Chambers: Enhanced UK Teletext Moves Towards Still Pictures
`
`ENHANCED UK TELETEXT MOVES TOWARDS STILL PICTURES“
`
`John P. Chambers, BBC Research Department, Kingswood Warren,
`TADWORTH, Surrey, KT20 GNP. UK.
`
`1.
`
`Introduction
`
`The UK teletext system has been a full public
`service for almost four years, and the specification
`of the system has remained unaltered. «
`It
`is
`expected that by the end of 1980 there will be
`more than 100 000 teletext receivers in use. There
`is no doubt that the system rests on a firm tech-
`nical foundation and the deliberate attempt made
`to stretch technology at the time the specification
`was formulated means that the system is giving the
`highest possible information transfer rate consis-
`tent with ruggedness and freedom from errors.
`
`In the UK the only significant criticism of
`teletext has been that
`the access time is slow.
`
`Typically, with two data lines per field, a broadcast
`magazine will contain a major cycle of about 90
`pages repeating about every 20 seconds, and con-
`sequently the
`average waiting time compares
`unfavourably with that of an interactive viewdata
`service using the telephone line, such as Prestel.
`
`In Europe the UK teletext system as it stands
`is not appropriate to the character requirements
`of many of the languages used. Moreover, there is
`a desire to be able to mix languages within a single
`teletext page. A survey of the requirements of
`the members of the European Broadcasting Union1
`shows that a common—core alphabet of about 200
`characters is needed for languages using the Latin
`alphabet.
`A requirement to underline text has
`also been identified.
`
`By reference to a particular proposal, which is
`annexed, this paper indicates how the UK teletext
`Reprinted by permission from BBC Research Department Report
`BBC RD 1980/4. June 1980. Copyright 1980. BBC Research Dept,
`Kingswood Warren. Tadworth, Surrey KT20 6 NP, England.
`
`0098-3063/80/0527-0554$00.75©1980|EEE
`
`It is accepted that UK teletext is the begin-
`ning of a line of development
`leading to an
`enhanced teletext
`system whose capability is
`limited only by the display device itself, and
`capable of producing a sequence of still pictures
`with a full range of colour, and definition superior
`to that of the current broadcast television systems.
`Such a system would, of course, be capable of
`handling material from systems such as Antiope
`and Telidon should this prove necessary.
`An
`enhanced teletext
`system should also provide
`for particular
`types of auxiliary data such as
`programme labelling, as well as allowing arbitrary
`data to be sent as part of a teletext magazine
`or on totally independent channels within the
`teletext format.
`
`system can be extended to meet all these require-
`ments, including improved access, while retaining
`full compatibility with, and the ruggedness of, the
`existing system.
`The techniques are described in
`terms of the 625/50 television system, the adapta-
`tion to 525/60 systems has been discussed else-
`where.2
`
`2.
`
`Improved access
`
`The UK teletext system already includes ample
`provision for uniquely addressing individual pages.
`Within each of eight
`independent magazines,
`selected by a single digit, 100 pages can be selected
`by a further two-digit page number and up to 3200
`versions of each page can be selected by a further
`four-digit
`subcode (formerly known as
`‘Time-
`Code‘).
`So even using the existing keyboards and
`decoders over 2% million different pages can be
`individually accessed.
`The addition of a ‘don’t-
`care' key (see A.2.1.4-) allows groups or sequences
`of pages to be accessed.
`
`Already teletext decoder chip sets are avail-
`able in the UK which support more than one page
`store.
`This means that while a page is being
`viewed several other pages can be captured and
`stored ready for instant access.
`This poses two
`questions; which other pages should the decoder
`capture and how does it know when it’s caught
`them?
`
`2.1. Linked Pages
`
`One way of deciding on the choice of pages
`is to give the viewer the facility to preprogramme
`a popular selection into the decoder, preferably
`with non-volatile storage associated with each tele-
`vision channel.
`Then, when the set is switch on
`and a channel is selected, the chosen set of pages is
`captured at
`the earliest opportunity, ready for
`viewing. A sequence of pages could be stored so
`that while any one page is being read, the next
`one or two are being captured.
`This approach
`can be coupled with a quick-select facility so that
`pre-programmed pages, which normally require
`a three- or seven-digit number to select them, can
`be accessed using a single keystroke as‘in a reper-
`tory telephone dialler.
