Abstract:
A system for decorrelating signals including a first all pass filter, having a first delay length, processing a first signal. A second all pass filter, having a second delay length, is connected to the first all pass filter and processes the first signal after processing by the first all pass filter. A third all pass filter, having a third delay length, processes a second signal. A fourth all pass filter, having a fourth delay length, is connected to the third all pass filter and processes the second signal after processing by the third all pass filter. The first delay length, the second delay length, the third delay length and the fourth delay length each have a unique value, and the sum of the first delay length and the second delay length is equal to the sum of the third delay length and the fourth delay length.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to systems and methods for processing signals, and more specifically to a system and method for time-domain decorrelation for sound field widening, elimination of audio artifacts and other suitable purposes. 
       BACKGROUND OF THE INVENTION 
       [0002]    Systems and methods for processing signals are known in the art. Decorrelation in particular has been achieved in a variety of ways, both by all pass filter and by filters containing both frequency and phase shaping. In the past, all pass filters have been used to decorrelate signals to some extent, however with uncompensated inter-channel delay and without an ability to change the amount of decorrelation in a simple fashion. 
       SUMMARY OF THE INVENTION 
       [0003]    A system and method are provided that create a varying phase shift as a function of frequency for different channels of audio data to eliminate phase correlations between channels of audio data. 
         [0004]    In particular, a system and method are provided that can be used to widen the perceived sound field, eliminate audio artifacts and perform other suitable functions by eliminating undesired time-domain correlations between channels of audio data. 
         [0005]    In accordance with an exemplary embodiment of the invention, a system for decorrelating signals is provided. The system includes a first all pass filter, having a first delay length, processing a first signal. A second all pass filter, having a second delay length, is connected to the first all pass filter and processes the first signal after processing by the first all pass filter. A third all pass filter, having a third delay length, processes a second signal. A fourth all pass filter, having a fourth delay length, is connected to the third all pass filter and processes the second signal after processing by the third all pass filter. The first delay length, the second delay length, the third delay length and the fourth delay length each have a unique value, and the sum of the first delay length and the second delay length is equal to the sum of the third delay length and the fourth delay length. 
         [0006]    Those skilled in the art will further appreciate the advantages and superior features of the invention together with other important aspects thereof on reading the detailed description that follows in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a diagram of a system for sound field widening and phase decorrelation in accordance with an exemplary embodiment of the present invention; 
           [0008]      FIG. 2  is a diagram of a system for multi-channel phase decorrelation in accordance with an exemplary embodiment of the present invention; 
           [0009]      FIG. 3  is a diagram of a system for performing sound field widening and phase decorrelation in accordance with an exemplary embodiment of the present invention; 
           [0010]      FIG. 4  is a diagram of a method for processing a plurality of channels of audio data to decorrelate phase in accordance with an exemplary embodiment of the present invention; 
           [0011]      FIG. 5  is a diagram of a system for performing phase modification of an audio channel in accordance with an exemplary embodiment of the present invention; 
           [0012]      FIG. 6  is a diagram of a system for performing phase decorrelation and sound field widening in accordance with an exemplary embodiment of the present invention; and 
           [0013]      FIG. 7  is a diagram of a system for sound field widening and phase decorrelation with phase modulation in accordance with an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0014]    In the description which follows, like parts are marked throughout the specification and drawing with the same reference numerals, respectively. The drawing figures may not be to scale and certain components may be shown in generalized or schematic form and identified by commercial designations in the interest of clarity and conciseness. 
         [0015]      FIG. 1  is a diagram of a system  100  for sound field widening and phase decorrelation in accordance with an exemplary embodiment of the present invention. System  100  can be used to increase the perceived width of a sound field by phase decorrelation of the audio channel signals being processed. 
