Patent Publication Number: US-10764704-B2

Title: Multi-channel subband spatial processing for loudspeakers

Description:
FIELD OF THE DISCLOSURE 
     Embodiments of the present disclosure generally relate to the field of audio signal processing and, more particularly, to spatially enhanced multi-channel audio. 
     BACKGROUND 
     Surround sound refers to sound reproduction of an audio signal including multiple channels with loudspeakers positioned around a listener. For example, 5.1 surround sound uses a six channels for a front speaker, left and right speakers, a subwoofer, and rear (or “surround”) left and rear right speakers. In another example, 7.1 surround sound uses eight channels by separating the rear left and right speakers of the 5.1 surround sound configuration into four separate speakers, such as a left surround speaker, a right surround speaker, a left rear surround speaker, and a right rear surround speaker. Audio channels of the multi-channel audio signal may be associated with an angular position that corresponds with the location of the speaker to which the audio channels are output. Thus, the multi-channel audio signals allow a listener to perceive a spatial sense in the sound field when the audio signals are output to speakers at different locations. However, the spatial sense may be lost when the multi-channel audio signals for surround sound are output to stereo (e.g., left and right) loudspeakers or head-mounted speakers. 
     SUMMARY 
     Example embodiments relate to processing a (e.g., surround sound) multi-channel input audio signal into a stereo output signal for left and right speakers, while preserving or enhancing the spatial sense of the sound field of the multi-channel input audio signal. Among other things, the processing results in a listening experience whereby each channel of audio signal is perceived as originating from the same or similar direction as would occur if the audio signal were rendered on a surround sound system (e.g., 5.1, 7.1, etc.). 
     In some example embodiments, a multi-channel input audio signal including a left input channel, a right input channel, a left peripheral input channel, and a right peripheral input channel is received. A subband spatial processing is performed on the left input channel, the right input channel, the left peripheral input channel, and the right peripheral input channel to create spatially enhanced channels. The subband spatial processing may include gain adjusting mid and side subband components of the left input channel, the right input channel, the left peripheral input channel, and the right peripheral input channel. Crosstalk cancellation is performed on the spatially enhanced channels to create a crosstalk cancelled left channel and a right crosstalk cancelled channel. A left outpout channel is generated from the left crosstalk cancelled channel and a right output channel is generated from the right crosstalk cancelled channel. 
     The left and right peripheral channels may include a left surround input channel and a right surround input channel, and/or a left surround rear input channel and a right surround rear input channel. The multi-channel input audio signal may further include a center channel and a low frequency channel that may be combined with the output of the crosstalk cancellation. 
     In some embodiments, the subband spatial processing is performed on each of the corresponding pairs of left right channels. For example, subband spatial processing may be performed by gain adjusting the mid subband components and the side subband components of the left input channel and the right input channel, gain adjusting the mid subband components and the side subband components of the left peripheral input channel and the right peripheral input channel, and combining the gain adjusted mid subband components and the gain adjusted side subband components of the left input channel, the right input channel, the left peripheral input channel, and the right peripheral input channel into a left combined channel and a right combined channel. The crosstalk cancellation is performed on the left and right combined channels to generate the output channels. 
     In some embodiments, the subband spatial processing is performed on combined left and right channels. For example, the subband spatial processing may include combining the left input channel and the left peripheral input channel into a left combined channel, combining the right input channel and the right peripheral input channel into a right combined channel, and gain adjusting mid subband components and the side subband components of the left combined channel and the right combined channel to create a left spatially enhanced channel and a right spatially enhanced channel. The crosstalk cancellation is performed on the left and right spatially enhanced channels to generate the output channels. 
     In some embodiments, a binaural filter is applied to at least a portion of the input channels. For example, a binaural filter is applied to the peripheral input channels to adjust for angular positions associated with the peripheral input channels. In some embodiments, a binaural filter is applied to any input channel as suitable to adjust for the angular positions associated with the input channel, including the left or right input channels. 
     Some embodiments may include a system for processing a multi-channel input audio signal. The system includes circuitry configured to: receive the multi-channel input audio signal including a left input channel, a right input channel, a left peripheral input channel, and a right peripheral input channel; perform subband spatial processing on the left input channel, the right input channel, the left peripheral input channel, and the right peripheral input channel to create spatially enhanced channels, the subband spatial processing including gain adjusting mid and side subband components of the left input channel, the right input channel, the left peripheral input channel, and the right peripheral input channel; perform crosstalk cancellation on the spatially enhanced channels to create a left crosstalk cancelled channel and a right crosstalk cancelled channel; and generate a left output channel from the left crosstalk cancelled channel and a right output channel from the right crosstalk cancelled channel. 
     Some embodiments may include a non-transitory computer readable medium storing program code. The program code may be software comprised of executable instructions. The program code may be executed by one or more processors. The program code, when executed by a processor, causes the processor to receive a multi-channel input audio signal including a left input channel, a right input channel, a left peripheral input channel, and a right peripheral input channel. When executed, the program code when executed by the processor may cause the processor to perform subband spatial processing on the left input channel, the right input channel, the left peripheral input channel, and the right peripheral input channel to create spatially enhanced channels. The subband spatial processing may include gain adjusting mid and side subband components of the left input channel, the right input channel, the left peripheral input channel, and the right peripheral input channel. The program code when executed by the processor may cause the processor to perform crosstalk cancellation on the spatially enhanced channels to create a left crosstalk cancelled channel and a right crosstalk cancelled channel. The program code when executed by the processor also may cause the processor to generate a left output channel from the left crosstalk cancelled channel and a right output channel from the right crosstalk cancelled channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a surround sound stereo audio reproduction system, according to one embodiment. 
         FIG. 2  illustrates an example of an audio system, according to one embodiment. 
         FIG. 3  illustrates an example of a subband spatial processor, according to one embodiment. 
         FIG. 4  illustrates an example of a crosstalk cancellation processor, according to one embodiment. 
         FIG. 5  illustrates an example of a method for enhancing an audio signal with the audio system shown in  FIG. 2 , according to one embodiment. 
         FIG. 6  illustrates an example of an audio system, according to one embodiment. 
         FIG. 7  illustrates an example of a method for enhancing an audio signal with the audio system shown in  FIG. 6 , according to one embodiment. 
         FIG. 8  illustrates an example of a computer system, according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
     The Figures (FIG.) and the following description relate to the preferred embodiments by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the present invention. 
     Reference will now be made in detail to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein. 
     Example Surround Sound Stereo and Example Audio System 
     The audio systems discussed herein provide crosstalk processing and spatial enhancement for multi-channel surround sound audio signal for output to stereo (e.g., left and right) speakers. The signal processing results in the preserving or enhancing of the spatial sense of the sound field encoded in the multi-channel surround sound audio signal. Among other things, the spatial sense achieved using multi-speaker surround sound systems is achieved using stereo loudspeakers. 
