11
`
`I
`·iJ· "'1
`
`:1
`
`1li
`
`New isotope
`scarnningr
`technicaue
`
`! ;
`
`11
`
`j I l
`
`n
`
`¥
`
`\)'
`
`1
`
`~i:
`
`1
`
`\& .. \
`
`Page i
`
`PMC Exhibit 2118
`Apple v. PMC
`IPR2016-01520
`Page 1
`
`

`

`J
`
`J
`]
`]
`J
`J
`J
`]
`]
`
`]
`]
`
`]
`
`]
`]
`J
`J
`]
`]
`]
`
`wireless
`world,
`
`·Electronics, Television, ~adio, Audio
`
`MAY 1977
`
`Vol83
`
`-No 1497
`
`Contents
`
`\
`
`35 Surround sound - tim~ to consolidate
`36 Radio in the '80s by Duncan MacEwan
`41 BBC Matrix H by P. A. Ratliffe and D. J. Meares
`46 World of amateur radio
`47
`Automatic electrolytic tester by A. Drummond-Murray
`50 Variomatrix adapter for System 45J and Matrix H
`by Michael A Gerzon
`51 H.F. predictions
`52 Logic design - 4 by B. Holdsworth and D. Zissos
`55 Viewdata- 4 by S. Fedida
`60 Letters to the editor
`Mobile radio planning
`Do-it-yourself biofeedback
`Audibility of phase effects
`63 News of the month
`Annan andtechnology
`ITU Conference .results
`British Rail high-speed track measurements
`67 Two-stage linear amplifier by Helge Gronberg
`71 Power semiconductors - 2 by Mike Sagin
`79 Circuit ideas
`Linear voltage/frequency converter
`Pulse-counting frequency comparator
`Op-amppower output stage
`82 New tomography machine by John Dwyer
`85 New products
`APPOINTMENTS VACANT
`INDEX TO ADVERTISERS
`
`\..
`
`./
`
`Current issue price· 40p. back issues (if available) 50p. at· Retail and Trade Counter, Paris Garden~
`London SELBy post, current issue 55p. back issues (if available) 50p, order and payment to Room II.
`Dorset House. London SEI 9LU. ·.
`·
`Editorial ·&-Advertisliii offices: Dorset Howie. Stamford Street. London SEl 9LU· .
`.Telephones: Editorial 01-,261 8620: Advertising 01-261 8339 ..
`Telegrams/Telex, Wiwotld Bisnespres 25137 BlSPRS G. Cables. "Ethaworld. London SEL
`Subscription rates: 1 year: £7.00 Ul< ana overseas ($18.20 USA and Canada). Student rate: I year, £3.50
`UK and overseas ($9.10 USA and Canada).
`. Distrib_uti?n: 40 Bowling Green Lane, London EClR ONE .. Telephone 01-837 3636.
`Subscnpttons: Oakfield House. Perrymount Rd. Haywards Heath. Sussex RH 16 3DH. Telephone
`0444 59188. Subscribers are-requested to notify a change of address.~ 0 · I.P.C. Business· Press Ltd, 1977.
`"USA mailing agents: Expedi.ters of the Printed Word Ltd., 527 Madison Avenue, Suite 1217, New York,
`\..NY 10022. 2nd-class postage paid at New York.
`
`Front cover is a pdnt produced
`by the Tomoscanner, described
`by John Dwyer on page 82 of'
`this issue. Print supplied by J. &
`P. Engineering (Reading) Ltd.
`
`IN OUR NEXT iSSUE
`Loudspeakers and rooms. A
`discussion by James Moir of the
`interaction between the output of
`a loudspeaker and the acoustic
`performance of
`the
`listening
`room.
`
`Matrix H decoding. Circuit
`details of a matrix H variable
`matrix decoder, a development of
`Sansui's Variomatrix, for use with
`experimE?ntal surround-sound
`programmes.
`
`Using a microprocessor. The;
`start of a series of articles on the
`design of a typical processor-bas(cid:173)
`ed control system, starting with
`no assu'mptions of prior knowled(cid:173)
`ge on the reader's part.
`
`ISSN 0043 6062
`
`1 ibpa 1
`
`lnlllll.ilmnill Bu!ont!~
`P•e1< .\Jsoc,~rt!
