Patent Publication Number: US-10313820-B2

Title: Sub-band spatial audio enhancement

Description:
BACKGROUND 
     Field of the Disclosure 
     Embodiments of the present disclosure generally relate to the field of audio signal processing and, more particularly, to spatial enhancement of stereo and multi-channel audio produced over loudspeakers. 
     Description of the Related Art 
     Stereophonic sound reproduction involves encoding and reproducing signals containing spatial properties of a sound field. Stereophonic sound enables a listener to perceive a spatial sense in the sound field from a stereo signal. 
     SUMMARY 
     A subband spatial audio processing method enhances an audio signal including a left input channel and a right input channel. The left input channel and the right input channel are processed into a spatial component and a nonspatial component. First subband gains are applied to subbands of the spatial component to generate an enhanced spatial component, and second subband gains are applied to subbands of the nonspatial component to generate an enhanced nonspatial component. The enhanced spatial component and the enhanced nonspatial component are then combined into a left output channel and a right output channel. 
     In some embodiments, the processing of the left input channel and the right input channel into the spatial component and the nonspatial component includes processing the left input channel and the right input channel into spatial subband components and nonspatial subband components. The first subband gains can be applied to the subbands of the spatial component by applying the first subband gains to the spatial subband components to generate enhanced spatial subband components. Similarly, the second gains can be applied to the subbands of the nonspatial component by applying the second subband gains to the nonspatial subband components to generate enhanced nonspatial subband components. The enhanced spatial subband components and the enhanced nonspatial subband components can then be combined. 
     A subband spatial audio processing apparatus for enhancing an audio signal having a left input channel and a right input channel can include a spatial frequency band divider, a spatial frequency band processor, and a spatial frequency band combiner. The spatial frequency band divider processes the left input channel and the right input channel into a spatial component and a nonspatial component. The spatial frequency band processor applies first subband gains to subbands of the spatial component to generate an enhanced spatial component, and applies second subband gains to subbands of the nonspatial component to generate an enhanced nonspatial component. The spatial frequency band combiner combines the enhanced spatial component and the enhanced nonspatial component into a left output channel and a right output channel. 
     In some embodiments, the spatial frequency band divider processes the left input channel and the right input channel into the spatial component and the nonspatial component by processing the left input channel and the right input channel into spatial subband components and nonspatial subband components. The spatial frequency band processor applies the first subband gains to the subbands of the spatial component to generate the enhanced spatial component by applying the first subband gains to the spatial subband components to generate enhanced spatial subband components. The spatial frequency band processor applies the second subband gains to the subbands of the nonspatial component to generate the enhanced spatial component by applying the second subband gains to the nonspatial subband components to generate enhanced nonspatial subband components. The spatial frequency band combiner combines the enhanced spatial component and the enhanced nonspatial component into the left output channel and the right output channel by combining the enhanced spatial subband components and the enhanced nonspatial subband components. 
     Some embodiments include a non-transitory computer readable medium to store program code, the program code comprising instructions that when executed by a processor cause the processor to: process a left input channel and a right input channel of an audio signal into a spatial component and a nonspatial component; apply first subband gains to subbands of the spatial component to generate an enhanced spatial component; apply second subband gains to subbands of the nonspatial component to generate an enhanced nonspatial component; and combine the enhanced spatial component and the enhanced nonspatial component into a left output channel and a right output channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , comprising  FIGS. 1A and 1B , illustrates an example of a stereo audio reproduction system, according to one embodiment. 
         FIG. 2  illustrates an example of an audio system  200  for enhancing an audio signal, according to one embodiment. 
         FIG. 3A  illustrates an example of a spatial frequency band divider of the audio system, according to some embodiments. 
         FIG. 3B  illustrates an example of a spatial frequency band divider of the audio system, according to some embodiments. 
         FIG. 3C  illustrates an example of a spatial frequency band divider of the audio system, according to some embodiments. 
         FIG. 3D  illustrates an example of a spatial frequency band divider of the audio system, according to some embodiments. 
         FIG. 4A  illustrates an example of a spatial frequency band processor of the audio system, according to some embodiments. 
         FIG. 4B  illustrates an example of a spatial frequency band processor of the audio system, according to some embodiments. 
         FIG. 4C  illustrates an example of a spatial frequency band processor of the audio system, according to some embodiments. 
         FIG. 5A  illustrates an example of a spatial frequency band combiner of the audio system, according to some embodiments. 
         FIG. 5B  illustrates an example of a spatial frequency band combiner of the audio system, according to some embodiments. 
         FIG. 5C  illustrates an example of a spatial frequency band combiner of the audio system, according to some embodiments. 
         FIG. 5D  illustrates an example of a spatial frequency band combiner of the audio system, according to some embodiments. 
         FIG. 6  illustrates an example of a method for enhancing an audio signal, according to one embodiment. 
         FIG. 7  illustrates an example of a subband spatial processor, according to one embodiment. 
         FIG. 8  illustrates an example of a method for enhancing an audio signal with the subband spatial processor shown in  FIG. 7 , according to one embodiment. 
         FIG. 9  illustrates an example of a subband spatial processor, according to one embodiment. 
         FIG. 10  illustrates an example of a method for enhancing an audio signal with the subband spatial processor shown in  FIG. 9 , according to one embodiment. 
         FIG. 11  illustrates an example of a subband spatial processor, according to one embodiment. 
         FIG. 12  illustrates an example of a method for enhancing an audio signal with the subband spatial processor shown in  FIG. 11 , according to one embodiment. 
         FIG. 13  illustrates an example of an audio system  1300  for enhancing an audio signal with crosstalk cancellation, according to one embodiment. 
         FIG. 14  illustrates an example of an audio system  1400  for enhancing an audio signal with crosstalk simulation, 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 Audio System 
       FIG. 1  illustrates some principles of stereo audio reproduction. In a stereo configuration, speakers  110   L  and  110   R  are positioned at fixed locations with respect to a listener  120 . The speaker  110  convert a stereo signal comprising left and right audio channels (equivalently, signals) into sound waves, which are directed towards a listener  120  to create an impression of sound heard from an imaginary sound source  160  (e.g., a spatial image), which may appear to be located between loudspeakers  110   L  and  110   R , or an imaginary source  160  located beyond either of the loudspeakers  110 , or any combination of such sources  160 . The present disclosure provides various methods for enhancing the perception of such spatial images-processing of the left and right audio channels. 
