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`Teletext and Viewdata
`
`Steve A Money
`
`T.Eng(CE/), MBCS, MITE
`
`
`
`Newnes Techhical Books
`
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`Newnes Technical Books
`
`is an imprint of the Butterworth Group
`which has principal offices in
`
`London, Sydney, Toronto, Wellington, Durban and Boston
`
`First published 1979
`Reprinted 1981
`
`1979
`© Butterworth 84 Co (Publishers)
`All rights reserved. No part of this publication maybe reproduced or transmitted
`in any form or by any means, including photocopyiing and recording, without the
`written permission of the copyright holder, application‘for which should be
`addressed to the Publishers. Such written permissionmust also be obtained before
`any part of this publication is stored in a retrieval systemof any nature.
`
`This book is sold subject to the Standard Conditions of Sale of Net Books and
`may not be re-sold in the UK below the net price given by the Publishers in their
`current price list.
`5;
`r
`I
`,
`'
`.
`‘
`,
`.
`I
`-
`
` British Library-Cataloguing in Publication Data
`
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`1"
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`_
`‘ Teletext and viewdzita,‘ ,. j-
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`l. Teletext and vieyvdjat "
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`’
`L Title
`I ,,
`
`621.388 _.
`..
`
`
`
` Ty‘pes‘otby Bo "
`
`ISBN 0-4’08~Qo'3:7;9;1..
`
`'
`
`Printed in
`
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`
`
`
`
`
`
`Chapter 9
`
`Teletext Production
`
`in this book we have examined the techniques inVOIVed in
`So far
`decoding and displaying teletext. Now it may be interesting to see how
`the teletext service is handled at the broadcasting centres.
`
`Ceefax
`
`High over Wood Lane in west London, on the seVenth floor of the BBC
`Television Centre,
`is the editorial suite from which the Ceefax service
`is controlled.
`
`in a relatively quiet room away from the bustle of the
`it is here,
`television studios on the floors below, thata team of journalists compile
`the pages of text for the two Ceefax magazines. From a corner of the
`room there is an intermittent chattering as one of the bank of‘four or
`fiVe teleprinters types out a news item. These printers provide much of
`the news information that you will later see on the Ceefax pages. some
`~ printers are linked to news agencies, such as Reuters, whilst others may
`be collecting information from other departments of the BBC. A printer
`may be connected to Exchange Telegraph for financial and Stock
`Market news whilst another may be linked to a sports news service to
`giVe the latest racing or football results. Journalists will now take suitable
`items from these teleprinter machines and edit them to fit into the
`format of the teletext pages.
`'
`'
`Next comes the process of composing the actual page of text that is
`to be transmitted. This is done by using a typewriter style keyboard
`which is
`linked to a computer system in another part of the building.
`in front of the journalist, as he composes the page, is a colour television
`- monitor which is also controlled by the computer. On this monitor will
`be displayed the page of text as the journalist builds it up.
`
`105
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`106 Teletext Production
`
`Composing the page
`
`Often the journalist will start of by typing in his copy without any
`commands to select a display colour. This produces a display of white
`text on a black background, At this stage he is interested in gettingthe
`text correct with an acceptable layout of the page. Next he may decide
`to add a page title with large letters. These extra large letters will be
`built up by keying in a series of graphics shapes. Sometimes if the
`title is one that
`is frequently used the set of graphics patterns will
`already be stored in the computer memory and the journalist merely
`keys in a command to insert the stored title into the page of text.
`Very, often a page that is currently being transmitted in the magazine
`In
`may need to be edited. A copy of the page is set up in the computer"
`memory and the journalist then alters it by adding or deleting sections
`of the text to produce an edited version. Whilst this process is going on
`the page being transmitted is not affected since only the copy in the
`computer is being altered.
`_
`'
`Having composed his page of text the journalist may new key in
`colour commands to change the colour of parts of the text or he may
`add some more graphics to improve the appearance of the page. When
`he is satisfied he will key in a command which tells the computer to
`insert the new page into the current magazine and within a second or
`two the new page will go out over the air to the viewers.
`
`The computer
`
`To handle the editing and storage of the pages of text in a teletext
`service a small digital computer is used. This may well be a minicomputer
`such as one of the Digital Equipment PDPii series computers or a
`similar machine by one of the many minicomputer manufacturers.
`Data for the pages of text will usually be stored on a magnetic disk
`memory which uses
`a magnetic oxide coated disk as the memory
`device. As the disk'rotates a magnetic head moves radially across it in
`much the same fashion as the pickup on a record player. Data is written
`in a series of concentric tracks on the disk and the head moves over the
`disk to select any desired track to read or write the data. A typical disk
`may store seVeral million bytes of data, sufficient for several complete
`teletext magazines.
