GB2187614A - Teletext systems - Google Patents

Abstract

A teletext decoder which is adapted to produce a teletext display using characters of a dynamically redefinable character set (DRCS). The decoder interrogates additional data in a received teletext page to obtain pseudo-pages which contain data representing the character shapes of characters for character codes in the basic page. The decoder processes this character shape data into a dot matrix format in a DRCS memory which is then addressed by character codes in the basic page to produce character generating signals for driving a display device. <IMAGE>

Description

SPECIFICATION Teletext Systems This invention relates to teletext systems, that is, systems of a type in which digitally coded data representing alpha-numeric text and graphics for display, or other teletext information, is transmitted in a television signal in television lines where no picture signals representing normal television picture information are present.

Ateletext system may be adapted for so-called "full-channel" operation in which all of the available television lines of a television signal are used for transmitting the teletext information. However, in existing television systems embodying teletext transmission, operating in the United Kingdom and elsewhere, only certain television lines in the field-blanking intervals are used for the transmission of the teletext information which is thus, in effect, multiplexed with the normal picture information in the television signal.

The teletext information transmitted in this way can be received and stored in a teletext decoder. This teletext information can then be used to provide a teletext display on the screen of a CRT (cathode ray tube) or other display device. The teletext decoder may form part of, or be otherwise associated with, a television receive. on the screen of which the teletext display is provided as a selectable alternative to the normal picture display. A teletext transmission can use over-air broadcast or satellite transmission, or it can use a cable as the transmission medium.

A teletext television system is described in United Kingdom patent specification 1 370 535. Also, the document "Broadcast Teletext Specification", September 1976, published jointly by the British Broadcasting Corporation, Independent Broadcasting Authority and British Radio Equipment Manufacturers Association, gives details of a teletext specification on which are based the Ceefax and Oracle (Registered Trade Marks) teletext television systems operated in the United Kingdom by BBC and IBA, respectively.

In the document "British Teletext Specification", a quantity of teletext information to be displayed as an entity is termed a page and will be so termed in the present specification. Pages which are available for display are normally transmitted in a recurrent cycle, with or without updating page information as appropriate. At a teletext decoder, any such page can be requested for display, and the digitally coded data representing the page information is then acquired by the teletext decoder from the cyclic transmission the next time they occur therein and stored in a page memory of the decoder for as long as the page is to be displayed.

Normally, the page format consists of 24 data rows each having 40 character positions. The alpha-numeric text and graphics which form the page information are composed of discrete characters which are displayed at respective character positions. The characters are represented by respective digital codes which are stored in the page memory at respective locations which correspond to the character positions in the displayed page. The shapes of the characters are defined by selected dots of a dot matrix which constitutes a character format. There is provided in a teletext decoder a character memory in which is stored character information identifying the character shapes that are available for display.The character information is addressed selectively in accordance with the digital codes stored in the page memory and the character information read out is used to produce character generating signals for driving a display device to produce the teletext display.

In order to facilitate this selective addressing, it is convenient to store the character information that identifies the patterns of discrete dots which define the character shapes as corresponding patterns of data bits in respective character memory cell matrices. With this form of storage, the dot pattern of a character shape as displayed at the relevant character position can have a one-to-one correspondence with the stored bit pattern for the character. Such character shapes are, for instance, the shapes of alpha-numeric characters.However, for other character shapes which require less resolution for their display, for instance the shapes of so-called graphics characters which can be used to display simple diagrams and mosaics composed of "large" character dots, rather than text, the corresponding stored bit patterns need not conform to the one-to-one correspondence, provided that the addressing of these latter stored bit patterns is appropriate for such a character shape as displayed to fill the entire character position concerned.

As set forth in the document "Broadcast Teletext Specification", a limited range of colour choice for displayed characters is made possible by the use of control characters. With a view to extending the colour choice, a more recent proposal is to use more than a single stored data bit for each dot of a character in order to encode a colour choice uniquely for that character dot. With this proposal, the stored character information for a character shape will consist of more than one bit pattern.

Hereinafter, character bit patterns of different size and multiple will be considered as pertaining to different character modes which are identifiable according to their size and multiple. For instance, an mxnxb character mode defines a bit matrix format containing mxn bits which is repeated b times to provide b-bits per displayed character dot of the character shape concerned.

In a teletext decoder, the amount of character memory that can be provided viably imposes a limitation on the number of character shapes of different size and colour choice that are available for a teletext display. One or more fixed character sets may be provided permanently in a "read-only" character memory of the teletext decoder. Additionally, it has now been proposed to provide a teletext system with a number of so-called "dynamically redefinable character sets" (DRCS), the shapes of which are transmitted as required to teletext decoders for temporary storage therein in a random access read/write character memory. The use of such DRCS characters provides a virtually unlimited choice of different character shapes within the character format used, for all or only part of a teletext display.

However, this selective use of DRCS characters poses problems in the logic organisation within a teletext decoder for dealing with received DRCS characters. It is an object of the present invention to provide means for overcoming these problems.

According to the present invention, there is provided a teletext decoder for processing coded digital data, representing alpha-numeric text for display or other teletext information, which are transmitted in a television signal in television lines where no picture signals representing normal picture information are present, the teletext information comprising pages which are transmitted in a recurrent cycle, each page comprising a plurality of data rows of which a first includes the page number and each data row has an individual row address: said teletext decoder comprising: means for selecting a first page by its page number, means for detecting a selected page when received, - a page memory in which data in each received data row of a selected page is stored in a respective memory portion which is identified by the row address of the data row, means responsive to received and stored data in a given row to cause the selection, detection and storage of at least one further teletext page containing data that defines characters appertaining to character codes contained in the first page selected, means responsive to the character codes in the first page selected to address the stored character defining data; and means responsive to resulting output signals to produce character generating signals for driving a display device.

The invention thus provides a teletext decoder whose logic operations permit DRCS characters to be used in a simple manner.

In carrying out the invention, the teletext decoder can include a character dot memory, means being provided for converting character data in the further teletext page into a bit matrix format and storing it in this format as the character defining data in the character dot memory.

The teletext decoder may further include means for converting the character codes in the first (selected) page into display and attribute codes which are stored in a display memory, and means for addressing the display memory to produce output signals for addressing selectively either said character dot memory or a fixed character memory, to produce said resulting output signals. The address selection of the character dot memory or the fixed character memory may be in accordance with data in a second given row of the first (selected) page.

In orderthatthe invention may be more fully understood, reference will now be made byway of example to the accompanying drawings, of which: Figure 1 illustrates diagrammatically seven different DRCS characters modes; Figure 2 illustrates diagrammatically the general organisation of a teletext pseudo-page for transmitting DRCS characters; Figure 3 illustrates diagrammatically a code table for a set of DRCS characters; Figures 4 to 6 illustrate diagrammatically the organisation of teletext pseudo-pages for transmitting DRCS characters of different modes; Figure 7 is a block diagram of a teletext system of the type referred to; Figure 8 is a block diagram of a teletext television receiver; and Figure 9 is a block diagram illustrating the operation of a teletext decoder.

Referring to the drawings, for the purposes of the present invention it is assumed that there are seven different DRCS character modes P to V as listed in the following Table 1 and as illustrated diagrammatically in Figure 1.

TABLE 1 No. of Bits Number No. of Dots No. of Dot Per Dot of Per Row (H) Rows (V) (or Pixel) D-bytes P 12 10 1 20 Q 12 10 2 40 R 6 10 1 10 S 6 10 2 20 T 6 10 4 40 U 6 5 2 10 V 6 5 4 20 It can be seen that each character mode is composed of 1,2 or 4 bit layers with a character size of 12x10, or 6x 10 or 6x5 dots as the case may be. Consideration of these character modes shows that each is composed of 10, 20 or 406-bit patterns. These 6-bit patterns will be referred to hereinafter as D-bytes and the number of bytes required for each character mode is also given in the above Table 1. In Figure 1, there is shown by way of example two possible D-bytes DB1 and DB2 for a character shape having the character mode P.

As specified in the document "Broadcast Teletext Specification", a teletext data line contains 40 8-bit character bytes. These character bytes can be utilised to transmit (or download) respective D-bytes of DRCS characters. For this purpose, in the present example, the six least significant bits of an 8-bit character byte are used. In hexadecimai notation, this means that the 64 codes 4/0 to 7/15 (i.e. 7/F in hexadecimal notation) are used to represent all the 64 different possible bit patterns that the D-bytes can have.

Using one 8-bit character byte for each D-byte, it becomes possible to transmit 40 D-bytes in a teletext row. A teletext page containing 24 data rows can therefore transmit 40x24=960 D-bytes. Hereinafter, a teletext page to be used for this purpose will be referred to as a "pseudo-page". In order to obtain a measure of conformity in the transmission of the D-bytes of different character modes, a pseudo-page is considered to consist of 48 "Pattern Transfer Units" (PTUs) each containing 20 D-bytes. The organisation of such a pseudo-page is illustrated in Figure 2. This pseudo-page is designated PP and comprises a page-header (PH) row R0 and twenty-four data rows R1 to R24 which will hereinafter be termed "packets".

Each packet contains two PTU's, and each PTU contains 20 D-bytes DB.

Figure 3 illustrates a code table CT for a set of up to 96 DRCS characters which have respective locations in the table as defined by DRCS character codes 2/0 to 7/15. In this table only a few of the character codes have actually been shown. Each code is a composite of two 4-bit bytes b1 to b4 and b5 to b8. The bit b8 is not shown because it is normally a parity bit. The PTU's in a pseudo page are associated with the character locations in the code table of Figure 3 such that there is always a simple relationship between the packet number/PTU number of a pseudo page and the DRCS character codes. Figures 4, 5 and 6 illustrate diagrammatically the allocation of the packets and PTUs to the bit patterns for the characters having the character codes 2/0 to 7/15 given in the code table of Figure 3, to achieve the simple relationship.

More specifically, for mode P (i.e. 12x10x1), one PTU of 20 D-bytes is sufficient to define exactly one character. Two pseudo pages can thus be used to define a complete set of 94 (or 96) mode P characters. As shown in Figure 4, in a first pseudo page PP1, PTU 1 in packet PKT1 contains the 20 D-bytes for the character code 2/0, PTU2 in packet PKT1 contains the 20 D-bytes DB for the DRCS character code 2/1, PTU3 in packet PKT2 contains the 20 D-bytes for the DRCS character code 2/2, PTU4 in packet PKT2 contains the 20 D-bytes for the DRCS character code 2/3, and so on. Thus, the bit patterns for the DRCS characters having the codes 2/0 to 4/15 are transmitted in the pseudo-page PP1 and the bit patterns for the DRCS characters having the codes 5/0 to 7/15 are transmitted in a second pseudo-page PP2.By way of example, with respect to this bit pattern transmission, Figure 4 also shows that in packet PKT17, the pattern transfer unit PTU33 contains the 20 D-bytes for the DRCS character having the code 4/0. Only the D-bytes DB1, DB2 and DB12 of this PTU33 are shown. These may be, for instance, the correspondingly designated D-bytes for the mode P character shown in Figure 1.

Considering now the other DRCS character modes, it can be seen from Table 1 above that characters in modes S(6x10x2) and V(6x5x4) also require 20 D-bytes for their definition and can therefore be transmitted using exactly one PTU per character. Therefore, the relationship between the packet number/PTU number and the character code for a mode S or mode V character being defined can be made the same as for a mode P character. The organisation of Figure 4 can therefore apply to character modes P, SandV.

The DRCS characters in modes Q(12x10x2) and T(6x10x4) each require 40 D-bytes for their definition.

For these two modes, compatability of the simple relationship between the packet number/PTU number of a pseudo page and the character code is maintained by using the two PTUs in the same packet to transmit the bit patterns for a single (even) character code. The second PTU in each packet is now not available for the following add character code and this limits the number of character codes that can be identified using two pseudo pages to one half the previous number. The add character codes can be transmitted by using the second PTU in one packet and the first PTU in the succeeding packet together. The organisation for even character codes is illustrated in Figures 5a and 5b.In the pseudo page PP1, PTU1 and PTU2 in packet PKT1 contain the character code 2/0, character code 2/1 is omitted, PTU3 and PTU4 in packet PKT2 contain the character code 2/2, character code 2/3 is omitted, and so on. The same organisation continues on the pseudo page PP2 so that every second code is transmitted in the same location as corresponding codes in modes P, S and V. The corresponding organisation for odd character codes is shown in Figures Sc and 5d. In order to give all character codes the same transmitted locations, PTU2 in packet PKT1 and PTU3 in packet PKT2 contain the character code 2/1, PTU4 in packet PKT2 and PTU5 in packet PKT3 contain the character code 2/3, and so on. This means that each character code in modes Q and T has a (first) PTU number that is the same as that for corresponding character codes in modes P, S and V.There is the limitation that a character for code 7/15 cannot be defined in modes Q and T because this code has no second PTU available to it.

