Price £1.00
`
`Broadcast Tel'etext
`
`Specjfication
`
`September 1976
`
`Publianed jointly by the
`
`BRITISH BROADCASTING CORPORATION
`
`INDEPENDENT BROADCASTING AUiHORITY
`
`BRITISH RADIO EQUIPMENT MANUFACTURERS ' ASSOCIATION
`
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`

`Contents
`
`Sll:iJUJWY
`
`••
`
`Introduction
`
`Television Da.ta-Linl:'l
`
`Organisation of PJges and Rows
`
`P<lge Display
`
`Defutition of Teletext Tenns ..
`
`Figures
`I.
`
`Teletext Data-lines ..
`
`2.
`
`3.
`
`4.
`5.
`6.
`
`Data. Levels
`Data Timing
`..
`An Approximate Spectrum of a Data Pulse
`An Approximate One-Bit Data Pulse
`Synchronisation and Hamming Codes at start of Page-Header md Row ttansmissions
`
`..
`
`7a.
`
`Page·Header Format ..
`
`7b. Row Fonnat
`
`..
`
`S.
`9.
`
`Examples of Alphanumerics and Graphks Dhplays
`
`Idealised operation of Framing Code
`
`•.
`
`Tables
`I a. Hamming Cod.e Bytes
`l b. Hamming Codes-tests for odd parity
`I c. Hamming Codes-decoding action
`2.
`Display Modes and Control Characters .•
`
`..
`
`3.
`
`Teletext Charact!r Cod~s ..
`
`3
`
`4
`
`4,5
`
`5,6
`
`6, 7,8
`
`8, 9, 10
`
`..
`
`II
`
`.. 12
`
`!2
`..
`.. 13
`
`.. 13
`.. 14
`
`.. 15
`
`15
`.. 16
`
`.. 17
`
`.. 18
`.• 18
`
`.. 18
`.. 19
`.. 20
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`FOREWORD
`
`This document describes the parameters of the Teleted signals transmitted in the
`It
`United Kingdom by the BBC and the Independent Television networks.
`supersedes earlier specifications. The BBC uses the name Ce.efax. and the
`Independent Television Companies the name Oracle for their public information
`services. The technical specifications of the Oracle and Ceefax signals are
`identical.
`
`Based on experience gained in the initial years of the transmissions, changes
`have been made since
`the first specification published
`in October 1974.
`Additional Control Characters have been allocated to provide facilities which can
`be U:'led to enhance the display of infonnation. These changes have been made in
`such a way that future transmissions remain compatible with Teletext decoders
`based on the later specification dated January !3th 1976 .
`
`Distortions, noise and spurious signals inevitably degrade the signal to a
`greatet or less extent. An important point to note is that an increase in
`magnitude of these effects will cause a gradual deterioration in analogue television
`while a digital signal, such as Teletext, can still be decoded until the disturbances
`exceed a critical level. Field studies have confirmed that in almost all cases this
`critical level for Teletext occurs when television reception is already poor.
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`INTRODUCTION
`
`This document defmez the Teletext broadcasting system. Much of the detailed information is carried by
`figures and tables. The desc ription is in four section.s.
`The first section describes how binary code groups are fonned into Data-Lines for inclusion in the tele·
`vision field-blanking interval .
`The second section desc ribes how the control and address infonnat iort carried on each Dat.a-!Jne,
`together with. the special Page-Header Data -Lines and the sequence of transmissi.on of the Data-Lines , allow
`the Data-Lines corresponding to the · Rows of a selected Page to be identified.
`The third section describes how the Character Codes received on the Data-Lines corresponding to the•
`Rows of the selected Page are interpreted to gi.ve the Page display.
`The fourth sec tion defines Teletext terms.
`
`L
`
`TELEVISJON DATA-LINES
`
`The television signal includes unuse d lines in the field(cid:173)
`blanking interval (see Figure I) to allow time for field
`flyback in receivers before each active field begins. The
`duration of this interval is usually 25 tines, and some of
`the later lines are used by broadcast·ers for test and
`signalling purposes.
`This system c:an use any of these unused lines as
`Data·Unes. Init iall y lines 17(330) and 18(331) are being
`used but other lines may be used.
`A line in the field-blanki ng interval 1S identified as a
`Teletext Data-Line by the presence of the Clock Run-In
`(see 1.2.1) fo ll owed by the Framing Code (see I .2.2) at an
`appropriate time .
`
`It
`is 444
`frequency ..
