[19]
`United States Patent
`4,538,174
`[11] Patent Number:
`Aug. 27, 1985
`Gargini et al.
`[45] Date of Patent:
`
`bution Systems”, British Telecommunications Engineer-
`ing, vol. 1, Jan. 1933, pp. 205-210.
`Mesiya et al, "Mini—l-Iub Addressable Distribution Sys-
`tem for Hi—Rise Application,“ First Annual Phoenix
`Conference on Computers and Communications. Phoe-
`nix Az.,-U.S.A.. (9-12 May 1932), IEEE, pp. 346-35].
`
`Primary Examine-—I(eith E. George
`Attorney, Agent. or F3r'rm—Lau1‘ence R. Brown
`
`[57]
`
`ABSTRACT
`
`A communications system comprising at least one ex-
`change,
`television signal sources located at
`the ex-
`change, subscriber's units each connected to the ex-
`change by a respective direct communications link and
`provided with a television signal receiver. and selectors
`located at the exchange for selectively connecting the
`television signal sources to the communications links.
`The subscriber's units have transmitters for applying
`data signals to their respective communications links for
`transmission to the exchange to control the selectors
`and for transmitting general data to the exchange. The
`subscriber’s units may each comprise two subscriber's
`stations each of which has a respective television re-
`ceiver and data signal
`transmitter. Each subscriber's
`station is associated with a respective exchange—located
`selector controlled by that subscriber's station transmit-
`ter. Signals transmitted to the subscriber's stations are
`freqnency division multiplexed, each subscriber’s sta-
`tion receiver being tuned to receive a respective one of
`the FDM signals. Signals transmitted to the exchange
`are time division multiplexed under the control of a
`timing signal transmitted from the exchange, the ex-
`change being responsive to the timing signal to identify
`the source of data signals received from each subscrib-
`er’s unit.
`
`19 Claims, 12 Drawing Figures
`
`[54] TWO-WAY SUBSCRIBER rv SYSTEM WITH
`MULTIPLE SUBSCRIBER’S stats
`
`[75]
`
`Inventors: Eric J. Gargini, West Drayton;
`Henry L. Baker, Northwood, both of
`England
`
`[73] Assignee:
`
`Communications Patents Limited,
`London, England
`
`[221 Appl. Nc.: 494,933
`
`[22] Filed:
`
`May 16, I983
`
`Foreign Application Priority Data
`[30]
`Mar. 1]. 1932 [GB] United Kingdom ............... .. 8306'.-'25
`May 14, 1982 [GB] United Kingdom
`8214159
`
`Int. CL3 ............................................. .. H04N 7/10
`[51]
`{52] U.S. C1. .................................... .. 358/86; 358/147;
`455/4; 455/5
`{S8} Field of Search ............. .. 358/34. 36, 147; 455/2,
`455/3, 4. 5, 6; 179/2 TV, 2 T5
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`455./6 X
`l/1975 Jannery et al.
`3,859,596
`358/84
`3.997.718 12/1976 Ricketts
`353/I-fl‘
`4,205,343
`5/1980 Barrett ..
`358/86
`4,302,771 11/ 1981 Gargini .................. ..
`353/1&1’?
`4,390,900 6/1983 Van Kampen ct al.
`.
`455/S X
`4,395,750 7/1983 Gohm ct al.
`........... ..
`4,450,‘?-T7
`S/1934 Lovett ................................. .. 358/36
`
`
`
`FOREIGN PATENT DOCUMENTS
`
`8/1982 PCT Int'lApp1. ................. .. 358/86
`W082/02639
`2063026 S/2981 United Kingdom .
`
`OTHER PUBLICATIONS
`
`W. G. Simpson, “Broadband Cable Systems”, vol. 3,
`Part 1, of British Telecommunications Engineering, pp.
`6-13, Apr. 1984.
