A METHOD AND APPARATUS FOR SWITCHING BETWEEN DIGITAL BITSTREAMS

Abstract: The present invention relates generally to the domain of digital broadcasting, and, more precisely, the methods and apparatus employed in switching between from a first digital bit-stream to a second digital bit-stream. The present invention solves some of the problems associated with such switching, and is of particular use in switching between digital subtitle streams.

Description: A METHOD AND APPARATUS FOR SWITCHING BETWEEN DIGITAL BITSTREAMS The present invention relates generally to digital video broadcasting (DVB) and more specifically to a method and apparatus for switching between digital bit-streams. In a digital broadcast system there are times when it becomes necessary to splice television programs together in the compressed domain. This might occur, for example, when broadcasts are switched from national broadcasts to regional broadcasts to accommodate regional variations in programme scheduling or advertisements, as well as when originating programmes from video servers. For the purposes of this document, splicing is defined as the switching from a first bit-stream to a second bit-stream. In the digital video broadcasting domain, splicing requires that the second bit-stream is modified so that, to a receiver, the streams appear identical. This is achieved by, amongst other things, changing the appropriate packet identifiers in the second bit-stream to match those in the first bit-stream. When splicing is necessary it is important to ensure that the individual elementary streams (video, audio, subtitles, etc.) are 'legal' and do not cause any problems for the decoder. A subtitles elementary stream consists of a multiplex of several individual subtitle streams (to accommodate multiple languages, for example), together with a common set of ancillary data, such as logos or graphics. In the temporal domain, a stream consists of a hierarchical structure. At the highest level is the 'epoch', indicating the start of a period, which typically lasts for the full length of a television programme. Inside each epoch there are a number of 'displays' which are actual displayed subtitle frames. The data forming each display is carried in 'segments'. The first part of a subtitle definition is known as a Page Composition Segment (PCS). This is used in defining which and how many areas of the display are to used for displaying subtitles. There are other components of a subtitle definition and these are described in more detail in European Standards ETS- 300743. The PCS contains a flag known as 'Page State' which is used to indicate to the subtitle decoder the required configuration of memory and resources that are needed to correctly display a given set of subtitles. The 'Page State' flag can take various values including 'Mode Change' and 'Acquisition Point'. At the start of every programme the 'Page State' flag is transmitted with a value of 'Mode Change'. This causes the receiver to reset the subtitle decoder, clear all the associated memory and reconfigure the subtitle decoder for the subtitle stream that will follow according to the configuration data following the 'Page State' flag. The 'Mode Change' is transmitted infrequently since, due to the requirement that the decoder discards its existing configuration of subtitle & resources upon reception of this flag, some disturbances to the display of subtitles may occur. It will typically be transmitted at the start of each programme although it may also be desirable to send additional 'Mode Changes' at intermediate times. If a viewer changes channel, say half way through a programme, then the subtitle decoder will already have missed and therefore will not receive the 'Mode Change'. To ensure that the decoder is correctly configured, however, throughout every programme the 'Page State' flag with a value of 'Acquisition Point' is transmitted every few seconds followed by a repetition of the configuration data sent at the start of the programme. If a 'Mode Change' has been received, the 'Acquisition Points' are ignored until either a new 'Mode Change' is received or the user changes channel. The channel change has the effect of resetting the subtitle decoder. Once the reset has occurred the subtitle decoder will take the information following the 'Acquisition Point' and use this to configure the subtitle decoder memory. When switching between two independent MPEG streams it is important for the change-over to appear transparent to the viewer. If there are subtitles being transmitted on both streams then it is also important that the changeover happens cleanly and that no corruption of subtitles occurs. When one MPEG stream is spliced into another the splice appears transparent to the receiver, i.e. all the stream and packet IDs remain identical. The problem that occurs, however, is that if the subtitles in the first stream require a different