Abstract:
Techniques for evaluating the audio quality of an audio test signal are disclosed. These techniques provide a quality analysis that takes into account spatial audio distortions between the audio test signal and a reference audio signal. These techniques involve, for example, determining a plurality of audio spatial cues for an audio test signal, determining a corresponding plurality of audio spatial cues for an audio reference signal, comparing the determined audio spatial cues of the audio test signal to the audio spatial cues of the audio reference signal, and determining the audio quality of the audio test signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     In general, the invention relates to sound quality assessment of processed audio files, and, more particularly, to evaluation of the sound quality of multi-channel audio files. 
     2. Description of the Related Art 
     In recent years, there has been a proliferation of digital media players (e.g., media players capable of playing digital audio files). Typically, these digital media players play digitally encoded audio or video files that have been “compressed” using any number of digital compression methods. Digital audio compression can be classified as ‘lossless’ or ‘lossy’. Lossless data compression allows the recovery of the exact original data that was compressed, while data compressed with lossy data compression yields data files that are different from the source files, but are close enough to be useful in some way. Typically, lossless compression is used to compress data files, such as computer programs, text files, and other files that must remain unaltered in order to be useful at a later time. Conversely, lossy data compression is commonly used to compress multimedia data, including audio, video, and picture files. Lossy compression is useful in multimedia applications such as streaming audio and/or video, music storage, and internet telephony. 
     The advantage of lossy compression over lossless compression is that a lossy method typically produces a much smaller file than a lossless compression would for the same file. This is advantageous in that storing or streaming digital media is most efficient with smaller file sizes and/or lower bit rates. However, files that have been compressed using lossy methods suffer from a variety of distortions, which may or may not be perceivable to the human ear or eye. Lossy methods often compress by focusing on the limitations of human perception, removing data that cannot be perceived by the average person. 
     In the case of audio compression, lossy methods can ignore or downplay sound frequencies that are known to be inaudible to the typical human ear. In order to model the human ear, for example, a psychoacoustic model can be used to determine how to compress audio without degrading the perceived quality of sound. 
     Audio files can typically be compressed at ratios of about 10:1 without perceptible loss of quality. Examples of lossy compression schemes used to encode digital audio files include MPEG-1 layer 2, MPEG-1 Layer 3 (MP3), MPEG-AAC, WMA, Dolby AC-3, Ogg Vorbis, and others. 
     Objective audio quality assessment aims at replacing expensive subjective listening tests (e.g., panels of human listeners) for audio quality evaluation. Objective assessment methods are generally fully automated, i.e. implemented on a computer with software. The interest in objective measures is driven by the demand for accurate audio quality evaluations, for instance to compare different audio coders or other audio processing devices. Commonly, in a testing scenario, the audio coder or other processing device is called a “device under test” (DUT).  FIG. 1  is a block diagram of an audio quality testing setup  100 . Reference audio signal  101  is input into the DUT  103 . The DUT  103  outputs a processed audio signal  105  (e.g., a digitally compressed audio file or stream that has been restored so that it can be heard). The processed audio signal  105  is then fed into the audio quality tester  107 , along with the original reference audio signal  101 . In the audio quality tester  107 , the processed audio signal  105  is compared to the reference audio signal  101  in order to determine the quality of the processed audio signal  105  output by the DUT  103 . A measure of output quality  109  is output by the audio quality tester  107 . 
     Transparent quality, i.e. best quality, is achieved if the processed audio signal  105  is indistinguishable from the reference audio signal  101  by any listener. The quality may be degraded if the processed signal  107  has audible distortions produced by the DUT  103 . 