`
`TABCl~
`
`I
`, MoM•o•T•I.,ut ..
`,, ....... - - J
`
`Page ii
`
`PMC Exhibit 2118
`Apple v. PMC
`IPR2016-01520
`Page 2
`
`

`

`Wireless Wor!1l, May 1977
`
`Viewdata·
`
`55
`
`4 -The Viewdata terminal 1n more detail
`
`by S. Fedida, B.Sc.(Eng.), M.Sc., F.I.E.E., A.C.G.I,
`
`Post Office Research Centre
`
`A Viewdata decoder may be considered
`as being made up of six parts, as shown
`from left to right in Fig l(a): a line
`isolation unit; a modem; a keypad; an
`input processor; a store (possibly
`r.a.m.); and an output processor. Indeed
`the breakdown of facilities is very simi(cid:173)
`lar to that of teletext, shown in Fig. l(b).
`This diagram also indicates that, apart
`from additional minor interconnections,
`parts common to Viewdata and teletext
`are the store and output processor.
`These are substantial components and
`therefore combined Viewdata/teletext
`receivers show important savings over
`two separate decoders for the two ser(cid:173)
`vices. This is a slightly over-simplified
`picture but the situation will be clarified
`later.
`Note however, an important differ(cid:173)
`ence. The input circuits in Viewdata,
`up to and including the store are bi(cid:173)
`directional, thus highlighting the inter(cid:173)
`active nature of the system. On teletext
`the input circuits are one way only.
`
`Line transmission
`The transmission code used over the
`telephone line between the Viewdata
`terminal arid the computer is at present
`8-bit, 10-unit asynchronous (or start
`stop), as shown in Fig. 2. Each character
`
`From & tn!fiBBB(cid:173)
`Iine
`
`Isolation
`
`Modem
`
`(a)
`
`10 unit code
`8 bit character
`10110011 = character M
`I
`Parity bit (odd parity)
`
`Fig. 2. Transmission code used between
`a Viewdata terminal and the computer
`is an 8-bit, 10-unit asynchronous code.
`
`consists of an 8-bit code, the first 7 bits
`containing the information while the
`8th bit is a parity bit. Preceding each
`character is a start bit, with a stop bit
`terminating the character. The cha(cid:173)
`racter illustrated in Fig. 2 is M, with odd
`parity. A 10-unit asynchronous system
`
`Fig. 1. Comparing the main sections of
`(a) a Viewdata decoder with (b) those of
`a .teletext_ decoder
`
`was chosen for simplicity. It is clearly
`not as efficient as a synchronous trans(cid:173)
`mission mode, in which characters fol(cid:173)
`low each other without the intervention
`of start and stop bits, but it is simpler to
`implement and js currently used by
`many time-sharing computer systems.
`In order to transmit this code over a
`telephone line, a modem (modulator(cid:173)
`demodulator) is required. Essentially
`this device modulates the code on to a
`voice frequency carrier, within the
`speech band, thus obviating the pro(cid:173)
`blems encountered with very low fre(cid:173)
`quency transmission over the telephone
`network. The modem also enables the
`go and return transmission to take place
`
`To tv
`display circuits
`
`To tv
`splay circuits
`
`Page 55
`
`PMC Exhibit 2118
`Apple v. PMC
`IPR2016-01520
`Page 3
`
`

`

`Wireless World, May 1977
`
`~-------,
`
`I
`tr=' =J
`I
`U
`0
`L
`o
`
`Teletext
`d isp Jay
`
`I
`I
`
`•
`
`unit _a
`r-T-
`~ ~ ~
`
`56
`
`.. ~---·-~
`7(
`Teletext data
`,
`>
`I
`' .::a £::::::~ c::=:l ~ c::::::J c::::::::l c::::::3 c=:J E:.::::'l r::::::::l ·z:::::::::~: r=::::{:::::=t
`~ata
`Data
`F
`......1'---.-l
`selector
`c::\
`, - - - - - - - - , fF.·===-,..,.---,
`u
`n
`....1
`I VieWdata I teletextrt=n~~a~.,....J"-.d
`11 ~----.1:1 0
`switch
`L - - - - - -
`0
`D
`A
`n
`!!.c n
`v
`== ~ =::. c= c:::::::l c= ~ c:~., c:::lo c::s c:::::l c:::::::l
`Teletext address
`c:::l ~ ~ c::::l c:::::l c:::::::llu
`~ c= c:::::l c::=i c:::::::a c:::::l ==:::11 c::::::t c::l ==.n c::::l c::::::l r:::= ~ == c:::::l c:::::::l
`0
`D
`u
`~
`~
`0
`u
`0
`0
`
`simultaneously over the two-wire tele(cid:173)
`phone line.
