Abstract:
Multiple passes are executed in the setup of an equalizer, and modification of the equalization is performed after each pass of an analysis phase. After an initial pass, the equalization is adjusted, based upon the location of peaks and valleys in the system response. This initial adjustment of equalization may tend to flatten most of the peaks and valleys to produce the desired uniform linear response. Inexact application of equalization corrections may introduce other artifacts into the system response and/or may not sufficiently normalize equalization. A second pass is then performed to measure the system response using the new equalization settings. The new peaks and valleys are measured, and the equalization adjusted to try to flatten response further. A proximity range may be applied to each pass, to reduce the likelihood that adjustment of one equalizer coefficient will create artifacts in the resulting system response.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority from Provisional U.S. Patent Application No. 60/612,474 filed on Sep. 23, 2004 (Cirrus Logic Docket No. 1537-DSP), and incorporated herein by reference. The present application is also a Continuation-In-Part of U.S. patent application Ser. No. 11/002,102 entitled “TECHNIQUE FOR SUBWOOFER DISTANCE MEASUREMENT”, filed on Dec. 3, 2004 (Cirrus Logic Docket No. 1538-DSP), and incorporated herein by reference. The present application is also a Continuation-In-Part of U.S. patent application Ser. No. 11/038,577, filed on Jan. 21, 2005 (Cirrus Logic Docket No. 1539-DSP), and incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method and apparatus for automatically equalizing a home theater or other audio system. In particular, the present invention is directed toward a technique for providing continuous or active adaptation of the equalization of a home theater or other audio system.  
       BACKGROUND OF THE INVENTION  
       [0003]     Home theater systems, which once were expensive luxury items, are now becoming commonplace entertainment devices. Complete Home Theater systems, known as a Home Theater In a Box (HTIB), are available to consumers at reasonable prices. However, properly setting up such Home Theater systems can sometimes be problematic for the consumer.  
         [0004]     Home theater systems provide a number of components, which may be located in various parts of the room. The components include the home theater receiver/amplifier, front stereo speakers (left and right), rear surround sound speakers (left and right), a center speaker, and a subwoofer. Various other combinations of speakers may also be used, including additional or fewer speakers. One such home theater system is described, for example, in U.S. Pat. No. 5.930,370, issued Jul. 27, 1999 to Ruzicka, incorporated herein by reference.  
         [0005]      FIG. 1  depicts a diagrammatic view of the home theater surround sound speaker system (the surround sound system)  10  arranged in accordance with the principles of the present invention. The surround sound system  10  includes a source of a preferably amplified stereo signal, shown in  FIG. 4  as television (“TV”) set  12 . The stereo audio source may be any of a number of audio signal sources. It should, thus, be noted that the source of a stereo audio signal is represented herein as television  12 , but the audio signal source may also be a stereo receiver, a car stereo, a portable compact disk or tape player, a portable boom-box type stereo, or any other source of a stereo signal.  
         [0006]     Television  12  outputs an amplified audio signal to interconnect module  14  via a multi-conductor cable  16 . Multi-conductor cable  16  typically includes two conductor pairs for conducting the left and right channels of the stereo signal output by television  12  to interconnect module  14 . Interconnect module  14  receives the audio signals from television  12  and assembles the component left and right channel signals for selective distribution to particular component speakers of the surround sound system  10 .  
         [0007]     The component speakers typically include a sub-woofer  18 , which receives full range left and right signals, but only reproduces the low frequency components of the audio signal. Interconnect module  14  also outputs an audio signal to front center speaker  20 . Front center speaker  20  receives both the left and right component signals of the stereophonic signal and reproduces the (L+R) summation signal. Preferably, front center speaker  20  is located in proximity to television  12  and projects the acoustic output of the (L+R) summation signal toward the listener  28 .  
         [0008]     Interconnect module  14  also outputs the left channel signal to left satellite speaker  22  and right channel signal to right satellite speaker  24 . Left satellite speaker  22  and right satellite speaker  24  may be relatively small speakers and need only reproduce mid range and/or high frequency signals. Left and right satellite speakers are preferably oriented so that the primary axis of radiation of the speaker points upward along a vertical axis; however, other orientations of the satellite speakers may also provide satisfactory performance. Interconnect module  14  also outputs an audio signal to rear ambience speaker  26 . Rear ambience speaker  26  typically receives an audio signal in the form of a left channel minus right channel (L−R) or a right channel minus left channel (R−L) difference signal. As will become apparent throughout this detailed description, several embodiments of the invention described herein enable interconnect module  14  to generate a variety of signals to be output to left satellite speaker  22 , right satellite speaker  24 , and/or rear ambience speaker  26 . It should be noted at the outset that the term speaker refers to a system for converting electrical input signals to acoustic output signals where the system may include one or a number of crossover networks and/or transducers.  
