EP2643974A1

EP2643974A1 - A method and apparatus for controlling a display on a host device

Info

Publication number: EP2643974A1
Application number: EP11779697.9A
Authority: EP
Inventors: Olivier Deprez; Conor Ryan
Original assignee: Nagravision SA
Current assignee: SmarDTV SA
Priority date: 2010-11-24
Filing date: 2011-11-10
Publication date: 2013-10-02
Also published as: EP2458885A1; WO2012069321A8; WO2012069321A1

Abstract

The present invention may be deployed in a system comprising a Common Interface Conditional Access Module (CICAM) connected to a host device having a high definition display in which the CICAM and the host device communicate using a Common Interface Plus (CI+) Standard. A method and an apparatus are disclosed for using the CICAM to interactively control the display of high definition graphics on the host device's display by converting the high definition graphics into a subtitle track, send ing the subtitle track from the C ICAM to the host as part of the transmission stream including the audio video content, decoding the subtitle track and the video track in the host device and combining the resulting graphics for display. The generation of the high definition graphics in the CICAM is subject to user activity information fed back from the host to the CICAM.

Description

A METHOD AND APPARATUS FOR CONTROLLING A DISPLAY ON A HOST

DEVICE

INTRODUCTION

The present invention relates to the domain of multimedia and more particularly to the aspect of interactive or dynamic display of information.

STATE OF THE ART

Many households nowadays have a television set or a video display connected to a set top box for viewing som e kind of m u ltimedia content or m ore specifical ly audio/video content. The television set or the set top box usually includes a receiver to receive broadcasts or other forms of transmission of content and a decoder to select a channel or service and may further comprise means to decrypt the service should it be transm itted in encrypted form . In the case where the viewing of the content is subject to the perception of a fee by a service provider or content owner it is usual for the set top box or television set to communicate with a security module, which is usually portable and detachable from the set top box or television set. The security module holds keys necessary to decrypt the content and rights proving that the user of the equipment has procured the necessary viewing rights from the provider.

In the Pay-TV industry, there exists a specification known as the Common Interface Specification which standardises the way in which a conditional access module or CAM communicates with a host device. By host device it is understood to include a set top box having a decoder and a receiver and being connected to a display unit or a television set with a receiver and a decoder. The CAM includes means to decrypt encrypted transm issions received from service providers via the receiver and is associated with a security module which holds the security information such as rights and cryptographic keys. A CAM which is configured to communicate according to the Common Interface Specification is known as a Common Interface Conditional Access Module or CICAM.

Common Interface Plus or CI+ is a specification which extends the Common Interface Specification and it has been widely accepted by many consumer electronics manufacturers and Pay-TV technology companies. This specification sets standards for communicating between the CICAM and the host allowing for a user to manage navigation through menus within an interactive TV application using a remote control device. In CI+ v1 .2 for example, the CICAM is authorised to take over the graphic layer of the host allowing the user, through his CICAM, to interact with the TV application by accessing menus or list displays or enter pin codes for example. In order to allow the user to interact with the TV by launching an application on the host, an MHEG-5 application for example, which allows for file exchange and data exchange between the CAM and the host in order to achieve interactivity between the user and the TV. Using this means the user can navigate through onscreen menus or enter data. The CI+ standard however only supports graphics resolution of up to 720x576 pixels and so no h igh definition (H D) graphics manipulation is possible i.e. no processing of graphics with resolution higher than 720x576 pixels. This resolution corresponds to what is generally accepted in the industry as being of standard definition (SD).

Since more and more high definition (HD) TV displays are being used and there is a growing demand for users to be able to take advantage of the high definition (HD) capabilities of the displays by using sophisticated graphics for interactivity sessions, the limitation posed by the CI+ standard is a major drawback. Furthermore, standard definition (SD) graphics rendering on high definition (HD) displays is very poor and since most displays being used now are of high definition type (HD), interactive sessions will be of poor quality. Another limitation is that right-to-left scripts such as Arabic and Hebrew are poorly represented.

