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1、 CEA Standard Digital Television (DTV) Closed Captioning CEA-708-D August 2008 NOTICE Consumer Electronics Association (CEA) Standards, Bulletins and other technical publications are designed to serve the public interest through eliminating misunderstandings between manufacturers and purchasers, fac
2、ilitating 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 preclude any member or nonm
3、ember 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, whether the standard is
4、 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, nor does it assume an
5、y obligation whatever to parties adopting the Standard, Bulletin or other technical publication. This CEA Standard is considered to have International Standardization implication, but the International Electrotechnical Commission activity has not progressed to the point where a valid comparison betw
6、een the CEA Standard and the IEC document can be made. This Standard 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 Standard to establish appropriate safety and health practices and to de
7、termine the applicability of regulatory limitations before its use. (Formulated under the cognizance of the CEAs R4.3 TV Data Systems.) Published by CONSUMER ELECTRONICS ASSOCIATION 2008 Technology Phone 800-854-7179; Fax 303-397-2740; Internet http:/; Email ISO/IEC Standards: Global Engineering Do
8、cuments, World Headquarters, 15 Inverness Way East, Englewood, CO USA 80112-5776; Phone 800-854-7179; Fax 303-397-2740; Internet http:/; Email IEC Central Office, 3, rue de Varembe, PO Box 131, CH-1211 Geneva 20, Switzerland; Phone +41 22 919 02 11; Fax +41 22 919 03 00; Internet http:/www.iec.ch;
9、Email pubinforiec.ch ISO Central Secretariat, 1, rue de Varembe, Case postale 56, CH-1211 Geneve 20, Switzerland; Phone +41 22 749 01 11; Fax +41 22 749 01 55; Internet http:/www.iso.ch; Email mbinfoiso.ch SMPTE Standards: Society of Motion Picture Email: engsmpte.org 2.2 Informative References The
10、following references contain provisions, which, through reference in this text, constitute informative provisions of this standard. At the time of publication, the edition indicated was valid. All standards are subject to revision, and parties to agreements based on this standard are encouraged to i
11、nvestigate the possibility of applying the most recent edition of the standard indicated below. 2.2.1 Informative Reference List ATSC A/53:2007 Part 4, with Amendment 1, MPEG-2 Video System Characteristics, 3 January 2007 Amendment 1 is in the ballot process ATSC A/65C, Program and System Informatio
12、n Protocol for Terrestrial Broadcast and Cable, with Amendment No. 1, May 9, 2006 ISO 639-2:1998, Codes for the representation of names of languagesPart 2: Alpha-3 code SCTE-21 2006, Standard for Carriage Of NTSC VBI Data in Cable Digital Transport Streams, SMPTE EG 43-2004, System Implementation of
13、 CEA-708-B and CEA-608-B Closed Captioning SMPTE 292M-1998, Television Bit-Serial Digital Interface for High-Definition Television Systems SMPTE 333-2008, DTV Closed-Caption Server to Encoder Interface (Revision of SMPTE 333M-1999) SMPTE 0334-1-2007, Vertical Ancillary Data Mapping of Caption Data a
14、nd Other Related Data CEA-708-D 5 SMPTE 0334-2-2007, Caption Distribution Packet (CDP) Definition SMPTE RP 2007-2007, Closed-Caption CDP and “Grand Alliance“ Serial Interfaces for DTV 2.2.2 Informative Reference Acquisition ATSC Standards: Advanced Television Systems Committee (ATSC), 1750 K Street
15、N.W., Suite 1200, Washington, DC 20006; Phone Phone 202-828-3130; Fax 202-828-3131; Internet http:/www.atsc.org SMPTE Standards: Society of Motion Picture Web: http:/www.smpte.org ISO Standards: Global Engineering Documents, World Headquarters, 15 Inverness Way East, Englewood, CO USA 80112-5776; Ph
16、one 800-854-7179; Fax 303-397-2740; Internet http:/; Email ISO Central Secretariat Office, 1, rue de Varembe, Case postale 56, CH-1211 Geneve 20, Switzerland; Phone +41 22 749 01 11; Fax +41 22 733 34 30; Internet http:/www.iso.ch; Email centraliso.ch CEA-708-D 6 3 Caption Channel Layered Protocol
17、A formal data communications channel protocol has been established for the DTVCC caption data channel. This formalization provides a framework for describing the caption communications hierarchy. Grouping the structures, concepts and features of this environment into the following hierarchical layer
18、s, aids in the understanding of the organizational aspects of the DTVCC system. There are 5 layers in the DTVCC data framework: DTVCC Transport Layer, DTVCC Packet Layer, DTVCC Service Layer, DTVCC Coding Layer, and DTVCC Interpretation Layer (see Figure 1). Similar to the OSI Reference Model, each
