Patent Publication Number: US-8972249-B2

Title: Decoding apparatus and method, encoding apparatus and method, and program

Description:
TECHNICAL FIELD 
     The present invention relates to a decoding apparatus, a decoding method, an encoding apparatus, an encoding method, and a program. More particularly, the present invention relates to a decoding apparatus, a decoding method, an encoding apparatus, an encoding method, and a program that can shorten the delay time caused by the band extension at the time of decoding, and restrain increases in resources on the decoding side. 
     BACKGROUND ART 
     As audio signal encoding techniques, the following transform coding techniques have been generally well known: MP3 (Moving Picture Experts Group Audio Layer-3), AAC (Advanced Audio Coding), and ATRAC (Adaptive Transform Acoustic Coding). 
     In such an encoding technique, results of encoding do not include a higher frequency spectrum containing a large amount of information, but include only the envelope of the higher frequency spectrum, so as to achieve a higher encoding efficiency. At the time of decoding in such a case, a lower frequency spectrum is duplicated by parallel translation, replication, or the like, to generate a higher frequency spectrum. Only the envelope of the generated higher frequency spectrum is made closer to the envelope of the original higher frequency spectrum contained in the results of encoding, to improve auditory quality. Such a decoding technique is called a band extension technique, and has been already known to the general public. 
       FIG. 1  is a block diagram showing an example structure of an encoding apparatus that has only the envelope of the higher frequency spectrum in the results of encoding. 
     The encoding apparatus  10  of  FIG. 1  includes a MDCT (Modified Discrete Cosine Transform) unit  11 , a quantizing unit  12 , and a multiplexing unit  13 . The encoding apparatus  10  is the same as a generally known transform coding apparatus, except that a higher frequency spectrum SP-H is not included in the results of encoding. For ease of explanation of the drawings, the quantizing unit  12  not only performs quantization but also extracts and normalizes objects to be quantized. 
     Specifically, the MDCT unit  11  of the encoding apparatus  10  performs a MDCT on a PCM (Pulse Code Modulation) signal that is an audio time-domain signal that is input to the encoding apparatus  10 . By doing so, the MDCT unit  11  generates a spectrum SP that is a frequency domain signal. The MDCT unit  11  supplies the generated spectrum SP to the quantizing unit  12 . 
     The quantizing unit  12  extracts envelopes from the higher frequency spectrum SP-H that is the higher frequency components of the spectrum SP supplied from the MDCT unit  11 , and from a lower frequency spectrum SP-L that is the lower frequency components of the spectrum SP. The quantizing unit  12  quantizes a higher frequency envelope ENV-H that is the extracted envelope of the higher frequency spectrum SP-H, and a lower frequency envelope ENV-L that is the extracted envelope of the lower frequency spectrum SP-L. The quantizing unit  12  supplies the quantized higher frequency envelope ENV-H and lower frequency envelope ENV-L to the multiplexing unit  13 . In this specification, the names (such as SP-L and SP-H) of signals are the same before and after quantization and encoding, for ease of explanation. 
     The quantizing unit  12  normalizes the lower frequency spectrum SP-L, using the lower frequency envelope ENV-L. The quantizing unit  12  quantizes the normalized lower frequency spectrum SP-L, and supplies the resultant lower frequency spectrum SP-L to the multiplexing unit  13 . 
     As described above, the quantizing unit  12  has the envelope and the normalized spectrum included in the results of encoding of the lower frequency components of the spectrum SP, but has only the envelope included in the results of encoding of the higher frequency components. Accordingly, the encoding efficiency becomes higher. 
     The multiplexing unit  13  multiplexes the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H, which are supplied from the quantizing unit  12 . The multiplexing unit  13  outputs the resultant bit stream. This bit stream is recorded on a recording medium (not shown), or is transferred to a decoding apparatus. 
       FIG. 2  is a flowchart for explaining an encoding operation to be performed by the encoding apparatus  10  of  FIG. 1 . This encoding operation is started when an audio PCM signal is input to the encoding apparatus  10 , for example. 
     In step S 11  of  FIG. 2 , the MDCT unit  11  performs a MDCT on a PCM signal that is an audio time-domain signal that is input to the encoding apparatus  10 , and generates the spectrum SP that is a frequency domain signal. The MDCT unit  11  supplies the generated spectrum SP to the quantizing unit  12 . 
     In step S 12 , the quantizing unit  12  extracts envelopes from the higher frequency spectrum SP-H that is the higher frequency components of the spectrum SP supplied from the MDCT unit  11 , and from the lower frequency spectrum SP-L that is the lower frequency components of the spectrum SP. 
     In step S 13 , the quantizing unit  12  normalizes the lower frequency spectrum SP-L, using the lower frequency envelope ENV-L. 
     In step S 14 , the quantizing unit  12  performs quantization on the extracted higher frequency envelope ENV-H, lower frequency envelope ENV-L, and on the normalized lower frequency spectrum SP-L. The quantizing unit  12  supplies the quantized higher frequency envelope ENV-H, lower frequency envelope ENV-L, and the normalized lower frequency spectrum SP-L to the multiplexing unit  13 . 
     In step S 15 , the multiplexing unit  13  multiplexes the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H, which are supplied from the quantizing unit  12 . The multiplexing unit  13  outputs the resultant bit stream. This operation then comes to an end. 
       FIG. 3  is a block diagram showing an example structure of a decoding apparatus that decodes bit streams encoded by the encoding apparatus  10  of  FIG. 1 . 
     The decoding apparatus  30  of  FIG. 3  includes a dividing unit  31 , an inverse quantizing unit  32 , an inverse MDCT unit  33 , and a band extending unit  34 . 
     The dividing unit  31 , the inverse quantizing unit  32 , and the inverse MDCT unit  33  of the decoding apparatus  30  decodes only the lower frequency components of PCM signals, like a conventional transform decoding apparatus. 
     Specifically, the dividing unit  31  obtains a bit stream encoded by the encoding apparatus  10 , and divides the bit stream into the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H. The dividing unit  31  then supplies the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H to the inverse quantizing unit  32 . 
     The inverse quantizing unit  32  performs inverse quantization on the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H, which are supplied from the dividing unit  31 . The inverse quantizing unit  32  then supplies the inversely-quantized lower frequency envelope ENV-L and lower frequency spectrum SP-L to the inverse MDCT unit  33 , and supplies the higher frequency envelope ENV-H to the band extending unit  34 . 
     Using the lower frequency envelope ENV-L supplied from the inverse quantizing unit  32 , the inverse MDCT unit  33  denormalizes the lower frequency spectrum SP-L. The inverse MDCT unit  33  performs an inverse MDCT on the lower frequency spectrum SP-L, which is a denormalized frequency domain signal, and obtains a PCM signal that is a time domain signal. This PCM signal is a PCM signal not containing higher frequency components, and is a PCM signal of auditorily muffled sound. The inverse MDCT unit  33  supplies the PCM signal to the band extending unit  34 . 