     Various conventional approaches to audio quality assessment are given by the recommendation outlined in ITU-R, “Rec. ITU-R BS.1387 Method for Objective Measurements of Perceived Audio Quality,” 1998, hereafter “PEAQ”, which is hereby incorporated by reference in its entirety. 
     PEAQ takes into account properties of the human auditory system. For example, if the difference between the processed audio signal  105  and reference signal  101  falls below the human hearing threshold, it will not degrade the audio quality. Fundamental properties of hearing that have been considered include the auditory masking effect. 
     However, objective assessment techniques do not employ appropriate measures to estimate deviations of the evoked auditory spatial image of a multi-channel audio signal (e.g., 2-channel stereo, 5.1 channel surround sound, etc.). Spatial image distortions are commonly introduced by low-bit rate audio coders, such as MPEG-AAC or MPEG-Surround. MPEG-AAC, for instance, provides tools for joint-channel coding, for instance “intensity stereo coding” and “sum/difference coding”. The potential coding distortions caused by joint-channel coding techniques cannot be appropriately estimated by conventional assessment tools such as PEAQ simply because each audio channel is processed separately and properties of the spatial image are not taken into account. 
       FIG. 2  is a block diagram of the PEAQ quality assessment tool, which only supports 1 channel mono or 2-channel stereo audio. More than 2 channels are not supported. 
     The objective quality assessment tool  200  implements PEAQ above is divided into two main functional blocks as shown in  FIG. 2 . The first block  201  is a psychoacoustic model, which acts as a distortion analyzer. This block compares corresponding monaural or stereophonic channels of a reference signal  203  and a test signal  205  and produces a number of Model Output Variables (MOVs)  207 . Both the reference signal  203  and the test signal  205  can be any number of channels, from monaural to multi-channel surround sound. The MOVs  207  are specific distortion measures; each of them quantifies a certain type of distortion by one value per channel. These values are subsequently averaged over all channels and output to the second major block, a neural network  209 . The neural network  209  combines all MOVs  207  to derive an objective audio quality  211 . 
     In PEAQ, since the distortions are independently analyzed in each audio channel, there is no explicit evaluation of auditory spatial image distortion. For many types of audio signals this lack of spatial image distortion analysis can cause inaccurate objective quality estimations, leading to unsatisfactory quality assessments. Thus, an audio signal may have a high quality rating according to the PEAQ standard, yet have severe spatial image distortions. This is highly undesirable in the case of high fidelity or high definition sound recordings where spatial cues are crucial to the recording, such as multi-channel (i.e., two or more channels) sound systems. 
     Accordingly, there is a demand for objective audio quality assessment techniques capable of evaluating spatial as well as other audio distortions in a multi-channel audio signal. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, the invention pertains to techniques for assessing the quality of processed audio. More specifically, the invention pertains to techniques for assessing spatial and non-spatial distortions of a processed audio signal. The spatial and non-spatial distortions include the output of any audio processor (hardware or software) that changes the audio signal in any way which may modify the spatial image (e.g., a stereo microphone, an analog amplifier, a mixing console, etc.) 
     According to one embodiment, the invention pertains to techniques for assessing the quality of an audio signal in terms of audio spatial distortion. Additionally, other audio distortions can be considered in combination with audio spatial distortion, such that a total audio quality for the audio signal can be determined. 
     In general, audio distortions include any deformation of an audio waveform, when compared to a reference waveform. These distortions include, for example: clipping, modulation distortions, temporal aliasing, and/or spatial distortions. A variety of other audio distortions exist, as will be understood by those familiar with the art. 
