Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04S—STEREOPHONIC SYSTEMS 
  • H04S3/00—Systems employing more than two channels, e.g. quadraphonic
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04S—STEREOPHONIC SYSTEMS 
  • H04S5/00—Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation 
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R5/00—Stereophonic arrangements
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R5/00—Stereophonic arrangements
  • H04R5/02—Spatial or constructional arrangements of loudspeakers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04S—STEREOPHONIC SYSTEMS 
  • H04S2400/00—Details of stereophonic systems covered by H04S but not provided for in its groups
  • H04S2400/01—Multi-channel, i.e. more than two input channels, sound reproduction with two speakers wherein the multi-channel information is substantially preserved
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04S—STEREOPHONIC SYSTEMS 
  • H04S2420/00—Techniques used stereophonic systems covered by H04S but not provided for in its groups
  • H04S2420/01—Enhancing the perception of the sound image or of the spatial distribution using head related transfer functions [HRTF's] or equivalents thereof, e.g. interaural time difference [ITD] or interaural level difference [ILD]

Definitions

• FIG. 1 shows a common binaural playback system that includes processing multiple channels of audio by a plurality of Head Related Transfer Function (HRTF) filters, e.g., FIR filters, so as to provide a listener 20 with the impression that each of the input audio channels is being presented from a particular direction.
• HRTF Head Related Transfer Function
• N a number, denoted N, of audio sources consisting of a first audio channel 11 (Channel 1), a second audio channel (Channel 2), ..., and an N'th audio channel 12 (Channel N) of information.
• the binaural playback system is for playback using a pair of headphones 19 worn by the listener 20.
• Each channel is processed by a pair of HRTF filters, one filter aimed for playback though the left ear 22 of the listener, the other played through the right ear 23 of the listener 20. So a first HRTF pair of filters 13, 14, up to an N'th pair of HRTF filters 15 and 16 are shown.
• each HRTF filter meant for the left ear 22 of the listener 20 are added by an adder 18, and the outputs of each HRTF filter meant for playback through the right ear 23 of the listener 20 are added by an adder 17.
• the direction of incidence of each channel perceived by the listener 20 is determined by the choice of HRTF filter pair that is applied to that channel. For example, in FIG. 1, Audio Channel 1 (11) is processed through a pair of filters 13, 14, so that the listener is presented with audio input via headphones 19 that will give the listener the impression that the sound of Audio Channel 1 (11) is incident to the listener from a particular arrival azimuth angle denoted ⁇ j, e.g., from a location 21.
• the HRTF filter pair for the second audio channel is designed such that the sound of Audio Channel 2 is incident to the listener from a particular arrival azimuth angle denoted ⁇ 2 , ...
• the HRTF filter pair for N'th audio channel is designed such that the sound of Audio Channel N (12) is incident to the listener from a particular arrival azimuth angle denoted ⁇ N .
• FIG. 1 shows only the azimuth angles of arrival, e.g., the angle of arrival of the perceived sound corresponding to Channel 1 from a perceived source 21.
• HRTF filters may be used to provide the listener 20 with stimulus corresponding to any arrival direction, specified by both an azimuth angle of incidence and an elevation angle of incidence.
• a HRTF filter pair is meant the set of two separate HRTF filters required to process a single channel for the two ears 22, 23 of the listener, one HRTF filter per ear. Therefore, for two channel sound, two HRTF filters pairs are used.
• the description herein is provided in detail primarily for a two-input-channel, i.e., stereo input pair system.
• FIG. 2 shows a stereo binauralizer system that includes two audio inputs, a left channel input 31 and a right channel input 32. Each of the two audio channel inputs are separately processed, with the left channel input being processed through one HRTF pair 33,34, and the right channel input being processed through a different FJDRTF pair 35, 36.
• the left channel input 31 and the right channel input 32 are meant for symmetric playback, such that the aim of binauralizing using the two EERTF pairs is to give the perception to the listener of hearing the left and right channels from respective left and right angular locations that are symmetrically positioned relative to the medial plane of the listener 20.
• the left channel is perceived from source 37 at an azimuth angle d?and the right channel is perceived to be from a source 38 at an azimuth angle that is the negative of the azimuth angle of the right perceived source 37, i.e., from an azimuth angle-ft [0007]
• the listener's head and sound perception is symmetric. That means that:
• the HRTF from the left source 37 to the left ear 22 is equal to the HRTF from the right source 38 to the right ear 23. Denote such an HRTF as HRTF near .