         [0009]     The components described in  FIG. 1  typically are arranged to optimize the surround sound effect to enhance the listening experience of the viewer  28 . The viewer  28  typically faces television  12  which has front center speaker  20  arranged in proximity to television  12  so that center speaker  20  and television  12  radiate their respective audio and video output in the general direction of viewer  28 . The left satellite speaker  22  typically is arranged to the left side of viewer  28  while right satellite speaker  24  is arranged to the right side of viewer  28 , both satellite speakers typically being located nominally midway between the viewer  28  and television  12 . Rear ambience speaker  26 , which contributes to creating a spacious audio effect, is typically located behind viewer  28 . Rear ambience speaker  26  is depicted as a single speaker, but multiple rear speakers  26  may be included in the system.  
         [0010]     One problem with these home theater systems is in adjusting the equalization of the system to compensate for room acoustics, speaker type, and other factors. Traditionally, a consumer adjusted equalization using a so-called graphic equalizer, where a number of narrow band-pass filters are provided, each with a corresponding slide switch. The consumer adjusts each slide switch to attenuate or amplify a particular frequency band. More modern systems may use electronic displays in place of the slide switches, but the overall functionality is the same.  
         [0011]     The purpose of an equalizer is to provide an audio response that is generally “flat” across the entire frequency spectrum. Due to limitations in system and speaker design, as well as room acoustics and interaction of room acoustics with speaker design and placement, various frequency ranges in a system may be attenuated or accentuated, resulting in a sound reproduction which is not faithful to the original recording.  
         [0012]     A “flat” response generally refers to the resulting frequency versus amplitude graph. If the system is properly equalized, the graph should look like a flat line though all frequency ranges. In reality, this goal is never entirely achieved due to limitations of audio components and room acoustics. However, with a graphic equalizer, it is possible to improve the response considerably such that the resulting sound is a more faithful reproduction of the original sound.  
         [0013]     The problem with manually operated graphic equalizers is that the equalization is based upon the consumer setting the various frequency levels based upon what the consumer hears and what the consumer thinks will create the proper equalization for the system. This manual solution is a largely empirical approach, as many consumers cannot properly isolate various frequencies “by ear” and understand how to adjust the equalizer properly. In addition, as the music type and sound changes, as well as the acoustic properties of the room, the equalization set at one level may be inappropriate for another audio environment.  
         [0014]     The early graphic equalizers generally had a fixed number of equalizer circuits, each adjusting a predetermined narrow band of frequencies. However, when attempting to equalize a system, it becomes readily apparent that certain frequency ranges may require finer incremental ranges of adjustment, whereas whole bands of frequencies can be adequately adjusted using a single circuit. Proving additional equalizer filter elements and switches to solve this problem is prohibitively expensive. The parametric equalizer helps solve this problem by allowing a limited number of equalizer elements to adjust audio levels in a flexible manner. Each level adjustment may be itself adjusted to control a different frequency range.  
         [0015]     Thus, each band-pass filter in the equalizer may be adjusted for width. Frequency ranges that require a fine granularity of adjustment may be more precisely controlled using a number of narrow-band elements in the parametric equalizer. Large frequency ranges that can be adjusted as a group can be controlled with one single wide-band element in the parametric equalizer. In this manner, the parametric equalizer can provide a more sophisticated and correct equalization to the frequency spectrum with the same number or even fewer control elements than a typical prior art fixed-frequency element graphic equalizer. Again, however, if a consumer attempts to manually control equalization, the results are often less than optimal, as the results are based upon the ability of the consumer to discern different frequency ranges.  
         [0016]     Equalization can be achieved in software as well as in hardware. For example, when decoding a digital data stream, such as from a CD, DVD, or other digital audio source, equalization may be applied to the data as part of the decoding process or in a separate step. Thus, the process of equalization, either using a fixed-bandwidth graphic equalizer or a variable-bandwidth parametric equalizer, can be achieved in software as well as in hardware, or in a combination of both.  
