Patent Publication Number: US-9406311-B2

Title: Encoding method, encoding apparatus, and computer readable recording medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-187570, filed on Aug. 30, 2011, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an encoding method and the like. 
     BACKGROUND 
     One of the coding schemes for an audio signal is High Efficiency-Advanced Audio Coding (HE-AAC). In HE-AAC, low-frequency components of an audio signal are encoded with AAC encoding, and high-frequency components are encoded with spectral band replication (SBR) encoding, thereby improving the coding efficiency. 
     An exemplary encoding apparatus of the related art will be described which encodes an audio signal with HE-AAC.  FIG. 23  is a diagram illustrating a configuration of an encoding apparatus  50  of the related art. As illustrated in  FIG. 23 , the encoding apparatus  50  includes a downsampler  10 , an AAC encoder  20 , an SBR encoder  30 , and a multiplexer  40 . 
     The downsampler  10  is a processor that performs downsampling on an audio signal. The downsampler  10  outputs the audio signal having a low-frequency component obtained through the downsampling, to the ACC encoder  20 . 
     The ACC encoder  20  is a processor that applies ACC to the audio signal having the low-frequency component so as to encode the audio signal having the low-frequency component. The ACC encoder  20  outputs the encoded audio signal having the low-frequency component to the multiplexer  40 . 
     The SBR encoder  30  is a processor that encodes the high-frequency component of the audio signal. The SBR encoder  30  outputs the encoded high-frequency component of the audio signal to the multiplexer  40 . The SBR encoder  30  controls quantization of the audio signal in such a manner that the time resolution is set to high when the audio signal has a transient, or that the frequency resolution is set to high when the audio signal is stationary. The state in which an audio signal has a transient means that, for example, the audio signal includes an abrupt amplitude change. 
     The multiplexer  40  is a processor that multiplexes the encoded audio signal having the low-frequency component and the encoded audio signal having the high-frequency component and that outputs the multiplexed audio signal to an external apparatus. 
     Now, an example of the SBR encoder  30  illustrated in  FIG. 23  will be described.  FIG. 24  is a diagram illustrating a configuration of the SBR encoder  30 . As illustrated in  FIG. 24 , the SBR encoder  30  includes an analysis filter bank  31 , a transient detector  32 , a grid information generator  33 , a spectrum estimator  34 , an additional information determiner  35 , a quantizer  36 , and a multiplexer  37 . 
     The analysis filter bank  31  is a processor that transforms an audio signal into a time-frequency spectrum. The analysis filter bank  31  outputs the audio signal subjected to a time-frequency-spectrum transformation to the transient detector  32 , the spectrum estimator  34 , and the additional information determiner  35 . 
     The transient detector  32  is a processor that analyzes the audio signal and that detects a state in which the audio signal has a transient. The transient detector  32  outputs the detection result to the grid information generator  33 . 
       FIG. 25  is a diagram for explaining a process performed by the transient detector  32 . As illustrated in  FIG. 25 , the transient detector  32  sets a detection range  60 , and divides the detection range  60  into  16  sections. The detection range  60  is set so as to start in a frame  1 A and end in a frame  2 A. The frame  1 A is a target frame to be subjected to SBR encoding, and the frame  2 A is subsequent to the frame  1 A. The transient detector  32  analyzes the detection range  60  and detects a section in which a signal having an abrupt amplitude change is included. Then, the transient detector  32  outputs the presence/absence of a transient and the position of the transient signal to the grid information generator  33 . The transient detector  32  determines presence/absence of a transient for each of the frames. 
     The grid information generator  33  is a processor that controls the quantizer  36  so that the time resolution is set to high when the audio signal has a transient, and the frequency resolution is set to high when the audio signal is stationary. 
     The spectrum estimator  34  is a processor that outputs, to the quantizer  36 , supplementary information used for replicating the high-frequency component from the low-frequency component. The additional information determiner  35  is a processor that outputs, to the quantizer  36  and the multiplexer  37 , additional information representing the high-frequency component of the audio signal. 
     The quantizer  36  is a processor that encodes the high-frequency component with the time resolution and the frequency resolution which are determined under the control of the grid information generator  33 . The quantizer  36  outputs the encoded high-frequency component of the audio signal to the multiplexer  37 . 
     The multiplexer  37  is a processor that multiplexes the encoded audio signal having the high-frequency component, which is output from the quantizer  36 , and the additional information, and outputs the multiplexed information. 
     However, in the related art described above, there is a problem in that the implementation scale and the processing load are large. 
     As illustrated in  FIG. 24 , since the transient detector  32  is implemented to detect a transient in an audio signal, the SBR encoder  30  has a large implementation scale. In addition, as illustrated in  FIG. 25 , since the detection of a transient is performed for each of frames, the transient detector  32  has a heavy processing load. 
     Regarding the related art, see Japanese Laid-open Patent Publication No. 2008-129541. 
