Patent Publication Number: US-9888319-B2

Title: Multichannel audio system having audio channel compensation

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. application Ser. No. 12/897,707, filed Oct. 4, 2010, now U.S. Pat. No. 9,100,766 issued Aug. 4, 2015, which claims the benefit of U.S. Provisional Application No. 61/248,760, filed Oct. 5, 2009, the disclosures of which are incorporated in their entirety by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to multichannel audio systems and, more particularly, to an audio channel compensation system for a multichannel audio system. 
     Related Art 
     The perception of sound provided by an audio system in an environment may be degraded by reflective surfaces in that environment. A listener in such an environment is presented with both the original sound and a delayed version of the sound, which results in constructive and destructive interference. This type of interference can produce deviations, such as a comb filtering effect, in a target frequency response. The frequency response of a comb filter includes a series of regularly-spaced peaks and troughs, giving the appearance of a comb. The listener therefore receives a sound having a different frequency response than the intended sound originally emitted by the sound system. 
     Deviations in the target frequency response, such as comb filtering, may be particularly noticeable in substantially enclosed environments, such as the passenger cabin of a vehicle having a multichannel audio sound system. Each listener in the cabin receives both direct and reflected sound associated with each channel, resulting in deviations such as complex comb filtering interactions that reduce enjoyment of the listening experience. 
     SUMMARY 
     A multichannel compensating audio system may correct deviations in a target response at one or more listening positions within a listening area using one or more compensation channels. Each of the one or more compensation channels may include a series connected delay circuit, a level adjuster circuit and frequency equalizer circuit that generates a compensated audio signal from an audio signal on a channel of an input audio signal. 
     The multichannel compensating audio system may drive a plurality of loudspeakers with corresponding audio signals provided from a sound source as a multichannel audio input signal. For example, a 5.1 channel input audio signal may drive Center, Right Front, Left Front, Right Rear and Left Rear speakers with corresponding audio signals provided on center, right front, left front, right rear, and left rear audio channels. Each of the one or more compensation channels may receive and process audio signal to generate a compensated audio signal. 
     In the case of a first channel and a second channel, and a corresponding first speaker and a second speaker, a listener in a listening location may psychoacoustically perceive deviations in a target frequency response due to output by the first speaker of the audio signal on the first channel. In this case, a compensation channel may generate a compensated audio signal from a first audio signal being supplied to the first speaker on the first channel based on a predetermined delay, a predetermined energy level adjustment and/or a predetermined equalization (EQ). The compensated audio signal may be electronically summed with a second audio signal being supplied to the second speaker on the second channel. When the first and second speakers operate in a listening space, the first audio signal output from the first speaker may be heard at the listening location in the listening space, and the listener at the listening location may perceptually localize the origination of the first audio signal as being from the first loudspeaker. When the summation of the compensated audio signal and the second audio signal are output from the second speaker, the listener may psychoacoustically perceive corrections to the deviations in the target response due to the first speaker. However, due to the multichannel compensating audio system, the listener in the listening position may not psychoacoustically perceive a change in the location of origin of the first audio signal. 
     Another interesting feature of the multichannel compensating audio system may involve equalizing the loudness of sound emitted from different loudspeakers as psychoacoustically perceived at a number of different listening locations in a listening space. Using the audio channels and compensated audio signals that are selectively produced from different speakers, the listeners at different listening locations may psychoacoustically perceive a substantially uniform level of spectral energy being produced by the speakers. Still another interesting feature involves movement of a listener perceived location of a source of audible sound using the audio signals and the compensated audio signals. 
     Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures 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 invention, and be protected by the following claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views. 
         FIG. 1  is an example multichannel compensating audio system. 
         FIG. 2  is a frequency response of a comb filter that may be associated with sound emitted from a speaker of the system of  FIG. 1 . 
         FIG. 3  is a multichannel compensating audio system having channel compensation associated with a single channel of the system. 
         FIG. 4  is the frequency response of the comb filter shown in  FIG. 2  as well as the compensated frequency response generated through use of the channel compensation shown in  FIG. 3 . 
         FIG. 5  is a multichannel compensating audio system having channel compensation for multiple channels of the audio system. 
         FIG. 6  is a single channel of a multichannel compensating audio system having a multichannel compensator. 
         FIG. 7  shows channel compensation for all channels of a multichannel compensating audio system. 
         FIG. 8  shows the channel speakers of a multichannel compensating audio system used in a passenger cabin of a vehicle. 
         FIG. 9  is a method for operating a multichannel compensating audio system having channel compensation. 
         FIG. 10  is an example multichannel compensating audio system used in a passenger cabin of a vehicle. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Deviations in a target frequency response at one or more listening positions within a listening space, such as passenger locations in a vehicle, may be at least partially addressed with selective frequency equalization of the audio signal. For example, a comb filtering effect associated with a channel may be at least partially addressed by providing equalization to the affected channel. Such equalization may involve providing frequency boosts and/or frequency reductions directly to the channel to correct for the dips and peaks representative of deviations in the target frequency response. Although deviations in the target frequency response for a given channel may depend on the location of a listener within the listening space or listening environment, a general frequency equalization setting may be provided on the channel based on the common areas in which the listener is positioned within the listening space or listening environment. 
     Application of equalization directly to an affected channel, may not provide satisfactory compensation for deviations in a target frequency response at one or more listening positions due to the equalized signal emitted by the channel still being subject to reflection. A listener positioned in a location within the listening space may receive both the equalized signal emitted by the channel and a delayed version of the equalized signal from the reflective surfaces. Thus, equalization can, for example, merely result in a change in the frequency response of a comb filter that does not adequately compensate for the degradation of the sound emitted from the channel. 