     The band extending unit  34  includes a band dividing filter  41 , a higher frequency component generating unit  42 , and a band combining filter  43 . The band extending unit  34  extends the frequency band of the PCM signal that is obtained by the inverse MDCT unit  33  and does not contain higher frequency components. By doing so, the band extending unit  34  performs a band extending operation to improve the sound quality of the PCM signal. 
     Specifically, the band dividing filter  41  of the band extending unit  34  divides the PCM signal supplied from the inverse MDCT unit  33  into higher frequency components and lower frequency components. Since this PCM signal does not contain higher frequency components, the band dividing filter  41  discards the higher frequency components of the divided PCM signal. The band dividing filter  41  also supplies a lower frequency PCM signal BS-L, which is the lower frequency components of the divided PCM signal, to the higher frequency component generating unit  42  and the band combining filter  43 . 
     Using the lower frequency PCM signal BS-L supplied from the band dividing filter  41  and the higher frequency envelope ENV-H supplied from the inverse quantizing unit  32 , the higher frequency component generating unit  42  generates a higher frequency PCM signal to be a pseudo higher frequency PCM signal BS-H. An example method of generating the pseudo higher frequency PCM signal BS-H is disclosed in Patent Document 1, which was filed by the applicant. The higher frequency component generating unit  42  supplies the pseudo higher frequency PCM signal BS-H to the band combining filter  43 . 
     The band combining filter  43  combines the lower frequency PCM signal BS-L supplied from the band dividing filter  41  with the pseudo higher frequency PCM signal BS-H supplied from the higher frequency component generating unit  42 , and outputs an entire-band PCM signal as the results of the decoding. 
     The sound corresponding to the entire-band PCM signal that is output in the above described manner is less muffled than the sound corresponding to the PCM signal not containing higher frequency components, and is a beautiful and comfortable sound. 
       FIG. 4  is a diagram for explaining the signals that are output from the inverse MDCT unit  33  and the band combining filter  43 . In  FIG. 4 , the abscissa axis indicates frequency, and the ordinate axis indicates signal level. This also applies to  FIGS. 7 ,  10 , and  12  through  16 , which will be described later. 
     The signal that is output from the inverse MDCT unit  33  is the PCM signal of the lower frequency spectrum SP-L denormalized by using the lower frequency envelope ENV-L, as shown in A in  FIG. 4 . The signal that is output from the band combining filter  43  is a PCM signal that contains lower frequency components as the PCM signal of the lower frequency spectrum SP-L denormalized by using the lower frequency envelope ENV-L, and higher frequency components as the pseudo higher frequency PCM signal BS-H generated from the higher frequency envelope ENV-H and the lower frequency PCM signal BS-L, as shown in B in  FIG. 4 . 
       FIG. 5  is a flowchart for explaining a decoding operation to be performed by the decoding apparatus  30  of  FIG. 3 . This decoding operation is started when a bit stream encoded by the encoding apparatus  10  is input to the decoding apparatus  30 , for example. 
     In step S 31  of  FIG. 5 , the dividing unit  31  divides the bit stream input to the decoding apparatus  30  into the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H. The dividing unit  31  then supplies the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H to the inverse quantizing unit  32 . 
     In step S 32 , the inverse quantizing unit  32  performs inverse quantization on the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H, which are supplied from the dividing unit  31 . The inverse quantizing unit  32  supplies the inversely-quantized lower frequency envelope ENV-L and lower frequency spectrum SP-L to the inverse MDCT unit  33 . The inverse quantizing unit  32  supplies the higher frequency envelope ENV-H to the band extending unit  34 . 
     In step S 33 , the inverse MDCT unit  33  denormalizes the lower frequency spectrum SP-L, using the lower frequency envelope ENV-L supplied from the inverse quantizing unit  32 . 
     In step S 34 , the inverse MDCT unit  33  performs an inverse MDCT on the lower frequency spectrum SP-L, which is a denormalized frequency domain signal, and obtains a PCM signal that is a time domain signal. The inverse MDCT unit  33  supplies the PCM signal to the band extending unit  34 . 
     In step S 35 , the band dividing filter  41  of the band extending unit  34  divides the PCM signal supplied from the inverse MDCT unit  33  into higher frequency components and lower frequency components. The band dividing filter  41  discards the higher frequency components of the divided PCM signal, and supplies the lower frequency PCM signal BS-L, which is the lower frequency components of the divided PCM signal, to the higher frequency component generating unit  42  and the band combining filter  43 . 
     In step S 36 , the higher frequency component generating unit  42  generates the pseudo higher frequency PCM signal BS-H, using the lower frequency PCM signal BS-L supplied from the band dividing filter  41  and the higher frequency envelope ENV-H supplied from the inverse quantizing unit  32 . The higher frequency component generating unit  42  supplies the pseudo higher frequency PCM signal BS-H to the band combining filter  43 . 
     In step S 37 , the band combining filter  43  combines the lower frequency PCM signal BS-L supplied from the band dividing filter  41  with the pseudo higher frequency PCM signal BS-H supplied from the higher frequency component generating unit  42 , to obtain the entire-band PCM signal. The band combining filter  43  outputs the entire-band PCM signal, and the operation comes to an end. 
     The above described band extension technique has been already used in HE-AAC (High-Efficiency Advanced Audio Coding), which is an international standard, and in the stereo high-quality mode of LPEC (trade name). 
     As described above, by the conventional band extension technique, the band extending operation is performed as the post processing for the decoding of the lower frequency spectrum SP-L. Accordingly, the degree of freedom of the pseudo higher frequency PCM signal BS-H can be made higher. That is, the pseudo higher frequency PCM signal BS-H can be generated not from the lower frequency spectrum SP-L, which is a frequency domain signal, but from the lower frequency PCM signal BS-L, which is a time domain signal. 
     The processing block sizes in the encoding operation and the decoding operation, and the processing block size in the band extending operation are arbitrarily set, so as to optimize frequency analysis precision and time resolving precision. 
     In a case where the pseudo higher frequency PCM signal is generated by the technique disclosed in Patent Document 1, complicated procedures need to be carried out to generate a noise spectrum from the higher frequency envelope ENV-H, generate a tonic spectrum from the higher frequency envelope ENV-H and the lower frequency PCM signal BS-L, and compare the two spectrums. 
     The process of generating the noise spectrum and the tonic spectrum is the necessary process in increasing the matching accuracy between the lower frequency spectrum and the higher frequency spectrum to generate sound with high auditory quality, and is also performed in the decoding apparatuses disclosed in Patent Documents 2 and 3. 
