JP2009134115A - Decoder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a decoder capable of correctly decoding a compressed speech data file in a file format such as an AAC, ADIF and an AAC raw data from an optional position without depending on an encoder, even in the first reproduction time. <P>SOLUTION: The decoder prestores a bit string for detecting a frame head position, composed of a bit string corresponding to a TERM bit string which is defined in an AAC standard, and a bit string corresponding to a data padding bit string, acquires the reproduction resume position, searches a bit string which matches the bit string for detecting the frame head position and which is the bit string on and after a reproduction resume position in the compressed speech data file, and detects a head position of the frame based on a position of the bit string which is acquired by the search. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a decoder that decodes a compressed audio data file in units of frames.

  Audio compression systems such as MP3 used in video data compression system MPEG-1 and AAC (Advanced Audio Coding) used in MPEG-2 or MPEG-4 are known. In the MP3 and AAC ADTS (Audio Data Transport Stream) format file formats, a header is provided for each frame, and the decoder detects the start position of the frame based on position information included in the header and plays back the frame. Can be synchronized for. On the other hand, in the file format of AAC ADIF (Audio Data Interchange Format) or AAC raw data format, a frame is composed only of data (raw data block).

  In the AAC ADIF format or AAC raw data format, when playback is temporarily stopped due to fast forward, rewind, or error occurrence, decoding and playback are resumed, so-called a parser. A device capable of interpreting the MPEG-4 format file format is required. However, when a parser is used, the resources of the playback device are consumed, such as an increase in the amount of playback processing and the amount of storage in the memory, and this is a factor that hinders downsizing the playback device. Therefore, there has been a demand for a technique that enables decoding and reproduction to be resumed without using a parser. Conventionally, when a parser is not used and a special technique for resuming playback is not used, the head position of the frame cannot be detected, so that it cannot be normally decoded and the compressed audio data file cannot be played.

  For example, in Patent Document 1, the AAC encoder embeds information for synchronization in an element called DSE (data stream element) in advance, and the AAC decoder synchronizes with the AAC encoder based on the information, thereby compressing information. A method for reproducing compressed information is disclosed. DSE is an element specified in the ISO / IEC 14496-3 standard, and the standard allows extensions by the user. Even if a method such as that disclosed in the document is used, the DSE deviates from the standard. The compressed information can be reproduced without any problem.

In Patent Document 2, the frame number and head address of a playback frame are held in the frame position information table as frame position information during the first playback process of the compressed audio signal, and a fast-forward instruction during the second and subsequent playback processes is provided. Accordingly, there is disclosed a compressed audio signal reproducing apparatus that refers to the frame information table and determines the read start address of the compressed audio signal after the fast-forward instruction. According to this apparatus, fast-forward reproduction of a compressed audio signal in the AAC standard ADIF format can be performed at high speed.
USP 6718507 JP 2002-041095 A

  However, in the compressed information reproduction method disclosed in Patent Document 1, since synchronization information is embedded in the DSE element in advance in the AAC encoder and the AAC decoder synchronizes with the AAC encoder based on the synchronization information, the synchronization information is included in the AAC decoder. It was necessary to set in advance what the information is. Therefore, the AAC decoder cannot be synchronized with an arbitrary encoder, and there is a problem that the combination of the AAC encoder and the AAC decoder is limited.

  In the case of the compressed audio signal reproducing device disclosed in Patent Document 2, information such as a frame number and a head address obtained by reproducing the compressed audio signal is held, and the information is referred to in the second and subsequent reproduction processing. Therefore, if playback is interrupted at the first playback, normal playback cannot be performed even if playback is resumed. It was.

  The present invention has been made in view of the above-described problems, and does not depend on an encoder. Even at the time of initial playback, the present invention can be applied from an arbitrary frame position of a compressed audio data file using a file format such as AAC ADIF or AAC raw data. An object is to provide a decoder capable of normal decoding.

  A decoder according to the present invention is a decoder that obtains audio data by decoding a compressed audio data file consisting of a plurality of consecutive frames, and includes a detection bit string storage unit that stores a frame start position detection bit string, and the compressed audio data A reproduction resumption position acquisition unit that obtains information indicating a reproduction resumption position of the data file, and a bit string that is a bit string after the reproduction resumption position in the compressed audio data file and that matches the frame head position detection bit string And a frame head position detecting unit for detecting the head position of the frame based on the position of the bit string obtained by the above.

  Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

<First embodiment>
FIG. 1 is a block diagram illustrating a decoder 10 according to the present invention. The decoder 10 includes a reproduction restart position acquisition unit 11, a detection bit string storage unit 12, a frame head position detection unit 13, and a decoding processing unit 14. The decoder 10 is a device that obtains audio data by decoding a compressed audio data file. The compressed audio data file is, for example, data configured in a file format of AAC ADIF (Audio Data Interchange Format) or AAC raw data format defined in the ISO / IEC 14496-3 standard.

  The reproduction resume position acquisition unit 11 obtains information indicating the reproduction resume position of the compressed audio data file (hereinafter referred to as reproduction resume position information ST). The reproduction restart position information ST is information indicating a position when reproduction is resumed after the reproduction of the compressed audio data file is interrupted due to fast forward / rewind or a reproduction error. For example, when the user designates a playback resume position after fast forward / rewind, the playback resume position acquisition unit 11 acquires playback resume position information ST input by the user from an input unit (not shown). When the reproduction is interrupted due to a reproduction error, the reproduction resume position acquisition unit 11 obtains reproduction resume position information ST given together with an instruction to resume reproduction from a control unit (not shown).

  Further, the reproduction restart position acquisition unit 11 also acquires the compressed audio data file AF. For example, when the file storage unit (not shown) stores the compressed audio data file AF, the reproduction restart position acquisition unit 11 acquires the compressed audio data file AF from the file storage unit. When a file reception unit (not shown) receives the compressed audio data file AF from a transmission device (not shown) via a communication line, the reproduction resume position acquisition unit 11 receives the compressed audio data from the file reception unit. A data file AF is acquired.

  The compressed audio data file AF consists of a series of frames. FIG. 2 is a diagram showing the components of the AAC ADIF and AAC raw data frames defined in the ISO / IEC 14496-3 standard. The frame is composed of SCE, CPE, CCE, LFE, DSE, PCE, FIL, and TERM elements. As shown in the figure, TERM (Terminator) is an element representing the end of a frame. The TERM value is defined by the standard as 0x7, that is, 111 in binary. Further, an element called data padding for aligning the data size of the frame in byte units is inserted following the TERM. The value of data padding is 0, and is represented by any one of 1 to 7 bits.

  The detection bit string storage unit 12 stores a detection bit string table composed of a frame head position detection bit string (hereinafter simply referred to as a detection bit string). FIG. 3 shows a detection bit string table. The detection bit string table shows seven detection bit strings. Each of the detection bit strings is a 10-bit bit string. These are defined to correspond to the values of TERM and data padding. As described above, the TERM value is a fixed value of 111, the data padding value is 0, and is represented by any one of 1 to 7 bits.

  For example, when the data padding in the frame is 7 bits, the bit string from the TERM to the data padding becomes 1110000000 from the TERM bit string 111 and the data padding bit string 0000000 (7-bit 0). Is stored in advance in the detection bit string table as a detection bit string (No. 7 in the figure). When the data padding in the frame is 1 bit, the bit string from the TERM to the data padding becomes 1110 from the TERM bit string 111 and the data padding bit string 0. Assuming that the value of the 6 bits ahead of the TERM bit string can be either 0 or 1, a 10-bit detection bit string xxxxxxxx 1110 is stored in advance in the detection bit string table (No. 1 in FIG. 0 or 1). Other detection bit strings are determined according to the same concept.

  The frame head position detection unit 13 receives the playback restart position information ST and the compressed audio data file AF from the playback restart position acquisition unit 11, and executes a frame head position detection process. FIG. 4 is a diagram centering on the Nth and N + 1th frames in a series of frames constituting the compressed audio data file AF (where N is a positive integer). Here, it is assumed that the reproduction resume position represented by the reproduction resume position information ST is a position indicated by a symbol ST represented in the figure. The reproduction restart position ST here is located in the middle of the Nth frame. The frame head position detector 13 searches for a bit string after the reproduction resuming position ST in the compressed audio data file AF that matches any of the seven detection bit strings shown in FIG. The head position FR of the frame is detected based on the position of the bit string obtained by.

