1、 CEA Bulletin Recommended Practice for Selection and Presentation of DTV Audio CEA-CEB21 June 2011 NOTICE Consumer Electronics Association (CEA) Standards, Bulletins and other technical publications are designed to serve the public interest through eliminating misunderstandings between manufacturers
2、 and purchasers, facilitating interchangeability and improvement of products, and assisting the purchaser in selecting and obtaining with minimum delay the proper product for his particular need. Existence of such Standards, Bulletins and other technical publications shall not in any respect preclud
3、e any member or nonmember of CEA from manufacturing or selling products not conforming to such Standards, Bulletins or other technical publications, nor shall the existence of such Standards, Bulletins and other technical publications preclude their voluntary use by those other than CEA members, whe
4、ther the bulletin is to be used either domestically or internationally. Standards, Bulletins and other technical publications are adopted by CEA in accordance with the American National Standards Institute (ANSI) patent policy. By such action, CEA does not assume any liability to any patent owner, n
5、or does it assume any obligation whatever to parties adopting the Standard, Bulletin or other technical publication. This CEA Bulletin is considered to have International Standardization implication, but the International Electrotechnical Commission activity has not progressed to the point where a v
6、alid comparison between the CEA Bulletin and the IEC document can be made. This Bulletin does not purport to address all safety problems associated with its use or all applicable regulatory requirements. It is the responsibility of the user of this Bulletin to establish appropriate safety and health
7、 practices and to determine the applicability of regulatory limitations before its use. This document is copyrighted by the Consumer Electronics Association (CEA) and may not be reproduced, in whole or part, without written permission. Federal copyright law prohibits unauthorized reproduction of thi
8、s document by any means. Organizations may obtain permission to reproduce a limited number of copies by entering into a license agreement with IHS (http:/). .Requests to reproduce text, data, charts, figures or other material should be made to the CEA. (Formulated under the cognizance of the CEA R4.
9、3 Television Data Systems Subcommittee) Published by CONSUMER ELECTRONICS ASSOCIATION 2011 Technology Specification for the use of Video and Audio Coding in Broadcasting Applications based on the MPEG-2 Transport Stream, 2009 10 ANSI/SCTE 54 2009, Digital Video Service Multiplex and Transport System
10、 for Cable Television, Society of Cable Telecommunications Engineers, 20092.2 Informative 11 ISO 639-2, “Codes for the representation of Names of Languages Part 2: Alpha-3 code, “ as maintained by the ISO 639/Joint Advisory Committee (ISO 639/JAC), http:/www.loc.gov/standards/iso639-2/iso639jac.html
11、 12 ISO/IEC IS 13818-1:2007, “Information technology Generic coding of moving pictures and associated audio information: Systems”, 2007 2.1.3 Informative Reference Acquisition ATSC Standards Advanced Television Systems Committee (ATSC), 1766 K Street N.W., Suite 200, Washington, DC 20006; Phone 202-
12、872-9160; Fax 202-872-9161; Internet http:/www.atsc.org/cms/index.php/standards/published-standards CEA Standards Global Engineering Documents, World Headquarters, 15 Inverness Way East, Englewood, CO USA 80112-5776; Phone 800-854-7179; Fax 303-397-2740; Internet http:/; Email: ETSI European Teleco
13、mmunications Standards Institute, 650, route des Lucioles, 06921 Sophia-Antipolis Cedex, France; Phone +33 (0)4 92 94 42 00; Fax +33 (0)4 93 65 47 16; Internet http:/www.etsi.org SCTE Standards Global Engineering Documents, World Headquarters, 15 Inverness Way East, Englewood, CO USA 80112-5776; Pho
14、ne 800-854-7179; Fax 303-397-2740; Internet http:/www.scte.org/standards 2.2 Definitions For the purposes of CEA-CEB21, the following definitions apply. Audio Stream The Transport Stream packets carrying the elementary stream which contains audio essence. Complete Main An audio service containing al
15、l audio components (dialogue, music, and effects) for the program. Described Video Described Video is the Canadian name for audio consisting of narrated descriptions of a television programs key visual elements. Such streams are properly signaled as Visually Impaired (VI) streams. Hearing Impaired (
16、HI) The HI service is a complete program mix containing music, effects, and dialogue with enhanced intelligibility. Visually Impaired (VI) - The VI service is a complete program mix containing music, effects, dialogue, and additionally a narrative description of the program content. 2.3 Symbols and
17、Abbreviations CM Complete Main DTV Digital Television EIT Event Information Table (A/65) HI Hearing Impaired CEA-CEB21 3 MVPD Multichannel Video Programming Distributor PMT Program Map Table PSIP Program and System Information Protocol (A/65) SAP Secondary Audio Program VI Visually Impaired 3 Audio