       FIG. 1  illustrates an example of a surround sound stereo audio reproduction system  100 , according to one embodiment. The system  100  is an example of a 7.1 surround sound system that provides audio signal reproduction to a listener  140 . The system  100  includes a left speaker  110 L, a right speaker  110 R, a center speaker  115 , a subwoofer  125 , a left surround speaker  120 L, a right surround speaker  120 R, a left surround rear speaker  130 L, and a right surround speaker  130 R. The center speaker  115  and subwoofer  125  may be positioned in front of the listener  140 , which defines a forward axis at 0°. The left speaker  110 L may be positioned at an angle between −20° to −30° relative to the forward axis, and the right speaker  110 R may be positioned at an angle between 20° to 30° relative to the forward axis. The left surround speaker  120 L may be positioned at an angle between −90° to −110° relative to the forward axis, and the right surround speaker  120 R may be positioned at an angle between 90° to 110° relative to the forward axis. The left surround rear speaker  130 L may be positioned at an angle between −135° to −150° relative to the forward axis, and the right surround speaker  130 R may be positioned at an angle between 135° to 150° relative to the forward axis. The system  100  may be configured to receive an audio signal including channels for each of the speakers  110 ,  115 ,  120 , and  130  and the subwoofer  125 . The multiple speakers and their positional arrangement provides for a spatial sense in the sound field that can be perceived by the listener  140 . As discussed in greater detail below, the audio system may be configured to process a multi-channel input audio signal for the surround sound system  100  into an enhanced stereo signal for left and right speakers (e.g., speakers  110 L and  110 R) that reproduces or simulates the spatial sense in the sound field generated by the surround sound system  100  using the multi-channel audio signal. 
       FIG. 2  illustrates an example of an audio system  200 , according to one embodiment. The audio system  200  receives an input audio signal including a left input channel  201 A, a right input channel  210 B, a center input channel  210 C, a low frequency input channel  210 D, a left surround input channel  210 E, a right surround input channel  210 F, a left surround rear input channel  210 G, and a right surround rear input channel  210 H. 
     The channels  210 E,  210 F,  210 G, and  210 H are examples of peripheral channels for surround speakers. Peripheral channels may include channels other than the left and right input channels. Peripheral channels may include channel pairs, such as left-right pairs, or front-back pairs, or other pair arrangements. For example, when the input audio signal is output by the surround sound stereo audio reproduction system  100 , the left surround speaker  120 L receives the left surround input channel  210 E, the right surround speaker  120 R receives the right surround input channel  210 F, the left surround rear speaker  130 L receives the left surround rear input channel  210 G, and the right surround rear speaker  130 R receives the right surround rear input channel  210 H. In some embodiments, the input audio signal has fewer or more peripheral channels. For example, an audio input signal for a 5.1 surround sound system may include only two peripheral channels, such as left and right surround input channels that may be output to left and right surround speakers. Similarly, the left speaker  110 L may receive the left input channel  210 A, the right speaker  110 R may receive the right input channel  210 B, the center speaker  115  may receive the center input channel  210 C, and the subwoofer  125  may receive the low frequency input channel  210 D. The input audio signal provides a spatial sense of the sound field when output by the surround sound stereo audio reproduction system  100 . 
     The audio system  200  receives the input audio signal and generates an output signal including a left output channel  290 L and a right output channel  290 R. The audio system  200  may combine the input channels of the input audio signal, and may further provide enhancements such as subband spatial processing and crosstalk cancellation, to generate the output audio signal. The left output channel  290 L may be provided to a left speaker and the right output channel  290 R may be output to a right speaker. The output audio signal provides a spatial sense of the sound field using the left and right speakers (e.g., left speaker  110 L and right speaker  110 R) that is typically achieved by outputting the input audio signal using a surround sound system including multiple (e.g., peripheral) speakers. 
     The audio system  200  includes gains  215 A,  215 B,  215 C,  215 D,  215 E,  215 F,  215 G, and  215 H, sub-band spatial processors  230 A,  230 B, and  230 C, a high shelf filter  220 , a divider  240 , binaural filters  250 A,  250 B,  250 C, and  250 D, a left channel combiner  260 A, a right channel combiner  260 B, a crosstalk cancellation processor  270 , a left channel combiner  260 C, a right channel combiner  260 D, and an output gain  280 . 
     Each of the gains  215 A through  215 H may receive a respective input channel  210 A through  210 H, and may apply a gain to an input channel  210 A through  210 H. The gains  215 A through  215 H may be different to adjust gains of the input channels with respect to each other, or may be the same. In some embodiments, positive gains are applied to the left and right peripheral input channels  210 E,  210 F,  210 G, and  210 H, and a negative gain is applied to the center channel  210 C. For example, the gain  215 A may apply a 0 db gain, the gain  215 B may apply a 0 dB gain, the gain  215 C may apply a −3 dB gain, the gain  215 D may apply a 0 db gain, the gain  215 E may apply a 3 dB gain, the gain  215 F may apply a 3 dB gain, the gain  215 G may apply a 3 dB gain, and the gain  215 H may apply a 3 dB gain. 
     The gain  215 A and gain  215 B are coupled to the subband spatial processor  230 . Similarly, the gains  215 E and  215 F are coupled to the subband spatial proricessor  230 B, and the gains  215 G and  215 H are coupled to the subband spatial processor  230 C. The subband spatial processors  230 A,  230 B, and  230 C each apply subband spatial processing to corresponding left and right channel pairs. 
     Each subband spatial processor  230  performs subband spatial processing on a left and right input channel by gain adjusting mid and side subband components of the left and right input channels to generate left and right spatially enhanced channels. The subband spatial processor  230 A performs the subband spatial processing on the left and right input channels, while other subband spatial processors  230 B and  230 C each perform the subband spatial processing to corresponding left and right peripheral channels. Depending on the number of peripheral channels in the input audio signal, the audio system  200  may include more or less subband spatial processors. In some embodiments, channels without left/right counterparts (such as the center input channel  210 C, the low frequency input channel  210 D, or other types of channels such as rear-center, overhead-center, etc.) can bypass SBS processing. 
     The subband spatial processor  230 B is coupled to the binaural filters  250 A and  250 B. The subband spatial processor  230 B provides a left spatially enhanced channel to the binaural filter  250 A, and provides a right spatially enhanced channel to the binaural filter  250 B. Similarly, the subband spatial processor  230 C is coupled to the binaural filters  250 C and  250 D. The subband spatial processor  230 C provides a left spatially enhanced channel to the binaural filter  250 C, and provides a right spatially enhanced channel to the binaural filter  250 D. Additional details regarding a subband spatial processor  230  are shown in  FIG. 3  and discussed below. 
     Each of the binuaral filters  250 A,  250 B,  250 C, and  250 D apply a head-related transfer function (HRTF) that describes the target source location from which the listener should perceive the sound of the input channel. Each binaural filter receives an input channel and generates a left and right output channel by applying a HRTF that adjusts for an angular position associated with the input channel. The angular position may include an angle defined in an X-Y “azimuthal” plane relative to listener  140  the as shown in  FIG. 1 , and may further include an angle defined in the Z axis, such as for an ambisonics signal or a channel-based format containing signals intended to be rendered above or below the X-Y plane relative to the listener  140 . For example, the binaural filter  250 A may be configured to apply a filter based on the left surround input channel  210 E being associated with the angle (defined in the X-Y plane) between −90° to −110° relative to the forward axis of the left surround speaker  120 L. The binaural filter  250 B may be configured to apply a filter based on the right surround input channel  210 F being associated the angle between 90° to 110° relative to the forward axis of the right surround speaker  120 L. The binaural filter  250 C may be configured to apply a filter based on the left surround rear input channel  210 G being associated with the angle between −135° to −150° relative to the forward axis of the left surround rear speaker  130 L. The binaural filter  250 D may be configured to apply a filter based on the right surround rear input channel  210 H being associated with the angle between 135° to 150° relative to the forward axis of the rear speaker  130 R. In some embodiments, the binaural processing may be bypassed entirely in order to preserve inter-channel spectral uniformity. One or more of the binuaral filters  250 A,  250 B,  250 C, and  250 D may be omitted from the audio system  200 . However, the binuaral filters  250 A,  250 B,  250 C, and  250 D may be used to enhance spatial imaging. In some embodiments, binaural filtering may be applied to channels other than peripheral input channels. For example, a binaural filter may be applied to each of the left and right spatially enhanced channels that are output from the subband spatial processor  230 A to adjust for different left and right output speaker location. In another example, if the input audio signal includes channels associated with other speaker locations (i.e. Overhead, Rear-Center, etc.), then binaural processing may be applied to the other input channels. In that sense, binaural processing may be applied to one or more of the left input channel  210 A, the right input channel  210 B, the center input channel  210 C, or the low frequency input channel  210 D. In some embodiments, HRTFs are not applied, and one or more of the binuaral filters  250 A,  250 B,  250 C, and  250 D may be bypassed or omitted from the system  200 . 
     An example binaural filter may be defined by Equation 1:
 