In European Patent Application Publication number EP 1 ,954,054 A1 , a system and a method are disclosed for allowing multimedia receivers to set up interactive services to be activated during the viewing of video content. To achieve this, the service provider generates an interactive mark associated with audio/video content and inserts the mark into Internet Protocol (IP) packets, which are associated with audio/video content packages. The interactive mark is intended to enable an interactive service during a period of the playing of the audio/video content. IP packets with the mark and with the content are transmitted to a plurality of users who can then synchronise the marks with their associated content. The document further teaches that the interactive mark may be embedded in the subtitle track. However, the interactivity of this system does not include receiving feedback information in order to influence the sequence of events involved in the interactive service.

A method and apparatus for dynamic displays for digital cinema is described in International Patent Application Publication number WO 10002361 A1 . The subject matter disclosed herein allows for the display of dynamic information, which may be a graphic object, on a digital display. Although the system described provides for a dynamic information display it does not provide for any interactivity in the sense that it does not react to a viewer's intervention. Furthermore, the information displayed dynamically is not integrated with the audio/video content and is not even displayed at the same time.

United States Patent Application Publication 2005/120374 discloses a method for passing an on-screen display (OSD) from a source device to a sink device over a serial interface, such as an I EE E-1394 interface. The OSD is encoded as an isochronous MPEG data stream at the source device and then passed to the sink device via the serial interface. This publication aims to solve a problem in the prior art whereby the OSD would be transmitted to the sink device as bit-map data, which is CPU intensive and not widely supported. The solution disclosed in the publication requires the OSD to be encoded as an MPEG data stream. In a case where the OSD needs to be displayed on top of, or combined with, some video content, and given that the video content also needs to be encoded as an MPEG data stream, it is not possible for a standard HD TV set (sink device) to simultaneously decode the two MPEG streams. For this reason, the publication describes having to (at best) time multiplex the OSD and the video content. Worse still, since the PMT has to be modified to point to a PID of either the OSD or the video, it follows that for the most part, processing (and displaying) of only the OSD or the video is possible at one time.

BRIEF SUMMARY OF THE INVENTION

It is a goal of the present invention to allow for a CICAM to control a host device to display high resolution graphics as part of an interactive graphical user interface or an interactive graphical display. The invention allows for a host device (a standard HD TV set for example, capable of processing one MPEG data stream at a time) to receive a high definition graphic from a control module via a transport stream interface at the same time as the video content. The host device is able to display the graphic and/or have the graphic modified (or updated) by the control module, responsive to user commands received by remote control device for example and transferred from the host device to the control module via an application interface. This is achieved by providing a method for controlling a host device to render a graphic having a first resolution on a display having the first resolution, said method using a control module adapted to communicate with a host device via a transport stream interface and an application interface, said method comprising:

receiving a first transport stream in a control module, said first transport stream comprising audio/video content;

executing a controller application in the control module, said execution at least:

generating data representing the graphic having the first resolution; converting said data to give a subtitle track; and

combining the subtitle track with the first transport stream to give a second transport stream comprising the audio/video content and the graphic having the first resolution;

transmitting the second transport stream from the control module to the host device via the transport stream interface;

receiving, via the application interface, by the host device, at least one instruction, from the control module, to execute a host application in the host device, said execution at least:

extracting at least a video track and the subtitle track from the second transport stream;

decoding the subtitle track in the host device to give the graphic having the first resolution;

decoding the video track in the host device to give the video content; combining said graphic having the first resolution with the video content in the host device to give an enhanced video content; and rendering the enhanced video content on the display.

The present invention further provides a control module comprising:

a memory to store a host application; means to load the host application into a host device via an application interface;

means to generate data representing a graphic;

means to receive a first transport stream via a transport stream interface;

means to convert the data representing the graphic to give a subtitle track; means to combine the subtitle track with the first transport stream to give a second transport stream; and

means to transmit the second transport stream to the host device via the transport stream interface.

