288
`
`IEEE Transactions on Consumer Electronics, Vol. CE-25, No.3, July 1979
`
`TELETEXT AND VIEWDATA SYSTEMS AND THEIR POSSIBLE EXTENSION
`TO EUROPE AND USA
`
`G. 0. Crowther
`Mullard Limited
`New Road, Mitcham, Surrey, U.K.
`
`INTRODUCTION
`In this paper it is proposed to explore the extension of the
`UK Viewdata and Teletext systems primarily into the USA
`but also to Europe. The paper is complementary to the
`paper by N.E. Tanton, BBC, entitled 'UK Teletext -
`Evolution and Potential'. In adapting and extending an
`existing system it is worth recognising the basic starting
`points. For the UK Teletext and Viewdata systems these
`can be summed up as follows:
`
`I) decoder costs,
`2) ease of use by the end user.
`
`The cost of the basic decoding system was considered to
`be one of the prime parameters of the whole system. The
`price increment of including Teletext into the domestic tv
`receiver was targetted at significantly less than 10% and
`20% for a combined Teletext and Viewdata system. It was
`considered that unless these targets were achieved, the
`market size would not be sufficient to achieve the economy
`in scale essential for the success of the project. For this
`reason the system specifications were a collaborative effort
`between the broadcasters, the Post Office, the set makers,
`and the IC manufacturers. There are already good signs that
`the initial concepts were correct.
`In setting up the systems specifications, to a large extent
`it is Teletext which imposes most of the constraints, and
`for this reason will be the major subject of the paper.
`
`THE BASIC TELETEXT/VIEWDATA DECODER
`To understand the consequences of changing the systems
`to NTSC standards, it is necessary to understand the basic
`decoder requirements. Fig.l shows the main sections of a
`combined decoder.
`The section marked Teletext demodulator is a linear
`circuit. It is in this section that the incoming video signal is
`converted into a digital signal and a synchronous clock
`generated. Further, if the full opportunities of Teletext are
`to be realised such as sub-titling and news-flash, then the
`display timing system of the decoder has to be locked to
`the incoming picture signal. This function is also performed
`within the Teletext demodulator.
`The acquisition circuit selects the user-requested page
`from the incoming serial bit stream. It then converts the
`
`signal into bytes of information with an appropriate address
`and writes the data into a definite location in the page
`memory. In any Teletext system this process, for speed
`reasons, has to be handled by dedicated logic.
`The Viewdata demodulator is a conventional modem in
`which the specification on signal-to-noise ratio has been
`slightly relaxed taking account that only short line wo'rkin'g
`will be required.
`The Viewdata acquisition again takes the incoming data
`stream and converts it into bytes of data with an appropriate
`address for direct insertion into memory. In the Viewdata
`system the acquisition circuit also converts the user
`requests to the central data base into an outgoing data
`stream. Finally all the automatic call set up, shut down, and
`identification procedures are handled in this section.
`The procedures employed in both these Viewdata
`sections are largely covered by international standards
`(CCITT) and this section could be transferred in principle
`directly to the US and European markets without signifi(cid:173)
`cant change.
`The page memory holds a complete page of data in
`coded form (ASCII) whilst the character generator converts
`the digitally-coded signal stored in memory into video
`signal for display on the picture tube.
`The timing section provides the timing signals {line
`flyback and field flyback pulses) and also addresses both
`the memory and the character generator to ensure the
`information is displayed on the correct location on screen.
`It is essential for economic reasons that the above three
`sections are common to both Teletext and Viewdata.
`
`+Video
`
`t PO line
`
`Fig.1 -Basic Teletext and Viewdata system
`
`RECEIVED JUNE 20, 1979.
`
`0098-3063/79/0400-0288$00. 75©19791EEE
`
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`Crowther: Teletext and Viewdata Systems and Their Possible Extension
`
`289
`
`Significant changes will be required in this area for opera(cid:173)
`tion on NTSC as will be discussed later.
`The user interface has to interpret user requests and
`instruct the rest of the decoder. In present systems this is
`performed by dedicated logic circuits but, in future, it
`could be handled with advantage by a micro-computer.