     In addition, regarding the related art, see Suzuki, Masanao, Ota, Yasuji, and Ito, Takashi, “Wansegu Housou Muke Audio Fugouka Gijutsu (Audio Coding Algorithm for One-Segment Broadcasting),”  FUJITSU. 58, 2, pp. 162-167, March 2007. 
     SUMMARY 
     According to an aspect of the embodiments, an encoding method executed by a computer, the method includes converting the computer information about a transient included in a low-frequency component of an audio signal into information about a transient included in a high-frequency component of the audio signal; detecting, by the computer the transient of the high-frequency component of the audio signal based on the high-frequency component of the audio signal and on the information about the transient of the high-frequency component obtained by the converting; and encoding, by the computer the high-frequency component of the audio signal based on the transient detected by the detecting. 
     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, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of an encoding apparatus according a first embodiment; 
         FIG. 2  is a diagram illustrating timing at which an audio signal is processed by encoders; 
         FIG. 3  is a functional block diagram illustrating a configuration of an AAC encoder and an SBR encoder according to the first embodiment; 
         FIG. 4  is a diagram illustrating an exemplary data structure of information about a transient of a low-frequency component according to the first embodiment; 
         FIG. 5  is a diagram for explaining a process performed by a transient information converter according to the first embodiment; 
         FIG. 6  is a diagram illustrating an exemplary data structure of information about a transient of a high-frequency component according to the first embodiment; 
         FIG. 7  is a flowchart of a procedure performed by the encoding apparatus according to the first embodiment; 
         FIG. 8  is a diagram illustrating a configuration of an encoding apparatus according to a second embodiment; 
         FIG. 9  is a functional block diagram illustrating a configuration of an AAC encoder and an SBR encoder according to the second embodiment; 
         FIG. 10  is a first diagram for explaining a process performed by a low-frequency transient detector according to the second embodiment; 
         FIG. 11  is a second diagram for explaining a process performed by the low-frequency transient detector according to the second embodiment; 
         FIG. 12  is a diagram illustrating an exemplary data structure of grouping information; 
         FIG. 13  is a diagram for explaining a process performed by a transient information converter according to the second embodiment; 
         FIG. 14  is a diagram illustrating an exemplary data structure of information about a transient of a high-frequency component according to the second embodiment; 
         FIG. 15  is a flowchart of a procedure performed by the encoding apparatus according to the second embodiment; 
         FIG. 16  is a diagram illustrating a configuration of an encoding apparatus according to a third embodiment; 
         FIG. 17  is a functional block diagram illustrating a configuration of an AAC encoder and an SBR encoder according to the third embodiment; 
         FIG. 18  is a diagram illustrating an exemplary data structure of information about a transient of a low-frequency component according to the third embodiment; 
         FIG. 19  is a diagram for explaining a process performed by a transient information converter according to the third embodiment; 
         FIG. 20  is a diagram illustrating an exemplary data structure of information about a transient of a high-frequency component according to the third embodiment; 
         FIG. 21  is a flowchart of a procedure performed by the encoding apparatus according to the third embodiment; 
         FIG. 22  is a diagram illustrating an exemplary computer which executes an encoding program; 
         FIG. 23  is a diagram illustrating a configuration of an encoding apparatus of the related art; 
         FIG. 24  is a diagram illustrating a configuration of an SBR encoder; and 
         FIG. 25  is a diagram for explaining a process performed by a transient detector. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of an encoding method, an encoding apparatus, and an encoding program which are disclosed herein will be described in detail below based on the drawings. These embodiments are not limited to the disclosure set forth herein. 
     First Embodiment 
       FIG. 1  is a diagram illustrating a configuration of an encoding apparatus according a first embodiment. An encoding apparatus  100  encodes the low-frequency component of an audio signal in accordance with AAC encoding, and encodes the high-frequency component in accordance with SBR encoding. As illustrated in  FIG. 1 , the encoding apparatus  100  includes a downsampler  110 , an AAC encoder  120 , an SBR encoder  130 , and a multiplexer  140 . 
     The downsampler  110  is a processor that performs downsampling on an audio signal. The downsampler  110  outputs the audio signal having a low-frequency component obtained through the downsampling, to the AAC encoder  120 . 
     The AAC encoder  120  is a processor that applies AAC to the audio signal having the low-frequency component so as to encode the audio signal having the low-frequency component. The AAC encoder  120  outputs the encoded audio signal having the low-frequency component to the multiplexer  140 . 
     The AAC encoder  120  determines whether or not the audio signal having the low-frequency component has a transient based on the audio signal. The AAC encoder  120  outputs, to the SBR encoder  130 , the determination result as to whether or not the audio signal has a transient. In the following description, the determination result as to whether or not the audio signal has a transient is referred to as transient information of the low-frequency component. 
