GB2503267A - Gapless video transition - Google Patents

Abstract

A synchronisation signal from a master node is used to control video stream transmissions between sending nodes 102, 108, 111 and receiving nodes 103, 112. The synchronisation signal comprises first synchronisation elements that have a frequency greater than the highest image frequency of the video stream transmitted by the sending nodes and second synchronisation elements that are used to enable the sending nodes to maintain synchronisation with the master node. Methods are disclosed that a) permit image data to be transmitted to receiving nodes according to the first synchronising elements, b) control synchronisation between sending nodes by using the first synchronisation elements to synchronise image data transmission and c) control video stream transmission by transmitting image data of the video stream from a sending node to a receiving node using the first synchronisation elements and maintaining synchronisation between the sending nodes and master nodes using the second synchronising elements. The method is designed to provide a gapless display transition between video streams of the sending nodes when the receiving node switches from one display to another.

Description

tM:;: INTELLECTUAL S... * PROPERTY OFFICE Application No. 0B1210997.1 R'T'M Date:18 October 2012 The following terms are registered trademarks and should be read as such wherever they occur in this document:

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Intellectual Ptoperty Office is an operaling name of Ihe Patent Office www.ipo.gov.uk The present invention relates to transmission of video streams by a plurality of video sources for receipt by a receiving device, notably for displaying the video streams. In particular, the present invention relates to switching from one video stream to another.

There is a need for having several video sources outputting respective video streams that may be selectively displayed on at least one display. For example, there may be such need in a conference room, in a TV (television) production studio or other cases.

Typically, an operator alternatively selects one video source to be displayed on the reference display using switching means. A smooth transition between images from the previous source and images from the new source is a requirement systems must meet. This requirement is known as "gapless transition".

A gapless transition is a transition that does not show any interruption between the image flow of the previous source and the image flow of the new source. A system that does not implement gapless transition inserts at least one blank image between the image flow of the previous source and the image flow of the new source.

In order to implement gapless transition, the first image of the new source has to be displayed right after the last image of the previous source.

Two problems may rise. The first problem consists in making sure that the first image of the new source is ready on time at least at the end of the last image of the previous source. The second problem lies in the management of image frequency difference between the two sources. If the display got synchronized to the first source image frequency then it has to change its synchronization to the new source image frequency. Synchronizing again is likely to provoke an interruption in the image flow displayed.

In order to overcome the above problems, video switch systems have been proposed.

Figure 1 shows a system comprising a central switch. Such systems have been developed for cable connections such as SDI (Serial Digital Interface) or HDMI (High Definition Multimedia Interface).

The central video switch 100 has wired video connections to video sources 101, 102 and 103 and wired video connections to displays 104 and 105. In addition to the wired video connections, the central video switch has wired synchronization connections 106 ("genlock" connections) to the video source.

The video switch receives all image flows simultaneously from all the video sources through the video connections (or links) from the video sources.

Therefore, the first image of the new video source is always available when switching from the previous source to the new one.

The "genlock" connections provide image synchronization signals from the video switch to the video sources. Thus, the video sources have the same image rate.

A drawback of the central architecture of the systems according to Figure 1 is the lack of scalability. The number of sources and displays that can be connected depends on the number of physical ports in the switch device. In addition, the video sources must have a synchronization input port for the genlock connection. Such synchronization (genlock) port is costly and is reserved for professional video sources.

Figure 2 illustrates another type of digital systems having a distributed architecture based on networking technologies. Document Shiral et 31. "Real time switching and streaming transmission of uncompressed 4K motion pictures" describes such systems. Several distant video sources 200, 201 are connected to a display node 202 through a long distance digital network 203. An operator located at the display node switches between the distant sources.

First, the display node receives a video stream from the video source 200. It also receives respective timecode packets TA and TB from both video sources 200 and 201. Next, a stream ending time ETA is sent to the video source 200, based on a user request. A stream starting time STB is also sent to the video source 201, ST8 being computed as ETA ± (TA -TB).

Such systems do not address the problem of source image rate difference. Also, in order to solve the problem of having the first image of the new video source ready to be displayed on time, a protocol based on frame image buffering is performed. However, image buffering techniques are costly because images represent a huge amount of data. This may not be appropriate, in particular for uncompressed high resolution video streams.

Document US 4,574,379 discloses a satellite transmission system for TV signals. The system uses an analog wireless transmission based on a TDMA (Time Division Multiple Access) scheme synchronized on a multiple of a reference image frequency. However, this document does not address the video stream switching problem.

There is thus a need for enhancing video stream transmissions in communication networks having several nodes. In particular, there is a need for enhancing gapless video transition (i.e. without display interruption).

According to a first aspect of the invention there is provided a method of transmitting image data of a video stream from a sending node of a communication network comprising the following steps: -receiving from a master node of the communication network, a synchronization signal comprising synchronization elements, -transmitting image data of the video stream to a receiving node of the network according to first synchronization elements of the synchronization signal, said first synchronization elements having a frequency above the highest image frequency of the video streams transmitted by the sending nodes of the communication network, and -maintaining synchronization with the master node using second synchronization elements of the synchronization signal.

The network may comprise at least a receiving node, for example connected to a display for displaying video streams received from sending nodes. The network may comprise several sending nodes transmitting respective video streams to the at least one receiving node. They may transmit video streams to other receiving nodes. The network may also comprise a master node for synchronizing video stream transmission by the sending nodes.