  Specifically, first, the frame head position detection unit 13 matches the bit string starting from the bit of the reproduction restart position ST with any of the seven detection bit strings stored in the detection bit string storage unit 12. It is determined whether or not to do so. If the frame head position detection unit 13 determines that they match, the position of the next bit in the matched bit string (the end of the Nth frame) is set as the head position FR of the (N + 1) th frame, and this is used as the decoding processing unit 14. Notify If the frame head position detection unit 13 determines that they do not match, the frame head position detection unit 13 detects the head position of the frame by changing the detection position in the compressed audio data file AF. At this time, the frame head position detection unit 13 obtains the reproduction resume position ST again from the reproduction resume position acquisition unit 11 and searches similarly. Further, the frame head position detection unit 13 determines, for example, whether a bit string starting from a bit advanced by a predetermined M bits from the bit of the reproduction restart position ST matches any of the detection bit strings. (M here is a positive integer). In this case, the frame head position detection unit 13 repeats the same determination process while increasing the value of M until a determination result indicating that they match is obtained. When it is finally determined that the frame head position detection unit 13 matches, the position of the next bit in the matched bit string is set as the frame head position FR, and this is notified to the decoding processing unit 14.

  For example, if the 10-bit bit string starting from the bit at the reproduction restart position ST shown in FIG. 4 is 0101010101, the bit string does not match any of the detection bit strings. In this case, the frame start position detection unit 13 repeats the determination process while shifting the start position of the bit string that is symmetrical to the determination from the position of the bit at the reproduction resume position ST in the frame progress direction until a determination result that they match is obtained. . For example, when the bit string from TERM to data padding is 1111000000 as shown in the figure, the frame head position detection unit 13 stores the No. stored in the detection bit string table. 7 is determined to be coincident with the detection bit string 11110000000, and the position of the bit next to the coincident bit string 11110000000 (that is, the end of the Nth frame), that is, the position of the first bit of the (N + 1) th frame is determined as the head of the frame. This is notified to the decoding processing unit 14 as the position FR.

  The decode processing unit 14 has a function of obtaining audio data by decoding a compressed audio data file AF composed of a plurality of consecutive frames. The decode processing unit 14 can receive the frame start position FR from the frame start position detection unit 13 together with the compressed audio data file AF, and can perform the decoding process on the (N + 1) th and subsequent frames using this as the synchronization position. In general, when the decoder decodes from the middle of the frame, normal audio data cannot be obtained. However, the decoding processing unit 14 determines the N + 1th frame based on the frame start position FR from the frame start position detection unit 13. The normal position can be decoded from the head position, and the N + 2 and subsequent frames can be normally decoded with the head position as a synchronization position.

  The decode processing unit 14 has an error determination processing function for the compressed audio data file AF. The decode processing unit 14 determines an error in the following two cases. One is a case where a data error is detected in the compressed audio data file AF. For example, when the compressed audio data file AF is stored in a hard disk (not shown), the reproduction resume position acquisition unit 11 receives the compressed audio data file AF from the hard disk via some hardware circuit (not shown). At this time, the compressed audio data file AF including erroneous data is acquired due to the influence of a reading error of the hard disk or noise in the hardware circuit, and when error determination processing is performed on this. Judged as an error. Alternatively, when the compressed audio data file AF is distributed from a distribution device (not shown) to the decoder 10 via a wired or wireless communication line, the reproduction resume position is acquired due to noise or a momentary interruption that occurs in the communication line. The unit 11 obtains the compressed audio data file AF including erroneous data. In this case as well, the unit 11 determines that an error has occurred when the error determination process is performed.

  The other is a case where information for decoding audio data is out of the specification range. Normally, the compressed audio data file AF includes information for decoding the audio data such as a compression method and a compression rate together with the audio data. If these compression methods and compression ratios do not conform to the specifications in the decoder 10, the decode processing unit 14 determines an error when performing an error determination process.

  The decoding processing unit 14 stores the audio data DA obtained by the decoding processing in a memory (not shown). An audio reproducing unit (not shown) can appropriately read out audio data DA from the memory and reproduce it as audio.

  FIG. 5 is a flowchart showing a decoding reproduction processing routine. Hereinafter, the decoding reproduction process will be described with reference to FIG.

  First, when the user designates a reproduction resume position after fast forward / rewind, the reproduction resume position acquisition unit 11 obtains reproduction resume position information ST input by the user from an input unit (not shown). Alternatively, when playback is interrupted due to a playback error, the playback restart position acquisition unit 11 acquires playback restart position information ST given together with a playback restart instruction from a control unit (not shown) (step S1). Further, the reproduction restart position acquisition unit 11 acquires the compressed audio data file AF together. When the file storage unit (not shown) stores the compressed audio data file AF, the reproduction resume position acquisition unit 11 communicates the compressed audio data file AF from the file storage unit or the file reception unit (not shown). When the data is received from a transmission device (not shown) via a line, the compressed audio data file AF is acquired from the file receiving unit.

