WO1986006238A1

WO1986006238A1 - Text receiver

Info

Publication number: WO1986006238A1
Application number: PCT/GB1986/000213
Authority: WO
Inventors: John Alan Eggleden; Clive William Warder Read; Graham Paul Pink; James Digby Collier; Carl John Orsborn; Jonathan Richard Robert Kimmitt; Julian Malcolm Coles
Original assignee: Telebeam International Limited
Priority date: 1985-04-16
Filing date: 1986-04-15
Publication date: 1986-10-23
Also published as: EP0217906A1; GB2184925A; GB2184925B; AU5694986A; KR880700377A; GB8622041D0

Abstract

A text receiver, particularly but not exclusively a teletext receiver, is capable of displaying a full page of information while having dimensions permitting it to be held in a user's hand. The receiver includes: (i) signal-receiving means including aerial means arranged to receive signals representing text information, (ii) signal-processing means arranged to process signals from the signal-receiving means and to provide information signals for display, (iii) a display device (2) capable of displaying a page of information, and, (iv) a keyboard for selecting the page of information for display. The display device includes a matrix of picture elements arranged to display characters as sub-matrices. The device is capable of displaying a full range of alpha-numeric characters with all or substantially all of the characters being depicted by a 5 x 5 matrix of picture elements. The receiver may include storage means for storing the number of a page of information that has just been displayed and control means operable by a user to cause the receiver system to display again that page of information.

Description

Text Receiver

The present invention relates to a text receiver and is particularly but not exclusively concerned with a teletext receiver for use in receiving teletext services such as those currently available in the United Kingdom and known under the trademarks CEEFAX and ORACLE.

One form of teletext information is transmitted during the field flyback periods of broadcast television transmissions. Since the introduction of this teletext service in the United Kingdom, there have been available television receivers which, in addition to being able to receive and display ordinary television pictures, have been able to select the teletext information from the television transmission, decode it and display it. At the present time, it is very common to find such tele¬ vision receivers, situated either in an office or in a private home.

It is envisaged in British patent specification No. 1 582 102 that it might be possible to provide a teletext receiver corresponding in size approximately to that of a pocket calculator. Such a receiver could have an alphanumeric display panel with data being displayed in accordance with the "newscaster or moving-screen principle". The specification provides no information as to how such a device might be made to work. It is an object of the invention to provide a text receiver that is more versatile than existing receivers.

In accordance with the present invention, a text receiver includes:-

(i) signal-receiving means including aerial means arranged to receive signals representing text information,

(ii) signal-processing means arranged to process signals from the signal-receiving means and to provide information signals for display,

(iii) a display device capable of^" displaying a page of information, and,

(iv) a keyboard for selecting the page of information for display, characterised in that the receiver has dimensions permitting it to be held in a hand.

A text receiver that can be hand-held and is therefore truly portable yet is able to display a page of information provides a solution to various deficien¬ cies of existing arrangements that have not previously been fully appreciated. For a travelling businessman, an important part of the teletext information, for example financial information, is of value only if it is up-to-date and therefore a fixed- location receiver has drawbacks. Similarly, in a domestic situation, it is a draw-back to have to be at home and, in many cases, in a particular room of the home in order to have access to certain of the infor¬ mation. The value of all kinds of news information, especially travel information, which is continually changing, is limited because it is only accessible at a fixed location. The present invention, by providing a text receiver that is truly portable, overcomes the deficiencies noted above of existing receivers and ^• thereby greatly increases the potential value of the teletext service.

Even having recognised the desirability of pro¬ viding a portable text receiver, it would not be apparent to the skilled person, without the benefit of - the teaching in the present specifcation, how to provide a commercially attractive text receiver capable of displaying a full page of text information and yet small enough to be hand-held. It is notable that British patent specification No. 1 582 102 does not attempt to provide a solution to this problem and apparently proposes a receiver in which only a single line of information can be viewed at any one time. The descrip¬ tion below with reference to the drawings, however, shows how a portable receiver capable of displaying a full page of information can be made. While it is an essential feature of the invention that the^" dimensions of the receiver are such that it can be held in a user's hand and is truly portable, it is not essential that the receiver actually be used in this way. For example it might be desirable to have a receiver according to the invention permanently installed in a motor vehicle.

In the case where the receiver is a teletext receiver and is arranged to receive a combined picture and teletext signal, it is preferred for the receiver to be a dedicated teletext receiver and not to provide any facility in the receiver for.displaying a picture derived from the picture signal. The display device preferably displays only information derived from the teletext signal.

Preferably, the receiver has an overall volume of less than 2000 cm³ and is therefore truly portable. A convenient shape for the receiver is a substantially cuboidal shape with one dimension of the cuboid (the depth) being less than 50 mm. This again enhances the portability of the receiver enabling it to be carried on a person's body without protruding too far. Preferably, the length of the cuboid is less than 300 mm and the width is less than 160 mm. Conveniently, the keyboard and the display device are provided on the same face of the cuboid; this makes it easy for an operator to look at the keyboard and the display device at the same time or at least glance from one to the other very quickly.

The receiver preferably further includes its own power supply means, for example battery means, to enable it to operate without connection to an external power supply. This allows the receiver to be operated in remote places away from a power supply. Preferably, the receiver further includes an arrangement for indicating

the state of a battery or batteries in the receiver so that a user is warned in advance of the need to replace the batteries. Preferably, the display device that displays the text information is arranged to display an indication of the state of the battery or batteries; this avoids the need to provide a separate display which would take up further space.

In the case where the receiver is a teletext ^• receiver, the power supply for at least a part of the signal-receiving means is preferably disconnected during periods corresponding to reception of signals carrying picture information. In the case of a receiver having its own power supply means, power consumption is important and the power consumed by the signal-receiving means significant. Because only short portions of the signal received by the device have to be decoded an important power saving can be achieved by disconnecting the power supply to at least part of the signal- receiving means at other times. The signal-receiving means preferably includes means arranged to effect storage of selected signal values during disconnection of the power supply. More particularly the signal-receiving means may include means arranged to effect storage of gain controlling signal values during disconnection of the power supply. A signal value that may be stored is one indicative of the strength of the signal to which the signal receiving means is tuned. The signal-receiving means may include means arranged to maintain decoupling capacitors in a charged state during disconnection of the power supply. Arrangements of the kind described above enable the operation of other parts of the receiver to be main¬ tained during disconnection of the power supply from at least part of the signal-receiving means. The receiver may have an active mode during which it is responsive to a predetermined signal received by the signal-receiving means and a dormant mode during . which the power supply for at least a part of the signal- receiving means is disconnected, the receiver being arranged to alternate between the active and dormant modes while the predetermined signal is not being detected by the signal-receiving means. Preferably the receiver is arranged to spend substantially more time, in one example to be described approximately ten times more time, in the dormant mode than in the active mode during alternation between the active and dormant modes. Such an arrangement reduces the power consumption of the receiver while searching for a channel of sufficient signal strength, while not unduly impeding the effectiveness of the search. The text receiver requires electrical energy at a plurality of voltage levels, and, since operation from low voltage energy sources is envisaged, voltage level conversion means may be provided. The voltage level conversion means is arranged, preferably, to include current-level setting switch means for drawing discrete packets of energy from a low voltage energy source and boosting the voltage level of each energy packet by holding it in inductive energy storage means before transferring it to voltage trans¬ forming means. Preferably, the voltage level conversion means includes a feedback loop arranged to vary the conduction period of the switch means in order to main¬ tain a fixed relationship between an output voltage level and a reference voltage level. The voltage level conversion means facilitates the extraction of energy from a battery with a declining output voltage level to effect the maximum utilisation of the battery energy.

In order to facilitate the switching off of the power supply to parts of the receiving system as an energy conserving measure, the voltage level conversion means preferably includes switch means, responsive to timing control means of the text receiver, arranged to disconnect at least selected parts of the voltage level conversion means from the receiving means of the text receiver.

The receiver preferably further includes an arrangement for indicating the strength of a signal of a channel to which- the receiver is tuned. This assists a user to find a channel which is being received at sufficient signal strength for -the signals to be read correctly. Again, it is preferable for the display device that displays the text information to be arranged to display an indication of the channel strength thereby avoiding the need to provide a separate display.

