1、Rec. ITU-R BT.653-3 1RECOMMENDATION ITU-R BT.653-3TELETEXT SYSTEMS*(Question ITU-R 72/11)(1986-1990-1994-1998)Rec. ITU-R BT.653-3The ITU Radiocommunication Assembly,consideringa) that several countries have developed and established satisfactory teletext systems;b) that it would be highly desirable
2、to assure the compatibility of such systems with the videotex (interactivevideography) systems;c) that a proliferation of such systems would add further complication to the interconnection of such systems,recommends1 that for a country wishing to initiate a teletext service, one of the four systems
3、in Annex 1 is to be preferred.ANNEX 1Characteristics of teletext systems1 IntroductionThis Annex provides information about the teletext systems, developed for use with television systems ofRecommendation ITU-R BT.470.An outline description of the essential elements of the teletext systems is given
4、in Table 1 and the accompanyingdiagrams (Figs. 6, 7, 8 and 9). The structure of the Table is based, as far as practicable, on the ISO reference model*.Table 2 lists the countries and systems used.2 Definition of the teletext serviceA digital data broadcasting service which may be transmitted either
5、within the structure of an analogue television signalor by using digital modulation systems. The service is primarily intended to display text or pictorial material in two-dimensional form reconstructed from coded data on the screens of suitably equipped television receivers.NOTE 1 At the present ti
6、me, the field-blanking interval is, in most cases, used for the data broadcasting service, but apossible option exists for extending the data broadcasting service to occupy all active lines in a television signal. Theeffect on protection ratios for television broadcasting has been studied for 625-li
7、ne systems and the results published inRecommendation ITU-R BT.655.3 A layered model for describing teletext systemsTeletext systems can be described, as shown in Fig. 1, according to the reference model for data broadcasting detailed inRecommendation ITU-R BT.807._*Also referred to as broadcast vid
8、eography.*ISO 7498 (1984) “Basic reference model for open systems interconnection”.2 Rec. ITU-R BT.653-3Layer 7 a) displayb) editingc) printingLayer 6 a) textsb) picturesc) display attributesLayer 5 a) page addressingb) page classificationc) page linkingd) page storageLayer 4 a) message formattingb)
9、 message identificationc) message control informationd) message data informationLayer 3 a) data channel addressingb) data packet sequencingLayer 2 a) byte synchronizationb) error control (framing, misdetection andb) false detection)c) data formattingLayer 1 a) RF modulation typeb) number of televisi
10、on lines for data communicationc) data signal levels, shape and bit rated) primary data codinge) bit synchronizationUserserviceDatacommunicationPhysical transmissionTransmissionlayersFIGURE 1A functional layered model for the description of teletext systemsD01FIGURE 1.D01 = 20.5 CMRec. ITU-R BT.653-
11、3 3According to this functional model, services may be delivered by arranging the information into logical groupings,delivering them to lower layers for transmission and, after reception, reconstituting the information into the proper formfor use by the recipient.In what follows, the names of the la
12、yers are those adopted by the ISO in ISO 7498 (1984) “Basic reference model foropen systems interconnection”. Some of these names are used in broadcasting technology to express different concepts.This particularly applies to the terms “network” and “link” and care must be taken to avoid confusion.La
13、yer 1: PhysicalWithin a given broadcast transmission system this layer relates to the electrical transmission of the data signal andincludes such items as bit rate and pulse shaping.Layer 2: LinkThis layer includes logical functions related to the data transmission such as digital frame synchronizat
14、ion techniques,data formatting and error control procedures.Layer 3: NetworkThis layer includes logical functions related to multiplexing and demultiplexing of data packets belonging to differentcommunication flows. Examples of such functions are data channel addressing and data packet sequencing.La
15、yer 4: TransportThis layer provides the function of arranging the data in a way suitable for transfer from one point to another, by suchmeans as segmenting data into groups of information, delivering them to the lower layers for transmission to the distantpoint and there reconstituting the groups of
16、 information and arranging them in a proper sequence.Layer 5: SessionThis layer includes data handling functions which are intended to assist the user to gain access to services. Examples ofsuch functions are access control and page classification.Layer 6: PresentationThis layer comprises data prese
17、ntation functions. Examples are the codings used for the presentation of text, pictures andsound.Layer 7: ApplicationThis layer refers to practical use of the potential facilities provided by the lower layers for a given type of service.Examples are captioning, telesoftware and cyclic teletext.4 Tra