18、Metadata Elements in the Transport Stream The following list describes data structures that contain the audio metadata addressed in CEA-CEB21: AC-3_audio_stream_descriptor() This descriptor defines parameters relevant to a given AC-3 audio stream program element including the type of audio stream, l
19、anguage, sampling rate, surround mode (number of channels), and bit-rate. See A/52 1 ISO_639_language_descriptor() Until March, 2008, the ISO_639_language_descriptor() (MPEG-2 Systems 12) was used to identify the language of a given audio program element. The language of an audio program element is
20、now signaled by the three-byte ISO-639 language field in the AC-3_audio_stream_descriptor().See A/52 1. component_name_descriptor() This is used to associate textual labels with an Elementary Stream component of a program. Its structure is in a tag-length-data format and the data portion is a compon
21、ent_name_string (see below).The component_name_descriptor() is required to be present whenever multiple audio tracks of the same type (same bsmod value) and language are present in a program. Receivers are expected to use the text in the component_name_descriptor() in the user interface offering the
22、 user to choose which audio track to decode. See A/65 4. component_name_string A Multiple String Structure giving the name of the associated Elementary Stream component. See A/65 4. 3.1 PMT Construction and Comments Programs delivered in the digital multiplex may include more than one audio service.
23、 The receiver should not infer anything from the order in which audio streams are listed in the PMT. 3.1.1 Mandatory Elements ATSC A/53 Part 3:2009 Section 6.3 2 requires at least one audio service be designated as a Complete Main (CM). All audio services are required to be constructed so that recei
24、vers can render each independently of any other (complete mixes). Some may be identified as Hearing Impaired (HI) or Visually Impaired (VI) which are authored for improved accessibility. When two or more audio streams of the same language and bsmod value are present, receivers can expect the compone
25、nt_name_descriptor() to be present in the audio metadata (as required in ATSC A/65 4). The component_name_string provides information describing the differences between the audio streams. In some cases, the component_name_descriptor() is a mandatory element that should be used to distinguish between
26、 audio streams (see Example 3 on page 11). In other cases, presence of the component_name_descriptor() could allow a receiver to build a more informative user interface. (See A/69 5, Annex I) 3.1.2 Other PMT Considerations Audio language is signaled in the AC-3_audio_stream_descriptor() by the 3-byt
27、e language field. In earlier versions of the ATSC standards, audio language was signaled in the AC-3_audio_stream_descriptor() by fields called langcod and langcod2. It is expected that broadcasters will follow ATSC A/79 6 to indicate which audio stream is primary or secondary; but this signaling is
28、 for MVPD equipment, and is not intended for consumer product use. CEA-CEB21 4 As of the date of this bulletin, some broadcasters and Multichannel Video Program Distributors manage PMT and PSIP audio metadata using systems that might not create real-time updates to transmitted tables. These systems
29、might not alter the signaling data to exactly match the audio metadata of interstitial insertions (e.g. commercials). The result can be momentary inconsistencies in the transmitted metadata. 3.2 Audio Stream Considerations Receivers should ignore any langcod and langcod2 information present in the t
30、ransport stream. These fields can appear in the audio elementary stream or in the AC-3 descriptor. See ATSC A/52 1. Note Receivers are expected to gracefully handle legacy transport streams containing one or more AC-3_audio_stream_descriptor()s that have non-null langcod or langcod2 fields. Once an
31、audio stream is selected for decoding, the metadata in the stream itself provides all information the decoder needs to render that stream. 3.3 Inconsistencies Between PSIP and PMT In the event that AC-3_audio_stream_descriptor() data is inconsistent between EIT-0 and PMT, receivers should use AC-3_a
32、udio_stream_descriptor() data from the PMT. Situations may be encountered in which the broadcast emission does not follow established standards. Receivers may encounter broadcast streams that have only a single audio stream with no audio metadata present or broadcast streams that do not have a prima
33、ry audio stream assigned. If this occurs, the receiver should decode an audio stream, and not produce an error message. 3.4 Use of Metadata A receiver should parse the PMT and each elementary streams fields and descriptors to build an understanding of each streams content. For the purposes of displa
