Patent Publication Number: US-9424830-B2

Title: Apparatus and method for encoding audio signal, system and method for transmitting audio signal, and apparatus for decoding audio signal

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-267142, filed on Dec. 6, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed in the specification are related to techniques for encoding, decoding, and transmitting an audio signal. 
     BACKGROUND 
     In multimedia broadcasting for mobile application, there is a demand for low-bit-rate transmission. For an audio signal such as that of a sound, an encoding is employed in which only a perceivable sound, for example, is encoded and transmitted taking a human auditory characteristic into consideration. 
     As a conventional technique for encoding, the following technique is known (for example, Japanese Patent Laid-Open No. 9-321628). An audio encoding apparatus includes: an input data memory for temporarily storing input audio signal data that is split into a plurality of frames; a frequency division filter bank for producing frequency-divided data for each frame; a psycho-acoustic analysis unit for receiving i number of frames with a frame which is sandwiched between the i number of frames, and for which a quantization step size is to be calculated, and calculating the quantization step size by using the result of a spectrum analysis for a pertinent frame and a human auditory characteristic including an effect of masking; a quantizer for quantizing an output of the frequency division filter bank with the quantization step size indicated by the psycho-acoustic analysis unit; and a multiplexer for multiplexing the data quantized by the quantizer. The psycho-acoustic analysis unit includes a spectrum calculator for performing a frequency analysis on a frame, a masking curve predictor for calculating a masking curve, and a quantization step size predictor for calculating the quantization step size. 
     Further, as another conventional technique, the following technique is known (for example, Japanese Patent Laid-Open No. 2007-271686). In the case of an audio signal such as that of music, many of the signal components (maskees) eliminated by compression are attenuated components that were maskers before. Thus, by giving reverberation to a decompressed audio signal, signal components that were maskers before but are now maskees are incorporated into a current signal to restore the audio signal of an original sound in a pseudo manner. Since a human auditory masking characteristic varies depending on frequency, the audio signal is divided into sub-band signals in a plurality of frequency bands, and reverberation of a characteristic conforming to a masking characteristic of each frequency band is given to the sub-band signal. 
     Moreover, the following technique is known (for example, National Publication of International Patent Application No. 2008-503793). In an encoder, an audio signal is divided into a signal portion with no echo and information on the reverberant field relating to the audio signal, and the audio signal is preferably divided with an expression using a very slight parameter such as a reverberation time and a reverberation amplitude. Then, the signal with no echo is encoded with an audio codec. In a decoder, the signal portion with no echo is restored with the audio codec.
     [Patent Document 1] Japanese Laid-open Patent Publication No. 09-321628   [Patent Document 2] Japanese Laid-open Patent Publication No. 2007-271686   [Patent Document 3] Japanese National Publication of International Patent Application No. 2008-503793   

     SUMMARY 
     According to an aspect of the embodiments, an audio signal encoding apparatus includes: a quantizer for quantizing an audio signal; a reverberation masking characteristic obtaining unit for obtaining a characteristic of reverberation masking that is exerted on a sound represented by the audio signal by reverberation of the sound generated in a reproduction environment by reproducing the sound; and a control unit for controlling a quantization step size of the quantizer based on the characteristic of the reverberation masking. 
     According to an aspect of the embodiments, there is provided an advantage of enabling an even lower bit rate. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration example of a common encoding apparatus for improving the sound quality of an input audio signal in encoding of the input audio signal; 
         FIG. 2  is a schematic diagram illustrating an operation and effect of the encoding apparatus according to the configuration of  FIG. 1 ; 
         FIG. 3  is a block diagram of an encoding apparatus of a first embodiment; 
         FIG. 4  is an explanatory diagram illustrating a reverberation characteristic  309  in the encoding apparatus of the first embodiment having the configuration of  FIG. 3 ; 
         FIG. 5A  and  FIG. 5B  are explanatory diagrams illustrating an encoding operation of the encoding apparatus of  FIG. 3  in the absence of reverberation and in the presence of reverberation; 
         FIG. 6  is a block diagram of an audio signal encoding apparatus of a second embodiment; 
         FIG. 7  is a diagram illustrating a configuration example of data stored in a reverberation characteristic storage unit  612 ; 
         FIG. 8  is a block diagram of a reverberation masking calculation unit  602  of  FIG. 6 ; 
         FIG. 9A ,  FIG. 9B , and  FIG. 9C  are explanatory diagrams illustrating an example of masking calculation in the case of using frequency masking that reverberation exerts on the sound as a characteristic of reverberation masking; 
         FIG. 10A  and  FIG. 10B  are explanatory diagrams illustrating an example of masking calculation in the case of using temporal masking that reverberation exerts on the sound as the characteristic of the reverberation masking; 
         FIG. 11  is a block diagram of a masking composition unit  603  of  FIG. 6 ; 
         FIG. 12A  and  FIG. 12B  are operation explanatory diagrams of a maximum value calculation unit  1101 ; 
         FIG. 13  is a flowchart illustrating a control operation of a device that implements, by means of a software process, the function of the audio signal encoding apparatus of the second embodiment having the configuration of  FIG. 6 ; 
         FIG. 14  is a block diagram of an audio signal transmission system of a third embodiment; 
         FIG. 15  is a block diagram of a reverberation characteristic estimation unit  1407  of  FIG. 14 ; 
         FIG. 16  is a flowchart illustrating a control operation of a device that implements, by means of a software process, the function of the reverberation characteristic estimation unit  1407  illustrated as the configuration of  FIG. 15 ; 
         FIG. 17  is a flowchart illustrating a control process of an encoding apparatus  1401  and a decoding and reproducing apparatus  1402  in the case of performing a process in which a reverberation characteristic  1408  of a reproduction environment is transmitted in advance; and 
         FIG. 18  is a flowchart illustrating a control process of the encoding apparatus  1401  and the decoding and reproducing apparatus  1402  in the case of performing a process in which the reverberation characteristic  1408  of the reproduction environment is transmitted periodically. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the invention will be described in detail below with reference to the drawings. 
     Before describing the embodiments, a common technique will be described. 
       FIG. 1  is a diagram illustrating a configuration example of a common encoding apparatus for improving the sound quality of an input audio signal in encoding of the input audio signal. 