The master node may be a sending node, a receiving node or a device dedicated to synchronization of video transmission by the sending nodes.

The network may be a wireless network. The network may be a distributed network.

According to embodiments, the synchronization elements are broadcasted by a master node thereby enabling each sending node of the network to synchronize transmission of images to the receipt of the synchronization elements.

The first synchronization elements have a frequency above the highest image frequency of the video streams transmitted by the sending nodes.

Hence, the sending nodes can synchronize transmission of video data to the master node and any sending node that can receive the synchronization signal may be selected for video transmission to the receiving node in a gapless fashion.

The first synchronization elements are provided so the sending nodes can make image transmission coincide with receipt of the first synchronization elements.

A receiving node may transparently receive the new image from a new source following the last image from the previous source. Therefore, there is no image interruption for the receiving node Any sending node that can receive the synchronization signal may be added to the network and a receiving node can receive image or video data from it.

The synchronization signal gives to receiving nodes a unique image frequency reference to be synchronized on whatever the video source is.

By synchronizing the receiving node to the single synchronization signal, the receiving node does not have to change the display synchronization when changing the video source. The frequency of the images remains the same whatever the source is.

Compared to a central video switch, image transition may be performed without extra synchronization cables running from the switch to the source and display devices.

Embodiments of the invention may provide shorter synchronization intervals both for transmission window and video synchronization. Also, thanks to the synchronization signal, image starts may be indicated.

Shorter synchronization intervals enable to keep a good synchronization of the display to the source. Better synchronization makes it possible to have less buffering means and to use cheaper clock generation means.

Shorter transmission intervals make it possible to keep medium access control tightly synchronized so that overhead is reduced in the guard intervals. The guard interval are periods where no one is allowed to access the shared medium because of uncertainty of the synchronization, this prevents collisions.

The synchronization signal comprises second synchronization elements, such as frequency synchronization elements that enable the sending node to keep a precise synchronization with the master node without waiting for the first synchronization elements.

According to embodiments, the method further comprises the following steps: -determining whether image data of a current image of the video stream is ready for transmission when a first synchronization element is received, and -if said image data of said current image is ready for transmission, transmitting said image data upon receipt of a next first synchronization element, or -if said image data of said current image is not ready for transmission, transmitting image data of a previous image of the video stream upon receipt of said next first synchronization element.

The image data of said current image may be buffered when the current image data is not ready for transmission in order to transmit the buffered data upon receipt of another first synchronization element when ready.

Hence, if one image cannot be transmitted before next first synchronization element receipt, then the previous image is duplicated.

This makes the sending node synchronized to the reference synchronization signal.

According to embodiments, the sending node sends a complete image between receipts of two first synchronization elements.

This makes the sending node synchronized to the reference synchronization signal.

In other words, image data of a given image of the video stream is transmitted between receipts of two successive first synchronization elements.

For example, the sending node may receive a start signal indicating a first synchronization element upon receipt of which it should start transmitting an image.

Thus, the sending node is able to send its first image on time.

The first synchronization elements give to sending nodes a common reference for starting transmission of images. Therefore, a new source can start sending the new video at the exact same time where the previous source node stops emitting the last image.

According to embodiments, the sending nodes receive data stream start and stop commands from the master node.

Thus, the new image from the new video stream can arrive right at the end of the last image of the previous video stream.

For example, the method further comprises the following steps: -receiving a start signal for starting transmitting image data of the video stream, said start signal identifying a synchronization element upon receipt of which image data of a first image of the video stream should be transmitted, -determining whether said image data of said first image can be ready for transmission upon receipt of said synchronization element identified, and -outputting a response to said start signal, according to a result of the determination step, said response comprising an indication as whether image data transmission can be started upon receipt of the synchronization element identified.

The start signal may be received from the master node and the response may be transmitted to the master node.

The sending nodes may thus delay a start stream command thereby avoiding an interruption of the video stream at the receiving node.

For example, the stop signal for stopping transmitting image data of the video stream to a receiving node identities a synchronization element after receipt of which image data transmission should be stopped.

The stop signal may be received from the master node.

According to embodiments, the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second elements.

For example, the network follows a TDMA (Time Division Multiple Access) protocol.

For example, the synchronization elements are transmitted as part of beacon frames (for example TDMA beacon frames).

The beacon frames may comprise a field indicating a frequency of receipt of synchronization elements.

Thus, the sending nodes may more accurately synchronize transmission of image data.

For example, the beacon frames comprise a field indicating a type of synchronization element they comprise (such as image or frequency synchronization types).

A first type may correspond to the first elements and be image synchronization. A second type may correspond to the second elements and be frequency synchronization.

The beacon frames may be part of superframes comprising other network configuration parameters.

For example, the method further comprises transmitting to the master node a message indicating an image frequency of the video stream.

Thus, the master node may adapt the frequency of the synchronization signal. For example, the master node may adapt the frequency of the elements of the synchronization signal, or the first synchronization elements in case a new node with a slower image frequency appears.

According to embodiments, the method further comprises measuring a duration of a current image of the video stream to transmit and transmitting the duration measured as part of the message indicating the image frequency of the video stream.

Hence, the master node may be informed of the current image frequency of the sending node and may adapt the frequency of the synchronization signal.