`Transmission rates selected for
`Viewdata during the present experi(cid:173)
`mental phase are 1200 bits per second
`from computer to terminal and 75 bits
`per second in the reverse direction.
`in the computer-to-terminal direction
`as high a transmission rate as possible is
`'desirable in order to achieve a fast
`picture build-up. 1200 bits per second
`was chosen to fit in with a well tried and
`readily available modem. For the majo(cid:173)
`rity of Viewdata displays, consisting for'
`example of mainly alphanumeric cha(cid:173)
`racters, the picture build-up is much
`·faster than can be read by the user, and
`hence quite adequate from this point of
`view. Where,. however, large uniform
`areas of graphics are displayed, the
`build-up. may appear rather slow (the
`display shows repetitive information),
`and improvements to the build-up in
`this case may be obtained by using
`special means. But in general the
`additional complexity is not really
`worthwhile.
`In· the direction from terminal to
`computer the bit rate of 75 bits per
`second (7.5 characters per second) is
`quite adequate for hand keying.
`The frequencies used in line trans(cid:173)
`mission are as follows:
`Forward channel:
`binary I = 390 Hz
`(from terminal
`to computer)
`
`binary 0 = 450 Hz
`
`Return chanpel:
`(from computer
`to terminal)
`
`binary I= 1300 Hz
`
`binary 0=2100 Hz
`
`When no data transmission is taking
`place on the line the terminal is trans(cid:173)
`mitting continuously at 390Hz and the
`computer at 1300Hz. These tones are
`used in the modems at either end of the
`line to. provide an indication of conti(cid:173)
`nuity; which as we shall see below is of
`some Importance in the operation of the
`whole system.
`When data is being· transmitted the
`
`Fig. 3. Simplified block diagram of a
`Viewdata terminal, with adaptation to
`teletext shown in broken lines. The
`number and bar on certain connecting
`lines indicate that the line is carrying
`parallel information on. that number of
`_wires.
`
`'carrier is frequency modulated (fre-·
`quency shift keying), between the
`binary I and binary 0 frequencies, the
`change being smoothed out to give a·
`gradual transition between the fre(cid:173)
`quencies.
`The transmission arrangement used
`at present is duplex, with "echoing"
`facilities provided from the computer to
`the terminal. In a duplex system trans(cid:173)
`mission may take place in both direc(cid:173)
`tions at once over the telephone with no
`mutual interference (hence, of course,
`the choice of frequencies). Characters
`· keyed at the terminal are· first trans-·
`mitted by the modem to the computer
`and displayed only when they are
`"echoed" back. This arrangement gives
`some important advantages. First, it
`provides a measure of error detection,
`the user being aware of any corruption
`in t-ransmission, errors in the computer
`or mis-keying errors. Secondly, duplex
`working also increases the user's confi(cid:173)
`dence in the working of the system, as
`"echoed" characters provide a continu(cid:173)
`ous indication that the whole system is
`in satisfactory order.
`"Echoing" from the terll?inal to the
`computer is not necessary. A parity
`check is sufficient to provide for the
`deteCtion of the majority of errors, the
`computer usually responding in these
`cases by requesting a repetition of the
`instruction. The computer also moni- ,
`tors continuously the terminal carrier,
`thus ensuring that a·Jine break is noted
`as soon as it occurs. This avoids the
`possibility of the user being incorrectly
`charged for using the system after the
`occurrence of a line interruption.
`
`0
`Viewdata
`0
`read address
`Mixed blanking 0
`Cursor
`
`0
`D.
`
`0
`D
`0
`
`11
`
`r---,_~R
`
`1--I~G
`l._~.........J.-...... 8
`
`Experimental Viewdata terminal
`The experimental Viewdata terminal at
`present in use is best introduced in two.