     With some multichannel audio sound systems the corresponding listening environments may have a limited amount of space. One such environment is the passenger cabin of a vehicle. When space in the listening environment is limited, the quality and placement of the speakers within the cabin may likewise be limited. For example, a speaker for an audio channel may necessarily be located at a less than optimal position within a vehicle cabin due to the design constraints imposed by the overall design of the cabin. Further, speakers having different speaker qualities with respect to one another may be used based on cost constraints, available space for a speaker, and other criterion. Such variations in quality and placement of speakers in a listening environment may also contribute to deviations from a target frequency response at the listening positions unless appropriate channel compensation is applied. 
       FIG. 1  is an example multichannel compensating audio system that may employ channel compensation. Two channels of the multichannel compensating audio system are shown in  FIG. 1 , although more channels may be employed. The multichannel compensating audio system of  FIG. 1  is shown without channel compensation enabled. As used herein, the term “multichannel” describes two or more audio channels provided within an input audio signal to drive two or more loudspeakers. Example multichannel audio signals include a stereo audio signal, a 5.1 channel audio signal, a 6.1 channel audio signal, a 7.1 audio signal, or any other audio signal that includes two or more audio channels. 
     The multichannel compensating audio system may include one or more processors such as a digital signal processor and memory. Operation of the multichannel compensating audio system may be based on instructions, software or code stored in the memory that are executable by the processor, electronic hardware, and devices and systems controlled by the processor, or some combination. The memory can include volatile, non-volatile, flash, magnetic, or any other form of non-transient memory capable of storing the executable instructions, information/parameters of the audio system, user specific configuration information, and data such as audio content, audio-visual content, or any other information capable of being stored and accessed. The multichannel compensating audio system may also include a user interface, capable of receiving user inputs and providing information to a user of the system. In addition, the multichannel compensating audio system may include amplifiers, audio sources, and wired or wireless interfaces to external devices, as well as functionality such as navigation, telecommunications, satellite communications, desktop computing, and any other functions or capabilities. 
     The multichannel compensating audio system may include a first audio signal  110  provided without compensation to a first speaker  115 . A second audio signal  120  may be provided to a second speaker  125  without compensation. The first and second audio signals  110  and  120  may represent audio content present on different audio channels within an input audio signal of the multichannel audio system, such as a stereo, 5.1, 6.1, or 7.1 audio channels. Sound emitted from each speaker  115  and  125  is dispersed in a complex manner in a listening environment  127  and may involve multiple interactions between the reflective surfaces within the listening environment  127 , the direct  140  and reflected  145  sound from speaker  115 , and the direct  150  and reflected  155  sound from the second speaker  125 . 
     For simplicity, only a very basic interaction of the sound emitted from speaker  115  in the listening environment  127  is illustrated. In this simplified representation, a listener positioned in a listening location  135  within the listening environment  127  receives the direct sound  140  from speaker  115  and sound  145  from speaker  115  that is reflected from reflective surface  130 . As such, a listener at the listening position  135  in the listening environment  127  is presented with both the direct sound  140  and a delayed version of the sound  145 , which can result in constructive and destructive interference that may produce deviations in a target frequency response, such as a comb filtering effect. In other examples, more loudspeakers, more listening positions, and more reflective surfaces may be present. 
     An exemplary comb filtering response representative of a deviation in a target frequency response is shown in  FIG. 2 . As shown, the frequency response  200  of the comb filter includes a series of regularly-spaced peaks  205  and troughs  210 , giving the appearance of a comb. The listener at the listening location  135  receives a sound having a different frequency response than the original sound emitted by the speaker  115 . As used herein, deviations in a target frequency response refers to audible sound received by a listener at a listening position within a listening space that does not come within a desired range of frequency response. Comb filtering is but one example describing deviation from a target frequency response, but as discussed herein should be considered a non-limiting example representative and interchangeable with other forms of deviations from a target frequency response psychoacoustically perceived by a listener at a listening position in a listening space. As used herein, the terms “psychoacoustically perceived” or “perceived” or “perception” or “psychoacoustical perception” refers to a listener&#39;s awareness, observation, and discernment of a sound field being experienced by the listener within a listening area or listening space. 
       FIG. 3  shows another example of the multichannel compensating audio system of  FIG. 1  with compensation for a single channel. In  FIG. 3 , the first audio signal  110  is provided to speaker  115  as audio content of a single channel in the input audio signal. As in  FIG. 1 , a listener at the listening position  135  in the listening space  127  receives both a direct sound  140  and reflected sound  145  from speaker  115  being driven by the first audio signal  110 . To compensate for the direct and indirect sounds occurring in listening environment  127 , audio signal  110  is also provided to the input of a compensation channel  305 . 
     Compensation channel  305  may include a series connected delay circuit  310 , a level adjuster circuit  313 , and an equalizer circuit  315  through which the audio signal  110  is processed. The delay circuit  310 , the level adjuster circuit  313 , and the equalizer circuit  315 , may be modules consisting of instructions stored in memory and executable by a processor, hardware such as electronic circuits, registers, and electrical circuit devices, or come combination of instructions and hardware. The delay circuit  310  may be used to selectively add delay to the frequencies or different ranges of frequencies included in the audio signal  110 . As described later, the delay may be used to preserve a physical direction or location of sound being produced in a listening space. The level adjuster circuit  313  may be used to globally adjust the spectral energy of the audio signal to increase or attenuate the energy level of the audio content across the entire range of frequencies represented in the audio signal  110 . As described later, the adjustment of the energy level of an audio signal may decrease or increase the overall magnitude of audible sound output by a speaker. The equalization circuit  315  may be used to selectively increase and attenuate the energy level of individual frequencies or different ranges of frequencies included in the audio signal  110 . In some examples, the equalization circuit  315  may also perform global adjustment of the audio signal, and the level adjuster circuit  313  may be omitted. 
