SUBTITLING TRANSMISSION SYSTEM

Abstract: A method of simultaneously transmitting a video signal and encoded data representing graphic images is disclosed. The invention is particularly applicable for transmitting multilingual subtitles with a video program. The graphic images are rectangular regions within the active video area. They are transmitted in the form of bitmaps. The invention not only allows any character font or size to be displayed, but also the definition of e.g. a program provider's logo. The encoded data includes a time stamp to specify the time at which a subtitle is to be displayed. Preferred embodiments of the invention include the transmission of colour-look-up-table (CLUT) data and a compatibility code indicating a required minimum number of entries of said colour-look-up-table. For receivers with a CLUT having more entries than necessary, a map table is transmitted for mapping the pixel data width to the input width of the relevant CLUT. The method is applicable to Direct Video Broadcast systems wherein the video signal is MPEG2 encoded and the graphic images are accommodated in a private data stream of an MPEG2 Transport Stream.

Description: Subtitling transmission system. FIELD OF THE INVENTION The invention relates to a method of transmitting encoded data defining a graphic image in the form of a rectangular region within an active video area. The invention also relates to a method of receiving said graphic images, and a transmitter and receiver arranged to carry out said methods. BACKGROUND OF THE INVENTION A prior art method of transmitting graphic images is generally known by the name "teletext". The World System Teletext standard has been adopted in many countries for the simultaneous transmission of textual and graphic information with a conventional analog television signal. The teletext system is particularly used for the transmission of subtitles associated with a television program. Meanwhile, the MPEG2 (ISO/IEC 13818) digital television standard will be applied worldwide for the broadcast of digital television programs to the end-user by satellite, cable, terrestrial networks, and by packaged media such as tape or disc. A multitude of integrated circuits for MPEG2 decompression and demultiplexing are on the market and will be applied in receiver decoders for the purpose of receiving and decoding of broadcast television programs or decoding of packaged (multi-)media. The MPEG2 compression and multiplexing technology will provide the end-user a very large choice in programs (typically in the order of hundreds), which will require an easy-to-use "man-machine interface". It is expected that the end-user will associate "digital" with high quality. High quality must therefore be manifest in all aspects of the service that will be provided. Typical of satellite broadcast is the large footprint of its reception area, which usually encompasses multiple language areas. Many of the television programs to be broadcast will therefore also be targeted at multiple language areas, which requires multi-language subtitling capabilities. The attractiveness and readability of subtitles will have a major impact on the end-user's willingness to watch a television program in a language spoken other than his or her own. The prior art teletext system is very limited in quality and performance. It is a character-based transmission system, in which coded data is transmitted to define which characters are to be displayed. The appearance of the subtitles (in terms of character font, spacing, graphic capabilities, colours) is fixed in hardware. Further, the present teletext system is not able to support complex character sets such as used in the People's Republic of China and Japan. The character-based system would require very expensive and complex character generators in case of supporting multiple character sets, or in case of a single but very complex character set. The prior art teletext system thus provides only a basic solution for multi-language subtitling. OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to overcome the above mentioned disadvantages of the prior art system. It is a further object to provide a universal solution for multi-lingual subtitling, in particular, for an MPEG2 environment. The method according to the invention is characterized in that the pixels forming said region are individually defined by the encoded data, the size and position of said region being included in the encoded data. T.ie method renders it possible for the program providers to control the appearance of the graphic images pixel-wise. The "Region based Graphics" system provides bitmapped rather than character coded regions overlaying the video signal. There are no restrictions as regards character size and character font, and there is a capability to overlay other graphics than text, e.g. the program provider's