lower harmonics that characterize amplifier distortion or other loudspeaker nonlinearities. Conventional Total Harmonic Distortion tests may not detect rub and buzz, because they do not take into consideration the order of the harmonics when diagnosing the problem. Techniques for determining rub and buzz, on the other hand, focus on examining the power of the high order harmonics that the loudspeaker produces as a response to a given test signal. 
         [0006]    At present, specialized tests are used to detect rub and buzz distortion in a speaker produced in a factory. When a speaker exhibits significant rub and buzz, it is discarded as part of the manufacturer&#39;s quality assurance process. However, it has been recognized by the inventors that speakers in many applications, such as automotive loudspeakers, are subjected to many stresses and strains while in use, so that structural or mechanical defects can be introduced over the course of time. Based on the recognition of this problem, it has been determined that the need exists to electronically address the problem of rub and buzz distortion in a speaker over time, and not just at the factory. 
         [0007]    In one exemplary embodiment, the disclosed system and method for minimizing the occurrence of rub and buzz distortion is based on the assumption that rub and buzz noise is created when the loudspeaker displacement exceeds a given threshold. This assumption has been corroborated by empirical evidence, and can be explained by the fact that the distortion is caused by a mechanical constriction that inhibits the speaker&#39;s movement. Because the loudspeaker displacement can vary in frequency, one exemplary embodiment of the algorithm for correcting rub and buzz distortion utilizes the displacement transfer function, where the total displacement at any given time can be calculated from the input signal alone. 
         [0008]    Techniques such as clipping, automatic gain control, phase modification, or a combination thereof can be used to minimize the incidence of rub and buzz distortion. In one exemplary embodiment, a phase modification system compresses the peak amplitude of the audio signal without changing its spectral characteristics, where individual sinusoidal tracks in speech are monitored. In another exemplary embodiment, tracks with low power are discarded. In yet another exemplary embodiment, the overall signal is attenuated based on the peak amplitude after the phase modification. 
         [0009]    Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]    Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0011]      FIG. 1  is a diagram of a typical prior art audio output system; 
           [0012]      FIG. 2  is a diagram of a clipping circuit in an audio path in accordance with an exemplary embodiment of the present invention; 
           [0013]      FIG. 3A  is a diagram of a basic analog implementation of automatic gain control (AGC) in an audio path in accordance with an exemplary embodiment of the present invention; 
           [0014]      FIG. 3B  is a diagram of a digital implementation of an AGC in accordance with an exemplary embodiment of the present invention; 
           [0015]      FIG. 4A  is a diagram of an audio driver using phase modification to reduce peak amplitudes in accordance with an exemplary embodiment of the present invention; 
           [0016]      FIG. 4B  is a diagram of an audio driver using speech based phase modification in accordance with an exemplary embodiment of the present invention; 
           [0017]      FIG. 5  is a diagram of an audio driver using the phase modification described in  FIG. 4B  and the AGC shown in  FIG. 3  in accordance with an exemplary embodiment of the present invention; and 
           [0018]      FIG. 6  is a flow chart of a phase modification process with optional enhancements in accordance with an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A detailed description of embodiments of the present invention is presented below. While the disclosure will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure. 
         [0020]    Rub and buzz can be created by a mechanical fault that disturbs the normal movement of the loudspeaker&#39;s cone. This disruption can result in an impulsive non-linear interference whenever the cone is displaced by a certain distance(s) from its point of origin. The rub and buzz can be ameliorated by decreasing the signal peak amplitude, so as to avoid the disturbance. In one exemplary embodiment, attenuation, dynamic range compression or automatic gain control can be used to result in a decrease in loudness, but this approach is undesirable in practical applications. 
         [0021]    Because the human hearing mechanism is somewhat insensitive to differences in the phase of a signal, two waveforms with a completely different waveform envelope may be indistinguishable to a human listener. This allows the peak amplitude to be reduced by separating the signal into overlapping windowed frames, analyzing the signal in each frame as a collection of sine waves at different amplitudes, frequencies and phases, altering the phases of the sine waves so that the sum of the modified sine waves will have an envelope that has lower peaks, and then combining the modified sine waves to synthesize a signal that is more dispersed, while maintaining the magnitude response of the signal. In this manner, the spectral shape or the gain of the signal is not changed, and the change in phase will result in an acceptable level of reduced distortion. 