`
`This approach, however, depends on the tele-
`text magazine of a particular television channel
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`IEEE Transactions on Consumer Electronics, Voi. CE~26. August I980
`
`From time to time in
`having a regular structure.
`the UK there is pressure to, for example, always
`put
`the Newsflash on page 250 and the initial
`index on page 100.
`Such a practice could, how-
`ever, place an increasingly severe constraint on
`the editor and sacrifice one of the great strengths
`of teletext, which is its flexible structure that
`can be changed instantly to accommodate both
`the expected and the unexpected.
`A much
`more adaptable technique invokes linked pages
`using the page service row (Annex B) to provide
`details of up to six pages related to any particular
`page, and the television service data-line (Annex
`C)
`to indicate the initial
`teletext page of that
`television channel.
`This allows the teletext
`
`editor to Structure the pages into decision trees
`in such a way that, while any page is being read,
`up to six related pages are being captured. When
`a choice is made the new page is immediately
`
`Fig. 1 - A Ceefax page with linking
`infannation added
`
`available by an abbreviated code of one or two
`keystrokes.
`Fig. 1 shows a Ceefax news headlines
`page with information added to show how linked
`pages could be indicated.
`Fig. 2 shows how the
`operation #4 leads to a new page and a further
`selection. There is a variety of ways in which the
`structure could be organised, for example, along
`string of pages could include provision for jumps
`to the next, the previous, the first or the last page
`of the string.
`
`2.2. Page Check Word
`
`Particularly when a multiple page store is
`used, a definite indication is required to signify
`that any partiCular page has been correctly and
`completely received.
`This operation should
`not require human intervention and judgement, as
`future uses of teletext systems may involve the
`dumping of several hundred pages at a time into
`a mass store.
`In Annex B there is a proposal that
`an optional service row associated with each page
`should include a cyclic redundancy check (CRC)
`word of the entire page content,
`including pre-
`sumed characters occupying any rows that were
`not
`transmitted.
`A 16-bit check word would
`identify 99.99896 of all possible error patterns, and
`an even higher proportion of all
`likely error
`patterns.
`In an automatic page capture routine
`this test would allow a correct page to be protected
`in the store and the routine could continue with
`the next
`task.
`When a page is being viewed
`directly the viewer could be given an indication
`whether the page had a check word and whether
`the check was successful, a successful check would
`indicate a high confidence that the displayed page
`is correct and complete.
`
`If the page check fails the only possibility
`for error correction in the basic teletext system
`depends on the cyclic repetition of pages.
`A
`method of using an auxiliary ‘flag’ bit associated
`with each byte of the page store to allow such
`error correction has been described elsewhere3
`together with examples of its use.
`
`cept of a ‘fixed-format’ where every address in the
`
`3. Character repertoire
`
`Table A3a in the annexed document shows the
`
`basic display character set of the UK teletext
`system, and Table A421 lists the differences be—
`tween this set and two other national-variant
`character sets already in use with the UK teletext
`system elsewhere in Europe.
`
`The UK teletext system is based on the con—
`
`Fig. 2 - A Ceefax page linked to that in Fig. 1
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`Chambers: Enhanced UK Teletext Moves Towards Still Pictures
`
`529
`
`header be used for this purpose. There is no need
`to reduce the overall character repertoire or to
`provide different character generators because of
`these variants; all that is required is a permutation
`of some of the character codes under the control
`of these header control bits, as indicated in Table
`A4.
`
`3.2. Implications of Use of Eight-Bit Codes
`
`The use of the full eight bits of every trans-
`mitted character byte, which is presumed in the
`above extended character repertoire, means that
`many transmitted bytes will not satisfy an odd-
`parity check. At present all UK Teletext decoders
`use this check to prevent the writing of character
`bytes with 1, 3, 5 or 7 errors into storage. This
`prevents the appearance of wrong characrers (other
`than the spaces which normally appear in place of
`characters not yet written) on the display when
`the received data is subject to isolated single bit
`errors. A decoder responding to eight-bit codes
`will,
`in general, display a wrong character when-
`ever the received byte is in error.
`
`It can be argued that the use of the page
`check word, together with possible use of the ‘flag
`bit’ technique,
`is sufficient to counter errors in
`teletext;
`in the vast majority of homes in the UK
`a properly functioning teletext installation Suffers
`no errors at all on most days.
`If it is desired
`to retain the ‘parity’ type of error detection rows
`26 or 27 of a page (see Annex B) can be used“
`to send, by a direct addressing technique,
`the
`locations of the few even-parity sites within a page.