  The frame head position detection unit 13 receives the playback restart position information ST and the compressed audio data file AF from the playback restart position acquisition unit 11, and executes a frame head position detection process (step S2). The frame head position detection unit 13 searches for a bit string that corresponds to one of the seven detection bit strings stored in the detection bit string storage unit 12 after the reproduction restart position ST in the compressed audio data file. Then, the start position FR of the frame is detected based on the position of the bit string obtained by the search.

  First, the frame head position detection unit 13 determines whether the bit string starting from the bit at the reproduction restart position ST matches any of the seven detection bit strings stored in the detection bit string storage unit 12. Is discriminated (step S3). If the frame head position detection unit 13 determines that they match, the position of the bit next to the matched bit string (the end of the Nth bit string) is set as the head position FR of the (N + 1) th frame, and this is used as the decoding processing unit 14. Notify

  When it is determined that they do not match (step S3), the frame head position detection unit 13 acquires the playback resume position ST from the playback resume position acquisition unit 11 again and executes the same determination process, or the playback restart position. It is determined whether or not the bit string starting from the bit advanced by M bits (M is a positive integer) from the ST bit matches any of the detection bit strings (step S2). The frame head position detection unit 13 repeats the same determination process until the reproduction restart position ST is re-acquired or the value of M is increased until a determination result that matches is obtained. When it is finally determined that the frame head position detection unit 13 matches, the position of the bit next to the matched bit string (the end of the Nth frame) is set as the head position FR of the (N + 1) th frame, and this is decoded. Notification to the unit 14.

  The decode processing unit 14 receives the frame start position FR from the frame start position detection unit 13 together with the compressed audio data file AF, decodes the (N + 1) th frame from the start position, and uses the start position as the synchronization position and the (N + 2) th and subsequent frames. The same frame is also decoded. The frame is decoded from the head position (step S4). At this time, the decoding processing unit 14 also performs error determination processing on the compressed audio data file AF. When the decoding is not normally executed due to an error or the like (step S5), the reproduction resume position acquisition unit 11 obtains another reproduction resume position information ST (step S1) and repeats the same processing. .

  If no error is detected and decoding is executed normally (step S5), the decoding processing unit 14 stores the audio data DA obtained by the decoding processing in a memory (not shown). An audio reproducing unit (not shown) can appropriately read out the audio data DA from the memory and reproduce it as audio (step S6).

  As described above, the decoder according to the present embodiment stores in advance a frame head position detection bit string including a bit string corresponding to the TERM bit string defined by the AAC standard and a bit string corresponding to the data padding bit string. The decoder acquires the playback restart position, searches for a bit string (the end of the Nth frame) that matches the frame head position detection bit string after the playback restart position in the compressed audio data file, and obtains it by the search. The position of the next bit of the obtained bit string is set as the head position of the (N + 1) th frame. The decoder can normally decode the (N + 1) th and subsequent frames using the start position of the (N + 1) th frame as a synchronization position. As described above, the decoder according to the present embodiment starts decoding normally from an arbitrary frame position and resumes reproduction even when reproduction of the compressed audio data file is interrupted due to fast-forward / rewind or reproduction error. Can do.

  The frame start position detection bit string is determined so as to satisfy the ISO / IEC 14496-3 standard, and the encoder that compresses the compressed audio data file only needs to satisfy the standard. There is no need to perform special processing such as embedding specific information. Therefore, the decoder according to the present embodiment can normally decode the compressed audio data file without depending on the encoder. Also, the decoder according to the present embodiment can normally decode the compressed audio data file regardless of whether it is the first reproduction or the second reproduction or later.

<Second Embodiment>
The decoder 10 in this embodiment is shown in FIG. 1 as in the first embodiment. Hereinafter, parts different from the first embodiment will be described. It is assumed that the decoder 10 can decode data of two channels.

  FIG. 6 is a diagram mainly showing the Nth and N + 1th frames in a series of frames of the two channels Ch1 and Ch2 constituting the compressed audio data file AF. Here, it is assumed that the reproduction resume position represented by the reproduction resume position information ST is a position indicated by a symbol ST represented in the figure. The reproduction restart position ST is located in the middle of the Nth frame. In a series of frames of the channel Ch1, an element SCE (Single Channel Element) at the head of the (N + 1) th frame is shown following the end portion TERM and data padding of the Nth frame. SCE is known as an element that stores a front center signal in an audio signal. SCE is defined by the ISO / IEC 14496-3 standard as 0x0, that is, 000 in a 3-bit binary representation. In a series of frames of the channel Ch2, an element CPE (Channel Pair Element) at the head of the (N + 1) th frame is shown following the terminal TERM and data padding of the Nth frame. CPE is known as an element that stores a front left / right signal and a surround signal among audio signals. CPE is defined as 0x1 by the same standard, that is, 001 in a 3-bit binary representation.