`
`times
`
`the nominal
`
`television
`
`line
`
`I. LJ
`Data Ti11Ung
`timing reference point is the peak of the
`The data
`'I' of the Clock Run-In sequence (see
`penultimate
`Figure 3). This point has been selected to reduce the
`effect of any transient d istortion ~ ar 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 tranSITiitted
`shall be 12 ·0 ( +0·4/- I ·O)#S after the line time reference.
`from Data-Line w
`The data
`liming may vary
`Data-Line .
`
`l.l
`Data-Line Waveform
`Each Data-Une contain~ binary elements (bits) as a
`(Non -Re turn -to-Zero) signal , suitably
`two-level NRZ
`shaped by a filter .
`
`1. 1 .l Data Levels
`The binary signalling levels are defined on a scaJe where
`television black level is 0% and white le vel 100% (~ee
`Figure 2). The binary '0" level is then O(;t2)% and the
`binary "I' level is 66(±6)%. The difference between these
`Levels is the basic data amplitude . The da ta waveform will
`conuin overshoots so the peak-to-peak da ta amplitude
`will exceed the basic data amplitude .
`The basic data amplitude may vary fmm Data-Line to
`Data-Line.
`
`1. L~ Bit Rate
`The binary element signalling f:}le is 6 ·9375 Mbit/ s (±25
`parts per million).
`
`4
`
`l.l .4 Dab PuJse Sha~
`The spectrum of the generated data pulses, which is the
`product of the spectrum of the basic NRZ data waveform
`and that of a phase-corrected shaping niter, is indicated
`in Figure 4 . To minimise 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
`Figure 5.
`
`is
`
`indicated
`
`in
`
`Data-Line Structure
`1.2
`Each D;~ta-Line comprises 360 bits which may be
`considered as 45 eight-bit Byles.
`The first three Bytes, which have even pa.rity, serve to
`synchro nise the bit and Byte recovery operation in the
`receiver. The remaining 42 Bytes have odd parity and
`carry address and control information, and the codes for
`a Cha.racter Row (5ee Figure 6).
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`Th~ use of odd p~rity during the variable part of the
`Data-line ensures th3t there are never more than 14 bit
`periods between the data level transitions in the wave(cid:173)
`form. nus simplifies the recovery of the bit-rate clock
`directly from the data w~vefarm.
`All the address and Page control information is trans(cid:173)
`mitted using Hamm ing Code Bv te.s 10
`reduce
`the
`po>sibility of the wrong Cha,acter Rows being stored in
`the receiver.
`
`Clock Run-In
`1.2.1
`lim two l:lytcs o f every DJtJ-Line compri~c t.he
`The
`Cock Ru n-In sequence of al tern ating bits, beg in ning
`101010 ..... 10 indicJ ie the presence o f a Data-Line Jnd
`to es1ahli sh
`the
`timing of the b1ts on that line (sc~
`Figure 6).
`In some ci r, umsr ance~ the first one or two binary
`'l's m3y be ahsent.
`
`1.2-2
`Framing Code
`The th ird Byte of every Data-Li ne comprises the Fram ing
`I 1 I 00 I 00.
`This code has been selec ted
`to
`Code
`enable Byte synchronisation to be established e\'crl if one
`bit of the Fram ing Code has been wro ngly rec eived ..
`Figure 9 indicates how in com ing data are compared
`with the Framing Code panern. It shows rha t a test for
`3ny seven corresponding bits will give :1 co rrect indic;Jtio n
`of the Fru111ing C'otle in the presence of a sin gle error.
`
`Hamming Codes
`1.2.3
`The fo urth and nfth Byte of e,·ery D~ta-Li ne. and ~
`fur ther eight By 1cs of the Page-Header DatJ-Li n·e~. are
`Hammin g Codes :.:ontJining fou r 'message' bits inte rleaved
`with four ' prore..: tion' bits de pe nden t on the me-ssage bits
`a.> lis1eJ
`in Ta bk
`I J.
`Th e hits are
`tr~ n sm 1ne d in
`nume ric'-11 order fr ·•m bt to hs.
`T:tblc I h detoi h four parity tests that ..:an be m~de on
`the received Byte . TJbie 1<: shows how 1hc result s of
`these IC ~t; on hL· used to co r rc~t single errors in the
`reccivc t! lly tc ~ml detect multiple error~ (w lte n :!. 4 o r 6
`bits a re m errnr). \\llten there are J. 5. 7 or 8 errors in the
`Byte
`this pro~ed u re result£ in a false me~sage being
`Jc ~ ndc d .