`There are two main areas to be examined in converting
`the present UK Teletext and Viewdata systems for NTSC
`based television sets:
`
`1) transmission format of Teletext,
`2) the display format.
`
`The latter point affects both Viewdata and Teletext.
`However, the implication on Viewdata occurs only in the
`main data base software and the decoder character genera(cid:173)
`tor but not the transmission system. In Teletext it has
`also a direct implication on the transmission strategy and
`the acquisition circuits.
`
`TELETEXT SYSTEM CONSTRAINTS
`In designing a Teletext system two major factors have to be
`recognised.
`
`1) Teletext unlike other data communication systems is
`non-interactive.
`2) The data rate is 1 M byte/s.
`
`The non-interactive nature of the Teletext system is
`perhaps the more important factor. It affects the method of
`transmission adopted and decoder design; in particular, the
`techniques adopted for transmission error protection and
`the need to give apparent user interaction.
`There are essentially two ways in which the end user can
`obtain a piece of desired information from the Teletext
`system. The first is that he be told in advance of the time of
`transmission, in much the same way as a tv programme, and
`therefore be ready to request the page at that time. Alterna(cid:173)
`tively, all the data is transmitted cyclically and the user has
`
`to wait for the time in the cycle for the specific data to be
`transmitted. Both these strategies have been built into the
`UK system.
`The user feedback and error strategies have been based
`on the cyclical nature of the majority of information.
`Single transmission of timed information can only be
`achieved reliably and economically in any Teletext system
`if the data is transmitted two or three times.
`The high data rate, particularly if a whole tv channel is
`devoted to Teletext, requires special consideration from the
`decoder designer's point of view. There is no software-based
`system that could process and sort data at this rate, and
`consequently the coding system adopted has to minimise
`the dedicated logic necessary in the decoder to select the
`data requested and transfer it direct to memory in an
`ordered manner.
`
`ANALYSIS OF BASIC TRANSMISSION SYSTEM
`ADOPTED IN UK
`The basic format of the data transmission is shown in Fig.2.
`Essentially, the data stream is located on the tv line in the
`space normally occupied by video. NRZ coding has been
`employed for coding a binary bit and transmitted as a
`raised cosine, see Fig.3. The data stream has been divided
`up into bytes of 8 bits each and these are further divided as
`shown into three groups: sync, address, and data. The line
`flyback pulses are an important part of the Teletext signal
`and would be required if the whole channel were given over
`to test transmission.
`In essence, the line sync pulses give reliable marking of a
`packet ofdata and act as a flag for the synchronisation data
`(both bit and byte synchronisation). This allows the
`decoder to readily and reliably keep the data clock in
`synchronism with the data.
`The line flyback pulse is also employed in UK Teletext
`to flag the address information. Again, to simplify decoder
`design, the address gives adequate information to the
`decoder of the nature and precise location in memory of
`the following data and as a consequence its location on the
`
`10 address
`
`30 data
`
`3 sync
`
`,_, I !
`
`Ro format
`
`Slicing
`levels
`1\
`
`(
`
`7
`
`Lr
`
`40 data
`
`3 sync 2 address
`I
`I I
`,_,~-~-------------------*ur
`R1 to R23 format
`
`'\ 7 Ki
`
`:
`
`'
`
`Incoming
`data
`
`Recovered
`data
`
`Recovered
`clock
`
`1 1 0
`
`0
`
`Fig.2- UK Teletext packet formats
`
`Fig.3 - Data and clock recovery
`
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`290
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`IEEE Transactions on Consumer Electronics, Vol. CE-25 , No. 3, July 1979
`
`Data -----~
`
`Clock run in
`& address
`
`Tile quick brown fox jumped
`
`Fig.4- Direct relationship between data on tv line and display row
`
`final display, see Fig.4. In this way only address data has to
`be processed at the high incoming data rate.
`It is recognised that a page of data will be made up of
`several packets of data and that certain address information
`would be common to every packet. To economise in trans(cid:173)
`mission time, two types of packet are defmed as shown in
`Fig-2.
`The R0 packet contains all the common address inform(cid:173)
`ation for a page of text consisting of several packets of
`data. The Ro packet marks the ~tart and finish of a page.