     CITATION LIST 
     Patent Documents 
     Patent Document 1: Japanese Patent No. 3861770 
     Patent Document 2: Japanese Patent No. 3646938 
     Patent Document 3: Japanese Patent No. 3646939 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     As described above, the conventional band extension technique has been studied, developed, and put into practice in such a manner that the band extending operation is performed as the post processing for the decoding of the lower frequency spectrum SP-L. Therefore, the entire-band PCM signal is output after the processing time required by the band extending unit  34  has passed (time T 1  in the example illustrated in  FIG. 3 ) from the end of the conventional decoding operation performed by the dividing unit  31 , the inverse quantizing unit  32 , and the inverse MDCT unit  33  (time T 0  in the example illustrated in  FIG. 3 ). 
     This does not cause a serious problem, if the decoding apparatus  30  is provided in a reproducing apparatus that reproduces only sound. In a case where the decoding apparatus  30  is provided in a reproducing apparatus that reproduces video images in synchronization with sound, however, there is a difference in the output time of the entire-band PCM signal between a case where only the conventional decoding is performed and a case where the band extension is also performed. As a result, outputting video images in synchronization with sound becomes difficult. 
     To solve this problem, the timing to reproduce video images needs to be delayed. However, video image buffering requires a memory with a larger capacity than that for sound buffering, resulting in an increase in resources. The synchronizing timing between video images and sound may be delayed in advance. However, whether to perform only the conventional decoding and whether to perform the band extension as well as the conventional decoding depend on the reproducing apparatus to be used. Therefore, it is difficult to constantly designate the optimum synchronizing timing. 
     The decoding apparatus  30  needs to additionally include the band extending unit  34  for the band extension, resulting in more resources than in a decoding apparatus that does not perform the band extension. 
     In view of the above, decoding apparatuses that perform the band extension are expected to shorten the delay time caused by the band extension and restrain increases in resources. 
     The present invention has been made in view of the above circumstances, and the object thereof is to shorten the delay time caused by the band extension at the time of decoding, and restrain increases in resources on the decoding side. 
     Solutions to Problems 
     A decoding apparatus according to a first aspect of the present invention includes: an obtaining unit configured to obtain, as encoding results, a lower frequency envelope of an audio signal, a lower frequency spectrum normalized by using the lower frequency envelope, a higher frequency envelope of the audio signal, and a degree of concentration of a higher frequency spectrum of the audio signal; a generating unit configured to generate a spectrum by using the normalized lower frequency spectrum and the higher frequency envelope in the encoding results obtained by the obtaining unit; a randomizing unit configured to randomize a phase of the spectrum, based on the degree of concentration, the spectrum being generated by the generating unit; and a combining unit configured to denormalize the lower frequency spectrum by using the lower frequency envelope in the encoding results obtained by the obtaining unit, and combine the spectrum randomized by the randomizing unit or the spectrum generated by the generating unit with the denormalized lower frequency spectrum, a result of the combination being used as a spectrum of an entire band. 
     A decoding method and a program of the first aspect of the present invention correspond to the decoding apparatus of the first aspect of the present invention. 
     In the first aspect of the present invention, the lower frequency envelope of an audio signal, the lower frequency spectrum normalized by using the lower frequency envelope, the higher frequency envelope of the audio signal, and the degree of concentration of the higher frequency spectrum of the audio signal are obtained as encoding results. A spectrum is generated by using the lower frequency spectrum and the higher frequency envelope in the obtained encoding results. Based on the degree of concentration, the phase of the spectrum is randomized. The lower frequency spectrum is denormalized by using the lower frequency envelope in the obtained encoding results. The randomized spectrum or the generated spectrum is combined with the denormalized lower frequency spectrum, and the combination result is used as the spectrum of the entire band. 
     A decoding apparatus according to a second aspect of the present invention includes: an obtaining unit configured to obtain, as encoding results, a lower frequency envelope of an audio signal, a lower frequency spectrum normalized by using the lower frequency envelope, and a higher frequency envelope of the audio signal; a generating unit configured to generate a spectrum by using the normalized lower frequency spectrum and the higher frequency envelope in the encoding results obtained by the obtaining unit; a determining unit configured to determine a degree of concentration of the lower frequency spectrum, based on the normalized lower frequency spectrum in the encoding results obtained by the obtaining unit; a randomizing unit configured to randomize a phase of the spectrum, based on the degree of concentration determined by the determining unit, the spectrum being generated by the generating unit; and a combining unit configured to denormalize the lower frequency spectrum by using the lower frequency envelope in the encoding results obtained by the obtaining unit, and combine the spectrum randomized by the randomizing unit or the spectrum generated by the generating unit with the denormalized lower frequency spectrum, a result of the combination being used as a spectrum of an entire band. 
     A decoding method and a program of the second aspect of the present invention correspond to the decoding apparatus of the second aspect of the present invention. 
     In the second aspect of the present invention, the lower frequency envelope of an audio signal, the lower frequency spectrum normalized by using the lower frequency envelope, and the higher frequency envelope of the audio signal are obtained as encoding results. A spectrum is generated by using the normalized lower frequency spectrum and the higher frequency envelope in the obtained encoding results. Based on the normalized lower frequency spectrum in the obtained encoding results, the degree of concentration of the lower frequency spectrum is determined. Based on the determined degree of concentration, the phase of the generated spectrum is randomized. The lower frequency spectrum is denormalized by using the lower frequency envelope in the obtained encoding results. The randomized spectrum or the generated spectrum is combined with the denormalized lower frequency spectrum, and the combination result is used as the spectrum of the entire band. 
     An encoding apparatus according to a third aspect of the present invention includes: a determining unit configured to determine a degree of concentration of a higher frequency spectrum of an audio signal, based on the higher frequency spectrum; an extracting unit configured to extract an envelope of a lower frequency spectrum and an envelope of the higher frequency spectrum from a spectrum of the audio signal; a normalizing unit configured to normalize the lower frequency spectrum by using the envelope of the lower frequency spectrum; and a multiplexing unit configured to obtain encoding results by multiplexing the degree of concentration determined by the determining unit, the envelope of the lower frequency spectrum and the envelope of the higher frequency spectrum extracted by the extracting unit, and the lower frequency spectrum normalized by the normalizing unit. 
     An encoding method and a program of the third aspect of the present invention correspond to the encoding apparatus of the third aspect of the present invention. 
     In the third aspect of the present invention, the degree of concentration of the higher frequency spectrum of an audio signal is determined, based on the higher frequency spectrum. The envelope of the lower frequency spectrum and the envelope of the higher frequency spectrum are extracted from the spectrum of the audio signal. The lower frequency spectrum is normalized by using the envelope of the lower frequency spectrum. The determined degree of concentration, the extracted envelope of the lower frequency spectrum, the extracted envelope of the higher frequency spectrum, and the normalized lower frequency spectrum are multiplexed, to obtain encoding results. 