         [0016]    System  100  includes all pass filter L 1   102 , all pass filter L 2   104 , all pass filter L 3   106  and all pass filter L 4   108 , each of which can be implemented in hardware, software, or a suitable combination of hardware and software, and can be one or more software systems operating on a general purpose processing platform. As used herein, “hardware” can include a combination of discrete components, an integrated circuit, an application-specific integrated circuit, a field programmable gate array, or other suitable hardware. As used herein, “software” can include one or more objects, agents, threads, lines of code, subroutines, separate software applications, two or more lines of code or other suitable software structures operating in two or more software applications or on two or more processors, or other suitable software structures. In one exemplary embodiment, software can include one or more lines of code or other suitable software structures operating in a general purpose software application, such as an operating system, and one or more lines of code or other suitable software structures operating in a specific purpose software application. 
         [0017]    All pass filter L 1   102  is coupled to all pass filter L 2   104 , and all pass filter L 3   106  is coupled to all pass filter L 4   108 . As used herein, the term “coupled” and its cognate terms such as “couples” or “couple,” can include a physical connection (such as a wire, optical fiber, or a telecommunications medium), a virtual connection (such as through assigned memory locations of a data memory device or a hypertext transfer protocol (HTTP) link), a logical connection (such as through one or more semiconductor devices in an integrated circuit), or other suitable connections. 
         [0018]    System  100  receives left channel audio data and right channel audio data, which can be a series of digital time domain samples of audio data or other suitable audio data. The left channel audio data is provided to all pass filter L 1   102 , which has a delay length of L 1 . In one exemplary embodiment, where the audio data is digital time domain samples of an audio signal, the delay length L 1  can be equal to the number of samples in the delay element of all pass filter L 1   102 . After the left channel audio data is processed with all pass filter L 1   102 , the processed data is then provided to all pass filter L 2   104 , which has a delay length L 2  that is different from delay length L 1 . The processed left channel audio data is then output, such as to speakers, for recordings or for other suitable applications. 
         [0019]    The right channel audio data is likewise provided to all pass filter L 3   106 , which has a delay length L 3  that is not equal to either delay length L 1  or L 2 . The processed right channel data is then provided to all pass filter L 4   108 , which has a delay length L 4  that is not equal to any of delay lengths L 1 , L 2  or L 3 . The processed right channel data is then output, such as to speakers, for recording, transmission, rendering in headphones or for other suitable applications. 
         [0020]    In one exemplary embodiment, all pass filter L 1   102 , all pass filter L 2   104 , all pass filter L 3   106  and all pass filter L 4   108  can each be sparse Nth order pole zero filters having an impulse response that is determined by the filter coefficients. The delay lengths of all pass filter L 1   102 , all pass filter L 2   104 , all pass filter L 3   106  and all pass filter L 4   108  are selected so that the total delay length of a signal processed by all pass filter L 1   102  and all pass filter L 2   104  is the same as the total delay length of a signal processed by all pass filter L 3   106  and all pass filter L 4   108 . In one exemplary embodiment, the delay lengths of all pass filter L 1   102  and all pass filter L 3   106  can be a first prime pair, and the delay lengths of all pass filter L 2   104  and all pass filter L 4   108  can be a second prime pair, such that the total length of the all pass filters can be set to an equal value by selection of suitable prime pairs. In one exemplary embodiment, the delay lengths of the first prime pair can be  41  and  43  and the delay lengths of the second prime pair can be  71  and  73 , such that all pass filter L 1   102  can have a delay length of  41  and all pass filter L 2   104  can have a delay length of  73 , and all pass filter L 3   106  can have a delay length of  43  and all pass filter L 4   108  can have a delay length of  71 . In this manner, the overall processing delay for both the left channel audio data (41+73) and the right channel audio data (43+71) will be equal (114). 
