WO2018002369A1

WO2018002369A1 - Traitor tracing

Info

Publication number: WO2018002369A1
Application number: PCT/EP2017/066393
Authority: WO
Inventors: Emeric STUDER; Pierre Sarda
Original assignee: Nagravision S.A.
Priority date: 2016-07-01
Filing date: 2017-06-30
Publication date: 2018-01-04

Abstract

The present disclosure provides a system and method of traitor tracing by identifying an original receiver of a media stream that has been re-transmitted comprises detecting sets of content items, such as adverts, that are embedded in the media stream. The sets of content items can then be used to determine receivers of the media streams comprising the detected sets of content items.

Description

TRAITOR TRACING

FIELD The present disclosure relates to a system and method of identifying an original receiver of a media stream, for example, when the media stream has been re-transmitted by an original receiver without authorisation.

BACKGROUND

Audio and/or video content can be transmitted e.g. multicast or webcast etc., from a content provider in a media stream to receivers that receive the media stream. For example, a radio program or television program may be multicast in a media stream to a receiver. Typically, the receivers are authorised to receive the media stream. However, some receivers re-transmit the media stream when they are not authorised to do so, leading to a pirate copy of the media stream being distributed. The pirate version of the media stream can then be received by unauthorised receivers. The pirate media stream is generally re-transmitted in real time, or with a short delay from when the media stream was originally received by the authorised receiver.

Generally, content providers seek to prevent unauthorised receivers from receiving the pirate media stream. This can be achieved by traitor tracing identifying the original receiver of the media stream, and then preventing that receiver from receiving further media streams.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are now described with reference to the drawings in which:

Figure 1A illustrates an environment in which re-transmission of a unicast media stream is taking place;

Figure 1 B illustrates an alternative environment in which re-transmission of a broadcast media stream is taking place;

Figure 2A illustrates a system for identifying re-transmitted media streams;

Figure 2B illustrates a database used by the system of Figure 2A;

Figure 2C illustrates a further database;

Figure 3 illustrates a flowchart of a method of identifying an original receiver of a media stream; Figure 4 illustrates a schematic diagram of audiences of content items;

Figure 5 illustrates a flowchart of a method of assigning sets of content items;

Figure 6 illustrates a schematic diagram of sets of content items;

Figure 7 illustrates a flowchart of a method of assigning sets of content items; Figure 8A illustrates an environment in which re-transmission of a media stream is taking place;

Figure 8B illustrates an alternative environment in which re-transmission of a broadcast media stream is taking place;

Figure 9 illustrates a system for transmitting media streams;

Figure 10 illustrates a system for identifying re-transmitted media streams;

Figure 1 1 illustrates a system for receiving media streams;

Figure 12 illustrates a flow chart of a method of identifying an original receiver of a media stream;

Figure 13A illustrates an example data flow in the method of Figure 1 1 ;

Figure 13B illustrates an example data flow in the method of Figure 1 1 ;

Figure 13C illustrates an example data flow in the method of Figure 1 1 ; and

Figure 14 shows a schematic diagram illustrating identification of a retransmitted video stream.

Figure 15 illustrates a block diagram of one implementation of a computing device.

DETAILED DESCRIPTION OF THE DRAWINGS

In overview, a system for and method of traitor tracing is disclosed. An original receiver of a media stream that has been re-transmitted is identified by detecting sets of content items, such as adverts, that are embedded in the media stream. The sets of content items are then used to determine receivers of the media streams comprising the detected sets of content items. A media stream that has been retransmitted may be identified by identifying a content item, such as an advert, that is transmitted in one of a subset of receivers. The content item can then be used to determine the subset comprising an original receiver of the media stream. The media streams may comprise one or more items of main content, for example a television series, a movie, a live event such as a sporting event etc., and the content items are typically inserted within the main content, for example during advertisement breaks. In some aspects of the disclosure a method comprises a method of identifying an original receiver of a media stream. The media stream was transmitted by a content provider to the original receiver. The media stream may have been transmitted such that a content provider provides a different media stream directly to each receiver over a corresponding connection or communication channel. Alternatively, the media stream may be transmitted by broadcast such that a content provider provides the same media stream to different receivers. The media stream comprises main content and time slots in breaks in the main content. The method comprises receiving a retransmitted media stream. The re-transmitted media stream is a re-transmission of the media stream originally transmitted by the content provider. The method further comprises detecting a set of content items embedded in the time slots of the received media stream. The set of content items is one of a plurality of sets of content items. Information associating content items with original receivers of a media stream containing the content items is accessed and used to determine an original receiver of the media stream comprising the detected set of content items.

By identifying media streams based on inserted content items rather than, for example, modifying the main content of the media stream in which the content items are inserted for example by watermarking one or more segments of the main content, a simple method of traitor tracing is provided that uses less processing power compared to a traitor tracing method where a unique identifier for each receiver is embedded into the media stream. For example, when a watermark is added to a video component of the media stream with a unique identifier corresponding to each receiver, this requires the content of the media stream to be modified, either before transmission by a content provider, or by a receiving device. This modification of the media stream requires significant processing power, especially if there are thousands of receivers that need to be identified. Further, simple watermarking can be removed on re-transmission by cropping the video frames in the media stream. Removal of the or all content items to remove identifying information from the media stream is much more difficult for a traitor if the media stream is re-transmitted in real time or near real time. Thus, the disclosed method may be more efficient (for example in that both inserting and detecting content items is relatively simple) but also more effective (for example in that it does not impact the video quality of what the user wants to watch). In some embodiments, the plurality of sets of content items a first set of content items and a second set of content items, and each content item comprises a video segment that is not part of the main content. The video segment of a content item for one of the time slots in the first set of content items is different to the video segment of a content item for the corresponding time slot in the second set of content items. The content items may be visually dissimilar, rather than being, for example, watermarked variants of the same video segment.

Each video segment may comprise a storyline (i.e. narrative). A difference between the content item in the time slot of the first set compared to the content item in the corresponding time slot of the second set may be the storylines of the respective video segments.

In some embodiments, at least 50%, say at least 60%, 70%, 80% or 90%, of the pixels of a frame of the video segment of the content item in the one time slot of the first set are different to the pixels in a corresponding frame of the video segment of the content item in the corresponding time slot of the second set. Difference between the pixels of the frame and the corresponding frame may be determined by comparing the values of pixels in corresponding positions in the frames. The percentage difference may be determined based on the total number of pixels in the frame. For example, a content item may be watermarked in two different ways to form two versions of the content item, however, the watermark may account for less than 5% of the total pixels and so the two versions would not be determined to be different in at least 50% of the pixels.

In some embodiments, each target group of receivers is predetermined based on the demographic of the receivers. In some embodiments, additionally or alternatively, sets of content items are predetermined and assigned to predetermined subgroups of authorised receivers before being transmitted to the authorised receivers. Assigning sets of content items to a subgroup of authorised receivers (effectively a target audience of the set), rather than content item by content item independently enables the subgroups and content item sets to be generated by the content provider with identifying re-transmitters in mind. Thus, by controlling the distribution of content items across the subgroups, the pirate transmitter can be identified more quickly compared to using a target group of receivers independently for each content item, for example, based on the demographic of the receivers, where demographic information means that some target audiences contain more intended receivers than other target audiences. In some embodiments, media items are assigned using a tree algorithm. A group of receivers comprising the original receiver of the re-transmitted media stream is split into subgroups and a different content item is assigned to each subgroup for transmission in a first time slot of a media stream. Then, once or repeatedly, each subgroup is split into further subgroups and a different content item is assigned to each further subgroup for transmission in a subsequent time slot. The media streams are transmitted with content items in the time slots according to the corresponding subgroup. Transmission may take place in parallel to the assignment of content items, that is the media streams are transmitted continuously and each time slot filled with content items in time for transmission with the assignment for the subsequent time slot occurring once the previous time slot has passed. Alternatively, content items may be assigned to time slots in advance so that a set of content items assigned to time slots is known in advance, possibly in advance of the start of transmission of the media streams, for example, in advance of an event to which the main content of the media stream relates. In some embodiments, the media streams are transmitted with metadata instead of each content item itself. The metadata may comprise an identification of each content item and the position (or time slot) of the content item to cause the receiver to access to the content items (eg. through a network connection, or to a content already pushed and stored to the local PVR) and insert the content items into the media streams to the appropriate time slots.

In any of the embodiments, naturally, demographic information can also be taken into account in assigning content items to subgroups or target audiences. Further aspects of the disclosure relate to a system and non-transitory computer readable storage media for carrying out the method described above.

In some aspects of the disclosure a method of identifying an original receiver of a media stream comprises splitting a set of receivers into subsets of receivers and associating a respective content item with the receivers in each subset. The media stream is transmitted by a content provider to the original receiver, and the set comprises the original receiver. The media stream comprises main content and time slots in breaks in the main content. The method comprises causing insertion, in the media stream, of the respective content item for each subset of receivers, identifying the respective content item in a re-transmitted media stream, and identifying the subset associated with the identified content item. The re-transmitted media stream is a re-transmission of the media stream. Then the identified subset of receivers is split into further subsets of receivers and a further respective content item is associated with the receivers of each further subset. The method comprises causing insertion, in the media stream, of the further respective content item for each further subset of receivers, identifying the further respective content item in the re- transmitted media stream, and identifying the further subset of receivers associated with the identified further respective content item.

It will be understood that reference to a content item or a further content item includes, but is not limited to, a reference to a plurality of content items or further content items, as the case may be. For example, additional content items may be associated with the subset or further subset of receivers and caused to be inserted at each iteration and may be used for identification, and/or a content item or further content item may comprise one or more component content items, for example a concatenation of component content items.