• the HRTF from the left source 37 to the right ear 23 is equal to the HRTF from the right source 38 to the left ear 22. Denote such a HRTF as HRTF far .
• the HRTF filters are typically found by measuring the actual HRTF response of a dummy head, or a human listener's head. Relatively sophisticated binaural processing systems make use of extensive libraries of HRTF measurements, corresponding to multiple listeners and/or multiple sound incident azimuth and elevation angles.
• HRTF ⁇ , L and HRTF ( ⁇ , R) are the measured HRTFs for to the left and right angle, respectively, for a perceived source at angle ⁇ . Therefore, by the near and far HRTFs are meant the actual measured or assumed HRTFs for the symmetric case, or the average HRTF's for the non-symmetric case.
• such a binauralizer simulates the way a normal stereo speaker system works, by presenting the left audio input signal though an HRTF pair corresponding to a virtual left speaker, e.g., 37 and the right audio input signal though an HRTF pair corresponding to a virtual right speaker, e.g., 38.
• This is known to work well for providing the listener with the sensation that sounds, left and right channel inputs, are emanating from left and right virtual speaker locations, respectively.
• Monolnput center panned e.g., split between the two channel inputs.
• :LeftAudio and RightAudio are created as:
• results of a so center panned signal for stereo speaker reproduction is meant to be perceived as a signal emanating from the front center.
• LeftEar and RightEar are input to the binauralizer of FIG. 2, the left ear 22 and right ear 23 are fed signals, denoted LeftEar and RightEar, respectively, with:
• ® denotes the filtering operation, e.g., in the case that HRTF near is expressed as an impulse response, and LeftAudio as a time domain input, HRTF mar ® LeftAudio denotes convolution. So, by combining the equations above,
• a signal that is meant to appear to come from the center rear typically will not be perceived to come from the center rear when played back on headphones via a binauralizer that uses symmetric rear HRTF filters aimed at placing the rear speakers at symmetric rear virtual speaker locations.
• Described herein in different embodiments and aspects are a method to process audio signals, an apparatus accepting audio signals, a carrier medium that carried instructions for a processor to implement the method to process audio signals, and a carrier medium carrying filter data to implement a filter of audio signals.
• the inputs include a panned signal
• each of these provide a listener with a sensation that the panned signal component emanates from a virtual sound source at a center location.
• One aspect of the invention is a method that includes filtering a pair of audio input signals by a process that produces a pair of output signals corresponding to the results of: filtering each of the input signals with a HRTF filter pair, and adding the HRTF filtered signals.
• the HRTF filter pair is such that a listener listening to the pair of output signals through headphones experiences sounds from a pair of desired virtual speaker locations.
• the filtering is such that, in the case that the pair of audio input signals includes a panned signal component, the listener listening to the pair of output signals through headphones is provided with the sensation that the panned signal component emanates from a virtual sound source at a center location between the virtual speaker locations.
• Another method embodiment includes equalizing a pair of audio input signals by an equalizing filter, and binauralizing the equalized input signals using HRTF pairs to provide a pair of binauralized outputs that provide a listener listening to the binauralized output via headphones the illusion that sounds corresponding to the audio input signals emanate from a first and a second virtual speaker location.
• the elements of the method are arranged such that the combination of the equalizing and binauralizing is equivalent to binauralizing using equalized HRTF pairs, each equalized HRTF of the equalized HRTF pairs being the corresponding HRTF for the binauralizing of the equalized signals equalized by the equalizing filter.
• the average of the equalized HRTFs substantially equals a desired HRTF for the listener listening to a sound emanating from a center location between the first and second virtual speaker locations.
• the pair of audio input signals includes a panned signal component
• the listener listening to the pair of binauralized outputs through the headphones is provided with the sensation that the panned signal component emanates from a virtual sound source at the center location.
• Another aspect of the invention is a carrier medium carrying filter data for a set of HRTF filters for processing a pair of audio input signals to provide a listener listening to the processed signals via headphones the illusion that sounds approximately corresponding to the audio input signals emanate from a first and a second virtual speaker location, the HRTF filters designed such that the average of the HRTF filters approximates the HRTF response of the listener listening to a sound from a center location between the first and a second virtual speaker locations.
• Another aspect of the invention is a carrier medium carrying filter data for a set of HRTF filters for processing a pair of audio input signals to provide a listener listening to the processed signals via headphones the illusion that sounds corresponding to the audio input signals emanate from a first and a second virtual speaker location, such that a signal component panned between each of the pair of audio input signals provides the listener listening to the processed signals via headphones the illusion that the panned signal component emanated from a center location between the first and a second virtual speaker locations.