`parts: (a) the data transmission unit,
`which deals with the Viewdata signal
`between the telephone line and the
`internal store, and (2) the display unit,
`which deals with the Viewdata signal
`between the store and display device
`(the c.r.t. of a television set). As'
`explained earlier, much of the display
`part is common with teletext.
`A typical arrangement of a Viewdata
`· terminal is shown in Fig. 3. There are
`four major units as follows: the data
`transmission unit (1); the address selec(cid:173)
`tor (2); the random access memory (3);
`and the display unit (4).
`The address selector (2) is the ,only
`unit which interconnects the input and
`output processors, essentially for the
`purpose of preventing mutual interfer(cid:173)
`ence. Unlike the situation in teletext
`data is received at random .times from
`the
`telephone
`line, completely
`unsynchronized with the operation of
`the display. It is therefore necessary to
`organise the access to .the memory for
`reading out and display on the one
`hand, and writing-in incoming charact(cid:173)
`ers on the other hand, without j::ross(cid:173)
`interference. This function is carried
`out by the address selector. The· write
`address generated in the data transmis(cid:173)
`sion unit (1) and the read address gene(cid:173)
`rated in the display unit (4) are both
`available at the address selector.
`A mixed blanking waveform, also
`generated in the display unit, indicates
`the times at which character's are
`required to be extracted from the
`memory for display purposes essentially
`during 40 microseconds of every line·
`period, excluding blank lines at the top
`and bottom margins of the display.
`During these times incoming characters
`are made to dwell a little longer in an
`input character buffer in the data
`transmission unit and the address sup(cid:173)
`plied to the memory is the read address.
`At other times the write address is
`Page 56
`
`PMC Exhibit 2118
`Apple v. PMC
`IPR2016-01520
`Page 4
`
`

`

`Wireless World, May 1977
`
`switched to the memory. The address
`selector als·o notes· the coincidence be(cid:173)
`tween the read address and the write
`address when it delivers a pulse to the
`display L\llit to initiate the generation of
`the cursor display (see Part 3).
`Shown also in Fig. 3 in broken lines,
`are the units required for interfacing
`Viewdata with teletext. In a receiver
`already fitted with a teletext decoder,
`one additional unit is required: the data
`selector (5), while the Viewdata display
`unit may be dispensed· with and the
`teletext displ~y unit (6) used instead.
`The connections required are shown
`also· as broken lines. A· Viewdata/tele(cid:173)
`text switch unit (7) is also shown. This
`sets data and address selectors to
`Viewdata or teletext as required.
`In the teletext mode the address and
`data selectors switch the memory to the
`teletext input circuits, while in the
`-viewdata mode the memory is available
`to Viewdata. The read address,
`however, is now provided by the tele(cid:173)
`text address, which scans the memory
`during the mixed blanking period.
`
`Data transmission unit
`The data transmission unit is shown in
`more detail in Fig. 4. This consists of a
`line isolator (1) and a modem (2, 3,. 4),
`the lastcmentioned including a modula(cid:173)
`tor ( 4) which transforms· the outgoing
`data stream to a voice frequency signal,
`a demodulator (3) which accepts a voice
`frequency signal from \ine and extracts
`the data stream from it, and a control
`circuit (2) which switches the connec(cid:173)
`tion of the telephone line to the tele(cid:173)
`phone receiver or to the modem.
`The transmission control unit (6),
`which is synchronized by the clock unit
`(5), accepts the demodulated data in
`serial form, checks character parity and
`
`)
`offers assembled characters to the con(cid:173)
`trol codes decoder (9). It also triggers
`the operation of the timing unit (10)
`which generates the necessary wave(cid:173)
`forms used throughout the data trans(cid:173)
`mission unit. The control codes decod,er
`recognises the special control charact(cid:173)
`ers used in Viewdata, initiates the cor(cid:173)
`responding control functions and
`enables the memory (8) to store the
`appropriate characters. It also controls
`the memory address unit (11), which
`maintains .a'record of the addresses at
`wtiich incoming characters are to be
`stored and instructs the terminal iden(cid:173)
`tifier (12), to generate the automatic
`identification code in reply to an
`enquiry signal received· from the View(cid:173)
`data computer.