In some embodiments, an individual media stream is transmitted to each receiver with the corresponding content item embedded by the content provider prior to transmission. Accordingly, causing insertion of the respective content item and the further respective content item may comprise inserting, in respective media streams for each receiver, the respective content item or further respective content item associated with the subset of the receiver. The respective media streams may then be transmitted to the receivers.

In some embodiments, an individual media stream is transmitted to each receiver with metadata indicating the corresponding content item to be inserted into the media stream by the receiver. The metadata may comprise an identification of each content item, for example, a server location to access to the item, or specific content item name for the receiver to retrieve the content item from local storage, when the receiver downloads the content items in advance of receiving the media stream, and the position (or time slot) of the content item to cause the receiver to insert the content items into the media streams. Accordingly, causing insertion of the respective content item and the further respective content item may comprise transmitting, in respective media streams for each receiver, metadata indicating the respective content item or further respective content item to be inserted at the receiver. The indicated respective content item or further respective content item may be inserted into the media stream at the receivers using the metadata. In some embodiments, the media stream is broadcast to the receivers with metadata indicating the content item associated with each subset of receivers. As the media stream is broadcast, the metadata is the same for all receivers. The metadata may comprise an identification of each content item, for example, a server location to access to the item, or specific content item name for the receiver to retrieve the content item from local storage, when the receiver downloads the content items in advance of receiving the media stream, and the position (or time slot) of the content item to cause the receiver to insert the content items into the media streams. Respective identifications and/or positions may be specified for each receiver by associating the respective identifications and/or positions with a receiver identifier enabling the intended receiver to retrieve the relevant content item. For example, in a conditional access system (CAS), the identifications and/or positions may be provided as part of an entitlement management message (EMM) or, more generally, a similar kind of message. Accordingly, causing insertion of the respective content item and the further respective content item may comprise transmitting in the media stream metadata indicating the respective content items or further respective content items, and their association with the receivers in each subset. The respective content item or further respective content item associated with each receiver may be inserted into the media stream at the receivers.

In some embodiments, the media stream is broadcast to the receivers with no indication of the content item to be inserted, for example, the metadata may provide time slot information but not an identification of any content item for the time slot(s). The receiver may request the corresponding content item, or information identifying the content item, to be inserted for each time slot, the request including information identifying the receiver making the request. The content item may then be retrieved from a server location or from a local storage of a receiver, when the receiver downloads the content items in advance of receiving the media stream. Accordingly, causing insertion of the respective content item and the further respective content item may comprise transmitting the media stream, and receiving a request from a receiver for the respective content item or further respective content item associated with the receiver to be inserted into the media stream. The request may comprise an identifier of the receiver. Further, by assigning a respective further content item in each of a subset of the identified subset of receivers, receivers other than those in the identified subset can be ruled out as comprising the re-transmitting receiver stream, and so are excluded from further consideration. This provides the content provider with the ability to freely select content items for the receivers other than those in the identified subset.

By repeating the splitting, assigning and identifying, the further subsets become successively smaller until a desired size of subset is reached, for example until an individual receiver is identified, i.e. the size of the subset is equal to one receiver. By identifying the original receiver and/or a corresponding subscriber to the main content, appropriate counter-measures can be taken. Identifying the respective content items and/or further respective content items may, in some embodiments, comprise receiving information identifying the respective content item or respective further content item. The information may comprise an identifier identifying the content item, a fingerprint of video and/or acoustic content of the content item, the content item itself, a portion of the content item, or any other information allowing identification of the content item. For example, a fingerprint as used in Automatic Content Recognition technologies may be used, and may comprise a condensed digital summary of at least a part of the content item, deterministically generated from an audio/video component of the content item. Further aspects of the disclosure relate to a system for carrying out the method described above.

Some specific embodiments are now described by way of illustration with reference to the accompanying drawings in which like reference numerals refer to like features.

With reference to Figure 1A, a content provider 102 transmits a media stream 104a, 104b, 104c, 104d to each of a plurality of authorised receivers 106a, 106b, 106c, 106d. The media stream is transmitted such that a content provider provides a different media stream 104a-d directly to each receiver 106a-d. The content provider 102 transmits the media streams 104a-d to the receivers via respective communication channels. For example, each media stream may be transmitted to its receiver via an internet protocol connection such as TCP/IP. The receivers may be arranged to output the media stream to a screen and/or speakers for consumption by a user of the authorised receiver. The authorised receivers 106a-d are authorised by the content provider to receive their respective media stream. The content provider 102 may authorise the receiver using a logged-in user account, a pre-authorised device issued by the content provider, and/or a smart card for the receiver (e.g. a set- top box smart card). Accordingly, the authorised receivers 106a-d are a defined set of devices that are at least temporarily registered with the content provider 102 to receive content. Content may be delivered on demand, i.e. on-demand content, or may be provided on a scheduled basis, i.e. linear content.

Each media stream 104a-d comprises the same main content and a different set of content items 108a, 108b, 108c, 108d relative to each other. Each set of content items 108a-d comprises a plurality of content items. Each content item may be an advertisement, a movie trailer, a station identifier such as a TV ident, a news flash, a weather update, etc. The content items may be interleaved in main content delivered with the media streams 104a-d. The main content may be a radio show, a television programme, a live event such as a sports match, a live event, a movie, etc.

The content items 108a-d may be allocated to time slots in the media streams 104a-d in breaks between the main content of the media stream. Each content item may be of equal length with the benefit that the content items can be easily interchanged between time slots.

In some embodiments, the content provider 102 broadcasts a media stream to receivers instead of respective point to point communication channels described above with reference to Figure 1A. In this setting, as each receiver receives the same media stream, the individualised content items cannot be embedded before transmission of the media stream because the same media stream is received by all receivers. Instead the content items are embedded by the receiver before the media stream is output, e.g. decoded and displayed, as described below.

With reference to Figure 1 B, the content provider 102 broadcasts a media stream 1 14, and it is received by a plurality of authorised receivers 106a, 106b, 106c, 106d, each having a receiver identifier, for example a unique ID of the receiver, or a unique ID of a peripheral such as a smart card associated with the receiver, or a logged-in subscriber ID. The media stream 1 14 comprises the main content and metadata 1 16 indicating different sets of content items and their time slots for each receiver 106a-d. For example, the metadata may comprise a list of individual receiver identifiers and their corresponding set of content items, or the metadata may define a set of content items for a range of receiver identifiers, e.g. receiver identifier numbers 1 to 10,000, 10,001 to 20,000 etc. Each receiver 106a-d is arranged to read the metadata and insert the set of content items corresponding to its receiver identifier into a media stream for output. For example, receiver 106b is instructed by the metadata 1 16 to output the media stream with a set of content items 1 18.

A content item server 1 15 stores a collection of content items that can be inserted into media streams for retrieval by the receivers 106a-d. The receivers 106a-d may comprise a memory for storing content items retrieved from the content item server 1 15. Alternatively or additionally, the receivers 106a-d may retrieve a content item indicated in the metadata from the content item server 1 15 at the time of inserting into a media stream.

In some embodiments, the metadata indicates a time slot for a content item but does not specify the content item to be inserted. Instead, the receivers 106a-d request a content item for each time slot by transmitting the receiver identifier to the content item server 1 15 and a content item is selected and transmitted by the content item server 1 15 based on the receiver identifier.

In some embodiments, the receivers 106a-d comprise an appropriate set of instructions which uses the receivers unique ID and the metadata to select the appropriate content item to be inserted inside the content and at which time slot.

It will be understood that metadata can be inserted in the respective media streams instead of the set of content items in embodiments where the content provider 102 transmits the media streams as described with reference to Figure 1A. The receivers 106a-d may read the metadata and output the media stream with the indicated set of content items embedded. Since individualised respective media streams are transmitted, the metadata may be different for each receiver. Of course, the metadata may be the same for all receivers but indicate different sets of content items for each receiver, e.g. identified by an identifier of the receiver or subscriber, as described with reference to Figure 1 B.

In the embodiments of both Figures 1A and 1 B, one of the authorised original receivers 106b, referred to as the re-transmitting receiver 106b below re-transmits its media stream 104b, 1 14 to further receivers 1 10a, 1 10b, 1 10c in re-transmitted media streams 1 12a, 1 12b, 1 12c, 1 19, i.e. pirate transmissions of the media stream. The re-transmitting receiver 106b is not authorised by the content provider 102 to retransmit the media stream 104b, and the further receivers may not be authorised by the content provider to receive the main content in the re-transmitted media stream 1 12a-c. Accordingly, the further receivers 1 10a-c are an open set of devices unknown to the content provider 102. It will be understood that the media stream 104b may not be received directly from the re-transmitting receiver 106b or any one of the authorised receivers. Instead, the media stream 104b may be received from a source that is sent a media stream from one of the authorised receivers, and it will be understood that the principles of the present disclosure equally apply to this situation.

The re-transmitted media streams 1 12a-c, 1 19 may be in a different data format to the original media stream 104b received by the re-transmitting receiver. For example, the re-transmitted media stream may have a lower data rate or use different audio or video codecs than the original media stream received by the re-transmitting receiver. The re-transmitted media streams 1 12a-c, 1 19 may even be modified, e.g. a video portion may be cropped, to circumvent any watermarking applied to the media stream 104b, without affecting the principle of the present disclosure. In other words, the present disclosure does not rely on, and do not need, watermarking in the media steam in order to identify the traitor. Specifically, disclosed methods do not need to watermark the main content.

