1、)45G134 4 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%2-).!,G0G0%15)0-%.4G0G0!.$G0G002/4/#/,3G0G0 amended at Malaga-Torremolinos, 1984)CONTENTSPreamble1 Purpose and scope of this Recommendation2 General Videotex coding structure3 Common features4 Representation of alphanumeric characters in a Vid
2、eotex system5 Alphamosaic option6 Alphageometric option7 Dynamically redefinable character sets (DRCS) option8 Alphaphotographic option9 Service enhancements10 Line and end-to-end protocols11 Interworking with other servicesAnnex A The extension scheme of ISO 2022Annex B Repertoire of graphic charac
3、tersPreambleThe CCITT,considering(a) that there is increasing interest in public network-based new interactive information retrieval services usingdomestic television receivers suitably supplemented, or other apparatus, as terminal equipment;(b) that the CCIR is studying standards for broadcast Tele
4、text services for general reception and has expresseda view that it is desirable that terminal equipment compatibility should exist between broadcast Teletext systems forgeneral reception and public network-based data bank systems;(c) that such services should be provided over public networks in acc
5、ordance with CCITT Recommendationsand may be required to operate as an international service;(d) that such services may interwork with terminals provided for text communication services (Teletex forexample);(e) that some Administrations intend to have an early introduction of, or have already introd
6、uced, publicinteractive Videotex services;unanimously recommendsthat the following technical provisions be applied for international information exchange for interactiveVideotex service.2 Fascicle VII.5 - Rec. T.1001 Purpose and scope of the Recommendation1.1 Purpose1.1.1 The purpose of this Recomme
7、ndation is:a) to facilitate an orderly introduction of early Videotex services (including the continuation of existingservices, with a clear identification of potential enhancements) that need to be considered in futuredevelopments;b) to identify parameters needed to design Videotex terminals; andc)
8、 to provide technical recommendations desirable for potential interworking of other services with Videotexservices.1.2 Scope1.2.1 This Recommendation describes the characteristics of coded information that is exchanged between countriesparticipating in the international interactive Videotex service
9、(as described in Recommendation F.300) and defines thedisplay features corresponding to its various elements.1.2.2 Videotex systems are text communication systems having in addition the capability of a given level of pictorialrepresentation and a repertoire of display attributes. The text and the pi
10、ctures obtained are intended to be displayedusing the current television (TV) raster standards of the different countries.1.2.3 Different options are offered as a choice for the Administrations to implement their national services.Substantial degrees of compatibility exist between these options, but
11、 some transcoding may be necessary to facilitateinterworking.1.2.4 For the international service, four different options for representing pictorial information have been recognized:a) mosaic character sets;b) geometric system;c) dynamically redefinable character sets;d) photographic representation.T
12、hese options are not mutually exclusive and it is possible that systems may develop using two or moreoptions.1.2.5 For international interworking, two categories of TV systems have to be considered:a) systems having a vertical resolution of 525 lines per TV frame at 30 TV frames per second;b) system
13、s having a vertical resolution of 625 lines per TV frame at 25 TV frames per second.1.2.6 Interworking problems at the pictorial level between countries having different recognized pictorial systemsand/or television standards require further study.1.2.7 This Recommendation is structured as follows:
14、1, 2 and 3 deal with the features common to all the options; 4 deals with the coding of characters of the Videotex alphanumeric repertoire defined in Annex B; 5 deals with the alphamosaic option; 6 deals with the alphageometric option; 7 deals with the dynamically redefinable character sets (DRCS) o
15、ption; 8 deals with the alphaphotographic option;Fascicle VII.5 - Rec. T.100 3 9 deals with future enhancements and identifies features requiring further study such as: audio,downloaded software, motion, etc.; 10 deals with line and end-to-end protocols; 11 deals with interworking with other service
16、s.Some of these parts have not been completed, and therefore contain guidelines towards future extensions ratherthan a complete technical specification.2 General Videotex coding structure2.1 General2.1.1 The basis of the coding structure for the Videotex service is Recommendation T.50 and the intern
17、ationalstandards ISO 2022, ISO 6937 for the 7-bit environment. Specifically, the shift-in code SI (0/15) invokes the G0 set foralphanumeric text mode of operation, and the shift-out code SO (0/14) invokes the G1 set, for all the models (seeAnnex A). The use of the 8-bit coding scheme is for further