     The decoding apparatus of the first or second aspect and the encoding apparatus of the third aspect may be independent of each other, or may be internal blocks constituting an apparatus. 
     Effects of the Invention 
     According to the first and second aspects of the present invention, the delay time caused by the band extension at the time of decoding can be shortened, and increases in resources can be restrained. 
     According to the third aspect of the present invention, encoding can be performed so that the delay time caused by the band extension at the time of decoding can be shortened, and increases in resources on the decoding side can be restrained. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing an example structure of an encoding apparatus. 
         FIG. 2  is a flowchart for explaining an encoding operation to be performed by the encoding apparatus of  FIG. 1 . 
         FIG. 3  is a block diagram showing an example structure of a decoding apparatus. 
         FIG. 4  is a diagram for explaining the signals that are output from the inverse MDCT unit and the band combining filter. 
         FIG. 5  is a flowchart for explaining a decoding operation to be performed by the decoding apparatus of  FIG. 3 . 
         FIG. 6  is a block diagram showing an example structure of a first embodiment of an encoding apparatus to which the present invention is applied. 
         FIG. 7  is a diagram for explaining the signals that are output from the MDCT unit and the quantizing unit of  FIG. 6 . 
         FIG. 8  is a flowchart for explaining an encoding operation to be performed by the encoding apparatus of  FIG. 6 . 
         FIG. 9  is a block diagram showing an example structure of a decoding apparatus that decodes bit streams encoded by the encoding apparatus of  FIG. 6 . 
         FIG. 10  is a diagram for explaining the signal that is output from the inverse MDCT unit of  FIG. 9 . 
         FIG. 11  is a diagram for explaining the difference in decoding results between a case where phase randomization is performed and a case where phase randomization is not performed. 
         FIG. 12  is a diagram for explaining the characteristics of the higher frequency spectrum SP-H. 
         FIG. 13  is a diagram for explaining the characteristics of the higher frequency spectrum SP-H. 
         FIG. 14  is a diagram for explaining the characteristics of the higher frequency spectrum SP-H. 
         FIG. 15  is a diagram for explaining the characteristics of the higher frequency spectrum SP-H. 
         FIG. 16  is a diagram for explaining the characteristics of the higher frequency spectrum SP-H. 
         FIG. 17  is a flowchart for explaining a decoding operation to be performed by the decoding apparatus of  FIG. 9 . 
         FIG. 18  is a block diagram showing an example structure of a second embodiment of a decoding apparatus to which the present invention is applied. 
         FIG. 19  is a flowchart for explaining a decoding operation to be performed by the decoding apparatus of  FIG. 18 . 
         FIG. 20  is a diagram showing an example structure of a computer. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     First Embodiment 
     Example Structure of First Embodiment of Encoding Apparatus 
       FIG. 6  is a block diagram showing an example structure of a first embodiment of an encoding apparatus to which the present invention is applied. 
     In the structure shown in  FIG. 6 , the same components as those shown in  FIG. 1  are denoted by the same reference numerals as those shown in  FIG. 1 , and the same explanation will not be repeated. 
     The structure of the encoding apparatus  50  of  FIG. 6  differs from the structure of  FIG. 1  in that the quantizing unit  12  and the multiplexing unit  13  are replaced with a quantizing unit  51  and a multiplexing unit  52 . The encoding apparatus  10  generates a bit stream by multiplexing a random flag RND (described later in detail) as well as a lower frequency envelope ENV-L, a lower frequency spectrum SP-L, and a higher frequency envelope ENV-H. 
     Specifically, the quantizing unit  51  of the encoding apparatus  50  includes a determining unit  61 , an extracting unit  62 , a normalizing unit  63 , and a partial quantizing unit  64 . 
     Based on the higher frequency spectrum SP-H of a spectrum SP supplied from a MDCT unit  11 , the determining unit  61  determines the degree of concentration D of the higher frequency spectrum SP-H according to the following equation (1):
 
 D =max( SP - H )/ave( SP - H )  (1)
 
     In the equation (1), max(SP-H) represents the maximum value of the higher frequency spectrum SP-H, and ave(SP-H) represents the average value of the higher frequency spectrum SP-H. 
     According to the equation (1), in a case where the tone characteristics of the higher frequency components of the sound to be encoded are prominent and the distribution of the higher frequency spectrum SP-H has a high degree of bias, the degree of concentration D is high. In a case where the noise characteristics of the higher frequency components of the sound to be encoded are prominent and the distribution of the higher frequency spectrum SP-H is uniform, the degree of concentration D is low. 
     The determining unit  61  determines the random flag RND, based on the degree of concentration D. The random flag RND is a flag that indicates whether to randomize the phase of the spectrum to approximate the higher frequency spectrum SP-H generated from the lower frequency spectrum SP-L and the higher frequency envelope ENV-H in a band extending operation in a later described decoding apparatus. 
     In a case where the degree of concentration D is higher than a threshold value that is set in the encoding apparatus  50  in advance, or where the tone characteristics of the higher frequency spectrum SP-H are prominent, for example, the random flag RND is set to 0, which indicates that randomization is not to be performed. In a case where the degree of concentration D is equal to or lower than the predetermined threshold value, or where the noise characteristics of the higher frequency spectrum SP-H are prominent, the random flag RND is set to 1, which indicates randomization is to be performed. The determining unit  61  supplies the determined random flag RND to the multiplexing unit  52 . 
     Like the quantizing unit  12  of  FIG. 1 , the extracting unit  62  extracts envelopes from the higher frequency spectrum SP-H and the lower frequency spectrum SP-L of the spectrum SP supplied from the MDCT unit  11 . 
     Like the quantizing unit  12 , the normalizing unit  63  normalizes the lower frequency spectrum SP-L, using the lower frequency envelope ENV-L. 
     The partial quantizing unit  64  performs quantization on the normalized lower frequency spectrum SP-L, and supplies the resultant lower frequency spectrum SP-L to the multiplexing unit  52 . Like the quantizing unit  12 , the partial quantizing unit  64  also quantizes the extracted higher frequency envelope ENV-H and lower frequency envelope ENV-L. Like the quantizing unit  12 , the partial quantizing unit  64  supplies the quantized higher frequency envelope ENV-H and lower frequency envelope ENV-L to the multiplexing unit  52 . 
     The multiplexing unit  52  multiplexes the random flag RND supplied from the determining unit  61  of the quantizing unit  51 , as well as the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H, which are supplied from the partial quantizing unit  64 . The multiplexing unit  52  outputs the resultant bit stream. This bit stream is recorded on a recording medium (not shown), or is transferred to a decoding apparatus. 