In the setting of Figure 1A, the re-transmitted media streams 1 12a-c each comprise the same set of content items 108b as received by the re-transmitting receiver 106b because the re-transmitted media streams 1 12a-c are identical to or derived from the media stream 104b received by the re-transmitting receiver 106b. Therefore, as each of the media streams 104a-d transmitted to the authorised receivers comprise different sets of content items 108a-d, traitor tracing can be carried out to identify the media stream 104b that was re-transmitted by inspecting the set of content items in any of the re-transmitted media streams 1 12a-c. The re-transmitting receiver 106b may then be identified as the original receiver of the media stream 104b that was retransmitted. It will be understood that in practice there may be many more original receivers 106a-d, for example, thousands of original receivers and that depending on the number of content items analysed, individual receivers or only a subgroup of original receivers can be identified, as discussed further below.

In the setting of Figure 1 B, the re-transmitted media streams 1 19 each comprise the same set of content items 1 18 as indicated in the metadata 1 16 for receiver 106b. Therefore, as each of the media streams output by the authorised receivers comprise different sets of content items as indicated by the metadata 1 16, traitor tracing can be carried out to identify the re-transmitted receiver 106b by inspecting the set of content items in any of the re-transmitted media streams 1 19, as described above. In both Figures 1A and 1 B, one of the further receivers 1 10a, referred to as the identifying receiver 1 10a below, is arranged to identify the media stream that is being re-transmitted in order to determine the re-transmitting receiver.

With reference to Figure 2A, the identifying receiver 1 10a comprises a receiver module 120, an identifying module 130, a processor 140 and a database 150. It will be understood that these modules can be provided in hardware or software, or a combination thereof, and can be provided in a single device or a distributed system, as described in detail below.

The receiver module 130 is configured to receive the re-transmitted media stream 1 12a, the identifying module 130 is configure to analyse the received media stream to identify it and hence its original receiver and accesses database 150 to do so. The processor 140 is configured to coordinate between the modules and to process the result of identification, e.g. to raise a traitor alarm such as by generating an email alert or etc. so that the content provider 102, or other entity, can take action. Further, the processor 140 is configured to cause the receiver module 120 to access and receive different media streams, for example for suspected websites or internet links so that the identifying receiver 1 10a can scan for pirate media streams.

In some embodiments, the database 150 comprises predetermined content items. With reference to Figure 2B, the database 150 comprises a content item record for each content item. Each content item record comprises a target audience 152, and an acoustic fingerprint 154 and/or a video fingerprint 156. In some embodiments, the content item records do not include an acoustic fingerprint 154 and/or a video fingerprint 156 and may include the content items or excerpts of the content items instead. The target audience 152 includes a set of intended receivers for the content item in question. In some embodiments, the target audience 152 for each content item is predetermined by the content provider 102, for example based on demographic information. In some embodiments, the identifying receiver 1 10a is arranged to determine intended receivers, i.e. the target audience 152, for example, based on previously received content items and associated target audiences. The receivers in the target audience may be identified using a MAC address, an IP address, a logged in user account, or any combination of these. Alternatively or additionally, in some embodiments, the database 150 comprises subgroup records. With reference to Figure 2C, database 150 comprises a plurality of subgroup records 162, each subgroup record having a set of content items 164, a set fingerprint 166, e.g. an acoustic fingerprint and/or a video fingerprint for the set of content items, and a set of receivers 168 in the subgroup. The receivers in the target audience may be identified using a MAC address, an IP address, a user account, or any combination of these.

The target audience 152 and the sets of receivers 168 may comprise receivers that receive the same main content, for example, a group of receivers that are subscribed to receive a sports event as the main content.

A method implemented by the identifying receiver 1 10a to identify the media stream that was re-transmitted is now described with reference to Figure 3. At step 202, the identifying receiver 1 10a receives a re-transmitted media stream 1 12a from the re-transmitting receiver 106b. At step 204, content items embedded in the received re-transmitted media stream 1 12a are detected. The detection of content items may be carried out using automatic content recognition as is known in the art to recognise and identify content items present in the media stream from the database 150 of content items. For example, an acoustic fingerprint of a portion of the media stream may be generated and compared against the acoustic fingerprints 154 of the content items in the database 150 in order to determine a matching content item. The content items detected in the media stream form the detected set of content items in the media stream. For example, one content item may be detected and added to the detected set of content items each time step 204 is carried out.

At step 206, the identifying receiver 1 10a accesses information from the database 150 of content items to determine, for each content item in the detected set of content items, the associated target audience 152. Accordingly, a set of target of audiences corresponding to the detected set of content items is determined.

At step 208, the one or more receivers that have received all content items in the set, that is the information of all respective target audiences, are determined using the target audience 152 information in the database 150. The one or more receivers that are common to the whole set of target audiences is the original receiver(s) of the media stream that was re-transmitted. Figure 4 illustrates a plurality of target audiences 170a, 170b, 170c, 170d, each target audience having a plurality of intended receivers 172, 172a, typically corresponding to identified receivers of the target audience. Each receiver 172, 172a is included in one or more target audiences 170a-d, and hence some of the receivers are common to more than one target audience. In the example of Figure 4, if the plurality of target audiences 170a-d are determined as the set of target audiences at step 206, then receiver 172a is determined at step 208 to be common to the whole set of target audiences. This is because receiver 172a is an intended receiver of each of the target audiences 170a-d.

Returning to Figure 3, at step 210, if the number of original receivers identified in step 208 is below a predetermined size, then the process ends. On the other hand, if the number of original receivers identified in step 208 is greater than a predetermined size, then the process loops back to step 202 to continue receiving the media stream to detect further content items. As the number of content items in the detected set of content items increases, the set of target of audience correspondingly increases, and reduces the number of receivers that are common to the whole set of target audiences. In other words, by increasing the size of the set of content items, fewer receivers would be determined in step 208.

In some embodiments, at step 204, sets of content items embedded in the received re-transmitted media stream 1 12a are detected using the database 150 of subgroups. For example, an acoustic fingerprint of a portion of the media stream may be generated and compared against the set fingerprints 166 of the content items in the database 160 in order to determine a matching set of content items 164. In some embodiments, each content item in the set is individually detected, and the order in which the content items appear in the media stream is used to determine the matching set of content items. Then, at steps 206 and 208, the identifying receiver 1 10a accesses information from the database 160 of subgroups to determine the subgroup 162 associated with the detected set of content items in the media stream.

In some embodiments, each target audience 152 is predetermined based on the demographic of the receivers. In other embodiments, each target audience is a subgroup of the authorised receivers, where the subgroups are generated by the content provider with identifying re-transmitters in mind. Of course, it will be understood that these embodiments may take account of demographic information to assign content items, as well. A method implemented in some embodiments by the content provider 102 to generate subgroups and their corresponding content items is now described with reference to Figure 5. In some embodiments, the method may be implemented by a separate system that embeds content items into media streams. In either case, the content provider 102 or other system may be implemented using one or more computing devices as described below.

At step 402, the content provider 102 splits the authorised receivers into different subgroups each with substantially equal number of receivers. In other embodiments, the number of receivers in each subgroup may differ. By controlling the distribution of the size of the subgroups, the pirate transmitter can be identified more quickly compared to using target audiences based solely on the demographic of the receivers, where demographic information means that some target audiences contain more intended receivers than other target audiences. For example, if the content provider 102 suspects an authorised receiver to be a traitor, the content provider 102 can determine a small size subgroup comprising the suspected traitor. This can reduce the number of iterations of the method described with reference to Figure 3 to reach the predetermined number of receivers at step 210. If the subgroups are substantially equal sizes and the receivers are randomly divided in the subgroups, then each subgroup has approximately the same probability of comprising the traitor. At step 404, a set of content items is determined for each subgroup, such that each set of content items is different. Then at step 406, a set of content items is assigned to each subgroup. A master media stream for each subgroup may comprise the same main content, and the assigned content item may be inserted into the master media stream to form the media streams for the receivers in the subgroup.

In embodiments where respective media streams 104a-d are transmitted for the receivers, at step 408, the set of content items are inserted into media streams corresponding to each subgroup. Alternatively, metadata indicating the set of content items are inserted into media streams corresponding to each subgroup. The media streams comprising the set of content items or the metadata indicating the set of content items are transmitted to each subgroup of receivers. Once the media streams are transmitted to the authorised receivers at step 408, the process described above with reference to Figure 3 can be carried out by the identifying receiver 1 10a to identify which subgroup of original receivers received the media stream that was re-transmitted. In embodiments where the media stream 1 14 is broadcast, at step 408, the media stream is transmitted with metadata 1 16 defining the sets of content items determined for each subgroup. The receivers then read the metadata and determines the timeslots and available content items to insert. Then by using an appropriate set of instructions and its unique identifier insert at the appropriate timeslots the set of content items corresponding to their receiver identifier into the media stream to be output. Figure 6 illustrates a specific method of implementing, in some embodiments, the method described above with reference to Figure 5. At step 450, the content provider determines the number of time slots in the media stream, e.g. in the advertisement breaks between the main content, that require content items to be assigned, and selects an initial time slot to be filled. The number of time slots may be determined as a minimum number of time slots to identify a subgroup based on the desired size of the subgroup, the number of available content items and the number of authorised receivers, as discussed below in greater detail.