     The SBR encoder  130  is a processor that encodes the high-frequency component of the audio signal. The SBR encoder  130  outputs the encoded high-frequency component of the audio signal to the multiplexer  140 . The SBR encoder  130  controls quantization so that the time resolution is set to high when the audio signal has a transient, and the frequency resolution is set to high when the audio signal is stationary. 
     The SBR encoder  130  converts the transient information of the low-frequency component obtained from the AAC encoder  120  into transient information of the high-frequency component, and determines whether or not the audio signal has a transient based on the transient information of the high-frequency component. 
       FIG. 2  is a diagram illustrating timing at which an audio signal is processed by the encoders. In  FIG. 2 , the horizontal axis represents a time axis. A signal  70   a  is an audio signal received by the encoding apparatus  100 . A signal  70   b  is an audio signal obtained through downsampling. A signal  70   c  is an audio signal obtained through frequency conversion performed by the SBR encoder  130  by using, for example, a quadrature mirror filter (QMF). The AAC encoder  120  performs AAC encoding on the signal  70   b , and the SBR encoder  130  performs SBR encoding on the signal  70   c.    
     The phase or the like of the audio signal to be analyzed by the AAC encoder  120  is different from that of the audio signal to be analyzed by the SBR encoder  130 . In the example illustrated in  FIG. 2 , the phase in which the AAC encoder  120  processes the nth frame is different by TA from the phase in which the SBR encoder  130  processes the nth frame. The nth frame corresponds to a frame which is located as the nth frame from the first frame. 
     Because of this, the SBR encoder  130  adjusts the phase in the transient information of the low-frequency component, thereby converting the transient information of the low-frequency component into that of the high-frequency component. The SBR encoder  130  sets the timing obtained by shifting by TA the timing at which a transient is detected for the low-frequency component, as the timing at which a transient occurs in the high-frequency component. The detailed description about the SBR encoder  130  will be made below. 
     The multiplexer  140  is a processor that multiplexes the encoded audio signal having the low-frequency component and the encoded audio signal having the high-frequency component and that outputs the multiplexed audio signal to an external apparatus. 
     Now, an exemplary configuration of the AAC encoder  120  and the SBR encoder  130  which are illustrated in  FIG. 1  will be described.  FIG. 3  is a functional block diagram illustrating a configuration of the AAC encoder  120  and the SBR encoder  130  according to the first embodiment. 
     As illustrated in  FIG. 3 , the AAC encoder  120  includes a low-frequency transient detector  121 , a low-frequency converter  122 , and a low-frequency encoder  123 . The SBR encoder  130  includes a high-frequency converter  131 , a transient information converter  132 , a high-frequency transient detector  133 , and a high-frequency encoder  134 . 
     The low-frequency transient detector  121  sequentially obtains the frames of the audio signal obtained through the downsampling, and divides each of the frames into eight subframes. The low-frequency transient detector  121  analyzes each of the subframes and detects a subframe including a transient. For example, the low-frequency transient detector  121  detects a subframe having an abrupt amplitude change, as a subframe including a transient. The low-frequency transient detector  121  outputs the detection result to the transient information converter  132  as transient information of the low-frequency component. In addition, the low-frequency transient detector  121  outputs the detection result to the low-frequency converter  122 . 
       FIG. 4  is a diagram illustrating an exemplary data structure of transient information of the low-frequency component according to the first embodiment. As illustrated in  FIG. 4 , the transient information of the low-frequency component includes data on the presence/absence of a transient, the frame number, and the subframe number. For example, when the second subframe in the (n−2)th frame includes a transient, the data on the presence/absence of a transient is “presence”, the data on the frame number is “n−2”, and the data on the subframe number is “2”. 
     The low-frequency converter  122  is a processor that performs frequency conversion on the audio signal in accordance with the detection result obtained by the low-frequency transient detector  121 . The low-frequency converter  122  outputs the audio signal obtained through the frequency conversion, to the low-frequency encoder  123 . 
     Now, the SBR encoder  130  will be described. The high-frequency converter  131  is a processor that performs frequency conversion on an audio signal. The high-frequency converter  131  outputs the audio signal obtained through the frequency conversion, to the high-frequency transient detector  133  and the high-frequency encoder  134 . 
     The transient information converter  132  is a processor that converts the transient information of the low-frequency component into the transient information of the high-frequency component.  FIG. 5  is a diagram for explaining a process performed by the transient information converter  132  according to the first embodiment. The horizontal axis in  FIG. 5  corresponds to the time axis. For example, assume that the transient information of the low-frequency component indicates that the second subframe in the (n−2)th frame of the signal  70   b  includes a transient. 
     The transient information converter  132  determines which frame in the signal  70   c  corresponds to the time point obtained by adding a certain time period to the time point of the second subframe in the (n−2)th frame of the signal  70   b . In the example illustrated in  FIG. 5 , the time point obtained by adding a certain time period to the time point of the second subframe in the (n−2)th frame of the signal  70   b  corresponds to the nth frame of the signal  70   c . That is, it is found that the nth frame of the signal  70   c  includes a subframe including a transient. 