• Another aspect of the invention is a method that includes accepting a pair of audio input signals for audio reproduction, shuffling the input signals to create a first signal ("sum signal”) proportional to the sum of the input signals and a second signal (“difference signal”) proportional to the difference of the input signals, and filtering the sum signal through a filter that approximates the sum of an equalized version of a near ear HRTF and an equalized version of a far ear HRTF.
• the near ear and far ear HRTFs are for a listener listening to a pair of virtual speakers at corresponding virtual speaker locations.
• the equalized versions are obtained using an equalization filter designed such that the average of the equalized near ear HRTF and equalized far ear HRTF approximates a center HRTF for a listener listening to a virtual sound source at a center location between the virtual speaker locations.
• the method further includes filtering the difference signal through a filter that approximated the difference between the equalized version of the near ear HRTF and the equalized version of the far ear HRTF for the listener listening to the pair of virtual speakers.
• the method further includes unshuffling the filtered sum signal and the filtered difference signal to create a first output signal proportional to the sum of the filtered sum and filtered difference signals and a second output signal proportional to the difference of the filtered sum and filtered difference signals.
• Another aspect of the invention is a method that includes filtering a pair of audio input signals for audio reproduction, the filtering by a process that produces a pair of output signals corresponding to the results of filtering each of the input signals with a HRTF filter pair, adding the HRTF filtered signals, and cross-talk cancelling the added HRTF filtered signals.
• the cross-talk cancelling is for a listener listening to the pair of output signals through speakers located at a first set of speaker locations.
• the HRTF filter pair are such that a listener listening to the pair of output signals experiences sounds from a pair of virtual speakers at desired virtual speaker locations.
• the filtering is such that, in the case that the pair of audio input signals includes a panned signal component, a listener listening to the pair of output signals through the pair of speakers at the first set of speaker locations is provided with the sensation that the panned signal component emanates from a virtual sound source at a center location between the desired virtual speaker locations.
• Another aspect of the invention is a method that includes accepting a pair of audio input signals for audio reproduction, shuffling the input signals to create a first signal ("sum signal”) proportional to the sum of the input signals and a second signal (“difference signal”) proportional to the difference of the input signals, filtering the sum signal through a filter that approximates twice a center HRTF for a listener listening to a virtual sound source at a center location, filtering the difference signal through a filter that approximates the difference between a near ear HRTF and a far ear HRTF for the listener listening to a pair of virtual speakers, and unshuffling the filtered sum signal and the filtered difference signal to create a first output signal proportional to the sum of the filtered sum and filtered difference signals and a second output signal proportional to the difference of the filtered sum and filtered difference signals.
• the method is such that in the case that the pair of audio input signals includes a panned signal component, the listener listening to the first and second output signals through headphones is provided with the sensation that the panned signal component emanates from the virtual sound source at the center location.
• the filter that approximates twice the center HRTF is obtained as the sum of equalized versions of the near ear HRTF and the far ear HRTF, respectively, obtained by filtering the near ear HRTF and the far ear HRTF, respectively, by an equalizing filter, and wherein the filter that approximates the difference between the near ear HRTF and the far ear HRTF is a filter that has a response substantially equal to the difference between the equalized versions of the near ear HRTF and the far ear HRTF.
• the equalizing filter is an inverse filter for a filter proportional to the sum of the near ear HRTF and the far ear HRTF.
• the equalizing filter response is determined by inverting in the frequency domain a filter response proportional to the sum of the near ear HRTF and the far ear HRTF.
• the equalizing filter response is determined by an adaptive filter method to invert a filter response proportional to the sum of the near ear HRTF and the far ear HRTF.
• the filter that approximates twice the center HRTF is a filter that has a response substantially equal to twice a desired center HRTF.
• the audio input signals include a left input and a right input
• the pair of virtual speakers are at a left virtual speaker location and a right virtual speaker location symmetric about the listener
• the listener and listening are symmetric such that near HRTF is the left virtual speaker to left ear HRTF and the right virtual speaker to right ear HRTF, and such that far HRTF is the left virtual speaker to right ear HRTF and the right virtual speaker to left ear HRTF.
• the audio input signals include a left input and a right input
• the pair of virtual speakers are at a left virtual speaker location and a right virtual speaker location
• the near HRTF is proportional to the average of the left virtual speaker to left ear HRTF and the right virtual speaker to right ear HRTF
• the far HRTF is proportional to the average of the left virtual speaker to right ear HRTF and the right virtual speaker to left ear HRTF.