`For editing or transmission, a page of data is read from the disk into
`the computer's main memory where it is more readily accessible. After
`editing has been completed the page of data may again be transferred to
`the disk unit for long term storage. A page for transmission is fed from
`the computer memory to a teletext encoder where the data is converted
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`Teletext Production
`
`107
`
`into a serial stream and the synchronisation patterns added. The serial
`data is then inserted into the programme video signai and passed on to
`the transmitter.
`
`Figure 9.7 shows the general arrangement of the teletext computer
`
`DlSK
`
`MEMORY
`
`
`
`
`
`COMPUTER
`‘
`
`TELETEXT
`
`ENCODER
`
`DA TA
`
`lNSERTER
`
`To
`TX
`
`VDU
`
`+
`
`
`Sync.
`Piciure
`pulses
`video
`
`
`
`KEYBOARD
`
`
`
`Figure 9.7 Basic Teletext computer system
`
`and editing system. Programs within the computer system, known as
`the teletext software, control
`the editing, storage and data transfer
`processes within the system in accordance with commands keyed in
`from the journalist’s keyboards.
`
`Updating Ceefax
`
`,
`
`,
`
`,
`
`From time to time the information transmitted in the Ceefax magazines
`is updated. News pages will usually be updated every hour but if some
`special news item occurs this may be inserted into the magazine as
`soon as the page has been composed. Some of the other topical pages
`may be updated two or three times a day.
`Pages such as the television programme guide may be changed on a
`daily basis as would the weather report or a diary of events for the day.
`Some pages sueh as the Top Ten pop music chart'might be changed
`once a Week whilst a few pages giving sueh information as postal rates
`or useful addresses would be changed very infrequently.
`At the present time the BBC uses Ceefax ‘l which is carried on the
`EEG channel as a form of daily magazine with most ofjts pages
`updated on a day to day basis. News is updated more frequently of
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`108
`
`Teletext Production
`
`course. Meanwhile ceefax 2 on the BBC2 channel acts as a weekly
`magazine with the contents updated once or twice a week.
`Pages on Ceefax l are generally numbered from 100 to 199 whilst
`those on BBC2 have numbers from 200 to 299.
`'
`
`Oracle
`
`The Oracle service on lTV is a bit more complex in its organisation
`because of the number of independent programme companies that
`form the network. At present the three programme companies based
`in London provide between them the pages that make up the current
`Oracle service.
`
`2
`
`lndependent Television News from its headquarters just north of
`ORfo'rd Street in London is the source for Oracle pages dealing with
`news, sport and weather. Here lTN’s newsgathering organisation passes
`on to a team of journalists those items of news to be put in the Oracle
`pages. The process of editing and composing the text is much the same
`as that at the BBC Ceefax centre. Once again visual display units and
`keyboards linked to a computer are used.
`
`London Transmitter
`
`ITV Network
`
`
`
`
`DATA
`DATA
`DATAV
`INSERT
`lNSERT
`lNSERT
`
`
`
`DATA
`INSERT
`
`
`
`
`
`
`TV
`L.W.
`COMPUTER
`SYSTEM
`
`Link
`
`THAMES TV
`COMPUTER
`SYSTEM
`
`Link
`
`I T N
`COMPUTER
`SVSTEM
`
`Figure 9.2
`
`The London Oracle network system
`
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`Teletext Production
`
`109
`
`A little further north at the Thames Television studios near Euston
`station a second team of journalists prepare the general interest pages
`for their section of the Oracle service whilst on the south bank of the
`Thames not far from Waterloo station a third team of journalists at
`the London Weekend Television centre put together the remaining
`pages of the magazine.
`To produce the Oracle service all of these different sources of text
`pages have to be linked together in a network as shown in Figure 9.2.
`During the week from Monday till Thursday the London areas
`programmes are produced by Thames Television and all
`the data for
`the pages of Oracle generated by [TN and London Weekend are routed
`through to the computer at the Thames Television centre. From. this
`4.,computer the data for all of the Oracle pages is then added to the
`programme video signal and sent out to the London area transmitters.
`At the same time the signal is also sent out to the other [TV stations
`around the country via the lTV network.
`At Weekends When London Weekend take over the London area
`programming the Oracle signals fromThames and lTN are transferred
`to the LWT computer for insertion into the lTV network and for
`transmission via the London stations. Data is transferred between the
`three computer systems by means of a Post Office data line.
`
`m
`
`Dara bridges
`
`in the early days of the Oracle Service the text was only broadcast from
`regional stations when they were transmitting a programme networked
`by either Thames TV or London Weekend TV. At this“:time there was
`no means by which these local stations could inject the‘vtezt signals
`into their own programmes.
`To overcome this problem and to make Oracle available to regional
`viewers devices known as Data Bridges were made and installed at the
`major regional studios. These units made use of the fact that most of
`the regional centres had a video signal feed coming from the London
`programme eVen when they were transmitting their own local pro-
`grammes.