     In order to include degradations of an auditory spatial image into quality assessment schemes, a set of spatial image distortion measures that are suitable to quantify deviations of the auditory image between a reference signal and a test signal are employed. According to one embodiment of the invention, spatial image distortions are determined by comparing a set of audio spatial cues derived from an audio test signal to the same audio spatial cues derived from an audio reference signal. These auditory spatial cues determine, for example, the lateral position of a sound image and the sound image width of an input audio signal. 
     In one embodiment of the invention, the quality of an audio test signal is analyzed by determining a plurality of audio spatial cues for an audio test signal, determining a corresponding plurality of audio spatial cues for an audio reference signal, comparing the determined audio spatial cues of the audio test signal to the audio spatial cues of the audio reference signal to produce comparison information, and determining the audio spatial quality of the audio test signal based on the comparison information. 
     In another embodiment of the invention, the quality of a multi-channel audio test signal is analyzed by selecting a plurality of audio channel pairs in an audio test signal, selecting a corresponding plurality of audio channel pairs in an audio reference signal, and determining the audio quality of the multi-channel audio test signal by comparing each of the plurality of audio channel pairs of the audio test sample to the corresponding audio channel pairs of the reference audio sample. 
     In still another embodiment of the invention, the quality of a multi-channel audio test signal is analyzed by determining a plurality of audio spatial cues for a multi-channel audio test signal, determining a corresponding plurality of audio spatial cues for a multi-channel audio reference signal, downmixing the multi-channel audio test signal to a single channel, downmixing the multi-channel audio reference signal to a single channel, determining audio distortions for the downmixed audio test signal, determining audio distortions for the downmixed audio reference signal, and determining the quality of the audio test signal based on the plurality of audio spatial cues of the multi-channel audio test signal, the plurality of audio spatial cues of the multi-channel audio reference signal, the audio distortions of the downmixed audio test signal, and the downmixed audio reference signal. 
     Other aspects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG. 1  is a block diagram of an audio quality testing setup. 
         FIG. 2  is a block diagram of the PEAQ objective quality assessment tool. 
         FIG. 3  is a block diagram of a spatial image distortion determiner according to one embodiment of the invention. 
         FIG. 4  is a flow diagram of a spatial image distortion evaluation process according to one embodiment of the invention. 
         FIG. 5  is an illustration of various multi-channel audio configurations and corresponding audio channel pairs according to one embodiment of the invention. 
         FIG. 6  is a block diagram of a spatial image distortion evaluation process according to one embodiment of the invention. 
         FIG. 7A  is a block diagram of an exemplary audio quality analyzer according to one embodiment of the invention. 
         FIG. 7B  is a block diagram of an exemplary audio quality analyzer according to one embodiment of the invention. 
         FIG. 8  is an exemplary spatial cue analyzer according to one embodiment of the invention. 
         FIG. 9  is an exemplary time-frequency grid according to one embodiment of the invention. 
         FIG. 10A  is an exemplary spatial cue analyzer according to one embodiment of the invention. 
         FIG. 10B  is an exemplary diagram showing the integration of spatial distortion measures according to one embodiment of the invention. 
         FIG. 10C  is an exemplary diagram showing an artificial neural network according to one embodiment of the invention. 
         FIG. 11  is an exemplary diagram showing one option for generating conventional distortion measures according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Broadly speaking, the invention pertains to techniques for assessing the quality of processed audio. More specifically, the invention pertains to techniques for assessing spatial and non-spatial distortions of a processed audio signal. The spatial and non-spatial distortions include the output of any audio processor (hardware or software) that changes the audio signal in any way which may modify the spatial image (e.g., a stereo microphone, an analog amplifier, a mixing console, etc.) 
     According to one embodiment, the invention pertains to techniques for assessing the quality of an audio signal in terms of audio spatial distortion. Additionally, other audio distortions can be considered in combination with audio spatial distortion, such that a total audio quality for the audio signal can be determined. 