`The transmission control unit, the
`timing unit and the page transmission
`unit (7) together control the transmis(cid:173)
`sion of a complete page from the termi•
`nal to the computer. The keypad unit
`(13) generates and encodes the terminal
`responses and outputs these direct to
`the modem, for transmission to the
`computer.
`The data transmission unit operates
`in two different modes: reception mode
`and transmission mode.
`
`Reception of Viewdata signals
`Isolator and modem. The Viewdata
`signal enters the terminal from the
`telephone line, after passing through
`the isolator. This may consist simply of
`
`Fig. 4. Data transmission unit at
`Viewdata terminal. The number and
`bar on certain connecting lines ·
`·
`indicate that the line is carrying
`parallel information on that number
`of wires.
`
`57
`two pairs of opposite polarity gas
`·discharge· tubes, each pair connecting
`one of the telephone wires to earth. It
`ensures that voltages originating from
`the terminal are limited to safe values
`before entering the telephone. network.
`It also contains fuses, in series with each
`telephone wire and on either side of the
`gas discharge tubes, to limit the current
`flowing. The gas discharge tubes have a
`striking voltage of about 150V, to avoid
`breakdown in the presence of ringing
`tones originating in the telephone line.
`Following the isolator is the modem
`control unit, which contains a relay
`operated by the "data" button on the
`telephone: When this button
`is
`depr~ssed it switches the telephone line
`from the telephone receiver to a hybrid
`transformer .within the control unit.
`This separates the go and return chan(cid:173)
`nels connected to the modulator and
`. demodulator respectively.
`The incoming Viewdata signal is
`superimposed on an f.s:k. (frequency
`shift keying) carrier, binary 1 corre(cid:173)
`sponding to a frequency of 1300Hz and
`binary 0 to a frequency of 2100Hz. The
`incomin{ carrier first goes through two
`stages of bandpass filtering to eliminate
`unwanted signals. After this it is fre(cid:173)
`quency shifted by 10kHz, thus becom(cid:173)
`ing a frequency modulated carrier
`centred on 11.7kHz with a deviation of
`±400Hz, the modulation rate being
`1200 per second. Frequency shifting the
`carrier by 10kHz makes the demodula(cid:173)
`tion process much easier by virtue of
`increasing the number of carrier cycles
`per modulation cycle.
`Theincomihgcarrier is now applied to
`'an unbalanced discriminator and a
`d~tector which extracts the data modu(cid:173)
`lation. After filtering, amplification,
`sq~aring and level changing the data
`
`/~
`
`2-wire
`line
`
`Data
`switch
`
`Isolator
`(1)
`
`Control
`unit
`(2)
`Carrier~
`fail
`
`CF
`
`I
`
`Memory
`(8)
`
`MA
`
`WE
`
`ODH,
`~
`
`IDH
`
`jr-
`'r!
`
`11, ,
`
`Memory
`address
`(11)
`
`~
`' 2
`
`Reset MA
`
`~6
`
`{ '"";";, '0""'
`1MHz clock
`Reset character
`Reset row
`Upfdown
`Disable character
`
`Demodulator ~ IDH
`Transmission
`(3)
`control
`DA
`(6)
`
`Modulator J-+- Clock
`
`(4)
`
`(5)
`
`7~
`
`,
`
`control codes
`decoder
`(9)
`
`-
`
`'4
`
`ENQ
`
`Device
`control
`latches
`
`CLK ~ wr:
`
`TBE
`
`IPE
`
`TBE
`
`3 OEF
`DA
`
`r-------------------
`1 14
`-,
`
`Keyboard
`
`Keyboard
`encoder &
`register
`
`-,--
`
`I
`I Char<;~cter
`L-----------------J
`
`Shift
`
`LED
`
`I
`
`shrft
`
`Timing
`unit
`
`Keypad (13) ·
`
`TXE r-
`
`2'
`
`Load RDA.
`
`'
`
`-
`
`ir Page
`
`transmission
`unit
`(7)
`
`I<(_
`
`Reset MA
`
`MA·
`
`PTE
`WD
`
`'
`--+ Timing unit:
`
`(10)
`
`Code
`
`Tx1 t
`
`Terminal
`identifier
`(12)
`
`~
`
`w""'r;
`
`Page 57
`
`PMC Exhibit 2118
`Apple v. PMC
`IPR2016-01520
`Page 5
`
`

`

`.58
`
`signal is fed out to the transmission
`control unit at a level of -6V for a
`frequency of 1300Hz (binary 1) and
`+6V for a frequency of 2100Hz (binary
`0).