It is preferable that the signal strength indica- tion is arranged to have a fast-increase and slow-decay response. Such an arrangement indicates to the user the maximum-strength signal available in the vicinity of the receiver.

The receiver may further include an arrangement for indicating the channel to which it ^"is tuned. Again it is preferable for the display device that displays the text information to be arranged to display an indica¬ tion of the channel to which the receiver is tuned.

The channel to which the receiver is tuned can preferably.be controlled by operation of the keyboard. In one embodiment of the invention, the channel is selected simply by pressing numbered keys of the key¬ board that correspond to the channel number. This facilitates channel selection. Preferably, the receiver further includes an automatic channel searching arrangement operative to tune to each of a plurality of channels in turn until a channel of sufficient signal strength is found. Such an arrangement is of particular value when a user is operating the receiver at a location where he does not know which channels can be received well. The signal-receiving means preferably includes a synthesized tuner. This is of particular value in a receiver that is intended specifically to display teletext information, that is, a receiver in which no picture is available for use as a tuning reference. The text receiver preferably includes timing control means arranged to provide time slots signifying periods during which respective parts of the text receiver are operative. Also, preferably, the timing control means is arranged to synchronise operation with data being received to operate independently when data is not being received, and to delay resynchronising its operation with received data, on the recommencement of data reception. The timing arrangement, referred to, is of particular value in a receiver that is intended to display teletext information, as the periodic resyn- chronisation of the receiver to the text transmission is prevented from interrupting the display phase of the operations. The interference that the resynchronisation referred to could cause the text display, which inter- ference would lead to corruption of the text displayed, is avoided by an arrangement whereby the timing control means maintains its time slots at current durations, or extends its time slots, for the duration of one field of a television transmission, before resynchronising with received data.

The reference above to the timing control means operating-independently means operation of the timing control means "off-air", that is, not synchronised to the transmission timing, and synchronised operation, also referred to above, may alternatively be called "on- air" operation. The timing control means may be arranged such that, when conditions are such as to require continued independent operation, it keeps the receiver operating independently and fully active for a first plurality of sequential time slots and, at the end of the first plurality of sequential time slots, switches off selected parts of the receiver for a second plurality of sequential time slots, the first plurality of time slots being substantially fewer than the second plurality of time slots. The display device may include a matrix of picture elements disposed in a plurality of rows and columns to display characters as sub-matrices. The rows and columns are preferably orthogonal.

The picture elements may be of oblong shape with their minor dimensions extending across the widths of the characters. We have found that a satisfactory character set can be obtained using oblong picture elements. In providing oblong elements whose major axes extend "down" the display, comparatively more space is^' provided to make connections to the sides of the display. In the embodiment of the invention -to be described, the major dimension of each oblong picture element is about 30% greater than its minor dimension.

Preferably, all or substantially all of the chara- ters are displayed in a sub-matrix occupying 5 rows and 5 columns of the matrix of picture elements. In the case where substantially all, but not all, the characters are displayed in a sub-matrix occupying 5 rows and 5 columns, the other characters are preferably displayed in a sub- matrix occupying 6 rows and 6 columns and more preferably a sub-matrix occupying 6 rows and 5 columns. It is sur- prising that clear characters can be obtained from a sub-matrix that is only 5 X 5; we have found, however, that this is possible and one particular set of charac¬ ters even including fractions (1/2, 1/4 and 3/4 in this particular example) is fully described below by way of example with reference to the drawings. The set of characters illustrated are suitable for English speaking countries. We have also devised modified versions of the sets for use in other countries where for example accents may have to be included. In the example des- cribed^" all of the upper case characters are within the 5 X 5 sub-matrix but some of the lower case characters extend below the 5 X 5 sub-matrix; while there are relatively few such charaters we have found that such an arrangement is satisfactory. An alternative arrangement is to employ upper case characters only. The use of such a small sub-matrix, apart from reducing the total number of picture elements, enables a smaller display device to be used for a given density of connec¬ tions to the display device. This is of particular advantage in the present invention where size is an important factor.

Adjacent sub-matrices for displaying characters are preferably separated from one another by further energisable picture elements. These picture elements may be used in displaying those lower case characters that require a sub-matrix bigger than 5 X 5. Adjacent sub-matrices in the same row may be separated from one another by a single column of picture elements and adjacent rows of sub-matrices may be separated from one another by a single row of picture elements. This arrangement permits the display of contiguous graphics shapes. Adjacent sub-matrices in the same row preferably have a centre-to-centre spacing of 6 columns. Similarly, adjacent sub-matrices in the same column preferably have a centre-to-centre spacing of 6 rows and this applies even when a lower case ^"character that occupies 6 rows is being displayed. By using the normally vacant row that spaces adjacent lines of text to accommodate those lower case letters that benefit from the extra row of picture elements, there is no increase in the overall number of picture elements required in the display over that which would be required^"if all the characters were depicted within 5 rows of picture elements. At the same time because only a few lower case characters require this extra row the overall clarity of the display is not impaired by what is only an occassional merging of adjacent lines of characters.

The picture elements may be liquid crystal elements.

The picture elements are preferably energised substantially at the same time in more than one region of the display. For example, picture elements in an upper and a lower region of the display may be energised at substantially the same time. Energising more than one region of the display at a given time has the effect of permitting the energisation of each picture element for long enough to provide a display with good contrast (and therefore good legibility) despite the relatively large size of the display.

It is an essential feature of the invention that the display device be able to display a page of informa¬ tion rather than merely one or two lines. Thus it is advantageous for the display device -to have a capacity of at least ten lines of text information. In the case where the receiver is a teletext receiver it is desir¬ able for the receiver to be able to display an entire page of teletext information at one time. Accordingly, the display device may have a capacity of approximately 24 lines of text information, each of approximately 40 characters, this being the capacity required for a standard page of teletext in the United Kingdom. The numbers quoted above are approximate because a very slightly smaller or larger display could provide a workable arrangement and, in the case of the number of characters, also because the first character of each line of a conventional teletext transmission is a control character and that character need not there¬ fore be displayed. A display of the above capacity may occupy an area of less than 100. cm².

The time allocated for applying data to the display device may exceed the time required to execute the display function. In such a case the display control system may be arranged to continue driving off the display during the period between completion of the display function and the end of the allocated time.

Preferably the receiver includes fixed data store means arranged to control the execution of operations within the receiver in response to command instructions received as keyboard entries. The fixed data store means may also include data representing the repertoire of characters to be displayed. The fixed data store means may be arranged to copy the character data to an alterable data store which then acts as a character generator for the system. Preferably, the receiver is arranged to store the number o^'f a page of information that has just been dis¬ played and in response to a command from a user, to display again that page of information. This allows a user to re-view the page he was previously viewing without the need to remember its number and can be particularly useful for enabling a user to go back to an index page that he has just consulted.

The feature referred to immediately above is -in itself a novel and inventive aspect of the present invention and is applicable to text receivers other than hand-held receivers. According to that aspect of the invention there is provided a text receiver including signal receiving means arranged to receive signals representing text information as a plurality of pages, signal-processing means arranged to process signals from the signal-receiving means and to provide information signals for display, a display device capable of displaying a page of information, storage means for storing the number of a page of information that has just been displayed, and control means operable by a user to cause the receiver to display again that page of information. Also according to that aspect of the invention there is provided a method of operating a text receiver in which signals representing text information as a plurality of pages are received, the signals are processed to provide information - ι - __

signals for display and a selected page of information displayed, wherein the number of a page of information that has just been displayed is stored and the user can give a direct command to the receiver to display again that page of information. It is advantageous that only the page number (rather than the page itself) is stored, since in this case very little- storage capacity is required. Furthermore, it is only necessary for one page number, namely that of the page last displayed, to be stored. One or more keys, preferably a single key, on the keyboard may control the generation of the command signal to display again that page of information.

According to the invention, there is also provided a method of operating a text receiver as defined above.

While the signal-receiving means is not detecting a predetermined signal, the receiver preferably alternates between an active mode during which it is responsive to the predetermined signal and a dormant mode during which the power supply for at least a part of the signal-receiving means is disconnected.

Where an automatic channel searching arrangement is provided, the method may include: (i) tuning the receiver to a channel,

(ii) checking the signal strength of the channel, and (iii) if the signal strength of the channel is not sufficient, repeating steps (i) and (ii) above, or/ if the signal strength of the channel is sufficient, maintaining the tuning at the channel being received.