     The transient information converter  132  generates transient information of the high-frequency component based on the determination result.  FIG. 6  is a diagram illustrating an exemplary data structure of the transient information of the high-frequency component according to the first embodiment. As illustrated in  FIG. 6 , the transient information of the high-frequency component includes data on the presence/absence of a transient and the frame number. For example, as described in  FIG. 5 , when the nth frame of the signal  70   c  includes a transient, the data on the presence/absence of a transient is “presence”, and the data on the frame number is “n”. The transient information converter  132  outputs the transient information of the high-frequency component to the high-frequency transient detector  133 . 
     The high-frequency transient detector  133  is a processor that narrows down a frame to be subjected to detection of the presence/absence of a transient, based on the transient information of the high-frequency component, and that detects a subframe including a transient from the narrowed-down frame. For example, the case where the high-frequency transient detector  133  obtains the transient information of the high-frequency component as illustrated in  FIG. 6  will be described. 
     For example, when the high-frequency transient detector  133  obtains the transient information of the high-frequency component as illustrated in  FIG. 6 , the high-frequency transient detector  133  divides the nth frame into 16 sections so as to generate subframes. Then, the high-frequency transient detector  133  analyzes the subframes and detects a subframe including a transient. For example, the high-frequency transient detector  133  detects a subframe having an abrupt amplitude change as the subframe including a transient. 
     The high-frequency transient detector  133  outputs the fame number and the subframe number at which a transient is included, to the high-frequency encoder  134 . 
     The high-frequency encoder  134  is a processor that encodes the high-frequency component of the audio signal based on the detection result obtained by the high-frequency transient detector  133 . The high-frequency encoder  134  encodes a frame including no transients with a high frequency resolution. For example, a frequency resolution which is equal to or more than a certain resolution is used. 
     In contrast, the high-frequency encoder  134  encodes the subframes in the frame including a transient with a high time resolution. For example, a time resolution which is equal to or more than a certain resolution is used. The high-frequency encoder  134  may encode a subframe including no transients with a high frequency resolution. The high-frequency encoder  134  outputs the encoded audio signal to the multiplexer  140 . 
     Now, a procedure performed by the encoding apparatus  100  will be described.  FIG. 7  is a flowchart of the procedure performed by the encoding apparatus  100  according to the first embodiment. The process illustrated in  FIG. 7  is executed when, for example, an audio signal is obtained. As illustrated in  FIG. 7 , the encoding apparatus  100  obtains an audio signal in operation S 101 , and generates transient information of the low-frequency component based on the low-frequency component of the audio signal in operation S 102 . The encoding apparatus  100  performs AAC encoding in operation S 103 . 
     The encoding apparatus  100  holds the transient information of the low-frequency component of the audio signal in operation S 104 , and converts the transient information of the low-frequency component into transient information of the high-frequency component in operation S 105 . The encoding apparatus  100  performs frequency conversion in operation S 106 , and specifies a corresponding frame in operation S 107 . In operation S 107 , the corresponding frame is a frame specified from the transient information of the high-frequency component. 
     The encoding apparatus  100  determines whether the subframes included in the corresponding frame include a transient in operation S 108 . The encoding apparatus  100  performs SBR encoding based on the determination result in operation S 109 , and generates a bit stream in operation S 110 . 
     Now, an effect of the encoding apparatus  100  according to the first embodiment will be described. The encoding apparatus  100  converts the transient information of the low-frequency component into the transient information of the high-frequency component, and estimates a frame including a transient, in the audio signal having the high-frequency component. Thus, the SBR encoder  130  does not necessarily detect the presence/absence of a transient for all of the frames of an audio signal having a high-frequency component, resulting in reduction in the processing load. 
     Second Embodiment 
     Now, an encoding apparatus according to a second embodiment will be described.  FIG. 8  is a diagram illustrating a configuration of the encoding apparatus according to the second embodiment. As illustrated in  FIG. 8 , an encoding apparatus  200  includes a downsampler  210 , an AAC encoder  220 , an SBR encoder  230 , and a multiplexer  240 . 
     The downsampler  210  is a processor that performs downsampling on an audio signal. The downsampler  210  outputs the audio signal having a low-frequency component obtained through the downsampling, to the AAC encoder  220 . 
     The AAC encoder  220  is a processor that applies AAC to the audio signal having the low-frequency component so as to encode the audio signal having the low-frequency component. The AAC encoder  220  outputs the encoded audio signal having the low-frequency component to the multiplexer  240 . 
     The AAC encoder  220  divides the audio signal having the low-frequency component into multiple subframes, and analyzes whether each of the subframes has a transient. The AAC encoder  220  separates the subframes into an arbitrary number of groups in accordance with the position of the transient, and outputs the determination result to the SBR encoder  230 . In the description below, the determination result as to whether or not each group has a transient is referred to as grouping information. 