• the audio input signals include a left input and a right input
• the pair of virtual speakers are at a left front virtual speaker location and a right front virtual speaker location to the front of the listener.
• FIG. 1 shows a common binaural playback system that includes processing multiple channels of audio by a plurality of HRTF filters to provide a listener with the impression that each of the input audio channels is being presented from a particular direction. While a binauralizer having the structure of FIG. 1 may be prior art, a binauralizer with filters selected according to one or more of the inventive aspects described herein is not prior art.
• FIG. 2 shows a stereo binauralizer system that includes two audio inputs, a left channel input and a right channel input each processed through a air of HRTF filters. While a binauralizers having the structure of FIG. 1 may be prior art, a binauralizer with filters selected according to one or more of the inventive aspects described herein is not prior art.
• FIG. 3 shows diagrammatically an example of HRTFs for three source angles for, a left virtual speaker, a right virtual speaker, and a center location.
• FIG. 4A shows a 0° HRTF
• FIG. 4B shows near ear HRTF
• FIG. 4C a far ear HRTF
• FIG. 4D shows the average of the near and far ear HRTFs.
• FIGS. 5A-5D show how equalization can be used to modify the near and far HRTF filters such that the sum more closely matches the desired 0° HRTF.
• FIG. 5A shows the impulse response of the equalization filter to be applied to the near and far HRTFs.
• FIGs. 5B and 5C respectively show near ear and far ear HRTFs after equalization
• FIG. 5D shows the resulting average of the equalized near and far ear HRTFs according to aspects of the invention.
• FIG. 6 shows the frequency magnitude response of an equalization filter designed according to an aspect of the present invention.
• FIGS. 7 shows a first embodiment of a binauralizer using equalized HRTF filters determined according to aspects of the present invention.
• FIG. 8 shows a second embodiment of a binauralizer using equalized HRTF filters determined according to aspects of the present invention using a shuffler network (a "shuffler").
• FIG. 9 shows another shuffler embodiment of a binauralizer using a sum signal filter that is the desired center HRTF filter, according to an aspect of the invention.
• FIG. 10 shows a crosstalk cancelled binauralizing filter embodiment including a cascade of a binauralizer to place virtual speakers at the desired locations, and a cross talk canceller.
• the binauralizer part incorporates aspects of the present invention.
• FIG. 11 shows an alternate embodiment of a crosstalk cancelled binauralizing filter that includes four filters.
• FIG. 12 shows another alternate embodiment of a crosstalk cancelled binauralizing filter that includes a shuffler network, a sum signal filter, and a difference filter network.
• FIG. 13 shows an DSP-device based embodiment of an audio processing system for processing a stereo input pair according to aspects of the invention.
• FIG. 14A shows a processing-system-based binauralizer embodiment that accepts five channels of audio information, and includes aspects of the present invention to create the impression to a listener that a rear center panned signal emanates from the center rear of the listener.
• FIG. 14B shows a processing-system-based binauralizer embodiment that accepts four channels of audio information, and includes aspects of the present invention to create the impression to a listener that a front center panned signal emanates from the center front of the listener and that a rear center panned signal emanated from the center rear of the listener.
• One aspect of the present invention is a binauralizer and binauralizing method that, for the case of a stereo pair of inputs, uses measured or assumed HRTF pairs for two sources at a first source angle and a second source angle to binuaralize the stereo pair of inputs for more than two source angles, e.g . to create the illusion that a signal that is panned between the stereo pair of inputs is emanating from a source at a third source angle between the first and second source angles.
• FIG. 3 shows an example of HRTFs for three source angles, a first azimuth angle, denoted 6> for a left virtual speaker, an angle for a right virtual speaker, which in FIG.
• the HRTF pair is denoted as the pair HRTF(O 1 L) and HRTF(O 1 R) respectively.
• the left virtual speaker HRTF pair is denoted as the pair HRTF( ⁇ ,L) and HRTF( ⁇ .R) respectively, and the right virtual speaker HRTF pair is denoted as the pair HRTF(- ⁇ ,L) and HRTF(- ⁇ ,R) respectively.
• HRTF(O 1 L) HRTF(O 1 R), and denote this quantity as HRTF clr . It is therefore desired that for the signal split into the left and right inputs,
• an equalizing filter is applied to the inputs.
