Patent Publication Number: US-6707918-B1

Title: Formulation of complex room impulse responses from 3-D audio information

Description:
FIELD OF THE INVENTION 
     The present invention relates to the utilization of sound spatialization in audio signals. 
     BACKGROUND OF THE INVENTION 
     The use of B-format measurements, recordings and playback in the provision of more ideal acoustic reproductions which capture part of the spatial characteristics of an audio reproduction are well known. 
     In the case of conversion of B-format signals to multiple loudspeakers in a speaker array, there is a well recognized problem due to the spreading of individual virtual sound sources over a large number of playback speaker elements. In the worst case, this can lead to significant errors in a listener&#39;s localization of these virtual sound sources, especially if the listener is situated off-center in the speaker array. Likewise, in the case of binaural playback of B-format signals, the approximations inherent in the B-format soundfield can lead to less precise localization of sound sources, and a loss of the out-of-head sensation that is an important part of the binaural playback experience. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide for an improved form of creation of impulse response models. 
     In accordance with a first aspect of the present invention, there is provided a method for the creation of acoustic impulse responses for utilization in rendering to an array of speakers comprising the steps of: measuring a room response function; extracting a series of discrete time arrivals from the measured room response function so as to leave a reverberant residual response function; separately rendering the extracted series and the reverberant residual response function to the array of speakers to form a discrete response and a residual response; combining the discrete response and the residual response to form an acoustic impulse response for the array of speakers. 
     The measuring step preferably can include measuring the room response function in a B-format. 
     The extraction step preferably can include extracting a direction and magnitude of each of the discrete time arrivals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 illustrates a simplified B-format impulse response; 
     FIG. 2 illustrates an example speaker output array; 
     FIG. 3 illustrates the process of extraction of target arrivals and their rendering as a series of speaker impulse responses; 
     FIG. 4 illustrates a resulting reverberant residual; 
     FIG. 5 illustrates the combining of the reverberant residual and speaker arrivals; and 
     FIG. 6 illustrates the steps of the preferred embodiment. 
    
    
     DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS 
     In discussion of the embodiments of the present invention, it is assumed that the input sounds and impulse response functions have a three dimensional characteristics and is in an “ambisonic B-format”. It should be noted however that the present invention is not limited thereto and can be readily extended to other formats such as SQ, QS, UMX, CD-4, Dolby MP, Dolby surround AC-3, Dolby Pro-logic, Lucas Film THX etc. 
     The ambisonic B-format system is a very high quality sound positioning system which operates by breaking down the directionality of the sound into spherical harmonic components termed W, X, Y and Z. The ambisonic system is then designed to utilise all output speakers to cooperatively recreate the original directional components. 
     For a description of the B-format system, reference is made to: 
     (1) The Internet ambisonic surround sound FAQ available at the following HTTP locations. 
     http://www.omg.unb.ca/˜mleese/ 
     http://www.york.ac.uk/inst/mustech/3d —   
     audio/ambison.htm 
     http://jrusby.uoregon.edu/mustech.htm 
     The FAQ is also available via anonymous FTP from pacific.cs.unb.ca in a directory/pub/ambisonic. The FAQ is also periodically posted to the Usenet newsgroups mega.audio.tech, rec.audio.pro, rec.audio.misc, rec.audio.opinion. 
     (2) “General method of theory of auditory localisation”, by Michael A Gerzon, 90 sec, Audio Engineering Society Convention, Vienna 24th-27th March 1992. 
     (3) “Surround Sound Physco Acoustics”, M. A. Gerzon, Wireless World, December 1974, pages 483-486. 
     (4) U.S. Pat. Nos. 4,081,606 and 4,086,433. 
     The preferred embodiment makes use of a convenient, measurement method (a soundfield microphone, used to measure B-format impulse responses) as a means for constructing accurate acoustic impulse responses for use in multiple-speaker or binaural playback environments. 
     The new technique makes use of the fact that, in the early part of the impulse response of an acoustic space, discrete sound arrivals (individual echoes) can be separately identified and isolated. FIG. 1 shows the early part of a typical B-format impulse response  1  having w, x, y, z components. The direct sound appears as a large peak  2  in the W (omni) channel and corresponding positive, negative or zero peaks in the X,Y and Z channels eg.  3 ,  4  indicate the direction of arrival of this direct sound. Likewise, several later sound arrivals (echoes in the acoustic space) can also be separately isolated  6 - 9 , and their amplitude, time delay, and direction of arrival can be determined. 
     As part of the reverberant tail, several other peaks eg.  10 ,  11  may be recognizable. 
     The preferred embodiment proceeds by an analysis of the impulse response functions so as to extract the discrete sound arrival information so as to provide for a better B-format rendering of the impulse response function. 
     It is assumed that playback is to occur on a series of speakers and illustrated in FIG. 2 arranged around a listener  15  with the speakers S 1 -S 4  being arranged so as to provide for simple B-format conversion. 
     Initially, each of the discrete sound arrivals is processed so as to determine a magnitude (W component and direction). This is utilized to determine how to pan the discrete sound arrival between the speakers S 1 -S 4 . For example, in FIG. 3, there is shown the corresponding panning  17 ,  18  of the initial discrete sound arrival of FIG.  1 . 
     Subsequently, the earlier frictions are also processed in the same way so as to produce signals  19 ,  20 . The arrivals detected in the reverberant tail are separately processed so as to produce corresponding arrivals  21 . The detected arrivals, as shown by way of example in FIG. 1, are then subtracted out of the B-format signals with the result being as illustrated by way of example in FIG. 3 with the subtraction often leading a number of small residuals eg.  30 - 32  in the B-format signal. The remaining overall B-formal signal is then utilized as a residual  33  and decoded to the speakers utilizing standard B-format decoding techniques. The separately encoded arrivals (FIG. 3) are then combined with the residuals as illustrated  40  in FIG. 5 so as to provide for impulse responses for each speaker. 
     It should be noted that, in practice, there is often a large number of identifiable reflections and the figures show a simplified example for clarity of discussion. 
     Turning now to FIG. 6, there is illustrated the steps  50  involved in the preferred embodiment. The steps include the initial measurement of the B-format impulse responses  51  which outputs  4  impulse responses. The impulse responses are analysed  52  to identify discrete arrivals and their likely direction and magnitude. A database of arrivals is determined  53  and utilized firstly, to subtract the arrivals  54  out of the initially measured impulse response functions so as to form a residual B-format impulse response function which is then linearly decoded  55  utilizing standard techniques. The database of arrival  53  is also separately utilized so as to synthesise the detected targets separately on the output speaker array. The two outputs are combined  58  so as to produce combined output impulse response functions for each speaker. The output impulse response functions can then be convolved with an audio signal (in addition to any convolution with speaker equalization functions) so as to produce an enhanced spatialization of an audio source in multiple dimensions. 
     In a further embodiment, the target format of the impulse response may be a 2-channel binaural format for headphone playback, or a 2-channel cross talk cancelled binaural format for stereo playback. 
     It would be further appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.