A hand-held teletext receiver in accordance with the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a plan view of the teletext receiver.

Fig. 2 is a block diagram representation of a teletext data reception and processing system suitable for operating a

multiplexed display device to provide the hand-held teletext receiver of Fig. 1, Fig. 3 is a schematic representation of the tuning system arrangement of the teletext data reception and processing system represented by Fig. 2, Fig. 4 is a schematic representation of the power supply arrangement of the teletext data reception and processing system represented by Fig. 2,

Fig. 5 is a block diagram representation of the timing and display control arrangements of the teletext data processing system • represented by Fig. 2,- Fig. 6 is a block diagram representation of the timing arrangement for synchronising the operation of the teletext data reception and processing system, represented by Fig. 2, with teletext data transmissions, Figs. 7a. to 7d illustrate the complete set of characters (excluding graphics) displayed by the receiver by means of a 5 X 5 matrix of picture elements, an "X" denoting an energised picture element, the matrix characters being accompanied by their non-matrix equivalents, Fig. 8a illustrates diagrammatically the display of sample characters on the device, and. Fig. 8b illustrates diagra metically the arrangement of the display device as a whole.

Referring to Fig. 1, the teletext receiver shown is a portable, personal teletext receiver and includes a generally rectangular-sided enclosure 1 of approximate dimensions 220 mm (length) by 130 mm (width) by 38 mm (depth) having openings in a major surface to accommo¬ date in one half of the surface, a display device 2 of approximate dimensions 82 mm (height) by 100 mm (width), and, in the other half of the surface, a plurality of push-key members. The size of the teletext receiver is therefore such that it can comfortably be held in an adult person's hand and is truly portable.

The push> key members include a power control switch 3, a first group 4 of keys labelled 0 to 9, respectively, a second group of keys 5 labelled HOLD, REVEAL, AUTO PG and LAST PG, respectively, a channel select key 6 labelled CHSELECT, and a SEARCH key 7. The teletext receiver also includes a scale 8 for a bar graph signal strength indicator and a retractable or plug-in aerial 11 (not shown in Fig. 1). The display device 2 is a liquid crystal display device consisting of a matrix of energisable picture elements capable of displaying a full page of teletext information which may include alphanumeric and graphics characters. The display device is described in more detail below.

Referring to Fig. 2, the functional components of the teletext receiver of Fig. 1 include the aerial 11, a tuning system 12, a computer 13, a control key¬ board 14, a data slicer 15, a data acquisition system 16, a timing signals generator and liquid crystal display driver 17, an alterable data store 18, a clock oscillator 20, an analogue-to-digital converter 21, and a power management circuit 22. The teletext receiver makes use of standard batteries.

A front end amplifier is included in the tuning system 12 in order to provide the necessary UHF signal- receiving capability in compact form.

The tuning system 12 is arranged to be tunable by digital commands, and, as shown in Fig. 2, is arranged to receive digital tuning commands from the computer 13. The tuning system 12 is of the kind generally known as ^'a synthesised tuner and is described in detail below. The computer 13 is arranged to carry out the following functions:-

(i) Setting up the tuning synthesiser (part of the tuning system 12) to receive a television signal on a specified channel.

(ii) Setting up the data acquisition system 16 to extract teletext signals from the video signal provided by the tuning system 12.

(iii) Responding to and executing commands entered through the keyboard 14, including the REVEAL operation.

(iv) Effecting modification of the contents of the alterably data store 18, as appropriate, to reflect commands entered through the keyboard 14.

(v) Clearing the display at certain times. (vi) Checking on the strength of the signal ^• being received by the tuning system 12 and the condition of the power supply batteries, and causing the display of the results of the checks.

Referring still to Fig. 2, the computer 13 includes a processor, a program memory, a data memory, reset control circuits, address decoding circuits, and input/output-control circuits. The computer 13 uses the clock oscillator 20 as the source of its timing pulses. Some of the component parts of the computer 13 are provided by a 80C49 type microprocessor circuit available from various manufacturers. The computer 13 includes a substantial proportion of CMOS devices as a means of minimising power consumption. Output control circuits of the computer 13 are disabled by a signal from the timing signals generator and display driver 17 when the computer is inactive, in order to ensure that the computer cannot corrupt data by placing data or address signals on to shared data and address buses at times when the computer is not active. The computer 13 also includes a character read only memory (ROM) the use of which is described below. The data slicer 15, the data acquisition system

16, and the alterable data store 18 are interconnected with each other and with the timing signals generator 17 and the computer 13. The timing signals generator 17 is arranged to repeat a cycle of operations of 20 milli- seconds duration which corresponds to the transmission period of a television field. Each cycle of operation includes periods for (i) transforming any available teletext signal into lines of teletext data and storing the data in the alterable data store 18, (ii) altering, b the computer 13, the contents of the alterable data store 18 or of registers included in the data acquisi¬ tion system 16 and the timing signals generator and display driver 17 (in accordance with keyboard presses, for example) and^' (iii) transforming, by means of the timing signals generator and display driver 17, the contents of the alterable data store 18 into signals used to operate the display system.

Referring still to Fig. 2, part of the alterable data store 18 is arranged to act as a character read only memory (ROM) and the remainder of the alterable data store 18 is used to store the page of data displayed as teletext information at any time. The alterable data store 18 consists of 2048 X 8 addressable storage elements (nominally 2K X 8), 1024 X 8 elements of which^* are used as the current page store, that is, as the store for the data being displayed as teletext information. The part of the alterable data store 18 arranged to act as a character ROM is made to receive the full range of ASCII control and display codes from a character and display code ROM located in the computer 13 when the receiver is first switched on. The timing signals generator and display driver 17 is involved .in the transfer of the ASCII teletext codes from the character ROM in the computer 13 to the appropriate addresses in the alterable data store 18. The alterable data store 18 is then used by the timing signals generator and display, driver 17 as the source of ASCII codes required to transform the teletext data, held in the remainder of the alterable data store 18, into data suitable for presentation to the liquid crystal display. The sequence in which data is read from the alterable data store 18 is dictated by the structure of the display itself, the display structure being described below with references to Figs. 8a and 8b.

Referring still to Fig. 2, the analogue-to- digital converter 21 is arranged, under the control of the computer 13, to sample the voltage level of the batteries in the power supply 22 and to digitise the sampled value for assessment by the computer 13 as to the battery condition. In terms of the actual circuit arrangement (not shown in Fig. 2), a resistive potential divider having two high-resistance elements is connected to the batteries and the junction of the two potential divider elements is connected to the analogue-to-digital converter. The analogue-to-digital converter includes a resistor-capacitor series circuit having a transistor switch connected across the capacitor and an operational amplifier arranged as a comparator, the junction of the two elements of the potential divider, referred to above, being connected to one input port of the compara¬ tor and the junction of the resistor-capacitor series circuit being connected to the other input port of comparator. Analogue-to-digital conversion is effected by measuring the time it takes for a reference voltage supply to charge the capacitor through the resistor to the battery sample voltage level, the switch across the capacitor being- used to discharge the capacitor fully, prior to the charging phase of the operation.

The analogue-to-digital converter, referred to above, is used also for providing the signal strength indication at the bottom of the liquid crystal display device 2 adjacent the scale 8 as a bar graph. More specifically, an automatic gain control (AGO voltage (which is a measure of signal strength) , available in the tuning system 12, is applied to a voltage-follower amplifier and the output port of the voltage-follower amplifier is connected, by way of a transistor switch, to the junction of the two high-resistance elements of the resistive potential divider, referred to above in connection with battery voltage sampling. The transis¬ tor switch is controlled by -the computer 13, and, when the transistor is conductive; the output port of the voltage-amplifier is connected to the input port of the analogue-to-digital converter. The "sampling" point for the battery voltage remains connected to the analogue- to-digital converter while the output port of the voltage- follower amplifier is connected to the analogue-to-. digital converter, but, because the voltage-follower amplifier represents a low-independence voltage source, the voltage-follower amplifer signal overrides the battery sample voltage signal, and, in effect, the analogue-to-digital converter is made to sample two signals by the provision of a single switch. The indication of received signal strength is provided during tuning of the receiver.