     The output of the compensation channel  305  constitutes a compensated audio signal  320 . The compensated audio signal  320  is provided to the input of a summing circuit  323  along with the second audio signal  120 , which is representative of audio content of another single channel included in the input audio signal. The summing circuit  323  adds and/or subtracts the second audio signal  120  and compensated audio signal  320  with respect to one another to generate an output signal  325  that is provided to speaker  125 . Speaker  125  emits sound  330  into the listening environment  127  that corresponds to a combination of both the second audio signal  120  and the compensated version  320  of the first audio signal  110 . As used herein, the term “signal” or “signals” is used interchangeably to describe either electrical signals, or audible sounds produced by mechanical operation of a respective speaker based on corresponding electrical signals. 
     In the multichannel audio system of  FIG. 3 , the amount of delay provided by delay circuit  310 , level adjustment provided by the level adjuster  313 , and equalization provided by equalizer circuit  315  may be selected to reduce the comb filtering effect shown in  FIG. 2 , while still maintaining a psychoacoustical perception by the listener  135  that the source of audible sound representative of the audio content in the single channel is the first speaker  115  or in the vicinity and/or coming from the direction where the first speaker  115  is physically located. 
     An example of the resulting frequency response of the compensated sound in the listening environment  127  is shown in  FIG. 4 . Response  200  corresponds to the un-compensated response for the system shown in  FIG. 1 . The frequency response of the compensated audio signal  325  as represented with the sound  330  emitted by speaker  125  is shown at  405 . Frequency response  405  includes peaks  410  occurring at the troughs  210  of frequency response  200 . Thus, frequency response  405  is constructively added to the frequency response  200 . Response  405  also includes troughs  415  occurring at peaks  205  of frequency response  200 . Frequency response  405  is not performing cancellation of any portion of frequency response  200 . Accordingly, exact alignment in phase of frequency response  405  and frequency response  200  is unnecessary. In addition, the range of frequencies in the frequency response  405  and the range of frequencies in the frequency response  200  may be overlapping to enable the filling of multiple troughs  210  by the peaks  410 . As such, equalization of the frequency response  405  may occur in frequencies or ranges of frequency that are also present in frequency response  200 . 
     Also illustrated in  FIG. 4 , is a first average energy level  420  of the compensated audio signal  325 , which is shown as increased by a determined amount with the level shifter circuit  313  to a second average energy level  425 . The compensated audio signal  325  may be increased (or decreased) so that the magnitude of the peaks  410  of the frequency response  405  are more closely aligned with respect to the magnitude of the peaks  205  of the frequency response  200 . As a result, the frequency response  405  can be maintained at or below a level of magnitude of the frequency response  200  to avoid being psychoacoustically detected (or psychoacoustically perceived) by a listener as being emitted from a different physical location from frequency response  200 , or causing the perceived location of frequency response  200  to shift in physical location. 
     When frequency responses  200  and  405  combine with one another in the listening environments  127 , the listener perceived comb filtering effect associated with sound emitted from speaker  115  may be substantially reduced. In one example, the compensation channel  305  delays, energy adjusts, and equalizes the first audio signal so that sound corresponding to the first audio signal is received by a listener in the listening environment with minimized combing effect, and is psychoacoustically perceived by the listener as being produced from the first speaker  115 . 
     Referring again to  FIG. 3 , an input signal  110  may drive the first speaker  115  to emit audible sound that, upon reaching the listening position  135 , is perceived by the listener as having deficiencies in the target frequency response. The perceived deficiencies may be a result of deficiencies in the performance of speaker  115  and/or acoustical interference between the direct path of direct sound  140  and the reflected path of reflected sound  145 , such as comb filtering at the listening position  135 . This results in unwanted dips and peaks in the frequency response at the listening position  135 . These deficiences perceived by the listener may be minimized by processing the input signal  110  through the compensation channel  305  and the summing circuit  323 . The processed output signal  325  may be sent to the second speaker  125  at a different location in the listening space  127 . Because the second speaker  125  is at a different location it is likely to have different interference and so may have different peaks and dips in its response at the listener position  135 . Therefore, the compensated signal emitted from the second speaker  125  may be used to try to fill in some of the “holes,” or troughs, in the frequency response due to the first speaker  115 . Thus, troughs  210  may be filled with peaks  410  of the audio output from the second speaker, while the peaks  205  are substantially unchanged. ( FIG. 4 ) 
     Such filling of the “holes” may be substantially unnoticed by the listener by taking advantage of psychoacoustics when trying to fill the “holes” in the response of first speaker  115  at the listening position. An audible sound produced by the first speaker  115  in response to the first input signal  110  will typically be perceived at the listening position as sound coming from that direction or location+. When using a compensated version of the first input signal  110  (compensated audio signal  320 ) to produce audible sound as compensating sound from the second speaker  125  to fill the “holes,” the compensation may be appropriately delayed and the energy level appropriately adjusted such that the user still perceives substantially all of the audible sound at the listening position as coming from first speaker  115 , or from the direction of the first speaker  115 . As such, the listener perceives no movement in the location of the sound source (the first speaker  115 ) whether the second speaker  125  is producing, or not producing the compensated audio signal to fill the “holes.” 
     Compensation of the first input signal  110  to accomplish substantially no change in the perceived location may include applying a predetermined delay to the compensated audio signal  320  that is emitted by the second speaker  125 . The delay may be chosen such that the compensating audible sound produced by the second speaker  125  arrives at the listening position  135  a predetermined period of time after the corresponding audible sound produced from the first speaker  115 . In addition, a predetermined energy level adjustment and/or predetermined equalization may be selectively applied to first input signal  110 , and/or the compensated audio signal  320  to adjust the spectral energy of the resulting audible sound produced by the first and second speakers  115  and  125 . When the combination of audible sound produced by the first and second speakers  115  and  125  reaches the listening position  135 , the human ear sums the energy of the delayed sound with the energy of the direct sound when perceiving the originating location and originating direction of the sound. As a result of how the human auditory system and brain works, the listener will still localize the audible sound received as substantially originating from the first speaker  115 . There may be limits regarding how loud and how delayed the audible sound produced from second speaker  125  can be with respect to the audible sound produced by the first speaker  115  in order to substantially maintain the location and direction of the sound as perceived by the listener. Such limits may be established by spectral analysis of a listening space, experimentation with test subjects, or any other procedure(s) or test equipment capable of determining limits for delay, energy level, and/or equalization with regard to psychoacoustic location and direction of a source of sound, such as those previously and later described. 