For example the duration measured is compared to an image duration of a previous image of the video stream and the message indicating the image frequency of the video stream is transmitted only in case the durations of the current and the previous images are different.

Hence, information is transmitted to the master node only when it is relevant, thereby saving bandwidth of the network, For example, image frequency of the video stream is locked in to the first synchronization elements frequency.

This makes the source node synchronization based on frame duplication only.

According to a second aspect of the invention there is provided a method of controlling synchronization between sending nodes of a communication network, for transmission by said sending nodes of image data of respective video streams, the method comprising the following steps: -determining the lowest image frequency of the video streams, and -transmitting by a master node of the communication system, a synchronization signal comprising first synchronization elements enabling the sending nodes to synchronize image data transmission, said first synchronization elements having a frequency above the highest image frequency of the video streams, the synchronization signal further comprising second synchronization elements enabling the sending nodes to maintain synchronization with the master node.

The method according to the second aspect corresponds to the H operation of the master node discussed above with respect to the first aspect.

The method may further comprise transmitting a start signal to a sending node, said start signal identifying a synchronization element upon receipt of which the sending node should start transmitting image data of the video stream.

According to embodiments, the method further comprises the following steps: -receiving a response to said start signal, said response comprising an indication as whether image data transmission can be started upon receipt of the synchronization element identified, -if image data transmission cannot be started, determining a new synchronization element upon receipt of which the sending node should start transmitting image data, and -transmitting a message identifying said new synchronization element identified.

According to embodiments, the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second elements.

The control of the synchronization may be performed according to a TDMA (Time Division Multiple Access) scheme.

For example, the synchronization elements are transmitted as part of respective beacon frames.

The beacon frames may comprise a field indicating a frequency of transmission of synchronization elements.

The beacon frames may comprise a field indicating a type of synchronization element they comprise.

A first type may correspond to the first elements and be image synchronization. A second type may correspond to the second elements and be frequency synchronization.

The method may further comprise receiving from a sending node a message indicating an image frequency of the video stream it transmits.

The method may further comprise transmitting a stop signal to a sending node for stopping transmitting a video stream.

According to embodiments, the method further comprises the following steps: -determining an image duration of a video stream transmitted by a sending node of the network, -comparing said image duration received to the smallest image duration among the image durations previously received, and -modifying a frequency of the synchronization signal, if said image duration received is below said smallest image duration.

Hence, the master node can keep a frequency of the synchronization signal that is relevant to the actual image rates of the sending nodes of the network.

Modifying the synchronization signal frequency may make the master node modify the frequency of the first synchronization elements.

For defining a period of the synchronization signal, the method may further comprise the following steps: -performing a first integer division of an image duration IMG_D of a video stream transmitted by a sending node of the communication network by a duration SF_DURATION of a superframe comprising a synchronization element, -performing a second integer division of the remainder Ri of the first integer division by the quotient Qi of the first integer division, and -defining a new period for the synchronization signal as SF_DURATION + Q2 +1, 02 being the quotient of the second integer division.

For example, the method further comprises the following steps: -comparing the new period defined to the current period of the synchronization signal, and -setting the period of the synchronization signal to the new period defined, if the new period is different from the current period.

According to a third aspect of the invention there is provided a method of controlling video stream transmissions in a communication network comprising the following steps: -transmitting, by a master node of the communication network to sending nodes of the communication network, a synchronization signal comprising first synchronization elements having a frequency above the highest image frequency of the video streams transmitted by the sending nodes of the communication network, -transmitting, by a sending node to a receiving node of the communication network, image data of a video stream according to said first synchronization elements, -receiving, by a receiving node of the communication network, said image data, and -maintaining, by the sending node, synchronization with the master node using second synchronization elements of the synchronization signal.

The method may further comprise the following steps: -transmitting, by the master node to the sending node, a start signal for starting transmitting image data to the receiving node, said start signal identifying a synchronization element at which image data of a first image of the video stream should be transmitted, -starting transmitting, by the sending node, to the receiving node, said image data upon receipt of the synchronization element identified.

The method may also comprise the following steps: -transmitting, by the master node to the sending node, a stop signal for stopping transmitting image data to the receiving node, said stop signal identifying a first synchronization element after receipt of which image data transmission should be stopped, -stopping, by the sending node, transmitting data to the receiving node, when the stop signal is received, upon receipt of the first synchronization element identified.

For example, the image data is transmitted according to the first aspect of the invention.

The synchronization between the sending nodes of the communication network may be controlled according to the second aspect of the invention.

For example the master node is a sending node or a receiving node.

Alternatively, or in combination, the master node may be elected among the nodes of the network using an election protocol.

For example, the master node is elected using a Best Master Selection Algorithm.

According to embodiments, the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second synchronization elements.

The image data may be transmitted according to a TDMA (Time Division Multiple Access) scheme.

According to embodiments, the method further comprises the following steps: -comparing an image frequency of the video stream to the first synchronization elements frequency, and -decreasing the image frequency in case the image frequency is greater than the first synchronization elements frequency, or -increasing the image frequency in case the image frequency is lower than the first synchronization elements frequency.

Thus, image frequency is synchronized with the master node.

According to a fourth, fifth and sixth aspects of the invention there are provided computer programs and computer program products comprising instructions for implementing methods according to the first, second and/or third aspect(s) of the invention, when loaded and executed on computer means of a programmable apparatus such as a sending node, a master node and/or a receiving node.