At step 452, the content provider splits the authorised receivers into different subgroups each with substantially equal number of receivers. In other embodiments, the number of receivers in each subgroup may differ. At step 454, a different content item is determined for each subgroup. Then at step 456, the content item determined for each subgroup is assigned to its corresponding subgroup. In embodiments where respective media streams 104a-d are transmitted for the receivers, at step 458, a media stream is transmitted to each subgroup. The media streams comprise the content items or metadata indicating the content item, assigned to the subgroup to which it is transmitted, and the content item is embedded in the time slot to be filled.

In embodiments where the media stream 1 14 is broadcast, at step 458, the media stream is transmitted with metadata 1 16 defining the content items determined for each subgroup. The receivers then read the metadata and insert the content item corresponding to their receiver identifier into the media stream to be output.

At step 460, the content provider checks whether all the time slots determined at step 450 have been filled. If the time slots have not all been filled, then at step 462, the next time slot to be filled is selected, then the process loops back to step 452. Following the check of step 460, if all the time slots determined at step 450 have been filled, then the process ends. While the embodiment described assigns content items to time slots and transmits media streams as time slots are filled, it will be appreciated that in some embodiments, some or all of the time slots are filled in advance of transmitting the media streams.

Once the media streams are transmitted to the authorised receivers at step 458, the process of Figure 3 can be carried out by the identifying receiver 1 10a to identify which subgroup of original receivers received the media stream that was retransmitted.

Figure 7 illustrates how sets of content items are assigned according to the method as described above. In each set of content items 480a, 480b, 480c, a content item is allocated in each time slot 1 to n. A group of receivers comprising the original receiver of the re-transmitted media stream is split into subgroups and a different content item is assigned to each subgroup for transmission in a first time slot to of a media stream. Then, once or repeatedly, each subgroup is split into further subgroups and a different content item is assigned to each further subgroup for transmission in a subsequent time slot. The media streams are transmitted with content items in the time slots according to the corresponding subgroup.

The number of different sets of content items available for the determination at step 404, u, depends on the number of different content items, n, the number of time slots for content items in the media stream, k, whether a content item can be repeated in a set, and whether the order of the content items in the set are taken into account in the detecting step at 204.

The total number of sets of content items that are available is given by:

u = n^k

Of this total number of sets, there are sets that comprise repeated content items. The different combinations of sets of content items that are available without repeated content items is given by:

n!

^{U =} k\ (n - k)\ If the order of the content items in the set are taken into account allowing repeated content items, the different combinations of sets of content items that are available is given by:

_ (n + k - iy.

U⁼ /c! (n - l)!

If the order of the content items in the set are taken into account without repeated content items in the set, the different permutations of sets of content items that are available is given by:

n!

^{U =} n - k)

In a particular example, there are 1 million subgroups, for example, 1 million subgroups or authorised receivers, each subgroup containing a single receiver. Each subgroup requires a different set of content items, and there are 20 content items in the database of content items, i.e. u = 1 ,000,000 and n = 20. If the order of the content items is taken into account, the minimum number of time slots, k, required to have a different set of content items for each subgroup is 5, i.e. k≥ 5, irrespective of whether content items can be repeated or not. In this example, the content provider 102 can uniquely identify a traitor with at least five time slots. The content provider 102 can define the number of content items, i.e which do not have to be in consecutive time slots, required to uniquely identify an individual traitor, depending on the total number of authorised receivers and the number of available content items.

By using the process described with reference to Figures 5 or 6 to generate subgroups, the size of the subgroups can be controlled by the content provider 102. As the desired size of the subgroup can be generated in accordance with either Figure 5 or 6 to be smaller than the predetermined number of identified original receivers, in the process of Figure 2, the check at step 210 whether the number of identified original receivers is below a predetermined size, may not be required because each subgroup would comprise fewer receivers than the predetermined number of receivers to which the identification is to be limited.

With reference to Figure 8A, a content provider 1 102 transmits a media stream 1 104a, 1 104b, 1 104c, 1 104d to each of a plurality of authorised receiver 1 106a, 1 106b, 1 106c, 1 106d. The media stream is transmitted such that a content provider provides a different media stream 1 104a-d directly to each receiver 1 106a-d. The content provider 1 102, for example a head-end or over the top media server, transmits the media streams 1 104a-d to the receivers via respective communication channels. For example, each media stream may be transmitted to its receiver via an internet protocol connection such as TCP/IP. The receivers may be arranged to output the media stream to a screen and/or speakers for consumption by a user of the authorised receiver. The authorised receivers 1 106a-d are authorised by the content provider to receive their respective media stream. The content provider 1 102 may authorise the receiver using a logged-in user account, a pre-authorised device issued by the content provider, and/or a smart card for the receiver, e.g. a set-top box smart card. Accordingly, the authorised receivers 1 106a-d are a defined set of devices that are at least temporarily registered with the content provider 1 102 to receive content. Each media stream 1 104a-d comprises the same main content and a different set of content items 1 108a, 1 108b, 1 108c, 1 108d relative to each other. Each set of content items 1 108a-d comprises a plurality of content items. Each content item may be an advertisement, a movie trailer, a station identifier such as a TV ident, a news flash, a weather update, etc.

The content items 1 108a-d may be allocated to time slots in the media streams 104a- d in breaks between the main content of the media stream. Each content item may be of equal length with the benefit that the content items can be easily interchanged between time slots. The main content may be a radio show, a television programme, a sports match, a live event, a movie, etc.

Figure 8A, as described above, relates to point-to-point communication channels. In some embodiments, the content provider 1 102 instead broadcasts a media stream to receivers. In this setting, now described with reference to Figure 8B, as each receiver receives the same media stream, the individualised content items cannot be embedded before transmission of the media stream because the same media stream is received by all receivers. Instead the content items are embedded by the receiver before the media stream is output, e.g. decoded and displayed, as described below. With reference to Figure 8B, the content provider 1 102 broadcasts a media stream 1 1 14, and it is received by a plurality of authorised receivers 1 106a, 1 106b, 1 106c, 1 106d, each having a receiver identifier, for example a unique ID of the receiver, a unique ID of a peripheral such as a smart card associated with the receiver, or a logged-in subscriber ID. The media stream 1 1 14 comprises the main content and metadata 1 1 16 indicating different sets of content items and their time slots for each receiver 1 106a-d. For example, the metadata may comprise a list of individual receiver identifiers and their corresponding set of content items, or the metadata may define a set of content items for a range of receiver identifiers, e.g. receiver identifier numbers 1 to 10,000, 10,001 to 20,000 etc. Each receiver 1 106a-d is arranged to read the metadata and insert the set of content items corresponding to its receiver identifier into a media stream for output. For example, receiver 1 106b is instructed by the metadata 1 1 16 to output the media stream with a set of content items 1 1 18.

A content item server 1 1 15 stores a collection of content items that can be inserted into media streams for retrieval by the receivers 1 106a-d. The receivers 106a-d may comprise a memory for storing content items retrieved from the content item server 1 1 15. Alternatively or additionally, the receivers 1 106a-d may retrieve a content item indicated in the metadata from the content item server 1 1 15 at the time of inserting into a media stream. The receivers may retrieve only the content items required for insertion, or may download and store a larger set of content items to select from locally.

In some embodiments, the metadata indicates a time slot for a content item but does not specify the content item to be inserted. Instead, the receivers 1 106a-d request a content item for each time slot by transmitting the receiver identifier to the content item server 1 1 15 and a content item is selected and transmitted by the content item server 1 1 15 based on the receiver identifier. The identifier may be a Media Access Control (MAC) address, an Internet Protocol (IP) address of the network connection used by the receiver, a serial number/code, a CAS Unique identifier, or any other identifier. It will be understood that metadata can be inserted in the respective media streams instead of the set of content items in embodiments where the content provider 1 102 transmits the media streams as described with reference to Figure 8A. The receivers 1 106a-d may read the metadata and output the media stream with the indicated set of content items embedded. Since individualised respective media streams are transmitted, the metadata may be different for each receiver. Of course, the metadata may be the same for all receivers but indicate different sets of content items for each receiver, e.g. identified by an identifier of the receiver or subscriber, as described with reference to Figure 8B.

In the embodiments of both Figures 8A and 8B, one of the authorised original receivers 1 106b (referred to as the re-transmitting receiver 1 106b below) re-transmits its media stream 1 104b, 1 1 14 to further receivers 1 1 10a, 1 1 10b, 1 1 10c in retransmitted media streams 1 1 12a, 1 1 12b, 1 1 12c, 1 1 19 ,i.e. pirate transmissions of the media stream. The re-transmitting receiver 1 106b is not authorised by the content provider 1 102 to re-transmit the media stream, and the further receivers may not be authorised by the content provider to receive the media stream. Accordingly, the further receivers 1 1 10a-c are an open set of devices unknown to the content provider 1 102. It will be understood that the re-transmitted media stream may not be received directly from the re-transmitting receiver 1 106b or any one of the authorised receivers. Instead, the re-transmitted media stream may be received from a source that is sent a media stream from one of the authorised receivers, and it will be understood that the principles of the present disclosure equally apply to this situation.

The re-transmitted media streams 1 1 12a-c, 1 1 19 may be in a different data format to the original media stream received by the re-transmitting receiver. For example, the re-transmitted media stream may have a lower data rate or use different audio or video codecs than the original media stream received by the re-transmitting receiver. The re-transmitted media stream 1 1 12a-c, 1 1 19 may even be modified, e.g. a video portion may be cropped, to circumvent any watermarking apply to the media stream, without affecting the principle of the present disclosure.