       FIG. 2  illustrates an example of an audio system  200  in which a subband spatial processor  210  can be used to enhance an audio signal, according to one embodiment. The audio system  200  includes a source component  205  that provides an input audio signal X including two input channels X L  and X R  to the subband spatial processor  210 . The source component  205  is a device that provides the input audio signal X in a digital bitstream (e.g., PCM data), and may be a computer, digital audio player, optical disk player (e.g., DVD, CD, Blu-ray), digital audio streamer, or other source of digital audio signals. The subband spatial processor  210  generates an output audio signal O including two output channels O L  and O R  by processing the input channels X L  and X R . The audio output signal O is a spatially enhanced audio signal of the input audio signal X. The subband spatial processor  210  is configured to be coupled to an amplifier  215  in the system  200 , which amplifies the signal and provides the signal to output devices, such as the loudspeakers  110   L  and  110   R , that convert the output channels O L  and O R  into sound. In some embodiments, the output channels O L  and O R  are coupled to another type of speaker, such as headphones, earbuds, integrated speakers of an electronic device, etc. 
     The subband spatial processor  210  includes a spatial frequency band divider  240 , a spatial frequency band processor  245 , and a spatial frequency band combiner  250 . The spatial frequency band divider  240  is coupled to the input channels X L  and X R  and the spatial frequency band processor  245 . The spatial frequency band divider  240  receives the left input channel X L  and the right input channel X R , and processes the input channels into a spatial (or “side”) component Y s  and a nonspatial (or “mid”) component Y m . For example, the spatial component Y s  can be generated based on a difference between the left input channel X L  and the right input channel X R . The nonspatial component Y m  can be generated based on a sum of the left input channel X L  and the right input channel X R . The spatial frequency band divider  240  provides the spatial component Y s  and the nonspatial component Y m  to the spatial frequency band processor  245 . 
     In some embodiments, the spatial frequency band divider  240  separates the spatial component Y s  into spatial subband components Y s ( 1 )-Y s (n), where n is a number of frequency subbands. The frequency subbands each includes a range of frequencies, such as 0-300 Hz, 300-510 Hz, 510-2700 Hz, and 2700-Nyquist Hz for n=4 frequency subbands. The spatial frequency band divider  240  also separates the nonspatial component Y m  into nonspatial subband components Y m ( 1 )-Y m (n), where n is the number of frequency subbands. The spatial frequency band divider  240  provides the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n) to the spatial frequency band processor  245  (e.g., instead of the unseparated spatial component Y s  and nonspatial component Y m ).  FIGS. 3A, 3B, 3C, and 3D  illustrate various embodiments of the spatial frequency divider  240 . 
     The spatial frequency band processor  245  is coupled to the spatial frequency band divider  240  and the spatial frequency band combiner  250 . The spatial frequency band processor  245  receives the spatial component Y s  and the nonspatial component Y m  from spatial frequency band divider  240 , and enhances the received signals. In particular, the spatial frequency band processor  245  generates an enhanced spatial component E s  from the spatial component Y s , and an enhanced nonspatial component E m  from the nonspatial component Y m . 
     For example, the spatial frequency band processor  245  applies subband gains to the spatial component Y s  to generate the enhanced spatial component E s , and applies subband gains to the nonspatial component Y m  to generate the enhanced nonspatial component E m . In some embodiments, the spatial frequency band processor  245  additionally or alternatively provides subband delays to the spatial component Y s  to generate the enhanced spatial component E s , and subband delays to the nonspatial component Y m  to generate the enhanced nonspatial component E m . The subband gains and/or delays can be different for the different (e.g., n) subbands of the spatial component Y s  and the nonspatial component Y m , or can be the same (e.g., for two or more subbands). The spatial frequency band processor  245  adjusts the gain and/or delays for different subbands of the spatial component Y s  and the nonspatial component Y m  with respect to each other to generate the enhanced spatial component E s  and the enhanced nonspatial component E m . The spatial frequency band processor  245  then provides the enhanced spatial component E s  and the enhanced nonspatial component E m  to the spatial frequency band combiner  250 . 
     In some embodiments, the spatial frequency band processor  245  receives the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n) from the spatial frequency band divider  240  (e.g., instead of the unseparated spatial component Y s  and the nonspatial component Y m ). The spatial frequency band processor  245  applies gains and/or delays to the spatial subband components Y s ( 1 )-Y s (n) to generate enhanced spatial subband components E s ( 1 )-E s (n), and applies gains and/or delays to the nonspatial subband components Y m ( 1 )-Y m (n) to generate enhanced nonspatial subband components E m ( 1 )-E m (n). The spatial frequency band processor  245  provides the enhanced spatial subband components E s ( 1 )-E s (n) and the enhanced nonspatial subband components E m ( 1 )-E m (n) to the spatial frequency band combiner  250  (e.g., instead of the unseparated enhanced spatial component E s  and enhanced nonspatial component E m ).  FIGS. 4A, 4B, and 4C  illustrate various embodiments of the spatial frequency band processor  245 , including spatial frequency band processors that process the spatial and nonspatial components and that process the spatial and nonspatial components after separation into subband components. 
     The spatial frequency band combiner  250  is coupled to the spatial frequency band processor  245 , and further coupled to amplifier  215 . The spatial frequency band combiner  250  receives the enhanced spatial component E s  and the enhanced nonspatial component E m  from the spatial frequency band processor  245 , and combines the enhanced spatial component E s  and the enhanced nonspatial component E m  into the left output channel O L  and the right output channel O R . For example, the left output channel O L  can be generated based on a sum of the enhanced spatial component E s  and the enhanced nonspatial component E m , and the right output channel O R  can be generated based on a difference between the enhanced nonspatial component E m  and the enhanced spatial component E s . The spatial frequency band combiner  250  provides the left output channel O L  and the right output channel O R  to amplifier  215 , which amplifies and outputs the signals to the left speaker  110   L , and the right speaker  110   R . 