logo. In an embodiment, the encoded data further includes a time stamp representing the time at which said region is to be displayed. This allows multilingual subtitles to be transmitted in advance and to be displayed in exact synchronism with the video signal. In the prior art teletext system, the subtitles in the selected language are immediately displayed when received. More or less precise synchronization is possible only if all subtitles in the plurality of languages are transmitted within a restricted period of time. Subsequent encoded data for a given region may include a visibility code to disable the display of said region. This allows for effectively erasing a graphic region. In further embodiments of the invention, the pixels of a region are encoded as entries of a colour-look-up-table. Map tables are provided for mapping n-bit pixel codes to a colour-look-up-table having more than 2 s entries. In an particularly advantageous embodiment, the encoded data is accommodated in a private transport stream according to the MPEG2 or a similar standard. Packetizing the encoded data in PES packets of an MPEG2 private transport stream has the advantage of easy synchronizing the bitmaps to an associated MPEG video stream. BRIEF DESCRIPΗON OF FIGURES Figs. 1-3 show examples of a display screen comprising one or more graphic image regions. Fig.4 shows a region with its relevant size and position parameters. Fig.5 shows a schematic diagram of a transmitter adapted to simultaneously transmit a video signal and graphic images according to the invention. Fig.6 shows a schematic diagram of a receiver for simultaneously receiving a video signal and graphic images according to the invention. Figs. 7 and 8 show embodiments of a colour generating circuit of the receiver shown in Fig.6. DESCRIPΗON OF EMBODIMENTS The invention will now first be described in terms of the transmission format. The syntax as described hereinafter is similar to the syntax used in the MPEG2 (ISO IEC 13818) standard. The { and } symbols of this syntax have been omitted as they are not significant to the readability. Also omitted are data elements which are not essential to the invention or merely serve the purpose of byte alignment. The encoded data defining the graphic images is packetized in PES (Packetized elementary Streams) packets and accommodated in an MPEG2 Transport Stream. As prescribed by the MPEG2 standard, the PES packets comprise a header and payload. The header comprises a plurality of data elements, the following of which are relevant to the invention: * The element streamjd is set to '1011 1101" so as to indicate that the graphic image is conveyed as private _data_stream_l . - The element PES_packet_length is set to a value such that each PES packet is aligned with an MPEG2 transport packet. * The element datajalignmentjndicator is set to T, indicating that the region data fields are aligned with the PES packets. * The PTS (Presentation Time Stamp) indicates the beginning of the presentation time of each (region related) data contained in this PES packet. The PTSs of regions within one multi page shall differ more than 40 msec. The payload of PES packet defining the graphic images is denoted PESjL aJield(). Table I shows the syntax of this field. Table I The following semantic definition applies to the data elements shown in this Table: * datajderuifier - Region based Graphics (RbG) data is identified by the values 0x20-0x2F of the datajderuifier field. * RbG_stream_id - Identifies the Region based Graphics stream from which data is stored in this PES packet. * stream Jevel_oj ^compatibility - Indicates the minimum Colour-Look-Up-Table (CLUT) that is required to decode regions from this stream. The value 0x01 refers to a 2-bit CLUT, the value 0x02 refers to a 4-bit CLUT, and the value 0x03 refers to an 8-bit CLUT. * single _page_displayjlag - If set to '1', indicates that this RbG stream contains only one display page. * region Jd - Identifies the region from which the data is contained in the immediately following region _datajield(). * region Jevel_of_compaήbility - Indicates the minimum CLUT that is required to decode this region. The values defined above apply. * region j datajength ~ Specifies the number of bytes in the immediately subsequent region _datajxeld() the syntax of which is given in Table II. Table π shows the syntax of the region_data_field(). The following semantic definition applies to the data elements shown in this Table: * regionjk ajype - Identifies the type of region data. The value 0x10 refers to an intra region (I-region), the value 0x11 refers to region attributes (A-region), the value OxFF indicates a region to be deleted. Table π visibility Jlag - If set to T, specifies that this region shall be displayed as defined by the coded data for this region. If set to '0' this region shall be transparent. horizontal