`ln the data bridge Oracle signals from London are decoded
`retimed and then inserted into the local video signals.
`The block diagram of a typical data bridge is shown in Figure 9.3.
`Video signals from the London programme company are received from
`the lTV network and fed into a data slicer after which the teletext
`signals are gated out in much the same way as in a normal
`teletext
`decoder. The Framing code is detected and teletext data is converted
`into the parallel format and written into a small memory. This memory
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`l i O Te/etext Production
`
`
`DATA
` SERML—
`SUCER
`AND
`' PARALLEL
`“ CLOCK GEN
`CONVERT
`
`
`
`
`
`
`
`Tmemxl
`lnpm
`
`FRAME
`SYNC
`
`CODE
`DETECTOR
`DETECT
`
`
`
`
`
`
`MEMORY
`
`Stump
`6.9375
`PARALLEL
` SYNC.
`
`
`Dotu
`Sync-
`a
`‘v-SERIAL
`MHz
`FILTER
` DETECTOR
`pulses
`Out
`
`
`CLOCK
`CONVERT
`
`Figure 9.3 Block diagram ofdata bridge
`
`
`
`
`
`will probably hold about four rows of the text data at a time. Magazine
`and row address data is stored as well as the text information.
`Sync signals from the local
`television programme are then Lised to
`lock a 6.9375 MHZ oscillator and to control the reading of data from
`the store. Text data is now read out word by word and conVerted into
`a serial bit stream, using a shift register, and then passed via a filter to
`a data insertion unit which adds the text signals to the local programme.
`The clock run-in and Framing code for the locally generated signal are
`also transferred as fixed bit patterns into the parallelwserial shift register
`at the start of each of the data lines so that the inserted data signal
`is
`the same as if it had been generated completely at the local centre.
`
`Oracle magazines
`
`Unlike Ceefax where one magazine number is used for all of the pages
`on a channel, the Oracle service makes use of some pages in nearly all
`of the magazines from 100 Lip to 899.
`Magazine 2 consisting of pages 200 to 299 is used by independent
`Television News and covers national and world news, sport, finance and
`business news. Page 250 is usually reserved for newsflashes.
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`Teletext Production
`
`ii 1
`
`Magazine 7 on pages 700 to 799 is aconsumer maaazine with pages
`covering fashion, records, books, hobbies and even horoscopes. Magazine
`3 is generally used for news about the regional stations of the lTV
`netowrk giving' etails of local programmes,
`local events and perhaps
`local news‘ Some of these pages are compiled by the local network
`stations but inserted into the magazine in London.
`Magazine 5 is the London magazine covering news and events in the
`London area. Magazines 1 and 6 tend to be used as general magazines.
`Often magazine 1 is used for special pages whilst magazine 6 tends to '
`be for children and also carries educational pages for schools.
`. Some pages may carry adVertisements either using the full page or
`inserted at the bottom of another page of text Pages have been used
`{to transmit pictures ranging from a simple cartoon to portraits of
`famous people.
`r
`Updating of the Oracle pages varies with their content in muah the
`same way as the pages in a Ceefax magazine and in general the policies
`on the two services tend to be similar in this respect.
`As the use of teietext expands, the variety of news and information
`presented will increase although the number or” pages is likely to remain
`about the same until moreoata lines are used because of access time
`limitations.
`
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`
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`Chapter 10
`
`Viewdata
`
`The second type of text information service which is available to the
`British public is the Post Office Prestel service which uses the viewdata
`system where data is sent via the public telephone network. Originally
`viewdata was the name chosen by the Post Office for their information
`service but this was later changed to Prestel and the term viewdata, like
`teletext, is used to describe the system of transmission used to provide
`the service.
`In many respects the teletext and viewdata systems are similar since
`they both use television displays to present the pages of text whilst the
`page format and data coding for the two systems are virtually identical.
`The major differences are in the techniques used to transmit the data
`signals and to select the page of text which is to be displayed.
`Teletext is basically a one way communication system in which the
`complete set of pages of text is transmitted at frequent intervals using
`data signals carried as part of the broadcast television programme. The
`only control available to the user is that he can select which particular
`page from the received set his decoder will accept, store and display.
`The need to repeat continuously the whole set of pages tends to limit
`the total number of pages to one or two hundred. With more pages the
`time needed to go through the whole set of pages and hence the access
`time for any particular page becomes unacceptable.
`Unlike teletext,
`the viewdata system does provide a two way com—
`munication link between the user and a viewdata centre. Now the
`desired page can be requested by sending a command signal directly to
`the computer at the local viewdata centre. This command is decoded by
`the computer and the data for the selected page is then transmitted to
`the user and may be stored and displayed in much the same way as for
`a teletext page.
`Because only the data for the selected page is sent to the user, the
`data transmission rate can be relatively low thus allowing the signals to
`112
`
`
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