 S   o ( z )= H (θ, z ) S   i ( z )  Eq. (1)
 
where S o  and S i  are the output and input signals, respectively. The argument θ encodes the angle of each channel in S i  and S o . The value z is an arbitrary complex number, of which our solution is a function, encoding frequency. H(θ,z) is therefore a function of both angle θ and z, returning a transfer function, itself a function of z, which may be selected or interpolated among a collection of transfer functions, perhaps derived from an anthropometric database. In this notation, the angle θ, as well as S and H(θ) as functions of z may evaluate to vectors if multichannel processing is desired. In this case, each coefficient in S(z), and H(θ,z) corresponds to a different channel, while each coefficient in θ associates an angle to each channel.
 
     In some embodiments, the input audio signal is an ambisonics audio signal defining a speaker-independent representation of a sound field. The ambisonics audio signal may be decoded into a multi-channel audio signal for a surround sound system. The channels may be associated with speaker locations at various locations, including locations that are above or below the listener. A binaural filter may be applied to each decoded input channel of the ambisonics audio signal to adjust for the associated position of the decoded input audio channel. 
     In some embodiments, the binaural filtering is performed prior to subband spatial processing. For example, a binaural filter may be applied to one or more of the input channels as suitable to adjust for angular positions associated with the channels. For each left-right input channel pair, the left output channels of the binaural filters may be combined, and right output channels of the binaural filters may be combined, and the subband spatial processing may be applied to the combined left and right channels. In some embodiments, binaural filters are applied to the center input channel  210 C or the low frequency input channel  210 D. In some embodiments, binaural filters are applied to each input channel except the low frequency input channel  210 D. 
     The left channel combiner  260 A is coupled to the subband spatial processor  230 A, and the binaural filters  250 A,  250 B,  250 C, and  250 D. The left channel combiner  260 A receives the left output channels of the subband subband spatial processor  230 A, and the binaural filters  250 A,  250 B,  250 C, and  250 D, and combines these channels into a left combined channel. The right channel combiner  260 B is also coupled to the subband spatial processor  230 A, and the binaural filters  250 A,  250 B,  250 C, and  250 D. The right channel combiner  260 B receives the right output channels of the subband subband spatial processor  230 A, and the binaural filters  250 A,  250 B,  250 C, and  250 D, and combines these channels into a right combined channel. 
     The crosstalk cancellation processor  270  receives left and right input channels and performs a crosstalk cancellation to generate left and right crosstalk cancelled channels. The crosstalk cancellation processor is coupled to the left channel combiner  260 A to receive a left combined channel, and the right channel combiner  260 B to receive a right combined channel. Here, the left and right combined channels processed by the crosstalk cancellation processor  270  represent mixed down left and right counterpart input channels. Additional details regarding the crosstalk cancellation processor  270  are shown in  FIG. 4  and discussed below. 
     The high shelf filter  220  receives the center input channel  210 C and applies a high frequency shelving or peaking filter. The high shelf filter  220  provides a “voice-lift” on the center input channel  210 C. In some embodiments, the high shelf filter  220  is bypassed, or omitted from the audio system  200 . The high shelf filter  220  may attenuate or amplify frequencies above a corner frequency. The high shelf filter  220  is coupled to the left channel combiner  260 C and the right channel combiner  260 D. In some embodiments, the high shelf filter  220  is defined by a 750 Hz corner frequency, a +3 dB gain, and 0.8 Q factor. The high shelf filter  220  generates a left center channel and a right center channel as output, such as by separating the center input channel into two separate left and right center channels. 
     The divider  240  receives the low frequency input channel  210 D, and separates the low frequency input channel  210 D into left and right low frequency channels. The divider  240  is coupled to the left channel combiner  260 C and the right channel combiner  260 D, and provides the left low frequency channel to the left channel combiner  260 C and the right low frequency channel to the right channel combiner  260 D. 
     The left channel combiner  260 C is coupled to the crosstalk cancellation processor  270 , the high shelf filter  220 , and the divider  240 . The left channel combiner  260 C receives the left crosstalk channel from the crosstalk cancellation processor  270 , the left center channel from the high shelf filter  220 , and the left low frequency channel from the divider  240 , and combines these channels into a left output channel. 
     Right channel combiner  260 D is coupled to the crosstalk cancellation processor  270 , the high shelf filter  220 , and the divider  240 . The right channel combiner  260 D receives the right crosstalk channel from the crosstalk cancellation processor  270 , the right output channel from the high shelf filter  220 , and the right low frequency channel from the divider  240 , and combines these channels into a right output channel. 
     In some embodiments, the left center channel from the high shelf filter  220  and the left low frequency channel from the divider  240  are combined by the left channel combiner  260 A with the left spatially enhanced channel from the subband spatial processor  230 A and the left output channels of the binaural filters  250 A,  250 B,  250 C, and  250 D to generate the left combined channel. Similarly, the right output channel from the high shelf filter  220  and the right low frequency channel from the divider  240  are combined by the right channel combiner  260  with the right spatially enhanced channel from the subband subband spatial processor  230 A and the right output channels of the binaural filters  250 A,  250 B,  250 C, and  250 D to generate the right combined channel. The left and right combined channels are input into the crosstalk cancellation processor  270 . Here, the center and low frequency channels receive the crosstalk cancellation operation. The left channel combiner  260 C and right channel combiner  260 D may be omitted. In some embodiments, one of the center or low frequency channels receives the crosstalk cancellation operation. 
     The output gain  280  is coupled to left channel combiner  260 C and the right channel combiner  260 D. The output gain  280  applies a gain to the left output channel from the left channel combiner  260 C, and applies a gain to the right output channel from the right channel combiner  260 D. The output gain  280  may apply the same gain to the left and right output channels, or may apply different gains. The output gain  280  outputs the left output channel  290 L and the right output channel  290 R which represent the channels of the output signal of the audio system  200 . 
     Example Subband Spacial Processor 
       FIG. 3  illustrates an example of a subband spatial processor  230 , according to one embodiment. The subband spatial processor  230  is an example of the subband spatial processors  230 A,  230 B, or  230 C of the audio system  200 . The subband spatial processor  230  includes a spatial frequency band divider  340 , a spatial frequency band processor  345 , and a spatial frequency band combiner  350 . The spatial frequency band divider  340  is coupled to the spatial frequency band processor  345 , and the spatial frequency band processor  345  is coupled to the spatial frequency band cominber  350 . 
     The spatial frequency band divider  340  includes an L/R to M/S converter  312  that receives a left input channel X L  and a right input channel X R , and converts these inputs into a spatial component X m  and the nonspatial component X s . The spatial component X s  may be generated by subtracting the left input channel X L  and right input channel X R . The nonspatial component X m  may be generated by adding the left input channel X L  and the right input channel X R . 
     The spatial frequency band processor  345  receives the nonspatial component X m  and applies a set of subband filters to generate the enhanced nonspatial subband component E m . The spatial frequency band processor  345  also receives the spatial subband component X s  and applies a set of subband filters to generate the enhanced nonspatial subband component E m . The subband filters can include various combinations of peak filters, notch filters, low pass filters, high pass filters, low shelf filters, high shelf filters, bandpass filters, bandstop filters, and/or all pass filters. 
     In some embodiments, the spatial frequency band processor  345  includes a subband filter for each of n frequency subbands of the nonspatial component X m  and a subband filter for each of the n frequency subbands of the spatial component X s . For n=4 subbands, for example, the spatial frequency band processor  345  includes a series of subband filters for the nonspatial component X m  including a mid equalization (EQ) filter  362 ( 1 ) for the subband ( 1 ), a mid EQ filter  362 ( 2 ) for the subband ( 2 ), a mid EQ filter  362 ( 3 ) for the subband ( 3 ), and a mid EQ filter  362 ( 4 ) for the subband ( 4 ). Each mid EQ filter  362  applies a filter to a frequency subband portion of the nonspatial component X m  to generate the enhanced nonspatial component E m . 
     The spatial frequency band processor  345  further includes a series of subband filters for the frequency subbands of the spatial component X s , including a side equalization (EQ) filter  364 ( 1 ) for the subband ( 1 ), a side EQ filter  364 ( 2 ) for the subband ( 2 ), a side EQ filter  364 ( 3 ) for the subband ( 3 ), and a side EQ filter  364 ( 4 ) for the subband ( 4 ). Each side EQ filter  364  applies a filter to a frequency subband portion of the spatial component X s  to generate the enhanced spatial component E s . 
     Each of the n frequency subbands of the nonspatial component X m  and the spatial component X s  may correspond with a range of frequencies. For example, the frequency subband ( 1 ) may corresponding to 0 to 300 Hz, the frequency subband ( 2 ) may correspond to 300 to 510 Hz, the frequency subband ( 3 ) may correspond to 510 to 2700 Hz, and the frequency subband ( 4 ) may correspond to 2700 Hz to Nyquist frequency. In some embodiments, the n frequency subbands are a consolidated set of critical bands. The critical bands may be determined using a corpus of audio samples from a wide variety of musical genres. A long term average energy ratio of mid to side components over the 24 Bark scale critical bands is determined from the samples. Contiguous frequency bands with similar long term average ratios are then grouped together to form the set of critical bands. The range of the frequency subbands, as well as the number of frequency subbands, may be adjustable. 
     In some embodiments, the mid EQ filters  362  or side EQ filters  364  may include a biquad filter, having a transfer function defined by Equation 2: 
                     H   ⁡     (   z   )       =         b   0     +       b   1     ⁢     z     -   1         +       b   2     ⁢     z     -   2               a   0     +       a   1     ⁢     z     -   1         +       a   2     ⁢     z     -   2                     Eq   .           ⁢     (   2   )                 
where z is a complex variable. The filter may be implemented using a direct form I topology as defined by Equation 3:
 