`
`Most UK Teletext decoders use the transitions
`of the data waveform for recovering the bit-rate
`clock, so it is prudent to restrict the use of the two
`bytes which could result in long periods without
`transitions (see A.1.2).
`In the code table these
`bytes have been assigned characters which have
`restricted use.
`
`3.3. Further Extension of the Character Repertoire
`
`Fig. 3 - An experimental extended character set
`
`page display store corresponds to a given character
`site within the page, and every stored code is either
`a display character or a control character.
`The
`present system uses seven-bit codes with 96 display
`characters and 32 control characters.
`In the
`annexed proposal it is assumed that eight-bit codes
`will be used in the page store, still with 32 control
`characters, but now with 224 display characters.
`This total
`is conveniently similar to the require-
`ment
`for
`a
`common-core
`alphabet
`for
`the
`languages using the Latin alphabet, and allows
`languages to be mixed within a page.
`Fig. 3,
`which corresponds to the display characters listed
`in Table A3, shows the extended code table
`currently used in experiments.
`Two particular
`characters require comment, code Elll is a super-
`script
`1 which can be used, together with super-
`script 2 and 3, to introduce footnotes or linked
`pages, and code EO/S is allocated to the European
`Currency Unit
`(ECU)
`for which a temporary
`symbol has been devised.
`
`The use of an eight-bit store also allows the
`size of the mosaic (formerly known as graphic)
`character repertoire to be more than doubled and
`an indication of the characters that could be used
`
`It has already been recog-
`is given in Table A3b.
`nised that
`the 48 diagonal mosaic patterns add
`greatly to the capabilities of the system.
`
`3.1. Existing National Variants
`
`If the existing national variants of the basic
`character sets, as listed in Table A4a, are to con-
`tinue in use indefinitely a future decoder operating
`with an enhanced UK teletext system should be
`able to recognise which variant
`is in use and
`respond accordingly.
`It is proposed in A.2.3.1
`that the three unused control bits in the page
`
`Eindhoven.
`
`Although a set of 96 characters is enough for
`many applications, and a common-core alphabet of
`some 220 characters is likely to meet the everyday
`needs of most broadcasters using languages written
`in a Latin alphabet, applications of broadcast tele-
`text are envisaged where an even fuller repertoire
`is needed.
`This presents a problem when several
`million decoders are already in service and con-
`
`'
`
`A technique proposed by Mr. W.J. Christie of N.V. Philips'.
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`sideration is given to adding more characters to the
`list, as it is impracticable to modify these decoders.
`
`IEEE Transactions on Consumer Electronics, Vol. CE-26. August 1980
`
`is under active discussion.
`
`The intention is to
`
`allow most, if not all, of the modes to be changed
`between characters so that, for example, every
`letter of a word may be in a different colour on a
`different background colour.
`Clearly additional
`storage is required to handle these commands and
`in the extreme case the page display store for a
`parallel attribute system would need to be three
`times the size of a basic teletext page store.
`
`If there is a need to accommodate parallel
`attributes they too can be provided in an enhanced
`UK teletext system by using the page-associated
`data channels to convey directly-addressed control
`words to modify the appropriate parts of the
`extended page display store. Such a system forms
`part of the ‘polyglot C’ proposal.
`
`A more complete Latin alphabet character
`repertoire,
`together with a coding scheme,
`is the
`subject of an international standard in course of
`preparation.4 If UK Teletext is required to handle
`this
`fuller
`repertoire,
`and its possible further
`extensions to other alphabets (Arabic, Cyrillic,
`Greek, Hebrew, etc), as part of a more general
`text
`transmission system,
`this could be done
`strictly in accordance with the ISO coding scheme
`and using the independent data channels available
`in teletext (see C.1).
`If these characters are to
`be used within conventional teletext pages it would
`be essential
`to have a ‘fall-back’ presentation on
`decoders not equipped with the increased and
`increasing repertoire.
`This is easily achieved
`using the concept of over—writing using directly
`addressed bytes carried by page-associated data
`channels (Annex B).
`A normal page is sent in
`the conventional
`fixed-format
`transmission, and
`this serves as the fall-back presentation on the
`normal decoder. A decoder capable of responding
`to the new characters will recognise their codes
`in the page-associated data channel and substitute
`them in place of the characters transmitted in the
`normal way.
