Patent Publication Number: US-7912225-B2

Title: Generating 3D audio using a regularized HRTF/HRIR filter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 09/190,207, filed on Nov. 13, 1998 now U.S. Pat. No. 7,085,393, which claimed the benefit of the filing date of U.S. provisional application No. 60/065,855, filed on Nov. 14, 1997, the teachings of both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to three-dimensional (3D) sound. More particularly, it relates to an improved regularizing model for head-related transfer functions (HRTFs) for use with 3D digital sound applications. 
     2. Description of the Related Art 
     Many high-end consumer devices provide the option for three-dimensional (3D) sound, allowing a more realistic experience when listening to sound. In some applications, 3D sound allows a listener to perceive motion of an object from the sound played back on a 3D audio system. 
     Atal and Schroeder established cross-talk canceler technology as early as 1962, as described in U.S. Pat. No. 3,236,949, which is explicitly incorporated herein by reference. The Atal-Schroeder 3D sound cross-talk canceler was an analog implementation using specialized analog amplifiers and analog filters. To gain better sound positioning performance using two loudspeakers, Atal and Schroeder included empirically determined frequency dependent filters. Without doubt, these sophisticated analog devices are not applicable for use with today&#39;s digital audio technology. 
     Interaural time difference (ITD), i.e., the difference in time that it takes for a sound wave to reach both ears, is an important and dominant parameter used in 3D sound design. The interaural time difference is responsible for introducing binaural disparities in 3D audio or acoustical displays. In particular, when a sound object moves in a horizontal plane, a continuous interaural time delay occurs between the instant that the sound object impinges upon one of the ears and the instant that the same sound object impinges upon the other ear. This ITD is used to create aural images of sound moving in any desired direction with respect to the listener. 
     The ears of a listener can be “tricked” into believing sound is emanating from a phantom location with respect to the listener by appropriately delaying the sound wave with respect to at least one ear. This typically requires appropriate cancellation of the original sound wave with respect to the other ear, and appropriate cancellation of the synthesized sound wave to the first ear. 
     A second parameter in the creation of 3D sound is adaptation of the 3D sound to the particular environment using the external ear&#39;s free-field-to-eardrum transfer functions, or what are called head-related transfer functions (HRTFs). HRTFs relate to the modeling of the particular environment of the user, including the size and orientation of the listeners head and body, as they affect reception of the 3D sound. For instance, the size of a listener&#39;s head, their torso, what they wear, etc., forms a form of filtering which can change the effect of the 3D sound on the particular user. An appropriate HRTF adjusts for the particular environment to allow the best 3D sound imaging possible. 
     The HRTFs are different for each location of the source of the sound. Thus, the magnitude and phase spectra of measured HRTFs vary as a function of sound source location. Hence, it is commonly acknowledged that the HRTF introduces important cues in spatial hearing. 
     Advances in computer and digital signal processing technology have enabled researchers to synthesize directional stimuli using HRTFs. The HRTFs can be measured empirically at thousands of locations in a sphere surrounding the 3D sound environment, but this proves to require an excessive amount of processing. Moreover, the number of measurements can be very large if the entire auditory space is to be represented on a fine grid. Nevertheless, measured HRTFs represent discrete locations in a continuous auditory space. 
     One conventional solution to the adaptation of a discretely measured HRTF within a continuous auditory space is to “interpolate” the measured HRTFs by linearly weighting the neighboring impulse responses. This can provide a small step size for incremental changes in the HRTF from location to location. However, interpolation is conceptually incorrect because it does not account for environmental changes between measured points, and thus may not provide a suitable 3D sound rendering. 
     Other attempted solutions include using one HRTF for a large area of the three-dimensional space to reduce the frequency of discontinuities which may cause a clicking sound. However, again, such solutions compromise the overall quality of the 3D sound rendering. 
     Another solution wherein spatial characteristic functions are combined directly with Eigen functions to provide a set of HRTFs is shown in  FIG. 3 . 
     In particular, a set N of Eigen filters  422 - 426  are combined with corresponding sets of spatial characteristic function (SCF) samples  412 - 416  and summed in a summer  440  to provide an HRTF (or HRIR) filter  450  which acts on a sound source  460 . The desired location of a sound image is controlled by varying the sound source elevation and/or azimuth in the sets of SCF samples  412 - 416 . Unfortunately, this technique is susceptible to discontinuities in the continuous auditory space as well. 
     There is thus a need for a more accurate HRTF model which provides a suitable HRTF for source locations in a continuous auditory space, without annoying discontinuities. 
     SUMMARY OF THE INVENTION 
     A head-related transfer function or head-related impulse response model for use with 3D sound applications comprises a plurality of Eigen filters. A plurality of spatial characteristic functions are adapted to be respectively combined with the plurality of Eigen filters. A plurality of regularizing models are adapted to regularize the plurality of spatial characteristic functions prior to the respective combination with the plurality of Eigen filters. 