34、y to the user and audio stream selection, receivers should use the AC-3_audio_stream_descriptor() as the source of metadata about the audio stream. For the purposes of rendering the audio stream, receivers should use the metadata fields from the elementary stream. While some additional metadata elem
35、ents are present in this elementary stream, use of those metadata elements to select audio is not addressed in this recommended practice. Examples include dynrng, dsurmod, fscod, and copyright bit. See ATSC A/52 1. For example, a receiver may find that the metadata contained in an elementary stream
36、will dynamically track the number of channels and bit-rate of the audio for an inserted commercial advertisement, while the corresponding information in the AC-3 descriptor may apply to the program as a whole. The PMT has data for the current stream, and the EIT has a subset of audio metadata for cu
37、rrent and future events. If a receiver allows selection of a future event or an event on a channel that is not currently tuned (through a timer feature e.g. DVR, timer feature, etc.), the receiver should identify the corresponding PMT at the time the program is broadcast, and use it to confirm the c
38、urrently selected the audio stream. In case the scheduled selections metadata does not match the PMT metadata, the receiver should use the PMT instead. A receiver that can make recordings should record all available audio streams including information that will allow users to make selections during
39、playback. In certain cases cached audio metadata may be used to reduce channel change time. See CEA-CEB12 8 for example strategies. 4 Recommendations This section provides recommendations for user setup of audio-related preferences and for selection of audio services while interacting with programmi
40、ng. CEA-CEB21 5 4.1 User Set-Up During setup, receivers should provide a mechanism for a user to identify and save audio decoding preferences. These preferences should include preferred language setting and stream type (CM, VI, or HI). In the event the user does not perform the user setup, then the
41、receiver should select an audio stream marked as Complete Main and produce audio (unless the consumer has silenced the receiver). See subsequent sections for guidance on which stream to select. 4.2 User Operation Receivers should consider the following items when providing for user selection of thei
42、r preferred audio stream: a) Stream type (CM, VI, or HI,) as signaled by the bsmod field in the AC-3_audio_stream_descriptor(). b) The language field encoded in the AC-3_audio_stream_descriptor(). c) The component_name_descriptor() to provide supplemental audio stream information to users, if needed
43、. If a receiver is presented with two or more audio streams that have the same ISO-639 language information and bsmod value, then the receiver should display information from the component_name_descriptor() to the user, allowing the user to select which audio stream to decode. Notethis descriptor ma
44、y contain compressed data per A/65 6.10 4. Any truncation of this field for display may result in the loss of information. Determination of the language of a given audio stream if both the AC-3_audio_stream_descriptor() and ISO_639_language_descriptor() are present is shown by the figure below. In t
45、he absence of user preference for one of the available audio streams, if one audio stream is marked as primary in the priority field of the AC-3_audio_stream_descriptor(), the receiver should select that audio stream.CEA-CEB21 6 Acquire AC-3_audio_stream_descriptor() Read language_flag language_flag
46、 Process ISO_639_language_code in MPEG ISO_639_language_descriptor() Process ISO 639 language bytes in AC-3_audio_stream_descriptor()1 0Figure 1 Transport Stream Language Information Receiver manufacturers should select a set of ISO language codes to be presented as the name of the language as shown
47、 in the column “English Name of Language” in ISO-639. (See http:/www.loc.gov/standards/iso639-2/). If multiple languages are supported for the user interface, then translation to those languages should be provided. If a receiver is unable to fully present the name of the language indicated, it shoul
48、d not malfunction; rather it should show the 3-byte code. The receiver should allow the user to select their choice regardless of whether or not the label can be interpreted (e.g. the three-letter code can be expanded to a language name). The consumers choice while viewing a program (language based,
49、 VI based, HI based) should be carefully considered before making an audio selection change. For example, a program offers both an English and a Spanish audio track. The user has set English as the default preference. While watching the program, the user may choose to decode the Spanish track. How long the receiver retains this non-default audio selection (e.g. for this program only; for this channel only, etc.) is outside the scope of this Bulletin. 4.3 User Interface Recommendation