     The SBR encoder  230  is a processor that encodes the high-frequency component of an audio signal. The SBR encoder  230  outputs the encoded high-frequency component of the audio signal to the multiplexer  240 . The SBR encoder  230  controls quantization so that the time resolution is set to high when the audio signal has a transient, and the frequency resolution is set to high when the audio signal is stationary. 
     The SBR encoder  230  converts the grouping information obtained from the AAC encoder  220  into transient information of the high-frequency component, and determines whether or not the audio signal has a transient based on the transient information of the high-frequency component. A process in which the SBR encoder  230  converts the grouping information into the transient information of the high-frequency component will be described below. 
     The multiplexer  240  is a processor that multiplexes the encoded audio signal having the low-frequency component and the encoded audio signal having the high-frequency component and that outputs the multiplexed audio signal to an external apparatus. 
     Now, an exemplary configuration of the AAC encoder  220  and the SBR encoder  230  which are illustrated in  FIG. 8  will be described.  FIG. 9  is a functional block diagram illustrating a configuration of the AAC encoder  220  and the SBR encoder  230  according to the second embodiment. 
     As illustrated in  FIG. 9 , the AAC encoder  220  includes a low-frequency transient detector  221 , a low-frequency converter  222 , and a low-frequency encoder  223 . The SBR encoder  230  includes a high-frequency converter  231 , a transient information converter  232 , a high-frequency transient detector  233 , and a high-frequency encoder  234 . 
     The low-frequency transient detector  221  sequentially obtains the frames of the audio signal obtained through the downsampling, divides each of the frames into eight subframes, and classifies the subframes into an arbitrary number of groups.  FIGS. 10 and 11  are diagrams for explaining a process performed by the low-frequency transient detector  221  according to the second embodiment. In the example illustrated in  FIG. 10 , the low-frequency transient detector  221  classifies subframes # 0  to # 3  into a group  1 , a subframe # 4  into a group  2 , and subframes # 5  to # 7  into a group  3 . 
     The low-frequency transient detector  221  analyzes subframes in each of the groups, and detects a subframe including a transient. In the example illustrated in  FIG. 11 , the low-frequency transient detector  221  has detected a transient in the subframe # 4 . Accordingly, the low-frequency transient detector  221  has classified the subframes # 0  to # 3  into the group  1 , the subframe # 4  into the group  2 , and the subframes # 5  to # 7  into the group  3  so as to perform grouping. The low-frequency transient detector  221  outputs the detection result to the transient information converter  232  as grouping information. In addition, the low-frequency transient detector  221  outputs the detection result to the low-frequency converter  222 . 
       FIG. 12  is a diagram illustrating an exemplary data structure of the grouping information. As illustrated in  FIG. 12 , the grouping information includes data on the presence/absence of a transient, the position of the transient, and the frame number. For example, when the low-frequency transient detector  221  determines that the subframe # 4  of the group  2  in the (n− 2 )th frame has a transient, the data on the presence/absence of a transient is “presence”, the data on the position of the transient is “group  2 , # 4 ”, and the data on the frame number is “n− 2 ”. The grouping information may include information for identifying the way in which subframes have been separated into groups. For example, the grouping information may include information describing that the subframes # 0  to # 3  are classified into the group  1 , the subframe # 4  is classified into the group  2 , and the subframes # 5  to # 7  are classified into the group  3 . 
     The low-frequency converter  222  is a processor that performs frequency conversion on the audio signal in accordance with the detection result obtained by the low-frequency transient detector  221 . The low-frequency converter  222  outputs the audio signal obtained through the frequency conversion, to the low-frequency encoder  223 . 
     Now, the SBR encoder  230  will be described. The high-frequency converter  231  is a processor that performs frequency conversion on an audio signal. The high-frequency converter  231  outputs the audio signal obtained through the frequency conversion, to the high-frequency transient detector  233  and the high-frequency encoder  234 . 
     The transient information converter  232  is a processor that converts the grouping information into the transient information of the high-frequency component.  FIG. 13  is a diagram for explaining a process performed by the transient information converter  232  according to the second embodiment. The horizontal axis in  FIG. 13  corresponds to the time axis. For example, assume that the grouping information indicates that the group  2  in the (n−2)th frame of the signal  70   b  includes a transient. 
     The transient information converter  232  determines which subframe in which frame of the signal  70   c  corresponds to the time point obtained by adding a certain time period to the time point of the group  2  in the (n−2)th frame of the signal  70   b . In the example illustrated in  FIG. 13 , the transient information converter  232  determines that the subframes # 9  to # 11  of the nth frame of the signal  70   c  correspond to the group  2 . The transient information converter  232  determines that the subframe # 9  which is the first subframe among the subframes # 9  to # 11  includes a transient. 