The present invention yet further provides a host device comprising:

a display device;

an application interface to:

receive at least one first instruction; and

send at least one second instruction, responsive to a user input received by the host device;

a transport stream interface to:

send a first transport stream comprising at least a video track; and receive a second transport stream comprising at least a subtitle track and a processed version of the video track;

a track separator to extract at least a video content from the processed version of the video track and a graphic from the subtitle track, responsive to the receipt of the first instruction; and

a picture merger to combine the graphic and the video content for display on the display device. The invention therefore overcomes a limitation whereby the standard means for manipulating graphics, i.e. the application interface, is not well suited for handling high resolution graphics. Neither is a high resolution display properly adapted to display low resolution graphics, whereas without the present invention, low resolution graphics wou ld be transferred and manipulated via the com mand interface. According to the invention, by using the subtitle track, the high resolution graphic can be sent to the host device, while only the commands for manipulating the graphic are transmitted via the application interface. The invention uses a standard host device/control module setup (CICAM module/HD TV set connected via CI+ Interface) adapted to handle one transport stream.

According to an embodiment of the present invention, the host device sends encrypted video content to the control module via the transport stream interface for decryption in the control module. The host device sends commands to the control module via the application interface, the commands being relative to control of a high definition graphic for display on a screen in the host device. The commands may be responsive to instructions input by a user via a remote control device for example. The control module generates a high resolution graphic, perhaps influenced by the commands, encodes the graphic as a subtitle track and includes it as an additional track in the data stream comprising the original (or processed/decrypted) video track and sends it via the transport stream interface to the host device. The host device in turn can process the video and the graphic and display them on the screen by either laying the graphic over the video or combining them in an appropriate manner. The host receives a command, from the control module via the application interface, to launch an application to extract/process the graphic. Through this method it is possible for the control module to prepare and send high resolution graphics to be displayed responsive to com mands issued by the user. According to other embodiments, taking advantage of features available using DVB subtitles, it is possible to link graphical objects to certain parts of the video content through references made in a page description. The page description m ay also be transmitted via the subtitle track. In this manner it is possible to send graphical objects from the control module to the host device ahead of time and have them displayed at a later time along with their referenced video content. A video track can be MPEG2-Video (IS012818-2) or H.264 (ISO14496-Part10) for example, whereas a subtitle track may be encoded according to a DVB Subtitling Standard (ETSI EN 743). The PMT of a transport stream sent back to the host from the control module will therefore indicate different stream types for the subtitle track and the video track, thereby easily allowing the host to identify the graphic and the video. According to embodiments of the present invention, it is possible to avoid any jitter caused by the addition of extra packets in the processed transport stream. This is done by adding stuffing packets to the subtitle track.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood thanks to the detailed description which follows and the accompanying drawings, which are given as non-limiting examples of embodiments of the invention, namely:

Fig. 1 illustrates a system in which an embodiment of the present invention may be deployed. Fig. 2 illustrates a method for avoiding program clock reference jitter according to an embodiment of the present invention.

Fig. 3 illustrates a system in which another embodiment of the present invention may be deployed.

DETAILED DESCRIPTION Fig. 1 shows a system in which an embodiment of the present invention may be deployed. A control module (C), which may be a conditional access module or CAM as generally employed in the Pay-TV domain, is connected to a host device (H) via a transport stream interface. It is worth noting that the control module (C) may be a CICAM. This transport stream interface allows for audio/video content, usually in the form of an input transport stream (TSI) comprising an audio track (AT) and a video track (VT), to be transmitted from the host device (H) to the control module (C) and for an output transport stream (TSO) to be transmitted from the control module (C) to the host device (H). Although Fig.1 shows the input transport stream being received from the host device, there are other embodiments of the present invention in which the input transport stream may be received from some other source.

The host device (H) either comprises or has access to a display unit (VDU). The present invention may preferably be deployed in a system com prising a high definition (HD) display unit. By high definition (HD) it is understood to mean a definition of substantially more than the definition generally accepted within the industry as being standard definition. Such standard definition displays are adapted for displaying graphics with a resolution of between 704x480 pixels and 720x576 pixels. It is well known that high definition (HD) displays give poor results when they are used to d isplay graph ics having a standard resolution. It is therefore advantageous to have a system where the control module is enabled to control a host device to display a high resolution graphic which was generated in the control module. This is an advantage not only because it allows for the control module to be able to generate desirably complex graphics for display but also because the alternative is to have the control module control the host to display only graphics having a standard resolution, which would be poorly displayed on a high definition display. However, as was explained in the prior art section, current technology does not allow for high resolution graphics on the display to be manipulated via the CICAM.