address Jlag - Indicates the presence of the region Jιorizøntal_address field. In I-regions this flag shall be T. vertical jaddress Jlag - Indicates the presence of the region_vertical address field. In I-regions this flag shall be T. number _ofj>ixels Jlag - Indicates the presence of the number _pf_pixels field. In I-regions this flag shall be T. * number _ofJines Jlag - Indicates the presence of the number j ofjines field. In I-regions this flag shall be '1'. * regionJu)rizontal_address - Specifies the horizontal address of the top left pixel of this region. Within the frame of 720*576 pixels only even pixels shall be addressed. The most left pixel of the 720 active pixels has index zero, and the pixel address increases from left to right. The horizontal address shall be such that each pixel of the region is within the active 720 pixels. * region _vertical_address - Specifies the vertical address of the top line of this region. The top line of the 720*576 frame is line zero, and the line index increases by one within the frame from top to bottom. The vertical address shall be such that each line of the region is within the 576 active lines. * numberj)fj>ixels - Specifies the number of horizontal pixels of this region. A value of zero is forbidden. * numberjofjines - Specifies the number of vertical lines of this region. A value of zero is forbidden. * number jof subj-egions - Specifies the number of sub-regions within this region. A value of zero is forbidden. * sub -egionjd - Identifies the sub-region. * number _ofjsub_regionJines Jag - Indicates the presence of the number jτf_sub_regionJines field. * clutjlag - Indicates the presence of the colour Jookj pj ataJield() field, the syntax of which is given in Table HI. * attribute jextension Jlag - If set to '1', indicates the presence of the attribute extension Jength field and the associated reserved fields. * codedjlatajlag - Indicates the presence of the coded _sυb egion_data() field which is defined in Table IV. * coded jtatajextension Jlag ~ If set to '1', identifies the presence of the coded jlatajextension Jength field and associated reserved fields. * number _of_sub_region lines - Specifies the number of lines in a sub-region. The total number of lines in all sub-regions within the region shall be equal to the number of lines in the region. * attribute jextension Jength - Specifies the number of immediately following reserved bytes. codedjkaa xxension Jength - Specifies the number of immediately following reserved bytes. Table HI shows the syntax of the colour Jookjφ_d aJield(). Table m The following semantic definition applies to the data elements shown in this Table: * numberjofjclutjupdates - Specifies the number of update cycles to be made. * number jof ntries — Specifies the number of CLUT entries that are updated. * 2Jbit_clutjiestinationJlag - If set to T, indicates that this entry is intended for a 2-bit CLUT. * 4J>itj:lutjiestinationJlag - If set to T, indicates that this entry is intended for a 4-bit CLUT. * 8J?it_clutjiestinationJlag - If set to T, indicates that this entry is intended for an 8-bit CLUT. * fi l angejlag - If set to T, indicates that the Y-value, U-value, V-value and T-value fields have full 8-bit resolution. If set to '0', then these fields contain only the 4 most significant bits. * entry _ycdue - Specifies the entry value of the CLUT. The first entry of the CLUT has the entry value zero. * Y_value - The luminance output value of the CLUT for this entry. * U_value - The first chrominance output value of the CLUT for this entry. * V_value - The second chrominance output value of the CLUT for this entry. * T_value - The transparency output value of the CLUT for this entry. A value of zero identifies no transparency. A value of 255 identifies full transparency. For all other values the level of transparency is defined by linear interpolation. Table IV shows the syntax of the coded jυb region_dma(). The following semantic definition applies to the data elements shown in this Table: * datajblock Jength - Specifies the number of bytes of the coded_sυb_regionJata field immediately following this field. * coding jnode - Specifies the applied coding mode. If set to '00', identifying the presence of the mode jextension Jield. * pixels Jnjnode - Specifies the number of pixels coded by a 2, 4 or 8 bit code as specified by the coding jnode field. * 2J>it_code - A two bit code specifying the colour of a pixel. * 2_stuffJ>its - two stuffing bits that shall be coded with 'IT. * 4Jbit_code - A four bit code specifying the colour of a pixel. * 4_stuffJ>its - Four stuffing bits that shall be coded with '1111'. * 8J>itj:ode - An eight bit code specifying the colour of a pixel. * mode jextension - Identifying the applied extension mode. If set to '00', identifies the presence of a command. If set to another value, identifies that run- length