     [Description of Signals in the Encoding Apparatus] 
       FIG. 7  is a diagram for explaining the signals that are output from the MDCT unit  11  and the quantizing unit  51  of the encoding apparatus  50  of  FIG. 6 . 
     As shown in A in  FIG. 7 , the spectrum SP that is output from the MDCT unit  11  is a spectrum of the entire band. On the other hand, the signal that is output from the quantizing unit  51  and excludes the random flag RND includes the lower frequency spectrum SP-L, the lower frequency envelope ENV-L, and the higher frequency envelope ENV-H, as shown in B in  FIG. 7 . 
     [Description of Operation of the Encoding Apparatus] 
       FIG. 8  is a flowchart for explaining an encoding operation to be performed by the encoding apparatus  50  of  FIG. 6 . This encoding operation is started when an audio PCM signal is input to the encoding apparatus  50 , for example. 
     In step S 51  of  FIG. 8 , the MDCT unit  11  performs a MDCT on the PCM signal that is an audio time-domain signal input to the encoding apparatus  50 , to generate the spectrum SP, which is a frequency domain signal, as in step S 11  of  FIG. 2 . The MDCT unit  11  supplies the generated spectrum SP to the quantizing unit  51 . 
     In step S 52 , based on the higher frequency spectrum SP-H of the spectrum SP supplied from the MDCT unit  11 , the determining unit  61  of the quantizing unit  51  determines the degree of concentration D of the higher frequency spectrum SP-H according to the above described equation (1). 
     In step S 53 , the determining unit  61  determines the random flag RND, based on the degree of concentration D. The determining unit  61  supplies the determined random flag RND to the multiplexing unit  52 , and the operation moves on to step S 54 . 
     The procedures of steps S 54  through S 56  are the same as the procedures of steps S 12  through S 14  of  FIG. 2 , and therefore, explanation of them is not repeated herein. 
     After the procedure of step S 56 , the multiplexing unit  52 , in step S 57 , multiplexes the random flag RND, the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H, which are supplied from the quantizing unit  51 . The multiplexing unit  52  outputs the resultant bit stream. The operation then comes to an end. 
     [Example Structure of the Decoding Apparatus] 
       FIG. 9  is a block diagram showing an example structure of the decoding apparatus that decodes bit streams encoded by the encoding apparatus  50  of  FIG. 6 . 
     The decoding apparatus  70  of  FIG. 9  includes a dividing unit  71 , an inverse quantizing unit  72 , a higher frequency component generating unit  73 , a phase randomizing unit  74 , and an inverse MDCT unit  75 . The decoding apparatus  70  performs a band extending operation at the same time as decoding of the lower frequency spectrum SPL. 
     Specifically, the dividing unit  71  (an obtaining unit) obtains a bit stream encoded by the encoding apparatus  50  of  FIG. 6 . The dividing unit  71  divides the bit stream into the random flag RND, the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H, which are then supplied to the inverse quantizing unit  72 . 
     Like the inverse quantizing unit  32  of  FIG. 3 , the inverse quantizing unit  72  performs inverse quantization on the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H, which are supplied from the dividing unit  71 . 
     The inverse quantizing unit  72  supplies the inversely-quantized lower frequency envelope ENV-L to the inverse MDCT unit  75 , and supplies the lower frequency spectrum SP-L to the inverse MDCT unit  75  and the higher frequency component generating unit  73 . The inverse quantizing unit  72  also supplies the higher frequency envelope ENV-H to the higher frequency component generating unit  73 , and supplies the random flag RND to the phase randomizing unit  74 . 
     Using the lower frequency spectrum SP-L and the higher frequency envelope ENV-H, which are supplied from the inverse quantizing unit  72 , the higher frequency component generating unit  73  generates a higher frequency spectrum to be a pseudo higher frequency spectrum. Specifically, the higher frequency component generating unit  73  duplicates the lower frequency spectrum SP-L, and deforms the duplicated spectrum by using the higher frequency envelope ENV-H, to form the pseudo higher frequency spectrum. 
     To generate this pseudo higher frequency spectrum, the technique disclosed in Patent Document 1, which was filed by the applicant, may be used, or some other technique may also be used. The higher frequency component generating unit  73  supplies the generated pseudo higher frequency spectrum to the phase randomizing unit  74 . 
     Based on the random flag RND supplied from the inverse quantizing unit  72 , the phase randomizing unit  74  randomizes the phase of the pseudo higher frequency spectrum supplied from the higher frequency component generating unit  73 . 
     Specifically, in a case where the random flag RND is 1, which indicates that randomization is to be performed, the phase randomizing unit  74  randomizes the sign (+ or −) of the pseudo higher frequency spectrum, according to the following equation (2):
 
 SP - H ( i )=−1^(rand( )&amp; 0×1)× SP - H ( i )  (2)
 
     In the equation (2), SP-H represents the higher frequency spectrum, and i represents the spectrum number. 
     According to the equation (2), the higher frequency spectrum SP-H is multiplied by “−1” the number of times indicated by the lowest 1 bit of the return value of the random function rand( ), so that −1 or 1 is randomly assigned to the sign of the higher frequency spectrum SP-H. 
     In a case where the random flag RND is 0, which indicates that randomization is not to be performed, the phase randomizing unit  74  does not randomize the phase of the pseudo higher frequency spectrum. 
     The phase randomizing unit  74  supplies the pseudo higher frequency spectrum having its phase randomized or the pseudo higher frequency spectrum not having its phase randomized to the inverse MDCT unit  75 . 
     The inverse MDCT unit  75  (a combining unit) denormalizes the lower frequency spectrum SP-L, using the lower frequency envelope ENV-L supplied from the inverse quantizing unit  72 . The inverse MDCT unit  75  combines the denormalized lower frequency spectrum SP-L with the pseudo higher frequency spectrum supplied from the phase randomizing unit  74 . The inverse MDCT unit  75  performs an inverse MDCT on the entire-band spectrum that is a frequency domain signal obtained as a result of the combination. By doing so, the inverse MDCT unit  75  obtains an entire-band PCM signal that is a time domain signal. The inverse MDCT unit  75  outputs the entire-band PCM signal as the results of the decoding. 
     As described above, the decoding apparatus  70  generates the pseudo higher frequency spectrum at the same time as decoding of the lower frequency spectrum SP-L. Accordingly, the time required for decoding in the decoding apparatus  70  is substantially the same as the time required for decoding in a conventional decoding apparatus that performs only decoding. That is, the decoding apparatus  70  of  FIG. 9  can output results of decoding after time TO has passed from the time of the bit stream input. In other words, any delay is not caused by a band extension in the decoding apparatus  70 . 