         [0017]     Other systems are known in the art wherein home theater systems, particularly more low-end units, provide a limited number of pre-set equalization patterns for different music types and listening styles. Thus, a home theater system may provide pre-set equalization levels for rock music, jazz, classical, rap, or for movie or DVD playback or the like. The equalization takes place in software within the home theater system. These pre-set levels do not take into account the room acoustics and provide only limited choices to the consumer. The consumer can only select the equalization setup that sounds best for the given circumstances. The system is not optimized for the room acoustics, speakers, and other factors affecting audio playback.  
         [0018]     More recently, one of the more popular features for home theater systems has been some form of automatic equalization setup to minimize adverse affects of speaker/room interactions. Most solutions, however, involve a one-time setup performed by the user when installing the system and/or prior to listening to music and/or watching a video or the like. An example of such a prior art equalization system is illustrated in U.S. Pat. No. 6,721,428, issued Apr. 13, 2004 to Allred et al. and incorporated herein by reference.  
         [0019]     These prior art automatic equalization setup systems typically have three phases. First, the system is analyzed from a single position or multiple positions in the room, usually by generating an audio signal through the speakers, and then receiving the audio signal through a remote microphone or the like to produce a system response. Second, the results of such analysis are translated to a run-time equalization setup and saved. Run-time equalization refers to the process of equalizing the audio signal during the digital decoding stage. Third, the saved settings are used by the equalizer at run-time as an additional post-processing step to whatever other audio processor software is running at the time.  
         [0020]     Examples of such other audio processor software include Dolby™ Digital™ AC-3, Digital Theater Systems (DTS), Pulse Code Modulation (PCM), bass management, delay control or the like. These various digital audio processing algorithms are known in the art and may be licensed from their respective producers, or comparable algorithms may be devised. The equalization algorithms of the Prior Art may thus be applied subsequent to the decoding step in the playback of a digital audio stream from a DVD, CD, or other audio source. Such audio processor software may include an existing equalization algorithm, which may receive an input based upon system response in the room.  
         [0021]     The extent to which the system can be corrected for the speaker/room acoustics is largely determined by the complexity of the run-time equalization. For many low- to mid-level systems, the run-time equalization is simply the parametric or graphic equalizer already present in the software, and thus correction possibilities may be limited. Thus, it remains a difficulty in the prior art as to how to best fit a fixed-band graphic equalizer or parametric equalizer to a predetermined frequency response.  
         [0022]     There are a number of prior solutions to the problem of fitting a fixed-band equalizer to a predetermined frequency response. One solution is a straightforward curve-fitting. For an n-band equalizer, the algorithm finds the n highest peaks and/or valleys in the frequency response and sets each band to correct the corresponding feature. Thus, if a particular frequency range is too high, it may be attenuated, and if a particular frequency range is too low, it may be boosted.  
         [0023]     There are at least two problems with the curve-fitting technique. Quite often the peaks found in a system response (here, the term “system response” refers to the response of the speaker and room) are at least partially due to phase-response issues, which may not respond in the desired manner to a frequency-based solution. Applying a −3 dB equalization to a 3 dB peak may not flatten the response as intended.  
         [0024]     The “phase response” of the speaker in the room is a function of frequency and is one part of the frequency response. The other part is the magnitude response (often inaccurately called the “frequency response”), which is the power level (Y-axis, usually in dB) plotted against frequency (X-axis in Hz).  
         [0025]     A second problem with the curve fitting technique is the limited granularity of the underlying equalizer. For run-time equalizers with a limited range of center frequencies (either a graphic equalizer or a limited-implementation of a parametric equalizer), it may not be possible to exactly “center” on the peak or valley in system response. Thus, more or less of the frequency response is affected as desired. If a −3 dB attenuation is applied to a 3 dB peak, but due to the limitations of the system, applied at a frequency slightly away from this peak, adjacent frequencies may be unnecessarily attenuated, and the desired “peak” not properly flattened.  
         [0026]     The second problem can be at least partially offset using a brute force approach. If the equalization software was provided with an enormous number of narrow-band parametric equalization elements, then individual peaks and valleys could be selectively eliminated in the system response. However, such an approach may be processor-, memory-, and hardware-intensive.  