`W. K. Ritchie, “Multi—Service Cable—Television Distri-
`
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`U.S. Patent Aug. 27,1935
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`U.S. Patent Aug. 27, 1985
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`U.S. Patent Aug. 27, 1935
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`U.S. Patent Aug.27, 1935
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`Sheetl0ofl0 4,538,174
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`
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`

`
`1
`
`4,538,174
`
`TWO-WAY SUBSCRIBER TV SYSTEM WITH
`MULTIPLE SUBSCRIBEITS SETS
`
`The present invention relates to a communications
`system in which subscribers to the system can both
`receive signals from and return signals to the system via
`a cable carrying either electrical or optical signals.
`BACKGROUND ART
`
`Communications systems of the above type devel-
`oped from the well known wired broadcasting systems
`in which television and sound signals are distributed to
`subscribers. Known wired broadcasting systems have
`generally followed one of two basic design approaches,
`that is the braodband approach or the star network
`approach.
`In broadband systems all signals which are to be
`available to subscribers are broadcast simultaneously on
`a coaxial cable and signal selection is effected by tuning
`of the subscribers receiver. Thus the broadcast signals
`are frequency division multiplexed. This approach has
`the advantage that the cable network is relatively sim-
`ple but the disadvantage that the number of different
`signals which can be transmitted is limited by the avail-
`able bandwidth. Furthermore there is now a demand
`not only for an increased number of different signals but
`also for two way communication, i.e. the capacity for
`subscribers to transmit data to the system, not merely to
`receive signals from it. Broadband systems which can
`provide the subscribers with the facility of transmitting
`signals and which can identify the subscriber source of
`transmitted signals are complex, particularly when the
`number of subscribers is large. Generally some form of
`time division multiplexing has to be employed to man-
`age the subscriber transmitted signals.
`In star network systems each subscriber is connected
`by his own cable to a signal distribution centre com-
`monly referred to as an exchange. At the exchange are
`located selectors controlled by the subscribers for con-
`necting the subscriber cables to desired signal sources
`all operating at the same signal frequency. There is thus
`no theoretical limit on the number of different signals
`which can be broadcast to the subscribers even though
`each subscriber has only one cable extending to his
`premises. Furthermore the source of data signals trans-
`mitted by a subscriber is identified by the cable on
`which the data signals appear. Thus star networks are
`inherently more suitable for use in high signal capacity
`two way systems. The basic cable network is however
`more complex as each subscriber must be linked di-
`rectly by his own cable to the exchange. In practice in
`all but small systems a series of exchanges are provided
`on a trunk cable network, each exchange serving a
`localized group of subscribers.
`It is now frequently necessary to be able to provide a
`single subscriber with more than one outlet on his prem-
`ises. and it is generally thought that a capacity to serve
`three outlets per subscriber is required when the cable
`for a new system is to be installed. In a broadband sys-
`tem this does not raise any real problems. providing that
`the problems inherent in high capacity two way braod-
`band systems can be overcome. In simple star network
`systems however it would be necessary to provide each
`subscriber with three cables in case he eventually de-"
`cided to use three outlets. The expense involved would
`be prohibitive. It will be appreciated that if subscribers
`were initially provided with only one cable it would not
`
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`be economically feasible to subsequently install further
`cables as subscriber demand required.
`Furthermore. in large systems comprising many ex-
`changes the volume of data to be transmitted from the
`exchanges to the head end is considerable. Although the
`source of data signals reaching the exchanges from
`subscriber stations is identified by the cable on which
`the signals appear it is nevertheless necessary to “label"
`the source of the signals when the data is transferred to
`the head end. If this is achieved by associating an ad-
`dress code with each time of data to be transmitted to
`the head end the data handling capacity of the system is
`strained further.
`
`It is an object of the present invention to provide a
`communications system in which the source of sub-
`scriber generated signals can be readily determined.