configuration to those in the second after the splice has taken place, then the receiver will not recognise that a change of streams has taken place - the reason being that the receiver would have already received a 'Mode Change' before the splice, but after the splice it would only receive 'Acquisition Points', until the start of the following programme, hence these would be ignored. This problem could lead to a subtitle decoder not being correctly configured after a splice of two subtitle streams has occurred. According to a first aspect of the present invention there is provided a method of switching from a first digital bit-stream to a second digital bit-stream, the second bit-stream comprising configuration flags some of which contain a value which will cause a reconfiguration of a receiver, the method comprising the steps of: switching from the first bit-stream to the second bit-stream; identifying a configuration flag in the second bit-stream after the switch has occurred; and checking whether the identified flag contains the said value and, if not, changing the identified flag to contain the said value. According to a second aspect of the present invention there is provided apparatus for switching from a first digital bit-stream and a second digital bit- stream, the second digital bit-stream comprising configuration flags some of which contain a value which will cause a reconfiguration of the receiver, the apparatus comprising: a switch for switching from the first bit-stream to the second bit-stream; a detector for identifying a configuration flag in the second bit-stream after the switch has occurred; an analyser for checking whether the identified flag contains the said value; and a modifier for changing the value of flag to contain the said value if the flag does not already contain the said value. The invention will now be described by reference to the accompanying drawings, in which: Figure 1 is a block diagram showing an overview of the present invention; Figure 2 is a diagram showing the splicing of two subtitle streams; and Figure 3 is a detailed diagram showing one embodiment of the present invention. Referring to Figure 1 , two input streams 10 and 11 are shown as inputs to a multiplexer 12. The input streams can represent any individual sort of bit- stream, although for the purposes of this example they are representative of a national broadcast and a local broadcast. A National stream 10 represents an MPEG stream for a national broadcast. A Regional stream 11 represents a broadcast for a regional programme. Under normal operation, national stream 10 would be transmitted through multiplexer 12 to a satellite up-link 14, via satellite 15 and down to one of many receivers 16. Alternative transmission means could also be employed in place of the satellite link, such as a terrestrial or cable link. When a switch-over is required from a national broadcast to a regional broadcast this can initiated by way of a splice control signal 13. This causes a splice to occur and for regional stream 11 to be transmitted to the receivers instead of national stream 10. The splice occurs in such a way that, to the receiver nothing has changed - i.e. all the stream IDs are changed to be identical to that of the first stream. Figure 2 is a diagram showing in more detail the composition of each of the input streams. National stream 20 represents a national transmission which is to be spliced with regional stream 21. Received stream 22 represents the stream which is received at a receiver. At the start of each programme in stream 20, the 'Page State' flag is sent with a value 'Mode Change' 23. When received at the receiver this will cause the subtitle decoder to reset and to reconfigure itself according to the information contained in the bit-stream after the 'Page State' flag. The 'Mode Change' is typically sent infrequently i.e. only at the start of each programme. Every few seconds, however, the 'Page State' flag is sent with the value of 'Acquisition Point' 24. This is normally ignored by the decoder if the decoder has already received a 'Mode Change'. However, if a user changes to a channel, say half way through a programme the decoder will not receive the 'Mode Change' as this will already have been missed (since it is generally sent only at the start of each programme). In this situation, after a channel change, the decoder will pick-up the 'Acquisition Point' which is followed by the same information regarding the memory configuration as follows the 'Mode Change'. This allows the decoder to reconfigure itself for the correct memory configuration. When the national broadcast is required to change to the regional broadcast the splice control signal 13 will be sent to the multiplexer indicating that a splice is to take place imminently. A splice can take place at any time, but it is most convenient if it occurs at the end of the current subtitle segment. The point