`
`The transmission control unit. The
`transmission control unit accepts data
`in serial form and, using a sampling
`technique controlled by the clock gene(cid:173)
`rator, recognises the start and stop bits
`of each 10-bit character sequence, and
`stores each character in a temporary
`buffer. This completed, it signals the
`event to the timing unit, and control
`codes decoder, i.e. that a character has
`been received and is av~ilable for
`transfer at the input data highway in a
`7 -bit parallel form. ·
`The transmission control unit also
`checks character parity and feeds out
`IPE (input parity error) to the control
`codes decoder if parity is found in error.
`
`The timing 'unit provides a number of'
`waveforms which control the storage of
`characters in the memory. On receipt of
`a "data available" signal from the
`transmission control unit, it transfers
`the intended location of the received
`character from memory address to
`memory, enables memory to accept the
`character, clocks memory address to
`the next character position and resets
`the transmission control unit to indicate
`that the character received has been
`accepted.
`
`The control codes decoder accepts
`incoming characters from the input
`·data highway, decodes the special con(cid:173)
`trol codes and initiates the appropriate
`·actions as follows. The unit is "trans(cid:173)
`parent" to all characters other than
`control codes, the former being ap(cid:173)
`plied direct to the memory to be stored·
`therein.
`. The control codes decoder performs
`the following functions. On receipt of:'
`(a) Non storing characters such as NUL,
`CR, LF, BS, FF, etc. it inhibits their
`storage in memory. (Write disable to
`timing unit.)
`(b) BS, it causes memory address to
`count down one character
`(c) VT, it causes memory address to
`count down one row.
`(d) CR, it causes memory address to be
`reset to character address of zero, leav(cid:173)
`ing row address unchanged.
`(e) LF it causes memory address to
`count up one row.
`(f) FF it causes memory address to be
`reset to character address of zero and
`row address of zero. It also causes the
`complete content of memory to be
`erased by setting the code on the input
`data highway to "space" and entering
`this in the whole memory.
`·
`(g) ESC it causes bit 7 of the received
`character to be changed from 1 to 0,
`before storage.
`(h) DC1 to DC4, it sets latches to control
`internal devices.
`The control codes decoder, when
`receiving input parity error, substitutes
`
`character 7115 for the character re-·
`ceived in error before it is entered in the
`memory. The imp.lementation of
`memory and memory address may be
`either in the form of a random access
`memory or a series of shift registers. A
`r.a.m. appears to lend itself to a rather
`simpler logic circuit than a shift register
`memory and because of this bas been
`assumed in the description of the ter-·
`min a!.
`The memory address consists of
`characters and row counters which are
`controlled by the control codes decoder
`to indicate the address at which tlie
`next character is to be stored in the
`memory.
`
`Transmission of Viewdata signals
`The transmission of Viewdata signals
`originates either from the keypad unit
`or the page transmission unit.
`.
`The keypad unit controls a keyboard
`connected in a cross-matrix of 5
`columns and 9 rows, with a shift button,
`which together with the 45 keys, pro(cid:173)
`Vide a maximum of 90 codes. The basic
`keypad with which most of the View(cid:173)
`da_ta facilities may be used provides only,
`12 codes, (0 to 9), * and #, with
`additional optional codes for automatic
`calling.
`In both cases the output of the key(cid:173)
`board matrix is applied to an encoder
`which generates codes appropriate to
`the keys selected, serializes the bit pat(cid:173)
`tern thus obtained, adds parity, start
`and stop bits and applies the resulting
`data stream directly to the modulator,
`under the control of an internal timing
`unit which generates the appropriate
`clock signals. Characters fed out are not
`displayed on the screen until they have
`been "echoed" backby the computer.
`The page transmission unit operates
`jointly with the transmission control
`unit and timing unit, and its operation is
`initiated manually by a push-button on
`the terminal. This causes the page
`transmission unit to reset memory_
`address zero and enables transmission
`buffer empty (TBE) sigrtal from the
`transmission control unit to start the.