     A Modified Discrete Cosine Transform (MDCT) unit  101  converts an input sound that is input as a discrete signal into a signal in a frequency domain. A quantization unit  102  quantizes frequency signal components in the frequency domain. A multiplex unit  103  multiplexes the pieces of quantized data that are quantized for the respective frequency signal components, into an encoded bit stream, which is output as output data. 
     An auditory masking calculation unit  104  performs a frequency analysis for each frame of a given length of time in the input sound. The auditory masking calculation unit  104  calculates a masking curve with taking into consideration the calculation result of the frequency analysis and masking effect that is the human auditory characteristic, calculates a quantization step size for each piece of quantized data based on the masking curve, and notifies the quantization step size to the quantization unit  102 . The quantization unit  102  quantizes the frequency signal components in the frequency domain output from the MDCT unit  101  with the quantization step size notified from the auditory masking calculation unit  104 . 
       FIG. 2  is a schematic diagram illustrating a functional effect of the encoding apparatus according to the configuration of  FIG. 1 . 
     For example, assume that the input sound of  FIG. 1  schematically contains audio source frequency signal components illustrated as S 1 , S 2 , S 3 , and S 4  of  FIG. 2 . In this case, a human has, for example, a masking curve (a frequency characteristic) indicated by reference numeral  201  with respect to the power value of the audio source S 2 . That is, presence of the audio source S 2  in the input sound causes the human to hardly hear a sound of frequency power components within a masking range  202  of which the power value is smaller than that of the masking curve  201  of  FIG. 2 . In other words, the frequency power components are masked. 
     Accordingly, since this portion is hardly heard by nature, it is wasteful, in  FIG. 2 , to perform quantization by assigning a fine quantization step size to each of the frequency signal components of the audio source S 1  and the audio source S 3  of which the power values are within the masking range  202 . On the other hand, it is preferable, in  FIG. 2 , to assign the fine quantization step size with respect to the audio sources S 2  and S 4  of which the power values exceed the masking range  202  because the human can recognize these audio sources well. 
     In view of this, in the encoding apparatus of  FIG. 2 , the auditory masking calculation unit  104  performs a frequency analysis on the input sound to calculate the masking curve  201  of  FIG. 2 . The auditory masking calculation unit  104  then makes the quantization step size coarse for a frequency signal component of which the power value is estimated to be within a range smaller than the masking curve  201 . On the other hand, the auditory masking calculation unit  104  makes the quantization step size fine for a frequency signal component of which the power value is estimated to be within a range larger than the masking curve  201 . 
     In this manner, the encoding apparatus having the configuration of  FIG. 1  makes the quantization step size coarse for a frequency signal component which is unnecessary to be heard finely, to reduce an encoding bit rate, improving the encoding efficiency thereof. 
     Consider a case, in such an encoding apparatus, where a sampling frequency of an input sound is 48 kHz, the input sound is a stereo audio, and an encoding scheme thereof is an AAC (Advanced Audio Coding) scheme. In this case, a bit rate of, for example, 128 kbps having a CD (Compact Disk) sound quality is supposed to provide enhanced encoding efficiency by using the encoding apparatus having the configuration of  FIG. 1 . But, under a low-bit-rate condition such as 96 kbps or less having a streaming audio quality, or to an extent of a telephone communication quality of a mobile phone, a sound quality of an encoded sound deteriorates. It is therefore requested to reduce an encoding bit rate without deteriorating a sound quality even under such a low-bit-rate condition. 
       FIG. 3  is a block diagram of an encoding apparatus of a first embodiment. 
     In  FIG. 3 , a quantizer  301  quantizes an audio signal. More specifically, a frequency division unit  305  divides the audio signal into sub-band signals in a plurality of frequency bands, the quantizer  301  quantizes the plurality of sub-band signals individually, and a multiplexer  306  further multiplexes the plurality of sub-band signals quantized by the quantizer  301 . 
     Next, in  FIG. 3 , a reverberation masking characteristic obtaining unit  302  obtains a characteristic  307  of reverberation masking that is exerted on a sound represented by the audio signal by reverberation of the sound generated in a reproduction environment by reproducing the sound. For example, the reverberation masking characteristic obtaining unit  302  obtains a characteristic of frequency masking that reverberation exerts on the sound, as the characteristic  307  of the reverberation masking. Alternatively, for example, the reverberation masking characteristic obtaining unit  302  obtains a characteristic of temporal masking that reverberation exerts on the sound, as the characteristic  307  of the reverberation masking. Further, the reverberation masking characteristic obtaining unit  302  calculates, for example, the characteristic  307  of the reverberation masking by using the audio signal, a reverberation characteristic  309  of the reproduction environment, and a human auditory psychology model prepared in advance. In this process, the reverberation masking characteristic obtaining unit  302  calculates, for example, the characteristic  307  of the reverberation masking as the reverberation characteristic  309  by using a reverberation characteristic selected from among reverberation characteristics prepared for respective reproduction environments in advance. In this process, the reverberation masking characteristic obtaining unit  302  further receives selection information on the reverberation characteristic corresponding to the reproduction environment to select the reverberation characteristic  309  corresponding to the reproduction environment. Alternatively, the reverberation masking characteristic obtaining unit  302  receives, for example, a reverberation characteristic that is an estimation result of the reverberation characteristic in the reproduction environment based on a sound picked up in the reproduction environment and a sound emitted in the reproduction environment when the picked-up sound is picked up, as the reverberation characteristic  309 , to calculate the characteristic  307  of the reverberation masking. 
     In  FIG. 3 , a control unit  303  controls a quantization step size  308  of the quantizer  301  based on the characteristic  307  of the reverberation masking. For example, the control unit  303  performs control, based on the characteristic  307  of the reverberation masking, so as to make the quantization step size  308  larger in the case where the magnitude of a sound represented by the audio signal is such that the sound is masked by the reverberation, as compared with the case where the magnitude is such that the sound is not masked by the reverberation. 