According to an embodiment, information storage means readable by a computer or a microprocessor store instructions of a computer program, that it makes it possible to implement a method according the first, second and/or third aspect(s) of the invention.

According to a seventh aspect of the invention there is provided a sending node device for transmitting image data of a video stream over a communication network comprising: -a communication module configured to receive from a master node of the communication network, a synchronization signal comprising synchronization elements, -a communication module configured to transmit image data of the video stream to a receiving node of the network, and -a processing module configured to control transmission of the image data according to first synchronization elements of the synchronization signal, said first synchronization elements having a frequency above the highest image frequency of the video streams transmitted by the sending nodes of the communication network, and to maintain synchronization with the master node using second synchronization elements of the synchronization signal.

The processing module may be further configured to determine whether image data of a current image of the video stream is ready for transmission when a current first synchronization element is received, and if said image data of said current image is ready for transmission, to transmit said image data upon receipt of a next first synchronization element, or if said image data of said current image is not ready for transmission, to transmit image data of a previous image of the video stream upon receipt of said next first synchronization element.

The device may further comprise a buffering module configured to buffer said image data of said current image when said current image data is not ready for transmission, for transmission of the buffered data upon receipt of another first synchronization element.

The processing unit may be further configured to transmit the image data of a given image of the video stream between receipts of two successive first synchronization elements.

The device may further comprise: -a communication module configured to receive a start signal for starting transmitting image data of the video stream, said start signal identifying a synchronization element upon receipt of which image data of a first image of the video stream should be transmitted, -a communication module configured to output a response to said start signal, said response comprising an indication as whether image data transmission can be started upon receipt of the synchronization element identified, wherein the control unit is further configured to determine whether said image data of said first image can be ready for transmission upon receipt of said synchronization element identified, and wherein the response outputted by the communication module depends on the result of the determination.

The start signal may be received from the master node and the response is transmitted to said master node.

The device may further comprise a communication module configured to receive a stop signal and wherein the processing module is further configured to stop transmission of the image data of the video stream to a receiving node, said stop signal identifying a,first synchronization element after receipt of which image data transmission should be stopped.

The stop signal may be received from the master node.

The first synchronization elements and the second synchronization elements may be regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second synchronization elements.

The transmission of the image data of the video stream may be performed according to a TDMA (Time Division Multiple Access) scheme.

The synchronization elements may be transmitted as part of beacon frames.

The beacon frames may comprise a field indicating a frequency of receipt of synchronization elements.

The beacon frames may comprise a field indicating a type of synchronization element they comprise.

For example, a first type, corresponding to the first elements, is image synchronization.

For example, a second type, corresponding to the second elements, is frequency synchronization.

The device may further comprise a communication module configured to transmit to the master node a message indicating an image frequency of the video stream.

The processing module may be further configured to measure a duration of a current image of the video stream to transmit and wherein the duration measured is transmitted as part of the message indicating the image frequency of the video stream.

The processing unit may be further configured to compare the duration measured to an image duration of a previous image of the video stream and wherein the message indicating the image frequency of the video stream is transmitted only in case the durations of the current and the previous images are different.

The processing unit may be further configured to compare an image frequency of the video stream to the first synchronization elements frequency, and to decrease the image frequency in case the image frequency is greater than the first synchronization elements frequency, or to increase the image frequency in case the image frequency is lower than the first synchronization elements frequency.

The processing module may be further configured to lock in an image frequency of the video stream to the first synchronization elements frequency.

According to an eighth aspect of the invention there is provided a master node device for controlling synchronization between sending nodes of a communication network, for transmission by said sending nodes of image data of respective video streams, the device comprising: -a processing unit configured to determine the lowest image frequency of the video streams, and -a communication unit configured to transmit a synchronization signal comprising first synchronization elements enabling the sending nodes to synchronize image data transmission, said first synchronization elements having a frequency above the highest image frequency of the video streams, the synchronization signal further comprising second synchronization elements enabling the sending nodes to maintain synchronization with the master node.

The device may further comprise a communication module configured to transmit a start signal to a sending node, said start signal identifying a first synchronization element upon receipt of which the sending node should start transmitting image data of its video stream.

The device may further comprise: -a communication module configured to receive a response to said start signal, said response comprising an indication as whether image data transmission can be started upon receipt the synchronization element identified, and -a communication module configured to transmit a message identifying a new synchronization element, wherein the processing module is further configured to determine said new synchronization element upon receipt of which the sending node should start transmitting image data, if image data transmission cannot be started.

The first synchronization elements and the second synchronization elements may be regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second synchronization elements.

The control of the synchronization may be performed according to a TDMA (Time Division Multiple Access) scheme.

The synchronization elements may be transmitted as part of respective beacon frames.

The beacon frames may comprise a field indicating a frequency of transmission of synchronization elements.

The synchronization elements may be transmitted as part of beacon frames.

The beacon frames may comprise a field indicating a type of synchronization element they comprise.

For example, a first type, corresponding to the first elements, is image synchronization.

A second type, corresponding to the second elements, may be frequency synchronization.

The device may further comprise a communication module configured to receive from a sending node a message indicating an image frequency of the video stream it transmits.

The device may further comprise a communication module configured to transmit a stop signal to a sending node for stopping transmitting a video stream.