                     Y   ⁡     [   n   ]       =           b   0       a   0       ⁢     X   ⁡     [     n   -   1     ]         +         b   1       a   0       ⁢     X   ⁡     [     n   -   1     ]         +         b   2       a   0       ⁢     X   ⁡     [     n   -   2     ]         -         a   1       a   0       ⁢     Y   ⁡     [     n   -   1     ]         -         a   2       a   0       ⁢     Y   ⁡     [     n   -   2     ]                   Eq   .           ⁢     (   3   )                 
where X is the input vector, and Y is the output. Other topologies might have benefits for certain processors, depending on their maximum word-length and saturation behaviors.
 
     The biquad can then be used to implement any second-order filter with real-valued inputs and outputs. To design a discrete-time filter, a continuous-time filter is designed and transformed it into discrete time via a bilinear transform. Furthermore, compensation for any resulting shifts in center frequency and bandwidth may be achieved using frequency warping. 
     For example, a peaking filter may include an S-plane transfer function defined by Equation 4: 
                     H   ⁡     (   s   )       =         s   2     +     s   ⁡     (     A   Q     )       +   1         s   2     +     s   ⁡     (     A   Q     )       +   1               Eq   .           ⁢     (   4   )                 
where s is a complex variable, A is the amplitude of the peak, and Q is the filter “quality” (canonically derived as:
 
                 Q   =       f   c       Δ   ⁢           ⁢   f         )     .         
The digital filters coefficients are:
 
               b   0     =     1   +     α   ⁢           ⁢   A                     b   1     =       -   2     *     cos   ⁡     (     ω   0     )                       b   2     =     1   -     α   ⁢           ⁢   A                     a   0     =     1   +     α   A                     a   1     =       -   2     ⁢           ⁢     cos   ⁡     (     ω   0     )                       a   2     =     1   +     α   A             
where ω 0  is the center frequency of the filter in radians and
 
     
       
         
           
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                   sin 
                   ⁡ 
                   
                     ( 
                     
                       ω 
                       0 
                     
                     ) 
                   
                 
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Q 
                 
               
               . 
             