     The term “substantially” refers to the less than exact correction of deviations in the target response due to the first speaker  115  at the listening location  135 , since exact matching of the phase and magnitudes of the signals from speakers  115  and  125  is unnecessary to achieve the desired perceptual effect by the listener. In other words, since cancellation of spectral energy is not being performed, exact matching of the phase of the signals from the speakers  115  and  125  is unnecessary, since addition to the existing spectral energy produced by the first speaker  115  (see  FIG. 4 ) does not require exact matching of the phase of the signals. In addition, “substantially” maintaining the location and direction of sound is desireable to increase the area of the listening location in order to avoid the correction only being accurate at a precise location in the listening space such that relatively small movements by the listener may lessen or defeat the correction. This may be particularly true at relatively higher frequencies of sound that are compensated, where wavelengths are shorter. 
     By substantially filling the “holes” in the frequency response due to the first speaker  115 , the listener perceived response of the first speaker  115  may be improved. Filling, or minimizing, at least some of the troughs in the frequency response due to the first speaker  115  results in improvements in the psychoacoustically perceived magnitude response of the first speaker  115 . The processing to add delay to the compensated audio signal  320 , relies on how the human ear works to integrate signals from the two different sound sources, such as two different speakers. For example, the human ear may integrate delayed audible sound from the second speaker  125  formed with the compensated audio signal  325  with original audio sound from the first speaker  115  formed with the audio signal  110  such that the delayed sound is not heard as a separate event, and all of the sound appears to come from the direction of the first speaker  115 . 
     This desireable combination of audio sound generated from the first and second speakers  115  and  125  may effectively minimize deviations in the targeted frequency response so long as the delay is not greater than a predetermined amount, such as between 0 milliseconds and about 40 milliseconds to about 80 milliseconds with respect to the corresponding audio content of the audio signal driving the first speaker  115 , and the energy level of the audible sound from second speaker  125  is a predetermined amount, such as in a range between about +10 dB and about −20 dB relative to the energy level of the corresponding audio content included in the audible sound generated from the first speaker  115 . The predetermined amount of delay may be dependent on frequency of the audio signal being delayed. 
     By striving to substantially minimize deviations in the target response, instead of completely eliminating such deviations, correction of deviations within the audio system may be more robust, and the effect on the compensation due to movements by the listener may be minimized. As a result, the correction may substantially minimize deviations over a relatively large listening position  135 , such as a seating location in a vehicle regardless of the height, movement and head orientation of the listener occupying the listening position  135 . Such changes in a listener&#39;s position within a listening position  135  may not result in perceptible changes in the magnitude of the response, but can result in changes to the phase of the response. However, since the human ear is less sensitive to differences in phase, listener perceived changes in the minimization of deviations in the target response due to movement within the listening location are advantageously reduced. 
     The amount of delay provided by delay circuit  310  and equalization provided by equalizer circuit  315  may also be selected to psychoacoustically correct for the audible sound generated by the system in one or more listening locations when the audio system uses speakers having different frequency response characteristics, when the listening space has different reflective surface characteristics, or any other environmental or hardware related characteristics that affect audible sound received from the loudspeakers at the listening positions in a listening space. 
       FIG. 5  is an example of a multichannel compensating audio system where each channel may include compensation. Compensation channel  305  may be applied in a similar manner as described with reference to  FIG. 3 . In  FIG. 5 , a compensation channel is also associated with the second audio signal  120  to compensate for reflected sound  505  emitted from speaker  125 . The second audio signal  120 , representing one of the channels in a multi-channel audio signal, may be applied to the input of a second compensation channel  510 , which includes a series connected second delay circuit  515 , a level adjuster circuit  517  and a second equalization circuit  520 . The compensation channel  510  generates a second compensated audio signal  525  from the second audio signal  120 . The first audio signal  110  and the second compensated audio signal  525  may be applied to the input of a summing circuit  530 . The summing circuit  530  adds and/or subtracts the first audio signal  110  and the compensated audio signal  525  with respect to one another to generate a second output signal  535  that is provided to drive the first speaker  115 . The first speaker  115  emits sound  140  into the listening environment  127  that corresponds to both the first audio signal  110  and the compensated version  525  of the second audio signal  120  (compensated audio signal  525 ). 
     A listener at the listening location  135  may psychoacoustically perceive the location and direction of sound as coming from the respective first and second loudspeakers  115  and  125 . However, in reality, the direct and reflected sound  140  and  145  is being compensated to fill holes in the listener perceived soundfield at the listening position  135  using the second speaker  125  and the audio compensated signal  320 . Similarly, the direct and reflected sound  330  and  505  is being compensated to fill holes in the listener perceived soundfield at the listening position  135  using the first speaker  115  and the compensated audio signal  525 . In other example systems having additional speakers, two or more of the speakers and corresponding compensated audio signals may be used to fill holes in the listener perceived soundfield at the listening position  135  as compensation for either the first or the second speaker  115  and  125 . 
       FIG. 6  is an example multichannel compensating audio system that includes a compensation system extended to further channels. In such a multichannel compensating audio system, a plurality of audio channels may each provide a respective audio signal. A plurality of compensation channels may be provided that are each respectively associated with the audio signal of a respective audio channel. Each audio compensation channel includes a series connected delay circuit, a level adjuster circuit, and a frequency equalizer circuit that generates a compensated audio signal from the audio signal of the respective audio channel associated with the compensation channel. A plurality of summing circuits may be used to generate audio output signals for provision to corresponding speakers for each channel of the multichannel audio system. The plurality of summing circuits may have inputs for receiving the audio signal from a respective one of the plurality of audio channels and a plurality of compensated audio signals for a remaining plurality of the plurality of audio channels. 