     A method of determining spatial characteristic sets for use in a head-related transfer function model or a head-related impulse response model comprises constructing a covariance data matrix of a plurality of measured head-related transfer functions or a plurality of measured head-related impulse responses. An Eigen decomposition of the covariance data matrix is performed to provide a plurality of Eigen vectors. At least one principal Eigen vector is determined from the plurality of Eigen vectors. The measured head-related transfer functions or head-related impulse responses are projected to the at least one principal Eigen vector to create the spatial characteristic sets. 
     In one embodiment, the present invention is a method for generating a 3D sound signal. The method comprises (a) providing a regularized head-related transfer function (HRTF) filter and (b) applying an input sound signal to the regularized HRTF filter to generate the 3D sound signal. The regularized HRTF filter is generated by (1) generating a plurality of sets of spatial characteristic function (SCF) samples, (2) applying a corresponding regularizing model to each of one or more of the sets of SCF samples using a corresponding smoothness factor that trades off between smoothness and localization for the corresponding set of SCF samples, (3) combining each set of SCF samples with a corresponding Eigen filter, and (4) summing the results of the combining to generate the regularized HRTF filter. 
     In another embodiment, the present invention is a method for generating a 3D sound signal. The method comprises (a) providing a regularized head-related impulse response (HRIR) filter and (b) applying an input sound signal to the regularized HRIR filter to generate the 3D sound signal. The regularized HRIR filter is generated by (1) generating a plurality of sets of spatial characteristic function (SCF) samples, (2) applying a corresponding regularizing model to each of one or more of the sets of SCF samples using a corresponding smoothness factor that trades off between smoothness and localization for the corresponding set of SCF samples, (3) combining each set of SCF samples with a corresponding Eigen filter, and (4) summing the results of the combining to generate the regularized HRIR filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. 
         FIG. 1  shows an implementation of a plurality of Eigen filters to a plurality of regularizing models each based on a set of SCF samples, to provide an HRTF model having varying degrees of smoothness and generalization, in accordance with the principles of the present invention. 
         FIG. 2  shows a process for determining the principle Eigen vectors to provide Eigen filters used in the Eigen filters shown in  FIG. 1 , in accordance with the principles of the present invention. 
         FIG. 3  shows a conventional solution wherein spatial characteristic functions are combined directly with Eigen functions to provide a set of HRTFs. 
     
    
    
     DETAILED DESCRIPTION 
     Conventionally measured HRTFs are obtained by presenting a stimulus through a loudspeaker positioned at many locations in a three-dimensional space, and at the same time collecting responses from a microphone embedded in a mannequin head or a real human subject. To simulate a moving sound, a continuous HRTF that varies with respect to the source location is needed. However, in practice, only a limited number of HRTFs can be collected in discrete locations in any given 3D space. 
     Limitations in the use of measured HRTFs at discrete locations have led to the development of functional representations of the HRTFs, i.e., a mathematical model or equation which represents the HRTF as a function of frequency and direction. Simulation of 3D sound is then performed by using the model or equation to obtain the desired HRTF. 
     Moreover, when discretely measured HRTFs are used, annoying discontinuities can be perceived by the listener from a simulated moving sound source as a series of clicks as the sound object moves with respect to the listener. Further analyses indicates that the discontinuities may be the consequence of, e.g., instrumentation error, under-sampling of the three-dimensional space, a non-individualized head model, and/or a processing error. The present invention provides an improved HRTF modeling method and apparatus by regularizing the spatial attributes extracted from the measured HRTFs to obtain the perception of a smooth moving sound rendering without annoying discontinuities creating clicks in the 3D sound. 
     HRTFs corresponding to specific azimuth and elevation can be synthesized by linearly combining a set of so-called Eigen-transfer functions (EFs) and a set of spatial characteristic functions (SCFs) for the relevant auditory space, as shown in  FIG. 3  herein, and as described in “An Implementation of Virtual Acoustic Space For Neurophysiological Studies of Directional Hearing” by Richard A. Reale, Jiashu Chen et al. in  Virtual Auditory Space: Generation and Applications , edited by Simon Carlile (1996); and “A Spatial Feature Extraction and Regularization Model for the Head-Related Transfer Function” by Jiashu Chen et al. in  J. Acoust. Soc. Am.  97 (1) (January 1995), the entirety of both of which are explicitly incorporated herein by reference. 
     In accordance with the principles of the present invention, spatial attributes extracted from the HRTFs are regularized before combination with the Eigen transfer function filters to provide a plurality of HRTFs with varying degrees of smoothness and generalization. 
       FIG. 1  shows an implementation of the regularization of a number N of SCF sample sets  202 - 206  in an otherwise conventional system as shown in  FIG. 3 . 
     In particular, a plurality N of Eigen filters  222 - 226  are associated with a corresponding plurality N of SCF samples  202 - 206 . A plurality N of regularizing models  212 - 216  act on the plurality N of SCF samples  202 - 206  before the SCF samples  202 - 206  are linearly combined with their corresponding Eigen filters  222 - 226 . Thus, in accordance with the principles of the present invention, SCF sample sets are regularized or smoothed before combination with their corresponding Eigen filters. 