         [0022]    Common forms of non-linearity are studied almost exclusively in the frequency domain, using measurements such as Total Harmonic Distortion (THD). Based on the assumption that rub and buzz occurs when the loudspeaker reaches a certain peak displacement and that the displacement of the loudspeaker isn&#39;t constant across frequencies, it follows that rub and buzz has both time and frequency domain aspects that should be taken into account when searching for a solution. 
         [0023]    Another characteristic of rub and buzz is that it tends to occur suddenly. Specifically, there is not a gradual rise in rub and buzz distortion as the amplitude increases, but rather there is an onslaught of rub and buzz distortion whenever the amplitude of the signal exceeds a certain threshold. 
         [0024]    Using this novel observation, techniques for maintaining a signal below a threshold can be utilized to control rub and buzz distortion. In one exemplary embodiment involving analog circuitry, such as amplifiers or radio receivers, many components have a linear range of operation with respect to magnitude, frequency, or other signal parameters, so it is important to maintain the magnitude, frequency or other signal parameter of the processed signal within the linear range to control rub and buzz distortion. 
         [0025]      FIG. 2  is a diagram of a clipping circuit  200  in an audio path in accordance with an exemplary embodiment of the present invention. The clipping circuit  200  and other circuits and systems described herein can be collections of discrete components, integrated circuits, or can be one or more software systems operating on a suitable hardware platform. As used herein and by way of example and not by limitation, “hardware” can include a combination of discrete components, an integrated circuit, an application-specific integrated circuit, a field programmable gate array, a general purpose processing or server platform, or other suitable hardware. As used herein and by way of example and not by limitation, “software” can include one or more objects, agents, threads, lines of code, subroutines, separate software applications, one or more lines of code or other suitable software structures operating in one or more software applications or on one or more processors, or other suitable software structures. In one exemplary embodiment, software can include one or more lines of code or other suitable software structures operating in a general purpose software application, such as an operating system, and one or more lines of code or other suitable software structures operating in a specific purpose software application. 
         [0026]    One technique for maintaining a signal&#39;s amplitude within a certain range is clipping. Whenever a signal amplitude exceeds a threshold, a signal value at or slightly below the threshold is supplied, otherwise the signal is allowed to pass. Mathematically, a clipping function           can be expressed as: 
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         [0027]    A digital clipping circuit can also be incorporated as part of the digital processing circuitry. While this technique will prevent rub and buzz distortion, it does have the disadvantage that other forms of distortion are introduced. In fact, clipping itself is usually considered an unacceptable form of distortion. 
         [0028]      FIG. 3A  is a diagram of a basic analog implementation of AGC in an audio path in accordance with an exemplary embodiment of the present invention. AGC is used to control rub and buzz distortion by attenuating the signal based on its amplitude.  FIG. 3A  shows a basic analog implementation of AGC inserted into the audio path. AGC  310  includes amplifier  302  and gain control circuit  304 . Gain control circuit  304  measures the output of amplifier  302  and adjusts amplifier  302  on the basis of the measured signal amplitude. 
         [0029]      FIG. 3B  is a diagram of a digital implementation of an automatic gain controller (AGC) in accordance with an exemplary embodiment of the present invention. Digital AGC  320  is placed between DAC  104  and digital processing module  102 , and can also or alternatively be incorporated as part of digital processing module  102 . Although an AGC does not generally create unacceptable distortion in the manner that clipping does, an AGC compresses the dynamic range of the audio signal, which can impair the listening experience. 
         [0030]    Another exemplary embodiment of a solution for addressing rub and buzz distortion is to maintain the signal amplitude below a threshold while preserving its spectral characteristics. Phase modification is one exemplary approach that can be used to reduce the overall peak amplitudes without significantly altering the spectral characteristics. In this exemplary embodiment, phase modification can selectively add phase shifts to different frequency components of a signal to prevent the peaks at each frequency from aligning. 