     A single channel of an example multichannel compensating audio system, such as a 5.1 audio system, is shown in the example of  FIG. 6 . Only a single channel speaker  605  is illustrated for simplicity. For purposes of the following discussion, it is assumed that speaker  605  is the right front (RFC) speaker and is associated with the audio signal  610  of the right front channel of the audio system. The audio signals for the remaining channels other than the RFC of the audio system are provided to a multichannel compensator  615  that is respectively associated with the RFC. 
     The multichannel compensator  615  includes a compensation channel for each audio signal other than the RFC. In other examples, the multichannel compensator  615  may include compensation channels for less than the entirety of the remaining audio channels. In  FIG. 6 , compensation channel  620  receives an audio signal  625  corresponding to the center front channel (CFC) of the audio system and generates a corresponding compensated CFC audio signal at  630 . Compensation channel  635  receives an audio signal  640  corresponding to the left front channel (LFC) of the audio system and generates a corresponding compensated LFC audio signal at  640 . Compensation channel  650  receives an audio signal  655  corresponding to the left rear channel (LRC) of the audio system and generates a corresponding compensated LRC audio signal at  660 . Compensation channel  665  receives an audio signal  670  corresponding to the right rear channel (RRC) of the audio system and generates a corresponding compensated RRC audio signal at  675 . Compensation channel  680  receives an audio signal  685  corresponding to the low frequency effects (LFE) channel of the audio system and generates a corresponding compensated LFE audio signal at  690  that is representative of the low frequency portion of the audio signal. 
     Audio signal  610  and each compensated audio signal  630 ,  645 ,  660 ,  675 , and  690  are provided to a summing circuit  693 . The summing circuit  693  adds and/or subtracts the audio signals at its input to generate an output signal  695  that is provided to speaker  605 . As such, the audio signal  695  provided to speaker  605  corresponds to a non-compensated version of audio signal  610  for the audio channel as well as compensated audio signals for each of the remaining audio channels. Depending on the design criterion, compensated audio signals for certain channels need not be provided by the multichannel compensator  615 . 
     The system topology may be extended to each audio channel of the remaining audio channels as shown in  FIG. 7 . For example, the speaker  705  for the CFC channel accepts an output signal  707  corresponding to a non-compensated version of the CFC audio signal  625  and compensated versions of the RFC, LFC, RRC, RLC, and LFE audio signals  713  provided from multichannel compensator  715 . The speaker  720  for the LFC accepts an output signal  723  corresponding to a non-compensated version of the LFC audio signal  640  and compensated versions of the RFC, CFC, RRC, RLC, and LFE audio signals  717  provided from multichannel compensator  727 . The speaker  730  for the RRC channel accepts an output signal  733  corresponding to a non-compensated version of the RRC audio signal  655  and compensated versions of the RFC, CFC, LFC, RLC, and LFE audio signals  731  provided from multichannel compensator  737 . The speaker  740  for the RLC accepts an output signal  743  corresponding to a non-compensated version of the RLC audio signal  670  and compensated versions of the RFC, CFC, LFC, LLC, and LFE audio signals  741  provided from multichannel compensator  747 . The speaker  750  for the LFE channel accepts an output signal  753  corresponding to a non-compensated version of the LFE audio signal  685  and compensated versions of the RFC, CFC, LFC, LLC, and RRC audio signals  751  provided through multichannel compensator  757 . Although the multichannel audio system of  FIG. 6  and  FIG. 7  is described in the context of a 5.1 channel system, this topology may be extended to multichannel audio systems having a larger number of audio channels, such as a 6.1 or 7.1 system, or fewer number of audio channels, such as a stereo system. 
       FIG. 8  is an example of the placement of speakers of a multichannel compensating audio system, such as a 5.1 system, in a vehicle  805 . The speakers of the system of  FIG. 8  emit sound into a listening environment  815  formed by the passenger cabin of the vehicle  805 . In this example, a listening position  820  in the form of the drivers seat is located in the listening environment  815 . 
     Each compensation channel of the audio system may have its own unique delay, level adjustment and equalization characteristics. These characteristics may be selected based on on the psychoacoustic perceptions of the listener in the listening position  820  within the listening environment  815 . To this end, the listener in the listening position  820  may be replaced by a binaural dummy head. The binaural dummy head may be placed at a fixed and/or multiple listening locations within the listening environment  815 , such as a driver position, front passenger position, and rear passenger positions. The delay, energy level, and equalization characteristics of the compensation channels may be adjusted using sound measurements detected at the binaural dummy head. The sound measurements at the binaural dummy head may be compared with a variety of sound measurements associated with various psychoacoustic properties. The delay, energy level and equalization for the compensation channels may be varied until the sound measurements detected at the binaural dummy head correspond with the desired psychoacoustic properties at each of the listening positions. 
     The binaural dummy head may be moved to multiple listening locations within the listening environment  815  while varying the delay, level adjustment, and equalization characteristics of the compensation channels. In this way, the delay. energy level, and equalization values of the compensation channels may be set to values that provide psychoacoustic perception properties that would be acceptable to all of the listeners in different listening positions within the listening environment  815 . 
     The multichannel audio system of vehicle  805  may include multiple delay, energy level, and equalization settings that are optimized for psychoacoustic perception of audio by a listener at one or more listening locations in the listening environment  815 . To this end, the listener in a particular listening position may be provided with selections associated with a listener at one or more of the listening positions within the environment  815  (i.e., driver position, rear cabin, passenger position, all). In  FIG. 8 , the listening position  820  is at the driver&#39;s position, which corresponds to selection of “driver position” on the audio system user interface. When selected, the delay, energy level and equalization values of the compensation channels may be used to substantially minimize deviations in the target response in the listening position  820  with respect to all, or some of the speakers  605 ,  705 ,  720 ,  730 ,  740 ,  750  while maintaining the perceived locations and directions of the sound as coming from the speakers  605 ,  705 ,  720 ,  730 ,  740 ,  750 . 