     In addition to the above configuration, the auditory masking characteristic obtaining unit  304  further obtains a characteristic of auditory masking that the human auditory characteristic exerts on a sound represented by the audio signal. Then, the control unit  303  further controls the quantization step size  308  of the quantizer  301  based also on the characteristic of the auditory masking. More specifically, the reverberation masking characteristic obtaining unit  302  obtains a frequency characteristic of the magnitude of a sound masked by the reverberation, as the characteristic  307  of the reverberation masking, and the auditory masking characteristic obtaining unit  304  obtains a frequency characteristic of the magnitude of a sound masked by the human auditory characteristic, as a characteristic  310  of the auditory masking. Then, the control unit  303  controls the quantization step size  308  of the quantizer  301  based on a composite masking characteristic obtained by selecting, for each frequency, a greater characteristic from between the frequency characteristic of the characteristic  307  of the reverberation masking and the frequency characteristic of the characteristic  310  of the auditory masking. 
       FIG. 4  is an explanatory diagram illustrating the reverberation characteristic  309  in the encoding apparatus of the first embodiment having the configuration of  FIG. 3 . 
     On a transmission side  401 , an encoding apparatus  403  encodes an input sound (corresponding to the audio signal of  FIG. 1 ), resulting encoded data  405  (corresponding to the output data of  FIG. 1 ) is transmitted to a reproduction device  404  on a reproduction side  402 , and the reproduction device  404  decodes and reproduces the encoded data. Here, in a reproduction environment where the reproduction device  404  emits a sound to a user through a loud speaker, reverberation  407  is typically generated in addition to a direct sound  406 . 
     In the first embodiment, a characteristic of the reverberation  407  in the reproduction environment are provided to the encoding apparatus  403  having the configuration of  FIG. 3 , as the reverberation characteristic  309 . In the encoding apparatus  403  having the configuration of  FIG. 3 , the control unit  303  controls the quantization step size  308  of the quantizer  301  based on the characteristic  307  of the reverberation masking obtained by the reverberation masking characteristic obtaining unit  302  based on the reverberation characteristic  309 . More specifically, the control unit  303  generates a composite masking characteristic obtained by selecting, for each frequency, a greater characteristic from between the frequency characteristic of the characteristic  307  of the reverberation masking and the frequency characteristic of the characteristic  310  of the auditory masking obtained by the auditory masking characteristic obtaining unit  304 . The control unit  303  controls the quantization step size  308  of the quantizer  301  based on the composite masking characteristic. In such a manner, the encoding apparatus  403  performs control of outputting the encoded data  405  such that frequencies buried in the reverberation are not encoded as much as possible. 
       FIG. 5A  and  FIG. 5B  are explanatory diagrams illustrating an encoding operation of the encoding apparatus of  FIG. 3  in the absence of reverberation and in the presence of reverberation. 
     In the case where the reverberation is absent, as illustrated in  FIG. 5A , and an audio signal includes two audio sources P 1  and P 2 , for example, a range of the auditory masking is composed of ranges indicated by reference numerals  501  and  502  corresponding to the respective audio sources P 1  and P 2 . In this case, since both of the power values of the audio sources P 1  and P 2  exceed the range of the auditory masking, the control unit  303  of  FIG. 3  needs to assign a fine value as the quantization step size  308  to each of the frequency signal components corresponding to the respective audio sources P 1  and P 2  based on the characteristic of the auditory masking. 
     On the other hand, in the presence of the reverberation, as described in  FIG. 4 , the user is influenced by the reverberation  407  in addition to the direct sound  406 , therefore receiving the reverberation masking in addition to the auditory masking. 
     Accordingly, the control unit  303  of  FIG. 3  controls the quantization step size  308  for each frequency signal component taking into consideration a range  503  of the reverberation masking based on the characteristic  307  of the reverberation masking besides the ranges  501  and  502  of the auditory masking based on the characteristic  310  of the auditory masking. Specifically, consider a case where the reverberation is present, as illustrated in  FIG. 5B , and the range  503  of the reverberation masking entirely includes the ranges  501  and  502  of the auditory masking, that is, the case where the reverberation  407  is significantly large in the reproduction environment, as illustrated in  FIG. 4 . Further consider a case, with respect to the frequency signal component of the audio source P 2 , where the power value of the range  503  of the reverberation masking is greater than the power values of the ranges  501  and  502  of the auditory masking, and the power value of the audio source P 2  is within the range  503  of the reverberation masking. In this case, the control unit  303  of  FIG. 3  makes the quantization step size  308  for the frequency signal component corresponding to the audio source P 2  coarse based on the characteristic  310  of the auditory masking and the characteristic  307  of the reverberation masking. 
     As a result, in the case where the characteristic  307  of the reverberation masking is greater than the characteristic  310  of the auditory masking, encoding is performed such that frequencies buried in the reverberation are not encoded as much as possible. In such a manner, the encoding apparatus of the first embodiment of  FIG. 3  encodes only an acoustic component that is not masked by the reverberation, enabling the enhancement of the encoding efficiency as compared with the encoding apparatus having the common configuration that performs control based on only a characteristic of the auditory masking, as described in  FIG. 1 . This enables the improvement of the sound quality at the low-bit-rate. 
     According to an experiment, on the condition that the input sound is a speech sound, and the reproduction environment is an interior or the like in which the reverberation is large, the proportion of masked frequency bands to all frequency bands of the input sound accounted for about 7% when only the auditory masking was taken into consideration, whereas the proportion accounted for about 24% when the reverberation masking was also taken into consideration. Thus, under the aforementioned condition, the encoding efficiency of the encoding apparatus of the first embodiment is about three times greater than that of the encoding apparatus in which only the auditory masking is taken into consideration. 
     According to the first embodiment, an even lower bit rate is achieved. Specially, there is provided an advantage of lowering a bit rate requested to achieve the same S/N in the presence of the reverberation. According to the first embodiment, a reverberation component is not actively encoded and added on the reproduction side, but a portion buried in the reverberation generated on the reproduction side will not be encoded. 
       FIG. 6  is a block diagram of an audio signal encoding apparatus of the second embodiment. The audio signal encoding apparatus selects a reverberation characteristic of a reproduction environment based on an input type of the reproduction environment (a large room, a small room, a bathroom, or the like), and enhances the encoding efficiency of an input signal by making use of the reverberation masking. The configuration of the second embodiment may by applicable to, for example, an LSI (Large-Scale Integrated circuit) for a multimedia broadcast apparatus. 