• the filtering of such an equalizing filter may be applied (a) to the left and right channel input signals prior to binauralizing, or (b) to the measured or assumed HRTFs for the listener for the left and right virtual speaker locations, such that the average of the resulting near and far HRTFs approximates the desired phantom center HRTF. That is,
• HRTF' liear and HRTF f ' ar are the HRTF mar and HRTF far filters that include equalization.
• EQ 0 the equalizing filter response, e.g., impulse response. Applying this filter to the left and right channel inputs prior to binauralizing is equivalent to binauralizing with HRTF' near and HRTF f ' ar filters determined from the ⁇ and -0HRTF pairs denoted HRTF near and HRTR far , and the equalizing filter as follows, assuming symmetry:
• the equalizing filter is obtained by an equalizing filter that is the combination of the desired HRTF filter and an inverse filter.
• Eq. 13 is satisfied by an equalizing filter given by:
• inverse () denoted the operation of inverse filtering, such that, if X and Fare filters specified in the time domain, e.g., as impulse responses, Y—inverse(X) implies Y® X is a delta function, where ⁇ 8> is convolution.
• inverse filtering is also known in the art as deconvolution.
• X and Fare for FIR filters specified by a finite length vector representing the impulse response one fo ⁇ ns a Toeplitz matrix based on Y, denoted Toeplitz(F)-
• the vector X is a finite length vector chosen so that ToeplitzfT) ® Toeplitz(-Y) is close to a delta function. That is, Toeplitzr ⁇ Toeplitz(Z) is close to an identity matrix, with error being minimized in a least squares sense.
• the present invention is not restricted to any particular method of determining the inverse filter.
• One alternate method structures the inverse filtering problem as an adaptive filter design problem.
• a FIR filter of impulse response X, length m ⁇ is followed by a FIR filter of impulse response Y of length T&I-
• a reference output of delaying an input is subtracted from the output of the cascaded filters X and Y to produce an error signal.
• the coefficients of Y are adaptively changed to minimize the mean squared error signal.
• This is a standard adaptive filter problem, solved by standard methods such as the least mean squared (LMS) method, or a variation called the normalized LMS method. See for example, S. Haykim, "Adaptive Filter Theory," 3rd Ed., Englewood Cliffs, NJ: Prentice Hall, 1996.
• Other inverse filtering determining methods also may be used.
• Yet another embodiment of the inverse filter is determined in the frequency domain.
• the inventor produces a library of HRTF filters for use with binauralizers. These predetermined HRTF filters are known to behave smoothly in the frequency domain, such that their frequency responses are known to be invertible to produce a filter whose frequency response is the inverse of that of the
• HRTF near + HRTF far HRTF filter The method of creating an inverse filter is to invert for such
• the filter — is inverted in the frequency
• a first embodiment includes using an equalization filter denoted EQ C , that in one embodiment is computed as:
• HRTF' near and HRTF f ' ar are now no longer equal to HRTF( ⁇ ,L) and HRTF(QR), i.e., HRTF near and HRTF far as would be ideal.
• the left and right channel audio input signals now have an overall equalization applied to them.
• FIG. 4A shows the measured 0° HRTF, which is the desired center filter denoted HRTF center
• FIG. 4B shows the measured 45° near ear HRTF
• HRTF near used in the binauralizer HRTF near used in the binauralizer
• FIG. 4C shows the measured 45° far ear HRTF, HRTF f ar used in the binauralizer
• FIG. 4D shows the average of the near and far ear 45° HRTFs. It can be seen the sum of the near and far HRTFs does not match the desired 0° HRTF.
• FIGS. 5A-5D show how equalization can be used to modify the near and far HRTF filters such that the sum more closely matches the desired 0° HRTF.
• FIG. 5 A shows the impulse response of the equalization filter EQ C to be applied to HRTF near and HRTFf ar .
• FIG. 5B shows the 45° near ear HRTF after equalization, that is, HRTF' near .
• FIG. 5C shows the 45° far ear HRTF after equalization, that is, HRTF' near
• FIG. 5D shows the resulting average of the equalized near HRTF and equalized far HRTFs. Comparing FIG. 5D with FIG. 4A, it can be seen that the average of the equalized near and far HRTFs closely matches the measured 0° HRTF.
• FIG. 6 shows the frequency magnitude response of the equalization filter EQ C .
• FIGS. 7 and 8 show two alternate implementations of binauralizers using such determined equalized HRTF filters.
• FIG. 7 shows a first implementation 40 in which four filters: two near filters 41 and 44 of impulse responses HRTF' near and two far filters 42 and 43 of impulse responses HRTF f' ar are used to create signals to be added by adders 45 and 46 to produce the left ear signal and right ear signal.