           
         
       
     
     The spatial frequency band combiner  350  receives mid and side components, applies gains to each of the components, and converts the mid and side components into left and right channels. For example, the spatial frequency band combiner  350  receives the enhanced nonspatial component E m  and the enhanced spatial component E s , and performs global mid and side gains before converting the enhanced nonspatial component E m  and the enhanced spatial component E s  into the left spatially enhanced channel E L  and the right spatially enhanced channel E R . 
     More specifically, the spatial frequency band combiner  350  includes a global mid gain  322 , a global side gain  324 , and an M/S to L/R converter  326  coupled to the global mid gain  322  and the global side gain  324 . The global mid gain  322  receives the enhanced nonspatial component E m  and applies a gain, and the global side gain  324  receives the enhanced spatial component E s  and applies a gain. The M/S to L/R converter  326  receives the enhanced nonspatial component E m  from the global mid gain  322  and the enhanced spatial component E s  from the global side gain  324 , and converts these inputs into the left spatially enhanced channel E L  and the right spatially enhanced channel E R . 
     Example Crosstalk Cancellation Processor 
       FIG. 4  illustrates a crosstalk cancellation processor  270 , according to one example embodiment. The crosstalk cancellation processor  270  receives the left spatially enhanced channel E L  as input from the left channel combiner  260 A and the right spatially enhanced channel E R  as input from the right channel combiner  260 B, and performs crosstalk cancellation on the channels E L , E R  to generate the left output channel O L , and the right output channel O R . 
     The crosstalk cancellation processor  270  includes an in-out band divider  410 , inverters  420  and  422 , contralateral estimators  430  and  440 , combiners  450  and  452 , and an in-out band combiner  460 . These components operate together to divide the input channels T L , T R  into in-band components and out-of-band components, and perform a crosstalk cancellation on the in-band components to generate the output channels O L , O R . 
     By dividing the input audio signal E into different frequency band components and by performing crosstalk cancellation on selective components (e.g., in-band components), crosstalk cancellation can be performed for a particular frequency band while obviating degradations in other frequency bands. If crosstalk cancellation is performed without dividing the input audio signal E into different frequency bands, the audio signal after such crosstalk cancellation may exhibit significant attenuation or amplification in the nonspatial and spatial components in low frequency (e.g., below 350 Hz), higher frequency (e.g., above 12000 Hz), or both. By selectively performing crosstalk cancellation for the in-band (e.g., between 250 Hz and 14000 Hz), where the vast majority of impactful spatial cues reside, a balanced overall energy, particularly in the nonspatial component, across the spectrum in the mix can be retained. 
     The in-out band divider  410  separates the input channels E L , E R  into in-band channels E L,In , E R,In  and out of band channels E L,Out , E R,Out , respectively. Particularly, the in-out band divider  410  divides the left enhanced compensation channel E L  into a left in-band channel E L,In  and a left out-of-band channel E L,Out . Similarly, the in-out band divider  410  separates the right enhanced compensation channel E R  into a right in-band channel E R,In  and a right out-of-band channel E R,Out . Each in-band channel may encompass a portion of a respective input channel corresponding to a frequency range including, for example, 250 Hz to 14 kHz. The range of frequency bands may be adjustable, for example according to speaker parameters. 
     The inverter  420  and the contralateral estimator  430  operate together to generate a left contralateral cancellation component S L  to compensate for a contralateral sound component due to the left in-band channel E L,In . Similarly, the inverter  422  and the contralateral estimator  440  operate together to generate a right contralateral cancellation component S R  to compensate for a contralateral sound component due to the right in-band channel E R,In . 
     In one approach, the inverter  420  receives the in-band channel E L,In  and inverts a polarity of the received in-band channel E L,In  to generate an inverted in-band channel E L,In ′. The contralateral estimator  430  receives the inverted in-band channel E L,In ′, and extracts a portion of the inverted in-band channel E L,In ′ corresponding to a contralateral sound component through filtering. Because the filtering is performed on the inverted in-band channel E L,In ′, the portion extracted by the contralateral estimator  430  becomes an inverse of a portion of the in-band channel E L,In  attributing to the contralateral sound component. Hence, the portion extracted by the contralateral estimator  430  becomes a left contralateral cancellation component S L , which can be added to a counterpart in-band channel E R,In  to reduce the contralateral sound component due to the in-band channel E L,In . In some embodiments, the inverter  420  and the contralateral estimator  430  are implemented in a different sequence. 
     The inverter  422  and the contralateral estimator  440  perform similar operations with respect to the in-band channel E R,In  to generate the right contralateral cancellation component S R . Therefore, detailed description thereof is omitted herein for the sake of brevity. 
     In one example implementation, the contralateral estimator  430  includes a filter  432 , an amplifier  434 , and a delay unit  436 . The filter  432  receives the inverted input channel E L,In ′ and extracts a portion of the inverted in-band channel E L,In ′ corresponding to a contralateral sound component through a filtering function. An example filter implementation is a Notch or Highshelf filter with a center frequency selected between 5000 and 10000 Hz, and Q selected between 0.5 and 1.0. Gain in decibels (G dB ) may be derived from Equation 5:
 
 G   dB =−3.0−log 1.333 ( D )  Eq. (5)
 
where D is a delay amount by delay unit  1556 A/B in samples, for example, at a sampling rate of 48 KHz. An alternate implementation is a Lowpass filter with a corner frequency selected between 5000 and 10000 Hz, and Q selected between 0.5 and 1.0. Moreover, the amplifier  434  amplifies the extracted portion by a corresponding gain coefficient G L,In , and the delay unit  436  delays the amplified output from the amplifier  434  according to a delay function D to generate the left contralateral cancellation component S L . The contralateral estimator  440  includes a filter  442 , an amplifier  444 , and a delay unit  446  that performs similar operations on the inverted in-band channel E R,In ′ to generate the right contralateral cancellation component S R . In one example, the contralateral estimators  430 ,  440  generate the left contralateral cancellation components S L , S R , according to equations below:
 
 S   L   =D [ G   L,In   *F [ E   L,In ′]]  Eq. (6)
 
 S   R   =D [ G   R,In   *F [ E   R,In ′]]  Eq. (7)
 
where F[ ] is a filter function, and D[ ] is the delay function.
 