In the presence of a strong input signal to the tuner system 12, the timing signals generator and display driver 17 receives a composite sync signal from the data slicer 15 and derives, from the composite sync signal, two time slot signals one of which is passed to the data acquisition system 16 and the other of which goes to the computer 13. The data acquisition system 16 and the computer 13 are rendered active at the same time by the timing signals generator and display driver 17 by the application of the time slot signals referred to. The time slot signals are arranged to cover lines 7 to 22, of the odd fields, and lines 320 to 335, of the even fields, of a television transmssion (these being the teletext-carrying lines). The timing signals generator and display driver 17 provides a short (.5μS) synchronising pulse at the beginning of each of the teletext-carrying lines referred to, and the data slicer

15 and the data acquisition system 16 are arranged to extract data carried in lines "marked" by the synchro- nising pulse from the timing signals generator and display driver 17. On receipt of the synchronising pulse, the data slicer 15 halts its clock and waits for teletext data from its connection to the tuning system 12. It then passes on received data, with its now regenerated clock signals, to the data acquisition system 16 which examines the run-in sequence that precedes teletext data (alternate l's and 0's trans¬ mitted at the beginning of each teletext line). The data acquisition system 16 issues a new synchronising pulse to the data slicer 15 to alter the data slicer clock and synchronise it to the teletext data pattern. The data acquisition system 16 is partly controlled by the computer 13 which instructs it to select particular pages of a teletext magazine. The data acquisition system 16 transfers, to the alterable data store 18, teletext pages identified to the data acquisition system

16 by the computer 13. When appropriate, time information is copied to the alterable data store 18. The inter¬ connection of the computer 13, the data acquisition system 16, and the alterable data store 18 is such that the computer 13 may arrange either for the data acqui- sition system 16 to transfer data to the alterable data store 18 to provide up-to-date information for display or for the data acquisition system 16 to transfer data to a non-existent store, thereby dumping that data and holding the previous data in the alterable data store 18 for display.

The teletext receiver has two modes of operation, one being the tuning mode during which channel selection is achieved, and the other being the teletext mode during which the selection and display of the teletext data available on the selected channel is achieved.

It should be pointed out that channel selection on the receiver is precise, being accomplished by means of a digitally controlled phase locked loop tuning system employing a reference oscillator and digital divider techniques. The user can therefore be confident that the receiver will be correctly tuned when a desired channel has been selected by means of the control key¬ board. The problem of achieving channel selection in the present teletext-only receiver is not encountered in a combined picture-and-teletext receiver in which the picture transmission is available for fine setting of the tuner system by the user.

The data slicer 15 may be a Plessey type SL 9100 integrated circuit and the data acquisition system may be a Plessey type MR 9710 integrated circuit. . The tun¬ ing system 12 may include a Plessey type SP 5000 synthesiser integrated circuit, a type U 341 tuner, a SW 153A surface acoustic wave filter, and a type

TDA 2540/2541/3541 intermediate frequency amplifier with demodulator (synchronous detector). The power management circuit 22 effects shut down of parts of the tuning system during the transmission of picture (that is, non-teletext) information.

As has been pointed out above the receiver is a teletext-only receiver in which tuning is accomplished by means of a digitally controlled phase locked loop tuning system. The teletext-only receiver is arranged to select only those parts of a television transmission which contain teletext data by detecting the field synchronising patterns and accepting lines 7 to 22 in the even field and lines 320 to 335 in the odd field, the lines specified being the teletext-carrying lines. The average power consumption of the teletext receiver is reduced by switching off, or powering down, selected parts of the system for a major part of each television field. In particular, the powered down parts are switched on for a period of about l.^"4 milliseconds in order to be active during the transmission of the teletext-carrying lines, and in addition are switched on for the order of about 2.2 milliseconds in advance of the expected arrival of field synchronising pulses, giving a total switched on time of the order of 3.6 milliseconds in each 20 milliseconds occupied by a television field. The additional period of about 2.2 milliseconds is provided for the powered down circuits to settle to their respective working levels before the arrival of teletext data. The saving in average power is of the order of 80% for the parts powered down.

In putting powering down into effect in the teletext-only receiver, performance is maintained by including arrangements for retaining the charge on - selected automatic gain control (AGO and decoupling capacitors which would otherwise be discharged, during the periods of power down. In some instances, the necessary isolation of the capacitors referred to is provided by the use of analogue switches but transitor switches are used in situations where the "on" resis¬ tance of an analogue switch would degrade the circuit performance.

Powering-down is performed by means of a series switching transistor included ^'in the power management circuit 22 (Fig. 2) under the direction of the timing signals generator and display driver 17 (Fig. 2). Allowance is made for conditions under which no strong television signal is available, in which case the teletext-only receiver attempts to locate a field synchronising pulse by remaining fully active for the duration of three television fields. The teletext-only receiver reverts to the powered down pattern described above if it locates a field synchronising pulse during the period of extended activity, and, if a synchronising pulse is not located during the period of extended activity, powers down for a-period of half a second (about the duration of 29 fields), after which it operates for another period of extended activity. The timing signals generator and display driver 17 controls the timing of all the operations described above using the system's crystal-controlled clock oscillator (which is also used by the frequency synthesised tuning arrangement) .

The tuning system arrangement, the power manage¬ ment circuit, and the timing signals generator and display driver, effecting the operations, outlined above, are described below in detail.

Referring to Fig. 3, the tuning system arrange¬ ment 12 (Fig. 2) includes an input radio-frequency amplifier 120, a tuner 121, an intermediate frequency (I.F.) preamplifier 122, an intermediate frequency amplifier and synchronous detector 123, and a tuner controller 124. The radio-frequency amplifier 120 has an input-signal receiving port 134 which in practice is connected to the aerial 11 (Fig. 2). The radio- frequency amplifier 120 also has a power supply input port PS1. The output signal port of the radio frequency amplifier 120 is connected to the input signal port of the tuner 121 which has a first power supply input port PS4 connected to its front end amplifier stage, a second power supply input port PF2 connected to its oscillator stage, an automatic-gain- control' (AGO input port connected through a transistor emitter-follower 125 to an AGC capacitor 126, a tuning control input port connected to a tuning control trans¬ istor 127, an oscillator-output-sample port connected to the tuner controller 124 by way of a capacitor 135, and an intermediate signal output port connected to the intermediate frequency preamplifier 122. The AGC capacitor' 126 is positioned in the base circuit of the transistor 125 which has a collector supply port PF5. The AGC capacitor 126 is connected by way of a switch 140 to a resistor network energisable by way of a power supply input port PS2 and a connection 136 from the I.F. amplifier and synchronous detector 123. The inter¬ mediate frequency preamplifier 122 has a power supply input port PS5 and a signal output port connected to the signal input port of the I.F. amplifier and synchronous detector 123. The I.F. amplifier and synchronous detector 123 provides an AGC signal by way. of the connection 136 and the switch 140 to the AGC capacitor . 126, and has a power supply input port PS6, a signal output port (the output signal from_which passes through a switch 138), and an AGC-output-sample port connected by way of a controllable switch 137 to an AGC storing capacitor 132. A decoupling capacitor 130, for the I.F. amplifier and synchronous detector 123, is connected in the collector circuit of a transistor 129 which is controllable by way of its base driver input port PS7 to effect disconnection and. connection of the capacitor 130 to signal earth. The tuner controller 124 has a reference crystal 128 controlling a reference oscilla¬ tor and a data input bus 133 linking it to the computer 13 (Fig. 2). The crystal 128 is also used by the • computer 13 by way of a connection represented by the line 139. Also shown in Fig. 3 is a switchable control element 245 which has a first input port PFl for connec¬ tion to a source of fixed voltage, a second input port PD for connection to a switching signal output port of the computer 13 (Fig. 2), and an output port PSO which is connected to the power supply ports PS1 to PS9 of the remainder of the circuit -of Fig. 3. The switchable control element 245, in fact, forms part of the power management circuit 22 which is described below. The switchable control element 245 is included in Fig. 3 because it is material to the operation of the tuning system arrangement.