In the setting of Figure 8A, the re-transmitted media streams 1 1 12a-c each comprise the same set of content items 108b as received by the re-transmitting receiver 1 106b because the re-transmitted media streams 1202a-c are versions of the media stream 1 104b received by the re-transmitting receiver 1 106b. Therefore, as each of the media streams transmitted to the authorised receivers comprise different sets of content items 1 108a-d, traitor tracing can be carried out to identify the original receiver of the media stream 1 104b that was re-transmitted by inspecting the set of content items in any of the re-transmitted media streams 1202a-c. The retransmitting receiver 1 106b may then be identified as the original receiver of the media stream 1 104b that was re-transmitted. Similarly, in the setting of Figure 8B, the re-transmitted media streams 1 1 19 each comprise the same set of content items 1 1 18 as indicated in the metadata 1 1 16 for the receiver 1 106b and inserted at the receiver 1 106b. Therefore, as each of the media streams output by the authorised receivers comprise different sets of content items as indicated by the metadata 1 1 16, traitor tracing can be carried out to identify the re-transmitted receiver 1 106b by inspecting the set of content items in any of the re-transmitted media streams 1 1 19, as described above.

In both the settings of Figures 8A and 8B, one of the further receivers 1 1 10a (referred to as the identifying receiver 1 1 10a below) is arranged to identify the original receiver of the media stream that is being re-transmitted in order to determine the re-transmitting receiver, or to supply information to the content provider 1 102 that enables the identification as described below. The content provider 1 102 and the identifying receiver 1 1 10a are communicatively coupled, for example by a network connection so that the content provider 1 102 can receive information related to the identity of the original receiver.

The content provider 1 102 and the identifying receiver 1 1 10a will now be described in greater detail with reference to Figures 9 and 10 respectively.

With reference to Figure 9, the content provider 1 102, in some embodiments, comprises a splitter module 1202, a transmitter module 1204, an identifying module 1206, a communications module 1210, and a processor 1212. In some embodiments, the content provider 1 102 comprises an insertion module 1208. It will be understood that these modules can be provided in hardware or software, or a combination thereof, and can be provided in a single device or a distributed system, as described in detail below.

The splitter module 1202 is configured to split receivers into subsets and assign the same content item to receivers within the same subset. The insertion module 1208 is configured to insert content items into media streams. The identifying module 1206 is configured to analyse the received media stream to identify content items and/or information associated with content items in the received media stream. In some embodiments, the identifying module 1206 is arranged to receive information associated with content items from the identifying receiver 1 1 10a in order to identify the content item. The transmitter module 1204 is arranged to transmit media streams to authorised receivers 1 106a-d. The communications module 1210 is configured to communicate with the identifying receiver 1 1 10a. The processor 1212 is configured to coordinate between the modules and process the result of identification, e.g. to raise a traitor alarm such as by generating an email alert, suspend an account corresponding to the identified media stream or take any other suitable action to deal with the breach of content rights.

With reference to Figure 10, the identifying receiver 1 1 10a comprises a receiver module 1300, an identifying module 1302, a communications module 1304, and a processor 1306. It will be understood that these modules can be provided in hardware or software, or a combination thereof, and can be provided in a single device or a distributed system, as described in detail below. The receiver module 1302 is configured to receive the re-transmitted media stream 1202a, the identifying module 1302 is configured to analyse the received media stream to identify content items and/or information associated with content items in the received media stream. The communications module 1304 is configured to communicate with the content provider 1 102. The processor 1306 is configured to coordinate between the modules. Further, the processor 306 is configured to cause the receiver module 1300 to access and receive different media streams, for example for suspected websites or internet links so that the identifying receiver 1 1 10a can scan for pirate media streams. With reference to Figure 1 1 , the receivers 1 106a-d each comprise a receiver module 1350, and a processor 1352. In some embodiments, the receivers also comprise an inserter module 1354 and/or a memory 1356. It will be understood that these modules can be provided in hardware or software, or a combination thereof, and can be provided in a single device or a distributed system, as described in detail below. The receiver module 1350 is configured to receive the media stream 1 104a-d, 1 1 14. The memory 1356 may store content items. The insertion module 1354 is configured to insert content items into media streams, either from a remote source or from the memory 1356, as described above. The processor 1352 is configured to coordinate between the modules. Processor 1352 or insertion module 1354 may be arranged to buffer and delay the main content by a sufficient length of time to enable the necessary management of the metadata, and the correct insertion of the content items at the appropriate position in the media stream. A method of identifying an original receiver of a media stream is now described with reference to Figure 12. At step 1402, the splitter module 1202 of the content provider 1 102 splits a set of all the receivers into different subsets, each subset having substantially equal number of receivers. In other embodiments, the number of receivers in each subset may differ.

At step 1404, for each subset, the same content item is assigned to each receiver in the subset by the splitter module 1202 of the content provider 1 102 such that every receiver in a subset is assigned the same content item. The content items may be embedded in the media streams in time slots determined by the insertion module 1208 of the content provider 1 102. Assigning a content item to a media stream may include the insertion module 1208 of the content provider 1 102 inserting the content item into each media stream. A master media stream for each subset may comprise the same main content, and the assigned content item, or metadata indicating the assigned content item, may instead be inserted into the master media stream to form the media stream(s) to be transmitted or broadcast to the receivers in the subset.

At step 1406, insertion of the content item into the media stream is caused. The insertion may be carried out by the content provider 1 102 or at the receivers 1 106a- d. For example, the content provider 1 102 may provide information, i.e. metadata, to the receivers 1 106a-d that causes the insertion modules 1354 of the receivers 1 106a-d to insert the respective content item for the receivers in the subset, as described above.

In embodiments where respective media streams 1 104a-d are transmitted to the receivers, at step 1406, the respective content item (or metadata indicating the respective content item) is inserted into the respective media stream by the content provider 1 102, and the media streams are transmitted to their respective receiver by the transmitter module 1204 of the content provider.

In embodiments where the media stream 1 1 14 is broadcast, at step 1406, the media stream is transmitted with metadata 1 1 16 defining the content item determined for each subset. The receivers 1 106a-d then read the metadata and insert the content item corresponding to their receiver identifier in the metadata into the media stream to be output, as described above. At step 1408, a re-transmitted media stream from the re-transmitting receiver is received by the receiver module 1300 of the identifying receiver 1 1 10a. In other embodiments, the re-transmitted media stream is not received directly from the retransmitting receiver 1 106b or one of the original receivers, instead the re-transmitted media stream is received from a source that is sent a media stream from one of the authorised receivers.

At step 1410, the content item embedded in the received re-transmitted media stream is detected either by the identifying module 1206 of the content provider 1 102 or by the identifying module 1302 of the identifying receiver 1 1 10a as described below. The detection of the content item may be carried out using automatic content recognition as is known in the art to recognise and identify the content item present in the media stream. For example, an audio fingerprint of a portion of the media stream may be generated and compared against the audio fingerprints of the assigned content items in order to determine a matching content item.

At step 1412, the subset of receivers that comprise the detected content item is identified to determine the subset that was assigned the detected content item. At step 1414, the processor 1212 of the content provider checks if the number of receivers in the identified subset from step 1412 is below a predetermined size, then the process ends. On the other hand, if the number of receivers in the identified subset from step 1412 is greater than a predetermined size, then the process loops back to step 1402 to divide the identified subset into further subsets of receivers. In this way, the process continues until the number of receivers in the identified subset is reduced to below the predetermined size. For example, the predetermined size of the subset may be one receiver in order to uniquely identify the traitor.

Following step 1412, receivers in subsets other than the identified subset are determined not to comprise the re-transmitting receiver 1 106b, and so are excluded from further consideration in iterations of the process of Figure 12. This provides the content provider 1 102 with the ability to freely select content items for the receivers in subsets other than the identified subset. With reference to Figure 13A, an example data flow between the content provider 1 102, the re-transmitting receiver 1 106b and the identifying receiver 1 1 10a, in a particular embodiment, is now discussed. The content provider 1 102 splits the receivers into subsets (step 1402 in Figure 12) and assigns the same content item to each subset (step 1404). Further, step 1404 comprises the content provider 1 102 communicating to the identifying receiver 1 1 10a information associated with the content items. For example, information associated with the content items may be a list of the content items assigned to an upcoming time slot such that the list is received by the identifying receiver 1 1 10a before the upcoming time slot. In some embodiments, the information associated with the content items may comprise acoustic and/or video fingerprints of the assigned content items, at least a portion of the content items themselves, or an identifier identifying the content items of fingerprints in a database for retrieval by the identifying module 1302 of the identifying receiver 1 1 10a.

In embodiments where respective media streams 1 104a-d are transmitted to the receivers, at step 1406, the insertion module 1208 of the content provider 1 102 inserts the respective content item (or metadata indicating the respective content item), and the transmitter module 1204 transmits the media streams comprising the assigned content items (or metadata indicating the assigned content items) to their respective authorised receivers 1 106a-d (including the re-transmitting receiver 1 106b). In embodiments where the media stream 1 1 14 is broadcast, at step 1406, the transmitter module 1204 of the content provider 1 102 transmits the media stream with metadata 1 1 16 defining the content item determined for each subset. The receivers 1 106a-d then read the metadata and insert the content item corresponding to their receiver identifier in the metadata into the media stream to be output. The retransmitting receiver 1 106b re-transmits its media stream 1 104b and the receiver module 1302 of the identifying receiver 1 1 10a receives the re-transmitted media stream at step 1408.

