Patent Publication Number: US-2007098188-A1

Title: Equalization setting determination for audio devices

Description:
CROSS REFERENCE TO RELATED PATENTS  
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
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     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC  
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     BACKGROUND OF THE INVENTION  
      1. Technical Field of the Invention  
      This invention relates generally to audio devices and more particularly to audio processing.  
      2. Description of Related Art  
      As is known, the audio frequency spectrum is from approximately 20 Hz to 20 KHz and that the human ear is “tuned” to about 1 KHz (i.e., audio signals at 1 KHz are more easily heard than audio signals at the same power level, but at the edges of the audio frequency spectrum). While this is the natural occurrence of human reception of audio signals, audio playback devices attempt to provide audio signals throughout the audio frequency spectrum at levels for humans to hear as easily as we hear audio signals around 1 KHz.  
      One technique that audio playback devices (e.g., CD players, MP3 players, radio, computer audio codecs, etc.) utilize to provide more uniform perception of audio signals is non-linear quantization when converting between analog audio signals and digital audio signals. As is known, a non-linear quantization system may be generally viewed as a linear system having a compander added thereto. In such a system, an analog audio signal is first compressed using a non-linear law (e.g., A-law, μ-law) to produce a non-linear compressed signal and then linearly quantized to produce a digital audio signal. To convert a digital audio signal into an analog audio signal, the reverse process is followed using an inverse of the non-linear law.  
      In addition to non-linear quantization, many audio devices provide equalization. In a basic audio device, equalization is achieved by dividing the audio frequency spectrum into at least two regions (e.g., bass and treble). For each region, the user of the audio device may set the equalization level (i.e., the gain for the corresponding frequency spectrum) to make the signals within the region more or less perceivable to the human ear. In more complex audio devices, the audio frequency spectrum is divided into more than two regions with user control to set the equalization for each region.  
      A conflict arises with battery powered audio devices between providing maximum audio outputs over the audio frequency spectrum and battery life extension. As is generally known, battery life of a battery powered audio device is extended by reducing power consumption, which is at least partially achieved by reducing the operating voltage of audio device. This, however, may cause audio output drivers to clip when equalization and/or volume settings are set at near maximum levels. Such clipping is most problematic for bass signals (i.e., audio signals in lower portion of the audio frequency spectrum), which, when clipped, produce distortion within the audio frequency spectrum. Such distortion reduces the signal quality of the audio device.  
      Therefore, a need exists for a method and apparatus for automatically reducing the potential of distortion caused by clipping of lower frequency audio signals. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       FIG. 1  is a schematic block diagram of an audio device in accordance with the present invention;  
       FIG. 2  is a schematic block diagram of another audio device in accordance with the present invention;  
       FIG. 3  is a graph of human perceptibility of an audio signal spectrum;  
       FIGS. 4-8  are frequency spectrum graphs of equalization settings in accordance with various embodiments of the present invention; and  
       FIG. 9  is a schematic block diagram of an audio output stage in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  is a schematic block diagram of an audio device  10 , which may be included in an MP3 player, a computer audio system, a CD player, a television audio output, a DVD audio player, et cetera. The audio device  10  includes memory  14 , a processing module  12 , a digital-to-analog conversion (DAC) module  16 , an audio output stage  18  and a headphone jack  20 . The processing module  12  may be a single processing device or a plurality of processing devices. Such a processing device may be a microprocessor, micro-controller, digital signal processor, microcomputer, central processing unit, field programmable gate array, programmable logic device, state machine, logic circuitry, analog circuitry, digital circuitry, and/or any device that manipulates signals (analog and/or digital) based on operational instructions. The memory  14  may be a single memory device, a plurality of memory devices, and/or embedded circuitry of the processing module. Such a memory device may be a read-only memory, random access memory, volatile memory, non-volatile memory, static memory, dynamic memory, flash memory, cache memory, and/or any device that stores digital information. Note that when the processing module  12  implements one or more of its functions via a state machine, analog circuitry, digital circuitry, and/or logic circuitry, the memory storing the corresponding operational instructions may be embedded within, or external to, the circuitry comprising the state machine, analog circuitry, digital circuitry, and/or logic circuitry. Further note that, the memory  14  stores, and the processing module  12  executes, operational instructions corresponding to at least some of the steps and/or functions illustrated in  FIGS. 1-9 .  