     In general, audio distortions include any deformation of an audio waveform, when compared to a reference waveform. These distortions include, for example: clipping, modulation distortions, temporal aliasing, and/or spatial distortions. A variety of other audio distortions exist, as will be understood by those familiar with the art. 
     In order to include degradations of an auditory spatial image into quality assessment schemes, a set of spatial image distortion measures that are suitable to quantify deviations of the auditory image between a reference signal and a test signal are employed. According to one embodiment of the invention, spatial image distortions are determined by comparing a set of audio spatial cues derived from an audio test signal to the same audio spatial cues derived from an audio reference signal. These auditory spatial cues determine, for example, the lateral position of a sound image and the sound image width of an input audio signal. 
       FIG. 3  is a block diagram of a spatial image distortion determiner  300  according to one embodiment of the invention. Audio test signal  301  is input into an audio spatial cue determiner  303 , which outputs a set of audio spatial cues  305  for the audio test signal  301 . Audio reference signal  307  is also input into the audio spatial cue determiner  303 , yielding a set of audio spatial cues  309 . These audio signals  301  and  307  can be any multi-channel input (e.g., stereo, 5.1 surround sound, etc.) 
     According to one embodiment of the invention, three spatial cues are output for each input. For example, the spatial cues can be an inter-channel level difference spatial cue (ICLD), an inter-channel time delay spatial cue (ICTD), and an inter-channel coherence spatial cue (ICC). Those familiar with the art will understand that other spatial distortions can additionally or alternatively be determined. 
     The audio spatial cues  305  for the audio test signal  301  and the audio spatial cues  309  for the audio reference signal  307  are compared in spatial image distortion determiner  311 , and a set of spatial image distortions  313  are output. The set of spatial image distortions  313  has a distortion measure for each spatial cue input. For example, according to the above embodiment, a spatial image distortion can be determined for each of the ICLD, ICTD, and ICC audio spatial cues. 
       FIG. 4  is a flow diagram of a spatial image distortion evaluation process  400  according to one embodiment of the invention.  FIG. 4  begins with the selection  401  of an audio signal to analyze. The audio signal will be compared to a reference audio signal in order to determine spatial and other audio distortions. For example, the audio signal can be an MP3 file and the reference audio signal can be the original audio from which the MP3 was created. Next, one or more spatial image distortions, for example, those derived from comparisons of audio spatial cues ICLD, ICTD, and ICC as discussed above in reference to  FIG. 3  can be determined  403 . 
     Spatial image distortions rarely occur in isolation—they are usually accompanied by other distortions. This is especially true for audio coders, which typically trade off image distortions and other types of distortions to maximize overall quality. Thus, spatial image distortion measures can be combined with conventional distortion measures in order to assess overall audio quality. The spatial image distortion evaluation process  400  continues with a determination  405  of conventional audio distortions, for instance non-spatial audio distortions such as compression artifacts. Next, the audio distortions and spatial image distortions are used to determine  407  a spatial audio quality of the audio signal. There are various ways to determine the spatial audio quality of the audio signal. For instance, as one example, the spatial audio quality may be determined by feeding the spatial image distortions and other audio distortions, for example the PEAQ MOVs  207  discussed above in reference to  FIG. 2 , into an artificial neural network that has been taught to evaluate audio quality based on how the human auditory system perceives sound. Typically, the neural network&#39;s parameters are derived from a training procedure, which aims at minimizing the difference between known subjective quality grades from listening tests (i.e., as determined by human listeners) and the neural network output. 