         [0027]     The first problem can be resolved by using an equalizer that targets both magnitude and phase, if the system designer is not limited to the use of an existing magnitude-only equalization algorithm already present in a product.  
         [0028]     U.S. Pat. No. 6,721,426 to Allred et al. discloses an automatic loudspeaker equalizer. First digital data is provided for a tolerance range for a target response curve of sound level versus frequency for the loudspeaker. Second digital data is generated for an actual response curve of sound level versus frequency for the loudspeaker. The first digital data is compared with the second digital data, and it is determined whether the actual response curve is within the tolerance range. If the actual response curve is not within the tolerance range, digital audio filters are iteratively generated, and the digital audio filters are applied to the second digital data to generate third digital data for a compensated response curve. The frequency, amplitude and bandwidth of the digital audio filters are automatically optimized until the compensated response curve is within the tolerance range or a predetermined limit on the number of digital audio filters has been reached, whichever occurs first.  
         [0029]     The iterative approach of Allred improves equalization of the audio system, resulting in a flatter system response. However, the iterative approach can take considerable time to achieve. In particular, in the system of Allred, only one equalization element is adjusted with each iteration. As a result, it will take at least as many iterations as equalization elements to properly adjust all equalization elements and insure each equalization adjustment does not introduce new artifacts into the equalization. For a consumer electronic system (e.g., Home Theater system), this solution may not be acceptable, as the process may continue on for some time. The consumer may get impatient or believe the process if not functioning properly and terminate the process prior to completion.  
         [0030]     Thus, it remains a requirement in the art to provide an equalization technique that more accurately equalizes a home theater or other audio system while using a limited number of equalization elements and/or working within the parameters of an existing equalization algorithm. It remains a further requirement in the art to provide an equalization technique that optimizes the use of equalization elements for a given audio environment. It remains a further requirement in the art to provide an equalization technique that can optimize equalization settings without requiring a large number of iterative time-consuming processes.  
       SUMMARY OF THE INVENTION  
       [0031]     Rather than basing the entire equalization setup on only one pass of the analysis phase, multiple passes are executed in the equalization setup and modification of the equalization is performed after each pass of the analysis phase. This modification allows the software to modify its initial settings to compensate for the unexpected effects of the original equalization. The number of passes can vary widely, as can the equalization adjustment or setting at each step.  
         [0032]     After an initial pass, the equalization is adjusted, as in the Prior Art, based upon the location of peaks and valleys in the system response. This initial adjustment of equalization may tend to flatten most of the peaks and valleys to produce the desired uniform linear response. However, as noted above, this inexact application of equalization corrections may introduce other artifacts into the system response and/or may not sufficiently normalize equalization.  
         [0033]     A second pass is then performed to measure the system response using the new equalization settings. The new peaks and valleys are measured, and the equalization adjusted to try to further flatten response. Any number of subsequent passes may be made to further normalize the equalization. However, in practice, the number of passes may be limited to reduce the amount of time needed for the equalization process.  
         [0034]     The optimal number of iterations may be a tradeoff between test time and accuracy. For the most accurate results, one equalizer band per pass may be adjusted, so the number of iterations would be at least the number of equalization bands in the system. However, such a technique may take an excessive amount of time. In the preferred embodiment, two or three (or more) equalization bands are adjusted at the same time with each pass—the number of passes equals the number of total equalization bands in the equalizer divided by the bands set per pass. Thus, for example, with a nine-band equalizer, setting three bands per pass yields three passes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1  is a block diagram of the home theater surround sound speaker system in accordance with the Prior Art.  
         [0036]      FIG. 2  is a simplified block diagram of the equalization system of the present invention.  
         [0037]      FIG. 3  is a graph illustrating initial system response before equalization correction is applied identifying peaks and valleys in the system response.  
         [0038]      FIG. 4  is a graph illustrating initial system response, identifying peaks for equalization adjustment and illustrating application of a proximity range to determine which peaks and valleys will be equalized in a first pass.  
         [0039]      FIG. 5  is a graph illustrating system response after a first pass of equalization adjustment is applied.  
         [0040]      FIG. 6  is a graph illustrating system response after a second pass of equalization adjustment is applied.  