`DISCLOSURE OF THE INVENTION
`
`According to the present invention there is provided
`a communications system comprising at least one ex-
`change, a plurality of television signal sources making
`signals available at the exchange, a plurality of sub-
`scriber units each connected to the exchange by a re-
`spective direct communications link and each provided
`with a television signal receiver. Selectors are located at
`the exchange for selectively connecting the television
`signal sources to the communications links, the sub-
`scriber units having transmitters for applying data sig-
`nals to the respective communications link for transmis-
`sion to the exchange to control
`the selectors. Each
`subscriber unit is associated with a respective exchange
`located selector controlled by that subscriber units‘
`transmitter, the selector associated with said at least one
`subscriber unit applying television signals to the respec-
`tive communications link at a frequency to which the
`subscriber units receiver is ttined. The subscriber unit
`transmitter being responsive to a timing signal transmit-
`ted on the communications link from the exchange to
`apply data signals to the respective communications
`link, the exchange comprising means responsive to said
`timing signal to identify the source of data signals re-
`ceived from the subscriber units.
`Preferably, at least one of the subscriber units com-
`prises two subscriber stations each of which has a re-
`spective television receiver and data signal transmitter,
`and each subscriber station is associated with a respec-
`tive exchange located selector controlled by that sub-
`scriber stations transmitter.
`the selectors associated
`with the subscriber stations of said at least one sub-
`scriber unit playing frequency division multiplexed
`television signals to the respective communications link.
`and each subscriber station receiver being tuned to
`receive a respective one of the television signals applied
`to the communications link. The subscriber station
`transmitters apply time division multiplexed data signals
`to the communications link so that the source of the
`data signals can be determined.
`Thus the present invention retains the inherent ad-
`vantages of star networks for two-way communication
`systems and yet provides high capacity without over-
`loading the available bandwidth or dramatically in-
`creasing system complexity.
`Preferably, the exchange comprises a timing signal
`transmitter providing an AC timing signal periodically
`clamped to each of a plurality of DC voltages in a pre-
`determined sequence. Each subscriber station of the
`subscriber unit comprises a detector for detecting a
`respective one of the plurality of DC voltages. and data
`
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`4
`FIG. 4 illustrates time-related waveforms appearing
`in the equipment of FIGS. 2 and 3;
`FIG. 5 illustrates a data signal waveform received at
`the exchange equipment of FIG. 2;
`FIG. 6 schematically illustrates data signal stores
`‘located at the exchange;
`FIG. 7 is a schematic diagram of a large scale system
`according to the invention;
`FIG. 8 is a schematic diagram of a data concentrator
`and a switching center serving one home in the system
`of FIG. 7;
`FIG. 9 is a detailed diagram of data storage equip-
`ment provided for one subscriber unit in the switching
`center;
`FIGS. 10 and 11 respectively illustrate a transmitter
`and a receiver for communicating data between the data
`concentrator and switching centers;
`and
`
`FIG. 12 schematically illustrates a system of the type
`described above incorporating further teletext features.
`THE PREFERRED EMBODIMENT
`
`Referring to FIG. I, the basic lay-out of a thirty-
`channel
`television broadcasting system Operating in
`accordance with 525 line NTSC standards is diagram-
`matically illustrated. The system also enables data to be
`transmitted from the subscribers to a central station or
`head-end. All television signals which are to be broad-
`cast over the system are provided by signal sources 1
`located at the central station. The television signals may
`for example be locally generated. derived from record-
`ings. or derived from radiated transmissions. Each tele-
`vision signal is applied via a respective one of thirty
`cables 2 to a mixer and combiner 3. The signals on the
`first five channels are mixed with respective ones of five
`fixed frequency signals provided by local oscillators 4
`and the five resultant signals are combined and applied
`to a coaxial cable 5. The remaining twenty five signals
`are divided into five groups of live, mixed and com-
`bined and applied to coaxial cables 6 to 10 respectively
`in the same manner as with the first five channels. Thus
`channels one to live appear on cable 5, channels six to
`ten on cable 6, channels eleven to fifteen on cable 7,
`channels sixteen to twenty on cable 8. channels twenty
`one to twenty five on cable 9. and channels twenty six
`to thirty on cable 10.