`Figure h sl1ows the locations :md lists the 'functions of
`all the Hamming 'messuge ' bits. 'Wl1en ·f.'rr or corre t uon is
`llll'Ssagc bi 1 may di ffcr ! rom
`used
`the decnded
`the
`tile D:lla ·Lil1c as the bits tl f the
`~orrespond i ng btl in
`l!~ n11ning (\Jde Byte arc intcrtle p-:m.lmt.
`
`Character Bytes
`I .2.4
`Th ~ r~ m a iu ing Bytes ol t~c h D:Ha-Lin~ are S£"tcn-bll
`Character Codes (see Tiiblc 3) with an ~dded odci-pari ty
`The bit s are
`trJnsmitted in numerical order
`hit b~ .
`from h, to be .
`
`2.
`
`ORGANISATION OF PAGES AND
`ROWS
`
`2.1
`
`Adl!resses
`
`2 .1 .I Magazine and Row Address Group
`Every Data-Line conlains two Hamming Codes signifying
`a three -bit Magazine num ber and a five-bit Row address
`(see Figure 6).
`The Magazine number is in the range 1·8 , Magazine 8
`corresponding to lh~ h11s 000 and the others bdng
`dtrectly the number obtained with the bit weights given in
`Figure 6.
`The Row address is normally in the ra nge 0-23 and it is
`directly the number obtained with the bit weights as given
`in Figure 6 . Row addresses in the range 24-31 may be
`transmitted but such Data·Lines must be ignored.
`
`2.1.2
`Page-Header
`Data-Lines with Row a.ddtess 0 are Plgc-Headers, wh ich
`contain eight addilional Hamming Code B;·ces with
`message bits relating to tha t Page, includ ing the two digits
`of the Page num ber and 1he fou,·d igit Time Code (;ee
`Ftgure 6). The dispby and control functions of the o ch~r
`message bits are det~i.led in ! .J below.
`
`Page Identification and Time Code
`2.1 .3
`Each P~ge is identified by ils single digit ~ Jga?inc num ner
`(1-8) and its two-digit PJgc number (00-99).
`Different Pages with the same Magazine an d Page
`numbers m:Jy be identifie d by invo king J f~>ur-digit Time
`Code whereby up lv 3:!00 versiom of that Page ma y be
`individuatty selected and hel d.
`The 'Huurs' and 't-linutes' of the Time Code are no t
`necessarily re la te d to • lock -time. The 'Hours Tens' ma y
`take any val u~ 0·.3 and th~ 'Minu te; Tens' m~y take any
`value0-7 .
`
`Page Seleclion
`2.1 .4
`A Page mJ y b~ sele.: ted by lis Mag a~i ne number an d Plge
`num be r. or by its Magaz.ine number. Page number and
`Time Code.
`Neither type of Page select ion should respond to Page
`number 'Un ns· or 'Tens' in the range l 0- [ 5. and select] on
`by Time Code should no! respond to ' Minu tes Uni t>' or
`'Hou rs Units' in that range. which may be used for orh~r
`purposes.
`
`2.2
`
`Transmission Sequence
`
`Pages
`2.2.1
`The transrni3sion of ~ selecled Page begins with , and
`includes. its Page -Header and ends with. and exdudes, lhe
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`next Page-Head~r of the se.lected Magazine number . AJJ
`intermediate Data-Uncs
`carrying
`the
`the
`selec ted
`Magazi ne number relate to the selected Page ,
`Pages rnay be transmilled in any order. OccJ~i.on ally
`incomplete Pages rnay be transmit ted . Rows from Pa.ges
`of differ•enl Magazine number may be
`interleaved in
`time.
`
`;!.2.2
`Row~
`The Rows of a Page may be ! r~nsm i r ted in any order.
`Rows, including
`the Page·Heade r. may be · repeated in
`which case the la~es t apparent!~· error- free in fo rm ation
`s.hould take precedence. Rows containing no informJtion
`for display need not be HJnsmiued.
`
`on a Page designated as a 'Newsflash' Page. whether
`All
`or not
`i1 currently contains
`information,
`information intended fo r displ~y on su~h a Plge will
`be Boxed (see 3 .. 1.5).
`
`C6 -Subtitle Indica tor. This Control Bit is set to 'I' on a
`Page designated as a 'Subtitle ' Page, whether or not it
`currently con tains
`informa tion . AJI
`inform ation
`intended for display on sud1 a Page will be Boxed
`(see 3 .1.5).
`
`c, - Suppress Header. This Control Bit is se t to T when
`the Page ]s better dis played wi thou t the characters of
`the Page-Header.