     In some embodiments, the spatial frequency band combiner  250  receives the enhanced spatial subband components E s ( 1 )-E s (n) and the enhanced nonspatial subband components E m ( 1 )-E m (n) from the spatial frequency band processor  245  (e.g., instead of the unseparated enhanced nonspatial component E m  and enhanced spatial component E s ). The spatial frequency band combiner  250  combines the enhanced spatial subband components E s ( 1 )-E s (n) into the enhanced spatial component E s , and combines the enhanced nonspatial subband components E m ( 1 )-E m (n) into the enhanced nonspatial component E m . The spatial frequency band combiner  250  then combines the enhanced spatial component E s  and the enhanced nonspatial component E m  into the left output channel O L  and the right output channel O R .  FIGS. 5A, 5B, 5C, and 5D  illustrate various embodiments of the spatial frequency band combiner  250 . 
       FIG. 3A  illustrates a first example of a spatial frequency band divider  300 , as an implementation of the spatial frequency band divider  240  of the subband spatial processor  210 . Although the spatial frequency band divider  300  uses four frequency subbands ( 1 )-( 4 ) (e.g., n=4), other numbers of frequency subbands can be used in various embodiments. The spatial frequency band divider  300  includes a crossover network  304  and L/R to M/S converters  306 ( 1 ) though  306 ( 4 ). 
     The crossover network  304  divides the left input channel X L  into left frequency subbands X L ( 1 )-X L (n), and divides the right input channel X R  into right frequency subbands X R ( 1 )-X R (n), where n is the number of frequency subbands. The crossover network  304  may include multiple filters arranged in various circuit topologies, such as serial, parallel, or derived. Example filter types included in the crossover network  304  include infinite impulse response (IIR) or finite impulse response (FIR) bandpass filters, IIR peaking and shelving filters, Linkwitz-Riley (L-R) filters, etc. In some embodiments, n bandpass filters, or any combinations of low pass filter, bandpass filter, and a high pass filter, are employed to approximate the critical bands of the human ear. A critical band may correspond to the bandwidth within which a second tone is able to mask an existing primary tone. For example, each of the frequency subbands may correspond to a consolidated Bark scale to mimic critical bands of human hearing. 
     For example, the crossover network  304  divides the left input channel X L  into the left subband components X L ( 1 )-X L ( 4 ), corresponding to 0 to 300 Hz for frequency subband ( 1 ), 300 to 510 Hz for frequency subband ( 2 ), 510 to 2700 Hz for frequency subband ( 3 ), and 2700 to Nyquist frequency for frequency subband ( 4 ) respectively, and similarly divides the right input channel X R  into the right subband components X R ( 1 )-X R ( 4 ) for corresponding frequency subbands ( 1 )-( 4 ). In some embodiments, the consolidated set of critical bands is used to define the frequency subbands. 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 crossover network  304  outputs pairs of the left subband components X L ( 1 )-X L ( 4 ) and the right subband components X R ( 1 )-X R ( 4 ) to corresponding L/R to M/S converters  306 ( 1 )- 306 ( 4 ). In other embodiment, the crossover network  304  can separate the left and right input channels X L , X R  into fewer or greater than four frequency subbands. The range of frequency subbands may be adjustable. 
     The spatial frequency band divider  300  further includes n L/R to M/S converters  306 ( 1 )- 306 ( n ). In  FIG. 3A , spatial frequency band divider  300  uses n=4 frequency subbands, and thus the spatial frequency band divider  300  includes four L/R to M/S converters  306 ( 1 )- 306 ( 4 ). Each L/R to M/S converter  306 ( k ) receives a pair of subband components X L (k) and X R (k) for a given frequency subband k, and converts these inputs into a spatial subband component Y m (k) and a nonspatial subband component Y s (k). Each nonspatial subband component Y m (k) may be determined based on a sum of a left subband component X L (k) and a right subband component X R (k), and each spatial subband component Y s (k) may be determined based on a difference between the left subband component X L (k) and the right subband component X R (k). Performing such computations for each subband k, the L/R to M/S converters  306 ( 1 )- 306 ( n ) generate the nonspatial subband components Y m ( 1 )-Y m (n) and the spatial subband components Y s ( 1 )-Y s (n) from the left subband components X L ( 1 )-X L (n) and the right subband components X R ( 1 )-X R (n). 
       FIG. 3B  illustrates a second example of a spatial frequency band divider  310 , as an implementation of the spatial frequency band divider  240  of the subband spatial processor  210 . Unlike the spatial frequency band divider  300  of  FIG. 3A , the spatial frequency band divider  310  performs L/R to M/S conversion first and then divides the output of the L/R to M/S conversion into the nonspatial subband components Y m ( 1 )-Y m (n) and the spatial subband components Y s ( 1 )-Y s (n). 
     Performing the L/R to M/S conversion and then separating the nonspatial component Y m  into the nonspatial subband components Y m ( 1 )-Y m (n) and the spatial component Y s  into the spatial subband components Y s ( 1 )-Y s (n) can be computationally more efficient than separating the input signal into left and right subband components X L ( 1 )-X L (n), X R ( 1 )-X R (n) and then performing L/R to M/S conversion on each of the subband components. For example, the spatial frequency band divider  310  performs only one L/R to M/S conversion rather than the n L/R to M/S conversions (e.g., one for each frequency subband) performed by the spatial frequency band divider  300 . 
     More specifically, the spatial frequency band divider  310  includes an L/R to M/S converter  312  coupled to a crossover network  314 . The L/R to M/S converter  312  receives the left input channel X L  and the right input channel X R , and converts these inputs into the spatial component Y s  and the nonspatial component Y m . The crossover network  314  receives the spatial component Y s  and the nonspatial component Y m  from the L/R to M/S converter  312 , and separates these inputs into the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n). The operation of crossover network  314  is similar to network  304  in that it can employ a variety of different filter topologies and number of filters. 
       FIG. 3C  illustrates a third example of a spatial frequency band divider  320  as an implementation of the spatial frequency band divider  240  of the subband spatial processor  210 . The spatial frequency band divider  320  includes an L/S to M/S converter  322  that receives the left input channel X L  and the right input channel X R , and converts these inputs into the spatial component Y s  and the nonspatial component Y m . Unlike the spatial frequency band dividers  300  and  310  shown in  FIGS. 3A and 3B , the spatial frequency band divider  320  does not include a crossover network. As such, the spatial frequency band divider  320  outputs the spatial component Y s  and the nonspatial component Y m  without being separated into subband components. 
       FIG. 3D  illustrates a fourth example of a spatial frequency band divider  330 , as an implementation of the spatial frequency band divider  240  of the subband spatial processor  210 . The spatial frequency band divider  330  facilitates frequency domain enhancement of the input audio signal. The spatial frequency band divider  330  includes a forward fast Fourier transform (FFFT)  334  to generate the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n) as represented in the frequency domain. 