According to an embodiment of the present invention, the host device further comprises a receiver (RX) for receiving transmissions or broadcasts or otherwise acquired audio/video content. According to another embodiment of the present invention, the control module comprises its own receiver for receiving transmissions or broadcasts or otherwise acquired audio/video content. Depending on the format of the received transmission, the content may be decoded to give the input transport stream (TSI). In one embodiment of the present invention the input transport stream is in encrypted format while in another embodiment it is in clear format. In the case where the input transport stream is in encrypted format it may be decrypted by the control module thanks to an associated security module having the necessary rights and cryptographic keys according to processes well known in the Pay-TV industry. According to an embodiment of the present invention, the control module comprises recoding means, to store the incoming transport stream, or a decoded version of the incom ing transport stream, onto a storage medium such as a memory, a ROM, a hard disk, a CD or the like, for playback at a deferred time. In this embodiment the recorded transport stream replaces the received input transport stream or the decoded received transport stream of embodiments which do not use the storage medium. According to an embodiment of the invention, the control module (C) comprises a user interface generator (UIGEN). This generator (UIGEN) has means for generating a graphic intended to be displayed on the display (VDU). To be more precise, the user interface generator generates data representing a particular graphic. When that data is processed by the host device for display, the particular graphic is shown on the display. By a graphic it is understood to include one or more graphics or graphical elements which may be combined to form more or less complex graphical images, scenes or virtual interfaces. The graphic is preferably of a high resolution format. This is referred to in general terms as a high definition graphic or a graphic having high resolution. The generator is further capable of converting the data representing the high definition graphic to a high definition subtitle track (HDST). Historically, subtitle tracks were used, as their name suggests, to provide textual versions of dialog in films or TV programmes and were usually displayed on the bottom of the screen or display unit. They were also useful in providing translation of the text in a language other than the spoken language of the dialog. Nowadays the specification for subtitle tracks is rather more elaborate and allows for much more freedom and creativity including graphics objects of various dimensions and visual properties and able to be assigned to various positions of the display. Embodiments of the present invention take advantage of the subtitle track in ways which were not previously exploited. Whereas the state of the art would have one attempt to graphics images into MPEG video streams to be transmitted via a serial interface, the present invention makes inventive use of the subtitle track for such purposes. As such, the control module (C) of the present invention is adapted to be able to generate data representing rather elaborate high resolution graphical images, scenes or virtual interfaces suitable for display on the host's (H) high definition (HD) display unit (VDU) in keeping with what is fast becoming a requirement from users in the industry. Thanks to the present invention, it is possible to simultaneously transm it the HD graphic and the video content along the same transport stream interface. Furthermore, it is possible for the graphic, once displayed on the host device, to be controlled, responsive to viewer input, to display and/or manipulate the graphic. In order to avoid jitter, stuffing packets may be included in the subtitle track.

The control module (C) can communicate with the host device (H) via file exchange and command exchange across an application interface (LNCH and FB) according to the Common Interface Specification, such as CI+ v1 .2 for example. The host device and the control module are both capable of running applications in an application environment. Although an application may run on the control module and another application may run on the host device, according to a preferred embodiment of the present invention, the term application environment is used to describe a set of rules governing how an application behaves and not the physical support upon which the appl ication runs (AP P DOM in F ig. 1 ). MHEG-5 i s a cost-effective, license-free standard i nterface TV m idd leware wh ich is very su itable as an application environment for running the applications or applets according to a preferred embodiment of the present invention. Support for MHEG-5 is indeed obligatory according to the Common Interface Specification CM v1 .2. As previously mentioned, with the current technological limitations of the Common Interface Specification it is not possible to process graphics objects of resolution of more than 720x576 pixels. Therefore it is not suitable for processing high resolution graphics. Other application environments which are generally known in the state of the art are MHP, HTML-5 and HbbTV. Generally speaking, the Com mon Interface includes the transport stream interface (TSO, TS I ) and the appl ication interface (LN CH , FB) and the Specification covers the communication protocols over these interfaces.