coding is applied. * WJbitjvn - Specifying the number of subsequent pixels with the colour identified by the immediately following 2J>itj:ode field. * SJ>it vn — Specifying the number of subsequent pixels with the colour identified by the immediately following 4J)it_code field. * 4J>itJ r un — Specifying the number of subsequent pixels with the colour identified by the immediately following δjbitjcode field. * command Jength — Specifies the number of bytes in this coded ub egionji aO field immediately following the command Jength field. If set to a value of 15, this number is not specified by the command Jength field. * long Jength - Specifies the number of bytes in this code ub egion_data() field immediately following the long Jength field. Table IV command Jd - Identifies the type of command. The value 0x10 denotes Load Map Table. This command is followed by a mapjablejdataQ field which is defined in Table V. The value Oxll denotes End of Line. Table V shows the syntax of the mapjablejiataQ. Table V The following semantic definition applies to the data elements shown in this Table: * number _of_entries - Specifies the number of Map Table entries that are loaded. * 2jo_4jnapjableJlag - If set to T, indicates that this entry is loaded into the "2 to 4 Map Table. * 2jo_8 napjableJlag - If set to T, indicates that this entry is loaded into the "2 to 8 Map Table. * 4jo_8jnapjableJlag - If set to '1', indicates that this entry is loaded into the "4 to 8 Map Table. * mapjablejype - Identifies the type of Map Table that is loaded. The value Or denotes a "2 to 4 Map Table", the value '10' denotes a "2 to 8 Map Table", the value '11' denotes a "4 to 8 Map Table". * entry jyalue - Specifies the entry of the Map Table. The first entry of the Map Table has the entry value zero. * mapjable_output_value - Specifies the output value of the Map Table for this entry. The above syntax and semantics will now be elucidated in a more common language. The PESjdataJield OTable I) specifies at least one "page" comprising one or more regions. Figs. 1-3 show some examples of an active video area comprising one or more regions. Fig.l shows a program provider's logo A and a current subtitle B. Fig.2 shows an additional subtitle C for hearing impaired viewers (hearing impaired people often need additional explanatory text such as "audience applauds", "swelling music"). Fig.3 shows the logo A and a subtitle Bi, the index i indicating a selectable one from a plurality of languages. The data for each region is accommodated in the region_daιajield (Table II). This field specifies the size and position of the respective region. Referring to Fig.4, the position of a region is denoted (x,y), x being specified by region Jtorizomaljiddress and y by region_vertical jaddress. These position parameters can be changed in time which allows for scrolling, by retransmitting encoded data for a given region (specified by its region Jd) without retransmitting pixel codes and colour-look-up-table data. The size of the region is p*n, p and n being specified by number _of_pixels and number jofjiπes, respectively. Each region comprises up to 16 sub-regions. In Fig.4, two subregions comprising n, and n 2 lines, respectively, are shown. The region _datajxeld() specifies colour-look-up-table data and/or pixel data for each subregion. Not all parameters and data mentioned above are compulsory. Their presence is indicated by respective flags. This allows already defined regions and/or subregions to be updated or modified by referring to their region Jd and/or subjregionjd, respectively. For example: the position of a region can be modified (which allows scrolling); an already displayed region can be made invisible; or a subregion can also be referred to by an other region and thus displayed elsewhere on screen without all image data being retransmitted. The colour-look-up-table is loaded or updated by the colour Jookjφ_data() field .(Table HI). This field specifies which luminance, chrominance and transparency output values are to be stored in which entries of the CLUT. The transparency defines to what extent the video and graphics pixels are to be mixed on screen. The pixel data for a subregion (or for a whole region is the number of subregions is 1) is accommodated in the field coded _sυbj-egionjiata() (Table IV). The pixels are defined as entries of the colour-look-up-table. The pixel values can be defined in 2, 4 or 8 bits. Three coding modes are provided. In a first coding mode (codingjnode is '01 ', '10', '11'), consecutive pixels are individually specified. The number of pixels thus transmitted is defined by pixels Jn node. In a second coding mode (codingjnode is '00', coding jnode jextension is '01', '10', '11'), runs of consecutive identical pixels are run- length encoded. The first and second coding modes can be mixed within a region. In order to minimise decoding