`
`Page Erasure Interval
`2.2.3
`Rows wiU be transm itied such as to allow ~1 ac tive tele (cid:173)
`vision field period betwe en an ini tial Page -HeJder and
`further Rows sufficient to complete the tra nsmission fo r
`that Page.
`This allows one displ~y pe riod for the rece iver Page
`store to be erased when necessa.ry.
`
`C8 - Update In dicator This Control Bit may be set to 'l'
`when part or all of a Page comains later informa tion
`thiln that in the previous transmission of the Page
`bearing the same Magazine and Page number. The
`'Up date' Page
`transmission may be
`incomplete,
`..: onta.ining only the upda ted Rows of a Page (see
`:U.l).
`
`2.3
`
`Page·Header Struct ure
`
`The Page -Header D.:Jf.l·Lines (see 2. 1 .2) corllain eigh t
`Hamming Code Bytes in pbce or the fir$t eight Character
`Bytes of the other Data-Lines (see Ftgures 6 and 7).
`There are thus on ly J1 Ch a r~cter Codes in a Page·
`Header . They arc used to present general informa ti o n for
`display, such as the Magazine an d Page num ber, the day
`and da te and the programme source .
`In particular, the
`last eigh t charac ter> are reserve d fo r the display of clock.
`rime . Examples of the con ten·t of a Page.Header are g.ive n
`in Figure 7a.
`The locati ons of the 32 add r~ss and ~ontrol message
`bits of !he eight Ham mi ng Codes peculiar to a Page(cid:173)
`Header are shown in Figure 6 . This also shows the binary
`weigh~ of the e.ight Page number. and 13 Tirrie Code, bits
`whose functions are described in 2.! . The remaining 11
`bits are Control Bits numbe re d C4 to C ,. whose fu nctions
`are described below.
`
`Control Bits (see Figure 6)
`2.3.1
`C.4 -Erase Page. This Control B i~ is se t to ' I' when the
`information on that Page is signifkamly different
`from that in the previous transmission of th e Page
`beating the sam~ Magazine and Page nt~mber, such
`that the two should no t be c.onfusr:d.
`Its use will al ways be foll owed by a Page erasure
`interval (see 2.2 .3).
`
`C9 - In ter rupted Sequence . This Con trol Bit is set to '1 '
`:when a Page is being
`transmitted out of st ric t
`numerical sequ ence in order to give i! priority (such
`as
`J Subtitle Page) or more frequent tmnsmission
`(~uch as an Index Page).
`It allows the Page-Header to be suppressed wnen
`to avoid di.'l·
`Rolling Heade rs
`are displayed,
`con t.in uities in th e di>playcd Page numbers.
`
`C 10 - lnhibir Di splay. This Con trol Bit is set to '1' whet~
`ihe contents of a Page cannot usefully be in terprete d
`as a Telete xt transmission. II can be used 10 inhibit
`the display -o f meaningless Pages.
`
`C 11 - Magaz.ine Serial. TI!.is Control Bit is set to 'J' when
`the transmission seque nce of Magazines and Pages is
`such that it is preferable to dispLay all Page-Headers as
`Rolling Headers rat her than only those of the selected
`Magazine.
`
`It is desirable thai these unlilocated
`C n} Unallocated.
`C13 Control Bits be accessible in decoders for future use
`C 14 . as d ispl ~y control functions.
`·
`
`3.
`
`PAGE DISPLAY
`
`The 24 Rows of a Pa ge are numberea sequentially from
`C5 - Newsflash Indicator. This Controt Bit is set to 'I' 0 (Page .Header. top :Row) to 23 . The 40 Charac ter
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`the 40
`to
`Rectangles of a Row are directly related
`Character Bytes of the correspond ing Data-Line< each
`being assumed to be sunned sequentially from left to
`right (see Figure 7).
`Every Character Byte contains a Character Code which
`represents e·ither a Display Charlcter or a Con1roJ
`Character. The Control Characters are used to establish
`Di.splay Modes. which may be changed between Character
`Rectangles wi!hin a Row.
`The Display Modes dete nnine how a Display Character
`Colle is interpreted as a character to be gener:ned in the
`corresponding Character Rectangle of the Page disp lay.
`The Charac ter Rectangles correspond ing
`!O Control
`Charac te r C de~ are gene rally displayed as Spaces (bu t ~ee
`J. L7).
`
`3.1
`
`Display Modes
`
`The Display Modes are listed in Tabl~ 2 as complementary
`p airs~ those on the left are j_,sumed ~ ~ the ;ta rt ot' ~~cry
`Row. The Control Character Codes lisred ~~o·ith eac h rnode
`are used to initiate that mode.