     Alternatively or in addition, the delay, energy level and equalization values of the compensation channels may be used to substantially minimize deviations in the target response and also generate one or more virtual channel speaker sounds that are psychoacoustically perceived by the listener at a location other than the location of the actual physical position of the corresponding channel speaker. For example, application of the delay and equalization values to the audio channels may result in virtual movement of speaker  705  for the CFC to the virtual speaker position shown at  830  and/or virtual movement of speaker  720  to the virtual speaker position shown at  832 . The new virtual speaker positions  830  and/or  832  effectively shifts the CFC and/or the LFC so that it is perceived at a location that is more appropriate for the CFC and/or LFC for a listener at the driver&#39;s listening position  820 . A similar virtual speaker shift may be provided for any one or more of the remaining speakers. In this manner, substantially all or some of the speakers may be psychoacoustically shifted (in this case, counterclockwise) with respect to the actual locations of the channel speakers so that the system is perceived by the listener in the listening position  820  as though the listener is positioned at a central location within the listening environment  815 . Other position optimizations may also be selected through the audio system interface. For example, when a user selects the “all” option, the compensation channels may be set to delay, energy level, and equalization values that provide psychoacoustic perception properties that would be generally acceptable to listeners in all of the listening positions in the environment  815 . 
     The speakers of a multichannel audio system may not necessarily have the same sound reproduction quality or frequency response range with respect to one another. The use of different quality speakers for different channels within the listening environment  815  may be imposed by system design constraints. For example, in the case of a listening space in a vehicle, the speaker  705  for the CFC may have its size constrained by the limited availability of space in the vehicle&#39;s dashboard. The remaining speakers may have additional space available to them so that higher quality speakers or speakers with a wider desireable frequency response range may be used for the other channels. As such, two or more speakers may have different psychoacoustically perceived audio frequency responses across an audio frequency range in the listening environment  815 . The delay, energy levels and frequency characteristics of the compensation channels may be used to alter the psychoacoustically perceived audio frequency response of at least one of the two or more speakers having different psychoacoustically perceived audio responses. 
     For purposes of this discussion, the CFC speaker  705  may have a generally irregular frequency response across the audio frequency range when compared to one or more of the other channel speakers of the audio system. The delay, energy level and frequency characteristics of the compensation signals provided by the other channels of the system may be used to correct for this “irregular” frequency response so that the psychoacoustically perceived frequency response of the CFC speaker  705  approaches a target frequency response, such as a substantially flat frequency response within a desired range of frequencies. Additionally, or alternatively, the delay and frequency characteristics of the compensation signals provided by the other channels of the system may be used to correct for this “irregular” frequency response so that the psychoacoustically perceived frequency response of the CFC speaker approaches the psychoacoustically perceived frequency response of the other channel speakers of the audio system, irrespective of whether the other channel speakers have a desired target frequency response, such as a generally flat frequency response over a desired range of frequencies. 
     Quality correction may also be made using the compensation to minimize undesireable speaker characteristics such as colouration, distortion, and any other undesireable speaker characteristics. Such correction for channel speakers having different performance characteristics in the audio system may also be extended to speakers other than the CFC speaker  705 . 
     An example method for operating a multichannel compensating audio system is illustrated in  FIG. 9 . At  905  the audio system receives a first audio signal, and a second audio signal is received at  910 . A first compensated audio signal corresponding to the first audio signal is generated at  915 . The first compensated audio signal corresponds to a delayed, level shifted, and equalized version of the first audio signal. A second compensated audio signal corresponding to the second audio signal is generated at  920 . The second compensated audio signal corresponds to a delayed, level shifted, and equalized version of the second audio signal. The first audio signal and second compensated audio signal are summed at  925  to generate a first output signal while the second audio signal and first compensated audio signal are summed to generate a second output signal at  930 . The first output signal is provided to a first speaker at  935 . The second output signal is provided to a second speaker at  940 . The delay, energy level shift, and equalization values used to generate the first and second compensated audio signals may be selected to correct for deviation in a desired targeted response at one or more listening locations without changing a psychoacoustically perceived location and direction of sound generated with the first and second speakers. In addition or alternatively, the first and second compensated audio signals may be used to generate a virtual speaker sound that is psychoacoustically perceived by a listener in a listening environment at a location other than the actual locations of the first and second speakers in that listening environment. Further, the delay, energy level shift and equalization values may be selected to correct for differences in the acoustic quality of the speakers used in the audio system. 
       FIG. 10  is another example multichannel compensating audio system included in a listening environment in the form of a vehicle. Although illustrated as a passenger compartment of a vehicle having five speakers, in other examples, any other listening area and any number of loudspeakers may be used. With further reference to  FIGS. 1 through 9 , consider a signal going to a center speaker  1003  and arriving at listener position  1002 . For at least two different reasons the frequency response at the listener position  1002  may deviate from a desired target response. One possible reason is that the center speaker  1003  may have a frequency response that is inherently different from the desired target response. For example, the center speaker  1003  may have dips and peaks in its response. Another example would be when speaker  1003  is physically small and therefore not able to adequately reproduce audio content having low frequencies. This may be the case for the center channel speaker in a vehicle. Under these circumstances, other speakers, such as a left front speaker  1001  may be used to generate compensation audio based on a compensated audio signal to try to improve the perceived response of center speaker  1003  at the first listening location  1002 . 
     As previously discussed, the center channel audio signal is sent to the center speaker  1003 . In addition, the center channel audio signal may be processed to create the compensated audio signal that is sent to the left front speaker  1001 . The processing is designed to make the perceived response of the center channel speaker  1003  appear to be closer to the target response at listening location  1002 . This correction in the perceived response may be specific to the listening location  1002 . 