`A detailed proposal for such a
`scheme, for use with the ISO draft standard for
`the Latin alphabet, and with Hamming protection
`for the address information, has been described
`by Philips’ and is known as ‘polyglot C’.2
`This
`technique can be used equally well with a 96—
`character basic-repertoire or with a 220-character
`basic repertoire, although with the larger basic
`repertoire there are fewer occasions when the
`auxiliary rows would be needed in the page trans—
`m1ss1on.
`
`resolution of the teletext mosaic mode (Fig. 4)
`
`5. Data broadcasting
`
`Data not intended for direct display of text on
`the screen may be coded in either of two ways,
`chosen according to the application, and in both
`cases the data transmissions will not interfere with
`
`the normal operation of teletext decoders designed
`to receive only pages of text.
`
`5.1. Pseudo~Pages
`
`When data is conveniently organised as blocks
`of up to 1K byte it can be treated as a teletext
`page which cannot be accessed by a normal
`decoder.
`As well as the 21/2 million addresses
`
`available for normal pages there are more than 10
`million additional addresses available for
`these
`
`Such data blocks
`‘pseudo-pages’ (see A.2.1.3.1).
`can be linked together using the method described
`in 2.1 so that long sequences of data can be sent as
`packets, each being tested for integrity by means
`of the page check word.
`The concept of linked
`pages allows pseudo-pages
`to be coupled with
`normal pages of text which can then be used to
`introduce and comment upon the contents of the
`pseudo-pages.
`
`Many uses can be found for such a data
`broadcasting system closely coupled to the tele-
`text pages, and perhaps the most attractive is its
`use for sending pictures more intricate than pages
`of text. A teletext page requires about a kilobyte
`of memory, a high-quality still television picture
`requires about a megabyte.
`Between these
`extremes there is a large variety of uses, and the
`challenge is to find an efficient hierarchy of coding
`which will allow all
`intermediate cases
`to be
`
`handled effectively.
`
`For example,
`
`the normal
`
`4. Display modes
`
`The display modes of UK teletext are known
`as ‘serial’ modes.
`Under the control of codes
`
`which occupy positions on the displayed page,
`these modes can be set and changed. The full list
`of modes and control codes is given in Table A2,
`and their action is detailed in A.3.1.
`In this table
`a new mode has been defined in addition to those
`
`this is the
`given in the current specification;5
`‘underline' mode which shares the same control
`
`codes as the separated mosaic mode. Another new
`feature is
`the provision for variable character
`spacing to improve the appearance of text, and a
`control code has been provided to allow tabulation
`to be preserved (see A3). _
`
`The provision of ‘parallel’ modes for teletext
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`IEEE Transactions on Consumer Electronics. Vol. CE-26. August 1980
`
`System using Direct Coding’, which is reproduced
`here as Annexes A, B and C, was prepared in
`January 1980. The structure of the main part of
`this document (Annex A) follows exactly that of
`the September 1976 Broadcast Teletext Specifi-
`cation.
`
`7. Conclusion
`
`This paper has shown how UK Teletext can be
`used as a basis for a hierarchy'of future develop-
`ments embracing all
`the anticipated features of
`other systems and leading to a capability to trans-
`mit colour pictures whose quality is limited only
`by the display device itself.
`Throughout such a
`path of development the efficiency and ruggedness
`of the basic UK Teletext system can be pre—
`served.
`
`8. Acknowledgement
`
`The author wishes to thank the Director of
`
`Engineering of the British Broadcasting Corpor—
`ation for permission to publish this paper.
`
`9. References
`
`1.
`
`European Broadcasting Union, Brussels, 1980.
`Technical Document No. 3232. Displayable
`character sets for broadcast teletext.
`
`CROWTHER, G.O. Adaptation of UK Tele-
`text System for 525/60 operation.
`IEEE
`Chicago Spring conference, 1980.
`
`Teletext alphabets and
`CHAMBERS, ].P.
`EBU Review, Technical,
`error protection.
`No. 173, February 1979.
`
`ISO document TC97/SC2/WG4/N222. Sixth
`
`working draft for an international standard.
`Part 2:
`Latin alphabets and non—alphabetic
`graphic
`characters.
`Edited
`by L].
`Zeckendorf, April 1980.
`
`Broadcast Teletext Specification BBC/IBA/
`
`data line does not interfere with the operation of
`normal teletext decoders.
`
`5.4. Framing Code
`
`the distinction
`It has been suggested that
`between different uses of a data broadcasting
`system can be made by using different framing
`codes.