         [0021]    In operation, the all pass filters create a phase shift in the left channel audio data and right channel audio data that depends on the frequency and the length of the all pass filters. Because the phase shift created by each all pass filter is different, any pre-existing phase correlations between the left channel audio data and the right channel audio data will be eliminated, which results in the apparent widening of the sound field to a listener, such as when the signals are amplified and transmitted over loudspeakers. Because correlation between the phase of individual frequency bins can result in the apparent location of that frequency bin as being in the center between the left and right channel speaker, decorrelation of such frequencies will widen the apparent sound field. In other words, correlation in phase results in a collapse of the sound field towards the center of the left and right channel speaker, whereas decorrelation in phase avoids the collapse of the sound field towards the center of the left and right channel speaker. Additional all pass filters can also or alternatively be utilized to create additional phase decorrelation, where suitable. 
         [0022]    Furthermore, it is possible to connect two different pairs of sections to each channel, and to modulate the filter coefficients while keeping the coefficients for one pair in quadrature with those of the other pair, so as to modulate the decorrelation by a third signal, creating a time-varying change in the phase shifts. This third signal can be a sine wave of low frequency, a narrowband noise signal of low frequency or other suitable signals, such that reflection and cancellation patterns within a room or a listening environment are made to move about the room in a fashion preventing the human auditory system from strongly noticing the cancellations or reflections. 
         [0023]      FIG. 2  is a diagram of a system  200  for multi-channel phase decorrelation in accordance with an exemplary embodiment of the present invention. System  200  can be implemented in hardware, software or a suitable combination of hardware and software, and can be one or more software systems operating on a general purpose processing platform. This system can be accordingly made time varying in the same fashion as above. 
         [0024]    System  200  includes a plurality of channels of audio data from first audio channel, second audio channel, to an Nth audio channel. All pass filter L 1   202  having a delay length of L 1  and all pass filter L 2   204  having a delay length of L 2  are used to process the first audio channel, and all pass filter L 3   206  having a delay length of L 3  and all pass filter L 4   208  having a delay length of L 4  are used to process the second audio channel. Likewise, each additional audio channel is processed by different all pass filters having different delay lengths, until the last or Nth audio channel is processed by all pass filter L 2 N  210  having a delay length of L 2 N and all pass filter L 2 N+1  212  having a delay length of L 2 N+1. The individual delay lengths of each of the all pass filters are different, but the total delay length of each channel is equal. In this manner, the phase shift applied to each of the individual frequency bins of each channel of audio data will be decorrelated, which can be used to avoid creation of audio artifacts, to prevent audio artifacts from being generated in a recording studio environment, such as where different microphones are used to record sound from the different parts of a recording sound stage, or in other suitable applications. 
         [0025]      FIG. 3  is a diagram of a system  300  for performing sound field widening and phase decorrelation in accordance with an exemplary embodiment of the present invention. System  300  includes a first channel signal processing circuit and an Nth channel signal processing circuit, such that system  300  can be used for stereo, four channel audio, 5.1 channel audio, 7.1 channel audio, 9.1 channel audio or any other suitable numbers of channels. 
         [0026]    System  300  includes summation unit  302 , gain unit  306 , gain unit  310 , delay  304 , and summation unit  308 , which form an all pass filter configuration that may be referred to as a Schroeder section, and which provides an all pass filter of length L 1  when the gain factors of gain unit  306  and gain unit  310  are equal and opposite in sign, and which will pass all frequencies but will also add a phase shift depending on the frequency being passed, the delay, and the gain factor of gain unit  306  and gain unit  310 . By selecting suitable values for the length of the delay and the gain of gain unit  306  and gain unit  310 , the amount of phase shift at each frequency can be controlled. In one exemplary embodiment, by setting the gain of gain unit  306  equal to +0.25 and the gain of gain unit  310  equal to −0.25, the amount of phase shift, such as for audio signals from 0 hertz to 20,000 hertz, will vary in a predetermined manner for a given delay value between −180 degrees to +180 degrees, such that a frequency dependent phase shift will be generated in the processed signal. The delay length L 1  can be selected based on a number of sampled signals that are processed and the sample rate. In one exemplary embodiment, digital audio signals that are sampled at a rate of 44.1 kHz can be processed, where the length of the delay L 1  is equal to the number of samples divided by 44,100, or can alternatively be specified by the integer number of samples processed. 