     The delay and level of the compensated audio signal can be set such that the sound source is psychoacoustically perceived by a listener at the listening location  1002  to still sound like it is coming from the center speaker  1003 . Thus, predetermined delay can be applied to the compensation audio signal at the left front speaker  1001  so that the sound source remains localized at the center speaker  1003  from the perspective of a listener at the listening position  1002 . In addition, a predetermined energy level should be set for the compensated audio signal so that the compensating audible sound generated from the left front speaker  1001  is loud enough to adequately fill in the “holes” (such as troughs) in the response from the center speaker  1003 . Therefore, the delay can be maintained below a threshold level to avoid the situation where the the compensation signal cannot be made loud enough without causing perception by the listener at the listening location  1002  that the apparent sound source has shifted away from center speaker  1003 . 
     In this example, the left front speaker  1001  is closest to the listening position  1002 , and thus may have the most effect on this listening location  1002  due to the loudness (level) of a speaker diminishing as a listener is positioned further away from the speaker, and due to obstacles in the listening area. For example, in a vehicle, such obstacles in the listening area may include the driver and the front seats  1031  and  1032 , which can act as acoustical barriers and attenuate the sound emanating from the left front speaker  1001  that reaches a second listening position  1012 . The compensation effects due to the left front speaker  1001  may be substantially inaudible at other listening positions in the vehicle for these reasons, which may provide less detrimental effects on the other listening locations in the vehicle. In other words, the correction for the listening position  1002  due to the left front speaker  1001  may be largely independent of corrections for other listening positions in the vehicle. 
     In the case of the second listening position  1012 , a different compensation process for the center speaker  1003  may be applied. For example, a listener in the second listening position  1012  may hear audio content produced from the center speaker  1003  but it may be attenuated when compared to listening position  1002  due to the greater distance and the front seats  1031  and  1032  acting as obstacles. The attenuation due to the front seats  1031  and  1032  may be frequency dependent. Therefore, a compensation signal may be applied to a right rear speaker  1011  to correct for the response of center speaker  1003  at the second listening location  1012 . The choice of delay and energy level for this compensation signal may be guided by the actual measurements, surveys, or any other mechanism, as previously discussed. In one example, more delay may be applied to left rear speaker  1011  than was applied to left front speaker  1001  due to a first distance from the left front speaker  1003  to the listening location  1012  being greater than a second distance from the right rear speaker  1011  to the listening location  1002 . Accordingly, a level of the audible sound produced by the right rear speaker  1011  may be relatively louder without the listener in the second listening position  1012  perceiving that the location of the center speaker  1003  has changed. In addition, since the right rear speaker  1011  is close in proximity to the second listening location  1012  as compared to the other listening locations, this speaker will have the greatest effect on the audible sound perceived by a listener positioned in the second listening location  1012 . 
     In another example, compensated audio signals may be used to enable a listener to perceive that the individual speaker channels sound substantially equally loud at substantially all listener locations. For this example, consider a LFC signal  1000  on a left front channel of a multichannel sound source. Such multichannel sound sources may include a compact disc, broadcast audio content, live audio content, a DVD, an MP3 file, or any other live or pre-recorded audio content provided as an input signal. In addition, multichannel sound sources may include any device or mechanism capable of creating multi-channel audio content, such as an upmixer for converting audio content having fewer audio channels to audio content having additional audio channels, or a downmixer for converting audio content having many audio channels to audio content having fewer audio channels. The LFC signal  1000  may be channeled to and emitted by the left front speaker  1001 . The acoustical energy level of the LFC signal  1000  may be much louder at the first listener location  1002  than it is at the second listener location  1012 . This is due to the difference in distance, as well as the acoustic barriers between the first and second listening locations  1001  and  1012 . Conversely, consider a RRC signal  1006  provided on a right rear channel from the sound source. The RRC signal  1006  may be emitted as audible sound by the right rear speaker  1011 . The acoustical energy level of the RRC signal  1006  may be much louder at the second listening location  1012  than it is at the first listening location  1002 . 
     Also as part of this example, consider a third listening location  1030  that is located at approximately the center of the listening area. At the third listening location  1030 , the sounds from each of the speakers  1001 ,  1003 ,  1004 ,  1011  and  1021  of this example can be perceived by a listener in the third listening position  1030  as being substantially equal. Although this is a desired result for optimal multichannel playback, in the example vehicle provided, not only is there no seating position for a listener at this location, but also the other listening positions within the listening area may not perceive a similar experience. 
     With a multichannel compensating audio system, all of the output channels from the sound source may be perceived by listeners in the listening locations as being substantially equally loud. In the first listener location  1002 , for example, the sound from the left front speaker  1001  can be made substantially equal in perceived loudness to the sound from the right rear speaker  1011  without the compensation system, by simply increasing the level of audible sound produced by the right rear speaker  1011  to offset attenuation that the audible sound produced by the right rear speaker  1011  experiences in its audio path to the first listening location  1002 . Although simply increasing the audible sound produced by the right rear speaker  1011  could indeed resolve unequal sound levels perceived at the first listener location  1002 , it could also aggravate unequal sound levels perceived at the second listener location  1012 . In some cases, at the second location  1012 , the signal from the right rear speaker  1011  may already be perceived by a listener as louder than the signal from the left front speaker  1001 . By increasing the level of audible sound produced by the right rear speaker  1011  to accommodate the first listening location  1002 , the imbalance in loudness may be made even worse at the second listening location  1012 . 