         [0031]      FIG. 4A  is a diagram of an audio driver using phase modification to reduce peak amplitudes in accordance with an exemplary embodiment of the present invention. In addition to the components already described, the driver comprises analysis module  404  and phase modification module  402 . Analysis module  404  determines the amplitude and phase of each of the frequency components and can subdivide the time domain signal into a plurality of frames. Analysis module  404  can also determine the optimal phase for each frequency component to result in the smallest overall signal magnitude. These phases are supplied to phase modification module  402 , which modifies each phase according to the optimal phase determined by analysis module  404 . 
         [0032]    While this approach provides certain advantages, the amplitude of a signal might still exceed a predetermined threshold, although this approach reduces the chances of this condition from occurring. Therefore, it is advantageous to compress the amplitude by phase modification. 
         [0033]    In some applications such as cellular telephones, cheap speakers are used which may exhibit rub and buzz distortion. However, in such an application, the audio is generally optimized for a particular type of audio signal, namely speech. Phase modification has been used in the past, particularly in speech applications, to minimize the peak amplitude of a signal when the audio signal is predominantly speech. One such method is disclosed in U.S. Pat. No. 4,856,068 which is incorporated by reference. 
         [0034]      FIG. 4B  is a diagram of an audio driver using speech based phase modification in accordance with an exemplary embodiment of the present invention. In addition to the components already described, the driver comprises analysis module  412 , phase modification module  414 , and synthesis module  416 . The speech-based phase modification approach breaks down the audio signal into sinusoidal tracks. Human speech can be modeled as a plurality of tracks which have a frequency, an amplitude and phase associated with them. Analysis module  412  subdivides a signal into frames and determines the frequency, amplitude and phase of each track in the frame. Phase modification module  414  uses the frequency, amplitude and phase information of each track, as well as the phase and magnitude response from the loudspeaker displacement transfer function to determine an optimal phase for each track in order to minimize the peak displacement. Across the frame, the frequency, amplitude and optimal phase are interpolated. These revised values are then used by synthesis module  416  to construct a new audio signal which causes lower peak speaker displacement. 
         [0035]    In another exemplary embodiment, some of the tracks with lower energy can be omitted to provide greater peak amplitude compression. 
         [0036]    Depending on the application, any suitable peak amplitude compressors can be employed to eliminate or reduce rub and buzz distortion. However, phase modification leads to peak amplitude compression with minimal spectral effects. 
         [0037]    Because phase modification cannot guarantee compression of the peak displacement below a given threshold, phase modification can be combined with other methods, such as an AGC. By doing so, the impact of the AGC on clipping distortion is reduced, while ensuring that the peak displacement remains below a given threshold. 
         [0038]      FIG. 5  is a diagram of an audio driver using the phase modification described in  FIG. 4B  and the AGC shown in  FIG. 3  in accordance with an exemplary embodiment of the present invention. The process of optimizing the peak amplitude in a frame can yield the peak amplitude over the frame, so gain control can be performed within the phase modification module. 
         [0039]      FIG. 6  is a flow chart of a phase modification process with optional enhancements in accordance with an exemplary embodiment of the present invention. In one exemplary embodiment,  FIG. 6  can be implemented as an algorithm operating on a general purpose processing platform, as an algorithm operating in conjunction with a plurality of discrete components under centralized control, or in other suitable manners. 
         [0040]    At step  602 , analysis module  404  subdivides the signal into frames (either overlapping or non-overlapping) and identifies the tracks. Phase modification module  402  or analysis module  404  can optionally eliminate low energy tracks at step  604 . At step  606 , phase modification module  402  can determine the optimal phases to minimize the peak displacement. Phase modification module  402  can take into account the signal, as well as the displacement transfer function of the loudspeaker. Optionally at step  608 , phase modification module  402  can determine the maximum displacement over the frame and at step  610  it can attenuate the entire signal based on the maximum displacement if it exceeds the predetermine threshold. Steps  608  and  610  together represent gain control process  620  mentioned previously. Finally at step  612 , synthesis module  406  reconstitutes the signal with appropriate phase shifts and signal attenuation. 
         [0041]    It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.