`Some Control Ch:arlcte rs have immedi ate effect ('set
`that the new mode obtains fo r and from the
`at') in
`corresponding Characte'r Rec!angle,
`others
`nav,;
`subsequent dfect ('set aft er') when ttl ~ new rnode obtains
`fo r and fro m the next C h~ racter R~ctan g l e. A later
`Control Character takes precedence over an earHer one.
`When a Control Character signifies a change to a mode
`a.Jready obtaining,
`that mode gener:tlly conttnues
`uninterrupted
`throughout th e corre~ponding Ch aracter
`Rcc; tJny,]e (hut .>ee } .I . .ll .
`ln general the Char~d er CoJes of a Row are suft 1t.:iem
`to define the entire display of th~t Ro'" (bu t see 3 .1 .6 ).
`The in terpretation of the Display Modes is detailed
`below_
`
`3.LI
`Character Se f
`Three ""crJapplng sets of 96 Display Characters are
`available
`for
`the
`in terpret~ tion of
`the 96 Displ~y
`Charac te r Codes.
`During the Alphanumerics .Mode the A.lphanum~rics
`Set applies. During the Graphics Mode the Con uguous
`Graphics Set applies during the Corniguous Mode and the
`Separated Grapl1ics Set applies du ring
`the Sepa rated
`Mode .
`
`Display Colour
`3.1.2
`One of the ~ven ~oio urs white. yellow, cyan, green,
`magen tJ, red. blue is used to depict the Display Character
`in the Character Rectangle.
`S~ven p<iirs of Con~roi
`Characters are available so that the Display Colour and/or
`the Aiphanumerics/Graph~cs Mode may be changed by a
`
`single Control CharJ~ter. TI1ere is a d irect correspondence
`between bits b t, b2 , ll 1 ot· these codes and the red. green
`and blue components of the ~olours.
`
`lbckground Colour
`3 .I .3
`The Background Colour of the Character Rectangles is
`biack during th~ Black Buckground mode.
`\Vhenever the
`new backgwund Control Character l/ 13 (see Table J)
`occurs the Olsplay Colour then obtaining is adopted ~s
`the Background Colo ur in the new back~round mode.
`
`Conceal an9 Flash
`3.1 .4
`Two modes are provided wherein all
`!he Display
`Chara..:ters are displayed ~s Spaces at c~rtain times . All
`ch:~racte rs in
`the Con<:eJJ mode are
`intended
`to be
`displayed as Sp~~es u n n l the Reveal rnoJe is res til red a fret
`the receiver ur hy user control. All
`~ time deJay
`in
`characters in the Ftas~1 mode are intended to be displayed
`al ternatelr a~ they wmild otherwise be displayed. and as
`Spaces. under
`th,· c(>ntroJ of l
`timing d~v ice in
`the
`receiver.
`
`Boxing
`3.1.5
`All ch a.racters inten ded fo r displily on News tla sh and
`Subtitle Pages will be in the Boxed Mode. which detin~s
`the part of t h~ Page wh ich is l o \:le
`tn ~c-1 into the normal
`televisio n pic tu re. This in~et ope r;uion may be controlled
`~utomatie3fty by tn~ C"nn! ml Bi ts r, or C, (see 2.3 .1).
`Some m Jtl or lhe ,·flara<:rers Orl ~rl\' <)ther p~ ~es may
`be 6m ed. the BoxeJ Mode tht:J1 de tl nes a part of the Page
`in5et imo the norm ~] television picture
`whi.:h may be
`under user co ntr0l as Jl! altecna1 ive to the disp[ay of th~
`.:omptete P~ge llnnc. nr superimpo~ec:! on the picwre.
`In order ~" !;(iVc pn•t~.:liun a~opinst spurious Boxmg,
`two co n~cc u t[vc St ;m H"x C<>ntr() l Character> 0./1 1 l>~e
`Table 3) will be transmitted to start the Boxed Mode and
`two consecutiv'e Er~d Box Control Characters 0/ l 0 will be
`transmine d 10 term i n~te thai mode. The n10de changes
`occur between the co rresponding ~orHCCIItLve Character
`Re~ tangles
`
`3.1.6
`Double Height
`Wheneve r the Douhle Height mode occurs the [nformation
`in th~t Row is sufficient to define the disp~ay of both that
`Row and the Row of next higher address . A receiver
`responding to th~ DO\Ib]e Height mode on a Row must
`ign o re any information received for th.e R.ow or nex t
`higher address. but a receh·er not responding to this mode
`will operate normally on bmh Row~.