     According to one embodiment of the invention, spatial image distortion measures, for example the spatial image distortions discussed above in reference to  FIG. 3 , are applied to audio signals with two or more channels. For instance, the spatial image distortions that are determined in block  405  in  FIG. 4  can be calculated for one or more channel pairs. In the case of multi-channel audio signals, a plurality of channel pairs are evaluated. 
       FIG. 5  is an illustration of various multi-channel audio configurations and corresponding channel pairs according to one embodiment of the invention. According to one embodiment of the invention, spatial image distortions, for example ICLD, ICTD, and ICC as discussed above in reference to  FIG. 3 , are independently calculated for each channel pair. A channel pair  501 , i.e., a signal supplied to a set of audio headphones (a binaural signal) is one exemplary configuration. Next, a channel pair  503  is another exemplary configuration. This configuration is supplied to a conventional stereo music system. Third, a five-channel audio group  505  is another exemplary configuration. This configuration as supplied to a surround-sound audio system is represented by six channel pairs, Left/Center  507 , Center/Right  509 , Left/Right  511 , Left-Surround/Right-Surround  513 , Center/Left-Surround  515 , and Center/Right-Surround  517 . Clearly, other pairs are possible. 
     According to one embodiment of the invention, spatial image distortions are independently calculated for each channel pair. Other multi-channel sound encoding types, including 6.1 channel surround, 7.1 channel surround, 10.2 channel surround, and 22.2 channel surround can be evaluated as well. 
       FIG. 6  is a block diagram of a spatial image distortion evaluation process  600  according to one embodiment of the invention. The spatial image distortion evaluation process  600  can determine these spatial image distortions from, for example, the three spatial image distortions (derived from ICLD, ICTD, and ICC) as discussed above in reference to  FIG. 3 . Further, the spatial image distortion evaluation process  600  can be performed, for example, on any of the channel configurations discussed above in reference to  FIG. 4 . 
     The spatial image distortion evaluation process  600  begins with selecting  601  of a multi-channel audio signal. For example, the audio signal can be a two-channel MP3 file (i.e., a decoded audio file) and the reference audio signal can be the unprocessed two-channel audio that was compressed to create that MP3 file. Next, a channel pair is selected  603  for comparison. After the channel pair is selected  603 , a time segment of the audio signal to be compared can be selected. 
     An analysis can then be performed to determine spatial image distortions of the multi-channel audio signal. This analysis can employ, for example, uniform energy-preserving filter banks such as the FFT-based analyzer in Christof Faller and Frank Baumgarte, “Binaural Cue Coding—Part II: Schemes and Applications,” IEEE Trans. Audio, Speech, and Language Proc., Vol. 11, No. 6, November 2003, pp. 520-531, which is hereby incorporated by reference in its entirety, or the QMF-based analyzer in ISO/IEC, “Information Technology—MPEG audio technologies—Part 1: MPEG Surround,” ISO/IEC FDIS 23003-1:2006(E), Geneva, 2006, and ISO/IEC, “Technical Description of Parametric Audio Coding for High Quality Audio,” ISO/IEC 14496-3-2005(E) Subpart 8, Geneva, 2005, both hereby incorporated by reference in their entirety. For complexity reasons, a filter bank with uniform frequency resolution is commonly used to decompose the audio input into a number of frequency sub-bands. Some or all of the frequency sub-bands are analyzed, typically those sub-bands that are audible to the human ear. In one embodiment of the invention, sub-bands are selected to match the “critical bandwidth” of the human auditory system. This is done in order to derive a frequency resolution that is more appropriate for modeling human auditory perception. 
     The spatial image distortion evaluation process  600  continues with selection  607  of a frequency sub-band for analysis. Next, the spatial image distortions are determined  609  for the selected frequency sub-band. A decision  611  then determines if there are more frequency sub-bands to be analyzed. If so, the next frequency sub-band is selected  613  and the spatial image distortion evaluation process  600  continues to block  609  and subsequent blocks to analyze the spatial image distortions for such frequency sub-band. 