18、study.2.1.2 In addition to the provisions made by ISO 2022, the transmission of alphabetic characters having diacriticalsigns is effected by transmitting the code representing the diacritical mark together with the code of the basicalphabetic character.2.1.3 The different options are designated (and
19、 invoked) by specific escape sequences.2.2 Designation and invocation in the context of the alphamosaic option2.2.1 Two different modes for the alphamosaic option have been identified. They differ in their display control sets.These control sets are designated as the C1 set by the following control
20、sequences: ESC 2/2 4/0 for the serial mode andESC 2/2 4/1 for the parallel mode, as assigned by ISO. Individual controls are represented by: ESC F, sequences.2.2.2 The mosaic graphics set is designated (in the parallel mode) as the G1 set by an escape sequence ESC 2/9 6/3as allocated by ISO.2.3 Desi
21、gnation and invocation in the context of the alphageometric option2.3.1 The alphageometric coding scheme is to be designated and invoked by the escape sequence ESC 2/5 (5/x) inaccordance with 5.3.8 of ISO 2022 standard. This designates and invokes a complete code with interpretation asfollows.2.3.2
22、All the meanings and interpretation of Recommendation T.50 and ISO 2022 remain the same, including C0, G0and G2 with the exception of SI and SO. The codes of the G1 set and their meanings and interpretations are asdescribed in 6.2.3.3 The designation and invocation of the complete code by the sequen
23、ce ESC 2/5 (5/x) is to be terminated only byESC 2/9 (F) or ESC 2/13 (F), designating a normal G1 set.2.4 Designation and invocation in the context of DRCS2.4.1 A DRCS is a set of characters whose shapes are sent from the service and down-loaded via the line. It may beused to represent alphabetic cha
24、racters, special symbols, or picture element symbols for constructing fine graphics.Once loaded, the DRCS are regarded as members of a library that can be designated by appropriate ESC sequences asG0, G1, G2, G3 sets. One scheme is described in 7 in the context of a general architecture.2.5 Designat
25、ion and invocation in the context of the alphaphotographic option(For further study.)4 Fascicle VII.5 - Rec. T.1003 Common features3.1 General3.1.1 The features pertaining to individual systems will be described in the corresponding paragraphs. The commonfeatures comprise common display features and
26、 common control functions.3.2 Common display features3.2.1 The defined display area is that rectangular position of the display in which all text and pictorial images may bepresented (see Figure 1/T.100).3.2.2 The border area is that part of the visible display of a terminal that is outside the defi
27、ned display area (seeFigure1/T.100).3.3 Common format effector and code extension control functions3.3.1 General3.3.1.1 The format effector control functions described for the Videotex system permit the active drawing position tobe moved on the visible display area. These are taken from the C0 set (
28、see Figure 2/T.100) together with the Spacecharacter 2/0. In order to permit interworking between Videotex and other text communications services, these controlfunctions have functional compatibility to the extent possible with the basic C0 control set utilized by these otherservices.3.3.2 Format ef
29、fector controls3.3.2.1 Some of the format effector control functions may be used from terminal to computer with different meanings.3.3.2.2 Active position backward (APB)This control function causes the active position to be moved one character position backwards on the same row.APB on the first char
30、acter position on the row moves the active position to the last character position of the precedingrow. APB on the first character position on the first row moves the active position to the last character position of thelast row.Fascicle VII.5 - Rec. T.100 53.3.2.3 Active position forward (APF)This
31、function causes the active position to be moved to the next character position forward on the same row. Atthe last position on the row, this control moves the active position to the first character position on the following row.APF on the last character position of the last row moves the active posi
32、tion to the first character position of the firstrow.3.3.2.4 Active position down (APD)This function causes the active position to be moved to the equivalent character position on the following row.APD on the last row moves the active position of the equivalent character position of the first row of
33、 the display frameor causes a roll-up to be made.6 Fascicle VII.5 - Rec. T.100 Fascicle VII.5 - Rec. T.100 73.3.2.5 Active position up (APU)This function causes the active position to be moved to the equivalent character position on the preceding row.APU on the first row moves the active position to