The processing module may be further configured to determine an image duration of a video stream transmitted by a sending node of the network, compare said image duration received to the smallest image duration among the image durations previously received, and to modify a frequency of the synchronization signal, if said image duration received is below said smallest image duration.

The processing module may be further configured to perform a first integer division of an image duration IMG_D of a video stream transmitted by a sending node of the communication network by a duration SF_DURATION of a superframe comprising a synchronization element, to perform a second integer division of the remainder Ri of the first integer division by the quotient 01 of the first integer division, and to define a new period for the synchronization signal as SF_DURATION + Q2 +1, Q2 being the quotient of the second integer division.

For example, the processing unit is further configured to compare the new period defined to the current period of the synchronization signal, and to set the period of the synchronization signal to the new period defined, if the new period is different from the current period.

According to a ninth aspect of the invention there is provided a system for controlling video stream transmissions in a communication network comprising: -a master node device configured to transmit to sending node devices of the communication network, a synchronization signal comprising first synchronization elements having a frequency above the highest image frequency of the video streams transmitted by the sending node devices of the communication network, -at least one sending node device configured to transmit to at least one receiving node device of the communication network, image data of a video stream according to said first synchronization elements, said sending node device being further configured to maintain synchronization with the master node device using second synchronization elements of the synchronization signal, and -a receiving node device configured to receive said image data.

For example, the master node device is further configured to transmit to the sending node device, a stop signal for stopping transmitting image data to the receiving node device, and wherein the sending node device is further configured to stop transmitting data to the receiving node device, The master node device may be further configured to transmit to the sending node device, a start signal for starting transmitting image data to the receiving node device, said start signal identifying a synchronization element at which image data of a first image of the video stream should be transmitted, and wherein the sending node device is further configured to start transmitting to the receiving node device, said image data upon receipt of the synchronization element identified.

The sending node device may be a device according to the seventh aspect.

The master node device may be a device according to the eighth aspect.

The master node device may be a sending node device or a receiving node device.

The master node device may be elected among the node devices of the network using an election protocol.

For example, the master node device is elected using a Best Master Selection Algorithm.

For example, the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second synchronization elements.

The image data may be transmitted according to a TDMA (Time Division Multiple Access) scheme.

The objects according to the second, third, fourth, fifth, sixth, seventh, eight, and ninth aspects of the invention provide at least the same advantages as those provided by the method according the first aspect of the invention.

The devices and system according to the seventh, eight, and ninth aspects may comprise means for implementing features presented for the methods according to the first, second and third aspects. Features presented in relation to any one of the aspects of the invention may be combined with other features presented in relation to another aspect.

Other features and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments, with reference to the appended drawings, in which, in addition to Figure 1 and 2: -Figure 3 is a schematic illustration of a context of implementation of embodiments; -Figure 4 is a schematic illustration of a communication node that may be used as a sending node and/or as a receiving node; Figure 5 is a time diagram illustrating synchronization according to embodiments; -Figure 6 illustrates a superframe according to embodiments; -Figures 7a and lb are flowcharts of steps for determining the period of the synchronization signal; -Figure 8 is a flowchart of steps performed for transmitting synchronization elements; -Figure 9 is a schematic illustration of a video packet transmitter module; -Figures ba and lOb are flowcharts of steps performed by pixel receipt and frame transmission modules of the packet transmitter module; -Figure 11 is a schematic illustration of the display unit of the communication node of Figure 4; -Figures 12 and 13 are flowcharts of steps performed by elements of the display unit, and -Figure 14 is an illustration of an example of video switching protocol according to embodiments.

In what follows there is A context of implementation of embodiments of the invention is illustrated in Figure 3.

Video players 101, 106, 109 broadcast respective video streams to be displayed one atatime on at least one display 104, 114. Three video players and two displays are represented in Figure 3. However, the invention is not limited to these numbers.

Sending nodes 102, 108 and 111 are respectively associated with video players 101, 106 and 109. For example, the video players are connected to the sending nodes through standard wired video links 117, 107 and 110 such as HDMI links.

Receiving nodes 103, 112 are respectively associated with the displays 104 and 114. For example, the receiving nodes are connected to the displays through standard wired video links 105, 113 such as HDMI links.

The sending nodes and the receiving nodes gain access to wireless channel 115 in order for the sending nodes to transmit data to the receiving nodes.

The nodes (for example receiving node 112) may be configured to receive user indications through a user interface 116. The receiver so configured may act as a master node in the overall system of Figure 3.

Figure 4 is a schematic illustration of a device according to embodiments. The device may be a communication node able to be used as a sending node 102, 108, 111 or as a receiving node 103, 112 as described with reference to Figure 3.

The device comprises a network controller module 207 in charge of implementing the wireless communication of the video data and control data.

The device also comprises a video controller module 204 which comprises a display module 205 and a source module 206.

Display module 206 is in charge of playing back the video received from network controller module 207 to a video display device. The source module is in charge of capturing a video stream from a video player device and to send it to the receiving node through network controller module 207.

Modules 204 and 207 are controlled by a CPU subsystem comprising a processing module (comprising a processor) 201, a memory module (comprising a RAM 202 and a ROM 203) and a user interface module 218.