     In  FIG. 6 , a Modified Discrete Cosine Transform (MDCT) unit  605  divides an input signal (corresponding to the audio signal of  FIG. 3 ) into frequency signal components in units of frame of a given length of time. MDCT is a Lapped Orthogonal Transform in which frequency conversion is performed while window data for segmentation of an input signal in units of frame is overlapped by half of length of the window data, which is a known frequency division method for reducing the amount of converted data by receiving a plurality of input signals and outputting a coefficient set of frequency signal components of which the number is equal to a half of the number of the input signals. 
     The reverberation characteristic storage unit  612  (corresponding to part of the reverberation masking characteristic obtaining unit  302  of  FIG. 3 ) stores a plurality of reverberation characteristics corresponding to the types of the plurality of reproduction environments. The reverberation characteristic is an impulse response of the reverberation (corresponding to the reference numeral  407  of  FIG. 4 ) in the reproduction environment. 
     A reverberation characteristic selection unit  611  (corresponding to part of the reverberation masking characteristic obtaining unit  302  of  FIG. 3 ) reads out a reverberation characteristic  609  corresponding to a type  613  of the reproduction environment that is input, from the reverberation characteristic storage unit  612 . Then, the reverberation characteristic selection unit  611  gives the reverberation characteristic  609  to a reverberation masking calculation unit  602  (corresponding to part of the reverberation masking characteristic obtaining unit  302  of  FIG. 3 ). 
     The reverberation masking calculation unit  602  calculates characteristic  607  of the reverberation masking by using the input signal, the reverberation characteristic  609  of the reproduction environment, and the human auditory psychology model prepared in advance. 
     An auditory masking calculation unit  604  (corresponding to the auditory masking characteristic obtaining unit  304  of  FIG. 3 ) calculates a characteristic  610  of the auditory masking being an auditory masking threshold value (forward direction and backward direction masking), from the input signal. The auditory masking calculation unit  604  includes, for example, a spectrum calculation unit for receiving a plurality of frames of a given length as the input signal and performing frequency analysis for each frame. The auditory masking calculation unit  604  further includes a masking curve prediction unit for calculating a masking curve being the characteristic  610  of the auditory masking with taking into consideration the calculation result from the spectrum calculation unit and a masking effect being the human auditory characteristic (for example, see the description of Japanese Patent Laid-Open No. 9-321628). 
     A masking composition unit  603  (corresponding to the control unit  303  of  FIG. 3 ) controls a quantization step size  608  of a quantizer  601  based on a composite masking characteristic obtained by selecting, for each frequency, a greater characteristic from between the frequency characteristic of the characteristic  607  of the reverberation masking and the frequency characteristic of the characteristic  610  of the auditory masking. 
     The quantizer  601  quantizes sub-band signals in a plurality of frequency bands output from the MDCT unit  605  at quantization bit count corresponding to the quantization step sizes  608  that are input from the masking composition unit  603  in accordance with respective frequency bands. Specifically, when the frequency component of the input signal is greater than a threshold value of the composite masking characteristic, the quantization bit count is increased (the quantization step size is made fine), and when the frequency component of the input signal is smaller than the threshold value of the composite masking characteristic, the quantization bit count is decreased (the quantization step size is made coarse). 
     A multiplexer  606  multiplexes pieces of data on sub-band signals of the plurality of frequency components quantized by the quantizer  601  into an encoded bit stream. 
     An operation of the audio signal encoding apparatus of the second embodiment of  FIG. 6  will be described below. 
     First, a plurality of reverberation characteristics (impulse responses) are stored in the reverberation characteristic storage unit  612  of  FIG. 6  in advance.  FIG. 7  is a diagram illustrating a configuration example of data stored in the reverberation characteristic storage unit  612 . The reverberation characteristics are stored in associated with the types of reproduction environments, respectively. As the reverberation characteristics, measurement results of typical interior impulse responses corresponding to the types of the reproduction environments are used. 
     The reverberation characteristic selection unit  611  of  FIG. 6  obtains the type  613  of the reproduction environment. For example, a type selection button is provided in the encoding apparatus, with which a user selects a type in accordance with the reproduction environment in advance. The reverberation characteristic selection unit  611  refers to the reverberation characteristic storage unit  612  to output the reverberation characteristic  609  corresponding to the obtained type  613  of the reproduction environment. 
       FIG. 8  is a block diagram of the reverberation masking calculation unit  602  of  FIG. 6 . 
     A reverberation signal generation unit  801  is a known FIR (Finite Impulse Response) filter for generating a reverberation signal  806  from an input signal  805  by using an impulse response  804  of the reverberation environment being the reverberation characteristic  609  output from the reverberation characteristic selection unit  611  of  FIG. 6 , based on Expression 1 below. 
     
       
         
           
             
               
                 
                   
                     
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     In the above Expression 1, x(t) denotes the input signal  805 , r(t) denotes the reverberation signal  806 , h(t) denotes the impulse response  804  of the reverberation environment, and TH denotes a starting point in time of the reverberation (for example, 100 ms). 
     A time-frequency transformation unit  802  calculates a reverberation spectrum  807  corresponding to the reverberation signal  806 . Specifically, the time-frequency transformation unit  802  performs Fast Fourier Transform (FFT) calculation or Discrete Cosine Transform (DCT) calculation, for example. When the FFT calculation is performed, an arithmetic operation of Expression 2 below is performed. 
     
       
         
           
             
               
                 
                   
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     In the above Expression 2, r(t) denotes the reverberation signal  806 , R(j) denotes the reverberation spectrum  807 , n denotes the length of an analyzing discrete time for the reverberation signal  806  on which the FFT is performed (for example, 512 points), and j denotes a frequency bin (a signaling point on a frequency axis). 
     A masking calculation unit  803  calculates a masking threshold value from the reverberation spectrum  807  by using an auditory psychology model  808 , and outputs the masking threshold value as a reverberation masking threshold value  809 . In  FIG. 6 , the reverberation masking threshold value  809  is provided as the characteristic  607  of the reverberation masking, from the reverberation masking calculation unit  602  to the masking composition unit  603 . 