     [Description of Signals in the Decoding Apparatus] 
       FIG. 10  is a diagram for explaining the signal that is output from the inverse MDCT unit  75  of the decoding apparatus  70  of  FIG. 9 . 
     The signal that is output from the inverse MDCT unit  75  is a PCM signal obtained after a frequency transform is performed on the result of the combination of the lower frequency spectrum SP-L normalized by using the lower frequency envelope ENV-L as shown in  FIG. 10 , and the pseudo higher frequency spectrum generated from the higher frequency envelope ENV-H and the lower frequency spectrum SP-L as shown in  FIG. 10 . 
     [Description of Effects of Phase Randomization] 
       FIGS. 11 through 16  are diagrams for explaining the effects of phase randomization performed by the phase randomizing unit  74  of  FIG. 9 . 
       FIG. 11  is a diagram for explaining the difference in decoding results between a case where phase randomization is performed and a case where phase randomization is not performed. 
     As shown in  FIG. 11 , the encoding apparatus  50  of  FIG. 6  encodes a PCM signal in each section called a frame having a constant length. Those frames normally overlap one another by 50%. Specifically, the (J−1)th frame and the Jth frame overlap each other by half a frame, as shown in  FIG. 11 . 
       FIG. 11  illustrates a case where a spectrum with distinctive tone characteristics is encoded, as shown on the left side of  FIG. 11 . 
     In this case, where the phase of the spectrum is not randomized at the time of decoding of the spectrums of the (J−1)th and Jth frames as shown in the upper right portion of  FIG. 11 , the phase of the spectrum of the overlapping period between the (J−1)th frame and the Jth frame is accurately restored by a combination of the signs and the spectrums of the (J−1)th and Jth frames. Accordingly, the restored spectrum of the overlapping period is a spectrum with distinctive tone characteristics. 
     Where the phase of the spectrum is randomized at the time of decoding of the spectrums of the (J−1) th and Jth frames as shown in the lower right portion, on the other hand, the signs of the spectrums of the (J−1) th and Jth frames are not always the same. Therefore, the phase of the spectrum of the overlapping period is not accurately restored. As a result, the restored signal of the overlapping period in the decoding apparatus  70  is a spectrum having poorer tone characteristics than the tone characteristics of the spectrum prior to the encoding. 
     As the tone characteristics of the spectrum become poorer, the energy originally concentrating on the specific spectrum leaks into the surrounding spectrums. Therefore, the peaks (tops) of the spectrum are more restrained compared with the original spectrum, and the energy of the bottoms of the spectrum is boosted by the energy leaking into the surroundings. As a result, the spectrum acquires noise characteristics. 
     As described above, where phase randomization is performed at the time of decoding, the spectrum having tone characteristics prior to encoding is transformed into a spectrum having noise characteristics. 
       FIGS. 12 through 16  are diagrams for explaining the characteristics of the higher frequency spectrum SP-H. 
     As shown in A in  FIG. 12 , where the tone characteristics of the lower frequency spectrum SP-L are distinctive, the tone characteristics of the higher frequency spectrum SP-H are often distinctive too. This can be deduced from the fact that instruments such as wind instruments and string instruments emit sound waves that are a combination of basic frequency and harmonic components that are integral multiples of the basic frequency. 
     In a case where band extension encoding is performed on the spectrum formed with the lower frequency spectrum SP-L and the higher frequency spectrum SP-H, which have distinctive tone characteristics, a pseudo higher frequency spectrum that is generated by simply replicating the lower frequency spectrum SP-L at the time of band extension decoding is a spectrum with distinctive tone characteristics as shown in B in  FIG. 12 . Accordingly, the sound corresponding to the results of decoding is hardly disagreeable to the ear. 
     Therefore, in a case where the degree of concentration D is higher than the predetermined threshold value, or where the higher frequency components of the sound to be encoded have tone characteristics, the encoding apparatus  50  of FIG.  6  sets the random flag RND to 0. Therefore, the phase of the pseudo higher frequency spectrum is not randomized in the decoding apparatus  70 . Accordingly, the sound corresponding to the results of decoding is hardly disagreeable to the ear. 
     In a case where the lower frequency spectrum SP-L has distinctive noise characteristics, the noise characteristics become more distinctive at higher frequencies, as shown in A in  FIG. 13  and A in  FIG. 14 . This can be deduced from the fact that vibrations of higher frequencies propagate in instruments such as cymbals and maracas that emits hit sound and impact sound with distinctive noise characteristic or without tone characteristics, and higher frequency sound has more distinctive noise characteristics, with the amplitudes and phases of the respective vibration factors being intricately intertwined. 
     In a case where band extension encoding is performed on a spectrum formed with the lower frequency spectrum SP-L and the higher frequency spectrum SP-H having distinctive noise characteristics as described above, a pseudo higher frequency spectrum generated by using the lower frequency spectrum SP-L at the time of band extension decoding is a spectrum with distinctive noise characteristics as shown in B in  FIG. 13 . Therefore, where phase randomization is not performed on the pseudo higher frequency spectrum as shown in B in  FIG. 13  or where phase randomization is performed as shown in B in  FIG. 14 , the noise characteristics of the pseudo higher frequency spectrum are distinctive, and the sound corresponding to the results of decoding is hardly disagreeable to the ear. 
     However, the lower frequency components of sound of instruments with distinctive noise characteristics such as cymbals or maracas might contain tonic vibrational components. Also, the frequencies of sound of instruments such as cymbals and maracas are mainly high frequencies, and there is a possibility that the lower frequency components also contain sound with distinctive tone characteristics. Therefore, even in a case where the noise characteristics of the higher frequency spectrum SP-H are distinctive, the tone characteristics of the lower frequency spectrum SP-L might be distinctive as shown in A in  FIG. 15  and A in  FIG. 16 . 
     In a case where band extension encoding is performed on a spectrum formed with the lower frequency spectrum SP-L with distinctive tone characteristics and the higher frequency spectrum SP-H with distinctive noise characteristics as described above, a pseudo higher frequency spectrum generated by using the lower frequency spectrum SP-L at the time of band extension decoding might contain tonic components, as shown in B in  FIG. 15 . Therefore, if the phase of the pseudo higher frequency spectrum is not randomized as shown in B of  FIG. 15 , the higher frequency sound corresponding to the results of decoding does not have the original noise characteristics, but have tone characteristics like the lower frequency sound, resulting in sound that is disagreeable to the ear. 
     In a case where the phase of the pseudo higher frequency spectrum is randomized, on the other hand, the pseudo higher frequency spectrum after the randomization have noise characteristics as shown in B in  FIG. 16 , even if the original pseudo higher frequency spectrum contains tonic components. Accordingly, the sound corresponding to the results of decoding is hardly disagreeable to the ear. 