At step 1410, the identifying module 1302 of the identifying receiver 1 1 10a identifies the content item using the information received at step 1404, and communicates which content item was identified to the content provider 1 102, for example, by sending a content item identifier to the content provider 1 102 via the respective communications modules 1210, 1304 of the identifying receiver 1 1 10a and the content provider 1 102. The identifying module 1206 of the content provider 1 102 identifies at step 1412 to which subset the detected content item was assigned. The processor 1212 of the content provider 1 102 checks at step 1414 whether the size of the subsets is below the predetermined size, and if not, then the content provider loops back to step 1402 and splits the identified subset into further subsets. The steps 1402 to 1414 are repeated to reduce the size of the further subsets until a predetermined size of the further subset is reached, as described above.

With reference to Figure 13B, a different example data flow between the content provider 1 102, the re-transmitting receiver 1 106b and the identifying receiver 1 1 10a in a particular embodiment is now discussed. The data flow is similar to the data flow described with reference to Figure 13A above. However, instead of communicating to the identifying receiver 1 1 10a information associated with only the upcoming content items at step 1404, information, such as fingerprints as described above, associated with all available content items is sent at step 1400, prior to step 1402. In other words, information that can be used to identify each of the possible content items is sent to the identifying receiver 1 1 10a in advance of commencing the method of identifying an original receiver of a media stream described with reference to Figure 12.

With reference to Figure 13C, an example data flow between the content provider 1 102, the re-transmitting receiver 1 106b and the identifying receiver 1 1 10a in a further particular embodiment is now discussed. The data flow is similar to the data flow described with reference to Figure 13A above, in which the identification of the content item in the re-transmitted media stream is carried out by the content provider 1 102. The content provider 1 102 does not communicate information associated with the assigned content items to the identifying receiver 1 1 10a at step 1404. Instead, at step 1410, the identifying receiver 1 1 10a determines information identifying the content item (e.g. an acoustic and/or video fingerprint etc.) and communicates this identifying information to the content provider 1 102. The content provider 1 102 then uses the information identifying the content item to identify the content item from the assigned content items. In other embodiments, at step 1410, the identifying receiver 1 1 10a extracts the content item, or a portion of the content item, from the retransmitted media stream and communicates this to the content provider 1 102 so that the content provider 1 102 can identify the content item.

It will be understood that many other data flows are equally possible, for example, the content provider 1 102 could, in the examples of Figures 13A and 13B, send information about the media streams and associated user accounts to the identifying receiver 1 1 10a for processing and raising of alerts (e.g. email or other alerts to the content provider 1 102) by the identifying receiver 1 1 10a.

In the setting of the embodiment of Figure 8A where respective media streams are transmitted to each receiver 1 106a-d, an example assignment of content items to subsets is discussed with reference to Figure 14. All receivers in a set 1600 initially comprise the same content, i.e. main content and content items. The set 1600 is split (at step 1402) into a plurality of subsets 1602a, 1602b, 1602c, where each subset comprises different receivers. The subsets 1602a-c are each assigned a different content item (at step 1404) to a time slot of the media streams. The media streams are transmitted (at step 1406) to the receivers. For example, content item 'Bd1 ' is embedded in the media streams for the receivers in subset 1602a, content item 'Bd2' is embedded in the media streams for the receivers in subset 1602b, and content item 'Bdn' is embedded in the media streams for the receivers in subset 1602c.

The identifying receiver 1 1 10a receives (at step 1408) the re-transmitted media stream, and detects (at step 1410) that the re-transmitted media stream comprises content item 'Bd2'. Therefore, the identifying receiver 1 1 10a identifies (at step 1412) subset 1302b as comprising the re-transmitting receiver 1 106b. The subset 1602b comprises more receivers than the predetermined number of receivers, and so subset 1602b is split into further subsets 1604a, 1604b, 1604c, where each subset comprises different receivers.

The further subsets 1604a-c are each assigned a different content item (at step 1404) to a time slot of the media streams. The media streams are transmitted (at step 1406) to the receivers. For example, content item 'Bd1 ' is embedded and transmitted to the receivers in further subset 1604a, content item 'Bd3' is embedded and transmitted to the receivers in subset 1604b, and content item 'Bdn' is embedded and transmitted to the receivers in subset 1604c.

The identifying receiver 1 1 10a continues to receive (at step 1408) the re-transmitted media stream, and detects (at step 1410) that the re-transmitted media stream comprises content item 'Bdn'. Therefore, the identifying receiver 1 1 10a identifies (at step 1412) subset 1604c as comprising the re-transmitting receiver 1 106b. The subset 1604c comprises more receivers than the predetermined number of receivers, and so subset 1604c is split into further subsets 1606a, 1606b, 1606c, where each subset comprises different receivers. The further subsets 1606a-c are each assigned a different content item (at step 1404) to a time slot of the media streams. The media streams are transmitted (at step 1406) to the receivers. For example, content item 'Bd1 ' is embedded and transmitted to the receivers in further subset 1606a, content item 'Bd3' is embedded and transmitted to the receivers in subset 1606b, and content item 'Bnd-1 ' is embedded and transmitted to the receivers in subset 1606c.

The identifying receiver 1 1 10a continues to receive (at step 1408) the re-transmitted media stream, and detects (at step 1410) that the re-transmitted media stream comprises content item 'Bd3'. Therefore, the identifying receiver 1 1 10a identifies (at step 1412) subset 1606b as comprising the re-transmitting receiver 1 106b. The subset 1606b comprises fewer receivers than the predetermined number of receivers, and so a desired size of subset comprising the re-transmitting receiver 1 106b has been identified.

It is to be understood that in the setting of the embodiment of Figure 8B where the media stream is broadcast to the receivers 1 106a-d, the assignment of content items described with reference to Figure 14 may involve metadata indicating the content item being embedded into the media stream instead of the content item itself. For example, metadata is embedded in the media stream for 'timeslot V indicating content item 'Bd1 ' for the receivers in subset 1602a, content item 'Bd2' for the receivers in subset 1602b, and content item 'Bdn' for the receivers in subset 1602c. The receivers then insert the content item corresponding to their subset 1602a-c.

Figure 15 illustrates a block diagram of one implementation of a computing device 500 within which a set of instructions, for causing the computing device to perform any one or more of the methodologies discussed herein, may be executed. In alternative implementations, the computing device may be connected (e.g., networked) to other machines in a Local Area Network (LAN), an intranet, an extranet, or the Internet. The computing device may operate in the capacity of a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The computing device may be a personal computer (PC), a tablet computer, a set-top box (STB), a web appliance, a server, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single computing device is illustrated, the term "computing device" shall also be taken to include any collection of machines (e.g., computers) that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. The example computing device 500 includes a processing device 502, a main memory 504 (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM), etc.), a static memory 506 (e.g., flash memory, static random access memory (SRAM), etc.), and a secondary memory (e.g., a data storage device 518), which communicate with each other via a bus 530.

Processing device 502 represents one or more general-purpose processors such as a microprocessor, central processing unit, or the like. More particularly, the processing device 502 may be a complex instruction set computing (CISC) microprocessor, reduced instruction set computing (RISC) microprocessor, very long instruction word (VLIW) microprocessor, processor implementing other instruction sets, or processors implementing a combination of instruction sets. Processing device 502 may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. Processing device 502 is configured to execute the processing logic (instructions 522) for performing the operations and steps discussed herein.

The computing device 500 may further include a network interface device 508. The computing device 500 also may include a video display unit 510 (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), an alphanumeric input device 512 (e.g., a keyboard or touchscreen), a control device 514 (e.g., a mouse or touchscreen), and an audio device 516 (e.g., a speaker). The data storage device 518 may include one or more machine-readable storage media (or more specifically one or more non-transitory computer-readable storage media) 528 on which is stored one or more sets of instructions 522 embodying any one or more of the methodologies or functions described herein. The instructions 522 may also reside, completely or at least partially, within the main memory 504 and/or within the processing device 502 during execution thereof by the computer system 500, the main memory 504 and the processing device 502 also constituting computer-readable storage media. The various methods described above may be implemented by a computer program. The computer program may include computer code arranged to instruct a computer to perform the functions of one or more of the various methods described above. The computer program and/or the code for performing such methods may be provided to an apparatus, such as a computer, on one or more computer readable media or, more generally, a computer program product. The computer readable media may be transitory or non-transitory. The one or more computer readable media could be, for example, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, or a propagation medium for data transmission, for example for downloading the code over the Internet. Alternatively, the one or more computer readable media could take the form of one or more physical computer readable media such as semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disc, and an optical disk, such as a CD-ROM, CD-R/W or DVD.

In an implementation, the modules, components and other features described herein (for example, receiver module 120 or identifier module 130 in relation to Figure 2A, or receiver module 1300 or identifier module 1302 in relation to Figure 10) can be implemented as discrete components or integrated in the functionality of hardware components such as ASICS, FPGAs, DSPs or similar devices as part of an individualization server.

A "hardware component" is a tangible (e.g., non-transitory) physical component (e.g., a set of one or more processors) capable of performing certain operations and may be configured or arranged in a certain physical manner. A hardware component may include dedicated circuitry or logic that is permanently configured to perform certain operations. A hardware component may be or include a special-purpose processor, such as a field programmable gate array (FPGA) or an ASIC. A hardware component may also include programmable logic or circuitry that is temporarily configured by software to perform certain operations.

Accordingly, the phrase "hardware component" should be understood to encompass a tangible entity that may be physically constructed, permanently configured (e.g., hardwired), or temporarily configured (e.g., programmed) to operate in a certain manner or to perform certain operations described herein. In addition, the modules and components can be implemented as firmware or functional circuitry within hardware devices. Further, the modules and components can be implemented in any combination of hardware devices and software components, or only in software (e.g., code stored or otherwise embodied in a machine-readable medium or in a transmission medium).