`The methods described above all use
`the normal data-line preamble (clock run-in and
`framing code)
`and
`rely
`on the
`subsequent
`Hamming-coded
`bytes
`for distinguishing the
`applications.
`Only when byte synchronisation
`is properly achieved can data lines be reliably
`accepted or rejected; when more than one fram—
`ing code is in use the possibilities of obtaining
`false
`byte
`synchronisation
`are
`significantly
`increased.
`
`6. Background to annexed draft specification
`
`BREMA, September 1976.
`
`During the discussions in the meeting of the
`EBU Sub-group V2
`(Data Broadcasting)
`two
`themes became apparent. First, there was a general
`desire that
`the different systems already in use
`and about to be used should be developed in the
`future in such a way that they converge in their
`capabilities, even though the particular strengths
`and weaknesses of the basic systems would remain.
`This convergence of capabilities ‘should as far as
`possible also apply to the viewdata (Videotex)
`systems.
`Secondly, there was a strong wish from
`the many countries already using UK Teletext in
`a more or less experimental way that a unique
`path of development of UK Teletext should be
`specified.
`Ideally,
`their urgent needs, notably
`the common-core character set, would be accom—
`modated immediately and the means by which
`other material,
`such as data not
`intended for
`direct display as pages of text and data for pro-
`gramme labelling,
`could be carried would be
`defined. The most important point was that any
`development of UK Teletext should not render
`existing decoders obsolete;
`the broadcaster should
`be able to continue to transmit current pages in
`the same way.
`This compatibility should still
`apply as enhancements are added.
`It is against this
`background that a working document GT V2 169,
`‘Draft Specification of an Enhanced Teletext
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`Chambers: Enhanced UK Telctcxt Moves Towards Still Pictures
`
`ANNEX A
`
`A DRAFT AMENDED UK TELETEXT SPECIFICATION
`
`SUMMARY
`
`the features of the existing UK
`The enhanced teletext system includes all
`(1)
`teletext system for transmitting pages oftext and elementary pictorial information in coded
`form on otherwise unused television lines during the field-blanking interval. Moreover, it
`provides an extended alphabet adequate for text in the majority oflanguages using the Latin
`alphabet, together with the possibility ofusing other alphabets.
`
`any redundancy and retaining the maximum compatibility with the basic system.
`
`This draft specification defines an enhanced teletext system for broadcasting
`information accompanying, but not necessarily related to, a television signal.
`The main
`technical features of the system are.-
`
`It is
`The system uses binary signalling at 6.93 75 Mbit/s during each data-line.
`(2)
`directly applicable to terrestrial broadcasting using 625/50 television systems with video
`bandwidths 5.0 MHz or greater.
`Using two data-lines per field the system allows at least
`four pages to be transmitted every second.
`
`Up to eight independent teletext magazines may be multiplexed onto a single
`(3)
`television signal.
`Each magazine can contain up to 320 000 different teletext pages
`addressable individually or in groups or sequences by the user. Overa million addresses in
`each magazine are available for sending blocks of data each up to one Kilobyte in size, and
`not necessarily related to teletext pages, as pseudo—pages.
`
`The address and control information is well protected by a rigid format and the
`(4)
`use of Hamming codes.
`This substantially eliminates the possibility of all or part ofan
`unwanted page being mistaken for wanted data.
`A check for complete and correct recep-
`tion of pages is provided. This can be used to freeze the correct page in storage, and to start
`the next operation in an automatic data capture process.
`
`Each teletext page can carry the address of up to five subsequent and one
`(5)
`previous related page.
`This allows a page structure based on a decision tree to be followed
`with minimum instructions from the user.
`In decoders equipped with multi-page memories
`this feature can be used to give apparently immediate access to the next wanted page.
`
`A facility for underlining text, and an extended character set to allow improved
`(6)
`presentation ofpictorial material, are provided.
`
`This can be used to send infor-
`The system provides a television data-line.
`(7)
`mation about the accompanying television programme for display on the screen and for
`automatic control purposes.
`It can contain the address of an initial teletext page to be
`selected automatically by a teletext decoder.
`It also can carry a statement ofcurrent time
`with supplementary information to allow the day and date to be displayed.
`
`The system offers 15 auxiliary data channels, independent of teletext and of
`(8)
`They may be subdivided at will, and used for any purpose, teletext acting
`each other.