  FIG. 7 shows a channel 1 detection bit string table. The table shows seven detection bit strings. These bit strings are obtained by adding the SCE value 000 to the end of each of the detection bit strings shown in FIG. FIG. 8 is a diagram showing a channel 2 detection bit string table. The table shows seven detection bit strings. These bit strings are obtained by adding the CPE value 001 to the end of each of the detection bit strings shown in FIG.

  Similarly to the first embodiment, the frame head position detection unit 13 searches for a bit string after the reproduction restart position ST in the compressed audio data file AF that matches the frame head position detection bit string. The bit string obtained by the search is a bit string including the TERM and data padding of the Nth frame and the SCE or CPE of the (N + 1) th frame. The frame head position detection unit 13 sets the position of the head bit of the bit string corresponding to the SCE bit string or CPE bit string in the compressed audio data file AF as the head position FR of the (N + 1) th frame.

  For example, as shown in Ch1 in the figure, when the bit string from the TERM of the Nth frame to the SCE of the (N + 1) th frame is 1110000000000000, the frame head position detection unit 13 stores the bit string in the detection bit string table for channel 1 No. 7 is determined to coincide with the detection bit string 1110000000000000, and the position of the head bit of the bit string 000 corresponding to the bit string of SCE is set as the head position FR of the (N + 1) th frame, and this is notified to the decoding processing unit 14. The decode processing unit 14 receives the frame start position FR from the frame start position detection unit 13 together with the compressed audio data file AF, decodes the (N + 1) th frame from the start position, and uses the start position as the synchronization position and the (N + 2) th and subsequent frames. The same frame is also decoded.

  The decoder according to the present embodiment performs the frame head position detection process based on the bit string corresponding to the SCE or CPE of the (N + 1) th frame in addition to the bit string corresponding to the TERM and data padding of the Nth frame. Compared with the first embodiment, the head position of the frame can be detected more accurately.

<Third embodiment>
The decoder 10 in this embodiment is shown in FIG. 1 as in the second embodiment. Hereinafter, a different part from a 2nd Example is demonstrated.

  In this embodiment, the frame head position detection unit 13 includes a re-search unit that re-searches a bit string after the position of the bit string first obtained by the frame head position detection process and that matches the bit string for detecting the frame head position. I have. The frame head position detection unit 13 includes determination means for determining whether or not the bit strings obtained by the initial search and the re-search are the same. The frame head position detection unit 13 determines the frame based on the position of the bit string obtained by the first search only when the discrimination means determines that the bit strings obtained by the first search and re-search are the same. Detect the start position.

  FIG. 9 shows a bit string from the first bit of the TERM of the Nth frame to the last bit of the SCE or CPE of the N + 1th frame as a symbol SYNC1, and the SCE of the N + 2th frame from the first bit of the TERM of the N + 1th frame. It is a figure showing a part of a series of frames which constitute a compression voice data file when a bit string which reaches the last bit of CPE is expressed as symbol SYNC2,. The ISO / IEC 14496-3 standard specifies that the TERM and SCE of the Nth frame are the same as the TERM and SCE of the (N + 1) th frame. Therefore, if the compressed audio data file AF satisfies the standard, SYNC1 and SYNC2 are the same. Here, it is assumed that the reproduction resume position represented by the reproduction resume position information ST is a position indicated by a symbol ST represented in the figure. The reproduction restart position ST is located in the middle of the Nth frame.

  Similarly to the second embodiment, the frame head position detector 13 searches for a bit string with reference to the channel 1 detection bit string table shown in FIG. 7 and the channel 2 detection bit string table shown in FIG. Hereinafter, the decoding / reproducing process will be described with reference to the decoding / reproducing process routine shown in FIG.

  First, the frame head position detection unit 13 acquires the playback restart position ST from the playback restart position acquisition unit 11 (step S1), and is a bit string after the playback restart position ST in the compressed audio data file AF and a frame head position detection bit string. Is searched for a bit string that matches (step S2). The first bit string obtained by the search is SYNC1. Subsequently, the frame head position detection unit 13 re-searches a bit string after the position of SYNC1 that matches the frame head position detection bit string (step S2). The first bit string obtained by the re-search is SYNC2.