34、 the equivalent character position on the last row of the same displayframe.3.3.2.6 Clear screen (CS)This function causes the screen to be cleared and causes the active position to be moved to the first characterposition on the first row.3.3.2.7 Active position return (APR)This function causes the a
35、ctive position to be moved to the first character position of the same row.3.3.2.8 Space (SP)A control function that causes the active position to be moved one character width forward on the same row. Itis also regarded as a graphic character with no foreground. In those systems that define an expli
36、cit background, thespace copies the background colour into the active position and moves the active position one character width forward.If used in conjunction with the inversion attribute it copies the foreground colour into the active position and moves theactive position one character width forwa
37、rd.3.3.2.9 Cancel (CAN)A control function that fills all the character positions of the row, after the active position, with spaces andreturns the active position to its original value.3.3.3 Code extension control functions3.3.3.1 Code extension control functions are used to expand the capability of
38、 the 7-bit code beyond 128 differentcharacters or functions. Code extension functions alter the meaning of a number of characters following them.3.3.3.2 Escape (ESC)A control character that is used to provide additional control functions other than transmission controlfunctionsand that alters the me
39、aning of a limited number of contiguously following bit combinations in the manner specified inRecommendation T.51.3.3.3.3 Control sequence introducer (CSI)A code extension control function that is used to provide coded representations for additional control functions,in particular for control funct
40、ions with parameters such as presentation control functions.3.3.3.4 Shift-out (SO)A control character that is used in conjunction with the Shift-in character to extend the graphic character set ofthe code and that alters the meaning of the bit combinations of columns 2-7 of the code table, until the
41、 occurrence ofthe shift-in character, except that the meaning of the bit combinations corresponding to the space character and thedelete character (positions 2/0 and 7/15) are unaffected.3.3.3.5 Shift-in (SI)A control character, used in conjunction with the shift-out character, that reinstates the f
42、ormer meanings of thebit combinations of columns 2-7 of the code table.3.3.3.6 Single shift (SS2)This character alters the meaning of the single-bit combination following it. That bit combination must be oneof those from columns 2-7 except 2/0 and 7/15. The meaning of the bit combination concerned i
43、s derived from anappropriately designated G2 graphic set.3.3.3.7 Single shift (SS3)This character alters the meaning of the single-bit combination following it. That bit combination must be oneof those from columns 2-7 except 2/0 and 7/15. The meaning of the bit combination concerned is derived from
44、 anappropriately designated G3 graphic set.8 Fascicle VII.5 - Rec. T.1003.4 Miscellaneous3.4.1 Null (NUL)This function may occur in non-transparent modes in the received bit stream at the terminal. It shall be regardedas a time filler and discarded.3.4.2 Enquiry (ENQ)A control character used as a re
45、quest for a response from a remote station, which response may include stationidentification and/or station status.3.5 Coding of control functions3.5.1 A proposed coding of the control functions described is shown in Figure 2/T.100 as a C0 set, except for CSIwhich is coded in the C1 set.4 Representa
46、tion of alphanumeric characters in a Videotex system4.1 General4.1.1 The repertoire for the Latin alphabet is shown in Annex B. The repertoire is derived from ISO 6937. Terminalscapable of displaying a subset of the Videotex repertoire shall be permitted.4.1.2 Character repertoires for non-latin bas
47、ed languages can be accommodated in a similar manner to the latinalphabet. (For further study.)4.2 Coding4.2.1 Section 4.2 describes the coding of characters the shape of which are stored in the terminal. Some languagesrequire that consecutive letters or diacritical marks will be joined and that no
48、space appear between the characters.When an intersymbol space is required, it will be part of the character description.4.2.2 The code tables are shown in Figures 3/T.100 and 4/T.100. The code combinations representing characters notincluded in the Videotex repertoire shall not be transmitted.4.2.3
49、All the permitted combinations may be expected in the international exchange of information between twonational services. It is the responsibility of Administrations to decide whether this exchange is a direct terminal to data-base operation or has to be performed through a gateway. See Recommendation F.300.4.2.4 The graphic characters from columns 2, 3, 5, 6 and 7 of the supplementary set are invoked one at a time bySS2.4.2.5 A character with a diacritical mark is transmitted by the sequence SS