     The transient information converter  232  generates transient information of the high-frequency component based on the determination result.  FIG. 14  is a diagram illustrating an exemplary data structure of the transient information of the high-frequency component according to the second embodiment. As illustrated in  FIG. 14 , the transient information of the high-frequency component includes data on the presence/absence of a transient, the frame number, and the subframe number. For example, as described in  FIG. 13 , when the subframe # 9  in the nth frame of the signal  70   c  includes a transient, the data on the presence/absence of a transient is “presence”, the data on the frame number is “n”, and the data on the subframe number is “# 9 ”. The transient information converter  232  outputs the transient information of the high-frequency component to the high-frequency transient detector  233 . 
     The high-frequency transient detector  233  is a processor that outputs the frame number and the subframe number, at which a transient is included, based on the transient information of the high-frequency component to the high-frequency encoder  234 . 
     The high-frequency encoder  234  is a processor that encodes the high-frequency component of the audio signal based on the information obtained from the high-frequency transient detector  233 . The high-frequency encoder  234  encodes a frame including no transients with a high frequency resolution. For example, a frequency resolution which is equal to or more than a certain resolution is used. 
     In contrast, the high-frequency encoder  234  encodes the subframes in the frame including a transient with a high time resolution. For example, a time resolution which is equal to or more than a certain resolution is used. The high-frequency encoder  234  may encode a subframe including no transients with a high frequency resolution. The high-frequency encoder  234  outputs the encoded audio signal to the multiplexer  240 . 
     Now, a procedure performed by the encoding apparatus  200  will be described.  FIG. 15  is a flowchart of the procedure performed by the encoding apparatus  200  according to the second embodiment. The process illustrated in  FIG. 15  is executed when, for example, an audio signal is obtained. As illustrated in  FIG. 15 , the encoding apparatus  200  obtains an audio signal in operation S 201 . Based on the low-frequency component of the audio signal, the encoding apparatus  200  detects the presence/absence of a transient and its position, and generates grouping information in operation S 202 . The encoding apparatus  200  performs AAC encoding in operation S 203 . 
     The encoding apparatus  200  holds the grouping information in operation S 204 , and converts the grouping information into transient information of the high-frequency component in operation S 205 . The encoding apparatus  200  performs frequency conversion in operation S 206 . The encoding apparatus  200  determines whether the high-frequency component of the audio signal include a transient based on the transient information of the high-frequency component in operation S 207 . 
     The encoding apparatus  200  performs SBR encoding based on the determination result in operation S 208 , and generates a bit stream in operation S 209 . 
     Now, an effect of the encoding apparatus  200  according to the second embodiment will be described. The encoding apparatus  200  converts the grouping information into the transient information of the high-frequency component, and detects a subframe including a transient, without performing an actual transient detection process on the audio signal having the high-frequency component. Accordingly, the SBR encoder  230  does not necessarily detect a transient directly from the audio signal, resulting in reduction in the implementation scale and the processing load. 
     Third Embodiment 
     Now, an encoding apparatus according to a third embodiment will be described.  FIG. 16  is a diagram illustrating a configuration of the encoding apparatus according to the third embodiment. As illustrated in  FIG. 16 , an encoding apparatus  300  includes a downsampler  310 , an AAC encoder  320 , an SBR encoder  330 , and a multiplexer  340 . 
     The downsampler  310  is a processor that performs downsampling on an audio signal. The downsampler  310  outputs the audio signal having a low-frequency component obtained through the downsampling, to the AAC encoder  320 . 
     The AAC encoder  320  is a processor that applies AAC to the audio signal having the low-frequency component so as to encode the audio signal having the low-frequency component. The AAC encoder  320  outputs the encoded audio signal having the low-frequency component to the multiplexer  340 . 
     The AAC encoder  320  divides the audio signal having the low-frequency component into multiple subframes. The AAC encoder  320  determines whether or not each of the subframes includes a transient, and outputs the determination result to the SBR encoder  330 . In the description below, the determination result as to whether or not each of the subframes has a transient is referred to as transient information of the low-frequency component. 
     The SBR encoder  330  converts the transient information of the low-frequency component obtained from the AAC encoder  320  into transient information of the high-frequency component, and determines whether or not the audio signal has a transient based on the transient information of the high-frequency component. A process will be described below in which the SBR encoder  330  converts the transient information of the low-frequency component into the transient information of the high-frequency component. 
     The multiplexer  340  is a processor that multiplexes the encoded audio signal having the low-frequency component and the encoded audio signal having the high-frequency component and that outputs the multiplexed audio signal to an external apparatus. 
     Now, an exemplary configuration of the AAC encoder  320  and the SBR encoder  330  which are illustrated in  FIG. 16  will be described.  FIG. 17  is a functional block diagram illustrating a configuration of the AAC encoder  320  and the SBR encoder  330  according to the third embodiment. 