Source module 206 comprises an HDMI receiver module 215 configured to receive an HDMI flow from a video source through an HDMI connector. HDMI receiver 215 outputs data over a pixel data bus along with three video synchronization signals: the pixel clock signal, the horizontal synchronization signal and the vertical synchronization signal. HDMI receiver module 215 may be, for example Analog Devices' AD9880 receiver.

Source module 206 also comprises a video packet transmitter module 216 configured to obtain video data from HDMI receiver 215. Video packet transmitter module 216 builds video frames and sends the video frames to network controller module 207. The video Packet transmitter module is further described with reference to figure 9, A Vsync sampler module 217 is also part of the source module. It is configured to sample the occurrences of the vertical synchronization signal received from HDMI receiver module 215. Such information is required for the training process as described hereinafter with reference to figure 7.

Display module 205 of the video controller module 204 comprises a video packet receiver module 214. The video packet receiver module receives video frames from network controller module 207, extracts pixel data for internal storage, decodes 136 (Image Start Beacon) frames and sends notifications to a receptor (RX) synchronization manager module 213 also part of the display module.

RX synchronization manager module 213 generates the video synchronization signals for an HDMI transmitter module 212 also part of the display module. Module 213 also reads the pixels stored in the video packet receiver module and sends them to the HDMI transmitter module.

Display module 205 is described hereinafter with reference to Figures 11, 12 and 13. Module 212 receives input data from a pixel bus and the three video synchronization signals (the pixel clock signal, the horizontal synchronization signal and the vertical synchronization signal). Module 212 outputs data for driving the HDMI connector. For instance, the HDMI transmitter module may be Analog Device's AD9889B circuit.

For example, HDMI transmitter and receiver modules 215 and 212 are controlled and initialized by CPU module 201 using an 12C bus (not represented). The CPU module implements an HDMI software driver as it can be obtained, for example, from the HDMI chip manufacturer.

The network controller module may be a standard 60 0hz communication module implementing either IEEE 802.15.3 or WirelessHD standards. It comprises a MAC module 208 a PHY module 209, a transmission (TX) antenna module 210 and a receiving (RX) antenna module 211. Details concerning such modules may be found in the IEEE 802.15.3 standard specification (amendment c) or obtained from the WirelessHD consortium.

Usual sending nodes have a video controller module 204 that includes a source module only and no display module.

Figure 5 illustrates synchronization according to embodiments Four time-axis respectively associated with four nodes are represented. Node 301 is a master node, nodes 302 and 303 are sending nodes and node 304 is a receiving node.

The master node's time diagram represents its network access as implemented by the network controller module 207. Each arrow represents a synchronization element of a synchronization signal broadcasted on the network. In what follows, the synchronization elements are part of beacon frames which may be image start beacon frames (comprising image synchronization elements, or "first synchronization elements") and standard beacon frames (comprising frequency synchronization elements, or "second synchronization elements").

The sending nodes' time diagrams represent their respective network access as implemented by the network controller module 207 and also (at the bottom) the video acquisition and display as implemented by the Video controller part 204.

The master node 301 may be any node in the network. For example, a user selects it as the master node using a user interlace. It can also be configured to be the master node and designated as such in its ROM memory.

The master node may also be designated using an election protocol such as the Best Master Selection Algorithm as defined in the IEEE 1588.2002 standard

specification.

The master node controls the access to the wireless network as defined by the PNC_DEV device specifications in the IEEE 802.15.3 TM standard specification (part 15.3) and according to the superframe as detailed in what follows with reference to Figure 6.

Using a training algorithm (as described with reference to Figure 7 in what follows) the master node 301 may slightly change the TDMA frequency to be strictly above, i.e. not equal and not below, a multiple of the highest image frequency of the video streams from the source nodes. Preferably, the TDMA frequency is changed to be close to said multiple of the highest image frequency. For example, it is changed to be as nxFmax + c, wherein Fmax is the highest image frequency, E is a small value below Fmax and n is an integer value.

The TOMA frequency is represented in Figure 5 by the T_TDMA period 305.

All the nodes in the network synchronize their access to the radio channel according to this TDMA period using beacon frames sent by the master node. The beacon frames may be as defined in the PNC DEV IEEE 802.15.3

TM standard specification (part 15.3).

As already indicated above, the master node may send two different types of beacon frames. One type is the image start beacon frame (ISB) and one type is the standard beacon frame (SBF). Each time a new image starts, the master node sends an ISB.

Also, every T_TDMA period, the master node sends a Standard Beacon Frame (SBF). When a new image starts, the master node sends another SB frame (as described hereinafter with reference to Figure 8). The network image duration reference 306 may be deduced from the time between two successive ISB frames. The nodes in the system may synchronize their respective image frequency to this reference.

The source node sends each image of the video stream in synchronization with the ISB frame sent by the master node. All sending nodes shall wait until receipt of an ISB frame before sending data corresponding to a new image 307, 308. This is described hereinafter with reference to figures 9 and 10.

Each sending node sends the data corresponding to a complete image between two successive receipts of SB frames (as illustrated in Figures 9 and 10).

Also, each sending node sends the data corresponding to the previous image if it cannot send the data corresponding to the current image 309 (as illustrated in Figures 9 and 10).

Each display node synchronizes its video output synchronization signal with the receipt of the ISB frames 310, 311 (as illustrated in Figures 11, 12 and 13).