     The configurations of the crosstalk cancellation can be determined by the speaker parameters. In one example, filter center frequency, delay amount, amplifier gain, and filter gain can be determined, according to an angle formed between two outputs speakers of the output signal with respect to a listener, or other features of the speaker such as relative position, power, etc. In some embodiments, values between the speaker angles are used to interpolate other values. 
     The combiner  450  combines the right contralateral cancellation component S R  to the left in-band channel E L,In  to generate a left in-band compensation channel U L , and the combiner  452  combines the left contralateral cancellation component S L  to the right in-band channel E R,In  to generate a right in-band compensation channel U R . The in-out band combiner  460  combines the left in-band compensation channel U L  with the out-of-band channel E L,out  to generate the left output channel O L , and combines the right in-band compensation channel U R  with the out-of-band channel E R,Out  to generate the right output channel O R . 
     Accordingly, the left output channel O L  includes the right contralateral cancellation component S R  corresponding to an inverse of a portion of the in-band channel T R,In  attributing to the contralateral sound, and the right output channel O R  includes the left contralateral cancellation component S L  corresponding to an inverse of a portion of the in-band channel T L,In  attributing to the contralateral sound. In this configuration, a wavefront of an ipsilateral sound component output by a right speaker (e.g., speaker  110 R) according to the right output channel O R  arrived at the right ear can cancel a wavefront of a contralateral sound component output by a right speaker (e.g., speaker  110 L) according to the left output channel O L . Similarly, a wavefront of an ipsilateral sound component output by the left speaker according to the left output channel O L  arrived at the left ear can cancel a wavefront of a contralateral sound component output by the right speaker according to right output channel O R . Thus, contralateral sound components can be reduced to enhance spatial detectability. 
     Example Audio Signal Enhancement Process 
       FIG. 5  illustrates an example of a method  500  for enhancing an audio signal with the audio system  200  shown in  FIG. 2 , according to one embodiment. In some embodiments, the method  500  may include different and/or additional steps, or some steps may be in different orders. 
     The audio system  200  receives  505  a multi-channel input audio signal. The multi-channel audio signal may be a surround sound audio signal including a left input channel, a right input channel, at least one left peripheral input channel, and at least one right peripheral input channel. The multi-channel audio signal may further include the center input channel  210 C and the low frequency input channel  210 D. For example, the input audio signal may be for a 7.1 surround sound system including the left input channel  210 A and the right input channel  210 B, and peripheral channels including the left surround input channel  210 E and the right surround input channel  210 F, and the left surround rear input channel  210 G, and the right surround rear input channel  210 H. In another example of an input audio signal for a 5.1 surround sound system, the peripheral channels may include a single left peripheral channel and a single right peripheral channel. 
     The audio system  200  (e.g., gains  215 A through  215 H) applies  510  gains to the channels of the multi-channel input audio signal. The gains  215 A through  215 H may vary to control the contribution of particular input channels to the output signal generated by the audio system  200 . In some embodiments, the center channel  210 C receives a negative gain while the peripheral input channels receive a positive gain. 
     The audio system  200  (e.g., subband spatial processor  230 A) generates  515  a left spatially enhanced channel and a right spatially enhanced channel by performing subband spatial processing on the left input channel and the right input channel. For example, the subband spatial processor  230 A generates the spatially enhanced channels by adjusting gains of n subbands of the mid component and the side component of the left input channel  210 A and the right input channel  210 B. 
     The audio system  200  (e.g., subband spatial processor  230 B and/or  230 C) generates  520  a left spatially enhanced peripheral channel and a right spatially enhanced peripheral channel by performing subband spatial processing on the left peripheral input channel and the right peripheral input channel. For example, the subband spatial processor  230 B adjusts gains of n subbands of the mid component and the side component of the left surround channel  210 E and the right surround channel  210 F to generate left and right spatially enhanced peripheral channels. The subband spatial processor  230 C adjusts gains of the n subband of the mid component and the side component of the left surround rear channel  210 G and the right surround rear channel  210 H to generate left and right spatially enhanced peripheral channels. 
     The audio system  200  (e.g., binaural filters  250 A through  250 D) applies  525  a binaural filter to each of the left and right spatially enhanced peripheral channels. For example, the binaural filter  250 A generates a left and right output channel from the left spatially enhanced peripheral channel output from the subband spatial processor  230 B by applying a head-related transfer function (HRTF). The binaural filter  250 B generates a left and right output channel from the spatially enhanced right channel output from the subband spatial processor  230 B by applying a HRTF. The binaural filter  250 C generates a left and right output channel from the spatially enhanced left channel output from the subband spatial processor  230 C by applying a HRTF. The binaural filter  250 D generates a left and right output channel from the spatially enhanced right channel output from the subband spatial processor  230 C by applying a HRTF. In some embodiments, the binaural filtering is bypassed. 
     The audio system  200  (e.g., high shelf filter  220 ) applies  530  a high shelf filter to the center input channel  210 C. In some embodiments, a gain is applied to the center input channel  210 C. Furthermore, the high shelf filter  220  separates the center input channel  210 C into a left center channel and a right center channel. 
     The audio system  200  (e.g., divider  240 ) separates  535  the low frequency input channel into left and right low frequency channels. 
     The audio system  200  (e.g., left channel combiner  260 A) combines  540  the left spatially enhanced channel from the subband subband spatial processor  230 A and the left output channels of the binaural filters  250 A,  250 B,  250 C, and  250 D to generate a left combined channel. For example, the left spatially enhanced channel may be added with the left output channels. 
     The audio system  200  (e.g., right channel combiner  260 B) combines  545  the right spatially enhanced channel from the subband subband spatial processor  230 A and the right output channels of the binaural filters  250 A,  250 B,  250 C, and  250 D to generate a right combined channel. For example, the right spatially enhanced channel may be added with the right output channels. 
     The audio system  200  (e.g., crosstalk cancellation processor  270 ) performs  550  a crosstalk cancellation on the left combined channel and the right combined channel to generate a left crosstalk cancelled channel and a right crosstalk cancelled channel. 
     The audio system  200  (e.g., left channel combiner  260 C and right channel combiner  260 D) combines  555  the left crosstalk cancelled channel from the crosstalk cancellation processor  270  with the left low frequency channel from the divider  240  and the left center channel from the high shelf filter  220  to generate a left output channel, and combines the right crosstalk cancelled channel from the crosstalk cancellation processor  270  with the right low frequency channel from the divider  240  and the right center channel from the high shelf filter  220  to generate a right output channel. Furthermore, the audio system  200  (e.g., output gain  280 ) may apply gains to each of the left and right output channels. The audio system  200  outputs an output audio signal including the left and right output channels  290 L and  290 R. 
     Example Audio System and Example Audio Processing Process 
       FIG. 6  illustrates an example of an audio system  600 , according to one embodiment. The audio system  600  may be similar to the audio system  200 , but may differ from the audio system  200  at least in that the left and right input channels are combined with the left and right peripheral channels prior to subband spatial processing for the audio system  600 . Here, a single subband spatial processor and corresponding subband spatial processing step may be used rather than separate subband spatial processors for left-right speaker pairs as shown for the audio system  200 . 