     As illustrated in  FIG. 17 , the AAC encoder  320  includes a low-frequency transient detector  321 , a low-frequency converter  322 , and a low-frequency encoder  323 . The SBR encoder  330  includes a high-frequency converter  331 , a transient information converter  332 , a high-frequency transient detector  333 , and a high-frequency encoder  334 . 
     The low-frequency transient detector  321  sequentially obtains the frames of the audio signal obtained through the downsampling, and divides each of the frames into eight subframes. The low-frequency transient detector  321  analyzes each of the subframes and detects a subframe including a transient. The low-frequency transient detector  321  outputs the detection result to the transient information converter  332  as transient information of the low-frequency component. In addition, the low-frequency transient detector  321  outputs the detection result to the low-frequency converter  322 . 
       FIG. 18  is a diagram illustrating an exemplary data structure of transient information of the low-frequency component according to the third embodiment. As illustrated in  FIG. 18 , the transient information of the low-frequency component includes data on the presence/absence of a transient, the position of the transient, and the frame number. For example, when the subframe # 1  in the (n−2)th frame includes a transient, the data on the presence/absence of a transient is “presence”, the data on the position of the transient is “# 1 ”, and the data on the frame number is “n−2”. 
     The low-frequency converter  322  is a processor that performs frequency conversion on the audio signal in accordance with the detection result obtained by the low-frequency transient detector  321 . The low-frequency converter  322  outputs the audio signal obtained through the frequency conversion, to the low-frequency encoder  323 . 
     Now, the SBR encoder  330  will be described. The high-frequency converter  331  is a processor that performs frequency conversion on an audio signal. The high-frequency converter  331  outputs the audio signal obtained through the frequency conversion, to the high-frequency transient detector  333  and the high-frequency encoder  334 . 
     The transient information converter  332  is a processor that converts the transient information of the low-frequency component into the transient information of the high-frequency component.  FIG. 19  is a diagram for explaining a process performed by the transient information converter  332  according to the third embodiment. The horizontal axis in  FIG. 19  corresponds to the time axis. For example, assume that the transient information of the low-frequency component indicates that the subframe # 1  in the (n−2)th frame of the signal  70   b  includes a transient. 
     The transient information converter  332  determines which subframe in which frame of the signal  70   c  corresponds to the time point obtained by adding a certain time period to the time point of the subframe # 1  in the (n−2)th frame of the signal  70   b . In the example illustrated in  FIG. 19 , the transient information converter  332  determines that the subframes # 8  to # 10  of the signal  70   c  correspond to the subframe # 1 . The transient information converter  332  determines that the subframe # 8  which is the first subframe among the subframes # 8  to # 10  includes a transient. 
     The transient information converter  332  generates transient information of the high-frequency component based on the determination result.  FIG. 20  is a diagram illustrating an exemplary data structure of the transient information of the high-frequency component according to the third embodiment. As illustrated in  FIG. 20 , the transient information of the high-frequency component includes data on the presence/absence of a transient, the frame number, and the subframe number. For example, as described in  FIG. 19 , when the subframe # 8  in the nth frame of the signal  70   c  includes a transient, the data on the presence/absence of a transient is “presence”, the data on the frame number is “n”, and the data on the subframe number is “# 8 ”. The transient information converter  332  outputs the transient information of the high-frequency component to the high-frequency transient detector  333 . 
     The high-frequency transient detector  333  is a processor that outputs the frame number and the subframe number, at which a transient is included, based on the transient information of the high-frequency component to the high-frequency encoder  334 . 
     The high-frequency encoder  334  is a processor that encodes the high-frequency component of the audio signal based on the information obtained from the high-frequency transient detector  333 . The high-frequency encoder  334  encodes a frame including no transients with a high frequency resolution. For example, a frequency resolution which is equal to or more than a certain resolution is used. 
     In contrast, the high-frequency encoder  334  encodes the subframes in the frame including a transient with a high time resolution. For example, a time resolution which is equal to or more than a certain resolution is used. The high-frequency encoder  334  may encode a subframe including no transients with a high frequency resolution. The high-frequency encoder  334  outputs the encoded audio signal to the multiplexer  340 . 
     Now, a procedure performed by the encoding apparatus  300  will be described.  FIG. 21  is a flowchart of the procedure performed by the encoding apparatus  300  according to the third embodiment. For example, The process illustrated in  FIG. 21  is executed when an audio signal is obtained. As illustrated in  FIG. 21 , the encoding apparatus  300  obtains an audio signal in operation S 301 . The encoding apparatus  300  generates transient information of the low-frequency component based on the low-frequency component of the audio signal in operation S 302 . The encoding apparatus  300  performs AAC encoding in operation S 303 . 