18、nsmission characteristicsThe logical structure of different elements of the teletext data and their relationship to the television signal are illustratedin Figs. 2, 3, 4 and 5.4.1 Data line (Fig. 2)A data line is a television line, the active part of which is assigned to digital data. The data conte
19、nt is subdivided into abit synchronization sequence followed by a data unit.4.2 Data unit (Fig. 3)A data unit is a logical unit of data, sudivided into a byte synchronization sequence and a data packet.4 Rec. ITU-R BT.653-34.3 Data packet (Fig. 4)A data packet is an identifiable information package
20、which comprises: a prefix providing for functions such as addressing, packet size indication, packet continuity indication anddesignation of packet type; a data block containing control signals or user information; in some systems, a suffix to perform the function of error detection or correction at
21、 the packet level.4.4 Data group (Fig. 5)A data group is an identifiable group of data blocks containing information from the same source.5 Presentation layer characteristicsIn specifying the presentation layer of teletext systems, substantive account has been taken of the work of the ex-CCITTon vid
22、eotex systems in its Recommendations T.100 and T.101 (Mlaga-Torremolinos, 1984). The work of ISOTC 97/SC 2 on character repertoires and coding for all writing systems and languages also has to be taken into account.5.1 Repertoires5.1.1 Alphabets and character setsa) Latin alphabetThe code tables for
23、 characters and pictorial commands for the presentation of Latin alphabet based alphanumeric andpictorial information are identical to the respective Videotex code tables of the annexes of ex-CCITT RecommendationT.101 (Malaga-Torremolinos, 1984), with the exception of additional characters as indica
24、ted in 5.1.2. For some codingformats, the controls, commands and instruction sequences are also identical to those of the respective Videotex codingstandards. For other formats a precisely equivalent set of controls and description/instruction sequences are defined.b) Cyrillic alphabetFor the Cyrill
25、ic alphabet all characters are ISO identified (ISO/DIS 6937-8) with the exception of two symbols.The system of coding the Cyrillic symbols for the transmission of Ukrainian texts is currently submitted to Study Group11 for consideration. Method of switching Russian/Ukrainian specific symbols is unde
26、r study.c) Chinese character setsChinese characters are very large in number and complicated in form. According to the National Standard of thePeoples Republic of China GB 2312 “Code of Chinese Graphic Character Set for Information Interchange PrimarySet”, the number of the first grade characters is
27、 3755, and that of the second grade characters is 3008. The stipulation onencoding and character forms in GB 1988 “Information processing 7-bit coded character set for informationinterchange”, GB 2311 “Information processing ISO 7-bit and 8-bit coded character sets Code extension techniques”and the
28、GB 5007.1-5007.2 “24 24 dot matrix font set and data set of Chinese ideograms for information interchange”will be will be followed . GB/T 12345 “Code of Chinese ideogram set of information interchange supplementary set”and the HK-subset is specially used in the teletext broadcasting of complex form
29、from Chinese ideograms.d) Japanese character setsThe Japanese language is written with mixed use of three types of Japanese characters, and sometimes with the additionof Latin alphabets. They are Katakana, Hiragana and Kanji. The Katakana and Hiragana character sets have a single-bytestructure based
30、 on ISO standards and respectively contain 86 and 83 Japanese phonetic characters. The Kanji characterset has a two-byte structure also based on the ISO standards and contains 2965 characters of level 1 and 3388 charactersof level 2 specified in Japanese Industrial Standard (JIS) C 6226. Kanji uses
31、ideographic characters which have a closerelation with Chinese characters.Rec. ITU-R BT.653-3 5GHJKFEAB CDFIGURE 2Data lineFIGURE 3Data unitFIGURE 4Data packetFIGURE 5Data groupF:G:H:J:K:data unitprefixdata blockdata packetsuffix where applicableD02line synccolour burstbit syncdata linebyte syncA:B:
32、C:D:E:FIGURE 5.D02 = 23.5 CM6 Rec. ITU-R BT.653-35.1.2 Special charactersCertain characters of importance for the captioning functions of teletext services are not included in any of thepresentation layer syntaxes defined in Annexes to ITU-T Recommendation T.101. These are:123Off-screen indicatorsTe
33、lephoneD03FIGURE 3.D03 = 3 CM5.2 Source coding5.2.1 Alphanumeric codingThese codes are used to display text. Alphanumeric graphic elements include alphabetic letters, syllabic characters andideographic characters with or without diacritical signs, figures, punctuation marks and special signs.5.2.2 M