complexity care has been taken that the run-length coded data is byte aligned. Each run-length code comprises two bytes: 2 bits for the coding mode, 2 bits for the coding mode extension and 12 bits for the run-length code. The latter defines a 10-bit run of 2-bit pixels, an 8-bit run of 4-bit pixels, or a 4-bit run of 8-bit pixels, as specified by the coding mode extension. A third coding mode (codingjnode is '00', coding jnod jextension is '00'), specifies commands such as map tables and end-of-line indicators which work on the run-length decoded data. The concept of map tables is disclosed in more details below. It is envisaged that existing On-Screen-Display chips are used for displaying the graphic images. Some of these chips allow for 2 bits per pixel, others for 4 bits per pixel or 8 bits per pixel. A potential problem arises if the pixels within a region are encoded using a number of bits per pixel different from the colour-look-up-table's input width. In order to solve this problem, the concept of map tables has been introduced. Map tables also increase the coding efficiency. They allow pixels to be encoded in less bits per pixel than the entry-width of the colour-look-up-table. These aspects will now be elucidated by way of an example. Suppose a (sub)region in which only four different colours are used. The respective colours are stored, in terms of luminance and chrominance, in four entries of a CLUT. The pixel data for said region is encoded in 2 bits pixel. The decoder, however, may have a 4-bit CLUT and thus require four bits per pixel. Now, the map table provides the mapping of the transmitted 2-bit pixel codes to a 4-bit CLUT entry. The map table thus informs a decoder which four of the 16 entries are to be used. Whether or not the 2 -bit/pixel coding indeed improves the efficiency depends on the overhead to change the coding mode or the map table. Thus, map tables are provided for mapping n-bit/pixel coding to an m-bit CLUT, n and m being different numbers. In view of this elucidation of the concept of map tables, the mapjablejlataQ field O^-ble V) needs no further explanation. For each entry value of the map table, an output value is defined. Fig.5 shows a schematic diagram of a transmitter according to the invention. The transmitter is adapted to simultaneously transmit a video signal and graphic images The transmitter receives a baseband video signal in YUV format and an associated audio signal AUD and comprises a MPEG2 video encoder 101 and MPEG2 audio encoder 102 for encoding said video and audio signal into packetized elementary streams PSV and PSA, respectively. The transmitter further comprises an editing terminal 103 for editing subtitles and/or other graphic images such as a program provider's logo. The editing terminal is coupled to a processing and storage device 104. A plurality of subtitles associated with the relevant television program, possibly in a plurality of different languages, is assumed to be stored in the processing and storage device in advance. The video signal is also applied to a time generator 105 which applies a time t to the processor and storage device 104. The time t can be the time which has elapsed since the start of the relevant television program, or can be a time code associated with the program and stored on the same tape as the television signal. In MPEG systems, the time t will correspond with MPEG's System Clock Reference SCR. The processor and storage device compares the time t with a time stamp associated with each stored subtitle. When a subtitle is due, the relevant data is applied to an encoding and packetizer 106 which forms a packetized elementary stream PSG defining encoded data according to the transmission format discussed hereinbefore. The packetized elementary streams PSV, PSA and PSG are multiplexed by MPEG multiplexer 107 so as to form a transport stream TS. The transport stream is transmitted to a receiver via a transmission or storage medium (not shown). Fig.6 shows a schematic diagram of a receiver according to the invention. The receiver comprises an MPEG2 demultiplexer 201 which receives the MPEG2 transport stream TS and selects the packetized audio stream PSA, video stream PSV and encoded graphics data PSG together constituting a selected television program. The audio stream is decoded by an MPEG2 audio decoder 202. The decoded audio signal AUD is applied to a speaker 203 for reproduction. The video stream is decoded by an MPEG2 video decoder 204 to obtain the decoded video signal in YUV format for display on a display screen 205. The encoded graphics data stream is applied to a data parser 206. This circuit parses the data accommodated in the PES packets PSG and provides the following output signals: the presentation time stamp PTS accommodated in the PES header, indicating the time at which the region is to be displayed; - the value of