19、of these layers provides particular services in the receiver, as shown in Table 1. Services in a Receiver CEA-708-D Section DTVCC Protocol Model CEA-608 datastream Specification of Window appearance and content Placement of Windows over Video Synchronization of Command Interpretation to the Display
20、of video Reset of Caption Service 8 DTVCC Interpretation Layer Parsing of Syntax 7 DTVCC Coding Layer Demultiplexing of Caption Services 6 DTVCC Service Layer Detection of Synchronization Loss Resynchronization of Data Stream Reset of Caption Decoder 5 DTVCC Packet Layer These layers are defined in
21、CEA-608-E. Extraction of closed caption data from parent transport system, e.g. MPEG user data Demultiplexing of CEA-608 datastream and DTVCC caption streams 4 DTVCC Transport Layer Table 1 DTVCC Protocol Stack The DTVCC Transport Layer is where DTVCC data leave the DTV Video subsystem and are intro
22、duced to the DTV Closed-Caption decoder. Within the DTVCC decoder, DTVCC data are further processed up through the remaining layers of the DTVCC Protocol Model. Both the CEA-608 datastream and DTVCC data are carried in this layer. NOTEThe DTVCC Transport layer should not be confused with the main sy
23、stem transport layer. For example, in MPEG-2 ATSC systems, the MPEG-2 transport layer is defined in ISO 13818-1, and this transport layer carries video, audio, data and control streams. DTVCC bits are CEA-708-D 7 encoded within the video signal conforming with ISO 13818-2. These bits enter the DTVCC
24、 decoder from the video streams user data structure. CEA-708-D specifies a method of placing a CEA-608 datastream, embedded in the CEA-708-D signal, into the correct bit positions and fields in an NTSC signal, but CEA-708-D does not specify any application or presentation level processes for CEA-608
25、 datastreams. Further processing of CEA-608 datastreams is specified in CEA-608-E. The DTVCC Packet Layer marks where DTVCC data enter the DTVCC decoder. This is a protocol data reassembly layer that buffers incremental bitstream data into a byte-aligned, multi-byte packet. This layer provides resyn
26、chronization for decoding the DTVCC stream, and is not dependent on the DTVCC Transport layer. Processing of the Caption Channel packet data begins in the DTVCC Service Layer. Caption Channel packets are broken up into the encapsulated sub-blocks of data to be routed to one or more separate caption
27、service processing routines within the decoder. Using information in the DTVCC Service Directory (see Section 4.5), TV viewers may choose to view the processed data for one or more DTVCC services at a time. For example, a caption channel may contain an English language service and a Spanish language
28、 service. The DTVCC Coding Layer breaks out the individual caption commands and caption text sequences from the service data blocks. The DTVCC Interpretation Layer processes the caption elements presented by the DTVCC Coding Layer. The DTVCC Interpretation Layer also provides for delaying the interp
29、retation of the command stream and reset of a specific Caption Service. CEA-708-D 8 Figure 1 DTV Closed-Captioning Protocol Model Coding Layer Interpretation Layer Service Layer Packet Layer Transport Layer DTVCC Data however, receiver implementation may use any process providing equivalent function
30、ality when receiving compliant streams. NOTEThis model turns off the run in clock; however, there is no requirement that the run in clock ever be turned off. Two FIFO-based Field Buffers (one for NTSC_Line21_field1_data and one for NTSC_Line21_field2_data), as shown in Figure 2, hold CEA-608 datastr
31、eam byte-pairs and associated cc_valid flags. Events Time Line21 Field 1 Buffer Level Bytes MPEG userdata() byte pairs added Field 1 removed Field 2 removed Field 1 Field 2 Field 1 Field 2 Field 1 Field 2 Field 1 Field 2 Field 1 Line21 Field 2 Buffer Level Bytes 24222 2222 0 0 Events Time Line21 Fie
32、ld 1 Buffer Level Bytes MPEG userdata() byte pairs added Field 1 removed Field 2 removed Field 1 Field 2 Field 1 Field 2 Field 1 Field 2 Field 1 Field 2 Field 1 Line21 Field 2 Buffer Level Bytes 24222 2222 0 0 Figure 2 Example of CEA-608-E Captioning Field Buffers Byte pairs are received from the DT
33、VCC Transport and written into the Field Buffers in the order that they appear in the cc_data() structure. Bytes are removed from Field Buffer “x” at the start of NTSC field “x”, and are transmitted on the NTSC signal in the order they are removed from the buffer. If Field “x” Buffer is empty at the
34、 transmit time of NTSC field “x”, a CEA-608 datastream caption waveform should be generated for that field with cc_data_1 = 0x80 and cc_data_2 = 0x80 (in CEA-608-E notation, two 0x00s with odd parity). CEA-708-D 11 If the byte pair at the output of the FIFO is marked “cc_valid=0”, no CEA-608 datastr