     In a case where the higher frequency spectrum SP-H has noise characteristics, randomization may be or may not be performed, if the lower frequency spectrum SP-L also has noise characteristics. In that case, however, randomization needs to be performed, if the lower frequency spectrum SP-L has tone characteristics. Therefore, in a case where the higher frequency spectrum SP-H has noise characteristics, randomization is constantly performed, so that decoding results that are hardly disagreeable to the ear can be achieved based on the degree of concentration D. 
     In view of this, in a case where the degree of concentration D is equal to or lower than the predetermined threshold value, or where the higher frequency components of the sound to be encoded have noise characteristics, the encoding apparatus  50  of  FIG. 6  sets the random flag RND to 1. As a result, the phase of the pseudo higher frequency spectrum is randomized in the decoding apparatus  70 . Accordingly, the sound corresponding to the results of decoding is hardly disagreeable to the ear. 
     Since there exists almost no sound that has distinctive noise characteristics at lower frequencies and distinctive tone characteristics at higher frequencies in nature, a spectrum formed with the lower frequency spectrum SP-L with distinctive noise characteristics and the higher frequency spectrum SP-H with distinctive tone characteristics is not discussed herein. 
     [Description of Operation of the Decoding Apparatus] 
       FIG. 17  is a flowchart for explaining a decoding operation to be performed by the decoding apparatus  70  of  FIG. 9 . This decoding operation is started when a bit stream encoded by the encoding apparatus  50  is input to the decoding apparatus  70 , for example. 
     In step S 71  of  FIG. 17 , the dividing unit  71  obtains the bit stream encoded by the encoding apparatus  50 , and divides the bit stream into the random flag RND, the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H. The dividing unit  71  supplies the random flag RND, the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H to the inverse quantizing unit  72 . 
     In step S 72 , the inverse quantizing unit  72  performs inverse quantization on the lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and the higher frequency envelope ENV-H, which are supplied from the dividing unit  71 . The inverse quantizing unit  72  supplies the inversely-quantized lower frequency envelope ENV-L to the inverse MDCT unit  75 , and supplies the lower frequency spectrum SP-L to the inverse MDCT unit  75  and the higher frequency component generating unit  73 . Also, the inverse quantizing unit  72  supplies the higher frequency envelope ENV-H to the higher frequency component generating unit  73 , and supplies the random flag RND to the phase randomizing unit  74 . 
     In step S 73 , the higher frequency component generating unit  73  generates a pseudo higher frequency spectrum by using the lower frequency spectrum SP-L and the higher frequency envelope ENV-H, which are supplied from the inverse quantizing unit  72 . The higher frequency component generating unit  73  supplies the generated pseudo higher frequency spectrum to the phase randomizing unit  74 . 
     In step S 74 , the phase randomizing unit  74  determines whether the random flag RND supplied from the inverse quantizing unit  72  is 1. If the random flag RND is determined to be 1 in step S 74 , the phase randomizing unit  74 , in step S 75 , randomizes the phase of the pseudo higher frequency spectrum supplied from the higher frequency component generating unit  73 , according to the above described equation (2). The phase randomizing unit  74  then supplies the pseudo higher frequency spectrum having its phase randomized to the inverse MDCT unit  75 , and the operation moves on to step S 76 . 
     If the random flag RND is determined not to be 1 or is determined to be 0 in step S 74 , the phase randomizing unit  74  does not randomize the phase of the pseudo higher frequency spectrum, and supplies the pseudo higher frequency spectrum as it is to the inverse MDCT unit  75 . The operation then moves on to step S 76 . 
     In step S 76 , the inverse MDCT unit  75  denormalizes the lower frequency spectrum SP-L by using the lower frequency envelope ENV-L supplied from the inverse quantizing unit  32 . 
     In step S 77 , the inverse MDCT unit  75  combines the denormalized lower frequency spectrum SP-L with the pseudo higher frequency spectrum supplied from the phase randomizing unit  74 , and performs an inverse MDCT on the resultant entire-band spectrum. By doing so, the inverse MDCT unit  75  obtains an entire-band PCM signal. The inverse MDCT unit  75  outputs the entire-band PCM signal as decoding results, and the operation comes to an end. 
     As described above, the decoding apparatus  70  generates the pseudo higher frequency spectrum by using the lower frequency spectrum SP-L prior to the inverse MDCT, and randomizes the pseudo higher frequency spectrum in accordance with the random flag RND determined based on the degree of concentration of the higher frequency spectrum SP-H. By doing so, the decoding apparatus  70  restores the higher frequency components of the spectrum of the sound to be encoded. 
     By using the lower frequency spectrum SP-L in the above manner, a spectrum that is relatively similar to the higher frequency spectrum SP-H can be restored as the higher frequency components of the spectrum of sound to be encoded. Accordingly, as the higher frequency components of the spectrum of sound to be encoded are restored by using the lower frequency spectrum SP-L, a decoding operation and a band extending operation can be simultaneously performed on the lower frequency spectrum SP-L, and the delay time caused by the band extension can be shortened. As a result, the entire-band PCM signal of sound that is not muffled and is beautiful and agreeable to the ear is output as the results of decoding after substantially the same period of time has passed as in a decoding apparatus not performing the band extension operation. 
     Also, the decoding apparatus  70  randomizes the phase of the pseudo higher frequency spectrum generated by using the lower frequency spectrum SP-L, to generate a pseudo higher frequency spectrum with noise characteristics. Accordingly, the decoding apparatus  70  can generate a pseudo higher frequency spectrum that is more similar to the higher frequency spectrum SP-H than in a case where a random spectrum is simply generated as a pseudo higher frequency spectrum. 
     Further, the decoding apparatus  70  generates the lower frequency components and the higher frequency components of a spectrum prior to the inverse MDCT. Therefore, the decoding apparatus  70  does not need to include the band dividing filter  41  and the band combining filter  43  for band extending operations, like the decoding apparatus  30  of  FIG. 3 . Accordingly, the processing for band extending operations, and the resources such as the circuit size and the code size can be reduced, compared with those in the decoding apparatus  30  of  FIG. 3 . 
     Second Embodiment 
     Example Structure of Second Embodiment of Decoding Apparatus 
       FIG. 18  is a block diagram showing an example structure of a second embodiment of a decoding apparatus to which the present invention is applied. 
     Of the components shown in  FIG. 18 , the same components as those shown in  FIGS. 3 and 9  are denoted by the same reference numerals used in  FIGS. 3 and 9 , and the same explanation will not be repeated. 
     The structure of the decoding apparatus  100  of  FIG. 18  differs from the structure of the decoding apparatus  70  of  FIG. 9  in that the dividing unit  71  and the inverse quantizing unit  72  are replaced with a dividing unit  31  and an inverse quantizing unit  32 , and a determining unit  101  is added. The decoding apparatus  100  determines a random flag RND, based on a lower frequency spectrum SP-L included in a bit stream encoded by the encoding apparatus  10  of  FIG. 1 . 