         [0041]      FIG. 7  is a graph illustrating how artifacts can be introduced into the system response if the proximity range is not applied.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]      FIG. 2  is a simplified block diagram of the equalization system of the present invention. The apparatus of  FIG. 2  may be incorporated into a Home Theater system such as that illustrated in  FIG. 1 , or an other type of audio system, including but not limited to a commercial audio systems, car audio systems, home stereo systems, and the like. For the sake of clarity, various elements that are not essential to the understanding of the invention are not illustrated.  
         [0043]     Referring to  FIG. 2 , a digital signal decoder  260  may receive data from a digital signal source and decode the data accordingly. Examples of such decoders, as noted above, include Dolby™ Digital™ AC-3 decoders, Digital Theater Systems (DTS) decoders, Pulse Code Modulation (PCM) decoders, and the like. Other types of decoders, including proprietary decoding systems, may also be used. Decoder  260  receives digital data from a digital signal source. For ordinary playback of audio, this digital sound source may include digital signal source  215  which may include a CD, DVD, HDTV digital audio track, digital radio, MP-3 data stream, or other digital audio data.  
         [0044]     For setup and testing purposes, a digital testing signal  205  may be used to generate a sound pattern for various testing and setup purposes. As set forth in co-pending applications Ser. Nos. 11/002,102 and 11/038,577 cited previously, the test signal may comprise gated nose, a Maximum Length Sequence (MLS) or the like. In some embodiments, almost any source signal may be used for calibration, testing, and setup, including digital signal source  215 .  
         [0045]     The output of digital signal decoder  260  may be fed to equalizer  210 . As previously noted, equalizer  210  may comprise a portion of digital signal decoder  260 . Moreover, all or part of both digital signal decoder  260  and equalizer  210  may comprise software or firmware components of the system, as opposed to dedicated hardware components. Thus, equalizer  210  may comprise a run-time equalizer that is executed subsequent to the process of digital signal decoder  260 .  
         [0046]     Equalizer  210  may be provided with equalizer coefficients  270  to adjust the amplitude of each equalizer element. If equalizer  210  comprises a parametric equalizer, then these coefficients may also include center frequency and frequency ranges for each equalizer element. When initialized, the system may default to predetermined coefficients. These coefficients may be 0 coefficients (e.g., neither amplifying or attenuating any frequency band) or some other predetermined values. To reduce the amount of time for calibration and setup, the default coefficients may be selected to represent proper equalization for a “typical” consumer setting or other area.  
         [0047]     The system may enter an equalization setup mode automatically when first powered up, or at the direction of the consumer (e.g., through infrared remote control, on-screen menu, or the like). Once the equalization calibration process begins, the digital testing signal  205  (or other signal) is fed to digital signal decoder  260  and equalizer  210 , which outputs a decoded and equalized digital audio signal to digital to analog converter (DAC)  220 . DAC  220  may then output an audio signal that may be amplified in amplifier  225  and then be reproduced in the room by speaker  230 .  
         [0048]     For purposes of illustration, only one speaker  230  is shown in  FIG. 2 . It will be appreciated by one of ordinary skill in the art that other numbers of speakers may be used, including, but not limited to left and right front speakers, center speakers, left and right rear speakers, surround sound speakers, subwoofers, and the like. Each speaker may be tested separately or in some combination. (Tests are typically done separately, except when testing the combination of a speaker and subwoofer)  
         [0049]     Microphone  240  receives the audio signal from the room. As set forth in co-pending applications Ser. Nos. 11/002,102 and 11/038,577 cited previously, microphone  240  might also be used for other testing purposes, such as measuring speaker location and determining time delay. Thus, the same components in the system may be used for more than one purpose in setting up the system.  
         [0050]     The output of microphone  240  may be fed to Analog to Digital Converter (ADC)  250  that in turn outputs a digital audio signal to frequency analyzer  280 . Frequency analyzer  280  may process the digital audio signal from ADC  250  and/or compare this signal with the source digital audio signal output from equalizer  210 . The result of this analysis is output as the system response  290 . System response  290  may be kept internal to the system; however, in some embodiments, system response  290  may be displayed on an on-screen display, LCD display or the like so that the consumer can better understand the process and view the results of the setup and calibration procedure.  