       FIG. 9  A,  FIG. 9B , and  FIG. 9C  are explanatory diagrams illustrating an example of masking calculation in the case of using a frequency masking that reverberation exerts on the sound as the characteristic  607  of the reverberation masking of  FIG. 6 . In  FIG. 9A ,  FIG. 9B , or  FIG. 9C , a transverse axis denotes frequency of the reverberation spectrum  807 , and a vertical axis denotes the power (db) of each reverberation spectrum  807 . 
     First, the masking calculation unit  803  of  FIG. 8  estimates a power peak  901  in a characteristic of the reverberation spectrum  807  illustrated as a dashed characteristic curve in  FIG. 9 . In  FIG. 9A , two power peaks  901  are estimated. Frequencies of these two power peaks  901  are defined as A and B, respectively. 
     Next, the masking calculation unit  803  of  FIG. 8  calculates a masking threshold value based on the power peaks  901 . A frequency masking model is known in which the determination of the frequencies A and B of the power peaks  901  leads to the determination of masking ranges, for example, the amount of frequency masking described in the literature “Choukaku to Onkyousinri (Auditory Sense and Psychoacoustics)” (in Japanese), CORONA PUBLISHING CO., LTD., p. 111-112 can be used. Based on the auditory psychology model  808 , the following characteristics can be generally observed. With regard to the power peaks  901  illustrated in  FIG. 9A , when a frequency is as low as the power peak  901  at the frequency A of  FIG. 9A , for example, a slope of a masking curve  902 A having a peak at the power peak  901  and descending toward the both side of the peak is steep. As a result, a frequency range masked around the frequency A is small. On the other hand, when a frequency is as high as the power peak  901  at the frequency B of  FIG. 9A , for example, a slope of a masking curve  902 B having a peak at the power peak  901  and descending toward the both side of the peak is gentle. As a result, a frequency range masked around the frequency B is large. The masking calculation unit  803  receives such a frequency characteristic as the auditory psychology model  808 , and calculates masking curves  902 A and  902 B as illustrated by triangle characteristics of alternate long and short dash lines of  FIG. 9B , for example, in logarithmic values (decibel values) in a frequency direction, for the power peaks  901  at the frequencies A and B, respectively. 
     Finally, the masking calculation unit  803  of  FIG. 8  selects a maximum value from among the characteristic curve of the reverberation spectrum  807  of  FIG. 9A  and the masking curves  902 A and  902 B of the masking threshold values of  FIG. 9B , for each frequency bin. In such a manner, the masking calculation unit  803  integrates the masking threshold values to output the integration result as the reverberation masking threshold value  809 . In the example of  FIG. 9C , the reverberation masking threshold value  809  is obtained as the characteristic curve of a thick solid line. 
       FIG. 10A  and  FIG. 10B  are explanatory diagrams illustrating an example of masking calculation in the case of using temporal masking that the reverberation exerts on the sound as the characteristic  607  of the reverberation masking of  FIG. 6 . In  FIG. 10A  or  FIG. 10B , a transverse axis denotes time, and a vertical axis denotes power (db) of the frequency signal component of the reverberation signal  806  in each frequency band (frequency bin) at each point in time. Each of  FIG. 10A  and  FIG. 10B  illustrates temporal changes in a frequency signal component in any one of the frequency bands (frequency bins) output from the time-frequency transformation unit  802  of  FIG. 8 . 
     First, the masking calculation unit  803  of  FIG. 8  estimates a power peak  1002  in a time axis direction with respect to temporal changes in a frequency signal component  1001  of the reverberation signal  806  in each frequency band. In  FIG. 10A , two power peaks  1002  are estimated. Points in time of these two power peaks  1002  are defined as a and b. 
     Next, the masking calculation unit  803  of  FIG. 8  calculates a masking threshold value based on each power peaks  1002 . The determination of the points in time a and b of the power peaks  1002  can lead to the determination of masking ranges in a forward direction (a time direction following the respective points in time a and b) and in a backward direction (a time direction preceding the respective points in time a and b) across the respective points in time a and b as boundaries. As a result, the masking calculation unit  803  calculates masking curves  1003 A and  1003 B as illustrated by triangle characteristics of alternate long and short dash lines of  FIG. 10A , for example, in logarithmic values (decibel values) in a time direction, for the power peaks  1002  at the respective points in time a and b. Each masking range in the forward direction generally extends to the vicinity of about 100 ms after the point in time of the power peak  1002 , and each masking range in the backward direction generally extends to the vicinity of about 20 ms before the point in time of the power peak  1002 . The masking calculation unit  803  receives the above temporal characteristic in the forward direction and the backward direction as the auditory psychology model  808 , for each of the power peaks  1002  at the respective points in time a and b. The masking calculation unit  803  calculates, based on the temporal characteristic, a masking curve in which the amount of masking decreases exponentially as the point in time is away from the power peak  1002  in the forward direction and the backward direction. 
     Finally, the masking calculation unit  803  of  FIG. 8  selects the maximum value from among the frequency signal component  1001  of the reverberation signal of  FIG. 10A  and the masking curves  1003 A and  1003 B of the masking threshold values of  FIG. 10A  for each discrete time and for each frequency band. In such a manner, the masking calculation unit  803  integrates the masking threshold values for each frequency band, and outputs the integration result as the reverberation masking threshold value  809  in the frequency band. In the example of  FIG. 10B , the reverberation masking threshold value  809  is obtained as the characteristic curve of a thick solid line. 
     Two methods have been described above as specific examples of the characteristic  607  (the reverberation masking threshold value  809 ) of the reverberation masking output by the reverberation masking calculation unit  602  of  FIG. 6  having the configuration of  FIG. 8 . One is a method of the frequency masking ( FIG. 9 ) in which masking in the frequency direction is done centered about the power peak  901  on the reverberation spectrum  807 . The other is a method of the temporal masking ( FIG. 10 ) in which masking in the forward direction and the backward direction is done centered about the power peak  1002  of each frequency signal component of the reverberation signal  806  in the time axis direction. 
     Either or both of the masking methods may be applied for obtaining the characteristic  607  (the reverberation masking threshold value  809 ) of the reverberation masking. 