     The audio system  600  receives an input audio signal. The input audio signal may include a left input channel  610 A, a right input channel  610 B, a center input channel  610 C, a low frequency input channel  610 D, a left surround input channel  610 E, a right surround input channel  610 F, a left surround rear input channel  610 G, and a right surround rear input channel  610 H. The channels  610 E,  610 F,  610 G, and  610 H are examples of peripheral channels that may be provided to surround speakers. In some embodiments, the audio system  600  may receive and process an input audio signal having fewer or more channels. 
     The audio system  600  generates an output signal including a left output channel  690 L and a right output channel  690 R using enhancements such as subband spatial processing and crosstalk cancellation on the input audio signal. The left output channel  690 L may be provided to a left speaker and the right output channel  690 R may be output to a right speaker. The output audio signal provides a spatial sense of the sound field associated with the surround sound input audio signal using left and right speakers (e.g., left speaker  110 L and right speaker  110 R). 
     The audio system  600  includes gains  615 A,  615 B,  615 C,  615 D,  615 E,  615 F,  615 G, and  615 H, a high shelf filter  620 , a divider  640 , binaural filters  650 A,  650 B,  650 C, and  650 D, a left channel combiner  660 A, a right channel combiner  660 B, a sub-band spatial processor  630 , a crosstalk cancellation processor  670 , a left channel combiner  660 C, a right channel combiner  660 D, and an output gain  680 . 
     Each of the gains  615 A through  615 H may receive a respective input channel  610 A through  610 H, and may apply a gain to an input channel  610 A through  610 H. The gains  615 A through  615 H may be different to adjust gains of the input channels with respect to each other, or may be the same. In some embodiments, positive gains are applied to the left and right peripheral input channels  610 E,  610 F,  610 G, and  610 H, and a negative gain is applied to the center channel  610 C. For example, the gain  615 A may apply a 0 db gain, the gain  615 B may apply a 0 dB gain, the gain  615 C may apply a −3 dB gain, the gain  615 D may apply a 0 db gain, the gain  615 E may apply a 3 dB gain, the gain  615 F may apply a 3 dB gain, the gain  615 G may apply a 3 dB gain, and the gain  615 H may apply a 3 dB gain. 
     The gain  615 A for the left input channel  610 A is coupled to the left channel combiner  660 A. The gain  615 B for the right input channel  610 B is coupled to the right channel combiner  660 B. The gain  615 C is coupled to the high shelf filter  620 . The gain  615 D is coupled to the divider  640 . The gains  615 E,  615 F,  610 G, and  610 H of the peripheral input channels are each coupled to a binaural filter  650 . In particular, the gain  610 E is coupled to the binaural filter  650 A, the gain  615 F is coupled to the binaural filter  650 B, the gain  615 G is coupled to the binaural filter  650 C, and the gain  615 H is coupled to the binaural filter  650 D. 
     Each of the binuaral filters  650 A,  650 B,  650 C, and  650 D apply a head-related transfer function (HRTF) that describes the target source location from which the listener should perceive the sound of the input channel. Each binaural filter receives an input channel and generates a left and right output channel by applying the HRTF. The discussion of the binaural filters  250 A,  250 B,  250 C, and  250 D of the audio system  200  may be applicable to the binaural filters  650 A,  650 B,  650 C, and  650 D. For example, each of the binaural filters  650 A through  650 D may apply an adjustment for the angular positions associated with their respective input channel. In some embodiments, one or more of the binaural filters  650 A through  650 D may be bypassed, or omitted from the audio system  600 . 
     The left channel combiner  660 A is coupled to the gain  615 A and the binaural filters  650 A through  650 D. The left channel combiner  660 A receives the left output channels of the binaural filters  650 A through  650 D, and combines the left output channels with the output of the gain  615 A. The right channel combiner  660 B is coupled to the gain  615 B and the binaural filters  650 A through  650 D. The right channel combiner  660 B receives the right output channels of the binaural filters  650 A through  650 D, and combines the right output channels with the output of the gain  615 B. 
     In some embodiments, the binaural filtering is performed subsequent to subband spatial processing. For example, a binaural filter may be applied to the left and right outputs of the subband spatial processor  630  as suitable to adjust for angular positions associated with the channels. In some embodiments, binaural filters are applied to the peripheral input channels as shown in  FIG. 6 . In some embodiments, binaural filters are applied to the center input channel  610 C or the low frequency input channel  610 D. In some embodiments, binaural filters are applied to each input channel except the low frequency input channel  610 D. 
     The subband spatial processor  630  performs subband spatial processing on a left and right input channel by gain adjusting mid and side subband components of the left and right input channels to generate left and right spatially enhanced channels as output. The subband spatial processor  630  is coupled to the left channel combiner  660 A to receive a left combined channel from the left channel combiner  660 A and is coupled to the right channel combiner  660 B to receive a right combined channel from the right channel combiner  660 B. Unlike the subband spatial processors  230 A,  230 B, and  230 C of the audio system  200  that each processes a corresponding left and right input channel, the subband spatial processor  630  processes the left and right channels after combination into the left and right combined channels. Thus, the audio system  600  may include only a single subband spatial processor  630 . In some embodiments, the subband spatial processor  230  shown in  FIG. 3  is an example of the subband spatial processor  630 . 
     The crosstalk cancellation processor  670  performs crosstalk cancellation on the output of the subband spatial processor  630 , which may represent a mixed down stereo signal of the input audio signal. The crosstalk cancellation processor  670  receives left and right input channels from the subband spatial processor  630 , and performs a crosstalk cancellation to generate left and right crosstalk cancelled channels. The crosstalk cancellation processor  670  is coupled to the left channel combiner  260 A and the right channel combiner  260 B. In some embodiments, the crosstalk cancellation processor  270  shown in  FIG. 4  is an example of the crosstalk cancellation processor  670 . 
     The high shelf filter  620  receives the center input channel  610 C and applies a high frequency shelving or peaking filter. The high shelf filter  620  provides a “voice-lift” on the center input channel  610 C. In some embodiments, the high shelf filter  620  is bypassed, or omitted from the audio system  600 . The high shelf filter  620  may attenuate frequencies above a corner frequency. The high shelf filter  620  is coupled to the left channel combiner  660 C and the right channel combiner  660 D. In some embodiments, the high shelf filter  620  is defined by a 750 Hz corner frequency, a +3 dB gain, and 0.8 Q factor. The high shelf filter  620  generates a left center channel and a right center channel as output. 
     The divider  640  receives the low frequency input channel  610 D, and separates the low frequency input channel  610 D into left and right low frequency channels. The divider  640  is coupled to the left channel combiner  660 C and the right channel combiner  660 D, and provides the left low frequency channel to the left channel combiner  660 C and the right low frequency channel to the right channel combiner  660 D. 
     The left channel combiner  660 C is coupled to the crosstalk cancellation processor  670 , the high shelf filter  620 , and the divider  640 . The left channel combiner  660 C receives the left crosstalk channel from the crosstalk cancellation processor  670 , the left center channel from the high shelf filter  620 , and the left low frequency channel from the divider  640 , and combines these channels into a left output channel. 
     Right channel combiner  660 D is coupled to the crosstalk cancellation processor  670 , the high shelf filter  620 , and the divider  640 . The right channel combiner  660 D receives the right crosstalk channel from the crosstalk cancellation processor  670 , the right center channel from the high shelf filter  620 , and the right low frequency channel from the divider  640 , and combines these channels into a right output channel. 
     In some embodiments, the left center channel from the high shelf filter  620  and the left low frequency channel from the divider  640  are combined by the left channel combiner  660 A with the left output channels of the binaural filters  650 A through  650 D and the output of the gain  615 A to generate a left combined channel. The right center channel from the high shelf filter  620  and the right low frequency channel from the divider  640  are combined by the right channel combiner  660 B with the right output channels of the binaural filters  650 A through  650 D and the output of the gain  615 B to generate a right combined channel. The left and right combined channels are input into the subband spatial processor  630  and the crosstalk cancellation processor  670 . Here, the center and low frequency channels receive the subband spatial processing and crosstalk cancellation operations. The left channel combiner  660 C and right channel combiner  660 D may be omitted. In some embodiments, one of the center or low frequency channels receives the subband spatial processing and crosstalk cancellation operations. 