`solely as the carrier.
`
`The system can be extended to provide for the transmission ofa sequence of
`(9)
`still colour television frames whose quality is limited only by the display device.
`Any
`intermediate requirement between this and teletext pages can be coded efficiently to exploit
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`IEEE Transacfions on Consumer Electronics, Vol. CE-Zé, August 1980
`
`INTRODUCTION
`
`This document defines an enhanced teletext system compatible
`with, and using the direct-coding principles of, the UK system. Much of the
`detailed information is carried by figures and tables. The description is in
`four sections.
`
`The first section describes how binary code groups are formed into
`Data-Lines for inclusion in the television field-blanking interval.
`
`The second section describes how the control and address infor-
`
`together with the special Page-Header
`mation carried on each Data-Line,
`Data-Lines and the sequence of transmission of the Data—Lines, allow the
`Data-Lines corresponding to the Rows of a selected Page to be identified.
`The Page structure can be used to send information other than Pages of text.
`Certain Data-Lines can be used for functions unrelated to Teletext.
`
`NRZ data waveform and that of a phase-corrected
`
`The third section describes how the Character Codes received on
`
`the Data-Lines corresponding to the Rows of the selected Page are inter-
`preted to give a Page display.
`
`The fourth section defines Teletext terms.
`
`A. 1. Television Data-Lines
`
`The television signal includes unused lines in
`the field-blanking interval to allow time for field
`flyback in receivers before each active field begins.
`The duration of this interval is usually 25 lines, and
`some of the later lines are used by broadcasters for
`test and signalling purposes.
`
`The basic data amplitude may vary from
`Data-Line to Data—Line.
`
`A.1.1.2. Bit Rate
`
`signalling rate
`binary element
`The
`6.9375 Mbit/s (:25 parts per million).
`
`is
`
`It
`
`is 444 times the nominal
`
`television line
`
`This system can use up to 16 of these unused
`lines as Data-Lines (see Fig. A1).
`Initially lines
`17(330) and 18(331) are being used but other lines
`may be used.
`
`frequency.
`
`A.1.1.3. Data Timing
`
`A line in the field-blanking interval is identi-
`fied as a Teletext Data-Line by the presence of the
`Clock Run—In (see A.1.2.1) followed by the Fram—
`ing Code (see A.1.2.2) at an appropriate time.
`
`A.1.1. Data- Line Waveform
`
`Each Data-Line contains binary elements
`(bits) as a two-level NRZ (Non-Retum-to-Zero)
`signal, suitably shaped by a filter.
`
`A.1.1.1. Data Levels
`
`The data timing reference point is the peak
`of the penultimate ‘1 ’ of the Clock Run—In se-
`quence (see Fig. A3). This point has been selected
`to reduce the effect of any transient distortions at
`the start of the Data—Line.
`
`The line time reference is the half-amplitude
`point of the leading edge of the line synchronising
`pulse.
`
`The data timing reference in the signal as
`transmitted shall be 12.0(+O.4/—1.0)/.rs after the
`line time reference.
`
`The binary signalling levels are defined in a
`scale where television black level is 0% and white
`
`The data timing may vary from Data-Line to
`Data-Line.
`
`level 100% (see Fig. A2). The binary ‘0’ level is
`then 0(i2)% and the binary ‘1’ level
`is 66(i6)%.
`The difference between these levels is the basic
`
`data amplitude. The data waveform will contain
`overshoots so the peak-to-peak data amplitude will
`exceed the basic data amplitude.
`
`A.1.1.4. Data Pulse Shape
`
`The spectrum of the generated data pulses,
`which is the produce of the spectrum of the basic
`
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`Chambers: Enhanced UK Teletext Moves Towards Still Pictures
`
`To mini-
`is indicated in Fig. A4.
`shaping filter,
`mise intersymbol interference the spectrum is sub-
`stantially skew-symmetrical about
`a frequency
`corresponding to one-half of the bit rate. There is
`minimal energy above 5.0 MHz.
`
`The corresponding one-bit pulse is indicated
`in Fig. A5.
`
`A.1.2.3. Hamming Codes
`
`The fourth and fifth Byte of every Data-
`Line, and a further eight Bytes of the Page-Header
`Data‘Lines, are Hamming Codes containing four
`‘message’ bits interleaved with four ‘protection’
`bits dependent on the message bits as listed in
`Table Ala. The bits are transmitted in numerical
`
`A.1.2. Data-Line Structure
`
`order from b1 to b8 .