of the splice is indicated at splice point 26. The regional broadcast 21 is then "spliced in' through the multiplexer. This will ensure that, to the receiver, the data stream appears identical to that before the splice. Looking now at regional broadcast stream 21 , the splice point 26 is shown. It cannot be guaranteed that 'Mode Change' associated with the start of regional program will appear immediately after the splice due to timing considerations. As shown in Figure 2, the 'Mode Change' 27 actually occurs before the splice has taken place. To ensure that the splice is effected cleanly the following important step needs to take place. As soon as the splice has taken place the multiplexer must change the first 'Page State' flag to 'Mode Change'. If the first 'Page State' flag is 'Mode Change' then obviously this does not need to be changed. However, if the first 'Page State' flag after the splice is 'Acquisition Point' then this has to be changed to 'Mode Change' to force the receiver to reconfigure itself. Turning to stream 22, the actual stream as received at the receiver is shown. Due to the timing of the splice it can be seen that the 'Mode Change' for the new stream was missed, as the splice took place after the 'Mode Change' in the regional stream 21. However, this does not cause a problem for the receiver as the present invention changes the first 'Page State' flag that occurs after the splice to a have a value of 'Mode Change'. This ensures that the receiver is correctly configured to receive the current subtitle stream. Figure 3 shows in more detail one operation of the multiplexer and splicer 12 during a splice. When a splice is signalled to take place splice control signal 13 causes a latch 30 to control a flag analyser 31 , a bit-stream modifier 32 and a router 33. The splice control signal 13 also controls the multiplexer 35 which changes the output stream 34 to the desired input stream. Once activated, the flag analyser 31 analyses the bit-stream 34 looking for 'Page State' flags. When it finds one it signals this to stream modifier 32 which checks to see whether the flag needs modifying to 'Mode Change' and if so then carries out the modification. The modified bit-stream 36 is fed to a router 33 which causes the modified bit-stream to be output to the up-link 14. After the splice has occurred the latch relinquishes control of the flag analyser 31 and stream modifier 32 and the stream 34 flows through router 33 to uplink 14. In this particular example the modification of the 'Page State' flag has to occur for the first 'Page State' flag in each of the individual subtitle streams inside the subtitle bit-stream - for example, in a multi-language subtitle stream there is a separate subtitle stream for each of the different languages.

Claims: CLAIMS 1. A method of switching from a first digital bit-stream to a second digital bit-stream, the second bit-stream including configuration flags some of which contain a value which will cause a reconfiguration of a receiver, the method comprising the steps of: switching from the first bit-stream to the second bit-stream; identifying a configuration flag in the second bit-stream after the switch has occurred; and checking whether the identified flag contains the said value and, if not, changing the identified flag to contain the said value. 2. The method of claim 1 , in which the step of identifying a configuration flag in the second bit-stream comprises identifying the first such flag after the switch has occurred. 3. The method of claim 1 or 2, applied to switching from a first digital bit- stream to a second digital bit-stream, each digital bit-stream consisting of a digital video broadcast subtitle stream. 4. Apparatus for switching from a first digital bit-stream and a second digital bit-stream, the second digital bit-stream including configuration flags some of which contain a value which will cause a reconfiguration of the receiver, the apparatus comprising: a switch for switching from the first bit-stream to the second bit-stream; a detector for identifying a configuration flag in the second bit-stream after the switch has occurred; an analyser for checking whether the identified flag contains the said value; and a modifier for changing the value of flag to contain the said value if the flag does not already contain the said value. 5. The apparatus of claim 4, wherein the detector is adapted to identify the first configuration flag in the second bit-stream after the switch has occurred. 6. The apparatus of claim 4 or 5, adapted for switching from a first digital bit-stream and a second digital bit-stream wherein the digital bit-streams each consist of a digital video broadcast subtitle bit-stream.

Inventors: DALLARD NIGEL STEPHEN, DALLARD, NIGEL, STEPHEN
Applicants: DALLARD NIGEL STEPHEN, NDS LTD, DALLARD, NIGEL, STEPHEN, NDS LIMITED
Application year: 1998
Main classification: H04N7
IPC codes: H04N7