`A receiver re.sponding to one or mo re oocurrence5 o f
`the Dou ble Height mode in Row 'R" will opna~e as other·
`wise during that Row except that in every Character
`Re ctangle during the Double Height mode only the upper
`half of whai would otherwise have been displayed is ·
`displayed, st re tched vertically to fill the rectangle.. On
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`'R-r l' the corresponding lower half of each of Boxed Mode-The Display Mode in which, under the
`Row
`is similarly displayed. The remaining Character
`user's control, the characters are intended to be inset
`these
`Ret:tan~le~ of Row ' R t I' are to be d i~played as unhuxed
`or adlinl to :r television picture. When a Nl'w3tlash "'
`the .. Subt i tle~ is tramrrritted this operation may be autmnatic
`in
`the
`same Backgroulll.l Colour
`as
`~ilao.:es
`corresponding Character Rectangles of Row 'R'.
`un der the control of Control Bits.
`
`3.1.7
`Hold Graphics
`Generally all Control Characters lre displayed as Spaces,
`implying at least one Space betw een rectangles with
`different Display Colours in the same Row. The Hold
`Graphics mude allo ws a limited range of lb rupt Display
`Colour changes by calling for •he displa]· of a Held
`Graphics Character in the rectangle correspond ing to any
`Co ntrol Ch aracter occurring during tile Graphics Mode_
`This Held Charac ter is dispia)' ed in the modes obtaining
`fo r the rectangle in which it is displared . except for the
`Contiguo us/ Separated Mode which forms part of the
`structure of the Held Cr~phics Character.
`The Hel d Graprrics Char~ctcr 1s only defined during the
`Graphics Mode . It is then the m051 recent ch aracter with
`b ~ =I tn its character ~ode. p ro~i ding that th~re has been
`the Alphanumerics/
`no
`Inte rv ening chlng.e
`in e ither
`Graphics or th e )':ormai/Double Heigh t modes. This
`is
`to be displaye d
`character
`in
`the Contiguous or
`Separated Mode as when it was first displayed.
`In the
`absence of such a characte r the Held Gra phics Characte r
`is taken to be a Sp~ce.
`
`4.
`
`DEFINITIQ. OF TELETEXT TERMS
`
`Access Time- The time be twee n sdec ting a Page at a
`receiver and the fi rst complete re cep tion of th:ll Page.
`
`the 96 Displ ay
`Alphanumerics Character-One o f
`Charact ers li~ted in columns 2, 3 . 4 ., 5. 6 and 7 of Table 3 .
`The shapes of til t characters are no1 de-fined but the y
`shou ld all be diffe ren l and rccogn isable.
`
`Broadcast Teletext - The information broadcasting sySt·em
`defined in this doc ument.
`
`Byte-A group of eight consecutive data bits intended to
`be lr·eated as an entity .
`
`Character Byte- The Byte obtained by appending an odd·
`parity bit to a Chancier Code.
`
`Chara<:ter Code- A seven-bit binary numbe r represen ting
`one of a set of Display Characters. or a Control
`Charac ter.
`
`Char.~cter Rectangle-One of lhe 960 units in the regular
`ma trix of 24 Row~ of 40 sites in which characters are
`generate d in the display of a Page.
`
`Character Row-see Row.
`
`Clock Run-In-A sequence of alternating bits at the start
`of a Data·-Une
`to allow a
`receiver
`to achieve bit
`synchronisation.
`
`Conceal-A Display Mode during which all charac ters,
`the receiver, a.re
`intended to be
`although sto red in
`displayed as Spaces until the viewer chooses to Reveal
`them .
`
`set of 96 Displav
`Contiguous Graphics Set- The
`the 64 Contiguous G[aphics
`Characters comprising
`Ch.arac ters listed in colurr ns 2a, 3a, 6a and 7a ofTable 3,
`toge ther wi th
`the 32 Blast-Through Alphanumerics
`Cha racters of columns 4 and 5 .
`
`Alphanumerics Mode - The Dis.play Mode ~n which the
`Display Characters are those of the Alpt13numerics Set.
`
`Contiguous Mode-The Display Mode in which the six
`the Graphics Characters fill
`c.ells of
`the Character
`Rectangle (see Figure 8).
`
`Alphanumerio Set - The se t o f 96 Display Charact ers
`comprising all the Alphanumerics Char ~~ ters.
`
`Background Colol!!- The colour ftl!lng the parts of the
`Cha[acter Rectangle not occupied by the character itself
`(see Figure 8). The Background Colour may be black or
`rt m ay be changed
`one of the seven Display Colours.
`within a Row by Co ntrol Characters.