`
`Each Data-Line comprises 360 bits which
`may be considered at 45 eight-bit Bytes.
`
`The codes used to send Pages avoid the bytes
`00000000 and 1111111 1. This ensures that there
`
`are never more than 14 bit periods between the
`data level
`transitions in the waveform.
`This
`
`simplifies the recovery of the bit—rate clock directly
`from the data waveform.
`
`All the address and Page control information
`transmitted using Hamming Code Bytes
`to
`is
`reduce the possibility of the wrong Data-Lines
`being stored in the receiver.
`
`A.1.2. 1. Clock Run-In
`
`two Bytes of every Data-Line
`The first
`comprise the Clock Run-In sequence of alternating
`bits, beginning 101010 .... ..,
`to indicate the pre-
`sence of a DataiLine and to establish the timing of
`the bits on that line (see Fig. A6).
`
`In some circumstances the first one or two
`
`binary ‘1’s may be absent.
`
`A.1.2.2. Framing Code
`
`The third Byte of every Data-Line com-
`prises the Framing Code 11100100.
`This code
`has been selected to enable Byte synchronisation
`to be established even if one bit of the Framing
`Code has been wrongly received.
`
`Fig. A9 indicates how incoming data are com-
`pared with the Framing Code pattern.
`It shows
`that a test for any seven corresponding bits will
`give a correct indication of the Framing Code in
`the presence of a single error.
`
`The first three Bytes, which have even parity,
`serve to synchronise the bit and Byte recovery
`operation in the receiver. The remaining 42 Bytes
`carry address and control information and, when
`forming part of a Teletext Page,
`the codes just
`sufficient for a complete Row of characters in the
`Page.
`
`30 and 31 relate to Independent Data-Lines (see
`
`Table Alb details four parity tests that can
`be made on the received Byte. Table Alc shows
`how the results of these tests can be used to
`
`correct single errors in the received Byte and detect
`double errors. When more than two bits of 3 Byte
`are in error
`this process may result
`in a false
`message being decoded.
`
`Fig. A6 shows the locations and lists the
`functions of
`all
`the Hamming ‘message’ bits.
`When error correction is used the decoded message
`bit may differ from the corresponding bit in the
`Data-Line as the bits of the Hamming Code Byte
`are interdependent.
`
`A.1.2.4. Character Bytes
`
`The remaining Bytes of each Data—Line are
`eight-bit Character Codes (see Table A3).
`
`The bits are transmitted in numerical order
`
`from b1 to b B .
`
`A.2. Organisation of Pages and Rows
`
`A.2.1. Addresses
`
`A.2.1.1. Magazine and Row Address Group
`
`Every Data—Line contains two Hamming
`Codes signifying a three-bit Magazine number and
`a five-bit Row address (see Fig. A6), or an Inde—
`pendent Data—Line.
`
`The Magazine number is in the range 1—8,
`Magazine 8 corresponding to the bits 000 and the
`others being directly the number obtained with the
`bit weights given in Fig. A6.
`
`The Row Address is in the range 0—29 and
`the number obtained with the message bit
`is
`weights given in Fig. A6.
`Data-Lines with Row
`address 0—23 are used for the 24 Rows of a Tele-
`
`The
`text Page, or as part of a Pseudo-Page.
`functions of Rows 24—29 are given in Annex B.
`The bit patterns representing Row Addresses
`
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`
`A.2.2. Transmission Sequence
`
`A.2.2.1. Pages
`
`The transmission of a selected Page begins
`with, and includes, its Page—Header and ends with,
`and excludes, the next Page-Header of the selected
`Magazine number.
`All
`the intermediate Data-
`Lines carrying Row addresses in the range 0—29,
`and the selected Magazine number, relate to the
`selected Page.
`
`Pages may be transmitted in any order.
`Occasionally incomplete Pages may be transmitted.
`Rows from Pages of different Magazine number
`may be interleaved in any way.
`
`A.2.2.2. Rows
`
`The Rows of a Page may be transmitted in
`any order.
`Rows,
`including the Page—Header,
`may be repeated in which case the latest infor—
`mation should take precedence. Rows containing
`no information for display need not be trans-
`mitted.
`
`A.2.2.3. Page Erasure Interval
`
`Rows will be transmitted such as to allow
`
`an active television field period between an initial
`Page-Header and further Rows sufficient to com-
`plete the transm