      In operation, the processing module  12  retrieves audio files  22  from memory  14 . The audio files  22  may be stored in an MP3 format, a DVD audio format, a WMA format, and/or any other type of audio encoding protocol. The processing module  12  decodes, in accordance with a particular encoding protocol, the audio files  22  to produce digital audio signals  24 .  
      The processing module  12  also generates an equalization setting  28  to avoid low frequency distortion of the audio output  32 . In one embodiment, the processing module  12  generates the equalization setting  28  by monitoring a volume setting  30  with respect to a range of volume settings to produce a monitored volume setting. The processing module  12  then establishes the equalization settings  28  based on the monitored volume setting. For example, the volume setting  30  may correspond to a 5-bit digital signal (e.g., a digital counter of an up/down switch). As such, the volume setting  30  may be one of thirty-two values. If the largest digital value corresponds to the maximum volume setting, then there is a volume setting below the maximum setting where clipping of the audio output may occur, thus producing low frequency distortion. At this volume setting, and for volume settings above this setting to the maximum setting, the processing module generates the equalization setting  28  such that clipping of the lower frequency components of the audio output  32  is substantially avoided, thereby avoiding the low frequency distortion.  
      In another embodiment, the processing module  12  may use the supply voltage  34  in additional to the volume setting  30  to generate the equalization setting  28 . In this embodiment, the processing module  12  uses the supply voltage  34  to determine whether the “volume setting threshold” as discussed in the example of the preceding paragraph should be adjusted. If the supply voltage  34  is nominal (e.g., 1.5 Volts to 3.3 Volts dependent on the fabrication process of the components of the audio device  10 ), the volume setting threshold is not adjusted for generating the equalization setting. If, however, the supply voltage  34  is less than nominal, the audio output is more likely to clip and produce noticeable low frequency distortion at the volume setting threshold, thus it is decreased. If the supply voltage is greater than nominal, the audio output is less likely to clip and produce noticeable low frequency distortion at the volume setting threshold, this it may be increased. The generation of the equalization setting  28  will be described in greater detail with reference to  FIGS. 3-8 .  
      The digital-to-analog conversion module  16  converts the digital audio signals  24  into analog audio signals  26  and provides the analog audio signals  26  to the audio output stage  18 . The audio output stage  18 , which may include one or more amplifiers and/or drivers, converts the analog audio signal  24  into the amplified audio output  32  based on the equalization setting  28  and the volume setting  30 . In one embodiment, the audio output stage  18  provides the amplified audio output  32 , which may be a monotone signal, a stereo signal and/or a surround sound signal, to one or more headphone jacks  20 . In other embodiments, the audio output stage  18  may provide the amplified audio output  32  to one or more speakers. If the audio output stage  18  is providing the amplified audio output  32  to a single speaker, the audio output stage  18  may mix a left and right channel of a stereo signal to produce a monotone signal.  
      In an alternative embodiment, the processing module  12  may provide the equalization setting  28  to the DAC module  16 . In this embodiment, the DAC module  16  converts the digital audio signal  24  into the analog audio signal  26  based on the equalization setting  28  such that the low frequency components of the audio signal are scaled in accordance with the equalization setting  28  to avoid low frequency distortion. In another embodiment, the processing module  12  may utilize the equalization setting  28  to scale low frequency components of the digital audio signal  24  to avoid the low frequency distortion.  
       FIG. 2  illustrates a schematic block diagram of another audio device  40  that may be included in an MP3 player, a computer audio system, a CD player, a television audio output, a DVD audio player, et cetera. The audio device  10  includes the processing module  12 , memory  14 , DAC module  16 , audio output stage  18 , and headphone jack  20 . In this embodiment, the processing  12  decodes audio files  22  to produce the digital audio signals  24 ; the DAC module  16  converts the digital audio signals  24  into the analog audio signals  26 ; and the audio output stage  18  amplifies the analog audio signals  26  based on the equalization setting  28  and the volume setting to produce the amplified audio output  32  as discussed with reference to  FIG. 1 .  