     On the other hand, if there are no more frequency sub-bands to analyze, the spatial image distortion evaluation process  600  continues with a decision  615  that determines if there are more time segments to analyze. If there are more time segments to analyze, the next time segment is selected  617  and the spatial image distortion process  600  continues to block  607  and subsequent blocks. Otherwise, if there are no more time segments to analyze, a decision  619  determines if there are more channel pairs to be analyzed. If there are, then the next channel pair is selected  621  and the spatial image distortion evaluation process  600  continues to block  603  and subsequent blocks. 
     If there are no more channel pairs to be analyzed, then the end of the multi-channel audio signal has been reached (i.e., the entire multi-channel audio signal has been analyzed), and the spatial image distortion evaluation process  600  continues with a evaluation  623  of the spatial image distortions for the multi-channel audio signal and the process ends. 
     Those familiar with the art will understand that the order in which the time-segment and frequency sub-bands loops are analyzed are matters of programming efficiency and will vary. For example, in  FIG. 6 , the time-segment loop is nested, but could alternatively be the outer loop instead of the channel-pair selection loop being the outer loop. 
       FIG. 7A  is a block diagram of an exemplary audio quality analyzer  700  according to one embodiment of the invention. 
     An audio test signal  701  and an audio reference signal  703  are supplied to the audio quality analyzer  700 . The audio test signal  701  can be, for example, a two-channel MP3 file (i.e., a decoded audio file) and the reference audio signal  703  can be, for example, the unprocessed two-channel audio that was compressed to create test audio signal  701 . The audio test signal  701  and the reference audio signal  703  are both fed into a spatial image distortion analyzer  705  and into an audio distortion analyzer  707 . 
     The audio quality analyzer  700  has a neural network  709  that takes outputs  711  from the spatial image distortion analyzer  705  and outputs  713  from the audio distortion analyzer  707 . The outputs  711  from the spatial image distortion analyzer  705  can be, for example, the spatial image distortions  313  of the spatial distortion determiner  300  described above in  FIG. 3 . The outputs  713  from the audio distortion analyzer  707  can be, for example, the PEAQ MOVs  207  described above in  FIG. 2 . 
     The neural network  709  can be a computer program that has been taught to evaluate audio quality based on how the human auditory system perceives sound. Typically, parameters used by the neural network  709  are derived from a training procedure, which aims at minimizing the difference between known subjective quality grades from listening tests (i.e., as determined by human listeners) and the neural network output  705 . Thus, the neural network output  715  is an objective (i.e., a calculatable number) overall quality assessment of the quality of the audio test signal  701  as compared to the reference audio signal  703 . 
       FIG. 7B  is a block diagram of an exemplary audio quality analyzer  750  according to a second embodiment of the invention. 
     A multi-channel audio test signal  751  and a multi-channel audio reference signal  753  are supplied to the simplified audio quality analyzer  755 . The multi-channel audio test signal  751  can be, for example, a two-channel MP3 file (i.e., a decoded audio file) and the multi-channel reference audio signal  753  can be, for example, the unprocessed two-channel audio that was compressed to create test audio signal  751 . The multi-channel audio test signal  751  is fed into a spatial image distortion analyzer  757 . 
     The multi-channel audio test signal  751  and the multi-channel audio reference signals are also down-mixed to mono in downmixer  759 . The monaural outputs of downmixer  759  (monaural audio test signal  761  and monaural audio reference signal  761 ′) are fed into an audio distortion analyzer  763 . This embodiment has the advantage of lower computational complexity in the audio distortion analyzer  763  as compared to the audio distortion analyzer  705  in  FIG. 7A  since only a single downmixed channel (mono) is analyzed. 
     The audio quality analyzer  750  has a neural network  765  that takes outputs  767  from the spatial image distortion analyzer  757  and outputs  769  from the audio distortion analyzer  763 . The outputs  757  from the spatial image distortion analyzer  757  can be, for example, the spatial image distortion outputs  313  of the spatial distortion determiner  300  described above in  FIG. 3 . The outputs  769  from the audio distortion analyzer  763  can be, for example, the PEAQ MOVs  207  described above in  FIG. 2 . 