`The data -to be transmitted may then be divided into a
`maximum of 32 packets with a unique address location in
`the decoder memory. In the packets labelled R1 to R31
`minimum addressing is incorporated to identify the packet.
`In this manner the throughput of data is maximised. It
`should be recognised that one unrequired address bit per
`packet represents the loss of approximately one packet of
`message data per page.
`The amount of data incorporated in a packet is depend(cid:173)
`ent on the bandwidth of the tv network. In Europe 45
`bytes can be reliably transmitted, which permits one row
`of the final display to be transmitted within one tv line.
`The bandwidth of the NTSC system will not allow this data
`rate and an alternative method of packaging will be
`required. This will be discussed later.
`
`DESIGN ADVANTAGES OF DIRECT ADDRESSING
`SYSTEM
`three instances where the fixed relationship
`There are
`between the transmitted data and display data on a tv line
`is of importance. The first is when the user requests a new
`page; the second when errors occur in the transmission; and
`lastly, in the future when full channel Teletext transmis(cid:173)
`sions are implemented.
`To cover the first aspect , all present LSI decoders
`display on screen every page header. Since the page number
`is always transmitted in the same location, it will appear on
`screen as a continually changing or rolling display. It has
`been arranged that the page number rolls from the instant
`
`the new page is requested until its acquisition. The user is
`given
`immediate feedback;
`the system
`is functi.oning
`correctly even though the page requested is not being trans(cid:173)
`mitted. Other variants on the facility are clearly possible
`but they can only be achieved economically if the one-to(cid:173)
`one relationship exists.
`More important, is to examine the effects of errors in a
`transmission. If errors occur in any Teletext system, either
`detected or undetected, the only possible action is to wait
`for a repeat transmission. Furthermore, since errors are
`likely to be caused by noise, probably aided by other
`distortion phenomena (reflections, asymmetric distortions
`in equipment, and co-channel interference), it is probable
`that the next reception of the required text will contain
`errors but in new locations. Advantage can be taken of this
`fact if the coding system is well chosen. It can be arranged
`that an integration of correct text automatically takes place
`by the use of simple parity checks over two or three
`receptions of the wanted data. For this to be achieved, it is
`vital that the page selection and the page formatting
`information are protected against disturbance. Hamming
`codes for the protection and correction of the address data
`are employed. The correction is important in that it ensures
`that the following valid "good" data is not rejected due to a
`1 bit error in the address with a consequent extension of
`the access time of a specific page.
`For decoder design, it is essential that the non-standard
`code (e .g. Hamming protected address) is marked by a flag
`that can be readily recognised.
`In the UK synchronous system the page formatting data
`always occurs at the start of a tv line and, in essence, the
`line sync pulses act as the flag. The normal flywheel tech(cid:173)
`niques give complete protection to this method of flagging.
`More recent work on multilanguage systems (Ref. I) has
`led to a technique of achieving greater security on the data
`than is achieved with the simple parity system by a small
`addition to the decoder as shown in Fig.S.
`The memory has been increased from 1K7 to 1K8 of
`which 1K7 are employed to store the page of data. The
`eighth bit is employed as a status bit for the associate byte
`
`I
`10 address
`
`8 data
`
`Video
`-+
`
`J
`
`Data
`
`Fig.5 - Strategy for polyglot system
`
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`Crowther: Teletext and Viewdata Systems and Their Possible Extension
`
`291
`
`Status of
`new data
`
`First
`reception
`Second &
`subsequent
`Second &
`subsequent
`Second &
`subsequent
`Second &
`subsequent
`
`Present Comparison New data New
`flag
`flag
`old &
`into
`status
`new data
`memory
`-
`-
`1
`
`Write
`
`Agrees
`
`Write
`
`0
`
`1
`
`0
`
`1
`
`0
`
`Agrees
`
`Write
`
`Disagrees
`
`No
`
`Disagrees
`
`Write
`
`1
`
`0
`
`0
`
`Fig.6 - Error strategy
`
`of data. Incoming information is only written into memory
`after a comparison between it and the data already stored
`and the status bit. The decision table is given in Fig.6.
`In conclusion, it is worth mentioning that conventional
`methods of protecting serial data such as BCC or CRC are
`not as effective as the combination of majority decision
`described above and Hamming protection, although they
`can be very effective in giving confidence that the data in
`memory is correct particularly if it contains important
`numeric data.