         [0051]     As will be discussed in more detail in connection with  FIGS. 3-5 , the system response may be analyzed by the system to determine which frequencies should be attenuated and which accentuated. The results of these decisions are used to alter the equalizer coefficients  270 .  
         [0052]     After an initial system response  290  has been determined, equalizer coefficients  270  may be adjusted and the process repeated. If individual elements of equalizer  270  are adjusted one at a time, it may take a large number of repeated processes to properly calibrate equalizer  270 . Moreover, if the number of processes is limited (due to testing time considerations), the resulting calibration may not be optimal. Thus, for example, if there are seven elements in equalizer  270 , and seven processes are repeated, one for each equalizer element, then each element is adjusted only once.  
         [0053]     In the present invention, a multiple number of equalizer elements may be adjusted in one process, and thus the overall testing time may be limited, while enhancing the adjustment of the equalizer elements. The optimal number of iterations may be a tradeoff between test time and accuracy. In the preferred embodiment, two or three (or more) equalization bands are adjusted at the same time with each pass—the number of passes equals the number of total equalization bands in the equalizer divided by the bands set per pass. Thus, for example with a nine-band equalizer, setting three bands per pass yields three passes.  
         [0054]     In addition, each band can be re-adjusted to compensate for the subsequent adjustment of other adjacent bands. Thus, an equalizer band may be initially adjusted, the results tested, and the band adjustment then fine-tuned to improve the overall system response. Additionally, in the preferred embodiment a proximity range may be applied to the initial peaks to be adjusted, such that adjacent or proximal bands of the equalizer are not adjusted simultaneously, resulting in artifacts in the resultant system response.  
         [0055]     Rather than basing the entire EQ setup on only one pass of the analysis phase, multiple analysis phases are executed with EQ setup and modification occurring after each analysis phase. This allows the software to modify its initial settings to compensate for unexpected effects of the original equalization. The number of passes can vary widely, as can the EQ adjustment or setting at each step. For example, one extreme might be:  
                                                   Do (number of bands)           {             Analyze             Set one EQ band             While(unsatisfied)             {               Analyze               Tweak EQ band             }           }                      
 
         [0056]     At the other extreme: 
        Analyze     Set all EQ bands     Analyze     Tweak all EQ bands        
 
         [0061]     Or 
        Analyze     Set half the EQ bands     Analyze     Set the other half        
 
         [0066]     Additionally, when setting a plurality of bands at one time, it may be beneficial to ensure that those bands are orthogonal so that they do not affect each other.  FIG. 3  is a graph illustrating an example of initial system response before equalization correction is applied. The X-axis represents frequency, on a logarithmic scale, while the Y-axis represents relative amplitude in dB. As previously discussed, an ideal system response may comprise a flat line at the 0 dB level, indicating that each frequency in the spectrum is reproduced faithfully and at the same level relative to all other frequencies in the spectrum.  
         [0067]     As illustrated in the example system response of  FIG. 3 , the overall response is anything but “flat”. Several peaks occur at different frequencies, representing frequencies that are overly amplified. Several valleys are illustrated that represent frequencies that are overly attenuated. In this example, major peaks  430 ,  450  and  480  are located at approximately 100 Hz, 150 Hz, and 1500 Hz, respectively. Significant valleys  470  and  490  are present at approximately 600 Hz and 4000 Hz, respectively. The rest of the spectrum is relatively flat, or outside the range of human hearing or system (particularly speaker) range.  
         [0068]      FIG. 4  is a graph illustrating initial system response, identifying peaks for equalization adjustment and illustrating peak width measurement as well as the proximity range applied in the present invention. In this example, when setting two bands between analysis phases, the first can be anywhere in the spectrum, but the second should be limited to anywhere in the spectrum except within a specified distance of the center frequency of the first, in order to prevent interference between the two corrections. Given the spectrum in  FIGS. 3 and 4 , the first large peak  430  may be targeted at 100 Hz with the first equalizer band, but selection for the second band would ignore the next peak  450  at 150 Hz because of its “close” proximity to the 100 Hz peak  430 .  
         [0069]     In this embodiment, a predetermined proximity range may be selected, for example, as four times (4×) the bandwidth of the first peak  430 . This range is represented in  FIG. 4  by solid lines  410  and  460 . The bandwidth of the first peak  430  is illustrated by solid lines  420  and  440 . Bandwidth of a peak, such as peak  430 , may be determined by the width of the peak at a particular predetermined dB cutoff level such as −3 dB from the peak, or by the width at a particular percentage of the peak (in this example 70%, or 4 dB).  