• FIG. 8 shows a second implementation 50 that uses the shuffler structure first proposed by Cooper and Bauck. See for example, U.S.
• a shuffler that includes an adder 51 and a subtracter 52 produces a first signal which is a sum of the left and right audio input signals, and a second signal which is the difference of the left and right audio signals.
• a sum filter 53 having an impulse response
• HRTF' mar + HRTF ⁇ ' r for the first shuffled signal the sum signal
• a difference filter 54 having an impulse response HRTF' near - HRTF'f ar for the second shuffled signal the difference signal.
• the resulting signals are now unshuffled in an unshuffler network (an "unshuffler") that reverses the operation of a shuffler, and includes an adder 55 to produce the left ear signal, and a subtracter 56 to produce the right ear signal.
• Scaling may be included, e .g., as divide by two attenuators 57 and 58 in each path, or a series of attenuators split at different parts of the circuit.
• the sum filter 53 has an impulse response that by equalizing the near and far HRTFs is approximately equal to the desired center HRTF filter response, 2*HRTF cen ( er This makes sense, since the sum filter followed by the unshuffler network 55, 56 and attenuators 57, 58 is basically an HRTF filter pair for a center panned signal.
• Such an implementation is shown in FIG. 9 and corresponds to: • Processing the first signal from the shuffler, i.e., the sum signal proportional to the sum of the left and right channel inputs, using a filter that forms a localized center virtual speaker image for a center panned signal component.
• Processing the second signal from the shuffler i.e., the difference signal proportional to the sum of the left and right channel inputs, so that the left and right inputs are approximately processed so as to localize at a desired left and a desired right virtual speaker locations.
• FIG. 9 achieves this by using a shuffler network that includes the adder 51 and subtracter 52 to produce the center and difference signals. While the embodiment of FIG. 9 uses Left and Right equalized HRTFs, then converts them into the sum and difference of the equalized HRTFs, the embodiment of FIG. 9 replaces the sum filter with a sum filter 59 that has twice the desired center HRTF response, and uses for the difference filter 60 a response equal to the unequalized difference filter. This method provides the desired high-quality center HRTF image, at the expense of some localization error in the Left and Right signals. [0105] Therefore, presented have been a first and a second set of embodiments as follows:
• a third set of embodiments combines the two versions 1. and 2. as follows: 3.
• the equalization filter e.g., mat of FIG. 6 for the virtual speakers at ⁇ 45°
• the equalization filter is modified, so as to be only partially effective, resulting in a set of HRTFs that have a slightly less clear center image than the HRTFs described in the first above-described set of embodiments, but with the advantage that the left and right signals are not colored as much as would occur with the equalized HRTF filters described in the first above-described set of embodiments.
• an equalizer is produced by halving (on a dB scale) the equalization curve of FIG. 6 so that, at each frequency, the effect of the filter is halved, and likewise, the equalization filter's phase response (not shown) is halved, while maintaining the well-behaved phase response, e.g., maintaining a minimum phase filter.
• the resulting filter is such that a pair of such equalization filters cascaded provide the same response as the filter shown in FIG. 6.
• This equalization filter is used to equalize the desired, e.g., measured HRTF filters for the desired speaker locations.
• center panning is known to correctly create the location of the center for a listener, i.e., to create a phantom center image for stereo speaker playback, only when the stereo speakers are placed symmetrically in front of the listener at no more than about ⁇ 45 degrees to the listener.
• aspects of the present invention provide for playback though headphones with front-center image location the virtual left/right speakers are up to +/-90 degrees to the listener.
• crosstalk refers to the left ear hearing sound from the right speaker, and also to the right ear hearing sound from the left speaker. Because normal sound cues are disturbed by crosstalk, crosstalk is known to significantly blur localization. Crosstalk cancellation reverses the effect of crosstalk.
• a typical cross-talk-cancelled filter includes two filters that process the mono input signal to two speakers, usually placed in front of the listener like a regular stereo pair, with the signals at the speakers intended to provide a stimulus at the listener's ears that corresponds to a binaural response attributable to a sound arrival from a virtual sound location.
• two actual speakers that are located at ⁇ 30° angles in front of a listener, and suppose it is desired to provide the listener with the illusion of a sound source at +60°.
• Cross-talk cancelled binauralization achieves this by both "undoing" the ⁇ 30° degree HRTFs that are imparted by the physical speaker setup, and binauralizing using 60 degree HRTF filters.