the visibility Jlag fTable II), denoted VF; the encoded pixel data as defined by the fields nJtit_code in Table IV (n =2,4,8); 8-bit pixel codes are denoted PX8, 4-bit pixel codes are denoted PX4, and 2-bit pixel codes are denoted PX2; the position (x,y) and size (p,n) of the currently received region as defined by the fields region Jωrizontal_address, region /erύcal jaddress, number jofjjixels, and number jjfjines, respectively (Table , Fig.4); map table data denoted MTD defined by the field mαpjαblejiαtαO fTable V), (this signal is not used in the present embodiment); a signal denoted LoC derived from the field region Jeveljof compαtibility (Table I), and colour-look-up-table data denoted CTD, defined by the field colour JookjipjαblejiαtαQ (Table HI). The pixel data PXn (n =2,4,8) received from the parser 206 is stored (when run-length encoded via a run-length decoder 207) in a pixel memory 208, the relevant memory locations being determined by position (x,y) and size (p,n) received from the parser. The pixel memory 208 is read out by display timing signals (not shown). The pixels PX2, PX4 or PX8 (a region may comprise both 2-, 4- and 8-bit pixels, but only one of them is active for the duration of a pixel) are thus applied to a colour generating circuit 209 in synchronism with the received video signal. The colour generating circuit 209 comprises a colour-look-up-table 2092. In this embodiment, said CLUT is assumed to have four entries only, i.e. its input is 2 bits wide. The CLUT holds, for each entry, an output signal denoted OSD in terms of an 8-bit luminance value Y and 8-bit chrominance values U and V, and an 8-bit transparency value T. The relevant YUV and T values are stored in the CLUT in response to the colour-look¬ up-table data CTD received from the parser. As the colour-look-up-table 2092 has a 2-bit input, the 2-bit pixels PX2 are directly applied thereto. The 4-bit pixels PX4 and 8-bit pixels PX8 are applied to die CLUT via a 4-to-2-bit reduction circuit 2093 and an 8-to-2-bit reduction circuit 2094, respectively. In a simple embodiment, said reduction circuits merely discard the least significant input bits. In an alternative embodiment, the most significant bit is unaltered (or some of the most significant bits are OR-ed) to obtain the first output bit whereas less significant input bits are OR-ed to obtain the second output bit. The reduction circuits 2093 and 2094 allow higher level CLUT data ( > 2 bits) to be compatibly decoded by lower level CLUTs (2 input bits). However, a program provider may prevent regions defined by more than 4 colours to be decoded by 2-bit CLUTs. This is achieved by a switch 2091 controlled by the signal LoC received from the parser, i.e. by the field region Jeveljofjcompaήbility (see Table I). If the switch is in the position denoted 4 or 8 (i.e. the program provider requires at least a 4-bit CLUT or 8-bit CLUT, respectively) the region will not be displayed. Only if the switch is in die position denoted 2 (i.e. the program provider accepts that pixels are displayed in four colours only), the pixel data is indeed applied to the CLUT. In response to the 2-bit pixel codes, the CLUT 2092 delivers die display output signal OSD and die transparency signal T. The output signal OSD is applied to a mixing stage 210 in which it is mixed, on a pixel-by-pixel basis, with the video signal YUV. The transparancy signal T determines the amount of transparency of graphic pixels. If T has the value 0, the video signal is blanked completely and the graphic pixels are not affected. If T has the value 255 (i.e. 100%), the relevant graphic pixel is fully transparent. For otiier values of T, the level of transparency is linearly interpolated. This allows the video signal to remain visible through the graphic image. The combined signal is then displayed on die display screen 205. However, whether or not the graphic image is indeed displayed depends on a switch 212 which is controlled by a display enable circuit 213. This circuit receives the presentation time stamp PTS and the visibility flag VF from me parser, as well as (not shown) die current time t as conveyed by MPEG's System Clock Reference. The display enable circuit 213 is adapted to enable die display of die graphics region as soon as t> PTS and to disable its display when die visibility Jlag for die relevant region is set to '0'. Fig.7 shows a further embodiment of the colour generating circuit 209 of die receiver. In this embodiment, the CLUT 2092 of die receiver has a 4-bit input, i.e. the CLUT comprises sixteen entries. Now, the 4-bit pixels PX4 are direcdy applied to die switch 2091 whereas the 8-bit pixels PX8 pass tiirough an 8-to-4-bit reduction circuit 2096 which, for example, discards die four least significant input bits. The 2-bit pixels pass through a 2- to-4-bit map table 2095, in fact being a memory having four 4-bit