         [0070]     As second peak  450  is within the 4× range lines  410  and  460 , for the first adjustment of equalizer coefficients  270 , peak  450  will be ignored. Instead, the second equalizer band may target one of the smaller valleys  470 ,  490  or the peak  480 . The use of the 4× proximity range prevents the adjustment of adjacent equalizer bands from interfering with each other and producing unexpected or undesirable results. Using this technique, each equalizer element can be adjusted once and still provide a reasonable equalization. Since more than one equalization element is adjusted during each stage, the overall numbers of cycles in the process is reduced.  
         [0071]     In this example, a 4× proximity range is utilized. However, other ranges may be used within the spirit and scope of the present invention. For example, the proximity range may be selected as a logarithmic scale based upon peak (or valley) center frequency. Alternately, a fixed proximity range or selected one of a number of fixed proximity ranges may be used. The proximity range can also be determined based upon peak (or valley) amplitude or other indicia. The main feature of the proximity range is to prevent one equalization adjustment from altering or affecting an adjacent equalization adjustment.  
         [0072]     In a subsequent cycle, peak  450  may be used to adjust another equalizer band to eliminate this peak. In each subsequent cycle, one or more equalizer elements may be adjusted until all the equalizer bands are optimized for the best system response (e.g., flat response or some other desirable response). In an alternative embodiment, the process may be repeated to fine-tune the equalizer band elements to provide an even better overall system response.  
         [0073]      FIG. 5  is a graph illustrating system response after equalization is applied. As illustrated in  FIG. 5 , peaks  430  and  480  have been largely attenuated, such that the overall system response is closer to the desirable flat response (in this example). Peak  450  and valleys  470  and  490  may be corrected in a subsequent cycle, provided they are not within the designated proximity range of one another. In this manner, more than one peak or valley may be corrected per cycle, without the corrections interfering with each other or otherwise creating new artifacts in the system response.  
         [0074]      FIG. 6  is a graph illustrating system response after a second pass of equalization adjustment is applied. In this example, after peaks  430  and  480  have been attenuated in a first round of adjustment of equalization coefficients, a second round of testing and adjustment may be performed. In this example, only peak  450  is eliminated though adjustment of the equalizer coefficients. Valleys  470  and  490  are left for a subsequent adjustment cycle or cycles.  
         [0075]      FIG. 7  is a graph illustrating how artifacts can be introduced into the system response if the proximity range is not applied. In  FIG. 5 , the new extent of peak  450  after the first pass of equalization has been applied is illustrated. In  FIGS. 3 and 4 , this peak is higher. If the proximity range were not applied in the first cycle of adjustment, and the system attempted to adjust equalizer coefficients  270  for the two adjacent peaks  430  and  450 , the net effect would be to over-attenuate peak  450 , resulting in a new valley  750 . Thus, a new valley is created, and the system will have to be “tweaked” further to eliminate this artifact.  
         [0076]     While the present invention may be implemented in a number of embodiments, a number of fundamental features are present in one or more of these embodiments. Adjusting multiple equalizer bands during one process cycle is one feature of the present invention. In addition, the use of the proximity range to determine which frequency ranges, which may be adjusted in one cycle without causing adjacent equalizer band interference, is another feature. The use of multiple cycles of the calibration process to fine-tune the equalizer coefficients is yet another feature of the present invention. There are other features of the present invention that may be used alone or in combination with any of the aforementioned features of the present invention.  
         [0077]     Note that multiple proximity ranges can be applied in each pass. Thus, in the example of  FIG. 4 , a second proximity range could be applied to peak  480 . This second proximity range, may, for example, indicate that valleys  470  and  490  are not to be compensated in this pass, as they are within 4× the bandwidth (or other criteria) of peak  480 . Thus, valleys  470  and  490  would not be corrected until a subsequent pass, as is illustrated in the Example of  FIG. 4 . The number of proximity ranges used in a given pass can vary and the width or formula used to determine the proximity range size can also be varied as previously noted.  
         [0078]     While the preferred embodiment and various alternative embodiments of the invention have been disclosed and described in detail herein, it may be apparent to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope thereof.