     The output gain  680  is coupled to left channel combiner  660 C and the right channel combiner  660 D. The output gain  680  applies a gain to the left output channel from the left channel combiner  660 C, and applies a gain to the right output channel from the right channel combiner  660 D. The output gain  680  may apply the same gain to the left and right output channels, or may apply different gains. The output gain  680  outputs the left output channel  690 L and the right output channel  690 R which represent the channels of the output signal of the audio system  600 . 
       FIG. 7  illustrates an example of a method  700  for enhancing an audio signal with the audio system  600  shown in  FIG. 6 , according to one embodiment. In some embodiments, the method  700  may include different and/or additional steps, or some steps may be in different orders. 
     The audio system  600  receives  705  a multi-channel input audio signal. The input audio signal may include a left input channel  610 A, a right input channel  610 B, at least one left peripheral input channel, and at least one right peripheral input channel. The multi-channel audio signal may further include the center input channel  610 C and the low frequency input channel  610 D. 
     The audio system  600  (e.g., gains  615 A through  615 H) applies  710  gains to the channels of the multi-channel input audio signal. The gains  615 A through  615 H may vary to control the contribution of particular input channels to the output signal generated by the audio system  600 . 
     The audio system  600  (e.g., binaural filters  650 A through  650 D) applies  715  a binaural filter to each of the left and right peripheral channels. For example, the binaural filter  650 A generates a left and right output channel from the left surround input channel  610 E by applying a head-related transfer function (HRTF). The binaural filter  650 B generates a left and right output channel from the right surround input channel  610 F by applying a HRTF. The binaural filter  650 C generates a left and right output channel from the left surround rear input channel  610 G by applying a HRTF. The binaural filter  650 D generates a left and right output channel from the right surround rear input channel  610 H by applying a HRTF. 
     The audio system  600  (e.g., high shelf filter  620 ) applies  720  a high shelf filter to the center input channel  610 C. In some embodiments, a gain is applied to the center input channel  610 C. Furthermore, the high shelf filter  620  separates the center input channel  610 C into a left center channel and a right center channel. 
     The audio system  600  (e.g., divider  640 ) separates  725  the low frequency input channel into left and right low frequency channels. 
     The audio system  600  (e.g., left channel combiner  660 A) combines  730  the left input channel  610 A and the left output channels of the binaural filters  650 A,  650 B,  650 C, and  650 D to generate a left combined channel. 
     The audio system  600  (e.g., right channel combiner  660 B) combines  735  the right input channel  610 B and the right output channels of the binaural filters  650 A,  650 B,  650 C, and  650 D, to generate a right combined channel. 
     The audio system  600  (e.g., subband spatial processor  630 ) generates  740  a left spatially enhanced channel and a right spatially enhanced channel by performing subband spatial processing on the left combined channel and the right combined channel. For example, the subband spatial processor  630  receives the left and right combined channels from the left channel combiner  660 A and the right channel combiner  660 B, and generates the spatially enhanced channels by adjusting gains of n subbands of the mid component and the side component of the left and right combined channels. 
     The audio system  600  (e.g., crosstalk cancellation processor  670 ) performs  745  a crosstalk cancellation on the left and right spatially enhanced channels from the subband spatial processor  630  to generate a left crosstalk cancelled channel and a right crosstalk cancelled channel. 
     The audio system  600  (e.g., left channel combiner  660 C and right channel combiner  660 D) combines  750  the left crosstalk cancelled channel from the crosstalk cancellation processor  670  with the left low frequency channel from the divider  640  and the left center channel from the high shelf filter  620  to generate a left output channel, and combines the right crosstalk cancelled channel from the crosstalk cancellation processor  670  with the right low frequency channel from the divider  640  and the right center channel from the high shelf filter  620  to generate a right output channel. Furthermore, the audio system  600  (e.g., output gain  680 ) may apply gains to each of the left and right output channels. The audio system  600  outputs an output audio signal including the left and right output channels  690 L and  690 R. 
     It is noted that the systems and processes described herein may be embodied in an embedded electronic circuit or electronic system. The systems and processes also may be embodied in a computing system that includes one or more processing systems (e.g., a digital signal processor) and a memory (e.g., programmed read only memory or programmable solid state memory), or some other circuitry such as an application specific integrated circuit (ASIC) or field-programmable gate array (FPGA) circuit. 
       FIG. 8  illustrates an example of a computer system  800 , according to one embodiment. The audio systems  200  and  600  may be implemented on the system  800 . Illustrated are at least one processor  802  coupled to a chipset  804 . The chipset  804  includes a memory controller hub  820  and an input/output (I/O) controller hub  822 . A memory  806  and a graphics adapter  812  are coupled to the memory controller hub  820 , and a display device  818  is coupled to the graphics adapter  812 . A storage device  808 , keyboard  810 , pointing device  814 , and network adapter  816  are coupled to the I/O controller hub  822 . Other embodiments of the computer  800  have different architectures. For example, the memory  806  is directly coupled to the processor  802  in some embodiments. 
     The storage device  808  includes one or more non-transitory computer-readable storage media such as a hard drive, compact disk read-only memory (CD-ROM), DVD, or a solid-state memory device. The memory  806  holds instructions and data used by the processor  802 . For example, the memory  806  may store instructions that when executed by the processor  802  causes or configures the processor  802  to perform the methods discussed herein, such as the method  500  or  700 . The pointing device  814  is used in combination with the keyboard  810  to input data into the computer system  800 . The graphics adapter  812  displays images and other information on the display device  818 . In some embodiments, the display device  818  includes a touch screen capability for receiving user input and selections. The network adapter  816  couples the computer system  800  to a network. Some embodiments of the computer  800  have different and/or other components than those shown in  FIG. 8 . For example, the computer system  800  may be a server that lacks a display device, keyboard, and other components. 
     The computer  800  is adapted to execute computer program modules for providing functionality described herein. As used herein, the term “module” refers to computer program instructions and/or other logic used to provide the specified functionality. Thus, a module can be implemented in hardware, firmware, and/or software. In one embodiment, program modules formed of executable computer program instructions are stored on the storage device  808 , loaded into the memory  806 , and executed by the processor  802 . 
     ADDITIONAL CONSIDERATIONS 
     The disclosed configuration may include a number of benefits and/or advantages. For example, a multi-channel input signal can be output to stereo loudspeakers while preserving or enhancing a spatial sense of the sound field. A high quality listening experience can be achieved without requiring expensive multi-speaker sound systems, such as on mobile devices, sound bars, or smart speakers. 
     Upon reading this disclosure, those of skill in the art will appreciate still additional alternative embodiments the disclosed principles herein. Thus, while particular embodiments and applications have been illustrated and described, it is to be understood that the disclosed embodiments are not limited to the precise construction and components disclosed herein. Various modifications, changes and variations, which will be apparent to those skilled in the art, may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the scope described herein. 
     Any of the steps, operations, or processes described herein may be performed or implemented with one or more hardware or software modules, alone or in combination with other devices. In one embodiment, a software module is implemented with a computer program product comprising a computer readable medium (e.g., non-transitory computer readable medium) containing computer program code, which can be executed by a computer processor for performing any or all of the steps, operations, or processes described.