     The neural network  765  can be a computer program that been taught to evaluate audio quality based on how the human auditory system perceives sound. Typically, the parameters used by the neural network  765  are derived from a training procedure, which aims at minimizing the difference between known subjective quality grades from listening tests (i.e., as determined by human listeners) and the neural network output  771 . Thus, the neural network output  771  is an objective (i.e., calculatable) overall quality assessment of the quality of the audio test signal  751  as compared to the reference audio signal  753 . 
     IMPLEMENTATION EXAMPLE 
     An exemplary implementation of a spatial audio quality assessment is described below. 
     The estimation of spatial cues can be implemented in various ways. Two examples are given in Frank Baumgarte and Christof Faller, “Binaural Cue Coding—Part I: Psychoacoustic Fundamentals and Design Principles,” IEEE Trans. Audio, Speech, and Language Proc., Vol. 11, No. 6, November 2003, which is hereby incorporated by reference in its entirety, and in “Binaural Cue Coding—Part II: Schemes and Applications,” referenced above. Alternative implementations can be found in ISO/IEC, “Information Technology—MPEG audio technologies—Part 1: MPEG Surround,” ISO/IEC FDIS 23003-1:2006(E), Geneva, 2006, and ISO/IEC, “Technical Description of Parametric Audio Coding for High Quality Audio,” ISO/IEC 14496-3-2005(E) Subpart 8, Geneva, 2005, both of which are hereby incorporated by reference in their entirety. 
     A spatial cue analyzer  800  is shown in  FIG. 8 . The input consists of the audio signals of a channel pair  801  with channels x 1 (n) and x 2 (n), where n is a time index indicating which time segment of the audio signal is being analyzed (as described in  605  of  FIG. 6 ). Each signal is divided into Z sub-bands  805  with approximately critical bandwidth in filter bank  803 . In each band, three spatial cues  809 ,  809 ′ and  809 ″ are calculated in spatial cue determiner  807 . Each of the three spatial cues  809  are then mapped with a psychoacoustically-motivated function ( 811 ,  811 ′, and  811 ″) so that the output is proportional to the perceived auditory image distortion. The mapping characteristics are different for each of the three cues. The outputs of the spatial analyzer consist of mapped spatial cues  813  (C L (q)),  813 ′ (C T (q)), and  813 ″ (C C (q)). These values may be updated at a lower rate than the input audio signal, hence the different time index q. 
     A specific set of formulas for spatial cue estimation are described. However, a different way may be chosen to calculate the cues depending on the tradeoff between accuracy and computational complexity for a given application. The formulas given here can be applied in systems that employ uniform energy-preserving filter banks such as the FFT-based analyzer in or the QMF-based analyzer in “Binaural Cue Coding—Part II: Schemes and Applications,” referenced above. The time-frequency grid obtained from such an analyzer is shown in  FIG. 9 , which shows a set of time-frequency tiles  901  illustrating the filter bank resolution in time (index k) and frequency (index m). The left side illustrates that several filter bank bands  903  are included in a critical band (index z). At the bottom, a time interval with index q can contain several time samples. Each of the uniform tiles illustrates the corresponding time and frequency of one output value of the filter bank. 
     For complexity reasons a filter bank with uniform frequency resolution is commonly used to decompose the audio input into a number of M sub-bands. In contrast, the frequency resolution of the auditory system gradually decreases with increasing frequency. The bandwidth of the auditory system is called “critical” bandwidth and the corresponding frequency bands are referred to as critical bands. In order to derive a frequency resolution that is more appropriate for modeling auditory perception, several neighboring uniform frequency bands are combined to approximate a critical band with index z as shown in  FIG. 9 . 
     The ICLD ΔL for a time-frequency tile (shown as bold outlined rectangle  901  in  FIG. 9 ) of an audio channel pair of channels i and j is computed according to (1). The tile sizes are controlled by the functions for the time interval boundary, k1(q) and k2(q), and the critical band boundaries, m1(z) and m2(z). The normalized cross-correlation Φ for a time-frequency tile is given in (2). The cross-correlation is calculated for a range of delays d, which correspond to an audio signal delay range of −1 to 1 ms. The ICTD τ is then derived from the delay d at the maximum absolute cross-correlation value as given in (3). Finally, the ICC Ψ is the cross-correlation at delay d=τ according to (4). 
     