       FIG. 11  is a block diagram of the masking composition unit  603  of  FIG. 6 . The masking composition unit  603  includes a maximum value calculation unit  1101 . The maximum value calculation unit  1101  receives the reverberation masking threshold value  809  (see  FIG. 8 ) from the reverberation masking calculation unit  602  of  FIG. 6 , as the characteristic  607  of the reverberation masking. The maximum value calculation unit  1101  further receives an auditory masking threshold value  1102  from the auditory masking calculation unit  604  of  FIG. 6 , as the characteristic  610  of the auditory masking. Then, the maximum value calculation unit  1101  selects a greater power value from between the reverberation masking threshold value  809  and the auditory masking threshold value  1102 , for each frequency band (frequency bin), and calculates a composite masking threshold value  1103  (a composite masking characteristic). 
       FIG. 12A  and  FIG. 12B  is an operation explanatory diagram of the maximum value calculation unit  1101 . In  FIG. 12A , power values are compared between the reverberation masking threshold value  809  and the auditory masking threshold value  1102 , for each frequency band (frequency bin) on a frequency axis. As a result, as illustrated in  FIG. 12B , the maximum value is calculated as the composite masking threshold value  1103 . 
     Note that, instead of the maximum value of the power values of the reverberation masking threshold value  809  and the auditory masking threshold value  1102 , the result of summing logarithmic power values (decibel values) of the reverberation masking threshold value  809  and the auditory masking threshold value  1102  each of which is weighted in accordance with the phase thereof may be calculated as the composite masking threshold value  1103 , for each frequency band (frequency bin). 
     In such a manner, according to the second embodiment, the unhearable frequency range can be calculated that is masked by both the input signal and the reverberation, and using the composite masking threshold value  1103  (the composite masking characteristic) enables even more efficient encoding. 
       FIG. 13  is a flowchart illustrating a control operation of a device that implements, by means of a software process, the function of the audio signal encoding apparatus of the second embodiment having the configuration of  FIG. 6 . The control operation is implemented as an operation in which a processor (not specially illustrated) that implements an audio signal encoding apparatus executes a control program stored in a memory (not specially illustrated). 
     First, the type  613  ( FIG. 6 ) of the reproduction environment that is input is obtained (step S 1301 ). 
     Next, the impulse response of the reverberation characteristic  609  corresponding to the input type  613  of the reproduction environment is selected and read out from the reverberation characteristic storage unit  612  of  FIG. 6  (step S 1302 ). 
     The above processes of the steps S 1301  and S 1302  correspond to the reverberation characteristic selection unit  611  of  FIG. 6 . 
     Next, the input signal is obtained (step S 1303 ). 
     Then, the auditory masking threshold value  1102  ( FIG. 11 ) is calculated (step S 1304 ). 
     The above processes of the steps S 1303  and S 1304  correspond to the auditory masking calculation unit  604  of FIG.  6 . 
     Further, the reverberation masking threshold value  809  ( FIG. 8 ) is calculated by using the impulse response of the reverberation characteristic  609  obtained in the step S 1302 , the input signal obtained in the step S 1303 , and the human auditory psychology model prepared in advance (step S 1305 ). The calculation process in this step is similar to that explained with  FIG. 8  to  FIG. 10 . 
     The above processes of the steps S 1303  and S 1305  correspond to the reverberation masking calculation unit  602  in  FIG. 6  and  FIG. 8 . 
     Next, the auditory masking threshold value  1102  and the reverberation masking threshold value  809  are composed to calculate the composite masking threshold value  1103  ( FIG. 11 ) (step S 1306 ). The composite process in this step is similar to that explained with  FIG. 11  and  FIG. 12 . 
     The process of the step S 1306  corresponds to the masking composition unit  603  of  FIG. 6 . 
     Next, the input signal is quantized with the composite masking threshold value  1103  (step S 1307 ). Specifically, when the frequency component of the input signal is greater than the composite masking threshold value  1103 , the quantization bit count is increased (the quantization step size is made fine), and when the frequency component of the input signal is smaller than a threshold value of the composite masking characteristic, the quantization bit count is decreased (the quantization step size is made coarse). 
     The process of the step S 1307  corresponds to the function of part of the masking composition unit  603  and the quantizer  601  of  FIG. 6 . 
     Next, pieces of data on the sub-band signals of the plurality of frequency components quantized in the step S 1307  are multiplexed into an encoded bit stream (step S 1308 ). 
     Then, the generated encoded bit stream is output (step S 1309 ). 
     The above processes of the steps S 1308  and S 1309  correspond to the multiplexer  606  of  FIG. 6 . 
     According to the second embodiment, similar to the first embodiment, an even lower bit rate is enabled. Moreover, by causing the reverberation characteristic storage unit  612  in the audio signal encoding apparatus to store the reverberation characteristic  609 , the characteristic  607  of the reverberation masking can be obtained only by specifying the type  613  of the reproduction environment, without providing the reverberation characteristic to the encoding apparatus  1401  from the outside. 
       FIG. 14  is a block diagram of an audio signal transmission system of a third embodiment. 
     The system estimates a reverberation characteristic  1408  of the reproduction environment in a decoding and reproducing apparatus  1402 , and notifies the reverberation characteristic  1408  to an encoding apparatus  1401  to enhance the encoding efficiency of an input signal by making use of reverberation masking. The system may be applicable to, for example, a multimedia broadcast apparatus and a reception terminal. 
     To begin with, configurations and functions of the quantizer  601 , the reverberation masking calculation unit  602 , the masking composition unit  603 , the auditory masking calculation unit  604 , the MDCT unit  605 , and multiplexer  606  that constitute the encoding apparatus  1401  are similar to those illustrated in  FIG. 6  according to the second embodiment. 
     An encoded bit stream  1403  output from the multiplexer  606  in the encoding apparatus  1401  is received by a decoding unit  1404  in the decoding and reproducing apparatus  1402 . 
     The decoding unit  1404  decodes a quantized audio signal (an input signal), that is transmitted from the encoding apparatus  1401  as the encoded bit stream  1403 . As a decoding scheme, for example, an AAC (Advanced Audio Coding) scheme can be employed. 
     A sound emission unit  1405  emits a sound including a sound of the decoded audio signal in the reproduction environment. Specifically, the sound emission unit  1405  includes, for example, an amplifier for amplifying the audio signal, and a loud speaker for emitting a sound of the amplified audio signal. 