Referring still to Fig. 3, the switchable control element 245 is switched on by the computer 13 (Fig. 2) at the beginning of the periods of a television field when teletext information will be transmitted with the result that the radio frequency amplifier 120 is then provided with a supply voltage by way of the port PS1 , the base circuit of the transistor 125 is provided with the supply voltage by way of the port PS2 (the switch 140 being turned on by way of its control input port which is also energised by PS2) , the front-end amplifier stage of the tuner 121 is provided with the supply voltage by way of the port PS4, the intermediate frequency preamplifier 122 is provided with the supply voltage by way of the port PS5, the I.F. amplifier and synchronous detector 123 is provided with the supply- voltage by way of the port PS6, the transistor 129 is provided with the supply voltage by way of the port PS6, the transistor 129 is provided with the supply voltage by way of the port PS7, and the controllable switch 137 is closed by the presence of the supply ^' voltage at the port PS8. The port PF2 of the tuner 121, which is the power supply port for the oscillator stage of the tuner 121, is provided with a fixed (that is, non-switched) voltage, as are the ports PF3, PF4 and PF5. The voltage applied to the ports PF2, PF3, PF4 and PF5 are not necessarily the same, and may be 12V, 12V, 30V and 12V respectively.

Referring still to Fig. 3, the radio frequency amplifier 120, when powered up, will- amplify a signal received on its input port 134 and pass the amplified radio frequency signal on to the front-end amplifier stage of the tuner 121 where the radio frequency signal is further amplified and transformed to the system's intermediate frequency by mixing with a fixed amplitude and frequency oscillation generated within the tuner 126. The gain of the front-end amplifier stage of the tuner 121 is set by the stored voltage on the capacitor 126 which is held isolated shortly before, and during, the time when selected parts of the system are switched off. The capacitor 126 is kept isolated for a short period after the selected parts of the system are switched back on. The timing of the connection and . disconnection of the capacitor 126 ensures that no AGC voltage samples reach the capacitor 126 during periods when transient disturbances, caused by the switching on and off of the selected parts of the system, are likely to occur. The transistor 125 acts as a buffer amplifer for the capacitor 126. The frequency-shifting- oscillation provided by the tuner 121 is controlled by the tuner controller 124 which receives a sample of the oscillation by way of the capacitor 135, divides the received sample by a set ratio which should give a resultant oscillation having the frequency and phase of the reference oscillator (the reference oscillator frequency being divided down from a standard frequency oscillator) controlled by the crystal 128, and adjusts the frequency of the tuner oscillator (a voltage controlled oscillator), as necessary, by varying the conductivity of the transistor 127 until the divided sample of the tuner oscillation has the required relationship to the reference oscillation provided by the tuner controller 124. The set ratio of division applied by the tuner controller 124 is dependent on data provided by the computer 13 (Fig. 2) on the data input port 133. The intermediate frequency signal is passed to the intermediate frequency preamplifier 122 which drives the intermediate frequency amplifier and^' synchronous detector 123. The decoupling capacitor 130 belonging to the intermediate frequency amplifier and synchronous detector 123 is grounded by way of the transistor 129 when the transistor 129 is powered up by way of its control port PS7 and prevented from discharging when the transistor 129 is powered down. The capacitor 132 is the AGC storage capacitor for he intermediate frequency and is connected to the inter¬ mediate frequency amplifier and synchronous detector 123 by way of the switch 137 during power up and supplies the AGC voltage to the AGC line 136. The capacitor 132 controls a voltage follower amplifier 131 the output signal of which is digitised (as described above) elsewhere in the system and is used as an indication of the received signal strength. The capacitor 132 stores the AGC value during power down. The controllable switch 138 having a control input port PS9 is arranged to isolate the output port of the intermediate frequency amplifier and synchronous detector 123 to avoid the propagation of a voltage step at switch off, since there is capacitive coupling to the following stage.

The tuner 121 shown in Fig. 3 is provided with separate supply ports PS4 and PF2 to the front-end amplifier and the oscillator, but some tuners may have a common supply port to the front-end amplifier and the oscillator, in which case better results are obtained by excluding the tuner from the parts powered down.

Referring to Fig. 4, the power supply arrangement includes a battery 220 the positive terminal of which is connected by way of a jack socket 221 to an indicator 222. The negative terminal of the battery 220 repre¬ sents earth potential for the system. The positive terminal of the battery 2^'20 is connected to one terminal of a push-switch 237 the other terminal of which is connected to the clock input port of a bistable D flip-flop 236 and to earth by way of a capacitor-resistor parallel network 250. The common terminal of the push-switch 237 and the battery 220 are connected to the SET input port of the bistable D flip- flop 236 by way of a capacitor which is part of a differentiating capacitor-resistor series netwok 235. The positive terminal of the battery 220 is further connected to the D input port of a bistable flip-flop 233 and to the emitter electrode of a PNP transistor

239 which is connected in series with the power supply input port of a comparator 238. Additionally, the posi- tive terminal of the battery 220 is connected to a group of four NOR gates two of which are shown as 231 and 232, and the other two of which are connected together to form an oscillator 234. The battery 220 is connected 5 also to a switched current source 224 consisting of a PNP transistor with an associated current-setting resis¬ tor and diodes, the base electrode of the transistor 224 being connected to the Q output port of the bistable D flip-flop 236. The switched current source 224 is

10 connected to a first current amplifier consisting of transistors 225 and 227 with an associated resistor and a second current amplifier, identical in arrangement to the first current amplifier, consisting of transistors 226 and 228 with an associated resistor. A control port - 15 of the first current amplifier is connected to the out¬ put port of the NOR gate 232 and to an arrangement of a transistor 229 with associated diode, resistor and capacitor for effecting rapid switch off of the first current amplifier when the output level of the NOR gate

20 232 goes high. The second current amplifier is arranged similarly to the first current amplifier by being connected to the NOR gate 231 and having a drive-off arrangement consisting of a transistor 230 with associated diode, resistor and capacitor. The collec-

25 tor electrode of the transistor 225 is connected to the terminal of the inductor 222 remote from the battery 220. The primary winding of a transformer 223 is connected in series with the inductor 222 and between the inductor 222 and the collector electrode of the transistor 226. A voltage stabilising diode 251 is connected to the junction of the inductor 222 and the primary winding of the transformer 223. The NOR gates 231 and 232 are two input-port gates, one input port from each of the gates 231 and 232 being connected to the Q output port of the D bistable flip-flop 236. The remaining input ports of the NOR gates 231 and 232 are connected respectively to the Q and Q output ports of the bistable flip-flop 233. The R input port of the bi¬ stable flip-flop 233 is connected to a first output port of the oscillator 234. A second output port of the oscillator 234 is connected to an input port of the- comparator 238 and the oscillator 234 has an enable/ disable input port connected to the Q output port of the D bistable flip-flop 236. The comparator 238 has a second input port connected to the output port of a differential input amplifier 240 which has a first input port connected to a resistive potential divider 243, 244 arranged to provide a proportion of the 5V supply, and a second input port connected to a voltage reference source 241 energised from the 12V supply by way of a resistor 242. The transformer 223 has a tapped secondary winding providing, by way of respec¬ tive rectifiers 246, ^'247, 248 and 249, output supplies of 5V, 12V, -15V, and 30V. The power supply arrange¬ ment also includes the switchable control element 245, described above with reference to Fig. 3, which acts as a switchable 12V source. ^' A first control input port PD of the power supply arrangement is connected to the control port of the switchable control element 245 and a second control input port PDD is connected to the clock input port of the D bistable flip-flop 326. The first control input port PD is connected to an output port of the timing signals generator and display driver 17 (Fig. 2) and the second control input port PDD is connected to an output port of the computer 13 (Fig. 2). Referring still to Fig..4, the primary electrical energy source for the power supply arrangement will generally be the battery 220, but the jack socket 221 permits connection to an external direct voltage energy source with resultant disconnection of the battery 220. The power supply is switched on and off by the operation of the push button switch 237 which cause the D bistable flip-flop 236 to provide alternate logic "1" and ^,r0" outputs at its- Q output port. Additionally, the power supply, when operative, is switched off by a pulse provided by the computer 12 (Fig. 2), on the input port PDD when the keyboard has not been used for longer than a set period. The power supply represented by Fig. 4 is arranged'to supply pulsed energy from the battery 220, or from an external d.c. source to the primary winding of the transformer 223. The voltage transformed output energy is applied to the rectifiers 246, 247, 248 and 249 to obtain transformed output d.c. The supply of energy to the primary winding of the transformer 223 is effected by making the transistor 226 conductive, con¬ trol of the transistor 226 being exercised by Q output signal from the bistable flip-flop 233 by way of the NOR gate 231, and the transistor 228, with the transistor 230 coming into action during the switching off of the - transistor 226. Conduction current for the transistor 228 is obtained by way of the transistor 224 which supplies a fixed current the level of which is deter¬ mined by the value for the emitter load resistor of the transistor 224 and the voltage drop across the diodes associated with the transistor 224. The voltage drop across the diodes associated with the transistor 224 will be relatively constant at about 1.4 volts. The effect of the arrangement, described above, for supply¬ ing pulsed energy to the transformer 223 is that the transistor 226 is supplied periodically with a set level of base current from the transistor 224 by way of the transistor 228, and the transistor 226 is rendered conductive by an amount dependent on the level of its base current. When the transistor 226 is in its conductive state it will permit the flow of current through the primary winding of the transformer 223. Conduction by the transistor 226 alternates with conduc¬ tion by the transistor 225_ which is arranged to be rendered conductive to substantially the same extent as the transistor 226 by having the current provided by the transistor 224 directed into its base by way of the transistor 227. When the transistor 225 is in its . conductive state it will permit the flow of current through the inductor 222. The alternate switching on of the transistors 225 and 226 takes place under the control of the bistable flip-flop 233 which, as shown, controls the transistor by means of its complementary output signals Q and Q . The alternate switching of the transistors 225 and 226 results in the inductor 222 being charged largely when the transistor.225 is conduc^J tive and then being made to discharge into the primary winding of the transformer 223 when the transistor 226 is conductive. The inductor 222 serves to boost the voltage level applied to the transformer 223 above that provided by the battery 220, permitting the use of battery energy when the battery voltage is quite low. The current drive arrangements for the transistors 225 and 226 further facilitate the use of the battery at low voltage levels. The transistors 229 and 230 provide action pull-down of the base voltages of the transistors 225 and 226, respectively, minimising turn- off delay df the transistors. 225 and 226. The timing of the operations within the power supply is controlled by the clock oscillator 234 which provides a pulse output to the R input port of the bistable flip-flop 233 and a triangular wave output to the comparator 238. The comparator 238 uses the triangular wave to generate a width-modulated pulse output to the clock input port of the bistable flip-flop 233 which is so connected as to change state on the leading edge of the width- modulated pulse and to be reset by the arrival of the clock pulse from the oscillator 234. The comparator 238 uses a measure of the level of the 5 volt supply pro¬ vided by the differential amplifier 240 as the other input signal in generating the width-modulated pulse referred to above.

The timing signals generator and display driver provides the main timing for the system and determines when the various parts of the system may use the system busses.

The timing signals generator and display driver breaks each t.v. scan period (20 ms) into three portions, and informs the other components of the system by means of time slot signals TS1 and TS2 (Fig. 5). if a strong television signal is present, the timing signals generator and display driver locks to the composite sync signal (CSI) extracted from the received video signal. The power saving arrangements remove most of the composite sync signal and the timing signals gen¬ erator and display driver has only frame synchronisation information.

The three portions (time slots) of the t.v. scan period, referred to above, are:-

(1) A first portion during which data acquisition is enabled for the 16 television lines which potentially contain teletext information, to allow the capture of teletext data, and, if wanted, its transfer to the system display memory.

During this time slot the memory enable. (MEMEN) signal is set inactive, the read/write (R/W) signal is set to read, the data and address busses will be left inactive (floating high) and the RAM/ROM signal will select the display memory. If the system is receiving a strong television signal, a half microsecond negative going pulse must be provided near the start of each of the television lines. The data acquisition and data slicer chips use this signal to prepare to receive teletext from the current television line.

(2) A second portion during which the computer is enabled. During this time slot the computer can write to the display memory and can access the control and status registers of the data acquisition chip and the timing signals generator and display driver. This is the only time during which the computer can write to the control register of the timing signals generator and display driver. In this period TSl and TS2 are set active, R/W is set to write but the timing signals generator and dis¬ play driver does not actually write to any portion of the memory, MEMEN is set inactive, the RAM/ROM signal is set to select RAM and the data and address busses are set inactive.

(3) A third portion during which the timing signals generator and display driver transforms the contents of the display RAM into the signals needed by the LCD driver chips to form a single refresh scan of the LCD. In this period the time slot signals TSl andTS2 are held inactive.

In order to keep the visual quality of the LCD display as high as possible it is important that the third period has the major share of the available time of each field scan period, and therefore the second period for the' computer must be short, the time needed for data acquisition (the first period) being fixed by the teletext standard.

When a television signal becomes available, the timing signals generator and display driver locks onto the composite sync signal to re-establish teletext receptio . It is arranged that during this transition the computer (as the system controller) and the data acquisition system do not receive time slots which are of shorter duration than normal, in order to avoid 5 corruption of the information being displayed.

The full range of function performed by the timing signals generator and display driver 17 may be understood by referring to Fig. 5, which shows that .the timing signals generator includes a video signals 0 generator 71, a. character counting and address logic circuit 72, a character decoder 73, and a liquid crystal display control circuit 74. The timing signals generator and display driver 17 includes various input and output multiplexers, associated with a sequence controller 75, 5 required for operation with buses, in known manner.

Referring to Fig. 5, the video signals generator 71 is arranged to receive composite sync television signals and also signals from the clock oscillator 20 (Fig. 2), and to provide, from the composite sync 0 signal, or in the absence of the composite sync signal, from the clock signal, the synchronising pulse and the time slot signal for the data acquisition system 16 and the data slicer 15 and the time slot signal for the computer 13. The video signals generator 71 also 5 provides the timing pulses for the character counting and address logic circuit 72 and the character decoder 73, which together with the fixed data store 19 (Fig. 2) effect the transformation of the contents of the alter¬ able data store 18 into the signals required to effect, by way of the display control circuit 74, the display of the data stored in the alterable data store 18 by means of a liquid crystal display device.

As explained below, data held in the alterable data store 18 are transformed for display in terms of picture element matrices by means of the character decoder 73, which actually transforms the data from

ASCII code to "matrix" code, assisted by the character count and address circuit 72 which supplies addresses for application to the alterable data store 18.

In the teletext data processing system the timing signals generator and display driver 17 provides control signals for a liquid crystal display (not shown in Fig. 2) having a fully populated matrix of display picture elements. The timing signals generator and display driver 17, as indicated above, controls the overall operation of the system on the basis of a

20 milliseconds cycle time, allocating some 86% of the available time to the display functions, 5% of the time to data acquisition, and the remainder of the time to the computer-related functions. Additionally, the timing signals generator and display driver 17 is capable of operation without the synchronising signals from a television transmission and is arranged to continue to generate the time slot signals for the data acquisition system 16 and the computer 13 in the absence of a television signal. Referring still to Fig. 5, the manner of opera¬ tion of the character count and address circuit 72 and the character decoder 73 is largely dictated by the manner in which the display device is operated. The alterable data store 18 (Fig. 2) includes a IK X 8 bits memory (actually 1024 X 8 bits) which contains a repre¬ sentation of 24 lines of 40 characters of teletext data. The character count and address circuit 72 controls the sequence in which data is read from the alterable data store 18 and applied to the character decoder 73. The data.stored in the alterable data store 18 includes additional information as to the form of each character or group of characters (lower case alphabetical charac¬ ters, upper case alphabetical characters, double height characters, and the like), and the additional informa- tion is also presented to the character decoder 73 in order that the specific form of each character is generated and displayed. The period allocated to applying a page of teletext data to the display device exceeds the time required to carry out the display function and the excess time is taken up by allowing the display control system to continue driving beyond the last line of picture elements, that is off the display device.

The timing signals generator and display driver 17 is capable of providing control signals for double height characters, which occupy two adjacent rows, by subtracting 40 from the address applied to the alter¬ able data store 18 in extracting the data for the lower of the two rows. Double height characters may be dis¬ played in either half of the display device, and a . double height character which occupies lines 12 (upper half) and 13 (lower half) of the display device is dealt with by noting the double height requirement through the end of page reset.

The liquid crystal control circuit 74 is arranged as a 480 stage shift register which is provided with data for display and clock pulses for driving the shift registers.

A further feature of the present teletext receiver is the arrangement whereby the receiver syn- chronises its internal timing to that of the received channel (that is, it goes "on-air") after it has operated independently for a time (that is, not synchronised to a transmission or "off-air"). The teletext receiver, on changing from "off- air" to "on-air" timing, obeys its internal "off-air" time slots at the current durations, or extends its time slots, for the duration of one frame of television transmission, and then synchronises to the television transmission pattern and adopts appropriate time slots. The technique of holding to,, or extending its time slots, before synchronising to the television trans- mission ensures that there can be no system "lock-up" or "jamming", and avoids the garbling of the displayed data during the change from "off-air" to "on-air" timing.

The timing arrangements leading to the teletext receiver either continuing to operate in its "off-air" mode or changing to its "on-air" mode, may be understood by referring to Fig. 6 which Shows a field synchro¬ nisation timer 501, a time slot counter 502, a self-clearing counter 503, AND gates 504 and 506, NAND gates 507 and 508, an OR gate 505, and a bistable flip-flop 509.

Referring to Fig. 6, the self-clearing counter 503 is a divide-by-128 counter which is provided with 2MHz clock- pulses (period .5μS) and provides output pulses at television line frequency (period 64μS). The output pulses from the self-clearing counter 503 are supplied to the field synchronisation timer 501 and to the time slot counter 502. The field synchronisation ^' timer 501 is arranged to count to more than 312 (the number of lines in a television field) , and, when it reaches its terminal count, it provides an output signal KILL and remains at its terminal state until cleared by an input pulse to its CLR input .port. The time slot counter 502 is arranged to provide an output signal END near the end of the display period and is held in its terminal state until cleared by a CLR pulse. The signals KILL and END are applied to the AND gate 504 which provides one input signal to the OR gate 505. The OR gate 505 is arranged to provide the CLR signal to the time slot counter 502. The NOR gate 507 and 508 and the bistable flip-flop 509 are arranged to receive the signals KILL and END, and an input signal FS indicating the receipt of a field synchronising pattern, and together to generate a signal LOCK under conditions described below. The signals FS and LOCK are applied to the AND gate 506 which-is connected to the other input port of the OR gate 505 and to the CLR input port of the self-clearing counter 503. The , signal FS is applied also to the CLR input port of the field synchronisation timer 501.

When a signal FS is made available because a field synchronising pattern (5 consecutive broad pulses) is detected by field synchronisation pattern detection circuitry, the- signal FS clears the field synchronisa¬ tion timer 501 and removes the signal KILL. The following possible conditions and results then occur: (i) If the system is in-lock ("on-air").

Provided that the signal FS has been received after the signal END (311 lines) and before the signal KILL (320.lines), the signal FS clears all the counters because the signal LOCK applied to the AND gate 506 opens the gate for the signal FS. If the signal FS is received before the signal END or after the signal KILL, the timing goes to the out-of-lock ("off-air") condition. If no signal FS is received, the time slot counter stops, gives the signal END after which the field synchronising timer 501 stops and gives the signal KILL; the signal KILL clears the LOCK condition and the signals END and KILL open the AND gate 504 to clear the time slot counter 502 by way of the OR gate 505; the system again goes to the out-of-lock condition, restarting at each END signal.

(ii) If the system is out-of lock ("off-air"). The signal FS clears the field synchronising timer 501 and removes KILL, but cannot affect the other counters. If the next signal FS arrives before the signal END, the field .synchronising timer 501 is again reset and the system remains out of lock. If the next signal FS arrives after the signal END and before the signal KILL, the LOCK condition is set and the system goes in-lock ('"on-air"). If KILL is given before the next signal FS, the system remains out-of-lock. Refering still to Fig. 6, the time slots, referred to above, and the start-of-display time, are set by the END signal from the time slot counter 502, by which means the teletext system is prevented from synchronising to the television transmission under conditions which would lead to possible corruption of the displayed data.

Referring to Fig. 8a_, which illustrates in detail a portion of a liquid crystal display device made up of a uniform matrix of picture elements which are selec¬ tively energisable to represent alphanumeric or graphics characters. Each alphanumeric character may be represented by a 5 X 5 sub-matrix of picture elements, adjacent columns of characters being separated by one column of picture elements, and adjacent rows of charac¬ ters being separated by one row of picture elements. The picture elements separating the characters are energisable elements permitting the display of graphics characters including the active separating elements to provide contiguous graphics shapes. The picture elements are of rectangular shape of height about 30 per cent greater than their width (Fig. 8a_ is not to scale in this respect). The display device consists of a matrix of 144 lines (lines numbered 0 to 143) of picture elements and 234 columns of picture elements, giving it a capacity of 24 lines each of 39 characters. Each picture element is, in this example, .35 mm wide by .45 mm high. Referring to Fig. 8b, the display device is divided electrically into an upper and a lower half each of 72 X 234 picture elements. The upper half and the lower half are so connected electrically as to be driven simultaneously line by line. That is, the data to be displayed is provided for the uppermost row of picture elements (row 0) at the same time as the data for row 72, followed by the data for rows 1 and 73, and so on, until the data has been supplied to all 144 rows (rows numbered 0 to 143).

A 5 X 5 sub-matrix is a relatively small matrix to use in generating a full range of alphanumeric charac- ters. We have, however, devised suitable sub-matrices for displaying a full range of upper case and lower case alphanumeric characters and these are indicated diagram- matically in Figs. 7a to 7d, where the picture elements that are energised to form each particular character are shown marked by the symbol "X" . ^• It will be seen that a few of the lower case letters make use of the picture elements in the row below the 5 X 5 sub-matrix and there¬ fore border directly on letters in the line below. We have found, however, that this does not unduly impair the legibility of the text while there are only a few such characters. In any case, it is not essential to employ lower case letters or those lower case letters that do make use of the picture elements in the row below the 5 X 5 sub-matrix may be shifted upwardly by one row of picture elements in which case they are accommodated wholly within the 5 X 5 sub-matrix.

The present teletext receiver is also capable of executing the HOLD GRAPHICS control command at a colour change in a modified form consistent with a monochrome display. In the execution of the HOLD GRAPHICS command at a colour change, the picture area corresponding to the colour change command is presented with the first four columns energised and the last two columns de-energised, providing a "break" between the two "colours" .

The operation of the teletext receiver by a user will now be described. Operation of the power control switch 3 turns the receiver alternately on and off. When switched on, the receiver behaves as if it had been instructed to search through the available teletext transmissions and halts at the first channel it receives at adequate strength for teletext reception, displaying the header page and channel number and providing an indication of signal strength.

The receiver will continue to display the header page and channel number of the channel at which it has stopped until the actuation of one of its control push- keys, at which time it will respond to the particular key actuated.

Operation of the CHSELECT push-key 6 will at any time clear the display 2 and provide, the information CHX in the top left hand corner of the display 2 (where X is the channel number to which the receiver is tuned at the time of operating CHSELECT), and the strength of the received siσnal relative to an internal reference will be shown by a bar graph at the bottom of the display adjacent the scale 8. A desired different channel (21to 68) can then be selected by pressing the appropriate two keys of the first group of keys 4 (the numeral keys labelled 0 to 9) . On the completion of channel selection, the information CHX will change to show the selected channel number, the receiver will tune to the selected channel, and will display the first line of the header page for the selected channel, even in relatively poor signal conditions, since the header page line referred to is transmitted in an error-correcting code. If a signal strong enough for general teletext reception is available (or is made available by suitable repositioning of the receiver), and the user presses LAST PAGE, the receiver will display every hundredth page of the magazine, in turn, while indicating "P-00", the effect being that the receiver scans through the index pages of the magazine (since pages 100, 200 etc. are index pages in the current UK teletext systems CEEFAX and ORACLE).

A specific page of the magazine of the channel being received (indicated in the display by its channel number) is selected by using the first group of keys 4 to enter the page number, the display then indicating the page number entered and displaying the required page as soon as it is available. The number of a page being " transmitted immediately before the selection of a specific page, meanwhile has been transferred to a last page number store (part of the alterable data store 18) and that page may be recalled by pressing the key LAST PG (belonging to the second group of push-keys 5), at which time the number of the page that was being displayed is transferred to the previous page store. The procedure of exchanging pages in this way may be repeated indefinitely. This "last page" facility is particularly useful for switching between two pages. nd also for switching between an index page and other pages since it makes suqh a process much easier for the user who does not need to memorise the page number; when switching between an index page and other pages, it is of course still necessary to wait for the transmission of the required page.