  The frame head position detection unit 13 determines whether or not the bit string SYNC1 obtained by the first search is the same as the bit string SYNC2 obtained by the re-search (step S3). Only when the frame head position detection unit 13 determines that these bit strings are the same, the position of the head bit of the bit string corresponding to the SCE bit string or the CPE bit string in the compressed audio data file AF is determined as the head of the (N + 1) th frame. Let it be position FR. For example, as shown in FIG. 9, when both the bit strings of SYNC1 and SYNC2 are 1110000000000000, the frame head position detection unit 13 sets the position of the head bit of the bit string 000 corresponding to the SCE bit string to the head of the N + 1th frame. Let it be position FR.

  The frame head position detection unit 13 gives the frame head position FR to the decode processing unit 14 together with the compressed audio data file AF. The decode processing unit 14 receives the frame start position FR from the frame start position detection unit 13 together with the compressed audio data file AF, decodes the (N + 1) th frame from the start position, and uses the start position as the synchronization position and the (N + 2) th and subsequent frames. The same frame is also decoded. (Step S4).

  The decoder according to this embodiment includes means for re-searching the start position of the frame, and detects the start position of the frame only when the bit string obtained by the initial search and the bit string obtained by the re-search are the same. Therefore, the head position of the frame can be detected more accurately than in the third embodiment.

  The first to third embodiments are examples in which the compressed audio data file has an AAC ADIF and AAC raw data file format. However, the present invention is not limited to all the bit strings at the beginning and end of a frame. Applicable to file format.

FIG. 3 is a block diagram illustrating a decoder according to the present invention. It is a figure showing the component of the flame | frame of AAC ADIF and AAC raw data. It is a figure showing the bit string table for a detection. It is a figure showing a part of a series of frames which constitute a compression audio data file. It is a flowchart showing a decoding reproduction processing routine. It is a figure showing a part of a series of frames which constitute a compression audio data file at the time of 2 channels. 6 is a diagram illustrating a detection bit string table for channel 1. FIG. 6 is a diagram illustrating a detection bit string table for channel 2. FIG. It is a figure showing a part of a series of frames which constitute a compression audio data file when a specific bit sequence is expressed as symbol SYNC.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Decoder 11 Reproduction | regeneration resumption position acquisition part 12 Bit stream memory | storage part 13 for detection Frame start position detection part 14 Decoding processing part AF Compressed audio signal DA Decoding audio | voice data FR Frame top position RP Reproduction data ST Reproduction resumption position

Claims (6)

A decoder for obtaining audio data by decoding a compressed audio data file consisting of a plurality of consecutive frames,
A detection bit string storage unit for storing a frame start position detection bit string;
A reproduction resumption position acquisition unit for acquiring information indicating a reproduction resumption position of the compressed audio data file;
A frame in which a bit string that coincides with the frame head position detection bit string in the compressed audio data file after the reproduction restart position is searched, and the head position of the frame is detected based on the position of the bit string obtained by the search And a head position detector. The frame head position detection bit string includes a bit string corresponding to a TERM bit string defined in the AAC standard and a bit string corresponding to a data padding bit string.
2. The decoder according to claim 1, wherein the frame head position detection unit sets a position of a bit next to a bit string that matches the frame head position detection bit string as a head position of the frame. The frame start position detection bit string includes a bit string corresponding to a TERM bit string defined in the AAC standard, a bit string corresponding to a data padding bit string, and a bit string corresponding to an SCE bit string or a bit string corresponding to a CPE bit string,
2. The decoder according to claim 1, wherein the frame head position detection unit sets a position of a head bit of a bit string corresponding to the SCE bit string or CPE bit string in the compressed audio data file as a head position of the frame. . The frame head position detector
Re-search means for re-searching a bit string after the position of the bit string obtained by the first search and matching the bit start position detection bit string;
Discriminating means for discriminating whether or not the bit strings obtained by the initial search and the re-search are the same,
2. The decoder according to claim 1, wherein the start position of the frame is detected based on the position of the bit string obtained by the first search only when the determination means determines that they are the same.   5. The decoder according to claim 1, further comprising: a decoding processing unit that decodes a frame subsequent to the frame with the head position of the frame as a synchronization position. 6.   The decoder according to claim 5, wherein the decoding processing unit performs error determination on the compressed audio data file.

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