     A sound pickup unit  1406  picks up a sound emitted by the sound emission unit  1405 , in the reproduction environment. Specifically, the sound pickup unit  1406  includes, for example, a microphone for picking up the emitted sound, and an amplifier for amplifying an audio signal output from the microphone, and an analog-to-digital converter for converting the audio signal output from the amplifier into a digital signal. 
     A reverberation characteristic estimation unit (an estimation unit)  1407  estimates the reverberation characteristic  1408  of the reproduction environment based on the sound picked up by the sound pickup unit  1406  and the sound emitted by the sound emission unit  1405 . The reverberation characteristic  1408  of the reproduction environment is, for example, an impulse response of the reverberation (corresponding to the reference numeral  407  of  FIG. 4 ) in the reproduction environment. 
     A reverberation characteristic transmission unit  1409  transmits the reverberation characteristic  1408  of the reproduction environment estimated by the reverberation characteristic estimation unit  1407  to the encoding apparatus  1401 . 
     On the other hand, a reverberation characteristic reception unit  1410  in the encoding apparatus  1401  receives the reverberation characteristic  1408  of the reproduction environment transmitted from the decoding and reproducing apparatus  1402 , and transfers the reverberation characteristic  1408  to the reverberation masking calculation unit  602 . 
     The reverberation masking calculation unit  602  in the encoding apparatus  1401  calculates the characteristic  607  of the reverberation masking by using the input signal, the reverberation characteristic  1408  of the reproduction environment notified from the decoding and reproducing apparatus  1402  side, and the human auditory psychology model prepared in advance. In the second embodiment illustrated in  FIG. 6 , the reverberation masking calculation unit  602  calculates the characteristic  607  of the reverberation masking by using the reverberation characteristic  609  of the reproduction environment that the reverberation characteristic selection unit  611  reads out from the reverberation characteristic storage unit  612  in accordance with the input type  613  of the reproduction environment. In contrast, in the third embodiment illustrated in  FIG. 14 , the reverberation characteristic  1408  of the reproduction environment estimated by the decoding and reproducing apparatus  1402  is directly received for the calculation of the characteristic  607  of the reverberation masking. It is thereby possible to calculate the characteristic  607  of the reverberation masking that more matches the reproduction environment and is thus accurate, this leads to more enhanced compression efficiency of the encoded bit stream  1403 , an even lower bit rate is enabled. 
       FIG. 15  is a block diagram of the reverberation characteristic estimation unit  1407  of  FIG. 14 . 
     The reverberation characteristic estimation unit  1407  includes an adaptive filter  1506  for operating by receiving data  1501  that is decoded by the decoding unit  1404  of  FIG. 14 , a direct sound  1504  emitted by a loud speaker  1502  in the sound emission unit  1405 , and a sound that is reverberation  1505  picked up by a microphone  1503  in the sound pickup unit  1406 . The adaptive filter  1506  repeats an operation of adding an error signal  1507  output by an adaptive process performed by the adaptive filter  1506  to the sound from the microphone  1503 , to estimate the impulse response of the reproduction environment. Then, by inputting an impulse to a filter characteristic on which the adaptive process is completed, the reverberation characteristic  1408  of the reproduction environment is obtained as an impulse response. 
     Note that, by using the microphone  1503  of which the characteristic is known, the adaptive filter  1506  may operate so as to subtract the known characteristic of the microphone  1503  to estimate the reverberation characteristic  1408  of the reproduction environment. 
     Accordingly, in the third embodiment, the reverberation characteristic estimation unit  1407  calculates a transfer characteristic of a sound that is emitted by the sound emission unit  1405  and reaches the sound pickup unit  1406  by using the adaptive filter  1506  such that the reverberation characteristic  1408  of the reproduction environment can therefore be estimated with high accuracy. 
       FIG. 16  is a flowchart illustrating a control operation of a device that implements, by means of a software process, the function of the reverberation characteristic estimation unit  1407  illustrated as the configuration of  FIG. 15 . The control operation is implemented as an operation in which a processor (not specially illustrated) that implements the decoding and reproducing apparatus  1402  executes a control program stored in a memory (not specially illustrated). 
     First, the decoded data  1501  ( FIG. 15 ) is obtained from the decoding unit  1404  of  FIG. 14  (step S 1601 ). 
     Next, the loud speaker  1502  ( FIG. 15 ) emits a sound of the decoded data  1501  (step S 1602 ). 
     Next, the microphone  1503  disposed in the reproduction environment picks up the sound (step S 1603 ). 
     Next, the adaptive filter  1506  estimates an impulse response of the reproduction environment based on the decoded data  1501  and a picked-up sound signal from the microphone  1503  (step S 1604 ). 
     By inputting an impulse to a filter characteristic on which the adaptive process is completed, the reverberation characteristic  1408  of the reproduction environment is output as an impulse response (step S 1605 ). 
     In the configuration of the third embodiment illustrated in  FIG. 14 , the reverberation characteristic estimation unit  1407  can operate so as to, on starting the decode of the audio signal, cause the sound emission unit  1405  to emit a test sound prepared in advance, and to cause the sound pickup unit  1406  to pick up the emitted sound, in order to estimate the reverberation characteristic  1408  of the reproduction environment. The test sound may be transmitted from the encoding apparatus  1401 , or generated by the decoding and reproducing apparatus  1402  itself. The reverberation characteristic transmission unit  1409  transmits the reverberation characteristic  1408  of the reproduction environment that is estimated by the reverberation characteristic estimation unit  1407  on starting the decode of the audio signal, to the encoding apparatus  1401 . On the other hand, the reverberation masking calculation unit  602  in the encoding apparatus  1401  obtains the characteristic  607  of the reverberation masking based on the reverberation characteristic  1408  of the reproduction environment that is received by the reverberation characteristic reception unit  1410  on starting the decode of the audio signal. 