`timing unit (using the page transmis(cid:173)
`sion enables signal). It also inhibits the
`writing into memory, via write disable
`to timing unit.
`On receipt of TBE, the timing unit
`generates a load signal to the transmis(cid:173)
`sion control unit which causes the latter
`·to accept a character from memory. and
`to clock it out in serial form at 75 bits/
`second, complete with start, stop and
`parity bits, to the modulator. The timing
`unit also increases the memory address(cid:173)
`count by one. When a character has
`been discharged from the transmission
`control unit, the next transmission
`buffer empty signal recommences the
`above cycle on the next character.
`When 960 characters have been· sent
`out, the page transmission unit notes
`the fact and resets 'the terminal to the
`quiscent state.
`At the beginning of a Viewdata ses(cid:173)
`sion the computer interrogates the
`
`Wireless World, May 1977
`
`built-in terminal identifier. The control
`codes decoder initiates the operation of
`this unit, which sends out an identifica(cid:173)
`tion code to the transmission control
`unit. This code is transmitted to the
`modulator, complete with start, stop
`and parity bits. The operation is similar.
`to that of the page transmission unit
`except that the identification code is
`stored in the terminal identifier.
`
`Display unit
`The display unit is shown in more·
`detail in Fig 5. The function of the
`display unit is to generate line and
`frame synchronising signal for the tele(cid:173)
`vision raster, to decode the special
`display control characters for colour
`and graphics and to generate alphanu(cid:173)
`meric and graphic symbols for display.
`As mentioned eariier, the display unit
`is nearly identical to the corresponding
`part in the teletext decoder. The major
`differences are in the line and frame
`synchronising generators and in the
`provision for the cursor, which is not
`required in teletext. With respect to the
`line and frame synchronising pulses,
`these are essential in a Viewdata-only
`receiver since it is required that the
`Viewdata service should be available at
`all times and not just during tv broad(cid:173)
`casting hours; thus it is not always
`possible to rely on the presence of tv line
`and frame sync to maintain the raster.
`The provision of line and frame sync
`pulses is also very useful in a combined
`Viewdata/teletext decoder, as indeed in
`a teletext-only decoder, since it is pro(cid:173)
`vided in teletext that viewers should be
`able to store a page of information
`transmitted during tv broadcasting
`hours and to view it later at their con(cid:173)
`venience, possible outside broadcasting
`hours.
`The display unit consists of a sync
`generator and memory scanner (1), a
`display control codes decoder (2), an
`alphanumeric character generator (3), a
`graphics generator ( 4), a character
`rounding unit (5), and an output unit
`(6).
`The sync generator and memory
`scanner generates line and frame
`synchronising pulses which are applied
`to the tv timebase generators, and row
`and character addresses which are
`applied to the r.a.m. via the address
`selector. The unit derives these wave(cid:173)
`forms from an 8MHz crystal controlled
`master oscillator followed by a chain of
`dividers. The extraction of characters
`from the memory and their display on
`the screen occurs at a rate of lMHz,
`which is derived directly from the BMHz
`clock by a divide-by-8 circuit, a further
`division by 64 providing the line
`synchronizing pulses. There is a certain
`amount of flexibility in the choice of
`master oscillator frequency; a lower
`frequency, say 7MHz or 6MHz, giving a
`wider character on the display, while
`not being quite so demanding on the
`width of the video pa~sband. The width
`of individual characters may also be
`altered by adjusting the blank margins.
`
`Page 58
`
`PMC Exhibit 2118
`Apple v. PMC
`IPR2016-01520
`Page 6
`
`

`

`Wireless World, May 1977
`
`59
`
`to the 'left and right of the page on
`display. The choice of 8MHz here is
`mainly of convenience to simplify the
`subsequent dividing circuits. The sync
`generator and memory scanned must
`also generate the mixed blanking
`waveform which provides the margins
`around the display area. Thus every lJlS
`a read signal is applied to the r.a.m.
`which then feeds out the character
`stored at the location indicated by the
`row and character addresses generated
`by the unit.