`Another important advantage of the simple relationship
`in decoder design arises when the system is extended by
`inserting data on more lines than the two tv lines currently
`employed.
`A number of organisations are already investigating
`systems which would dedicate a tv channel to Teletext on
`625/525 lines for at least part of the day. In these circum(cid:173)
`stances data will arrive at approximately 1 M byte/s. This
`has to be processed and sorted as it comes in unless severe
`constraints are imposed on the order in which data is
`transmitted and, in addition, considerable buffer stores
`incorporated in the decoder with the consequential cost
`penalty.
`With the present simple relationship the data can be
`readily processed as it arrives and present day decoders can
`already be adapted with minor change to full 625/525line
`operation.
`It is of interest to note that 625 line operation would
`give the possibility of 10 000 full pages with a cycle time of
`~ minute, whilst the decoders today already have the
`capability of selecting 1 out of 2~ million pages.
`One of the penalties often attributed to the flxed format
`concept is the use of serial picture attributes (colour, etc.)
`as opposed to parallel. Both definitions of picture attributes
`can be transmitted by the UK direct addressing method. It
`was, however, a conscious decision at the time of writing
`the UK specification to adopt serial attributes. Parallel attri(cid:173)
`butes increase the cost of the decoder (at least twice size of
`memory) and cause an indeterminate increase in the trans(cid:173)
`mission time.
`
`IMPLEMENTATION OF TELETEXT ON NTSC
`There are two inter-related aspects to be reviewed: the page
`format and the transmission data bit rate.
`In one solution the one-to-one relationship between data
`on tv line and the displayed row is retained. In the other·
`the two are optimised independently. In the first solution
`the existing UK hardware can be employed but an undesir(cid:173)
`able compromise has to be taken between two conflicting
`requirements. An alternative system is examined which
`avoids the conflict between data format and transmission
`data rate. The second proposal allows fleld trials to be
`undertaken over a wide range of data rates.
`
`Page Format
`In Europe the page format adopted is 40 characters/row
`and 24 rows/page. Experience in the UK suggests that
`editorially it is undesirable to reduce the number of
`displayed characters below 40. It is also recognised that
`there is a desire in the USA to implement Teletext via
`set-top adaptors as well as incorporating them with the tv
`receiver as is generally practised in Europe.
`Thus a bandwidth constraint is imposed on the created
`Teletext video signal as well as on the incoming data. The
`factors which determine the outgoing signal bandwidth are
`shown in Figs.7 and 8. It will be seen that a 40 character
`display is possible within the NTSC system provided it is
`not associated with the incoming signal rate.
`
`.-----------52~s --------~
`
`,-------------------- -------- --------..,
`' ;
`I'
`:
`i
`'
`' I ;
`I
`Desired
`Teletext display
`allowing 10"/.
`margin
`
`I
`
`Display time
`max. 42JIS
`
`1961ines
`=20rows
`of text
`
`..-
`
`i 2421ines
`j
`
`I
`I
`
`I
`I
`I
`I
`I
`' I
`' ' ' I
`' ' I
`
`Ll __ ------------------------ ____ j
`
`Normal picture size allowing 10'1o overscan
`
`Fig.7- Factors affecting display of Teletext
`
`No. of
`characters Display
`displayed
`time in
`per row
`JIS
`
`Display
`time per
`character
`JIS
`
`6 dots/character
`Display
`Highest
`fundamental
`clock rate
`freq. MHz
`MHz
`
`40
`
`32
`
`40
`45
`34
`40
`
`1-0
`1-125
`1·0
`1·25
`
`6·0
`5·3
`5·6
`4·8
`
`3·0
`2·6
`2·8
`2·4
`
`Fig.B - Display bandwidth for two format strategies
`
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`
`292
`
`IEEE Transactions on Consumer Electronics. Vol. CE-25. No.3, July 1979
`
`The number of rows to be displayed is limited by the
`number of tv lines visible on the screen. A minimum of 10
`lines is required to form a row of characters (upper and
`lower case) which leads to an optimum page format of 40
`characters per row and 20 rows.