Unless specifically stated otherwise, as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as "receiving", "determining", "comparing ", "identifying," or the like, refer to the actions and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other implementations will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific example implementations, it will be recognized that the disclosure is not limited to the implementations described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. The scope of the disclosure should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Further aspects and embodiments are disclosed in the following numbered items:

1. A method of identifying an original receiver of a media stream, wherein the media stream was transmitted by a content provider to the original receiver, the method comprising:

receiving a re-transmitted media stream, wherein the re-transmitted media stream is a re-transmission of the media stream;

detecting a set of content items embedded in the received media stream; accessing information associating content items with original receivers of a media stream containing the set of content items; and determining, using the information, an original receiver of the media stream comprising the detected set of content items.

2. The method of item 1 , further comprising, prior to receiving the re-transmitted media stream:

splitting a group of receivers into target subgroups, wherein the group comprises the original receiver;

determining sets of content items, wherein the sets of content items are mutually different;

assigning a set of content items of the sets of content items to each target subgroup; and

transmitting, in respective media streams, each set of content items to the receivers in the target subgroup corresponding to the set of content items, or

transmitting the media stream and causing insertion of each set of content items in the media stream at the receivers in the target subgroup corresponding to the set of content items.

3. The method of any of items 1 or 2, further comprising, prior to receiving the re-transmitted media stream:

splitting a group of receivers into first subgroups, wherein the group comprises the original receiver and assigning a different content item to each first subgroup for transmission in a first time slot;

once or repeatedly splitting each previous subgroup into further subgroups until the further subgroups are the target subgroups and assigning a different content item to each further subgroup for transmission in a subsequent time slot; and

transmitting, in respective media streams, each set of content items to the receivers in the target subgroup corresponding to the set of content items, or transmitting the media stream and causing insertion of each set of content items in the media stream at the receivers in the target subgroup corresponding to the set of content items.

4. The method of any of preceding item, wherein the number of receivers in each target subgroup is substantially equal across the target subgroups.

5. The method of any of preceding item, wherein the number of receivers in each target subgroup is one. 6. The method of any preceding item, wherein the sets of content items differ from each other with respect to the order of the content items in each set, and detecting comprises detecting the order of appearance of the content items.

7. The method of any preceding item, wherein each content item is of equal length, and wherein the media stream comprises predetermined time slots for the content items. 8. The method of any preceding item, wherein the media stream comprises an audio component, and wherein detecting a set of content items comprises detecting each content item in the set, and detecting a content item comprises comparing an audio fingerprint of the content item in the re-transmitted media stream with predetermined audio fingerprints associated with content items.

9. The method of any preceding item, wherein the media stream comprises a video component, and wherein detecting a set of content items comprises detecting each content item in the set, and detecting a content item comprises comparing a video fingerprint of the content item in the re-transmitted media stream with predetermined video fingerprints associated with content items.

1 1. The method of any preceding item, wherein accessing information associating content items with original receivers comprises accessing a database of content items each associated with a target audience of intended receivers.

12. The method of any preceding item, wherein accessing information associating content items with original receivers comprises accessing a database associating a set of content items associated with a subgroup of receivers. 13. The method of any of items 2 to 12, wherein causing insertion of each set of content items comprises transmitting metadata in the media stream indicating the set and locations or timeslots of content items to be inserted.

14. The method of any of items 2 to 13, wherein causing insertion of each set of content items comprises the receiver retrieving at least one content item in the set of content items from a memory of the receiver. 15. The method of any of item 2 to 14, wherein causing insertion of each set of content items comprises the receiver retrieving at least one content item in the set of content items from a remote source. 16. One or more non-transitory computer readable media having computer- executable instructions to cause one or more computer systems to perform the method of any of items 1 to 15.

17. A system for identifying an original receiver of a media stream, wherein the media stream was transmitted by a content provider to the original receiver, the system comprising:

a receiver arranged to receive a re-transmitted media stream, wherein the retransmitted media stream is a re-transmission of the media stream; and

a processor arranged to:

detect a set of content items embedded in the received media stream; access information associating content items with original receivers of a media stream containing the content items; and

determine, using the information, an original receiver of the media stream comprising the detected set of content items was transmitted.

18. The system of item 17, the processor further arranged to:

split a group of receivers into target subgroups, wherein the group comprises the original receiver;

determine sets content items, wherein the sets of content items are mutually different;

assign a set of content items of the sets of content items to each target subgroup; and

transmit, in respective media streams, each set of content items to the receivers in the target subgroup corresponding to the set of content items, or

transmit the media stream and cause insertion of each set of content items in the media stream at the receivers in the target subgroup corresponding to the set of content items.

19. The system of any of items 17 or 18, the processor further arranged to:

split a group of receivers into first subgroups, wherein the group comprises the original receiver and assigning a different content item to each first subgroup for transmission in a first time; once or repeatedly split each previous subgroup into further subgroups until the further subgroups are the target subgroups and assign a different content item to each further subgroup for transmission in a subsequent time; and

transmit the media stream and cause insertion of each set of content items in the media stream at the receivers in the target subgroup corresponding to the set of content items. 20. The system of any of items 17 to 19, wherein the number of receivers in each target subgroup is substantially equal across the target subgroups.

21. The system of any of items 17 to 20, wherein the number of receivers in each target subgroup is one.

22. The system of any of items 17 to 21 , wherein the sets of content items differ from each other with respect to the order of the content items in each set, and detecting comprises detecting the order of appearance of the content items. 23. The system of any of items 17 to 22, wherein each content item is one or more of: an advertisement; a movie trailer; a station identifier; a news flash; and a weather update

24. The system of any of items 17 to 23, wherein each content item is of equal length, and wherein the media stream comprises predetermined time slots for the content items.

25. The system of any of items 17 to 24, wherein the media stream comprises an audio component, and wherein the processor is arranged to detect a set of content items by detecting each content item in the set, and detecting a content item comprises comparing an audio fingerprint of the content item in the re-transmitted media stream with predetermined audio fingerprints associated with content items.

26. The system of any of items 17 to 25, wherein the media stream comprises a video component, and wherein the processor is arranged to detect a set of content items by detecting each content item in the set, and detecting a content item comprises comparing a video fingerprint of the content item in the re-transmitted media stream with predetermined video fingerprints associated with content items.

27. The system of any of items 17 to 26, wherein accessing information associating content items with original receivers comprises accessing a database of content items each associated with a target audience of intended receivers.

28. The system of any of items 17 to 27, wherein accessing information associating content items with original receivers comprises accessing a database associating a set of content items is with a subgroup of receivers.

29. The system of any of items 18 to 28, wherein the processor is arranged to cause insertion of each set of content items by transmitting metadata in the media stream indicating the set and locations, or timeslots, of content items to be inserted.

30. The system of any of items 18 to 29, wherein the processor is arranged to cause insertion of each set of content items by the receiver retrieving at least one content item in the set of content items from a memory of the receiver. 31. The system of any of items 18 to 30, wherein the processor is arranged to cause insertion of each set of content items by the receiver retrieving at least one content item in the set of content items from a remote source.

32. A method of assigning content items to media streams, the method comprising:

determining sets content items, wherein the sets of content items are mutually different;

transmitting, in respective media streams, each set of content items to the receiver in the target subgroup corresponding to the set of content items, or

transmitting the media stream and causing insertion of each set of content items in the media stream at the receivers in the target subgroup corresponding to the set of content items. 33. One or more non-transitory computer readable media having computer- executable instructions to cause one or more computer systems to perform the method of item 32.

34. A system for assigning content items to media streams, the system

comprising a processor arranged to:

transmit, in respective media streams, each set of content items to the receiver in the target subgroup corresponding to the set of content items.

35. A method of identifying an original receiver of a media stream, wherein the media stream is transmitted by a content provider to the original receiver, the method comprising:

a) splitting a set of receivers into subsets of receivers, wherein the set comprises the original receiver, and associating a respective content item with the receivers in each subset;

b) causing insertion, in the media stream, of the respective content item for each subset of receivers;

c) identifying the respective content item in a re-transmitted media stream, wherein the re-transmitted media stream is a re-transmission of the media stream;

d) identifying the subset of receivers associated with the identified content item;

e) splitting the identified subset of receivers into further subsets and associating a further respective content item with the receivers in each further subset;

f) causing insertion, in the media stream, of the further respective content item for each further subset of receivers;

g) identifying the further respective content item in the re-transmitted media stream; and h) identifying the further subset of receivers associated with the identified further respective content item.

36. The method of Item 35, further comprising repeating steps e to h to reduce the size of the identified further subset.

37. The method of Item 36, wherein steps e to h are repeated until the size of the further subset is one receiver. 38. The method of any of items 35 to 37, wherein identifying the respective content item in the re-transmitted media stream comprises:

receiving the re-transmitted media stream; and

detecting the respective content item in the received re-transmitted media stream.

39. The method of any of items 35 to 38, wherein identifying the further respective content item in the re-transmitted media stream comprises:

continuing to receive the re-transmitted media stream; and

detecting the further respective content items in the received re-transmitted media stream.

40. The method of any of items 35 to 39, wherein the number of receivers in each subset and/or further subset is substantially equal across the subsets. 41. The method of any of items 35 to 40, wherein the content items are one or more of: an advertisement; a movie trailer; a station identifier; a news flash; and a weather update

42. The method of any of items 35 to 41 , wherein each content item is of equal length, and wherein the media stream comprises predetermined time slots for the content items.