`
`Blast-Through Alphanumerics-The use o f
`the 3 2
`Alphanumerics Characters of columns 4 and 5 of Table 3
`during the Graphics Mode.
`
`Control Bits- Each Page-Header contains I I Conrr.ol Bits
`to regulat,e th e display of the Page and its header (see
`2.3. 1).
`
`Control Character-One of the 32 characters liste d in
`columns 0 and I of Ta ble 3 . Five of these are reserved
`for compatibility with other data codes. The others are
`used to alter the Display Modes. They are usually
`displayed as Spaces (but see 3 .I . 7.).. _
`
`Data-Line-One of the o therwise unused lines of the
`
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`television field-bl<!nking interva.l use d to carry info,rmation
`for one Teletext Character Row _ A Data-Une is identified
`by the Clock Run-In $equence followed by a Framing
`Code at the appropriate time on a line in the field interval.
`
`Display Character-One of 222 different shapes which can
`be generated in a Charac rcr Rectangle as pan of a Page.
`There are Alphanumerics Characters to provide text and
`Graphics Characters
`to provide elementary pic to rial
`rnformation. There are three ;ets , the Al phanume rics Set,
`the Contiguous Graphics Set and t.he Separated Graphics
`Sci , e;1c·h nf 'It) Display Characters, some of which are
`~;ore n non .
`
`Display Colour-One of the seven colours (white . yellow ,
`cya n, green, magenta, red, blue) used to depict a Display
`Character against the Background Colour in a Character
`Rec.tangle (see Figure 8). The Display Colour may be
`changed within a Row by Contra.! Characters.
`
`Display Mode - The way in which the Charac ter Codes
`corresponding to Display Characters are interpreted and
`displayed dep<!nds on Display Modes established by
`previous Control Characters {see Table 2). These modes
`may be changed with in a Row, and an initial set of modes
`is de lined for the start of a Row.
`
`in which wl characte r> are
`Flash - A Display Mo,1e
`inte nded
`to be displayed al terna te ly as
`they would
`otherwise be displayed, ;HJd a.s SpJ(;CS, un dc r the control
`uf ~ liming ucv i~:c in the rc<:.: •vc r.
`
`the Clock Run-In
`Framing Code- A Byte following
`sequence , selected to allow the receiver to achbe\'e Byte
`synchronisation even if one of its bits is wrongly decoded_
`
`Gr:aphics Character-One of 127 diffe rent Display
`Characters based on
`the divisi.on of the Charac1er
`Recta ngle into six cell s, the cells being Contiguous or
`Separated.
`The corresponding character codes have
`b6.=I ,
`there
`is a direct correspondence between the
`other six bits of the code and the states of the six cells
`of the Chancier Reel angl e, Example
`are given in
`Figure 8.
`
`Graphics Mode-The Display Mode in which the Display
`Characters are those of one or other of the Graphics Sets,
`depending_ on whether the Contiguous or Separated )\'lode
`obtains,
`
`Graphics Set ~see Contiguous Graphics Set and Separated
`Graphics Set.
`
`Hamming Code- In the Teletext system a Hamming Code
`is a Byte containing four message bits and four protection
`
`bits as indicated in Table I . A single bit error in such a
`Byle can be corrected . Hamming Codes are used for
`sending addrt..
`~d control infonnation.
`
`Hold Graphics-A Display Mode in which any Control
`Character occurring during the Graphics Mode results in
`the display of~ Held Graphics Character (see 3. l.7).
`
`Magaz.ine -A group of up to a hundred Pages. each
`carrying a fi;Omm on Magazine number in the range 1-S.
`Up to eight Magazi nes may be transmitted in sequence or
`independ ently on~ television programme channeL
`
`Newsflash Page ~ A Page in which aU the information for
`d i~play is Boxed . and Control Bit C1 is se~ to allow this
`information to be automa tically inset or added to a tele(cid:173)
`vision picture.
`
`Page-A group of 24 Rows of 40 characte rs in tended to be
`displayed as an entity on a television screen_
`
`Page-Header-A Page-Header D~ta-Line has Row address
`it separates the Pages of a Magazine in the
`'0' and
`sequence of traT\srnitted Data.-Lines. In place of the first
`eight Character Bytes it contains Hamming Coded address
`and conuol information relating to that Page. Thus the
`corresponding top Row of the Page has only 3 2 Charac1er
`Bytes. These ar,e used fo; the transmissioll of general
`inform~lion such as Magazine and Page number, da~- and
`date. programme ~m.H~e and clod.-t ime.