In a preferred embodiment of the present invention applications are stored in the control module. Some applications, controller applications, may be executed on the control module and others, host applications, may be transferred to the host device for execution thereon. In other embodiments, the control module and the host device both store the applications which are intended to be run on their respective engines and the control module giving a command to run a particular application on the host device. In yet other embodiments of the present invention, applications may be included along with the audio/video content as part of the broadcast or transmission of such content. In such cases, applications may be extracted from the broadcast or transm ission by the host device or by the control m od ule or even by both. Additionally, the host device may feed back information to the control module during the execution of an application on the host device in order to influence the execution of an application on the control module. Indeed, in a preferred embodiment of the present invention, feedback information relative to a user activity is transmitted to the control module during execution of a host application. For example, a user who presses keys on a remote control device would cause feedback information comprising codes corresponding to the keys that user presses to be sent back to the control module.

According to an embodiment of the present invention, a controller application (AP1 ) is executed causing the control module to generate the data representing the high resolution graphic as described above and to convert this to form a subtitle track and further causing the subtitle track (HDST) comprising the high resolution graphic to be combined with the input transport stream, for example by a track inserter (Tl), to form an output transport stream (TSO). The output transm ission stream (TSO) is transmitted to the host device via the transport stream interface. The control module (C) launches (LNCH) a host application (AP2) or applet on the host device (H) to switch on subtitle mode and to select the subtitle track included in the output transport stream. The host application (AP2) further causes the host (H) to separate (DMX) the subtitle track from the video track and to decode the subtitle track (DECST) to form a high definition graphic. In one embodiment of the present invention the high definition graphic is displayed on the high definition display. In another embodiment of the present invention the host device further decodes the video track (DECV) and then combines (CB) the decoded video track and the high definition graphic to form a complete high definition graphical user interface which is displayed on the display, the high resolution graphic being displayed on top of the video content. According to another embodiment of the present invention, the high resolution graphic and the video content are blended together rather than just overlaid.

Typically a transport stream comprises packets of data, each packet having a packet header. The packet header includes information about the data packet, including a packet identifier (PID) identifying the track to which the data packet belongs. The packet identifier (P ID) is useful for determining such things as the order of data packets, the type of data packets and the like. Similarly, a subtitle track comprises packets of subtitle data i.e. subtitle packets, also having packet identifiers. This information may be used either by the track inserter in combining the subtitle track with the audio/video tracks or in the host device to separate a particular subtitle track from the other audio/video tracks A data packet may comprise program clock reference (PCR) information. The PCR information is based on a value of the transmission system's internal clock when the data packet was generated or transmitted. The PCR information is useful for the host device to synchronise its internal clock to the transmission system's internal clock. In this manner the control module is able to ensure that audio/video content is processed in the control module at the same speed as it was assembled at the transmitting entity.

Furthermore, a data packet may comprise a presentation time stamp (PTS) which represents the time at which a particular video frame or audio frame in the data packet should be presented, the time being relative to the PCR.

The process of combining the subtitle track with the input transport stream (TSI) to form an output transport stream (TSO), as described above, may be accomplished by the track inserter (Tl). This is illustrated in Fig. 2. The figure shows that a transport stream can comprise data packets known as video packets or audio packets, which carry video information and audio information respectively. The transport stream may also comprise stuffing packets. Stuffing packets are generally known within the domain of digital media transm ission and more particularly in broadband packet switching networks. The use of such stuffing packets allows for a constant bit-rate transport stream to be generated from a variable bit-rate transport stream. In the track insertion process, care must be taken to maintain a constant delay (DEL) between the reception of a particular data packet from the input transport stream (TS I) and the transm ission of that transport packet in the output transport stream (TSO). This constant delay is determined based on the maximum time it takes for the control module to accomplish any processing involved in preparing the subtitle track and/or decrypting the data packets (PDEL) where necessary. The constant delay is therefore at least as high as the maximum processing and/or decoding time (PDEL). If this delay were not to be maintained constant, then the resulting output transport stream (TSO) would present jitter due to mismatches or other anomalies in program clock reference (PCR) information. According to an em bodiment of the present invention, the subtitle packets (SP) resulting from the conversion of the generated data representing the high resolution graphic are held in a queue. As long as data packets (DP) including video packets (VP) or audio packets (AP) are detected in the input transport stream (TSI), these packets are placed in corresponding positions in the output transport stream (TSO) and therefore have a characteristic delay (processing and/or decoding time (PDEL)) with respect to the input transport stream depending on whatever processing is made on the input transport stream. Whenever a stuffing packet (STF) is detected in the input transport stream (TSI), the next subtitle packet (SP) in the queue is placed at the corresponding position in the output transport stream (TSO). If further stuffing packets (STF) are detected in the input transport stream and no more subtitle packets are available in the queue, then the stuffing packet is placed at the corresponding position in the output transport stream.