      In this embodiment, the processing module  12  generates the equalization setting  28  based on a signal level of the digital audio signal  24  and/or of the analog audio signal  26 . To begin, the processing module  12  monitors the signal level of the digital audio signal  24  and/or the signal level of analog audio signal  26  to produce a monitored signal level. Note that the processing module  12  may monitor the signal level of the low frequency components of the signal  24  and/or  26  to produce the monitored signal level. The processing module  12  compares the monitored signal level with a low frequency distortion threshold (e.g., a signal level at which low frequency distortion might occur) to determine if the amplified audio output  32  might clip. If so, the processing module  12  generates the equalization setting  28  to scale the low frequency components of the amplified audio output  32 , the digital audio signal  24 , and/or the analog audio signal  26  to substantially avoid low frequency distortion.  
      In another embodiment, the processing module  12  may use the supply voltage  34  in additional to the signal level to generate the equalization setting  28 . In this embodiment, the processing module  12  uses the supply voltage  34  to determine whether the low frequency distortion threshold should be adjusted. If the supply voltage  34  is nominal, the low frequency distortion threshold is not adjusted for generating the equalization setting. If, however, the supply voltage  34  is less than nominal, the audio output is more likely to clip and produce noticeable low frequency distortion at the low frequency distortion threshold, thus it is decreased. If the supply voltage is greater than nominal, the audio output is less likely to clip and produce noticeable low frequency distortion at the low frequency distortion threshold, this it may be increased. The generation of the equalization setting  28  will be described in greater detail with reference to  FIGS. 3-8 .  
      In yet another embodiment, the processing module  12  may utilize the signal level, the supply voltage  34 , and/or the volume setting  30  to generate the equalization setting  28 .  
       FIG. 3  is a graph of human perceptibility to an audio signal frequency spectrum. As shown, the audio signal spectrum ranges from approximately 20 hertz to approximately 20 kilohertz with, respect to the human ear, peaks at approximately 1 kilohertz. In order for the human ear to perceive frequencies at the outer edges of the audio signal spectrum at the same level as signals near 1 kilohertz, increased gains may be used. Such increased gain may potentially cause clipping in the audio output stage of the lower and higher frequency components of an audio signal. If the lower frequency signal components are clipped, distortion in the audio output occurs. In order to avoid the low distortion caused by clipping of the low frequency signal components of an audio signal, the processing module  12  generates the equalization setting  28  as previously discussed and as further illustrated in the examples of  FIGS. 4-8 .  
       FIG. 4  is a graph of the frequency spectrum of an example of the equalization setting  28  generated by the processing module  12  based on the volume setting  30  with respect to a range of volume settings  50 . As shown, the frequency spectrum is divided into a low, mid and high range. Note that the frequency spectrum may be divided into more or less regions than three. In this example, when the volume level, with respect to the range of volume settings  50  is low, the equalization setting  28  is consistent (i.e., at the same level), throughout the frequency spectrum. Once the volume setting reaches the “volume setting threshold”, the lower range of the frequency spectrum is held at a lower gain setting than the gain setting of the mid and high ranges. As the volume setting increases above the volume setting threshold, the differential between the gain settings for the lower frequencies with respect to the mid and higher range frequencies is increased. With such an equalization setting  28 , gain of the lower frequency components of the digital audio signal  24 , the analog audio signal  26 , and/or the amplified audio output  32  is controlled such that low frequency distortion is substantially avoided.  
       FIG. 5  is a graph of the frequency spectrum of another example of the equalization setting  28  generated by the processing module  12  based on the signal level of the digital audio signal  24  and/or the analog audio signal  26 . In this embodiment, as long as the signal level of the digital or analog signal  24 ,  26  is below the low frequency distortion threshold, the equalization setting  28 , or gain, is substantially constant throughout the frequency spectrum. Once the signal level reaches, or exceeds, the threshold, the gain of the lower frequency spectrum is held at a particular value while the gain for the mid and high range frequency spectrum are allowed to increase corresponding to the desired signal level. Note that a combination of the signal level and volume setting may be used to establish the equalization setting  28 .  