locations. The map table 2095 maps the 2-bit pixels into 4-bit CLUT entries as specified by map table data MTD received from e parser. The concept of map tables has already been described before. If the switch 2091 is in the position denoted 8 (i.e. the program provider requires at least an 8-bit CLUT), me region will not be displayed. If die switch is in die position denoted 2 or 4 (i.e. the program provider accepts tiiat 8-bit pixels are displayed in sixteen colours only), die pixel data is indeed applied to the CLUT. Fig.8 shows an embodiment of the colour generating circuit 209 in which die CLUT 2092 has an 8-bit input, i.e. the CLUT comprises 256 entries. Now, the 8-bit pixels PX8 are direcdy applied to e switch 2091 whereas the 2-bit pixels PX2 and 4-bit pixels PX4 pass tiirough a 2-to-8-bit map table 2097 and 4-to-8-bit map table 2098, respectively. The map table 2097 maps 2-bit pixels into 8-bit CLUT entries as specified by map table data MTD received from the parser. The map table 2098 does the same for 4-bit pixels. Strictly speaking, the switch 2091 is redundant in this embodiment, as this receiver deals with all transmissions regardless of die transmitted CLUT requirements. As explained above, reduction circuits (2093, 2094, 2096) allow higher level CLUT data to be compatibly decoded by lower level CLUTs, whereas map tables O 96/19077 16 (2095, 2097, 2098) allow lower level CLUTs to be dynamically mapped in higher level CLUTs. Table VI summarizes the various options and indicates the various levels of performance. Table VI Table VI shows three rows indicating the performance of receivers having a 2-bit, 4-bit and 8-bit CLUT, respectively. Three columns indicate whedier a region comprises 2-bit, 4-bit or 8-bit pixels, respectively. LoC denotes the level of compatibility included in die encoded data. If a region is defined in 2 bits/pixel only, four colours can be displayed (performance denoted 0) irrespective of whedier the CLUT input is 2, 4 or 8 bits wide. It does not make sense for a program provider to require, for such a region, a better than 2-bit CLUT. This is indicated by LoC =2 in die respective column. If a region comprises 4-bit pixels, the program provider may allow such a region to be displayed using a 2-bit CLUT (LoC =2). Said 2-bit receivers display the region in four colours (performance denoted 0), whereas 4-bit and 8-bit receivers display 16 colours (performance denoted +). However, the program provider may also require die receiver to have a 4-bit CLUT (LoC =4). In tiiat case, a 2-bit receiver can not display die region (performance denoted -) whereas 4-bit and 8-bit receivers will display 16 colours (performance denoted ++). If a region comprises 8-bit pixels, the program provider may allow such a region to be displayed using a 2-bit CLUT (LoC =2). In tiiat case, 2-bit receivers display 4 colours (performance denoted 0), 4-bit receivers display 16 colours (performance denoted +), and 8-bit receivers display the full pallet of colours (performance denoted ++). The program provider may also require the receiver to have at least a 4-bit CLUT (LoC =4). In tiiat case, 2-bit receivers can not display die relevant region (performance denoted -), 4-bit receivers display 16 colours (performance denoted ++), and 8-bit receivers display the full pallet of colours (performance denoted + + +). If die program provider requires die receiver to have an 8-bit CLUT (LoC =8), neither 2-bit nor 4-bit receivers can display the relevant region (performance denoted -), whereas 8-bit receivers have an optimal performance (denoted + + ++). The latter performace is optimal because the program provider does not need to take the restrictions of lower level receivers into account. Summarizing the invention, a method of simultaneously transmitting a video signal and encoded data representing graphic images is disclosed. The invention is particularly applicable for transmitting multilingual subtides with a video program. The graphic images are rectangular regions witiiin die active video area. They are transmitted in the form of bitmaps. The invention not only allows any character font or size to be displayed, but also die definition of e.g. a program provider's logo. The encoded data includes a time stamp to specify the time at which a sub tide is to be displayed. Preferred embodiments of die invention include the transmission of colour-look-up-table (CLUT) data and a compatibility code indicating a required minimum number of entries of said colour- look-up-table. For receivers with a CLUT having more entries than necessary, a map table is transmitted for mapping the pixel data width to the input witdh of die relevant CLUT. The method is applicable to Direct Video Broadcast systems wherein me video signal is MPEG2 encoded and the graphic images are accommodated in a private data stream of an MPEG2 Transport Stream.