`
`Release Craphic~ -The Display Mude in which Control
`It is
`Characters are invariably displayed as Spaces_
`complementary 10 the Hold Graphics Mode .
`
`Reveal-The Display Mode complementary to the Conceal
`Mode .
`
`RoUing Headers-The use of the top Row of the Page to
`display a!l the Page -Headers of the selec ted Magazine {see
`2 .3 .!-Magazine Serial) as they are transmitted. This gives
`an indica tion of the Page transmission sequence while the
`user is watching, or awaiting. a selected Pag~ .
`
`Row- A Page comprises 24 Rows of characters. When
`displaye d on a television screen each Row rn;cupies about
`10 television display lines. Each Row is generated from
`lt is to
`the information on one television Data-Line_
`3void confusion with television 'lines' that Teletext Pages
`are said to contain 'Rows'.
`
`Row-Adaptive Transmission-Telete.xt
`in
`transmission
`which Rows containing no information are not trans(cid:173)
`mitted. This reduces the access times of the system. The
`non-transmitted Row~ are displayed as Rows of unboxed
`
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`

`black Spaces.
`
`Separated Graphics Set- The set of 96 Display Characters
`~ornprising the 64 Separated Graphics Cha ca~ters
`corresponding to
`the Contiguous Graphics Characters
`listed in columns 2a, 3a, 6a and 7a of Table 3, together
`with the 32 Blast-Through Alphanumerics Characters of
`columns 4 and 5.
`
`SepaRted Mode- The Display Mode in which there is a
`Background Colour boundary around and between the
`six cells of the Graphics Char2ctets within the Character
`Rectangle (see Figure 8).
`
`Space- A Character Rectangle entirely fl.lled by the back.
`ground colour .
`
`Subtitle Page- A Page in which all the info rmation for
`display is Boxed , and Control Bit C6 is su to allow this
`informatio n
`to be automatically inset or added to a
`telev ision picture.
`
`Teletext-An information transmission system using the
`data and display formats described in sections 2 and 3 of
`this document.
`
`Televwon Data-Line-see Data-Une:
`
`Tune-Coded Page-In addition to a Magazine number and
`Page number a Page may be assigned a 'Time Code' of one
`of 3200 numbers arranged as two 'Hours' digits and two
`'Minut~s· digits.. This code may be used to select one of
`many Pages, bearing the same Magazine and Page number,
`transmitted in sequence. When the transmission of each
`version of the Page is isolated or infrequent this code m.ay
`be made literally the 'Hours' and 'Minutes' of the dock·
`time at which it is transmitted.
`
`Tune Display- The last eight characters of every Page(cid:173)
`Header are reserved for clock-time. A receiver may be
`arranged to display these ch.amcters from the Rolling
`Headers to give a clock-time display.
`
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`~I
`
`22 I ~J 24
`
`21
`
`I
`I
`I
`I
`~
`I
`~ i
`I
`j
`
`I
`I
`
`I
`I
`I
`I
`I
`I
`I
`,.1 I
`I
`•I
`
`I
`I
`l
`
`
`
`!n11~ .~~~
`
`FIG. 1: TELETEXT DATA-LINES
`
`'Insertion Test Signal
`
`'
`
`I
`1
`•I
`
`lmes
`
`I
`Lmes 1
`
`1.
`
`,
`
`,. o_ata-.. 1• 1:rs·
`
`Initial
`
`possible Telete~o: t Data-Lines
`
`.
`
`1,
`
`field-blanking interval (25 lines)
`
`1
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`l00%_~HITE __ - - - _________ _
`LEVEL
`
`Bin~ry '1'
`
`Oata-l,ine
`17 (3301
`
`Binary ·o· level: 0(:!: 2)%
`Binary ' 1' level: 66(±6)%
`
`~:..::.- ·:..:.'y ·. ,·_::..:·.~-:~ :.:::,.;:;;..i:,:;_;:.J.:::.: _-.. ..:,·cf ·.-.::::::.::..
`waveform Oata· Line
`overshoou
`18 (.3311
`
`!
`
`ITS line
`19 (332 )
`
`Ba.sic Data
`Amplitude
`
`Peak-to-peak
`Data Amplitude
`
`FIG. 2: DATA LEVELS
`
`+0·4
`12-0 -1-0IJS
`
`LINE
`TIMING
`REFERENCE
`
`DATA
`TIMING
`REFERE NCE
`
`Line
`Syn.:;
`Pul51!
`
`Colour
`8urst
`
`I.
`
`.I.
`
`.J
`
`Clock Run-in
`
`Framing
`Code
`
`FIG . 3: DATA TIMING
`
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`1.0 ..... ____