     Use of compensated audio signals with adjusted delay and energy levels may solve such imbalanced loudness at different listening positions. For example, in  FIG. 10  consider the second listening location  1012  in a situation where the signal from the right rear speaker  1011  is louder than the signal from the left front speaker  1001 . In this example, the LFC signal  1000  on the left front channel may be processed through a compensation channel  1010 , which consists of the delay circuit, the level adjuster circuit, and the equalizer (EQ) circuit. The settings for compensation channel  1010  may be predetermined as previously discussed. The compensation delay may be set to be at least long enough so that the sound from the left front speaker  1001  reaches the second listener position  1012  before the compensated audio signal from the right rear speaker  1011 . More generally, the delay and energy level may be set so that the sound source continues to be psychoacoustically perceived by the listener in the second listening position  1012  as coming from speaker  1001 . The delay and energy level parameters may be set at a compensation channel  1010  so that the sound from the LFC signal  1000  of the sound source is psychoacoustically perceived by a listener at the second listener position  1012  as substantially equal in magnitude of spectral energy (substantially equally loud) as the sound from the RRC signal  1006  of the sound source. At the same time, the delay and energy level parameters may be set at a compensation channel  1040  so that the sound from the RRC signal  1006  of the sound source is perceived by a listener at the first listener position  1002  as equally loud to the sound from the LFC signal  1000  of the sound source. 
     The EQ may be set on the compensation channel  1010  to compensate for the response of speaker  1001  at the second listening location  1012 . The EQ of the compensation channel  1010  can also be used to attenuate the higher frequencies relative to the level of the lower frequencies. This may done to account for the fact that the human ear does not integrate higher frequencies as readily as lower frequencies. Therefore, for a given delay, the higher frequencies may be attenuated by a predetermined amount in order to prevent the compensation signal from being audible as a separate sound source, and/or to prevent LFC signal  1000  from shifting its perceived location away from its front-left location. 
     In some situations it may not be possible to make the compensated audio signal at the right rear speaker  1011  loud enough so that the LFC signal  1000  and the RRC signal  1006  of the sound source sound equally loud at the second listener position  1012 . There may be a limit as to how loud the compensation signal at the right rear speaker  1011  can become before the listener begins to experience a perceived shift in the sound image, or before the audible compensated audio signal from the right rear speaker  1011  is no longer integrated with the signal from the left front speaker  1001  by the listener&#39;s ear at the second listening location  1012 . When the compensation signal from the right rear speaker  1011  is no longer integrated with the signal from  1001 , then the signal from the right rear speaker  1011  will start to be heard as a separate sound source. To address this, additional compensation channels may be employed in order to try to increase the perceived loudness of the LFC signal  1000  at the second listener location  1012 . In  FIG. 10 , a second compensation channel  1020 , processes the LFC audio signal  1000  and creates a second compensation signal to be emanated from a left rear speaker  1021 . The second compensation signal may be used to supplement the first compensated audio signal from the right rear speaker  1011 . The delay, energy level and EQ may be predetermined as previously discussed. The nearest speaker to the listener location may be used as the first compensation channel for that listener location, with subsequent compensation channels configured in accordance with need and desireable effect on the perceived sound at the listener location. 
     In another example, it is desirable to move the perceived location of an individual speaker channel using the multichannel compensating audio system. In the example of a multichannel compensating audio system in a vehicle, consider the center speaker  1003  which is physically located in the front and center of the listening space, such as on the center of the dashboard in the vehicle. When the center channel signal from a sound source is sent to the center speaker  1003 , the listener at the first listening location  1002  may perceive the sound to come from the physical location of the center speaker  1003 . In some situations this is acceptable and desirable. However, some listeners may prefer to acoustically perceive the center channel sound as appearing to come from directly in front of them, even when the center speaker  1003  does not occupy that physical location. In addition, at the same time, the perceived center channel sound source should also be perceived by other listeners in other listening locations in the listening space as directly in front of all of those other listeners. 
     This may be accomplished with the multichannel compensating audio system by sending a center frequency (CFC) signal  1045  from the sound source to the center speaker  1003 . At the same time the CFC signal  1045  may be processed through a fourth compensation channel  1050  and the compensated audio signal may be provided to the left front speaker  1001 . Predetermined values of the delay, EQ, and the energy level may be chosen for the fourth compensation channel  1050  as previously discussed. In this case, it is possible to allow the compensation signal emitted by left front speaker  1001  to arrive at the first listener position  1002  before the signal from center speaker  1003  arrives at the first listening position  1002 . To achieve this, the CFC signal  1045  may be delayed in going to the center speaker  1003  using a delay circuit  1055 . 
     The compensating delay applied by the delay circuit  1055  for the center speaker  1001  could be positive or negative with respect to the time of arrival of the signal from the left front speaker  1003  at the first listening location  1002 . The predetermined level of the compensated audio signal emitted by the left front speaker  1001  may be chosen based on the chosen delay as well as the relative physical locations of the left front speaker  1001  and the center speaker  1003  with respect to the first listener position  1002 . In order to move the perceived sound source to a point directly in front of a listener in listening position provided by the seat  1032 , a substantially similar compensated audio signal may be provided to the right front speaker  1004 . A similar process may be used with left rear speaker  1021  and right rear speaker  1011  to provide a perceived center channel audio source for the second listening position, and other listening positions, such as in the rear seat of a vehicle. Also, multiple speakers may be used to move the position of a given audio source channel signal to a desired perceived location. 
     Using the compensation system, different listeners in different listening positions can have different perceived locations for the same sound source channels at the same time. For example, in a vehicle the driver may want the center channel audio signal from a sound source to be perceived as appearing directly in front of the driver seat, while the front seat passenger may want the center channel audio signal to be perceived as appearing to come from the center of the dashboard where the center speaker  1003  is physically located. 
     A similar process may be used on all of the sound source channel signals in order to make them appear to come from desired locations. In addition to moving a perceived speaker location from side-to-side, the compensation system may also provide for movement of a perceived speaker location forward or backwards in a listening area. Moreover, if the audio system includes one or more speakers that are physically positioned in an elevated location with respect to other speakers in the audio system, a perceived speaker location may be moved vertically up and down within a listening space. For example, where one or more speakers are physically positioned above one or more listening positions, such as mounted in the headliner of a vehicle, a perceived speaker location may be moved vertically up and down within the listening space of the vehicle. Accordingly, the perceived locations of the sound source channel signals may be selectively elevated. Similarly, the perceived locations of the sound source channel signals may be selectively lowered. 
     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.