     Specifically, based on the lower frequency spectrum SP-L inversely-quantized by the inverse quantizing unit  32 , the determining unit  101  determines the degree of concentration D′ of the lower frequency spectrum SP-L according to the following equation (3), for example:
 
 D ′=max( SP - L )/ave( SP - L )  (3)
 
     In the equation (3), max(SP-L) represents the maximum value of the lower frequency spectrum SP-L, and ave(SP-L) represents the average value of the lower frequency spectrum SP-L. 
     According to the equation (3), in a case where the tone characteristics of the lower frequency components of the sound to be encoded are distinctive and the distribution of the lower frequency spectrum SP-L has a high degree of bias, the degree of concentration D′ is high. In a case where the noise characteristics of the lower frequency components of the sound to be encoded are distinctive and the distribution of the lower frequency spectrum SP-L is uniform, the degree of concentration D′ is low. 
     The determining unit  101  determines the random flag RND, based on the degree of concentration D′. Specifically, in a case where the degree of concentration D is higher than a threshold value that is set in the decoding apparatus  100  in advance, or where the tone characteristics of the lower frequency spectrum SP-L are distinctive, the determining unit  101  determines the random flag RND to be 0. In a case where the degree of concentration D′ is equal to or lower than the predetermined threshold value, or where the noise characteristics of the lower frequency spectrum SP-L are distinctive, on the other hand, the determining unit  101  determines the random flag RND to be 1. The determining unit  101  supplies the determined random flag RND to the phase randomizing unit  74 . Accordingly, where the tone characteristics of the lower frequency spectrum SP-L are distinctive, the phase of a pseudo higher frequency spectrum is not randomized. Where the noise characteristics of the lower frequency spectrum SP-L are distinctive, the phase of the pseudo higher frequency spectrum is randomized. As a result, the sound corresponding to the results of decoding has a sufficiently high auditory quality. 
     [Description of Operation of the Decoding Apparatus] 
       FIG. 19  is a flowchart for explaining a decoding operation to be performed by the decoding apparatus  100  of  FIG. 18 . This decoding operation is started when a bit stream encoded by the encoding apparatus  10  of  FIG. 1  is input to the decoding apparatus  100 , for example. 
     In step S 91  of  FIG. 19 , the dividing unit  31  divides the bit stream encoded by the encoding apparatus  10  into a lower frequency envelope ENV-L, the lower frequency spectrum SP-L, and a higher frequency envelope ENV-H, which are then supplied to the inverse quantizing unit  32 . 
     The procedures of steps S 92  and S 93  are the same as the procedures of steps S 72  and S 73  of  FIG. 17 , and therefore, explanation of them is not repeated herein. 
     After the procedure of step S 93 , the determining unit  101 , in step S 94 , determines the degree of concentration D′ of the lower frequency spectrum SP-L according to the above described equation (3), based on the lower frequency spectrum SP-L inversely-quantized by the inverse quantizing unit  32 . 
     In step S 95 , the determining unit  101  determines the random flag RND, based on the degree of concentration D′. The determining unit  101  supplies the random flag RND to the phase randomizing unit  74 , and the operation moves on to step S 96 . 
     The procedures of steps S 96  through S 99  are the same as the procedures of steps S 74  through S 77  of  FIG. 17 , and therefore, explanation of them is not repeated herein. 
     Third Embodiment 
     Description of Computer to which the Present Invention is Applied 
     The above described series of encoding procedures and decoding procedures can be carried out by hardware or software. In a case where the series of encoding procedures and decoding procedures are carried out by software, the programs as the software are installed in a general-purpose computer or the like. 
       FIG. 20  shows an example structure of an embodiment of the computer in which the programs for carrying out the above described series of procedures are installed. 
     The programs can be recorded beforehand in a storage unit  208  or a ROM (Read Only Memory)  202  that are provided as recording media in the computer. 
     Alternatively, the programs may be stored (recorded) in a removable medium  211 . This removable medium  211  can be provided as so-called package software. Here, the removable medium  211  may be a flexible disc, a CD-ROM (Compact Disc Read Only Memory), a MO (Magneto Optical) disc, a DVD (Digital Versatile Disc), a magnetic disc, a semiconductor memory, or the like, for example. 
     The programs are installed in the computer from the above described removable medium  211  via a drive  210 . Alternatively, the programs may be downloaded into the computer via a communication network or a broadcast network, and be installed in the internal storage unit  208 . That is, the programs can be transferred wirelessly from a download site to the computer via an artificial satellite for digital satellite broadcasting, or can be transferred online to the computer via a network such as a LAN (Local Area Network) or the Internet, for example. 
     The computer includes a CPU (Central Processing Unit)  201 , and an input/output interface  205  is connected to the CPU  201  via a bus  204 . 
     When an instruction is input by a user operating an input unit  206  via the input/output interface  205 , the CPU  201  executes a program stored in the ROM  202 , in accordance with the instruction. Alternatively, the CPU  201  loads the program from the storage unit  208  into a RAM (Random Access Memory)  203 , and then executes the program. 
     With this arrangement, the CPU  201  performs operations according to the above described flowcharts or performs operations with the structures shown in the above described block diagrams. Via the input/output interface  205 , the CPU  201  outputs the results of the operations from an output unit  207 , or transmits the results from a communication unit  209 , or records the results into the storage unit  208 , for example, where necessary. 
     The input unit  206  is a keyboard, a mouse, a microphone, or the like. The output unit  207  is a LCD (Liquid Crystal Display), a speaker, or the like. 
     In this specification, procedures to be carried out by the computer in accordance with the programs are not necessarily carried out in chronological order by following the sequences shown in the flowcharts. That is, the procedures to be carried out by the computer in accordance with the programs include procedures to be carried out in parallel or independently of one another (such as parallel processing or processing by objects, for example). 
     The programs may be executed by a computer (or a processor), or may be executed by two or more computers in a distributed manner. Further, the programs may be transferred to a remote computer, and be executed by the remote computer. 
     Embodiments of the present invention are not limited to the above described embodiments, and various modifications may be made to them without departing from the scope of the invention. 
     REFERENCE SIGNS LIST 
       50  Encoding apparatus 
       52  Multiplexing unit 
       61  Determining unit 
       62  Extracting unit 
       63  Normalizing unit 
       70  Decoding apparatus 
       71  Dividing unit 
       73  Higher frequency component generating unit 
       74  Phase randomizing unit 
       75  Inverse MDCT unit 
       100  Decoding apparatus 
       101  Dividing unit 
       101  Determining unit