`
`Transmission Strategy for NTSC
`At this stage in the Teletext field trials in the USA, there is
`no information as to the maximum data rate that can be
`handled by the NTSC system. For this reason, a number of
`alternative solutions are given in Fig.9 whilst the basic
`principles are shown in Fig.1 0 by a single example.
`
`Display 40 characters/row and 20 rows/page
`
`Data bytes
`on received
`data line
`20
`24
`27
`30
`32
`
`Incoming
`data clock
`rate (MHz}
`3·8
`4·4
`4·9
`5·4
`5·7
`
`Pages per
`second at
`2 data/frame
`
`3
`3·6
`4
`4·5
`4·8
`
`Pages as
`"to of PAL
`systems
`-27
`-12
`0
`+10
`+17
`
`Fig.9- Selection of strategies for Teletext on NTSC
`
`• lJ
`
`Fig. tO- Strategy for reduced bandwidth defined Teletext transmis(cid:173)
`sion with 40 character display
`
`In the example of Fig.10, 32 message bytes are transmit(cid:173)
`ted on a tv line representing a data clock rate of~ 5.6 MHz.
`In the proposal, the first 20 packets (labelled 0 to 19)
`are employed to convey the data on the left-hand side of
`the screen. This permits an identical simple decoder design
`as currently employed to load
`the
`information into
`memory.
`The small sections on the right-hand side of the screen
`together into
`consisting of 8 characters are packaged
`sequential groups of 4 and transmitted on 5 further packets
`labelled, for instance, 27 to 31 inclusive.
`
`Code
`transmitted
`
`Location of data
`on display
`First 32 chara. Row 1
`,
`2
`, 3
`, 4
`Last 8 chara. Row 1 to 4
`
`First 32 chara. Row 5
`6
`7
`8
`Last 8 chara. Row 5 to 8
`
`First 32 chara. Row 17
`, 18
`, 19
`, 20
`Last 8 chara. Row17 to20
`
`Fig.11 -Typical transmission sequence for packets using 32 charac(cid:173)
`ters as example
`
`To improve the presentation of the data on first recep(cid:173)
`tion of the page, it is proposed to interleave these sub-groups
`within
`the main sequence. That
`is,
`the transmission
`sequence could be R0 R1 R3 R31 R4 R5 R6 R7 R3o Rg
`... etc., see Fig.11. Experiments have shown that visually
`no difference can be detected between the above strategy
`and the present European method of transmitting a com(cid:173)
`plete row.
`In Fig.9 five strategies are described with data clock
`rates from 3.8 to 5.6 MHz which, it is felt, should cover the
`NTSC requirements. A comparison is also made between
`the number of pages received per second for each strategy
`as compared with European systems.
`If it were so desired, clock rates above and below the
`above figures could be employed adopting the principles
`already described.
`The cost implications of the above proposals on the
`decoder are extremely small, and present-day decoders can
`be readily adapted to the scheme with a small peripheral
`circuit.
`
`Possible Future Extensions of the Teletext Service
`In the design of the Teletext service in the UK it was
`considered essential to be able to enhance the service as
`new features were devised and technology permitted.
`Already a number of enhancements are being considered.
`1) Multipage memory for rapid access of related
`information
`2) Full channel tv
`3) Multilingual operation to cater for all Latin languages
`and non-Latin language requirements
`4) Remote programming of character generator for
`specialised pages, for instance: mathematics,
`chemistry, and minority languages
`5) Remote programming of local home terminal.
`
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`
`Crowther: Teletext and Viewdata Systems and Their Possible Extension
`
`293
`
`Experiments in the UK have shown that all the above
`enhancements can be implemented with the present Teletext
`decoding scheme and with only minor modifications to the
`basic decoders.
`In enhancing the system, three aspects have to be taken
`into account:
`
`1) new decoders must be able to receive all services
`'
`-
`2) ensure old decoders will not be disturbed by the
`additional features,
`3) the simple decoder does not have an inbuilt cost
`penalty for the more sophisticated systems.
`
`The first requirement will be met by the appropriate
`redesign of the decoder, and to a large extent depends on
`the nature of the new service. The methods of achieving
`Points 2 and 3 are illustrated for the language extension in
`Figs.12 and 13.