34、osaic codingThese codes are used to construct drawings by means of block mosaic, smooth mosaic and line drawing characters. Eachelement defines part of a pattern and occupies one character position. Two forms of presentation are defined: separated: each element is surrounded by a border of the backg
35、round colour; contiguous: elements adjoin one another.5.2.3 Dynamically re-definable character sets (DRCS)Character sets in which some, or all, of the characters may be defined at the source and down loaded into the receiver,which can then use them as graphic elements.5.2.4 Geometric codingThese cod
36、es are used to construct drawings of various types by a succession of elements such as points, lines andsurfaces.5.2.5 Photographic codingThese codes are used to cause the generation of individual picture elements for the display of an image. Continuous toneimages as well as pattern oriented display
37、s including graphics characters are included.5.2.6 Musical sound dataThese codes are used to cause the generation of musical sounds. Pitch, tone duration, rhythm, timbre and harmonicrelationship are defined.Rec. ITU-RBT.653-3 7TABLE 1a Description of the essential elements of teletext systems specif
38、ied for 625/50 television systems Teletext system A B C D(1)Layer 1: Physical 1.1 Time slot usable for data Active part of any TV line subject to availability 1.2 Data positioning (relative to line sync. timing reference)(2)10.5 0.32 s Bit 13 is reference plus 12.0 s (-1.0, +0.4) 10.48 0.34 s 10.5-1
39、0.97 s(3)1.3 Data amplitude(2)logical 0 logical 1 S: sync D: pedestal A: data D/S = 0 ( 3%) A/S = 7/3(+0, 10%) for positive modulation A/S = 14/9(0, +6%) for negative modulation Black level 2% 66% ( 6%) of black-to-white excursion 0 IRE units 70 IRE units for negative modulation 100 IRE units for po
40、sitive modulation 0 2.5 IRE units 70 2.5 IRE units 1.4 Bit rate 6.203125 Mbit/s 0.005% 6.9375 Mbit/s 25 1065.734375 Mbit/s(4)(367 line frequency) 5.6427875 Mbit/s (14/11 fsc) 1.5 Data shaping(2)Sine square Spectral shaping is skew symmetrical about 0.5 bit rate Typically a raised cosine 100% roll-of
41、f spectrum, followed by a video low pass filter 100% cosine roll-off 1.6 Data coding Binary NRZ Binary NRZ Binary NRZ Binary NRZ 1.7 Data line including clock run-in 320 bits 360 bits 288 bits (The first 16 bits of alternating 1s and 0s constitute the clock run-in) 296 bits (bytes(5)1 to 37. Bytes 1
42、 and 2 comprise clock run-in) Layer 2: Link 2.1 Digital frame synchronization Byte 3 = 11100111 Byte 3 = 11100100 Byte BS = 11100111 Byte 3 = 11100101 2.2 Length of data unit 38 bytes 43 bytes 34 bytes (excluding clock run-in) 35 bytes 2.3 Format indicator Byte 8 (byte 5 in short prefix) Not require
43、d PS byte NOTE 1 For Notes see the end of Table 1b. 8 Rec. ITU-RBT.653-3TABLE 1a (Continued) Teletext system A B C D(1)2.4 Error detection/correction 2.4.1 Byte error detection parity Odd parity on teletext data bytes Odd parity for bytes 4 to 45 Even parity for bytes 1 to 3 Odd parity 2.4.2 Byte er
44、ror detection/correction 8/4 Hamming code on bytes 4 to 8 (4 and 5 in short prefix) 8/4 Hamming code for bytes 4 and 5; 8/4 and 24/18 for extension packets numbers 26, 27, 28 and 29 8/4 Hamming code on all bytes in the prefix, data group header, record header 2.4.3 Block error detection/correction N
45、o Bytes 44 and 45 of designated data blocks carry a cyclic redundancy check word (CRC)(6)Suffix bytes indicated by bits b8b6 of the PS byte (272,190) majority logic decodable difference set cyclic code on bytes 4 to 37 as a block Layer 3: Network 3.1 Data channel addressing Bytes 4, 5 and 6 Bytes 4
46、and 5 of all packets Bytes P1, P2, P3 Byte 4 and the data line position 3.2 Data packet sequencing Byte 7 Bytes 4 and 5 of all packets Byte C1 Byte 5 (bits 1 to 4) 3.3 Length of prefix 5 bytes (long prefix) or 2 bytes (short prefix) 2 bytes 5 bytes 14 bits (byte 4 and byte 5, bits 1 to 6) 3.4 Length
47、 of data block Given by value of byte 8 (byte 5 in short prefix) according to a look-up table 40 bytes 0, 26, 27 or 28 bytes indicated by bits b8b6 of PS byte 22 bytes (D-bytes(7)1 to 22) Layer 4: Transport 4.1 Group of data blocks Start = SOH-RS (0/1-1/14) End = ETX-EOT (0/3-0/4) In page-oriented s
48、ervices: start by page header packet, bytes 4 to 13 termination by next page header packet For independent data services: packets 30 and 31. Byte GT indentifying 16 types of data group Byte 5, bit 6 = 1 and D-byte 1 = 00/1 indicate the data block contains a data group header. D-bytes 2 to 7 constitu
49、te the data group header. Rec. ITU-RBT.653-3 9TABLE 1a (Continued) Teletext system A B C D(1)4.2 Data group size 1920 bytes max. 1024 bytes or multiples of 1024 bytes Bytes S1, S2, and F1, F2 D-bytes 4 and 5(8)4.3 Data group integrity 4.3.1 Continuity No Automatic Byte GC A data group is a series of data blocks sequentially transmitted in a data channel. (See 3.1 and 3.2) 4.3.2 Error detection/correction No Packet 27, bytes 44 and 45 of designated data blocks carry a cyclic redundancy check word (CRC)