An option in the procedure described above, of selecting a page for display, may be exercised by the use of the AUTO PG push-key belonging to the second group of keys 5. In the optional procedure, the selec- tion of AUTO PG in one digit position of a page number will cause the display of certain pages in sequence, those pages being those in which the digit values at the position corresponding to AUTO PG are 0 to 9. For example, if keys marked 1 and 2 of the first group of keys and the key marked AUTO PG are pressed in that order then pages 120 to 129 will be displayed in sequence. The page number of each page displayed will be shown on the display. The AUTO PG key may be used as one key in the page selection process, or two keys, or all three keys.

Operation of the REVEAL push key will cause the display of "hidden" data, that is, data which is stored with a code to keep it hidden. The REVEAL function is executed only for as long as the REVEAL push-key is held. In this way, the REVEAL function operates correctly for single pages and for sets of pages broadcast under the same number.

The operation of the HOLD push-key has the effect of preventing the reception of the current data for the page being displayed, thereby freezing the displayed page at any selected time. This is in contrast to the operation without HOLD where, if the data being trans¬ mitted to the receiver for the page being displayed is up-dated while that page is being displayed, the display will change as soon as the new data is received by the receiver. Operation of the HOLD key causes the computer 13 to arrange for the data acquisition system 16 to transfer data to the non-existent store thereby dumping that data. The word HOLD will be displayed with the data when the receiver is in the HOLD mode. The receiver is removed from the HOLD .mode by pressing the HOLD push-key again, or by pressing any other push-key. If desired, parts of the receiver not involved in the display of the page need not be powered while the receiver is in the HOLD mode. The receiver provides an indication of the received signal strength by means of the bar-graph signal strength indicator at the bottom of the display and the scale 8, during the channel selection procedure described above. The signal strength indicator includes an indication of the minimum signal level required to achieve teletext reception. The signal strength indica¬ tor acts as a peak level indicator and responds much more quickly to increases in signal strength than it does to decreases in signal strength.

The SEARCH push-key 7 of the receiver, when operated, results in the display being cleared of any information, CH being indicated at the top left corner of the display, and the signal strength indicator being activated. Then, starting at one above the number of the channel that was being received at the time of pressing the SEARCH push-key 7, the receiver will be tuned to each television channel between channels 21 and 68, both limits included, the channel number displayed after CH, the signal strength monitored, and the receiver will halt when tuned to a channel providing a signal transmission of strength adequate for teletext reception. The receiver will continue to search indefinitely (covering channels 21 to 68 and wrapping around from 68 to 21) if no signal of suitable strength is found, but may be halted by the operation of the CHSELECT push-key 6. If the receiver finds a signal of strength adequate for teletext reception, it halts to continue to recceive the transmissions on that channel and maintains the signal strength indication, and the operation is then controllable by each of the push-keys as described above.

As already mentioned, the receiver will also indicate that its battery voltage is inadequate for correct operation by displaying BATTERY LOW following switch-on, but will nonetheless attempt to operate with a low battery.

The receiver described above provides a user with a truly portable teletext receiver that can be used at any location. Because the receiver is able to run off relatively^' little power, it can be battery operated so that it can be used in remote places-. The channel strength indicator and the channel searching arrangement provided enable a user to tune the receiver without difficulty even without knowing the numbers of the channels that should provide a reasonable signal strength at the user's location. These facilities are of particular value because of the portable nature of the receiver.

The special design of the display device 2 is an important factor in keeping dovm the overall size of the device and thus making the receiver truly portable. By using a 5 X 5 sub-matrix of picture elements to repre¬ sent each character rather than a 5 X 7 sub-matrix as is typically used, the size of display required to display a full page of teletext information is significantly reduced.

While in the description with reference to the drawings a receiver for receiving signals from the U.K. teletext services known under the trademarks CEEFAX and ORACLE has been described by way of example, it should be understood that the invention may also be applied to a receiver for receiving signals from other countries* teletext services or to a receiver for any other service where text information transmitted from a remote source is to be received and displayed a' page at a time.

Claims

CLAIMS :

1. A text receiver including:

(iii) a display device capable of displaying a page of information, and,

2. A receiver as claimed in claim 1 in which the receiver is a dedicated teletext receiver.

3. A receiver as claimed in claim 2 which is capable of displaying an entire page of teletext information at one time.

4. A receiver as claimed in any preceding claim further including its own power supply means to enable the receiver to operate without connection to an external power supply.

5. A receiver as claimed in claim 4 further includ- ing an arrangement for indicating the state of a battery or batteries in the receiver.

6. A receiver as claimed in claim 5 in which the display device is arranged to display an indication of the state of the battery or batteries.

7. A receiver as claimed in claim 6 in which the receiver is a teletext receiver and the power supply- for at least a part of the signal-receiving means is dis¬ connected during periods corresponding to reception of signals carrying picture information.

8. A receiver as claimed in any preceding claim in which the receiver has an active mode during which it is responsive to a predetermined signal received by the signal-receiving means and a dormant mode during which the power supply for at least a part of the signal- receiving means is disconnected, the receiver being arranged to alternate between the active and dormant modes while the predetermined signal is not being detected by the signal-receiving means.

9. A receiver as claimed in any preceding claim further including an arrangement for indicating the strength of a signal of a channel to which the receiver is tuned.

10. A receiver as claimed in claim 9 in which the display device is arranged to display an indication of the channel strength.

11. A receiver as claimed in any preceding claim further including an arrangement for indicating the channel to which the receiver is tuned.

12. A receiver as claimed in claim 11 in which the display device is arranged to display ah indication of the channel to which the receiver is tuned.

13. A receiver as claimed in any preceding claim in which the channel to which the receiver is tuned can be controlled by operation of the keyboard.

14. A receiver as claimed in any preceding claim further including an automatic channel searching arrangement operative to tune to each of a plurality of channels in turn until a channel of sufficient strength is found.

15. A receiver as claimed in any preceding claim in which the signal-receiving means includes a synthesised tuner.

16. A receiver as claimed in any preceding claim in which the display device includes a matrix of picture elements disposed in a plurality of rows and columns to display characters as sub-matrices.

17. A receiver as claimed in claim 16 in which the picture elements are of oblong shape with their minor dimensions extending across the widths of the characters.

18. A receiver as claimed in claim 17, in which the major dimension of each picture element is about 30% greater than its minor dimension.

19. A receiver as claimed in any one of claims 16 to 18, in which all or substantially all of the characters are displayed in a sub-matrix occupying 5 rows and 5 columns of the matrix of picture elements.

20. A receiver as claimed in any of claims 16 to 19 in which adjacent sub-matrices for displaying characters are separated from one another by further energisable picture elements.

21. A receiver as claimed in claim 20, in which adjacent sub-matrices in the same row are separated from one another by a single column of picture elements.

22. A receiver as claimed in claim 20 or 21 in which adjacent rows of sub-matrices are separated from one another by a single row of picture elements.

23. A receiver as claimed in any of claims 16 to 22 in which the picture elements are liquid crystal elements.

24. A receiver as claimed in any preceding claim in which the display device is capable of displaying at least ten lines of text information.

25. A receiver as claimed in claim 24 in which the display device has a capacity of approximately 24 lines, each of approximately 40 characters.

26. A receiver, as claimed in any preceding claim, in which the receiver is arranged to store the number of a page of information that has just been displayed, and, in response to a command signal from a user, to display again that page of information.

27. A receiver as claimed in claim 26, in which one or more keys on the keyboard control the generation of the command signal to display again that page of information.

28. A method of operating a text receiver as claimed in any preceding claim.

29. A text receiver including signal receiving means arranged to receive signals representing text infor a- tion as a plurality of pages, signal-processing means arranged to process signals from the signal-receiving means and to provide information signals for display, a display device capable of displaying a page of informa¬ tion, storage means for storing the number of a page of information that has just been displayed, and control means operable by a user to cause the receiver to display again that page of information.

30. A method of operating a text receiver in which signals representing text information as a plurality of pages are received, the signals are processed to provide information signals for display and a selected page of information displayed, wherein the number of a page of information that has just been displayed is stored and the user can give a direct command to the receiver to display again that page of information.