`__
`It employs the fact that the page number transmitted in
`the page header Ro is transmitted as two BCD codes. The
`decoders have been designed so that the user can request
`only units 0 to 9 and tens 10 to 90. This means that binary
`codes 10 to 15 in both tens and units may be transmitted
`but never requested by an old decoder.
`In both illustrations it is assumed that the Page 151 is
`normal text suitable for both types of decoder, whilst Page
`152 is in Turkish and can only be decoded in second
`generation decoders.
`In the first method (Fig.12) Page 151 is terminated with
`a normal Ro, and would thus be received normally by both
`forms of decoder. If an old decoder were to request Page
`152, it would accept the page header with the message
`saying the text is in Turkish but the page would be termina(cid:173)
`ted by the next line which is a non-standard Ro.
`Since the old decoder can never request a non-standard
`
`page code, this line and all subsequent lines until the next
`standard Ro will be ignored. Thus, the old decoder is just
`informed that Page 152 is not available to the user.
`A new decoder can be trained t~ accept these non(cid:173)
`allowed codes and recognise that a new addressing structure
`applies and thus be able to receive the information. Usillg
`this technique at least 48 new extensions are possible.
`
`CONCLUSION
`The problems of adapting the Teletext and Viewdata
`systems to the NTSC have been analysed and the two
`significant changes, the data clock rate in Teletext and the
`page f~rmat, have been discussed.
`The reasons for adopting a direct addressing technique
`for Teletext have been analysed and the advantages in terms
`of decoder design cost and performance given. A number of
`strategies have been given to optimally match the concepts
`to the NTSC system. The option chosen must depend on
`the results of field trials. At present, it is suggested that a
`display format of 40 characters per row and 20 rows should
`be employed. The techniques described allow many other
`display formats if this were considered desirable.
`Finally, an indication of a number of directions of
`service ehancements being considered in the UK has been
`given. Techniques have been described of how the~e new
`features ~ould be introduced without interfering or causing
`obsolescence of the basic decoders.
`
`REFERENCE
`1. CHAMBERS, J.P., 'Teletext alphabets and error protec(cid:173)
`tion'. EBU Review- Technical No.173 (Feb. 1979).
`
`page 151
`
`page152
`page (15) 152
`
`R23
`Ro
`Ro
`R1
`Rz
`
`page?
`
`Received by normal and
`enhanced decoders
`
`}
`
`page151
`
`Ro
`R1
`
`Displayed on normal
`Page in Turkish } decoders
`
`New Ro format J Acts as page terminator
`
`for normal decoders
`
`Enhanced decoders only
`
`R23
`Ro
`R1
`
`Rzo
`
`Received by normal and
`enhanced decoders
`
`)
`} for normal decoders
`
`page (15) 122 New Ro format } Acts as page terminator
`
`Received by enhanced
`decoders only
`
`Fig.12- Transmission sequence to separate normal and polyglot
`terminals
`
`Fig.13 -Alternative transmission sequence to separate normal and
`polyglot terminals
`
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`
`294
`
`IEEE Transactions on Consumer Electronics, Vol. CE-25, No.3, July 1979
`
`BIOGRAPHY
`
`G. 0. Crowther received the B.Sc. in Engineering
`from Imperial College , London . He joined Mullard
`Limited in 1950 in their application laboratory . His
`present position is coordinator of consumer electronics
`in the Mullard Application Laboratory with responsibili(cid:173)
`ties for the applications of all components to television,
`radio, audio, and domestic appliances. He has a prime
`role within Philips for Teletext and Viewdata . His ex(cid:173)
`perience includes work on rad iation monitoring equip(cid:173)
`ment, simple control systems, electronic telephone
`exchanges, calculators, nano-second logic systems,
`computer interfaces, and consumer electronics. He has
`been part of and contributed to the evolution of Teletext
`and Viewdata since their initial conception some seven
`years ago.
`Mr. Crowther has some 20-30 patents on a wide
`range of subjects and a similar number of papers and
`lectures to learned bodies and magazines.
`
`G. 0 . Crowther
`
`PMC Exhibit 2059
`Apple v. PMC
`IPR2016-00755
`Page 7