`The timing of the whole display unit
`must take into account delays occurr(cid:173)
`ing in the r.a.m. and in the alpha(cid:173)
`numeric character generators. These
`delays may be each of the order of 200 to
`600 nanoseconds, depending on cost,
`the faster unit obviously being more
`expensive. Thus in order to take up
`these tolerances· and allow the cheaper
`units to be used, a 211s delay is allowed
`for from the instant a character is
`requested from memory to th.e time it is
`displayed.
`As in teletext, a row· of characters
`consists of 10 television lines in each
`frame (20 lines counting the interlace),
`made up of 7 display lines and 3 spacing
`lines, each character space in the
`horizontal direction consisting of 8 dots,
`5 display dots and 3 space dots, the dots
`occurring at the 8MHz rate.
`As each character is fed out from the
`memory it is transferred to the display
`control codes decoder which is pro(cid:173)
`grammed to recognise the characters in
`columns 0 and 1 of Fig 7 in the April
`issue, i.e. the special colour, graphics
`and other display control characters;
`provide blanking for the duration of
`these characters (since these are non(cid:173)
`display characters); and inhibit the
`character generator or graphics gener(cid:173)
`ators as appropriate.
`At the beginning of every row of
`characters all the latches are set to
`white, alphanumeric, steady according
`to the teletext convention. The output
`
`Fig. 5. Display unit at Viewdata
`terminaL The number and bar on
`certain cm:mecting lines indicate that
`the line is carrying parallel information
`on that number of wires. Some
`commercial Viewdata tv receivers may
`have clock frequencies other than 8
`MHz.
`
`of the decoder is applied the output unit
`which provides R, G, B signals to the
`guns of the cathode-ray tube.
`Non-control codes are applied to the
`alphanumeric character generator
`which generates the required character
`pattern. This generator also receives a
`, 4-bit line address from the sync _genera(cid:173)
`tor, which indicates which line out of
`the ten lines required for character
`display has been selected at any one
`time. When a line of dots is fed out from
`the character generator it is entered in:
`5-bit parallel form· in a 5-stage shift
`register and clocked out in the next l11s
`period at the 8MHz rate, under the
`control of the 8MHz clock.
`If a graphics control character is
`displayed, a latch is set in the display
`control codes decoder to indicate that
`all subsequent characters are graphics.
`The inhibition is lifted, however, in the
`case of the "blast-through" characters
`in columns 4 and 5 of Fig. 6 in the April
`issue.
`Generation of graphic symbols is car(cid:173)
`ried out under the control of vertical
`and horizontal bright-up waveforms,
`generated in the graphics generator.
`The horizonal bright-up waveform
`picks up left, right or both coloumns of
`the graphics symbol while the vertical
`bright-up waveform picks up one or
`more of the top, middle or bottom pair
`of squares in the graphics symbols. The
`7 -bit graphic character is decoded with
`the aid of these two waveforms. and
`control signals applied to the output
`unit.
`
`The display of the Viewdata cursor is
`initiated by the address selector, which
`notes the coincidence of input and out-
`· put memory addresses and enables an
`exclusive-OR gate in the output unit.
`This causes normal display of charact(cid:173)
`ers when the cursor is off, but inverted
`display (i.e. black on white) when the
`cursor is on. Thus characters on display
`may be read through the cursor.
`Character rounding is provided in the
`character rounding unit when this feat(cid:173)
`ure is required, i.e. mostly with Jarge
`screen displays. Character rounding is
`initiated by the odd/even signal gene(cid:173)
`rated together with the line interlace
`pulse in the ·sync generator unit. A
`.second alphanumeric character genera(cid:173)
`tor unit similar to unit (3) may be
`required, both units operating simulta(cid:173)
`neously out of step by one llne of the 7
`x 5 character matrix. The two outputs,
`one delayed with respect to the other,
`are compared in the character rqunding
`unit and additional dot pulses generated
`half way in the 8MHz dot interval and
`transmitted to the output unit to give
`the required result.
`The use of character rounding is not.
`necessary in the case of the small-size
`Viewdataphone display for ·use in the
`office, and this results in a useful sim(cid:173)
`plification.
`(To be continued)
`
`A limited number of commercial tele(cid:173)
`vision sets containing Viewdata/tele(cid:173)
`text decoders are now being manufac(cid:173)
`tured for marketing trials of Viewdata
`due to start in. March 1978. In a later
`issue we hope to publish an article
`outlining the main features of a typical
`commercial