     A frequency domain enhancement may be preferable in designs where many parallel enhancement operations are desired (e.g., independently enhancing 512 subbands vs. only 4 subbands), and where the additional latency introduced from the forward/inverse Fourier Transforms poses no practical issue. 
     More specifically, the spatial frequency band divider  330  includes an L/R to M/S converter  332  and the FFFT  334 . The L/R to M/S converter  332  receives the left input channel X L  and the right input channel X R , and converts these inputs into the spatial component Y s  and the nonspatial component Y m . The FFFT  334  receives the spatial component Y s  and the nonspatial component Y m , and converts these inputs into the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n). For n=4 frequency subbands, the FFFT  334  converts the spatial component Y s  and the nonspatial component Y m  in the time domain into the frequency domain. The FFFT  334  then separates the frequency domain spatial component Y s  according to the n frequency subbands to generate the spatial subband components Y s ( 1 )-Y s ( 4 ), and separate the frequency domain nonspatial component Y m  according to the n frequency subbands to generate the nonspatial subband components Y m ( 1 )-Y m ( 4 ). 
       FIG. 4A  illustrates a first example of a spatial frequency band processor  400 , as an implementation of the frequency band processor  245  of the subband spatial processor  210 . The spatial frequency band processor  400  includes amplifiers that receive the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n), and apply subband gains to the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n). 
     More specifically, for example, the spatial frequency band processor  400  includes 2n amplifiers (equivalently “gains,” as shown in the Figures), where n=4 frequency subbands. The spatial frequency band processor  400  includes a mid gain  402 ( 1 ) and a side gain  404 ( 1 ) for the frequency subband ( 1 ), a mid gain  402 ( 2 ) and a side gain  404 ( 2 ) for the frequency subband ( 2 ), a mid gain  402 ( 3 ) and a side gain  404 ( 3 ) for the frequency subband ( 3 ), and a mid gain  402 ( 4 ) and a side gain  404 ( 4 ) for the frequency subband ( 4 ). 
     The mid gain  402 ( 1 ) receives the nonspatial subband components Y m ( 1 ) and applies a subband gain to generate the enhanced nonspatial subband components E m ( 1 ). The side gain  404 ( 1 ) receives the spatial subband component Y s ( 1 ) and applies a subband gain to generate the enhanced spatial subband components E s ( 1 ). 
     The mid gain  402 ( 2 ) receives the nonspatial subband components Y m ( 2 ) and applies a subband gain to generate the enhanced nonspatial subband components E m ( 2 ). The side gain  404 ( 2 ) receives the spatial subband component Y s ( 2 ) and applies a subband gain to generate the enhanced spatial subband components E s ( 2 ). 
     The mid gain  402 ( 3 ) receives the nonspatial subband components Y m ( 3 ) and applies a subband gain to generate the enhanced nonspatial subband components E m ( 3 ). The side gain  404 ( 3 ) receives the spatial subband component Y s ( 3 ) and applies a subband gain to generate the enhanced spatial subband components E s ( 3 ). 
     The mid gain  402 ( 4 ) receives the nonspatial subband component Y m ( 4 ) and applies a subband gain to generate the enhanced nonspatial subband component E m ( 4 ). The side gain  404 ( 4 ) receives the spatial subband component Y s ( 4 ) and applies a subband gain to generate the enhanced spatial subband components E s ( 4 ). 
     The gains  402 ,  404  adjust the relative subband gains of spatial and nonspatial subband components to provide audio enhancement. The gains  402 ,  404  may apply different amount of subband gains, or the same amount of subband gains (e.g., for two or more amplifiers) for the various subbands, using gain values controlled by configuration information, adjustable settings, etc. One or more amplifiers can also apply no subband gain (e.g., 0 dB), or negative gain. In this embodiment, the gains  402 ,  404  apply the subband gains in parallel. 
       FIG. 4B  illustrates a second example of a spatial frequency band processor  420 , as an implementation of the frequency band processor  245  of the subband spatial processor  210 . Like the spatial frequency band processor  400  shown in  FIG. 4A , the spatial frequency band processor  420  includes gain  422 ,  424  that receive the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n), and applies gains to the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n). The spatial frequency band processor  420  further includes delay units that add adjustable time delays. 
     More specifically, the spatial frequency band processor  420  may include 2n delay units  438 ,  440 , each delay unit  438 ,  440  coupled to a corresponding one of 2n gains  422 ,  424 . For example, the spatial frequency band processor  400  includes (e.g., for n=4 subbands) a mid gain  422 ( 1 ) and a mid delay unit  438 ( 1 ) to receive the nonspatial subband component Y m ( 1 ) and generate the enhanced nonspatial subband component Y m ( 1 ) by applying a subband gain and a time delay. The spatial frequency band processor  420  further includes a side gain  424 ( 1 ) and a side delay unit  440 ( 1 ) to receive the spatial subband component Y s ( 1 ) and generate the enhanced spatial subband component E s ( 1 ). Similarly for other subbands, the spatial frequency band processor includes a mid gain  422 ( 2 ) and a mid delay unit  438 ( 2 ) to receive the nonspatial subband component Y m ( 2 ) and generate the enhanced nonspatial subband component E m ( 2 ), a side gain  424 ( 2 ) and a side delay unit  440 ( 2 ) to receive the spatial subband component Y s ( 2 ) and generate the enhanced spatial subband component E s ( 2 ), a mid gain  422 ( 3 ) and a mid delay unit  438 ( 3 ) to receive the nonspatial subband component Y m ( 3 ) and generate the enhanced nonspatial subband component E m ( 3 ), a side gain  424 ( 3 ) and a side delay unit  440 ( 3 ) to receive the spatial subband component Y s ( 3 ) and generate the enhanced spatial subband component E s ( 3 ), a mid gain  422 ( 4 ) and a mid delay unit  438 ( 4 ) to receive the nonspatial subband component Y m ( 4 ) and generate the enhanced nonspatial subband component E m ( 4 ), and a side gain  424 ( 4 ) and side delay unit  440 ( 4 ) to receive the spatial subband component Y s ( 4 ) and generate the enhanced spatial subband component E s ( 4 ). 
     The gains  422 ,  424  adjust the subband gains of the spatial and nonspatial subband components relative to each other to provide audio enhancement. The gains  422 ,  424  may apply different subband gains, or the same subband gains (e.g., for two or more amplifiers) for the various subbands, using gain values controlled by configuration information, adjustable settings, etc. One or more of the amplifiers can also apply no subband gain (e.g., 0 dB). In this embodiment, the amplifiers  422 ,  424  also apply the subband gains in parallel with respect to each other. 
     The delay units  438 ,  440  adjust the timing of spatial and nonspatial subband components relative to each other to provide audio enhancement. The delay units  438 ,  440  may apply different time delays, or the same time delays (e.g., for two or more delay units) for the various subbands, using delay values controlled by configuration information, adjustable settings, etc. One or more delay units can also apply no time delay. In this embodiment, the delay units  438 ,  440  apply the time delays in parallel. 
       FIG. 4C  illustrates a third example of a spatial frequency band processor  460 , as an implementation of the frequency band processor  245  of the subband spatial processor  210 . The spatial frequency band processor  460  receives the nonspatial subband component Y m  and applies a set of subband filters to generate the enhanced nonspatial subband component E m . The spatial frequency band processor  460  also receives the spatial subband component Y s  and applies a set of subband filters to generate the enhanced nonspatial subband component E m . As illustrated in  FIG. 4C , these filters are applied in series. 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. 
     More specifically, the spatial frequency band processor  460  includes a subband filter for each of the n frequency subbands of the nonspatial component Y m  and a subband filter for each of the n subbands of the spatial component Y s . For n=4 subbands, for example, the spatial frequency band processor  460  includes a series of subband filters for the nonspatial component Y m  including a mid equalization (EQ) filter  462 ( 1 ) for the subband ( 1 ), a mid EQ filter  462 ( 2 ) for the subband ( 2 ), a mid EQ filter  462 ( 3 ) for the subband ( 3 ), and a mid EQ filter  462 ( 4 ) for the subband ( 4 ). Each mid EQ filter  462  applies a filter to a frequency subband portion of the nonspatial component Y m  to process the nonspatial component Y m  in series and generate the enhanced nonspatial component E m . 
     The spatial frequency band processor  460  further includes a series of subband filters for the frequency subbands of the spatial component Y s , including a side equalization (EQ) filter  464 ( 1 ) for the subband ( 1 ), a side EQ filter  464 ( 2 ) for the subband ( 2 ), a side EQ filter  464 ( 3 ) for the subband ( 3 ), and a side EQ filter  464 ( 4 ) for the subband ( 4 ). Each side EQ filter  464  applies a filter to a frequency subband portion of the spatial component Y s  to process the spatial component Y s  in series and generate the enhanced spatial component E s . 
     In some embodiments, the spatial frequency band processor  460  processes the nonspatial component Y m  in parallel with processing the spatial component Y s . The n mid EQ filters process the nonspatial component Y m  in series and the n side EQ filters process the spatial component Y s  in series. Each series of n subband filters can be arranged in different orders in various embodiments. 
     Using a serial (e.g., cascaded) EQ filter design in parallel on the spatial component Y s  and nonspatial component Y m , as shown by the spatial frequency band processor  460 , can provide advantages over a crossover network design where separated subband components are processed in parallel. Using the serial EQ filter design, it is possible to achieve greater control over the subband portion being addressed, such as by adjusting the Q factor and center frequency of a 2 nd  order filter (e.g., peaking/notching or shelving filter, for example). Achieving comparable isolation and control over the same region of the spectrum using a crossover network design may require using higher order filters, such as 4 th  or higher order lowpass/highpass filters. This can result in at least a doubling of the computational cost. Using a crossover network design, subband frequency ranges should have minimal or no overlap in order to reproduce the full-band spectrum after recombining the subband components. Using a serial EQ filter design can remove this constraint on the frequency band relationship from one filter to the next. The serial EQ filter design can also provide for more efficient selective processing on one or more subbands compared to the crossover network design. For example, when employing a subtractive crossover network, the input signal for a given band can be derived by subtracting the original full-band signal from the resulting lowpassed output signal of the lower-neighbor band. Here, isolating a single subband component includes computation of multiple subband components. The serial EQ filters provides for efficient enabling and disabling of filters. However, the parallel design, where the signal is divided into independent frequency subbands, makes possible discrete non-scaling operations on each subband, such as incorporating time delay. 
       FIG. 5A  illustrates a first example of a spatial frequency band combiner  500 , as an implementation of the frequency band combiner  250  of the subband spatial processor  210 . The spatial frequency band combiner  500  includes n M/S to L/R converters, such as the M/S to L/R converters  502 ( 1 ),  502 ( 2 ),  502 ( 3 ) and  502 ( 4 ) for n=4 frequency subbands. The spatial frequency band combiner  500  further includes an L/R subband combiner  504  coupled to the M/S to L/R converters. 
     For a given frequency subband k, each M/S to L/R converter  502 ( k ) receives an enhanced nonspatial subband component E m (k) and an enhanced spatial subband component E s (k), and converts these inputs into an enhanced left subband component E L (k) and an enhanced right subband component E R (k). The enhanced left subband component E L (k) can be generated based on a sum of the enhanced nonspatial subband component E m (k) and the enhanced spatial subband component E s (k). The enhanced right subband component E R (k) can be generated based on a difference between the enhanced nonspatial subband component E m (k) and the enhanced spatial subband component E s (k). 
     For n=4 frequency subbands, the L/R subband combiner  504  receives the enhanced left subband components E L ( 1 )-E L ( 4 ), and combines these inputs into the left output channel O L . The L/R subband combiner  504  further receives the enhanced right subband components E R ( 1 )-E R ( 4 ), and combines these inputs into the right output channel O R . 
       FIG. 5B  illustrates a second example of a spatial frequency band combiner  510 , as an implementation of the frequency band combiner  250  of the subband spatial processor  210 . Compared to the spatial frequency band combiner  500  shown in  FIG. 5A , the spatial frequency band combiner  510  here first combines the enhanced nonspatial subband components E m ( 1 )-E m (n) into the enhanced nonspatial component E m  and combines the enhanced spatial subband components E s ( 1 )-E s (n) into the enhanced spatial component E s , and then performs M/S to L/R conversion to generate the left output channel O L  and the right output channel O R . Prior to M/S to L/R conversion, a global mid gain can be applied to the enhanced nonspatial component E m  and a global side gain can be applied to the enhanced spatial component E s , where the global gain values can be controlled by configuration information, adjustable settings, etc. 
     More specifically, the spatial frequency band combiner  510  includes an M/S subband combiner  512 , a global mid gain  514 , a global side gain  516 , and an M/S to L/R converter  518 . For n=4 frequency subbands, the M/S subband combiner  512  receives the enhanced nonspatial subband components E m ( 1 )-E m ( 4 ) and combines these inputs into the enhanced nonspatial component E m . The M/S subband combiner  512  also receives the enhanced spatial subband components E s ( 1 )-E s ( 4 ) and combines these inputs into the enhanced spatial component E s . 
     The global mid gain  514  and the global side gain  516  are coupled to the M/S subband combiner  512  and the M/S to L/R converter  518 . The global mid gain  514  applies a gain to the enhanced nonspatial component E m  and the global side gain  516  applies a gain to the enhanced spatial component E s . 
     The M/S to L/R converter  518  receives the enhanced nonspatial component E m  from the global mid gain  514  and the enhanced spatial component E s  from the global side gain  516 , and converts these inputs into the left output channel O L  and the right output channel O R . The left output channel O L  can be generated based on a sum of the enhanced spatial component E s  and the enhanced nonspatial component E m , and the right output channel O R  can be generated based on a difference between the enhanced nonspatial component E m  and the enhanced spatial component E s . 
       FIG. 5C  illustrates a third example of a spatial frequency band combiner  520 , as an implementation of the frequency band combiner  250  of the subband spatial processor  210 . The spatial frequency band combiner  520  receives the enhanced nonspatial component E m  and the enhanced spatial component E s  (e.g., rather than their separated subband components), 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 output channel O L  and the right output channel O R . 
     More specifically, the spatial frequency band combiner  520  includes a global mid gain  522 , a global side gain  524 , and an M/S to L/R converter  526  coupled to the global mid gain  522  and the global side gain  524 . The global mid gain  522  receives the enhanced nonspatial component E m  and applies a gain, and the global side gain  524  receives the enhanced spatial component E s  and applies a gain. The M/S to L/R converter  526  receives the enhanced nonspatial component E m  from the global mid gain  522  and the enhanced spatial component E s  from the global side gain  524 , and converts these inputs into the left output channel O L  and the right output channel O R . 
       FIG. 5D  illustrates a fourth example of spatial frequency band combiner  530  as an implementation of the frequency band combiner  250  of the subband spatial processor  210 . The spatial frequency band combiner  530  facilitates frequency domain enhancement of the input audio signal. 
     More specifically, the spatial frequency band combiner  530  includes an inverse fast Fourier transform (FFT)  532 , a global mid gain  534 , a global side gain  536 , and an M/S to L/R converter  538 . The inverse FFT  532  receives the enhanced nonspatial subband components E m ( 1 )-E m (n) as represented in the frequency domain, and receives the enhanced spatial subband components E s ( 1 )-E s (n) as represented in the frequency domain. The inverse FFT  532  converts the frequency domain inputs into the time domain. The inverse FFT  532  then combines the enhanced nonspatial subband components E m ( 1 )-E m (n) into the enhanced nonspatial component E m  as represented in the time domain, and combines the enhanced spatial subband components E s ( 1 )-E s (n) into the enhanced spatial component E s  as represented in the time domain. In other embodiments, inverse FFT  532  combines subband components in the frequency domain, then converts the combined enhanced nonspatial component E m  and enhanced spatial component E s  into the time domain. 
     The global mid gain  534  is coupled to the inverse FFT  532  to receive the enhanced nonspatial component E m  and apply a gain to the enhanced nonspatial component E m . The global side gain  536  is coupled to the inverse FFT  532  to receive the enhanced spatial component E s  and apply a gain to the enhanced spatial component E s . The M/S to L/R converter  538  receives the enhanced nonspatial component E m  from the global mid gain  534  and the enhanced spatial component E s  from the global side gain  536 , and converts these inputs into the left output channel O L  and the right output channel O R . The global gain values can be controlled by configuration information, adjustable settings, etc. 
       FIG. 6  illustrates an example of a method  600  for enhancing an audio signal, according to one embodiment. The method  600  can be performed by the subband spatial processor  210 , including the spatial frequency band divider  240 , the spatial frequency band processor  245 , and the spatial frequency band combiner  250  to enhance an input audio signal include a left input channel X L  and a right input channel X R . 
     The spatial frequency band divider  240  separates  605  the left input channel X L  and the right input channel X R  into a spatial component Y s  and a nonspatial component Y m . In some embodiments, spatial frequency band divider  240  separates the spatial component Y s  into n subband components Y s ( 1 )-Y s (n) and separates the nonspatial component Y m  into n subband components Y m ( 1 )-Y m (n). 
     The spatial frequency band processor  245  applies  610  subband gains (and/or time delays) to subbands of the spatial component Y s  to generate an enhanced spatial component E s , and applies subband gains (and/or delays) to subbands of the nonspatial component Y m  to generate an enhanced nonspatial component E m . 
     In some embodiments, the spatial frequency band processor  460  of  FIG. 4C  applies a series of subband filters to the spatial component Y s  and the nonspatial component Y m  to generate the enhanced spatial component E s  and the enhanced nonspatial component E m . The gains for the spatial component Y s  can be applied to the subbands with a series of n subband filters. Each filter applies a gain to one of the n subbands of the spatial component Y s . The gains for the nonspatial component Y m  can also be applied to the subbands with a series of filters. Each filter applies a gain to one of the n subbands of the nonspatial component Y m . 
     In some embodiments, the spatial frequency band processor  400  of  FIG. 4A  or the spatial frequency band processor  420  of  FIG. 4B  applies gains to separated subband components in parallel. For example, the gains for the spatial component Y s  can be applied to the subbands with a parallel set of n subband filters for the separated spatial subband components Y s ( 1 )-Y s (n), resulting in the enhanced spatial component E s  being represented as the enhanced spatial subband components E s ( 1 )-E s (n). The gains for the nonspatial component Y m  can be applied to the subbands with a parallel set of n filters for the separated nonspatial subband components Y m ( 1 )-Y m (n), resulting in the enhanced nonspatial component E m  being represented as the enhanced nonspatial subband components E m ( 1 )-E m (n). 
     The spatial frequency combiner  250  combines  615  the enhanced spatial component E s  and the enhanced nonspatial component E m  into the left output channel O L  and the right output channel O R . In embodiments such as the spatial frequency combiner shown in  FIG. 5A, 5B , or  5 D, where the spatial component E s  is represented by the separated enhanced spatial subband components E s ( 1 )-E s (n), the spatial frequency combiner  250  combines the enhanced spatial subband components E s ( 1 )-E s (n) into the spatial component E s . Similarly, if the nonspatial component E m  is represented by the separated enhanced nonspatial subband components E m ( 1 )-E m (n), the spatial frequency combiner  250  combines the enhanced nonspatial subband components E m ( 1 )-E m (n) into the nonspatial component E m . 
     In some embodiments, the spatial frequency band combiner  250  (or processor  245 ) applies a global mid gain to the enhanced nonspatial component E m  and a global side gain to the enhanced spatial component E s  prior to combination into the left output channel O L  and the right output channel O R . The global mid and side gains adjust the relative gains of the enhanced spatial component E s  and the enhanced nonspatial component E m . 
     Various embodiments of the spatial frequency band divider  240  (e.g., as shown by the spatial frequency band dividers  300 ,  310 ,  320 , and  330  of  FIGS. 3A, 3B, 3C, and 3D , respectively), the spatial frequency band processor  245  (e.g., as shown by the spatial frequency band processors  400 ,  420 , and  460  of  FIGS. 4A, 4B, and 4C , respectively), and the spatial frequency band combiner  250  (e.g., as shown by the spatial frequency band combiners  500 ,  510 ,  520 , and  530  of  FIGS. 5A, 5B, 5C, and 5D , respectively) may be combined with each other. Some example combinations are discussed in greater detail below. 
       FIG. 7  illustrates an example of a subband spatial processor  700 , according to one embodiment. The subband spatial processor  700  is an example of a subband spatial processor  210 . The subband spatial processor  700  uses separated spatial subband components Y s ( 1 )-Y s (n) and nonspatial subband components Y m ( 1 )-Y m (n), and n=4 frequency subbands. The subband spatial processor  700  includes either spatial frequency band divider  300  or  310 , either the spatial frequency band processor  400  or  420 , and either the spatial frequency band combiner  500  or  510 . 
       FIG. 8  illustrates an example of a method  800  for enhancing an audio signal with the subband spatial processor  700  shown in  FIG. 7 , according to one embodiment. The spatial frequency band divider  300 / 310  processes  805  the left input channel X L  and the right input channel X R  into the spatial subband components Y s ( 1 )-Y s (n) and the nonspatial subband components Y m ( 1 )-Y m (n). The frequency band divider  300  separates frequency subbands, then performs L/R to M/S conversion. The frequency band divider  310  performs L/R to M/S conversion, then separates frequency subbands. 
     The spatial frequency band processor  400 / 420  applies  810  gains (and/or delays) to the spatial subband components Y s ( 1 )-Y s (n) in parallel to generate the enhanced spatial subband components E s ( 1 )-E s (n), and applies gains (and/or delays) to the nonspatial subband components Y m ( 1 )-Y m (n) in parallel to generate the enhanced nonspatial subband components E m ( 1 )-E m (n). The spatial frequency band processor  400  can apply subband gains, while the spatial frequency band processor  420  can apply subband gains and/or time delays. 
     The spatial frequency band combiner  500 / 510  combines  815  the enhanced spatial subband components E s ( 1 )-E s (n) and the enhanced nonspatial subband components E m ( 1 )-E m (n) into the left output channel O L  and the right output channel O R . The spatial frequency band combiner  500  performs M/S to L/R conversion, then combines left and right subbands. The spatial frequency band combiner  510  combines nonspatial (mid) and spatial (side) subbands, applies global mid and side gains, then performs M/S to L/R conversion. 
       FIG. 9  illustrates an example of a subband spatial processor  900 , according to one embodiment. The subband spatial processor  900  is an example of a subband spatial processor  210 . The subband spatial processor  900  uses the spatial component Y s  and the nonspatial component Y m  without separation into subband components. The subband spatial processor  900  includes the spatial frequency band divider  320 , the spatial frequency band processor  460 , and the spatial frequency band combiner  520 . 
       FIG. 10  illustrates an example of a method  1000  for enhancing an audio signal with the subband spatial processor  900  shown in  FIG. 9 , according to one embodiment. The spatial frequency band divider  320  processes  1005  the left input channel X L  and the right input channel X R  into the spatial component Y s  and the nonspatial components Y m . 
     The spatial frequency band processor  460  applies  1010  gains to subbands of the spatial component Y s  in series to generate the enhanced spatial component E s , and gains to subbands of the nonspatial component Y m  in series to generate the enhanced nonspatial component E m . A first series of n mid EQ filters are applied to the nonspatial component Y m , each mid EQ filter corresponding with one of the n subbands. A second series of n side EQ filters are applied to the spatial component Y s , each side EQ filter corresponding with one of the n subbands. 
     The spatial frequency band combiner  520  combines  815  the enhanced spatial component E s  and the enhanced nonspatial component E m  into the left output channel O L  and the right output channel O R . In some embodiments, the spatial frequency band combiner  520  applies a global side gain to the enhanced spatial component E s , and applies global mid gain to the enhanced nonspatial component E m , and then combines E s  and E m  into the left output channel O L  and the right output channel O R . 
       FIG. 11  illustrates an example of a subband spatial processor  1100 , according to one embodiment. The subband spatial processor  1100  is another example of a subband spatial processor  210 . The subband spatial processor  1100  uses conversion between the time domain and frequency domain, with gains being adjusted to frequency subbands in the frequency domain. The subband spatial processor  1100  includes the spatial frequency band divider  330 , the spatial frequency band processor  400  or  420 , and the spatial frequency band combiner  520 . 
       FIG. 12  illustrates an example of a method  1200  for enhancing an audio signal with the subband spatial processor  1100  shown in  FIG. 11 , according to one embodiment. The spatial frequency band divider  330  processes  1205  the left input channel X L  and the right input channel X R  into the spatial component Y s  and the nonspatial components Y m . 
     The spatial frequency band divider  330  applies  1210  a forward FFT to the spatial component Y s  to generate spatial subband components Y s ( 1 )-Y s (n) (e.g., n=4 frequency subbands as shown in  FIG. 11 ), and applies the forward FFT to the nonspatial component Y m  to generate nonspatial subband components Y m ( 1 )-Y m (n). In addition to separation into frequency subbands, the frequency subbands are converted from the time domain to the frequency domain. 
     The spatial frequency band processor  400 / 420  applies  1215  gains (and/or delays) to the spatial subband components Y s ( 1 )-Y s (n) in parallel to generate the enhanced spatial subband components E s ( 1 )-E s (n), and applies gains (and/or delays) to the nonspatial subband components Y m ( 1 )-Y m (n) in parallel to generate the enhanced nonspatial subband components E m ( 1 )-E m (n). The gains and/or delays are applied to signals represented in the frequency domain. 
     The spatial frequency band combiner  520  applies  1220  an inverse FFT to the enhanced spatial subband components E s ( 1 )-E s (n) to generate the enhanced spatial component E s , and applies the inverse FFT to the enhanced nonspatial subband components E m ( 1 )-E m (n) to generate the enhanced nonspatial component E m . The inverse FFT results in the enhanced spatial component E s  and the enhanced nonspatial component E m  being represented in the time domain. 
     The spatial frequency band combiner  520  combines  1225  the enhanced spatial component E s  and the enhanced nonspatial component E m  into the left output channel O L  and the right output channel O R . In some embodiments, the spatial frequency band combiner  520  applies a global mid gain to the enhanced nonspatial component E m  and a global side gain to the enhanced spatial component E s , and then generates the output channels O L  and O R . 
       FIG. 13  illustrates an example of an audio system  1300  for enhancing an audio signal with crosstalk cancellation, according to one embodiment. The audio system  1300  can be used with loudspeakers to cancel contralateral crosstalk components of the left output channel O L  and the right output channel O R . The audio system  1300  includes the subband spatial processor  210 , a crosstalk compensation processor  1310 , a combiner  1320 , and a crosstalk cancellation processor  1330 . 
     The crosstalk compensation processor  1310  receives the input channels X L  and X R , and performs a preprocessing to precompensate for any artifacts in a subsequent crosstalk cancellation performed by the crosstalk cancellation processor  1330 . In particular, the crosstalk compensation processor  1310  generates a crosstalk compensation signal Z in parallel with the subband spatial processor  210  generating the left output channel O L  and the right output channel O R . In some embodiments, the crosstalk compensation processor  1310  generates spatial and nonspatial components from the input channels X L  and X R , and applies gains and/or delays to the nonspatial and spatial components to generate the crosstalk compensation signal Z. 
     The combiner  1320  combines the crosstalk compensation signal Z with each of left output channel O L  and the right output channel O R  to generate a precompensated signal T comprising two precompensated channels T L  and T R . 
     The crosstalk cancellation processor  1330  receives the precompensated channels T L , T R , and performs crosstalk cancellation on the channels T L , T R  to generate an output audio signal C comprising left output channel C L  and right output channel C R . Alternatively, the crosstalk cancellation processor  1330  receives and processes the left and right output channels O L  and O R  without crosstalk precompensation. Here, crosstalk compensation can be applied to the left and right output channels C L , C R  subsequent to crosstalk cancellation. The crosstalk cancellation processor  1330  separates the precompensated channels T L , T R  into inband components and out of band components, and perform a crosstalk cancellation on the inband components to generate the output channels C L , C R . 
     In some embodiments, the crosstalk cancellation processor  1330  receives the input channels X L  and X R  and performs crosstalk cancellation on the input channels X L  and X R . Here, crosstalk cancellation is performed on the input signal X rather than the output signal O from the subband spatial processor  210 . In some embodiments, the crosstalk cancellation processor  1330  performs crosstalk cancellation on both the input channels X L  and X R  and the output channels O L  and O R  and combines these results (e.g., with different gains) to generate the output channels C L , C R . 
       FIG. 14  illustrates an example of an audio system  1400  for enhancing an audio signal with crosstalk simulation, according to one embodiment. The audio system  1400  can be used with headphones to add contralateral crosstalk components to the left output channel O L  and the right output channel O R . This allows headphones to simulate the listening experience of loudspeakers. The audio system  1400  includes the subband spatial processor  210 , a crosstalk simulation processor  1410 , and a combiner  1420 . 
     The crosstalk simulation processor  1410  generates a “head shadow effect” from the audio input signal X. The head shadow effect refers to a transformation of a sound wave caused by trans-aural wave propagation around and through the head of a listener, such as would be perceived by the listener if the audio input signal X was transmitted from loudspeakers to each of the left and right ears of a listener. For example, the crosstalk simulation processor  1410  generates a left crosstalk channel W L  from the left channel X L  and a right crosstalk channel W R  from the right channel X R . The left crosstalk channel W L  may be generated by applying a low-pass filter, delay, and gain to the left input channel X L . The right crosstalk channel W R  may be generated by applying a low-pass filter, delay, and gain to the right input channel X R . In some embodiments, low shelf filters or notch filters may be used rather than low-pass filters to generate the left crosstalk channel W L  and right crosstalk channel W R . 
     The combiner  1420  combines the output of the subband spatial processor  210  and the crosstalk simulation processor  1410  to generate an audio output signal S that includes left output signal S L  and right output signal S R . For example, the left output channel S L  includes a combination of the enhanced left channel O L  and the right crosstalk channel W R (e.g., representing the contralateral signal from a right loudspeaker that would be heard by the left ear via trans-aural sound propagation). The right output channel S R  includes a combination of the enhanced right channel O R  and the left crosstalk channel W L  (e.g., representing the contralateral signal from a left loudspeaker that would be heard by the right ear via trans-aural sound propagation). The relative weights of the signals input to the combiner  1420  can be controlled by the gains applied to each of the inputs. 
     In some embodiments, the crosstalk simulation processor  1410  generates the crosstalk channels W L  and W R  from the left and right output channels O L  and O R  of the subband spatial processor  210  instead of the input channels X L  and X R . In some embodiments, the crosstalk simulation processor  1410  generates crosstalk channels from both the left and right output channels O L  and O R  and the input channels X L  and X R , and combines these results (e.g., with different gains) to generate the left output signal S L  and right output signal S R . 
     Upon reading this disclosure, those of skill in the art will appreciate still additional alternative embodiments of 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.