The master node allocates and de-allocates the image transmission time slots based on the network reference image boundary 312, 313 (as illustrated in Figure 12).

The receiving node switches to the video to be displayed based on the network reference image boundary 314 (as illustrated in Figure 12).

Figure 6 is an illustration of a TDMA superirame according to embodiments.

The superframe is a communication framework that is repeated each new TDMA cycle. The superframe is controlled by the master node 301.

Each new superframe comprises a beacon frame 401, a control time slot 402 and one or more video time slots 403, 404. The beacon frame 401 contains information about the configuration of the superframe as defined in the

IEEE 802.1 5.3 TM standard specification.

The configuration relates to the timing information and the channel access information. The control slot 402 (also defined as "CAP', acronym for Contention Access Period), is used by any node that wants to send information or requests to the master node. The duration of the control slot is defined in the beacon frame. The video slots are allocated to sending nodes for video transfer.

The video slots allocations are requested to the master node through the control slot 402.

The beacon frame is also defined in the IEEE 802.15.3 TM standard specification. For the sake of conciseness, only the fields used in the context of the present description are illustrated. Details concerning the other fields 405 can be found in the IEEE 502.15.3 TM standard specification. The beacon contains a piconet synchronization parameters field 406 and a MAC header

field 407.

In the Piconet synchronization parameters field, a superirame duration field 409 is used by the master node for indicating the duration of the superframe which is equivalent to the period of the TOMA (T TDMA 305).

According to the IEEE 802.15.3 TM standard specification, the superframe duration field is 16 bit wide and ranges from Oto 65535 ps.

In the MAC header field, a Frame type subfield 411 is used by the master node for indicating the frame's type. According to the IEEE 802.15.3 TM standard specification, the value 000" corresponds to a beacon frame.

According to embodiments, other values may be defined, For example, the value "101" may be reserved for the Image Start Beacon (ISB) and the value "110" for the Standard Beacon Frame (SBF).

Figures 7a and 7b are flowcharts for implementing algorithms to be executed by the sending nodes and the master node during a training procedure according to embodiments.

Figure Ta relates to the algorithm at the sending nodes. The algorithm may be carried out by the Vsync sampler module 217.

During a first step 501, the Vsync sampler 217 waits for a new image to start as indicated by a rising front of the Vsync signal as obtained from the video interface HDMI receiver 215. Upon a rising front of the Vsync signal, Vsync sampler 217 goes to step 502.

During step 502, Vsync sampler 217 starts a counter (for example in micro seconds) and goes to step 503.

During step 503 the Vsync sampler waits for the current image to end and for the next image to start as indicated by the next rising edge of the Vsync signal as received from the video interface HDMI receiver. At the end of step 503 the counter indicates the duration of the image (for example in micro seconds).

During step 504, the Vsync sampler checks whether the current measurement of the image duration is different from the previous measurements.

In case the current measurement is not different from the previous ones, the information is not sent to the master node since it is not considered as needed. Next, the Vsync sampler returns to step 502 in order to reset and start the counter again.

In case the newly measured image duration is different from the previous measurements, the Vsync sampler 217 informs the master node.

During step 506 the Vsync sampler 217 sends an image duration frame to the master node with the value from the counter. The frame is sent by the Network Controller 207 during the control slot 402 of the superframe.

Next, the Vsync sampler 217 returns to step 502 in order to reset and start the counter again.

Figure 7b relates to the algorithm at the master node. The algorithm may be carried out by the CPU.

During an initial step, the CPU waits for the receipt of an image duration frame from the Network Controller 207. Upon receipt of the frame, the CPU carries out step 521.

During step 521, the CPU extracts the image duration information contained within the image duration frame.

Next, it compares the image duration received with the smallest image duration information previously received.

In case the newly received image duration is greater or equal to the smallest image duration information previously received, the TDMA period (T_TDMA) of the superframe is kept as it is (it is considered that there is no need to change it). Next, the CPU returns to step 520 and waits for the next image duration frame.

In case the newly received image duration is smaller than the smallest image duration previously received, the master node computes a new TDMA period.

During step 522, the CPU computes the new TDMA period according to the new image duration information. Two parameters may be used for the computation: the superframe duration SF DURATION (for example in micro seconds), the image duration IMG_D (for example in micro seconds), as obtained during step 521.

The superframe duration is a system parameter read from the ROM memory at startup and modified during runtime (step 522). This parameter reflects the needs of the system for latency and synchronization precision. For example, a value of 1 millisecond for the superirame duration (SF DURATION) is often used in low latency and high precision systems.

The calculation formula may be as follows: NB_SF_PER_IMG = (IMGD div SF_DURATION) ("div" is for integer division) Remainder = IMGD -(SF DURATION * NBSF_PER_IMG) SF DURATION = SF_DURATION + (Remainder div NB_SF3ER_IMG) ÷1.

The value of SF_DURATION ensures that (SF DURATION * NB_SF_PER_IMG) is always slightly greater than For example, if IMG_D = 16666 ps and SF_DURATION = 1000 ps, then NB_SF_PER_IMG = 16, Remainder = 666 ps and SF_DURATION = 1042 PS.

During step 523, CPU 201 checks whether the newly computed SF_DURATION is different from the actual TDMA period used. In case it is not different, the CPU does not change the TDMA frequency. It returns to step 520 and waits for a new image duration frame receipt. Alternatively, the CPU performs step 524.