`The five program signals carried by each of cables 5
`to 10 have frequencies such that no second or third
`order spurious signals are produced. i.e. the live signals
`can be allotted to frequency channels having luma fre-
`quencies of 69.2. 15.2. 93.2.
`l23.2 and 135.2 MHz re-
`spectively.
`The six cables 5 to 100 are connected to a trunk cable
`which links the head end and each ofa series of identical
`exchanges 11 only one of which will be described in
`detail. Each exchange 11 supplies signals to a respective
`group of subscriber units 12. There may be for example
`several hundred subscriber units 12 connected to each
`exchange, and each subscriber unit may have up to
`three subscriber stations 13, each subscriber station
`comprising a television signal receiver 14 and a key pad
`15.
`_
`The exchange comprises an array of group selectors
`16 each controllable from a respective subscriber station
`13 to select the group ofchannels appearing on any one
`ofcables 5 to 10. and an array ofsingle channel selectors
`17 each controllable from a respective subscriber station
`to select one ofthe channels from the group ofchanncls
`
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`3
`signals are transmitted to the exchange at a predeter-
`mined time related to the timing of the detected DC
`voltage, i.e. immediately thereafter. At the exchange
`the received data signals and the timing signal are corre-
`lated to identify the source of the received data signals.
`Each subscriber station may be powered by the AC
`component of the timing signal. If the subscriber unit
`comprises three subscriber stations the timing signal
`may be periodically clamped to each of three DC volt-
`ages in turn.
`Preferably, where a plurality of exchanges are pro-
`vided each connected to a head end installation by a
`trunk communications network, each exchange com-
`prises a store in respect of each subscriber station in
`which data transmitted to the exchange from that sub-
`scriber station is stored. Also means are provided for
`sequentially addressing the stores to read out the stored
`data onto the communications link in a time division
`multiplexed data stream. the source of the read out data
`being identified by its position in the data stream.
`The exchanges may be connected in groups to a plu-
`rality of data concentrators that are in turn connected to
`the head end installation, each data concentrator com-
`prising means for storing data received from the head
`end to indicate which television channels are to be ac-
`cessible to individual subscriber stations, means for
`storing data received from the exchanges indicative of
`television channels selected by individual subscriber
`stations, means for comparing the stored data, and
`means for transmitting signals to the exchanges to con-
`trol the subscriber stations selectors to prevent access to
`television channels to which access is to be prevented.
`The invention also provides a communications sys-
`tem comprising a plurality of subscriber units each con-
`nected to a communications network capable of deliv-
`ery any one of a plurality of different signals on respec-
`tive television channels. wherein at least one subscriber
`unit is provided with a teletext receiver. Means are
`provided for supplying téletext signals on one television
`. channel of the system, and said at least one subscriber
`unit is allocated exclusive use of one teletext sub-page,
`the content of the said one sub-page being responsive
`only to data signals supplied by the said at least one
`subscriber unit.
`In one arrangement. a first television channel is dedi-
`cated to full field teletext signals, and a second channel
`is dedicated to provide interactive services, means being
`provided such that if the said one subscriber unit trans-
`mits data over the system in response to information on
`the second channel a system response is transmitted on
`the sub-page of teletext dedicated to that subscriber
`unit. The subscriber units selector at the exchange are
`automatically controlled to momentarily connect the
`teletext receiver to the said first channel such that the
`data transmitted on the dedicated sub-page is acquired
`and inlaid on the picture received from the said second
`channel.
`
`BRIEF DESCRIPTION OF THE DRAWING
`
`Embodiments of the present invention will now be
`described, by way of example, with reference to the
`accompanying drawings. in which:
`FIG. 1 is a schematic diagram of major portions of a
`system in accordance with the present invention;
`FIGS. 2 and 3 respectively show exchange located
`and subscriber station located equipment of the embodi-
`ment of FIG. 1;
`
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`selected by the associated group selector 16. The key
`pad 15 of each subscriber station may be manipulated to
`transmit data signals to the exchange. The data signals
`may be used to control the selectors. or for any other
`purpose. eg. to order goods or to register a response to
`a question transmitted to the subscriber.
`The oscillators, signal mixing and combining circuits
`of the head-end operate in a conventional manner Fami-
`lar to persons skilled in the field of wired broadcasting.
`Published British Patent Specification No. 2 063 026
`describes a system which is similar in many respects to
`the system of FIG. 1 although in the system described in
`detail in Specification No. 2 063 026 each subscriber
`unit has only one subscriber station. Thus although in
`the system described in detail in Specification No. 2 063
`026 only one television signal is transmitted at any one
`time to a particular subscriber unit and there is only one
`possible source of data signals received from a sub-
`scriber unit. In the system of FIG. 1 up to three differ-
`ent television signals can be transmitted simultaneously
`to each subscriber unit and the particular subscriber
`station sending data signals from a subscriber unit to the
`exchange must be determined. As a result in FIG. 1
`each subscriber station is tuned to a respective one of 2
`three television signal frequencies and television signals
`selected by a subscriber station must be transmitted at
`the appropriate frequency. Furthermore the data signals
`transmitted to the exchange are allotted to different
`time slots depending on their source. The basic televi-
`sion signal selection equipment, that is group selectors
`16 and single channel selectors 17,
`is similar to that
`described in Specification No. 2 063 026 however and
`therefore further details of this basic equipment are not
`given herein.
`Each key pad 14 is provided with buttons labelled 0
`to 9 and two further buttons labelled “star" and "cross-
`hatch". When a subscriber station is to transmit televi-
`sion signal selection data signals to the exchange, the
`user presses the star button and then the buttons identi-
`fying a channel from which it is desired to receive a
`television signal. When a subscriber station is to trans-
`mit to the exchange data signals not related to television
`signal selection, the user presses the crosshatch button
`and then the buttons identifying the data which it is
`desired to send to the exchange. The data signals are
`transmitted to the exchange in time slots allotted to each
`subscriber station as described in detail below and the
`transmitted signals are stored in a data store 18. All data
`signals are routed to a central data store 19 at the head
`end via cable 20 and television selection data signals are
`routed from the store 18 to the selectors 16, 17.
`is
`Referring now to FIG. 2, exchange equipment
`shown which is provided at the output of the single
`channel selectors 1'.-' for connecting the three selectors
`of each subscriber unit to the single coaxial cable ex-
`tending to the subscriber unit. The single ‘T5 ohm coax-
`ial cable 2l carries signals in both directions to and from
`a combiner and data filter 22. The combiner and filter 22
`receives television signals from a combiner 23 receiving
`on inputs 23a. 23b and 23¢‘ the three outputs of the sub-
`scriber units selectors, and also VHF Band II sound
`signals applied to line 24, and a polling signal applied to
`line 25. Data and television selection signals returned
`from the subscriber unit are delivered to a switching-
`network 26 controlled by signals applied to lines 27, 28
`and 29, the output of the switching network being ap-
`plied to the combiner 23 from which it is routed to the
`
`6
`data store 18 (FIG. 1) via the inputs 23a. 23b and 23c to
`the combiner 23 and low pass filters (not shown).
`The signals applied to the lines 25, 27, 28 and 29 are
`illustrated in FIGS. 4A. 4B, 4C and 4-D respectively.
`All of these signals are generated at the exchange or
`routed to each of the exchanges from the head end. The
`polling signal of FIG. 4A is an AC signal centered on
`+6 volts and having a peak-to-peak amplitude of at
`least 12 volts. At preset intervals of for example a few
`tens of milliseconds the AC signal is clamped to either
`zero volts, + I2 volts or +6 volts, the clamping volt-
`ages running in the sequence 12, 6, 0; 12. 6. 0; etc. On
`each occasion that the voltage is clamped to zero volts,
`a negative going pulse appears on line 27 (FIG. 433); on
`each occasion that the voltage is clamped to 12 volts a
`negative going pulse appears on line 28 (FIG. 4C); and
`on each occasion that the voltage is clamped to 6 volts
`a negative going pulse appears on line 29 (FIG. 4D).
`The switching network 26 operates such that data sig-
`nals received from the combiner and filter 22 are ap-
`plied to a respective one of outputs 3!}, 31 and 32, the
`output 30 being selected for the period t] between con-
`secutive pulses on lines 27 and 28, the output 31 being
`selected for the period t; between consecutive pulses on
`lines 28 and 29, and the output 32 being selected for the
`period t3 between consecutive pulses on lines 29 and 27.
`Thus data signals received during time slots ti, I2 and t3
`are switched to respective outputs 30, 31 and 32.
`FIG. 3 schematically illustrates equipment provided
`at each subscriber station. up to three sets of such equip-
`ment being provided for each subscriber unit. The
`equipment surrounded by dotted line 33 is a conven-
`tional keyboard arrangement which will not be de-
`scribed in detail as such arrangements are well known.
`The keyboard has twelve keys, that is 0 to 9, star and
`crosshatch. Depression of any one key closes one or
`more of the four switches 34 to form a four bit binary
`coded decimal signal unique to that key. This signal is
`introduced via a parallel to serial converter 35 to a shift
`register 36. After receipt of a reset pulse on input 37 the
`b.c.d signal is read out serially in response to the output
`of a clock circuit 38. The b.c.d signal controls output
`transistors 39 and 40 so that a waveform appears at
`point 41 which is normally at +6 volts but is clamped to
`+12 volts (logic “l”) or zero volts (logic “-3"} for the
`duration of each bit of the b.c.d signal.
`The polling signal (FIG. 4A) is received over the
`single cable connecting the three subscriber stations to
`the exchange and appears on line 42. On each occasion
`that the AC component of the polling signal disappears
`a negative going strobe pulse appears at point 43. The
`strobe pulse is inverted, delayed, amplified and then
`passed through a differentiator 44 to deliver a narrow
`positive going pulse (logic “l") to an AND gate 45. In
`addition the AC component of the polling signal is
`filtered out by CR network 46 such that the waveform
`of FIG. 4B appears at point 47, that is the voltage at
`point 47 is normally +6 volts but rises to +12 volts and
`falls to zero volts when the polling signal is clamped to
`those voltages.
`A link is connected in one of three possible positions
`4-8, 49 or 50. When the link is in position 48, 49 and 50
`respectively a positive going pulse (logic “l”) is applied
`to gate 45 when the signal of FIG. 4E is at +12 volts,
`+6 volts and zero volts respectively. A reset signal is
`generated by gate 45 only when it receives positive
`pulses on both its inputs. Each of the three keypads of
`each subscriber unit has the link in a respective one of
`
`PMC Exhibit 2058
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`Apple v. PMC
`
`|PR2016-00755
` Page 14
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`PMC Exhibit 2058
`Apple v. PMC
`IPR2016-00755
`Page 14
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`

`
`4,538,174
`
`If the subscriber wishes to transmit to the exchange
`data signals which are not related to television signal
`selection, he first presses "crosshatch" and then the
`buttons corresponding to the data he wishes to transmit.
`Each b.c.d signal is stored in turn in the first store 55.
`the stores 59 and 60 being unaltered as the store 55
`never contains the code corresponding to “star" which
`the gate 57 is arranged to detect.
`The read out of b.c.d data from the stores 55. 59 and
`60 is controlled by dividers 61. 62 and 63 driven by a
`common oscillator. Each divider is set to read out the
`content of its associated store at a different time such
`that the contents of the stores 55. 59 and 60 are read out
`sequentially onto a common bus bar 64. It will be appre-
`ciated that each subscriber unit has three sets of three
`associated dividers and that all the dividers of equip-
`ment connected to the common bus bar must read out at
`different times.
`The signals appearing on the bus bar 64 are routed to
`the appropriate data stores and television selection de-
`vices, the output of store 59 controlling the group selec-
`tor I6 of FIG. 1 and the output of store 60 controlling
`the single channel selector 1‘? of FIG. 1. As each signal
`on the bus bar is alloted to its own time slot as deter-
`mined by the common oscillator the distribution of 65
`these signals to the appropriate destination whether
`within the local exchange or to the head end is a rela-
`tively simple matter.
`
`50
`
`S5
`
`60
`
`7
`the three possible positions so that the keypads transmit
`their b.c.d signals in different time slots in response to
`the apearance of a reset signal at point 37. At the ex-
`change (FIG. 2) the b.c.d signal superimposed upon the
`AC waveform is filtered to provide a waveform at point
`51 such as that shown for the key number 3 in FIG. 5.
`The signal is a self clocking return to zero signal and is
`routed to an appropriate store by the switching network
`26 in dependence upon the time slot during which it
`arrives.
`It is obviously necessary to supply power to each
`keypad and this is achieved by rectifying and filtering
`the AC component of the polling signal to give a + l2
`volts source voltage at point 52 (FIG. 3) from which
`point the rest of the keypad is energized by appropriate
`connections (not shown). Thus the signalling system is
`not dependant upon the subscriber unit being locally
`supplied with power.
`Referring now to FIG. 6. a circuit suitable for use in
`the exchange data sorce 18 (FIG. 1) is schematically
`illustrated. The illustrated circuit represents the equip-
`ment required to process data signals from one sub-
`scriber station and therefore three sets of such equip-
`ment would be required for each subscriber unit having
`three subscriber stations.
`Data signals are received from the combiner 23 (FIG.
`2) at terminal 53 and the positive and negative pulses
`(relative to +6 volts) of the b.c.d signal are detected by
`the pair of differential amplifiers 54-. The first four bits
`of a received data signal, i.e. the first b.c.d signal keyed
`in. is delivered to a first store 55 via gate 56. If the store
`b.c.d signal corresponds to the “star” button on the
`respective keypad, in this case 14 (1110). the output of
`an AND gate 5‘? goes to logic ‘‘I’‘. As a result gate 56 is
`closed and gate 58 is opened. Two ‘subsequent b.c.d
`signals can then be written into second and third stores
`59 and 60. Thus by passing the star button and then two
`buttons to identify the desired channel the subscriber
`automatically enters data signals identifying the desired
`channel number in stores 59 and 60.
`
`5
`
`l0
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`8
`If the three star and three crosshatch buttons of each
`subscriber unit are given respective identifying codes.
`e.g. 10 to 15, monitoring of these identifying codes
`would detect any faults which resulted in confusion
`between the signals emanating from particular sub»
`scriber units. A high degree of security between sub-
`scriber stations is thus obtained.
`The contents of the stores 55, 59 and 60 are read out
`cyclically, readout during an update of any store being
`inhibited by gate 65. Thus it is not necessary for there to
`be any particular relationship between the keypad pol-
`ling times and the readout times.
`The AC component of the polling signal (FIG. 4A)
`may be for example square waves or sinusoidal and can
`itselfby frequency modulated at the exchange to enable
`information in addition to television signals to be di-
`rected to individual subscriber stations. For example,
`the polling signal could be modulated with a binary
`code unique to a particular subscriber station followed
`by a message code, the subscriber station being respon-
`sive only to message codes preceded by its own unique
`code. This facility enables the system to be used to
`operate equipment
`in the subscribers premises. e.g.
`VCR’s, meter reading devices, storage heaters or the
`like. or for example to confirm receipt of a keypad
`message ordering goods.
`In the description of FIG. 1 abo