Claims: Claims 1. A method of transmitting encoded data defining a graphic image in the form of a rectangular region witiiin an active video area, characterized in tiiat die pixels forming said region are individually defined by die encoded data, the size and position of said region being included in die encoded data. 2. A method as claimed in claim 1 , wherein the encoded data for said region further includes a time stamp representing die time at which said region is to be displayed. 3. A metiiod as claimed in claim 2, wherein subsequent encoded data for a given region includes a visibility code to disable die display of said region. 4. A method as claimed in claim 1, wherein a region comprises a plurality of subregions including an identifier code to allow a graphic image defined by said subregion to be referred to by a further region. 5. A method as claimed in claim 1, wherein me pixels for a region are encoded as entries of a colour-look-up-table, the encoded data including a compatibility code indicating a required minimum number of entries of said colour-look-up-table. 6. A method as claimed in claim 5, wherein the pixels are defined by n-bit pixel data, die encoded data further including a map table for mapping said n-bit pixel data to a colour-look-up-table having more than 2" entries. 7. A method as claimed in any of the preceding claims, wherein die encoded data is accommodated in a private transport stream according to the MPEG2 standard or extension diereof. 8. A method of receiving encoded data defining a graphic image in die form of a rectangular region for display within an active video area, characterized by die steps of decoding said encoded data into individual pixels constituting said region, decoding the size and position of said region, storing said pixels, and generating display signals representing said pixels. 9. A method as claimed in claim 8, further comprising die step of decoding a time stamp included in said encoded data, and generating the display signals from a time represented by said time stamp. 10. A method as claimed in claim 9, further comprising die steps of decoding a visibility code included in subsequent encoded data for a given region, and disabling the generation of said display signals for said region in response to said visibility code. 11. A method as claimed in claim 8, wherein a region comprises a plurality of subregions including an identifier code, comprising the step of including said subregion in a further region referring to said subregion. 12. A metiiod as claimed in claim 8, wherein the pixels for a region are encoded as entries of a colour-look-up-table and die encoded data includes a compatibility code indicating a required minimum number of entries of said colour-look-up-table, comprsising the step of disabling die generation of display signals if die required number of entries is less than available. 13. A method as claimed in claim 12, further comprising die steps of receiving map table data, and storing said map table data in a map table for mapping n-bit pixel data to a colour-look-up-table having more than 2" entries. 14. A method as claimed in any of claims 8-13, wherein me video signal is encoded according to an MPEG2 standard or extension tiiereof, and wherein die encoded data defining die graphic image is accommodated in packets of an MPEG2 private transport stream. 15. A transmitter comprising means for transmitting encoded data defining a graphic image in the form of a rectangular region within an active video area, characterized by means for encoding die individual pixels forming said region and including d e size and position of said region in the encoded data. 16. A receiver coupled to a display screen for receiving encoded data defining a graphic image in the form of a rectangular region within an active video area, characterized by means for decoding said encoded data into individual pixels constituting said region, means for decoding the size and position of said region, and display means for storing and displaying said pixels. 17. An image signal comprising encoded data defining a graphic image in me form of a rectangular region within an active video area, characterized in that the pixels forming said region are individually defined by the encoded data, die size and position of said region being included in the encoded data. 18. A storage medium on which an image signal is stored comprising encoded data defining a graphic image in the form of a rectangular region within an active video area, characterized in tiiat d e pixels forming said region are individuaUy defined by die encoded data, die size and position of said region being included in the encoded data.

Inventors: DE HAAN WIEBE, SPIERO RICHARD CEES, VAN DER MEER JAN, WEMELSFELDER ARMAND VICTOR, DE HAAN, WIEBE, SPIERO, RICHARD, CEES, VAN DER MEER, JAN, WEMELSFELDER, ARMAND, VICTOR
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H04N7
IPC codes: H04N5, H04N7