1、 1 Scope This standard specifies the mapping of A-law coded audio into a sound essence track of the MXF generic container. A-law coding is a sample-by-sample coding defined by ITU G-711 and used throughout the audio transmission chain. The MXF generic container is the native essence container of the
2、 material exchange format (MXF) file body. The MXF generic container is defined for the interchange of streamable audio-visual material. This standard defines the data structure at the signal interfaces of networks or storage media. This standard does not define internal storage formats for MXF comp
3、liant devices. 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on thi
4、s standard are encouraged to investigate the possibility of applying the most recent edition of the standards indicated below. SMPTE 377M-2004, Television Material Exchange Format (MXF) File Format Specification SMPTE 379M-2004, Television Material Exchange Format (MXF) Generic Container ITU-T Recom
5、mendation G.711, Pulse Code Modulation (PCM) of Voice Frequencies 3 Glossary of acronyms, terms and data types The general glossary of acronyms, terms and data types used in the MXF specification is given in SMPTE 377M and is supplemented in SMPTE 379M. These glossaries are not repeated here to avoi
6、d any divergence of meaning. 4 Introduction The MXF generic container (GC) is fully described in SMPTE 379M. This standard specifies a mapping of A-law coded audio as a sound element that may be used in the sound item of the MXF GC. A-law coding is defined in ITU G-711. Page 1 of 6 pages SMPTE 388M-
7、2004 Copyright 2004 by THE SOCIETY OF MOTION PICTURE AND TELEVISION ENGINEERS 595 W. Hartsdale Ave., White Plains, NY 10607 (914) 761-1100 Approved September 22, 2004 SMPTE STANDARD for Television Material Exchange Format (MXF) Mapping A-law Coded Audio into the MXF Generic Container SMPTE 388M-2004
8、 Page 2 of 6 pages This standard specifies the key, the length and the value fields of the A-law coded sound element together with the SMPTE Universal label (UL) that uniquely identifies this essence element mapping into the MXF GC. 4.1 A-law coding summary A-law coded audio is used for voice circui
9、ts and results in a low bit-rate with albeit a low quality, though almost no codec delay. A-law coded audio is used in some voice-only circuits within broadcast and telecommunications applications. Its applications in MXF lie in its use as a voice-over channel or as a proxy audio track. The source c
10、oding uses 8K samples per second with 14 bits for each sample. Each sample value is divided into coding ranges where each range is assigned an 8-bit code value. The result is a coded bit rate of 64 Kbps. If used with video, the number of bytes per frame for the commonly used frame rates are as defin
11、ed in table 1. Table 1 Bytes per frame for the A-law coded audio element Frame Rate (Hz) Coded Bytes per Frame Minimum Bytes per Frame Maximum Byte per Frame Frame Sequence Length 24/1.001 333.666(recurring) 333 334 3 24 333.333(recurring) 333 334 3 25 320 320 320 1 30/1.001 266.9333(recurring) 266
12、267 15 30 266.666(recurring) 266 267 3 50 160 160 160 1 60/1.001 133.4666(recurring) 133 134 15 The result is that decoders of video frame rates that have a frame sequence of greater than 1 will have to provide a small amount of additional buffering to handle the variable number of coded bytes per f
13、rame. 4.2 Application in the MXF generic container This mapping may be used in either a frame-wrapped or clip-wrapped generic container as defined in SMPTE 379M. 5 Key-length-value 5.1 Essence element key The values of the first 12 bytes of the essence element key are defined in SMPTE 379M. The valu
14、es of the last 4 bytes of the sound element key are given in table 2. SMPTE 388M-2004 Page 3 of 6 pages Table 2 Key value for the A-law sound element Byte No. Description Value (hex) Meaning 112 Defined in SMPTE 379M See SMPTE 379M 13 Item Type Identifier 16h GC sound item (as defined in SMPTE 379M)
15、 14 Essence Element Count kkh Count of sound elements in the sound item 15 Essence Element Type 08h 09h 0Ah A-law frame-wrapped sound element A-law clip-wrapped sound element A-law custom-wrapped sound element 16 Essence Element Number nnh The number (used as an Index) of this sound element in the s
16、ound item 5.1.1 Essence element count Byte 14 This is a count of the number of sound elements in the sound item of the generic container. 5.1.2 Essence element type Byte 15 The values are given in table 2. Three types are defined so that frame-wrapped, clip-wrapped and custom-wrapped essence can be