       
         
           
             
               
                 
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     The three spatial cues are then mapped to a scale, which is approximately proportional to the perceived spatial image change. For example, a very small change of a cross-correlation of 1 is audible, but such a change is inaudible if the cross-correlation is only 0.5. Or, a small change of a level difference of 40 is not audible, but it could be audible if the difference is 0. The mapping functions for the three cues are H L , H T , and H C , respectively.
 
 C   L ( q )= H   L (Δ L ( q ))  (5)
 
 C   T ( q )= H   T (τ( q ))  (6)
 
 C   C ( q )= H   C (Ψ( q ))  (7)
 
     An example of a mapping function for ICLDs: 
     
       
         
           
             
               C 
               L 
             
             = 
             
               
                 1 
                 
                   1 
                   + 
                   
                     ⅇ 
                     
                       
                         - 
                         0.15 
                       
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       L 
                     
                   
                 
               
               = 
               
                 
                   H 
                   L 
                 
                 ⁡ 
                 
                   ( 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     L 
                   
                   ) 
                 
               
             
           
         
       
     
     An example of a mapping function for ICCs:
 
 C   C =(1.0119−Ψ) 0.4   =H   C (Ψ)
 
     An example of a mapping for ITDs: 
     
       
         
           
             
               C 
               T 
             
             = 
             
               
                 1 
                 
                   1 
                   + 
                   
                     ⅇ 
                     
                       
                         - 
                         2000 
                       
                       ⁢ 
                       
                         τ 
                         ⁡ 
                         
                           [ 
                           s 
                           ] 
                         
                       
                     
                   
                 
               
               = 
               
                 
                   H 
                   T 
                 
                 ⁡ 
                 
                   ( 
                   τ 
                   ) 
                 
               
             
           
         
       
     
     In order to estimate spatial image distortions, the mapped cues of corresponding channel pairs p of the reference and test signal are compared as outlined in  FIG. 10A . A spatial cue analyzer  1001  is applied to a reference channel pair  1003  and a test channel pair  1005 . The magnitude of the difference of the output is then calculated  1007  and integrated  1009  over time (for the whole duration of the audio signal). The integration can be done, for instance, by averaging the difference over time. At the output of this stage we have spatial distortion measures  1011  based on ICLD, ICTD, and ICC for each channel pair p and each critical band z, namely dC L,tot (z,p), dC T,tot (z,p), and dC C,tot (z,p), respectively. 
     Next, the spatial distortion measures  1011  are integrated  1013  over frequency, as shown in  FIG. 10B . The integration can be done, for instance, by simple averaging over all bands. For the final distortion measures, the values for all channel pairs are combined into a single value. This can be done by weighted averaging, where, for instance, the front channels in a surround configuration can be given more weight than the rear channels. The final three values which describe the spatial image distortions  1015  of the test audio signal with respect to the reference audio signal are D L,tot , D T,tot , and D C,tot . 
     Spatial image distortions rarely occur in isolation—they are usually accompanied by other distortions. This is especially true for audio coders, which typically trade off image distortions and other types of distortions to maximize overall quality. Therefore, spatial distortion distortions  1015  can be combined with conventional distortion measures in order to assess overall audio quality. The system in  FIG. 10C  shows an example of an Artificial Neural Network  1019  that combines spatial distortion measures  1015  and other distortion measures  1017 . The Neural Network parameters are usually derived from a training procedure, which teaches the neural network to emulate known subjective quality grades from listening tests (i.e., those performed by human listeners) to produce an objective (i.e., calculatable) overall quality assessment  1021 . 
     If only the spatial image distortion measures  1015  are applied to the Neural Network  1019 , the objective audio quality  1021  will predominantly reflect the spatial image quality only and ignore other types of distortions. This option may be useful for applications that can take advantage of an objective quality estimate that reflects spatial distortions only. 
     The other distortion measures  1017  besides the spatial distortions  1015  can be, for instance, the MOVs of PEAQ, or distortion measures of other conventional models. Another option for generating conventional distortion measures is shown in  FIG. 11 . A multi-channel reference input  1101  and a multi-channel test input  1103  are each down-mixed to mono before the PEAQ analyzer  1105  is applied. The output MOVs  1107  can be used in combination with the spatial distortion measures. This approach has the advantage of lower computational complexity and it removes the spatial image, which is generally considered irrelevant for PEAQ. 
     The advantages of the invention are numerous. Different embodiments or implementations may, but need not, yield one or more of the following advantages. One advantage is that spatial audio distortions can be objectively analyzed. Another advantage is using a downmixed signal to analyze conventional audio distortions can reduces computational complexity. Still another advantage is unlike PEAQ and other similar audio analyses, the invention allows for the analysis of multi-channel audio signals. 
     The many features and advantages of the present invention are apparent from the written description and, thus, it is intended by the appended claims to cover all such features and advantages of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, the invention should not be limited to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention.