Abstract:
In one embodiment the present invention includes a method of improving audibility of speech in a multi-channel audio signal. The method includes comparing a first characteristic and a second characteristic of the multi-channel audio signal to generate an attenuation factor. The first characteristic corresponds to a first channel of the multi-channel audio signal that contains speech and non-speech audio, and the second characteristic corresponds to a second channel of the multi-channel audio signal that contains predominantly non-speech audio. The method further includes adjusting the attenuation factor according to a speech likelihood value to generate an adjusted attenuation factor. The method further includes attenuating the second channel using the adjusted attenuation factor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Patent Application No. 61/046,271, filed Apr. 18, 2008, hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The invention relates to audio signal processing in general and to improving clarity of dialog and narrative in surround entertainment audio in particular. 
     Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
     Modern entertainment audio with multiple, simultaneous channels of audio (surround sound) provides audiences with immersive, realistic sound environments of immense entertainment value. In such environments many sound elements such as dialog, music, and effects are presented simultaneously and compete for the listener&#39;s attention. For some members of the audience—especially those with diminished auditory sensory abilities or slowed cognitive processing—dialog and narrative may be hard to understand during parts of the program where loud competing sound elements are present. During those passages these listeners would benefit if the level of the competing sounds were lowered. 
     The recognition that music and effects can overpower dialog is not new and several methods to remedy the situation have been suggested. However, as will be outlined next, the suggested methods are either incompatible with current broadcast practice, exert an unnecessarily high toll on the overall entertainment experience, or do both. 
     It is a commonly adhered-to convention in the production of surround audio for film and television to place the majority of dialog and narrative into only one channel (the center channel, also referred to as the speech channel). Music, ambiance sounds, and sound effects are typically mixed into both the speech channel and all remaining channels (e.g., Left [L], Right [R], Left Surround [ls] and Right Surround [rs], also referred to as the non-speech channels). As a result, the speech channel carries the majority of speech and a significant amount of the non-speech audio contained in the audio program, whereas the non-speech channels carry predominantly non-speech audio, but may also carry a small amount of speech. One simple approach to aiding the perception of dialog and narrative in these conventional mixes is to permanently reduce the level of all non-speech channels relative to the level of the speech channel, for example by 6 dB. This approach is simple and effective and is practiced today (e.g., SRS [Sound Retrieval System] Dialog Clarity or modified downmix equations in surround decoders). However, it suffers from at least one drawback: the constant attenuation of the non-speech channels may lower the level of quiet ambiance sounds that do not interfere with speech reception to the point where they can no longer be heard. By attenuating non-interfering ambiance sounds the aesthetic balance of the program is altered without any attendant benefit for speech understanding. 
     An alternative solution is described in a series of patents (U.S. Pat. No. 7,266,501, U.S. Pat. No. 6,772,127, U.S. Pat. No. 6,912,501, and U.S. Pat. No. 6,650,755) by Vaudrey and Saunders. As understood, their approach involves modifying the content production and distribution. According to that arrangement, the consumer receives two separate audio signals. The first of these signals comprises the “Primary Content” audio. In many cases this signal will be dominated by speech but, if the content producer desires, may contain other signal types as well. The second signal comprises the “Secondary Content” audio, which is composed of all the remaining sounds elements. The user is given control over the relative levels of these two signals, either by manually adjusting the level of each signal or by automatically maintaining a user-selected power ratio. Although this arrangement can limit the unnecessary attenuation of non-interfering ambiance sounds, its widespread deployment is hindered by its incompatibility with established production and distribution methods. 
     Another example of a method to manage the relative levels of speech and non-speech audio has been proposed by Bennett in U.S. Application Publication No. 20070027682. 
     All the examples of the background art share the limitation of not providing any means for minimizing the effect the dialog enhancement has on the listening experience intended by the content creator, among other deficiencies. It is therefore the object of the present invention to provide a means of limiting the level of non-speech audio channels in a conventionally mixed multi-channel entertainment program so that speech remains comprehensible while also maintaining the audibility of the non-speech audio components. 
     Thus, there is a need for improved ways of maintaining speech audibility. The present invention solves these and other problems by providing an apparatus and method of improving speech audibility in a multi-channel audio signal. 
     SUMMARY 
     Embodiments of the present invention improve speech audibility. In one embodiment the present invention includes a method of improving audibility of speech in a multi-channel audio signal. The method includes comparing a first characteristic and a second characteristic of the multi-channel audio signal to generate an attenuation factor. The first characteristic corresponds to a first channel of the multi-channel audio signal that contains speech and non-speech audio, and the second characteristic corresponds to a second channel of the multi-channel audio signal that contains predominantly non-speech audio. The method further includes adjusting the attenuation factor according to a speech likelihood value to generate an adjusted attenuation factor. The method further includes attenuating the second channel using the adjusted attenuation factor. 
     A first aspect of the invention is based on the observation that the speech channel of a typical entertainment program carries a non-speech signal for a substantial portion of the program duration. Consequently, according to this first aspect of the invention, masking of speech audio by non-speech audio may be controlled by (a) determining the attenuation of a signal in a non-speech channel necessary to limit the ratio of the signal power in the non-speech channel to the signal power in the speech channel not to exceed a predetermined threshold and (b) scaling the attenuation by a factor that is monotonically related to the likelihood of the signal in the speech channel being speech, and (c) applying the scaled attenuation. 
     A second aspect of the invention is based on the observation that the ratio between the power of the speech signal and the power of the masking signal is a poor predictor of speech intelligibility. Consequently, according to this second aspect of the invention, the attenuation of the signal in the non-speech channel that is necessary to maintain a predetermined level of intelligibility is calculated by predicting the intelligibility of the speech signal in the presence of the non-speech signals with a psycho-acoustically based intelligibility prediction model. 
     A third aspect of the invention is based on the observations that, if attenuation is allowed to vary across frequency, (a) a given level of intelligibility can be achieved with a variety of attenuation patterns, and (b) different attenuation patterns can yield different levels of loudness or salience of the non-speech audio. Consequently, according to this third aspect of the invention, masking of speech audio by non-speech audio is controlled by finding the attenuation pattern that maximizes loudness or some other measure of salience of the non-speech audio under the constraint that a predetermined level of predicted speech intelligibility is achieved. 
     The embodiments of the present invention may be performed as a method or process. The methods may be implemented by electronic circuitry, as hardware or software or a combination thereof. The circuitry used to implement the process may be dedicated circuitry (that performs only a specific task) or general circuitry (that is programmed to perform one or more specific tasks). 
     The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a signal processor according to one embodiment of the present invention. 
         FIG. 2  illustrates a signal processor according to another embodiment of the present invention. 
         FIG. 3  illustrates a signal processor according to another embodiment of the present invention. 
         FIGS. 4A-4B  are block diagrams illustrating further variations of the embodiments of  FIGS. 1-3 . 
     
    
    
     DETAILED DESCRIPTION 
     Described herein are techniques for maintaining speech audibility. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein. 
     Various method and processes are described below. That they are described in a certain order is mainly for ease of presentation. It is to be understood that particular steps may be performed in other orders or in parallel as desired according to various implementations. When a particular step must precede or follow another, such will be pointed out specifically when not evident from the context. 
     The principle of the first aspect of the invention is illustrated in  FIG. 1 . Referring now to  FIG. 1 , a multi-channel signal consisting of a speech channel ( 101 ) and two non-speech channels ( 102  and  103 ) is received. The power of the signals in each of these channels is measured with a bank of power estimators ( 104 ,  105 , and  106 ) and expressed on a logarithmic scale [dB]. These power estimators may contain a smoothing mechanism, such as a leaky integrator, so that the measured power level reflects the power level averaged over the duration of a sentence or an entire passage. The power level of the signal in the speech channel is subtracted from the power level in each of the non-speech channels (by adders  107  and  108 ) to give a measure of the power level difference between the two signal types. Comparison circuit  109  determines for each non-speech channel the number of dB by which the non-speech channel must be attenuated in order for its power level to remain at least Θ dB below the power level of the signal in the speech channel. (The symbol “Θ” denotes a variable and may also be referred to as script theta.) According to one embodiment, one implementation of this is to add the threshold value Θ (stored by the circuit  110 ) to the power level difference (this intermediate result is referred to as the margin) and limit the result to be equal to or less than zero (by limiters  111  and  112 ). The result is the gain (or negated attenuation) in dB that must be applied to the non-speech channels to keep their power level Θ dB below the power level of the speech channel. A suitable value for Θ is 15 dB. The value of Θ may be adjusted as desired in other embodiments. 
     Because there is a unique relation between a measure expressed on a logarithmic scale (dB) and that same measure expressed on a linear scale, a circuit that is equivalent to  FIG. 1  can be built where power, gain, and threshold all are expressed on a linear scale. In that implementation all level differences are replaced by ratios of the linear measures. Alternative implementations may replace the power measure with measures that are related to signal strength, such as the absolute value of the signal. 
     One noteworthy feature of the first aspect of the invention is to scale the gain thus derived by a value monotonically related to the likelihood of the signal in the speech channel in fact being speech. Still referring to  FIG. 1 , a control signal ( 113 ) is received and multiplied with the gains (by multipliers  114  and  115 ). The scaled gains are then applied to the corresponding non-speech channels (by amplifiers  116  and  117 ) to yield the modified signals L′ and R′ ( 118  and  119 ). The control signal ( 113 ) will typically be an automatically derived measure of the likelihood of the signal in the speech channel being speech. Various methods of automatically determining the likelihood of a signal being a speech signal may be used. According to one embodiment, a speech likelihood processor  130  generates the speech likelihood value p ( 113 ) from the information in the C channel  101 . One example of such a mechanism is described by Robinson and Vinton in “Automated Speech/Other Discrimination for Loudness Monitoring” (Audio Engineering Society, Preprint number 6437 of Convention 118, May 2005). Alternatively, the control signal ( 113 ) may be created manually, for example by the content creator and transmitted alongside the audio signal to the end user. 
     Those skilled in the art will easily recognize how the arrangement can be extended to any number of input channels. 
     The principle of the second aspect of the invention is illustrated in  FIG. 2 . Referring now to  FIG. 2 , a multi-channel signal consisting of a speech channel ( 101 ) and two non-speech channels ( 102  and  103 ) is received. The power of the signals in each of these channels is measured with a bank of power estimators ( 201 ,  202 , and  203 ). Unlike their counterparts in  FIG. 1 , these power estimators measure the distribution of the signal power across frequency, resulting in a power spectrum rather than a single number. The spectral resolution of the power spectrum ideally matches the spectral resolution of the intelligibility prediction model ( 205  and  206 , not yet discussed). 
     The power spectra are fed into comparison circuit  204 . The purpose of this block is to determine the attenuation to be applied to each non-speech channel to ensure that the signal in the non-speech channel does not reduce the intelligibility of the signal in the speech channel to be less than a predetermined criterion. This functionality is achieved by employing an intelligibility prediction circuit ( 205  and  206 ) that predicts speech intelligibility from the power spectra of the speech signal ( 201 ) and non-speech signals ( 202  and  203 ). The intelligibility prediction circuits  205  and  206  may implement a suitable intelligibility prediction model according to design choices and tradeoffs. Examples are the Speech Intelligibility Index as specified in ANSI S3.5-1997 (“Methods for Calculation of the Speech Intelligibility Index”) and the Speech Recognition Sensitivity model of Muesch and Buus (“Using statistical decision theory to predict speech intelligibility. I. Model structure” Journal of the Acoustical Society of America, 2001, Vol 109, p 2896-2909). It is clear that the output of the intelligibility prediction model has no meaning when the signal in the speech channel is something other than speech. Despite this, in what follows the output of the intelligibility prediction model will be referred to as the predicted speech intelligibility. The perceived mistake will be accounted for in subsequent processing by scaling the gain values output from the comparison circuit  204  with a parameter that is related to the likelihood of the signal being speech ( 113 , not yet discussed). 
     The intelligibility prediction models have in common that they predict either increased or unchanged speech intelligibility as the result of lowering the level of the non-speech signal. Continuing on in the process flow of  FIG. 2 , the comparison circuits  207  and  208  compare the predicted intelligibility with a criterion value. If the level of the non-speech signal is low so that the predicted intelligibility exceeds the criterion, the gain parameter, which is initialized to 0 dB, is retrieved from circuit  209  or  210  and provided to the circuits  211  and  212  as the output of comparison circuit  204 . If the criterion is not met, the gain parameter is decreased by a fixed amount and the intelligibility prediction is repeated. A suitable step size for decreasing the gain is 1 dB. The iteration as just described continues until the predicted intelligibility meets or exceeds the criterion value. It is of course possible that the signal in the speech channel is such that the criterion intelligibility cannot be reached even in the absence of a signal in the non-speech channel. An example of such a situation is a speech signal of very low level or with severely restricted bandwidth. If that happens a point will be reached where any further reduction of the gain applied to the non-speech channel does not affect the predicted speech intelligibility and the criterion is never met. In such a condition, the loop formed by ( 205 , 206 ), ( 207 , 208 ), and ( 209 , 210 ) continues indefinitely, and additional logic (not shown) may be applied to break the loop. One particularly simple example of such logic is to count the number of iterations and exit the loop once a predetermined number of iterations has been exceeded. 
     Continuing on in the process flow of  FIG. 2 , a control signal p ( 113 ) is received and multiplied with the gains (by multipliers  114  and  115 ). The control signal ( 113 ) will typically be an automatically derived measure of the likelihood of the signal in the speech channel being speech. Methods of automatically determining the likelihood of a signal being a speech signal are known per se and were discussed in the context of  FIG. 1  (see the speech likelihood processor  130 ). The scaled gains are then applied to their corresponding non-speech channels (by amplifiers  116  and  117 ) to yield the modified signals R′ and L′ ( 118  and  119 ). 
     The principle of the third aspect of the invention is illustrated in  FIG. 3 . Referring now to  FIG. 3 , a multi-channel signal consisting of a speech channel ( 101 ) and two non-speech channels ( 102  and  103 ) is received. Each of the three signals is divided into its spectral components (by filter banks  301 ,  302 , and  303 ). The spectral analysis may be achieved with a time-domain N-channel filter bank. According to one embodiment, the filter bank partitions the frequency range into ⅓-octave bands or resembles the filtering presumed to occur in the human inner ear. The fact that the signal now consists of N sub-signals is illustrated by the use of heavy lines. The process of  FIG. 3  can be recognized as a side-branch process. Following the signal path, the N sub-signals that form the non-speech channels are each scaled by one member of a set of N gain values (by the amplifiers  116  and  117 ). The derivation of these gain values will be described later. Next, the scaled sub-signals are recombined into a single audio signal. This may be done via simple summation (by summation circuits  313  and  314 ). Alternatively, a synthesis filter-bank that is matched to the analysis filter bank may be used. This process results in the modified non-speech signals R′ and L′ ( 118  and  119 ). 
     Describing now the side-branch path of the process of  FIG. 3 , each filter bank output is made available to a corresponding bank of N power estimators ( 304 ,  305 , and  306 ). The resulting power spectra serve as inputs to an optimization circuit ( 307  and  308 ) that has as output an N-dimensional gain vector. The optimization employs both an intelligibility prediction circuit ( 309  and  310 ) and a loudness calculation circuit ( 311  and  312 ) to find the gain vector that maximizes loudness of the non-speech channel while maintaining a predetermined level of predicted intelligibility of the speech signal. Suitable models to predict intelligibility have been discussed in connection with  FIG. 2 . The loudness calculation circuits  311  and  312  may implement a suitable loudness prediction model according to design choices and tradeoffs. Examples of suitable models are American National Standard ANSI S3.4-2007 “Procedure for the Computation of Loudness of Steady Sounds” and the German standard DIN 45631 “Berechnung des Lautstärkepegels and der Lautheit aus dem Geräuschspektrum”. 
     Depending on the computational resources available and the constraints imposed, the form and complexity of the optimization circuits ( 307 ,  308 ) may vary greatly. According to one embodiment an iterative, multidimensional constrained optimization of N free parameters is used. Each parameter represents the gain applied to one of the frequency bands of the non-speech channel. Standard techniques, such as following the steepest gradient in the N-dimensional search space may be applied to find the maximum. In another embodiment, a computationally less demanding approach constrains the gain-vs.-frequency functions to be members of a small set of possible gain-vs.-frequency functions, such as a set of different spectral gradients or shelf filters. With this additional constraint the optimization problem can be reduced to a small number of one-dimensional optimizations. In yet another embodiment an exhaustive search is made over a very small set of possible gain functions. This latter approach might be particularly desirable in real-time applications where a constant computational load and search speed are desired. 
     Those skilled in the art will easily recognize additional constraints that might be imposed on the optimization according to additional embodiments of the present invention. One example is restricting the loudness of the modified non-speech channel to be not larger than the loudness before modification. Another example is imposing a limit on the gain differences between adjacent frequency bands in order to limit the potential for temporal aliasing in the reconstruction filter bank ( 313 ,  314 ) or to reduce the possibility for objectionable timbre modifications. Desirable constraints depend both on the technical implementation of the filter bank and on the chosen tradeoff between intelligibility improvement and timbre modification. For clarity of illustration, these constraints are omitted from  FIG. 3 . 
     Continuing on in the process flow of  FIG. 3 , a control signal p ( 113 ) is received and multiplied with the gains functions (by the multipliers  114  and  115 ). The control signal ( 113 ) will typically be an automatically derived measure of the likelihood of the signal in the speech channel being speech. Suitable methods for automatically calculating the likelihood of a signal being speech have been discussed in connection with  FIG. 1  (see the speech likelihood processor  130 ). The scaled gain functions are then applied to their corresponding non-speech channels (by amplifiers  116  and  117 ), as described earlier. 
       FIGS. 4A and 4B  are block diagrams illustrating variations of the aspects shown in  FIGS. 1-3 . In addition, those skilled in the art will recognize several ways of combining the elements of the invention described in  FIGS. 1 through 3 . 
       FIG. 4A  shows that the arrangement of  FIG. 1  can also be applied to one or more frequency sub-bands of L, C, and R. Specifically, the signals L, C, and R may each be passed through a filter bank ( 441 ,  442  and  443 ), yielding three sets of n sub-bands: {L 1  L 2 , . . . , L n }, {C 1 , C 2 , . . . , C n }, and {R 1 , R 2 , . . . , R n }. Matching sub-bands are passed to n instances of the circuit  125  illustrated in  FIG. 1 , and the processed sub signals are recombined (by the summation circuits  451  and  452 ). A separate threshold value Θ n  can be selected for each sub band. A good choice is a set where Θ n  is proportional to the average number of speech cues carried in the corresponding frequency region; i.e., bands at the extremes of the frequency spectrum are assigned lower thresholds than bands corresponding to dominant speech frequencies. This implementation of the invention offers a very good tradeoff between computational complexity and performance. 
       FIG. 4B  shows another variation. For example, to reduce the computational burden, a typical surround sound signal with five channels (C, L, R, ls, and rs) may be enhanced by processing the L and R signals according to the circuit  325  shown in  FIG. 3 , and the ls and rs signals, which are typically less powerful than the L and R signals, according to the circuit  125  shown in  FIG. 1 . 
     In the above description, the terms “speech” (or speech audio or speech channel or speech signal) and “non-speech” (or non-speech audio or non-speech channel or non-speech signal) are used. A skilled artisan will recognize that these terms are used more to differentiate from each other and less to be absolute descriptors of the content of the channels. For example, in a restaurant scene in a film, the speech channel may predominantly contain the dialogue at one table and the non-speech channels may contain the dialogue at other tables (hence, both contain “speech” as a layperson uses the term). Yet it is the dialogue at other tables that certain embodiments of the present invention are directed toward attenuating. 
     Implementation 
     The invention may be implemented in hardware or software, or a combination of both (e.g., programmable logic arrays). Unless otherwise specified, the algorithms included as part of the invention are not inherently related to any particular computer or other apparatus. In particular, various general-purpose machines may be used with programs written in accordance with the teachings herein, or it may be more convenient to construct more specialized apparatus (e.g., integrated circuits) to perform the required method steps. Thus, the invention may be implemented in one or more computer programs executing on one or more programmable computer systems each comprising at least one processor, at least one data storage system (including volatile and non-volatile memory and/or storage elements), at least one input device or port, and at least one output device or port. Program code is applied to input data to perform the functions described herein and generate output information. The output information is applied to one or more output devices, in known fashion. 
     Each such program may be implemented in any desired computer language (including machine, assembly, or high level procedural, logical, or object oriented programming languages) to communicate with a computer system. In any case, the language may be a compiled or interpreted language. 
     Each such computer program is preferably stored on or downloaded to a storage media or device (e.g., solid state memory or media, or magnetic or optical media) readable by a general or special purpose programmable computer, for configuring and operating the computer when the storage media or device is read by the computer system to perform the procedures described herein. The inventive system may also be considered to be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer system to operate in a specific and predefined manner to perform the functions described herein. 
     The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents will be evident to those skilled in the art and may be employed without departing from the spirit and scope of the invention as defined by the claims.