• FIG. 10 shows such a crosstalk cancelled binauralizing filter implemented as a cascade of a binauralizer to place virtual speakers at the desired locations, e.g., at ⁇ 60°.
• the binauralizer includes in the symmetric case (or forced symmetric case, e.g., per Eq.
• each near and far filter are added by adders 65, 66 to form the left and right binauralized signals.
• the binauralizer is followed by a cross-talk canceller to cancel the cross talk created at the actual speaker locations, e.g., at ⁇ 30° angles.
• the cross talk canceller accepts the signals from the binauralizer and includes in the symmetric case or forced symmetric case the near crosstalk cancelling filters 67, 68 whose impulse response is denoted Xnear and the far crosstalk cancelling filters 69, 70 whose impulse response is denoted Xf 3J , followed by summers 71 and 72 to cancel the cross talk created at the ⁇ 30° angles.
• the outputs are for a left speaker 73 and a right speaker 74.
• each of the near and far binauralizer and crosstalk cancelling filters is a linear time- invariant system
• the cascade of the binauralizer may be represented as a two-input, two output system.
• FIG. 11 shows an implementation of such a crosstalk cancelled binauralizer as four filters 75, 76, 77, and 78, and two summers 79 and 80.
• the four filters in the symmetric (or forced symmetric) case have two different impulse responses: a near impulse response denoted G near for filters 75 and
• Gf ar a far impulse response, denoted Gf ar for filters 77 and 78, wherein each of the G near and Gf ar are functions of the HRTF filters HRTF near and HRTFf 31 - and the crosstalk cancelling filters X near and X far .
• FIG. 12 shows a crosstalk cancelled binauralizer including a shuffling network 90 that has an adder 81 to produce a sum signal and a subtracter 82 to produce a difference signal, a sum signal filter 83 to filter the sum signal, such a sum signal filter having an impulse response proportional to G near + Gf ar , a difference filter 84 to filter the difference signal, the difference signal filter having an impulse response proportional to G near - Gf ar , followed by an un-shuffling network 91 that also includes a summer 85 to produce the left speaker signal for a left speaker 73 and a subtracter to produce a right speaker signal for a right speaker 74.
• a crosstalk cancelled binauralizing filter is implemented by a structure shown in FIG. 12, which is similar to the structures shown in FIG. 8 and FIG.9.
• the sum filter is designed to accurately reproduce a source located at the center, e.g., at 0°. Rather than calculate what such a filter is, one embodiment uses a delta function for such a filter, using the knowledge that a listener listening to an equal amount of a mono signal on a left and a right speaker accurately localizes such a signal as coming from the center.
• the cross-talk-cancelled filters are equalized to force the sum filter to be approximately the identity filter, e.g., a filter whose impulse response is a delta function.
• the sum filter is replaced by a flat (delta function impulse response) filter.
• Another aspect of the invention is correctly simulating a rear center sound source, by binauralizing to simulate speakers at angles ⁇ 90 degrees or more, e.g., having two rear virtual speaker locations, further locating a phantom center being localized at the 180 degree (rear-center) position, as if a speaker was located at the rear center position.
• a first rear signal embodiment includes equalizing the rear near and rear far HRTF filters such that the sum of the equalized rear near and rear far filters approximates the desired rear center HRTF filter.
• a binauralizer that uses a shuffler plus a sum signal HRTF filters that approximate a desired center rear HRTF creates playback signals that when reproduced through headphones appear to correctly come from the center, but with the left and right rear signals appearing to come from left and right rear virtual speakers that are slightly off the desired locations.
• Another embodiment includes combining front and rear processing to process both rear signals and front signals.
• surround sound e.g., four channel sound
• FIG. 13 shows a form of implementation of an audio processing system for processing a stereo input pair according to aspects of the invention.
• the audio processing system includes: a analog-to-digital (A/D) converter 97 for converting analog inputs to corresponding digital signals, and a digital to analog (D/A) converter 98 to convert the processed signals to analog output signals.
• the block 97 includes a SPDIF interface provided for digital input signals rather than the A/D converter.
• the system includes a DSP device capable of processing the input to generate the output sufficiently fast.
• the DSP device includes interface circuitry in the form of serial ports 96 for communicating with the A/D and D/A converters 97,98 without processor overhead, and, in one embodiment, an off-device memory 92 and a DMA engine that can copy data from the off-chip memory to an on-chip memory 95 without interfering with the operation of the input/output processing.
• the code for implementing the aspects of the invention described herein may be in the off-chip memory and be loaded to the on-chip memory as required.
• the DSP device includes a program memory 94 including code that cause the processor 93 of the DSP device to implement the filtering described herein. An external bus multiplexor is included for the case that external memory is required. [0126] Similarly, FIG.
• FIG. 14A shows a binauralizing system that accepts five channels of audio information in the form of a left, center and right signals aimed at playback through front speakers, and a left surround and right surround signals aimed at playback via rear speakers.
• the binauralizer implements HRTF filter pairs for each input, including, for the left surround and right surround signals, aspects of the invention so that a listener listening through headphones experiences a signal that is center rear panned to be coming from the center rear of the listener.
• the binauralizer is implemented using a processing system, e.g., a DSP device that includes a processor.
• a memory in included for holding the instructions, including any parameters that cause the processor to execute filtering as described hereinabove. [0127] Similarly, FIG.
• the binauralizer implements HRTF filter pairs for each input, including for left and right signals, and for the left rear and right rear signals, aspects of the invention so that a listener listening through headphones experiences a signal that is center front panned to be coming from the center front of the listener, and a signal that is center rear panned to be coming from the center rear of the listener.
• the binauralizer is implemented using a processing system, e.g., a DSP device that includes a processor.
• a memoiy in included for holding the instructions, including any parameters that cause the processor to execute filtering as described hereinabove.
• the methodologies described herein are, in one embodiment, performable by a machine that includes one or more processors that accept code segments containing instructions. For any of the methods described herein, when the instructions are executed by the machine, the machine performs the method. Any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine are included.
• a typical machine may be exemplified by a typical processing system that includes one or more processors.
• Each processor may include one or more of a CPU, a graphics processing unit, and a programmable DSP unit.
• the processing system further may include a memory subsystem including main RAM and/or a static RAM, and/or ROM.
• a bus subsystem may be included for communicating between the components.
• the processing system requires a display, such a display may be included, e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT) display.
• a display e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT) display.
• the processing system also includes an input device such as one or more of an alphanumeric input unit such as a keyboard, a pointing control device such as a mouse, and so forth.
• the term memory unit as used herein also encompasses a storage system such as a disk drive unit.
• the processing system in some configurations may include a sound output device, and a network interface device.
• the memory subsystem thus includes a carrier medium that carries machine readable code segments (e.g., software) including instructions for performing, when executed by the processing system, one of more of the methods described herein.
• the software may reside in the hard disk, or may also reside, completely or at least partially, within the RAM and/or within the processor during execution thereof by the computer system.
• the memory and the processor also constitute a carrier medium carrying machine readable code.
• the machine operates as a standalone device or may be connected, e.g., networked to other machines, in a networked deployment, the machine may operate in the capacity of a server or a client machine in a server-client network environment, or as a peer machine in a peer-to-peer or distributed network environment.
• the machine may be a personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine.
• PC personal computer
• PDA Personal Digital Assistant
• each of the methods described herein is in the form of a computer program that executes on a processing system, e.g., a one or more processors that are part of binauralizing system, or in another embodiment, a transaural system.
• embodiments of the present invention may be embodied as a method, an apparatus such as a special purpose apparatus, an apparatus such as a data processing system, or a carrier medium, e.g., a computer program product.
• the carrier medium carries one or more computer readable code segments for controlling a processing system to implement a method.
• aspects of the present invention may take the form of a method, an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects.
• the present invention may take the form of carrier medium (e.g., a computer program product on a computer-readable storage medium) carrying computer-readable program code segments embodied in the medium.
• the software may further be transmitted or received over a network via the network interface device.
• the carrier medium is shown in an exemplary embodiment to be a single medium, the term “carrier medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions.
• the term " carrier medium” shall also be taken to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by the machine and that cause the machine to perform any one or more of the methodologies of the present invention.
• a carrier medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media.
• Non-volatile media includes, for example, optical, magnetic disks, and magneto-optical disks.
• Volatile media includes dynamic memory, such as main memory.
• Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise a bus subsystem. Transmission media may also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications.
• carrier medium shall accordingly be taken to include, but not be limited to, solid-state memories, optical and magnetic media, and carrier wave signals.
• inventions are in the form of a carrier medium carrying computer readable data for filters to process a pair of stereo inputs.
• the data may be in the form of the impulse responses of the filters, or of the frequency domain transfer functions of the filters.
• the filters include two HRTF filters designed as described above. In the case that the processing is for headphone listening, the HRTF filters are used to filter the input data in a binauralizer, and in the case of speaker listening, the HRTF filters are incorporated in a crosstalk cancelled binauralizer.

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