From the discussion above, it follows that for a given input transport stream, the num ber of opportunities to be able to introduce a subtitle packet is equal to the number of stuffing packets present in the input transport stream . According to another embodiment of the present invention, the num ber of opportunities for including a subtitle packet, and therefore the capacity for including subtitle packets, is artificially increased by adding extra stuffing packets into the input transport stream, either by a dedicated stuffing packet inserter before the input transport stream gets to the track inserter, or as an extra process carried out within the track inserter. By adding, it is understood to mean inserting a new stuffing packet between data packets or existing stuffing packets in the input transport stream . This has the automatic effect of increasing the bit rate of the output transport stream with respect to the input transport stream since the track inserter will strive to maintain any timing constraints given in the input transport stream . As mentioned before, a data packet may comprise program clock reference information (PCR). By maintaining tim ing constraints, it is meant that any program clock reference information is respected such that the time difference between successive data packets having PCR information in the input transport stream is maintained, by the track inserter for example, in the output transport stream. This has the effect of increasing the bit rate in the output transport stream with respect to the input transport stream . Fig. 3 illustrates a system devised to carry out an embodiment such as this, with the control module comprising a stuffing packet inserter (STFI) to receive the input transport stream and to insert stuffing packets, thereby creating a stuffed input transport stream before going to the track inserter. The system of Fig. 1 could equally be used to carry out this embodiment, in which case the function of adding extra stuffing packets is carried out by the track inserter. According to a further embodiment of the present invention, the track inserter (Tl) is able to program a presentation time for a particular graphic. In order to do this a presentation time stamp (PTS) associated with the graphic is set in the subtitle packet comprising the data representing the graphic. In this way the graphic can be presented at the desired time.

The user may interact with the graphic display by performing an action, for example clicking an icon or entering characters into a field or moving an object on the screen, and the host sends feedback information (FB) depending on the user's action to the control module via the application interface. The user interface generator takes this feedback information as a further variable in the process of generating the graphic thereby completing a feedback loop where the user's actions will allow the user interface to be appropriately updated. For example, an action performed by a user may simply be the clicking of an icon. That icon may change colour to signify that it has been selected. The user may drag a boundary of an icon thus changing its shape or he may drag an entire icon thus changing its position. The feedback information therefore may describe changes to a graphic's attributes or properties, affecting the visual aspects of the graphic on the display. A drag of a boundary could represent a change in dimension of the object or a drag of a complete icon would represent a change in its position. A click on an icon could lead to a change in its colour and so on. The feedback information received from the host by the control module allows the execution of the controller application to be influenced to take into account the updates to a graphic's visual aspects such as colour, position or shape and the data representing the graphic, generated by the controller application, will reflect those updates.

Claims

1 . A method for controlling a host device (H) to render a graphic having a first resolution on a display (VDU) having the first resolution, said method using a control module (C) adapted to communicate with the host (H) device via a transport stream interface (TSO, TSI) and an application interface (LNCH , FB), said method comprising:

receiving a first transport stream in a control module (C), said first transport stream comprising audio/video content;

executing a controller application in the control module (C), said execution at least:

generating data representing the graphic having the first resolution and converting said data to give a subtitle track (HDST); and

combining the subtitle track (HDST) with the first transport stream to give a second transport stream comprising the audio/video content and the graphic having the first resolution;

transmitting the second transport stream from the control module (C) to the host device (H) via the transport stream interface (TSO, TSI);

receiving, via the application interface (LNCH, FB), by the host device (H), at least one instruction, from the control module (C), to execute a host application in the host device (H), said execution at least:

extracting at least a video track and the subtitle track (HDST) from the second transport stream;

decoding the subtitle track (HDST) in the host device (H) to give the graphic having the first resolution;

decoding the video track in the host device (H) to give the video content;

combining said graphic having the first resolution with the video content in the host device (H) to give an enhanced video content; and

rendering the enhanced video content on the display (VDU).