17、simply distinguished from at the first key in the MXF generic container. 5.1.3 Essence element number Byte 16 This is a number used as an index to identify this instance of the element type within the sound item. Each element within an item shall have a unique value between 00h and 7Fh as defined by
18、 SMPTE 379M, which shall remain constant throughout all content packages of an essence container. 5.2 Length The length field of the KLV coded element shall be 4 bytes BER long-form encoded (i.e., 83h.xx.yy.zz) for frame-wrapping. The length field of the KLV coded element shall be 8 bytes BER long-f
19、orm encoded (i.e., 87h.aa.bb.cc.dd.ee.ff.gg) for clip-wrapping. The length field of the KLV coded element shall be either 4 or 8 bytes BER long-form encoded for custom-wrapping. 5.3 Value In a clip-wrapped file, the element value shall be the complete A-law audio data. In a frame-wrapped file, the s
20、tart position of the first sample of the A-law audio data should be the same as the start position of the synchronized picture frame. When equal start position values cannot be achieved because of complex clocking relationships, the start position of the video access unit should fall within the dura
21、tion of the first sample or audio frame of the sound element. More information on wrapping to achieve synchronization is given in the MXF engineering guideline, SMPTE EG 41. SMPTE 388M-2004 Page 4 of 6 pages In a custom-wrapped file, the element value shall be defined by the primary element in the M
22、XF generic container. This wrapping kind is defined to allow this essence kind to be mapped along with a primary element that may define its own specialized or custom wrapping. The details of this wrapping are beyond the scope of this document and shall be defined by the wrapping of the primary esse
23、nce element. 6 SMPTE ULs for A-law codecs 6.1 Essence container UL The values for the essence container UL are given in table 3. Table 3 Specification of the essence container label Byte No. Description Value (hex) Meaning 1-12 Defined by Generic Container As defined in SMPTE 379M 13 Essence Contain
24、er Kind 02h MXF generic container 14 Mapping Kind 0Ah A-law sound element (as listed in SMPTE RP 224) 15 Content Kind 01h 02h 03h A-law frame-wrapped sound element A-law clip-wrapped sound element A-law custom-wrapped sound element 16 Reserved 00h The essence container UL is used within a batch of U
25、Ls in partition packs and the preface set and on its own in the essence descriptor. These UL values are listed in the SMPTE RP 224 (Registry of SMPTE Universal Labels). 6.2 Sound essence compression UL The values for the sound essence compression UL are given in table 4. Table 4 Specification of the
26、 sound essence compression label Byte No. Description Value (hex) Meaning 1-8 Registry Designator See SMPTE 379M Values are defined in SMPTE 379M, table 3 9 Parametric 04h Node used to define parametric data 10 Sound Essence 02h Labels for sound essence coding 11 Sound Coding Characteristics 02h Lab
27、els for sound coding characteristics 12 Compressed Sound Coding 02h Labels for compressed sound coding 13 Compressed Audio Coding 03h Labels for compressed audio coding 14 Companded Audio 01h Identifies sample-based non-linear companded coding 15 A-law Coded Audio (default) 01h Identifies A-law codi
28、ng(default variant) 16 Reserved 00h The sound essence compression UL is used in the generic sound essence descriptor. This UL is listed in SMPTE RP 224. SMPTE 388M-2004 Page 5 of 6 pages 7 Essence descriptor This mapping should use the generic sound essence descriptor as defined in SMPTE 377M to des
29、cribe this sound element. NOTE SMPTE 377M requires that an essence descriptor is used to describe an essence track in a top-level file package. At the time of writing, the generic sound essence descriptor is the most appropriate essence descriptor for use with this essence type. Future revisions to
30、the MXF standard may introduce a sub-class of the generic sound essence descriptor that may be more appropriate for use with this essence type. 8 Use of the KAG There are no specific KAG (KLV alignment grid) requirements for the A-law mapping. MXF encoders and decoders shall comply with the KAG rule
31、s in SMPTE 377M. SMPTE 388M-2004 Page 6 of 6 pages Annex A (informative) Bibliography ANSI/SMPTE 298M-1997, Television Universal Labels for Unique Identification of Digital Data SMPTE 336M-2001, Television Data Coding Protocol using Key-Length-Value SMPTE RP 224, SMPTE Labels Registry SMPTE EG 41-2004, Television Material Exchange Format ( MXF) Engineering Guideline