     The encoding apparatus  300  holds the transient information of the low-frequency component in operation S 304 , and converts the transient information of the low-frequency component into transient information of the high-frequency component in operation S 305 . The encoding apparatus  300  performs frequency conversion in operation S 306 . The encoding apparatus  300  detects a subframe including a transient based on the transient information of the high-frequency component in operation S 307 . 
     The encoding apparatus  300  performs SBR encoding based on the detection result in operation S 308 , and generates a bit stream in operation S 309 . 
     Now, an effect of the encoding apparatus  300  according to the third embodiment will be described. The encoding apparatus  300  converts the transient information of the low-frequency component into the transient information of the high-frequency component, and detects a subframe including a transient, without performing an actual transient detection process on the audio signal having the high-frequency component. Accordingly, the SBR encoder  330  does not necessarily detect a transient directly from the audio signal, resulting in reduction in the implementation scale and the processing load. 
     Now, an alternative process performed by the encoding apparatus  300  will be described. In the example illustrated in  FIG. 19 , it is determined that a transient is included in the subframe # 8  which is the first subframe among the subframes # 8  to # 10 . However, the determination is not limited to this. For example, the transient information converter  332  may output information describing the frame number n and the subframes # 8  to # 10  of the signal  70   c  to the high-frequency transient detector  333  as transient information of the high-frequency component. 
     In this case, the high-frequency transient detector  333  performs detection of a transient on the subframes # 8  to # 10  of the nth frame, and outputs the detection result to the high-frequency encoder  334 . Thus, the encoding apparatus  300  determines whether or not a transient is included, only for subframes including a transient, resulting in reduction in the processing load. 
     Now, an exemplary computer will be described which executes encoding programs for achieving functions similar to the encoding apparatuses described in the first to third embodiments.  FIG. 22  is a diagram illustrating the exemplary computer which executes the encoding programs. 
     As illustrated in  FIG. 22 , a computer  500  includes a central processing unit (CPU)  501  which executes various kinds of arithmetic processing, an input apparatus  502  which receives data input from a user, and a display  503 . The computer  500  also includes a readout apparatus  504  which reads out programs and the like from storage media, and an interface apparatus  505  which receives/sends data from/to other computers via a network. The computer  500  further includes a random-access memory (RAM)  506  which stores various kinds of information temporarily, and a hard disk apparatus  507 . The CPU  501 , the input apparatus  502 , the display  503 , the readout apparatus  504 , the interface apparatus  505 , the RAM  506 , and the hard disk apparatus  507  are connected to a bus  508 . 
     The hard disk apparatus  507  includes, for example, a downsampling program  507   a , an AAC program  507   b , an SBR program  507   c , and a multiplexing program  507   d . The CPU  501  reads out the downsampling program  507   a , the AAC program  507   b , the SBR program  507   c , and the multiplexing program  507   d , and develops them in the RAM  506 . 
     The downsampling program  507   a  functions as a downsampling process  506   a . The AAC program  507   b  functions as an AAC process  506   b . The SBR program  507   c  functions as an SBR process  506   c . The multiplexing program  507   d  functions as a multiplexing process  506   d.    
     For example, the downsampling process  506   a  corresponds to the downsamplers  110 ,  210 , and  310 . The AAC process  506   b  corresponds to the AAC encoders  120 ,  220 , and  320 . The SBR process  506   c  corresponds to the SBR encoders  130 ,  230 , and  330 . The multiplexing process  506   d  corresponds to the multiplexers  140 ,  240 , and  340 . 
     The downsampling program  507   a , the AAC program  507   b , the SBR program  507   c , and the multiplexing program  507   d  are not necessarily stored in advance in the hard disk apparatus  507 . For example, these programs are stored in a “portable physical medium”, such as a flexible disk (FD), a compact disk-read-only memory (CD-ROM), a digital versatile disk (DVD), a magneto-optical disk, or an integrated circuit (IC) card, which is inserted into the computer  500 . Then, the computer  500  may read out the downsampling program  507   a , the AAC program  507   b , the SBR program  507   c , and the multiplexing program  507   d  from the inserted medium and execute them. 
     Each of the downsampler  110 , the AAC encoder  120 , the SBR encoder  130 , and the multiplexer  140  illustrated in  FIG. 1  corresponds to, for example, an integrated device, such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA). In addition, each of the downsampler  110 , the AAC encoder  120 , the SBR encoder  130 , and the multiplexer  140  corresponds to, for example, an electronic circuit, such as a CPU or a micro processing unit (MPU). Furthermore, each of the downsampler  110 , the AAC encoder  120 , the SBR encoder  130 , and the multiplexer  140  may have a storage device. Similar descriptions are made for the downsampler  210 , the AAC encoder  220 , the SBR encoder  230 , and the multiplexer  240 , which are illustrated in  FIG. 8 , and the downsampler  310 , the AAC encoder  320 , the SBR encoder  330 , and the multiplexer  340 , which are illustrated in  FIG. 16 . 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation 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 the embodiments of the present invention 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.