43. The method of any of items 35 to 42, wherein the media stream comprises an audio component, and wherein identifying the respective content item and/or the further respective content item comprises comparing an audio fingerprint of the content item in the re-transmitted media stream with predetermined audio fingerprints associated with content items. 44. The method of any of items 35 to 43, wherein the media stream comprises a video component, and wherein identifying the respective content item and/or the further respective content item comprises comparing a video fingerprint of the content item in the re-transmitted media stream with predetermined audio fingerprints associated with content items.

45. The method of any of items 35 to 44, wherein identifying the respective content item and/or the further respective content item comprises receiving information identifying the content item.

46. The method of item 45, wherein the information identifying the content item is received over a communication network. 47. The method of any of items 35 to 46, wherein causing insertion of the respective content item and the further respective content item comprises:

inserting, in respective media streams for each receiver, the respective content item or further respective content item associated with the subset of the receiver; and

transmitting the respective media streams to the receivers.

48. The method of any of items 35 to 47, wherein causing insertion of the respective content item and the further respective content item comprises:

transmitting, in respective media streams for each receiver, metadata indicating the respective content item or further respective content item to be inserted at the receiver; and

causing insertion of the indicated respective content item or further respective content item into the media stream at the receivers. 49. The method of any of items 35 to 47, wherein causing insertion of the respective content item and the further respective content item comprises:

transmitting in the media stream metadata indicating the respective content items or further respective content items, and their association with each subset; and causing insertion of the respective content item or further respective content item associated with the receiver into the media stream at the receivers. 50. The method of any of items 35 to 47, wherein causing insertion of the respective content item and the further respective content item comprises:

transmitting the media stream; and

receiving a request from a receiver for the respective content item or further respective content item associated with the receiver to be inserted into the media stream, the request comprising an identifier of the receiver.

51. The method of any of items 48 to 50, wherein each receiver comprises a memory, and causing insertion of each content item comprises each receiver retrieving the respective content item or further respective content item from its memory.

52. The method of any of items 48 to 51 , wherein causing insertion of each content item comprises each receiver retrieving the respective content item or further respective content item from a remote source.

53. One or more non-transitory computer readable media having computer- executable instructions to cause one or more computer systems to perform the method of any of items 35 to 52.

54. A system for identifying an original receiver of a media stream, wherein the media stream is transmitted by a content provider to the original receiver, the system comprising a processor arranged to:

a) split a set of receivers into subsets, wherein the set comprises the original receiver, and associate a respective content item with the receivers in each subset;

b) cause insertion, in the media stream, of the respective content item for each subset of receivers;

c) identify the respective content item in a re-transmitted media stream, wherein the re-transmitted media stream is a re-transmission of the media stream; d) identify the subset of receivers associated with the identified content item;

e) split the identified subset of receivers into further subsets of receivers and associate a further respective content item with the receivers in each further subset;

f) cause insertion, in the media stream, of the further respective content item for each further subset of receivers; g) identify the further respective content item in the re-transmitted media stream; and

h) identify the further subset of receivers associated with the identified further respective content item.

55. The system of Item 54, the processor further arranged to repeat steps e to h to reduce the size of the identified further subset.

56. The system of Item 55, wherein steps e to h are repeated until the size of the further subset is one receiver.

57. The system of any of Items 54 to 56, wherein the step of identifying the respective content item in the re-transmitted media stream comprises:

receiving the re-transmitted media stream; and

detecting one of the respective content items in the received re-transmitted media stream.

58. The system of any of Items 54 to 57, wherein the step of identifying the further respective content item in the re-transmitted media stream comprises:

continuing to receive the re-transmitted media stream; and

detecting one of the respective further content items in the received retransmitted media stream.

59. The system of any of Items 54 to 58, wherein the number of receivers in each subset and/or further subset is substantially equal across the subsets.

60. The system of any of Items 54 to 59, wherein the content items are one or more of: an advertisement; a movie trailer; a station identifier; a news flash; and a weather update

61. The system of any of Items 54 to 60, wherein each content item is of equal length, and wherein the media stream comprises predetermined time slots for the content items. 62. The system of any of Items 54 to 61 , wherein the media stream comprises an audio component, and wherein identifying the respective content item and/or the further respective content item comprises comparing an audio fingerprint of the content item in the re-transmitted media stream with predetermined audio fingerprints associated with content items.

63. The system of any of Items 54 to 62, wherein the media stream comprises a video component, and wherein identifying the respective content item and/or the further respective content item comprises comparing a video fingerprint of the content item in the re-transmitted media stream with predetermined audio fingerprints associated with content items. 66. The system of any of Items 54 to 64, wherein identifying the content item comprises receiving information identifying the content item.

67. The system of Item 65, wherein the information identifying the content item is received over a communication network.

68. The system of any of Items 54 to 66, wherein the processor arranged to cause insertion of the respective content item and the further respective content item is arranged to:

insert, in respective media streams for each receiver, the respective content item or further respective content item associated with the subset of the receiver; and transmit the respective media streams to the receivers.

69 The system of any of Items 54 to 66, wherein the processor arranged to cause insertion of the respective content item and the further respective content item is arranged to:

transmit, in respective media streams for each receiver, metadata indicating the respective content item or further respective content item to be inserted at the receiver; and

cause insertion of the indicated respective content item or further respective content item into the media stream at the receivers.

70. The system of any of Items 54 to 66, wherein the processor arranged to cause insertion of the respective content item and the further respective content item is arranged to:

transmit in the media stream metadata indicating the respective content items or further respective content items, and their association with each subset; and cause insertion of the respective content item or further respective content item associated with the receiver into the media stream at the receivers.

71. The system of any of Items 54 to 66, wherein the processor arranged to cause insertion of the respective content item and the further respective content item is arranged to:

transmit the media stream; and

receive a request from a receiver for the respective content item or further respective content item associated with the receiver to be inserted into the media stream, the request comprising an identifier of the receiver.

72. The system of any of Items 68 to 70, wherein each receiver comprises a memory, and causing insertion of each content item comprises each receiver retrieving the respective content item or further respective content item from its memory.

73. The system of any of Items 68 to 71 , wherein causing insertion of each content item comprises each receiver retrieving the respective content item or further respective content item from a remote source.

Claims

1. A method of identifying an original receiver of a media stream, wherein the media stream was transmitted by a content provider to the original receiver, and the media stream comprises main content and time slots in breaks in the main content, the method comprising:

detecting a set of content items embedded in the time slots of the received media stream, wherein the set of content items is one of a plurality of sets of content items;

accessing information associating content items with original receivers of a media stream containing the set of content items; and

determining, using the information, an original receiver of the media stream comprising the detected set of content items.

2. The method according to 1 , further comprising, prior to receiving the retransmitted media stream:

determining a plurality of sets of content items to be embedded in time slots, wherein the sets of content items are mutually different;

3. The method according to claim 1 or 2, further comprising, prior to receiving the re-transmitted media stream:

splitting a group of receivers into first subgroups, wherein the group comprises the original receiver and assigning a different content item to each first subgroup for transmission in a first time slot; once or repeatedly splitting each previous subgroup into further subgroups until the further subgroups are the target subgroups and assigning a different content item to each further subgroup for transmission in a subsequent time slot; and

4. The method according to any of the preceding claims, wherein each content item is of equal length, and wherein the media stream comprises predetermined time slots for the content items.

5. The method of any preceding claim, wherein the plurality of sets of content items a first set of content items and a second set of content items, each content item comprises a video segment that is not part of the main content, and the video segment of a content item for one of the time slots in the first set of content items is different to the video segment of a content item for the corresponding time slot in the second set of content items.

6. The method according to claim 5, wherein each video segment comprise a storyline, and a storyline of the video segment of the content item in the one time slot of the first set is different to a storyline of the video segment of the content item in the corresponding time slot of the second set.

7. The method according to any of claims 5 or 6, wherein at least 50% of the pixels of a frame of the video segment of the content item in the one time slot of the first set are different to the pixels in a corresponding frame of the video segment of the content item in the corresponding time slot of the second set.

8. The method according to any of the preceding claims, wherein the media stream comprises an audio and/or video component, and wherein detecting a set of content items comprises detecting each content item in the set, and detecting a content item comprises comparing a fingerprint of the content item in the re- transmitted media stream with predetermined fingerprints associated with content items.

9. The method according to any of the preceding claims, wherein the main content is a sports match, a live event, a movie, or an episode of a television series.

10. The method according to any of the preceding claims, wherein each content item is an advertisement, a movie trailer, a station identifier, a news flash, or a weather update.

1 1. The method according to any of the claims 2 to 10, wherein causing insertion of each set of content items comprises transmitting metadata in the media stream indicating the set and locations or timeslots of content items to be inserted.

12. The method according to of any of the claims 2 to 1 1 , wherein causing insertion of each set of content items comprises the receiver retrieving at least one content item in the set of content items from a remote source.

13. The method according to any of the preceding claims, comprising:

g) identifying the further respective content item in the re-transmitted media stream; and

h) identifying the further subset of receivers associated with the identified further respective content item.

14. A system for identifying an original receiver of a media stream, wherein the media stream was transmitted by a content provider to the original receiver, and the media stream comprises main content and time slots in breaks between the main content, the system comprising:

a processor arranged to:

detect a set of content items embedded in the time slots of the received media stream, wherein the set of content items is one of a plurality of sets of content items;

access information associating content items with original receivers of a media stream containing the content items; and

15. A method of assigning content items to media streams, the media stream comprising main content and time slots in breaks between the main content, the method comprising:

determining sets content items to be embedded in the time slots, wherein the sets of content items are mutually different;