       FIG. 17  is a flowchart illustrating control processes of the encoding apparatus  1401  and the decoding and reproducing apparatus  1402  in the case of performing a process in which the reverberation characteristic  1408  of the reproduction environment is transmitted in advance, in such a manner. The control processes from the steps S 1701  to S 1704  are implemented as an operation in which a processor (not specially illustrated) that implements the decoding and reproducing apparatus  1402  executes a control program stored in a memory (not specially illustrated). Moreover, processes from the steps S 1711  to S 1714  are implemented as an operation in which a processor (not specially illustrated) that implements the encoding apparatus  1401  executes a control program stored in a memory (not specially illustrated). 
     First, when the decoding and reproducing apparatus  1402  of  FIG. 14  starts a decode process, a process for estimating the reverberation characteristic  609  of the reproduction environment is performed on the decoding and reproducing apparatus  1402  side, for one minute, for example, from the start (step S 1701 ). Here, a test sound prepared in advance is emitted from the sound emission unit  1405 , and picked up by the sound pickup unit  1406  to estimate the reverberation characteristic  1408  of the reproduction environment. The test sound may be transmitted from the encoding apparatus  1401 , or generated by the decoding and reproducing apparatus  1402  itself. 
     Next, the reverberation characteristic  1408  of the reproduction environment estimated in the step S 1701  is transmitted to the encoding apparatus  1401  of  FIG. 14  (step S 1702 ). 
     On the other hand, on the encoding apparatus  1401  side, the reverberation characteristic  1408  of the reproduction environment is received (step S 1711 ). Accordingly, a process is executed in which the aforementioned composite masking characteristic is generated to control the quantization step size, and thus achieving the optimization of the encoding efficiency. 
     On the encoding apparatus  1401 , thereafter, the execution of the following steps is repeatedly started: obtaining an input signal (step S 1712 ), generating the encoded bit stream  1403  (step S 1713 ), and transmitting the encoded bit stream  1403  to the decoding and reproducing apparatus  1402  side (step S 1714 ). 
     On the decoding and reproducing apparatus  1402  side, the following steps are repeatedly executed: receiving and decoding the encoded bit stream  1403  (step S 1703 ) when the encoded bit stream  1403  is transmitted from the encoding apparatus  1401  side, and reproducing the resulting decoded signal and emitting a sound thereof (step S 1704 ). 
     With the above advance transmission process of the reverberation characteristic  1408  of the reproduction environment, the audio signal that matches a reproduction environment used by a user can be transmitted. 
     On the other hand, instead of the aforementioned advance transmission process, the reverberation characteristic estimation unit  1407  can operate so as to, every predetermined period of time, cause the sound emission unit  1405  to emit a reproduced sound of the audio signal decoded by the decoding unit  1404  and cause the sound pickup unit  1406  to picked up the sound, in order to estimate the reverberation characteristic  1408  of the reproduction environment. The predetermined period of time is, for example, 30 minutes. The reverberation characteristic transmission unit  1409  transmits the estimated reverberation characteristic  1408  of the reproduction environment to the encoding apparatus  1401 , every time the reverberation characteristic estimation unit  1407  performs the above estimation process. On the other hand, the reverberation masking calculation unit  602  in the encoding apparatus  1401  obtains the characteristic  607  of the reverberation masking every time the reverberation characteristic reception unit  1410  receives the reverberation characteristic  1408  of the reproduction environment. The masking composition unit  603  updates the control of the quantization step size every time the reverberation masking calculation unit  602  obtains the characteristic  607  of the reverberation masking. 
       FIG. 18  is a flowchart illustrating a control process of the encoding apparatus  1401  and the decoding and reproducing apparatus  1402  in the case of performing a process in which the reverberation characteristic  1408  of the reproduction environment is transmitted periodically, in such a manner. The control processes from the steps S 1801  to S 1805  are implemented as an operation in which a processor (not specially illustrated) that implements the decoding and reproducing apparatus  1402  executes a control program stored in a memory (not specially illustrated). Moreover, processes from the steps S 1811  to S 1814  are implemented as an operation in which a processor (not specially illustrated) that implements the encoding apparatus  1401  executes a control program stored in a memory (not specially illustrated). 
     When the decoding and reproducing apparatus  1402  of  FIG. 14  starts the decode process, it is determined whether or not 30 minutes or more, for example, have elapsed after the previous reverberation estimation, on the decoding and reproducing apparatus  1402  side (step S 1801 ). 
     If the determination in the step S 1801  is NO because 30 minutes or more, for example, have not elapsed after previous reverberation estimation, the process proceeds to a step S 1804  to execute a normal decode process. 
     If the determination in the step S 1801  is YES because 30 minutes or more, for example, have elapsed after the previous reverberation estimation, a process for estimating the reverberation characteristic  609  of the reproduction environment is performed (step S 1802 ). Here, a decoded sound of the audio signal that the decoding unit  1404  decodes based on the encoded bit stream  1403  transmitted from encoding apparatus  1401  is emitted from the sound emission unit  1405 , and picked up by the sound pickup unit  1406 , in order to estimate the reverberation characteristic  1408  of the reproduction environment. 
     Next, the reverberation characteristic  1408  of the reproduction environment estimated in the step S 1802  is transmitted to the encoding apparatus  1401  of  FIG. 14  (step S 1803 ). 
     On the encoding apparatus  1401  side, the execution of the following steps is repeatedly started: obtaining an input signal (step S 1811 ), generating the encoded bit stream  1403  (step S 1813 ), and transmitting the encoded bit stream  1403  to the decoding and reproducing apparatus  1402  side (step S 1814 ). In the repeated steps, when the reverberation characteristic  1408  of the reproduction environment is transmitted from the decoding and reproducing apparatus  1402  side, the process is executed in which the reverberation characteristic  1408  of the reproduction environment is received (step S 1812 ). Accordingly, the aforementioned process is updated and executed in which the composite masking characteristic is generated to control the quantization step size. 
     On the decoding and reproducing apparatus  1402  side, the following steps are repeatedly executed: receiving and decoding the encoded bit stream  1403  when the encoded bit stream  1403  is transmitted from the encoding apparatus  1401  side (step S 1804 ), and reproducing the resulting decoded signal and emitting a sound thereof (step S 1805 ). 
     With the above periodic transmission process of the reverberation characteristic  1408  of the reproduction environment, even if the reproduction environment used by the user changes over time, the optimization of the encoding efficiency can follow the changes. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.