2. The method according to claim 1 , wherein said subtitle track (HDST) comprises at least one subtitle packet, said input transport stream comprises at least one data packet and at least one stuffing packet, said combining comprising: maintaining a subtitle packet queue having at least a front position, said subtitle queue comprising said at least one subtitle packet at said front position;

copying the at least one data packet from the input transport stream to a corresponding position in the output transport stream; and if the subtitle packet queue has a subtitle packet, then:

replacing the at least one stuffing packet by the subtitle packet, said replacement occurring at a corresponding position in the output transport stream; otherwise

copying the at least one stuffing packet from the input transport stream to a corresponding position in the output transport stream.

3. The method according to either of claims 1 or 2, wherein said combining further comprises inserting at least one further stuffing packet in the input transport stream.

4. The method according to any of claims 1 to 3, wherein said host application is loaded into the host device (H) from the control module (C) via the application interface (LNCH, FB).

5. The method according to any of claims 1 to 4, wherein the host application is configured to receive instructions to process graphics, said instructions to process graphics process graphics being received via the application interface (LNCH, FB) and being limited to processing graphics having a second resolution lower than the first resolution.

6. The method according to any of claims 1 to 5, wherein said control method is dependent on a user activity, said data representing the graphic including properties which determine at least a visual aspect of the graphic on the display (VDU), said control method further comprising:

generating feedback information by the host device (H), said feedback information being dependant on the user activity; and

using the feedback information to influence the execution of the controller application including updating the properties of the graphic.

7. The method according to claim 6, wherein said feedback information is received via the application interface (LNCH, FB).

8. The method according to any of claims 1 to 7, wherein the first transport stream is received from the host device (H).

9. The method according to any of claims 1 to 8, wherein the first transport stream is received via the transport stream interface (TSO, TSI)

10. The method according to any of claims 1 to 9, wherein the second transport stream is transmitted to the host device (H) via the transport stream interface (TSO, TSI).

1 1 . The method according to any of claims 1 to 10, wherein the first transport stream is encrypted and the control module (C) decrypts the first transport stream.

12. A control module (C) configured to perform the method according to claim 1 , said control module (C) comprising:

a memory to store a host application;

means to load the host application into a host device (H) via an application interface (LNCH, FB);

means to generate data representing a graphic;

means to receive a first transport stream via a transport stream interface (TSO, TSI);

means to convert the data representing the graphic to give a subtitle track (HDST);

means to combine the subtitle track (HDST) with the first transport stream to give a second transport stream; and

means to transmit the second transport stream to the host device (H) via the transport stream interface (TSO, TSI).

13. The control module (C) according to claim 12, wherein it further comprises: means to receive feedback information via the application interface (LNCH, FB); and

means to use said feedback information to influence the generation of the data representing the graphic.

14. The control module (C) according to either of claims 12 or 13, wherein said means to combine the subtitle track (HDST) with the first transport stream comprises means to insert at least one stuffing packet into the input transport stream.

15. A host device (H) configured to perform the method according to claim 1 , said host device (H) comprising:

a display device (VDU);

an application interface (LNCH, FB) to:

receive at least one first instruction; and

send at least one second instruction, responsive to a user input received by the host device (H);

a transport stream interface (TSO, TSI) to:

send a first transport stream comprising at least a video track; and receive a second transport stream comprising at least a subtitle track (HDST) and a processed version of the video track;

a track separator (DMX) to extract at least a video content from the processed version of the video track and a graphic from the subtitle track (HDST), responsive to the receipt of the first instruction; and

a picture merger (CB) to combine the graphic and the video content for display on the display device (VDU).