       FIGS. 6 &amp; 7  illustrate an example of adjusting the equalization setting  28  based on the supply voltage. Regardless of whether the equalization setting  28  was initially generated based on the volume setting  30  and/or the signal level, the processing module  12  may adjust the thresholds. For instance, as the supply voltage decreases, the potential for clipping of the lower frequency audio signal components is greater. Thus, the equalization setting  28  is adjusted to have the lower frequency component gain held at a lower level than the mid and high frequency ranges at a lower threshold as shown in  FIG. 6 . In this example, the reduction of the gain of the low frequency components of the equalization  28  is held low at a much lower threshold than in the examples of  FIGS. 4 and 5 .  
      Conversely, when the supply voltage is greater, there is less chance of clipping of the lower frequency signal components. Accordingly, the gain of the lower frequency components of the equalization setting  28  can rise to higher levels before being held at a constant value to avoid clipping. An example of this is illustrated in  FIG. 7 .  
       FIG. 8  is yet another example of the equalization setting  28  being held at a particular constant value throughout the spectrum when the signal level, or the volume setting, exceeds a corresponding threshold. In this example, there is a sharp roll-off between the gain of the low frequency section and the mid frequency range section. Note that the examples of  FIGS. 4-8  are merely examples and do not illustrate exact frequency responses. For example, the frequency response in the lower volume range does not have to be flat; it could be of any shape. Further, the corner frequencies for the lower frequency response changes could be at various frequencies and the roll off may be greater or less than illustrated. Further note that the equalization setting adjustment may to be used to avoid clipping of higher frequency signal components, but clipping of such components generally do not produce noticeable distortion.  
       FIG. 9  is a schematic block diagram of an embodiment of the audio output stage  18 . In this embodiment, the audio output stage  18  includes a low frequency equalization module  60 , a mid frequency equalization module  62 , and a high frequency equalization module  64 . Each module  60 - 64  may include one or more amplifiers and/or drivers, wherein the gain of the module  60 - 64  is adjusted based on the corresponding frequency component section of the equalization setting  28  and amplifies the analog audio signal  24  accordingly. The outputs of the modules  60 - 64  are summed to produce the amplified audio output  32 .  
      As one of ordinary skill in the art will appreciate, the term “substantially” or “approximately”, as may be used herein, provides an industry-accepted tolerance to its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from less than one percent to twenty percent and corresponds to, but is not limited to, component values, integrated circuit process variations, temperature variations, rise and fulll times, and/or thermal noise. Such relativity between items ranges from a difference of a few percent to magnitude differences. As one of ordinary skill in the art will further appreciate, the term “operably coupled”, as may be used herein, includes direct coupling and indirect coupling via another component, element, circuit, or module where, for indirect coupling, the intervening component, element, circuit, or module does not modify the information of a signal but may adjust its current level, voltage level, and/or power level. As one of ordinary skill in the art will also appreciate, inferred coupling (i.e., where one element is coupled to another element by inference) includes direct and indirect coupling between two elements in the same manner as “operably coupled”. As one of ordinary skill in the art will further appreciate, the term “operably associated with”, as may be used herein, includes direct and/or indirect coupling of separate components and/or one component being embedded within another component. As one of ordinary skill in the art will still further appreciate, the term “compares favorably”, as may be used herein, indicates that a comparison between two or more elements, items, signals, etc., provides a desired relationship. For example, when the desired relationship is that signal  1  has a greater magnitude than signal  2 , a favorable comparison may be achieved when the magnitude of signal  1  is greater than that of signal  2  or when the magnitude of signal  2  is less than that of signal  1 .  
      The preceding discussion has presented various methods and apparatus for reducing and/or avoiding low frequency distortion caused by clipping of low frequency signal components in an audio device. As one of average skill in the art will appreciate, other embodiments may be derived from the teaching of the present invention without deviating from the scope of the claims.