         [0027]    Likewise, a second all pass filter is provided using summation unit  312 , gain units  316  and  320 , delay  314  with length L 2 , and summation unit  318 . The phase modified signal received from the first all pass filter is then modified by the second all pass filter, which has different phase shift characteristics based on the delay length L 2  and the amplitude settings of gain units  316  and  320 . 
         [0028]    Likewise, for the Nth channel processing chain, summation units  322  and  328 , gain units  326  and  330 , and delay  324  are used to form a first all pass filter having a delay length of 2N, and summation units  332  and  338 , gain units  336  and  340 , and delay  334  are used to form a second all pass filter having a delay length of 2N+1. The delay length of delay  324  and  334  are different from the delay length of delays  304  and  314 , but the total delay for each filter chain is equal, such that the total delay length of delay  304  and  314  equals the total delay length of delay  324  and  334 . In this manner, sound being processed from the first channel to the Nth channel will be phase shifted so as to eliminate any inadvertent phase correlation and thus widen the perceived sound field where the sound is being provided to speakers, to decorrelate any inadvertent phase correlations and reduce audio artifacts, such as where the sound is being recorded, and for other suitable purposes. 
         [0029]    Any suitable all pass filter may be substituted, with proper mathematical treatment, for the Schroeder sections discussed above. 
         [0030]      FIG. 4  is a diagram of a method  400  for processing a plurality of channels of audio data to decorrelate phase in accordance with an exemplary embodiment of the present invention. Method  400  can be used to widen the sound field or for other suitable purposes related to phase decorrelation, and can be implemented as software on a general purpose processor. 
         [0031]    Method  400  begins at steps  402  and  404  in parallel, where a first channel data stream is received at  402  and an Nth channel data stream is received at  404 . Likewise, additional channels can also be separately received, so that any suitable number of channels can be processed using method  400 . 
         [0032]    The method then proceeds to  406  and  408  in parallel, where the first channel data is processed with an all pass filter having a delay length of L 1 , and the Nth channel data is processed with an all pass filter having a delay length of 2N. Likewise, additional channels can be provided, each channel having a delay length that is different from the delay length of the all pass filter for any other channel. The method then proceeds to  410  and  412  in parallel. 
         [0033]    At  410 , the first channel data that has been processed with all pass filter L 1  is then processed with all pass filter L 2 , and at  412  the Nth channel data that has been processed with all pass filter L 2 N is processed with all pass filter L 2 N+1. Likewise, additional channels can each be processed by a corresponding second all pass filter, where the lengths of each of the corresponding second all pass filters are different from the lengths of the other second all pass filters. Nevertheless, the sum of the delay lengths of the first and second all pass filters for any channel of audio data should equal the sum of the delay lengths of the all pass filters for all other channels, so as to avoid loss of synchronization between the channels when real-time processing is performed. When the audio data is being processed for storage, the delay lengths for each filter chain can be different as long as the processed audio data for each channel is subsequently correlated so as to allow the audio data for each of the channels to be synchronized. The method then proceeds to  414  and  416  in parallel. 
         [0034]    At  414  and  416 , the phase modified first channel data is output and the phase modified Nth channel of data is output. Likewise, additional channels can be output. In this manner, the data can be provided to speakers, so as to generate an audio signal that has an apparent sound field that is wider than the unprocessed audio signal, where phase correlations between frequency bends of each channel can cause the sound field to collapse towards the center. Likewise, the decorrelated audio channels can be recorded for subsequent processing, such as mixing, or other suitable purposes. 