     The particular level of smoothness desired can be controlled with a smoothness control to all regularizing models  212 - 216 , to allow the user to adjust a tradeoff between smoothness and localization of the sound image. The regularizing models  212 - 216  in the disclosed embodiment performs a so-called ‘generalized spline model’ function on the SCF sample sets  202 - 206 , such that smoothed continuous SCF sets are generated at combination points  230 - 234 , respectively. The degree of smoothing, or regularization, can be controlled by a lambda factor, with trade-offs of the smoothness of the SCF samples with their acuity. 
     The results of the combined Eigen filters  222 - 226  and corresponding regularized SCF sample sets  202 - 206 / 212 - 216  are summed in a summer  240 . The summed output from the summer  240  provides a single regularized HRTF (or HRIR) filter  250  through which the digital audio sound source  260  is passed, to provide an HRTF (or HRIR) filtered output  262 . 
     The HRTF filtering in a 3D sound system in accordance with the principles of the present invention may be performed either before or after other 3D sound processes, e.g., before or after an interaural delay is inserted into an audio signal. In the disclosed embodiment, the HRTF modeling process is performed after insertion of the interaural delay. 
     The regularizing models  212 - 216  are controlled by a desired location of the sound source, e.g., by varying a desired source elevation and/or azimuth. 
       FIG. 2  shows an exemplary process of providing the Eigen functions for the Eigen filters  222 - 226  and the SCF sample sets  202 - 206 , e.g., as shown in  FIG. 1 , to provide an HRTF model having varying degrees of smoothness and generalization in accordance with the principles of the present invention. 
     In particular, in step  102 , the ear canal impulse responses and free field response are measured from a microphone embedded in a mannequin or human subject. The responses are measured with respect to a broadband stimulus sound source that is positioned at a distance about 1 meter or farther away from the microphone, and preferably moved in 5 to 15 degree intervals both in azimuth and elevation in a sphere. 
     In step  104 , the data measured in step  102  is used to derive the HRTFs using a discrete Fourier Transform (DFT) based method or other system identification method. Since the HRTFs are either in a frequency or time domain form, and since they vary with respect to their respective spatial location, HRTFs are generally considered as a multivariate function with frequency (or time) and spatial (azimuth and elevation) attributes. 
     In step  106 , an HRTF data covariance matrix is constructed either in the frequency domain or in the time domain. For instance, in the disclosed embodiment, a covariance data matrix of measured head-related impulse responses (HRIR) are measured. 
     In step  108 , an Eigen decomposition is performed on the data covariance matrix constructed in step  106 , to order the Eigen vectors according to their corresponding Eigen values. These Eigen vectors are a function of frequency only and are abbreviated herein as “EFs”. Thus, the HRTFs are expressed as weighted combinations of a set of complex valued Eigen transfer functions (EFs). The EFs are an orthogonal set of frequency-dependent functions, and the weights applied to each EF are functions only of spatial location and are thus termed spatial characteristic functions (SCFs). 
     In step  110 , the principal Eigen vectors are determined. For instance, in the disclosed embodiment, an energy or power criteria may be used to select the N most significant Eigen vectors. These principal Eigen vectors form the basis for the Eigen filters  222 - 226  ( FIG. 1 ). 
     In step  112 , all the measured HRTFs are back-projected to the principal Eigen vectors selected in step  110  to obtain N sets of weights. These weight sets are viewed as discrete samples of N continuous functions. These functions are two dimensional with their arguments in azimuthal and elevation angles. They are termed spatial characteristic functions (SCFs). This process is called spatial feature extraction. 
     Each HRTF, either in its frequency or in its time domain form, can be re-synthesized by linearly combining the Eigen vectors and the SCFs. This linear combination is generally known as Karhunen-Loeve expansion. 
     Instead of directly using the derived SCFs as in conventional systems, e.g., as shown in  FIG. 3 , they are processed by a so-called “generalized spline model” in regularizing models  212 - 216  such that smoothed continuous SCF sets are generated at combinatorial points  230 - 234 . This process is referred to as spatial feature regularization. The degree of smoothing, or regularization, can be controlled by a smoothness control with a lambda factor, providing a trade-off between the smoothness of the SCF samples  202 - 206  and their acuity. 
     In step  114 , the measured HRIRs are back-projected to the principal Eigen vectors selected in step  110  to provide the spatial characteristic function (SCF) sample sets  202 - 206 . 
     Thus, in accordance with the principles of the present invention, SCF samples are regularized or smoothed before combination with a corresponding set of Eigen filters  222 - 226 , and recombined to form a new set of HRTFs. 
     In accordance with the principles of the present invention, an improved set of HRTFs are created which, when used to generate moving sound, do not introduce discontinuities causing the annoying effects of clicking sound. Thus, with empirically selected lambda values, localization and smoothness can be traded off against one another to eliminate discontinuities in the HRTFs. 
     While the invention has been described with reference to the exemplary embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention.