TELETEXT

Abstract: In order to decode full field teletext data using conventional teletext decoders which only recognise teletext data contained in lines of the vertical blanking interval, the full field data is divided into a plurality of channels each occupying no more than the number of lines in a vertical blanking interval. The data from a selected one of the channels is delayed before being applied to the decoder so that it occurs within the following vertical blanking interval.

Description: DESCRIPTION TELETEXT The invention relates to a method of and an arrangement for transmitting and/or decoding teletext signals. Normal teletext signals are transmitted in the vertical blanking interval (VBI) of a video signal occupying typically up to 15 lines per field. If however, there is no video signal present and the channel is used only for the transmission of teletext data then for a 625 line PAL television signal, about 300 lines per field can be used. This gives roughly 20 times the capacity of normal teletext transmission and allows more pages to be transmitted and/or reduced access times for each page. Teletext decoders as normally implemented in television receivers cannot receive full channel teletext data however. This arises as a normal teletext television receiver expects and can only receive teletext data in the VBI. Thus if a television set is connected to for example to a cable channel which is transmitting full field teletext then access to this data can only be achieved by having a special teletext decoder. This is clearly inconvenient to the user and likely to mean that the possible audience for a channel transmitting teletext over the full field is much reduced. In addition the possible user base for full field teletext information provided other than by cable television for example data pre recorded on a record carrier such as a video tape will also be reduced. It is an object of the invention to enable the decoding of full field teletext transmissions using normal teletext decoders. The invention provides a method of transmitting teletext data, the method comprising the steps of; a) transmitting said teletext data during at least part of the field period outside the vertical blanking interval (VBI) and b) dividing the teletext data into m separate data streams each occupying up to n lines where m and n are integers and n is less than or equal to the number of lines in the VBI. This method may be used particularly in a cable television system having a multiplicity of channels where one of the channels may be dedicated to the transmission of data. The data can be formatted into standard teletext form and transmitted not only during the VBI but also during the normal active picture period. By dividing the teletext data into a plurality of separate data streams each of which occupy a number of lines less than or equal to the number of lines in the VBI a standard teletext decoder may be used in the receiver to receive the teletext data. Clearly each data stream may contain a different number of lines provided they do not exceed n and the number of lines may vary depending on the quantity of data to be transmitted. The teletext data may be recorded on a record carrier. Thus a record carrier which may be a disc or a tape can be provided which is prerecorded with data which can be accessed and read using a standard television receiver. In order to allow satisfactory reproduction from teletext data recorded on a VHS video tape the teletext data may be recorded as a multilevel signal at a data rate lower than the standard teletext data rate. It is well known that standard VHS video recorders cannot satisfactorily record and replay standard teletext data due to the restricted bandwidth. An arrangement for allowing teletext data to be recorded and replayed by a VHS video recorder has been disclosed in our co-pending UK Patent Application No. 9605614.8 (PHB 34056). In this co- pending application an arrangement is disclosed in which the teletext data is recorded as a multilevel signal at a data rate lower than that of the standard teletext data rate. The recorder also includes circuitry which converts the received standard teletext data to the multilevel code and which reconverts the multilevel code to the standard teletext data rate on replay. The conversion from standard teletext data to the multilevel signal is not of course required of the video recorder if a prerecorded tape is played as the prerecorded teletext data will already be recorded as the multilevel signal. In that case only the arrangement for encoding the multilevel code into standard teletext code is required. Each of the data streams may occupy 15 lines, this being the number typically occupied by teletext transmissions in the vertical blanking interval. Where this is the case the number of data streams may be 20. This will occupy a total number of 300 lines per field. The invention further provides a record carrier bearing teletext data, the teletext data extending during at least part of a field period outside the VBI, wherein the teletext data is divided into m separate data streams each occupying up to n lines, where m and n are integers and n is less than or equal to the number of lines in the VBI. The record carrier may be a VHS tape and the teletext data may be stored as a multilevel code at a data rate lower than that of the standard teletext data. In this way data may be provided on a record carrier which is decodable by means of a standard teletext decoder in a normal television receiver, the major requirement being that the television receiver includes a teletext decoder. In the UK in particular most television sets are now provided with a teletext decoder and consequently data may be replayed by the television set from the record carrier and a greater amount of data than can be carried in the teletext service in the VBI is available. It is not of course essential that the teletext data occupies the whole of the field or that at all portions of the record carrier the teletext data extends outside the VBI interval. It may be convenient to have moving pictures interspersed with teletext data at the full field rate. Neither is it essential that all the lines within a data stream are used or that all the data streams contain the same number of lines. The invention still further provides a method of decoding teletext data transmitted by means of a method according to the invention or replayed from a record carrier according to the invention, the decoding method comprising the steps of; a) selecting one of the m data streams of the received teletext data and b) applying a teletext signal containing the selected data stream to a teletext decoder during a VBI. The decoding method according to the invention allows a standard teletext decoder to decode the data transmitted or replayed from the record carrier since one data stream is selected and time shifted so that it occurs during a VBI. Thus the standard teletext decoder recognises the teletext signal and can decode the signal to produce the appropriate display on a TV receiver display. The invention still further provides video replay apparatus for replaying teletext data recorded on a carrier, the video replay apparatus comprising means for selecting for replay one of said streams of teletext data and means for making available at an output the selected steam of teletext data during a VBI. Thus replay apparatus such as a video cassette recorder includes means for encoding the teletext data recorded on a video tape into the standard teletext format and delaying its output until the next VBI. In this way the television receiver will receive what is apparently a standard teletext signal during the VBI selected from the one of the several streams of teletext data which occur outside the VBI. The invention yet further provides a television receiver arranged to receive teletext data transmitted by a method according to the invention, the television receiver comprising means for selecting one of the m streams of teletext data and means for delaying the application of the selected stream to the teletext decoder until the occurrence of the next or a succeeding VBI. Thus by providing the television receiver with selection and delay means a selected stream of teletext data may be applied to the teletext decoder during a VBI. In this way a standard teletext decoder will recognise and decode the teletext signal presented to it. The invention yet further provides a decoder for decoding a television signal, the television signal comprising teletext data generated by a method according to the invention, said decoder including means for selecting one of the m data streams and for applying the selected data stream to a teletext decoder during a VBI. The decoder may for example be a so called set top box which may be present for decoding digital video broadcasts either from satellite or terrestrial transmitters or for decoding cable television signals. The decoder may conventionally include for example access control and charging means for charged for channels or pay per view applications. The above and other features and advantages of the invention will be apparent from the following description by way of example of embodiments of the invention, with reference to the accompanying drawings, in which : Figures 1 to 3 illustrate normal and full channel teletext data signals, Figure 4 is a timing diagram showing how a selected service may be time shifted, Figure 5 is a block diagram illustrating the replay of teletext data from a video cassette recorder according to the invention, Figure 6 is a block diagram of the service selector function of the video cassette recorder shown in Figure 5, Figure 7 shows the combination of teletext data services with a reduced height television picture and, Figure 8 illustrates a process for selecting a particular data stream from a multiplicity of such streams. Figure 1 represents one field of a typical television signal which includes teletext data transmitted in the vertical blanking interval represented by period A in Figure 1 and television picture information transmitted during the rest of the field period represented by period B in the Figure 1. Typically, a teletext decoder will be arranged to look for teletext data during lines 6 to 22 of field 1 and 319 to 335 of field 2. This is for a standard 625 line PAL television signal. In contrast a full channel teletext signal occupies the whole length of the field apart from a short field synchronisation sequence as shown in Figure 2. Thus teletext data will replace the television picture signal during the period B of the television field. Such a full channel teletext signal is known but cannot be decoded by a standard television receiver since the teletext decoder is set to look for teletext data only during the VBI. One reason for restricting the teletext decoder to look for teletext data only during the VBI is to prevent erroneous detection of teletext signals by the television picture signal mimicking teletext data. Thus a certain picture content could produce a signal which was similar to teletext data and the teletext decoder would then decode that as teletext data. However by ensuring that the teletext decoder only looks for teletext data during the VBI the possibility of such picture content causing a false detection of teletext data can be eliminated. It does however have the corresponding disadvantage that full channel teletext data cannot then be decoded. Figure 3 shows full channel teletext data which is divided into a plurality of separate data streams each having a duration which is less than the length of the VBI. Thus the teletext data may for example be split into 20 separate data streams each occupying 15 lines. It is, of course, not necessary that each of these 20 separate data streams occupies the same number of lines, provided that the number of lines of that data stream is less than or equal to the number of lines in the VBI. Further it is not essential that all the lines in any data stream are occupied by teletext data. Each of these 20 separate data streams will provide an individual service so that for example 20 services can be transmitted and the user can select between them. In order to decode the full channel teletext data one of the 20 separate data streams is selected and means are provided for shifting the occurrence of the selected data channel to a period corresponding with a VBI. Figure 4 illustrates this process. If it is assumed that data stream 6 is selected as shown in Figure 4a an enable signal, circuit as shown in Figure 4b, is generated which enables the data in data stream 6 to be written into a buffer memory. This is illustrated in Figure 4. Thus data is written into the buffer memory during lines which normally contain the TV picture signal but when full channel teletext is being transmitted is occupied by channel 6 of the teletext data. The buffer memory is then read out during the VBI of the next field and consequently a standard teletext decoder will recognise this as teletext data. Thus by splitting the full channel teletext data service into m streams each occupying n lines it is possible to select one of these data streams and apply it to a teletext decoder during a VBI. In the particular example m is equal to 20 and n is equal to 15. These numbers are not essential, the requirement being that the number of lines in each data stream is less than or equal to the number of lines in a VBI or is within the number of lines which the teletext decoder will assume can contain teletext data and the number of streams will depend on how many lines are in a television field and whether it is desired to totally occupy the television field by teletext data. The source of the teletext data may be a conventional television transmission either from a terrestrial transmitter, a satellite transmitter or a cable television service. Alternatively the source may be a record carrier for example, an optical disc or a magnetic tape. In the case of a cable television system or satellite broadcast or the proposed digital services from cable satellite or terrestrial transmitters there may well be a decoder box which is inserted before the standard television receiver. Such a decoder box may convert the transmitted signal from one encoding format to for example the PAL system or may be arranged to give an RGB output. In the former case the teletext decoder may be located within the television receiver as the television receiver will receive at least a CVBS signal which will contain the encoded teletext data or in the latter case the teletext decoder will have to be located in the decoder box since that is where the teletext signal exists and can be decoded. For playback from an optical record playback apparatus or from a video tape recorder the teletext decoder may be either in the playback apparatus or in the television receiver. If it is in the television receiver it is of course necessary that the connection from the playback apparatus to the television receiver is in CVBS form or modulated back to a PAL channel. It is well known that standard teletext data cannot be successfully recorded and replayed from a VHS video cassette recorder. This is because the bandwidth of the recorder is insufficient for the bandwidth of the teletext signal. Thus on replay a distorted signal which is impossible to decode is produced and satisfactory teletext reception cannot be achieved. It is however possible to record teletext data on a VHS tape using a technique disclosed in UK Patent Application No. 9605614.8 (PHB 34056). This application describes a VHS video recorder which includes an encoding apparatus which takes the teletext data and encodes it into a multilevel signal at half the standard teletext rate. Thus each collection of two bits is encoded using a four level code and this four level code is written onto the tape. On replay a complementary decoder is used which converts the four level code back to the standard teletext data which can then be applied to the teletext decoder which is located either in the VCR or in the television receiver to which the VCR is connected. Details of this encoding and decoding and of a video cassette recorder which is capable of performing these functions are given in our co-pending Application No. 9605614.8 (PHB34056) the contents of which are hereby incorporated by reference. Figure 5 is a block schematic diagram of the playback function of a VCR in which the teletext signal is encoded according to the scheme outlined in the above mentioned co-pending application and in which a full channel teletext service is provided. This block diagram shows only the replay function and not a recording function as the actual source of the data on the tape is unimportant to the invention. Thus it could be either from a prerecorded tape which is sold by a service provider or could be recorded by the user off air but the recording process is not relevant to the present invention. As shown in Figure 5, the playback function includes tape replay heads 1 which feed a luminance processor 2. The luminance processor is a purely standard VCR luminance processor. The output of the luminance processor is fed to a decoder 3 and to control and timing circuits 4 and a multiplexer 5. The output of the decoder 3 is fed to a buffer memory 6, whose output is fed to a teletext encoder 7 which converts the output of the buffer ram to the standard teletext data form. The control and timing circuit 4 takes the timing information from the signal produced by the luminance processor 2 and controls the operation of the decoder 3. Thus the decoder 3 will decode data output from the luminance processor at a appropriate times during each line period under the control of the control and timing circuit 4. Conveniently the decoder may be set to decode this data every line and the output of the decoder is read into the buffer memory 6 under the control of a service selector 8. Thus the service selector 8 will ensure that data is written into the buffer memory 6 only when a selected service is being output by the luminance processor 2. An output from the control and timing circuit 4 is also connected directly to the buffer memory 6 as a read enable signal which causes the data in the buffer ram 6 to be read to the teletext encoder at the normal VBI time. Thus data written into the buffer ram during the normal video period of a field is stored in the buffer memory 6 and read out during the succeeding VBI. In order to enable selection of a particular service the recorder includes a remote control receiver 15 which receives signals from a remote controlled handset 9. Thus instructions entered on the remote control handset 9 are detected by the receiver 8 and fed to the control and timing circuit 4. This will cause the service selector 8 to produce a write enable signal to the buffer memory 6 at the appropriate time to enable the selected data stream to be written into the buffer memory 6. The multiplexer 5 is controlled by a further output from the control and timing circuit 4 and selects either data from the teletext encoder 7 or video from the luminance processor 2 according to whether full channel teletext or mixed teletext and video signals are recorded on the tape. The output of the multiplexer is a CBVS signal which is either fed to an output 10 and connected via a SCART connector to the television set or is fed to a modulator 1 1 where it is modulated onto a UHF carrier and fed via an output 12 to the aerial input of an associated television receiver. Figure 6 is a block schematic diagram of an embodiment of a service selector 8 as shown in the VCR of Figure 5. The service selector has an input 80 which is fed to a register 81 which stores the service number requested by the user. The output of the register 81 is fed to a first input of a comparator 82 whose second input is connected to the output of a service counter 83. An input 84 of the service selector 8 receives pulses at line frequency which are fed to a line counter 85. The line counter will also receive a reset signal from an input 86 which resets the line counter at the end of each field. The line counter has an output which is connected to the service counter 83. Thus in the example given the line counter will increment the service counter every 15 lines. The output of the comparator 82 is fed to a first input of a multiplexer 87 whose second input receives a timing signal via an input 88. A control signal is fed to an input 89 which controls which input of the multiplexer 87 is fed to an output 90. The control signal 89 indicates whether a normal combined teletext and video signal is present or whether full channel teletex data is present. The timing signal on input 88 is the normal VBI timing which is selected when combined teletext/video signal is present. When full channel teletext is present the output of the comparator is fed to output 90 as a write enable signal for the buffer memory 6. This causes the decoded data to be read into the buffer memory 6 when the requested service stream is present. That is the output of the comparator will cause the write enable signal to be present for the 15 lines during which the selected service is being decoded. Figure 7 shows a situation where increased teletext data may be transmitted while a reduced height T.V. picture is also transmitted. This could be for example where a transmission of a letter box type picture was taking place, some of the active picture lines then becoming available for teletext data. Thus a 16 x 9 aspect ratio picture displayed on a 4 x 3 aspect ratio television would leave some lines at the top and bottom of the picture available for additional telext data services. The teletext data contained in these lines can be time shifted in the same way as the full channel teletext data is time shifted to the VBI. Figure 8 illustrates a process for selecting a particular data stream from the multiplicity of data streams transmitted in a full channel teletext signal according to the invention. Box 90 represents the step of reading data from the tape which indicates whether full channel data is available. Having read this data from the tape an on screen display is initiated, box 91 , which invites the user to select which channel to display. This is achieved by the user pressing a numbered key on the remote control unit. This is represented by box 92, which is the user entering digits from the remote control. The on screen display then confirms which service has been selected, represented by box 93. Thus the on screen display will say for example you have selected service 6. this number is then entered into the service number register 81 so that the service selector will select that particular data stream, box 94 From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design and use of television receiver and/or video recorders including teletext decoders and component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation of one or more of those features which would be obvious to persons skilled in the art, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims: CLAIMS: 1. A method of transmitting teletext data, the method comprising the steps of; a) transmitting said teletext data during at least part of the field period outside the vertical blanking interval (VBI), and b) dividing the teletext data into m separate data streams each occupying up to n lines, where m and n are integers and n is less than or equal to the number of lines in the VBI. 2. A method as claimed in Claim 1 in which the teletext data is transmitted over a cable television channel. 3. A method as claimed in Claim 1 in which the teletext data is recorded on a record carrier. 4. A method as claimed in Claim 3 in which the record carrier is a VHS video tape. 5. A method as claimed in Claim 4 in which said teletext data is recorded as a multilevel signal at a data rate lower than the standard teletext data rate. 6. A method as claimed in any preceding claim in which n = 15. 7. A method as claimed in any preceding claim in which m = 20. 8. A record carrier bearing teletext data, the teletext data extending during at least part of a field period outside the VBI, wherein the teletext data is divided into m separate data streams each occupying up to n lines, where m and n are integers and n is less than or equal to the number of lines in the VBI. 9. A record carrier as claimed in Claim 8, the record carrier being a VHS tape. 10. A record carrier as claimed in Claim 9 in which the teletext data is stored as a multilevel code at a data rate lower than the standard teletext data rate. 11. A method of decoding teletext data transmitted by a method according to any of Claims 1 to 7 or replayed from a record carrier as claimed in any of Claims 8 to 10, the method comprising the steps of ; a) selecting one of the m data streams of the received teletext data, and b) applying a teletext signal containing the selected data stream to a teletext decoder during a VBI. 12. Video replay apparatus for replaying teletext data recorded on a carrier as claimed in Claim 8 or Claim 9, said apparatus comprising means for selecting for replay one of said streams of teletext data, and means for making available at an output the selected stream of teletext data during a VBI. 13. Video replay apparatus as claimed in Claim 12 when dependent on Claim 9 comprising a VHS video recorder, wherein said video recorder includes means for encoding the multilevel code read from the tape into standard teletext data. 14. A television receiver connected to receive teletext data from video replay apparatus as claimed in Claim 12 or Claim 13. 15. A television receiver arranged to receive teletext data transmitted by a method as claimed in any of Claims 1 to 7, said television receiver comprising means for selecting one of the m streams of teletext data, and means for delaying the application of the selected stream to the teletext decoder until the occurrence of the next or a succeeding VBI. 16. A decoder for decoding a television signal, said television signal comprising teletext data generated by a method according to any of Claims 1 to 7, said decoder including means for selecting one of the m data streams and for applying the selected data stream to a teletext decoder during a VBI.

Inventors: KINGHORN JOHN RITCHIE, TARRANT DAVID ROBERT, KINGHORN, JOHN, RITCHIE, TARRANT, DAVID, ROBERT
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: H04N
IPC codes: H04N7