35、eam waveform shall be generated for field “x,” except that if captions are generated for field 2, CEA-608-E requires that clock run in and start bits shall also be generated for Field 1. . If a video frame has no associated cc_data() structure or if process_cc_data_flag=0, no byte pairs shall be add
36、ed to the Field Buffers. In steady-state operation, the proper Field Buffer always contains data at the start of a Field for all frame rates and repeated fields. Designers should understand that the use of frame rates of 60, 30 or 24 Hz may cause corrupted CEA-608-E caption sequences at start-up and
37、 across splices. CEA-608-E requires that a Field 1 waveform (run-in clock and start bit) be present whenever a Field 2 waveform is present. This data rate reduction can be done in an encoder by periodically sending cc_data() structures without cc_type = 00 or 01. See Table 3. The CEA-708-D decoder d
38、oes not put any data into the buffer when decoding this frame. This would normally produce a “captionless” frame (no captioning waveform on line 21). If captionless frames occur every 1001 frames, then, after startup, the buffer does not empty (assuming the decoder is locked to the DTVCC Transport t
39、imebase). The number of frames with a reduced cc_type = 00 or = 01 count at the beginning of the cc_data() structure should be minimized, but may be more frequent in portions of the program where splices have occurred. These reduced caption frequency portions of the sequence may create NTSC output w
40、ith corrupted or missing caption data during steady-state operation Encoding equipment shall not assume that decoders can buffer more than two byte-pairs (four bytes) of CEA-608-E field 1 data and two byte-pairs (four bytes) of CEA-608-E field 2 data, but shall send a CEA- 608 datastream at a substa
41、ntially constant data rate. The size of the decoder buffer shall be at least two byte-pairs for each type, since some cc_data() structures can have two byte-pairs of one type in them. Therefore, encoders may not assume a larger value when arranging the placement of a CEA-608 datastream in the cc_dat
42、a() structure. A field types buffer receives two or four bytes synchronized to the video frame, and two bytes are removed at the encoding time of the NTSC output. If there are exactly 1000 byte pairs of a type per 1001 video frames, then, under steady-state operation, the buffer always has two bytes
43、 available for each NTSC output. An NTSC frame could be encoded without a CEA-608 datastream at start-up, if a cc_data() structure with a reduced CEA-608 datastream fails to fill the buffer before an NTSC frame is output. This could cause corruption in the NTSC output signal, since CEA-608-E decodin
44、g rules include special cases for repeated control codes in adjacent frames. Insertion of an empty line 21 could change these to non-adjacent control codes, changing their interpretation. This is unavoidable and anticipated for FCC compliant CEA- 608-E caption decoders. 4.3.2.1 Alternating Transmiss
45、ion of CEA-608 datastream When a CEA-608 datastream is present in a stream, encoding equipment shall always include a cc_data() structure for each field, even if a decoder is being instructed not to generate the run-in clock and start bit on one field of CEA-608 datastream (by setting cc_valid to 0
46、for that field type). An exception occurs when the decoded frames produce a frame rate higher than 30/1.001, for example, when the frame rate is exactly 30 Hz. In this case, frames with a reduced number of cc_type = 00 or = 01 at the beginning of the cc_data() structure may occur to lower the averag
47、e rate to no more than 30/1.001 byte pairs of each field type per second. This allows downstream equipment to insert a legal CEA-608 datastream without moving DTVCC caption data between video frames. CEA-708-D 12 4.3.2.2 Suppressing Run-In Clock and Start Bit When No CEA-608-E Data Present When deco
48、ders do not have CEA-608-E cc_data_1 and cc_data_2 bytes to encode into NTSC outputs, decoders may suppress the run-in clock and start bit on the NTSC output signal, but there is no requirement that they do so. Likewise, if the entire cc_data() structure is missing from the video frame, decoders may
49、 suppress the run-in clock and start bit on the NTSC output signal, but there is no requirement that they do so. 4.4 DTV cc_data() structure Section 4.4 defines caption-related syntax elements in the cc_data() structure, and specifies their semantic rules for placing caption information into the cc_data() structure and interpreting that data. Syntax No. of Bits Format cc_data() reserved 1 1 process_cc_data_fl