         [0035]    In operation, method  400  can be used to decorrelate the phase of channels of data, such as to widen the apparent sound field to a listener, to avoid creation of audio artifacts, or for other suitable purposes. Although two exemplary filter stages are disclosed, additional filter stages can also or alternatively be used where suitable, as long as the delay length of each filter chain is the same for each channel of data. 
         [0036]      FIG. 5  is a diagram of a system  500  for performing phase modification of an audio channel in accordance with an exemplary embodiment of the present invention. System  500  can be implemented in hardware, software, or a suitable combination of hardware and software, and can be one or more software systems operating on a general purpose processing platform. 
         [0037]    System  500  includes summation units  502  and  508 , gain units  506  and  510  and delay  504 , having a length of 2N. Likewise, a second all pass filter unit is comprised of summation units  512  and  518 , gain units  516  and  520 , and delay  514 , having a length of 2N+1. Although a single Nth channel filter chain is shown, a suitable number of channels can be selected, as long as each have a total delay length that is equal to the delay length of each other filter chain, but where the length of any individual delay unit  504  and  514  is different from each other delay unit. 
         [0038]    In operation, system  500  provides a different architecture for performing stereo or multi channel phase decorrelation. By selecting suitable gain values and different delay lengths, each all pass filter will create phase variations as a function of frequency in the processed signal that are different from the phase variations of the other all pass filters, so as to eliminate unwanted phase correlation. System  500  can be used for sound field widening, elimination of audio artifacts, or other suitable purposes. Additional filters can also or alternatively be added to the filter chain, so long as the total delay lengths of all filter chains are equal. 
         [0039]      FIG. 6  is a diagram of a system  600  for performing phase decorrelation and sound field widening in accordance with an exemplary embodiment of the present invention. System  600  utilizes a very long cascade of second order all pass filters, and can be implemented in hardware, software, or a suitable combination of hardware and software, and can be one or more software systems operating on a general purpose processing platform. Other suitable forms of all pass filters can also be substituted, with proper mathematical treatment, for the Schroder sections. The calculation of the many stages of gain elements for the second-order cascade that will provide useful phase-shift can likewise be developed accordingly. 
         [0040]      FIG. 7  is a diagram of a system  700  for sound field widening and phase decorrelation with gain modulation in accordance with an exemplary embodiment of the present invention. 
         [0041]    System  700  includes summation unit  702 , gain unit  706 , gain unit  710 , delay  704 , and summation unit  708 , which form an all pass filter configuration that may be referred to as a Schroeder section, and which provides an all pass filter of length L 1  when the gain factors of gain unit  706  and gain unit  710  are equal and opposite in sign, and which will pass all frequencies but will also add a phase shift depending on the frequency being passed, the delay, and the gain factor of gain unit  706  and gain unit  710 . Likewise, a second all pass filter is provided using summation unit  712 , gain units  716  and  720 , delay  714  with length L 2 , and summation unit  718 . The phase modified signal received from the first all pass filter is then modified by the second all pass filter, which has different phase shift characteristics based on the delay length L 2  and the amplitude settings of gain units  716  and  720 . 
         [0042]    Gain modulation units  722  and  724  are used to change the filter coefficients, such as by ramping them between −0.25 and 0.25 or other suitable values. The coefficients for one pair of gain units are maintained in quadrature with those of the other pair, so as to modulate the decorrelation by a third signal, creating a time-varying change in the phase shifts. This third signal can be a low frequency sine wave, a narrowband noise signal of low frequency, or other suitable signals. In this manner, reflection and cancellation patterns within a room or a listening environment are moved relative to the listener so as to prevent the listener&#39;s auditory system from perceiving the cancellations or reflections. While two gain modulation units are shown, a single unit or other suitable numbers of units can also or alternatively be used. 
         [0043]    In view of the above detailed description of the present invention and associated drawings, other modifications and variations are apparent to those skilled in the art. It is also apparent that such other modifications and variations may be effected without departing from the spirit and scope of the present invention.