Characters in the remaining DRCS character modes R(6x10x1) and U(6x5x2) each require only 10 D-bytes for their definition. In order to maintain the simple relationship between the PTU used for a character code and the table location of the character code, only one half of each PTU of a pseudo page is actually used to define one set of mode R and U characters with codes 2/0 to 7/15. However, a corollary to this is that two such character sets of 94 (or) 96 characters can each be defined using two pseudo pages, so that then there is no redundancy. This organisation is illustrated in Figure 6a and 6b. In Figure 6a, in the pseudo page PP1, the first half (10 D-bytes) of PTU1 in packet PKT1 contains the character code 2/0, and the first half of PTU2 contains the character code 2/1.Similarly, the first half of PTU3 in packet PKT2 contains the character code 2/2, and the first half of PTU4 contains the character code 2/3, and so on, with character codes 4/14 and 4/15 being defined by the 10 D-bytes in the first half of PTU47 and PTU48, respectively. The pseudo-page PP2 is similarly organised for the remaining character codes. It will be apparent that a second character set of mode R or U characters can be transmitted in the second half of each PTU, with the simple relationship being maintained between the PTUs and character codes.

Because the PTU's are downloaded using the existing teletext transmission format, the D-bytes enjoy the parity protection which is already present for the 8-bit teletext character codes. Furthermore, the actual character codes have implicit Hamming protection because the packet numbers imply which character codes are which by their position in a pseudo-page. Transmission errors do not "propagate" throughout a DRCS character set as it being transmitted, but are confined to each character code in the same way as for normal teletext transmission. In normal teletext transmission, the space-code 2/0 is loaded into the page memory for each character that does not satisfy parity on its first reception. This space-code 2/0 is not in the code range 4/0 to 7/15 used forthe D-bytes, so that an error in a D-byte is immediately recognised.Double (or multiple) bit errors in a D-byte which satisfy parity fortuitously, would only corrupy the equivalent 6 dots of the character concerned, so that the visual effect of such serious transmission errors will be minimal.

There are the following eleven different possible downloading types of PTU.

1. PTU for a mode P character.

2. PTU for a mode S character.

3. PTU for a mode V character.

4. PTU for the first half of a mode 0 character.

5. PTU for the second half of a mode Q character.

6. PTU for the first half of a mode T character.

7. PTU for the second half of a mode T character.

8. First half of a PTU containing a mode R character and the second half of the PTU containing a mode R character of different set.

9. First half of a PTU containing a mode R character and the second half of the PTU containing a mode U character.

10. First half of a PTU containing a mode U character and the second half of the PTU containing a mode U character of a different set.

11. First half of a PTU containing a mode U character and the second half of the PTU containing a mode R character.

These eleven downloading types of PTU may be identified by a 4-bit binary code, and 48 of the relevant 4-bit code combinations are required to fully identify all the PTU's in a pseudo-page.

As specified in the document "Broadcast Teletext Specification", the data rows of a teletext page are identified by respective 5-bit row addresses. Only 24 (plus 1 page-header row) of the 32 possible 5-bit code combinations are used for downloading the PTU's. Therefore, a further 7 row addresses are available for transmitting other information and, of these one is used as an address for a "packet 28" which contains the identities of the different downloading types of PTU which are transmitted in a pseudo-page.

More specifically, the 40 8-bit data bytes which are contained in this packet 28 are divided into thirteen 3-byte groups. Each group is Hamming protected [24,18], that is, itcontains6protection bits and 18 useful data bits. The data bits in the first 3-byte group are used to define (a) that the "packet 28" is a DRCS pseudo-page, and (b) which half of a character set is defined by the PTU's contained in the page. The data bits in eleven of the remaining twelve 3-byte groups are used in 4-bit groups to identify the 48 PTU codes contained in the pseudo-page. The remaining 3-byte group is spare. The Hamming protection has the usual property that over each 4-bit PTU code, a single bit error is corrected and a double bit error is detected.

Thus, triple bit errors are necessary before an incorrect interpretation is placed on a 4-bit PTU code.

The strategy described above for downloading DRCS character sets has the advantage that the data is "fixed format" in the same way as the normal teletext transmission specified in the document "Broadcast Teletext Specification". In other words, there is a simple relationship between where on a data-line a D-byte is transmitted and where it ends up in memory in a teletext decoder. Thus, transmission errors are unlikely to result in "good" data ending up in the incorrect memory location and thus over-writing previouslyreceived "good" data. Also, protection is applied where it is most necessary; in that, the data identifying the all-important packet number is Hamming protected as well as being transmitted at the beginning of a data line where the bit and byte-synchronisation are likely to be at their strongest.

As shown generally, in Figure 7, a teletext system comprises a television transmitter 1 for transmitting a television video signal VS to television receivers such as receiver 2 via a conventional over-air broadcast or other transmission link 3. The transmitter 1 includes in known manner means for producing television picture information, means for producing teletext information, and further means for generating the television video signal VS containing picture signals representative of the picture information and data pulses representative of the teletext information, together with the usual synchronising, equalising and blanking pulses which are necessary for the operation of the television receiver.The television receiver 2 comprises the usual amplifying, tuning and i.f. detector circuits at its front end, together with video and data processing circuits as will now be considered with reference to Figure 8.

The television receiver shown in Figure 8 has its front end 4 connected to receive the incoming television video signal VS. For normal picture display by the television receiver, the demodulated video signal VS' is applied to a colour decoder which produces the R, G and B component signals for the picture display. Time base circuits for a display tube (not shown) receive the usual line and field synchronising pulses from a sync. pulse separator circuit which extracts these synchronising pulses from the video signal VS'. The element 5 represents the colour decoder and these other circuit elements which are provided for conventional picture display.

The demodulated video signal VS' is also applied to a teletext decoder section of the teletext television receiver which deals with the receipt and display of the alpha-numeric text and other teletext information that is received in digitally coded form. This section comprises a video processor circuit 6 which performs inter alia data slicing for retrieving teletext data pulses D from the video signal VS'. The video processor circuit 6 also produces input data clock pulses C from the data pulses D. The data pulses D are fed together with the clock pulses C to a data acquisition circuit 7 which is operable to feed selected groups D/G of the teletext data pulses to a memory 8 as address, control and display information. The memory 8 has a capacity for storing several pages of information, comprising a plurality of data rows.The page and row format laid down in the document "Broadcast Teletext Specification" is assumed.

A logic processor 9 is operable in accordance with page select signals S applied to it from a remote control arrangement 10 to control which groups of teletext data pulses are acquired by the data acquisition circuit 7. The arrangement 10 has a receiver part 1 Oa and a remote transmitter part comprising a transmitter 10b and a keypad 1 Oc. The processor 9 is further operable to read out from the memory 8 the stored teletext information for the selected page to provide display information for the page in a display controller circuit 11. The display controller 11 is operable in accordance with the display information to produce R, G and B component signals for the teletext display. A timing circuit 12 provides timing signals t1 to t3 for the circuit elements 7,8 and 11.These circuit elements and the timing circuit 12 are accessed by the processor 9 via an interface circuit 13. The operation ofthetiming circuit 12 is synchronised with the received video signal VS by a composite pulse signal VCS which contains the line and field synchronising pulses which are separated from the demodulated video signal VS' in the video processor 6.

In the teletext television receiver shown in Figure 8, only single line connections have been shown for the interconnections between the various circuit elements for the sake of simplicity. However, it will be apparent to a person skilled in the art that in practice most of these interconnections would be multi-line.

For instance, whereas the teletext line data pulses D retrieved from the video signal VS' would be applied serially to the data acquisition circuit 7 over a single connection, serial-to-parallel conversion would take place within this circuit 7, so that the groups D/G of teletext data pulses would be applied to the memory 8 in parallel over a multi-line connection. Also, the connection between the processor 9 and the interface 13 would be a multi-line bus. The processor 9 can be a commercially available microcomputer; e.g. from the 68000 Series. The circuit elements 7, 12 and 13 can be found in the integrated circuit EUROCCTtype SAA 5240 (Mullard); the circuit element 6 can be the integrated circuit VIP2 type SAA 5230 (Mullard); the circuit element 11 can be found in the integrated circuit EUROM type SAA 5350 and the memory 8 can be commercially available Random Access Memory (RAM)-for instance of 8K8 capacity.

Although a composite television receiver for receiving both normal picture information and teletext information is exemplified in Figure 8, it will be appreciated that the teletext decoder section together with the front end 4 may be provided as a separate teletext decoder which is adapted to feed either a CRT display monitor or a conventional television receiver.

For the performance of the invention the logic processor 9 is operable in accordance with a control strategy as illustrated in Figure 9. In this Figure 9, the rectangular elements represent hardware components of the teletext decoder, whilst the elliptical elements represent control functions which are performed by the processor. The inputs and outputs for these control functions are shown in broken lines. The connections between the various hardware components are shown in full lines.

The element D/AQU represents the data acquisition circuit (7-Figure 8) which receives the incoming teletext data TD. This element D/AQU is under the control of a user interface element US/I which represents the control functions performed by the processor in response to user generated signals from the remote control arrangement (lO--Fig u re 8). The element D/AQU is also under the control of further control functions, represented by the element AO/C, which are performed by the processor in response to control data which is provided in packet 27 of a basic teletext page that has been selected by a user and stored in the teletext decoder in a page memory BP.This control data identifies interalia the page numbers of DRCS pseudo pages which contain the DRCS character set(s) that are required for the display of the basic teletext page in the page memory BP. Figure 9 illustrates by way of example the association of two pseudo pages 1/PP and 2/PP with the basic page. These two pseudo pages are acquired by the teletext decoder on their next occurrence in the teletext transmission cycle and are stored in the decoder in memories 1/PP and 2/PP.

Each of these pseudo pages has a packet 28 which, as already described, contains data which identifies the DRCS character modes that are contained in the packets 1 to 24 of the two pseudo pages. This data is accessed by the processor which is responsive thereto to perform control functions as represented by the element FC to write the dot patterns for the DRCS characters, together with data which identifies their mode, into a DRCS dot and mode memory DRCS/MM.

The memory DRCS/MM now contains character information which supplements the character information already provided in a read-only memory ROM of the teletext decoder. The character codes received in the basic page and additional data contained in at least one packet 26 of the basic page are now read out by the processor and converted into display codes. The control functions for performing these operations are represented by the element CON/D. The resulting display codes are stored in the memories CM and AM. For non-DRCS characters, the memories CM and AM are addressed and output data therefrom addresses the read-only memory ROM which in turn addresses a colour choice element CCE that converts a single bit output from the read-only memory ROM into a 5-bit output that is used to address a colour map C/M. The colour map C/M provides 32 different colour display choices from a possible 4096 colour choices on a 12-bit output. This 12-bit output drives three digital-to-analogue converters D/A which provide RGB colour component signals forthe display.

For DRCS characters, ohe memory DRCS/MM is addressed by the output data from the memories CM and AM. This memory DRCS/MM has a 1, 2, or 4-bit output which addresses the colour choice element CCE, which again provides a 5-bit output that is used to address the colour CM. The basic page has a packet 28 the data in which is interpreted by the processor to control the operations of the element CCE. The element INT/D represents the control functions for this interpretation. Timing is not shown in Figure 9 in order to keep the diagram simple.

Claims (7)

1. A teletext decoder for processing coded digital data, representing alpha-numeric text for display or other teletext information, which are transmitted in a television signal in television lines where no picture signals representing normal picture information are present, the teletext information comprising pages which are transmitted in a recurrent cycle, each page comprising a plurality of data rows of which a first includes the page number and each data row has an individual row address: said teletext decoder comprising: means for selecting a first page by its page number, - means for detecting a selected page when received, - a page memory in which data in each received data row of a selected page is stored in a respective memory portion which is identified by the row address of the data row, means responsive to received and stored data in a given row to cause the selection, detection and storage of at least one further teletext page containing data that defines characters appertaining to character codes contained in the first page selected, means responsive to the character codes in the first page selected to address the stored character defining data; and means responsive to resulting output signals to produce character generating signals for driving a display device.

2. A teletext decoder as claimed in Claim 1, including a character dot memory, means being provided for converting character data in the further teletext page into a bit matrix format and storing it in this format as the character defining data in the character dot memory.

3. A teletext decoder as claimed in Claim 2, including means for converting the character codes in the first (selected) page into display and attribute codes which are stored in a display memory, and means for addressing the display memory to produce output signals for addressing selectively either said character dot memory or a fixed character memory, to produce said resulting output signals.

4. A teletext decoder as claimed in Claim 3, including means for determining the address selection of the character dot memory or the fixed character memory in accordance with data present in a second given row of the first (selected) page.

5. A teletext decoder as claimed in any one of Claims 2 to 4, wherein means are provided for determining from data in a given row of said further teletext page to which one of several different types of bit matrix format the character defining data for each character pertains.

6. A teletext decoder as claimed in any preceding claim, responsive to character defining data which is organised in pseudo-pages in the manner substantially as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings.

7. A teletext decoder substantially as hereinbefore described with reference to Figures 8 and 9 of the accompanying drawings.