During step 524, the CPU changes the TDMA period to the newly calculated SF_DURATION. The master node changes the field superframe duration in the Piconet synchronization parameter field of each beacon frame (ISB and SBF).

Figure 8 is a flowchart of steps carried out for performing an algorithm for sending beacon frames. For example, the steps are performed by the MAC module of the master node.

During an initial step 601, the MAC module reads the SF_DURATION parameter obtained from the ROM memory or set by the CPU during step 524.

Next, during step 602, the MAC module reads the parameter NB_SF_PER_IMG set by the CPU during step 524. In case the CPU has not yet executed step 524, the parameter is null.

During step 603, the MAC module reads the time slot assignment for video transmission as set by the CPU.

Next, during test 604, the MAC module checks whether the NB_SF_PER_IMG parameter is not null.

In case the NB_SF_PER_IMG parameter is null, it is considered that there is no video source ready for streaming data over the wireless network.

Therefore, no ISB (Image Start Beacon) frame is sent. The MAC module then performs step 605.

In case the NB_SF_PER_IMG parameter is not null, it is considered that there is at least one video source ready for streaming data over the wireless network. Hence, the MAC module sends one ISB (Image Start Beacon) frame for each new image start, The MAC module then performs step 607.

During step 605, the MAC module sends an SBF (Standard beacon frame) with the definition of the video time slots 403, 404 assignments obtained during step 603. The SBF is identified by the value "110" in the frame type field

of the MAC header field (see Figure 6).

Next, during step 606, the MAC module waits for the duration SF_DURATION obtained during step 601 to elapse. The MAC module 208 then returns to step 601.

During step 607, the MAC module sends an ISB (Image start beacon) frame with the definition of the video time slots 403, 404 assignments obtained during step 603. The ISB is identified by the value "101" in the frame

type field of the MAC header field.

Next, during step 608, the MAC module waits for the duration SF_DURATION obtained during step 601 to elapse.

During test 609, the MAC module checks whether the end of the image has been reached. The end of the image is reached when the number of beacon frames sent from the last ISB sent is equal to the NB_SF_PER_IMG parameter obtained in during step 602.

If the end of the image has been reached, the MAC module returns to step 601 If the end of the image has not been reached yet, the MAC module goes to step 610.

During step 610, the MAC module sends an SBF (Standard beacon frame) with the definition of the video time slots 403, 404 assignments obtained during step 603. Next, the MAC module goes back to step 608. The SBF is identified by the value "110" in the frame type field 411 of the MAC header field 407.

Figure 9 schematically illustrates the architecture of the video packet transmitter module 216.

The video packet transmitter module receives pixels 701 from the HDMI receiver module 215. The video packet transmitter module receives ISB (Image Start Beacon) frame receipt indications 711 and SBF (Standard beacon frame) receipt indications 712 from the MAC module.

The video packet transmitter module receives start (713) and stop (714) indications from the CPU for starting or stopping video streaming.

The video packet transmitter module sends MAC frames 708 to the MAC module.

An RX_pixel module 702 is in charge of storing the received pixels in a dual ported memory 704 using a write port 703.

A Tx_frame module 707 is in charge of reading a dual port memory 704 (using read port 706) in order to build frames 708 to be sent to MAC module 208.

Dual port memory 704 is divided into two memory portions. A first portion 709 dedicated to image A and a second portion 710 dedicated to image B. Rx_pixel module 702 manages a pointer module 705 that points to the last image received in the dual port memory. Tx_frame module 707 builds its frames by referring to the dual port memory 704 using the last pointer.

In case one full image has not been stored in the dual port memory 704 between two successive ISB frames, the pointer is not updated and the same image is sent two times, Figure lOa is a flowchart of steps performed by the Rx_pixel module of packet transmitter module 216.

During an initial step 801, Rx_pixel module 702 sets the pointer module 705 to null.

Next, during step 802, Rx_pixel module 702 waits for the receipt of the Vsync signal from the HDMI receiver module. The signal triggers the start of a new image.

In case the pointer was not previously set to null, the Rx_pixel module sets the pointer to image B during step 803.

Next, during step 804, the Rx_pixel module stores the incoming image pixels into memory portion 709 (image A).

Rx_pixel module 702 then waits during step 805 for a new image to start (as indicated by the Vsync signal from the HDMI receiver module).

Rxpixel module 702 changes the value of the pointer image A during step 806.

During step 807, Rx_pixel module 702 stores the incoming image pixels into the memory portion 710 (image B). Then, the process goes back to step 802.

Figure lOb is a flowchart of steps performed by the Tx_frame module of packet transmitter module 216.

During an initial step 830, Tx_frame module 707 waits for a start signal from the CPU.

Next, during step 831, Tx_frame module 707 waits for the indication 711 of an lSB frame from the MAC module.

Tx_frame module 707 then gets the last pointer value during step 832.

During test 833, Tx_frame module 707 checks whether the last pointer value is null. In case it is null, the process goes back to step 831 (it is considered that no image is ready for transmission). In case the last pointer value is not null, step 834 is executed.

During step 834, Tx_frame module 707 reads pixel data from the dual port memory at the memory address pointed by the pointer obtained during step 832.

The pixel data are then packed into a frame, for example as provided in the IEEE 802.15.3 TM standard specification (part 15.3), and sent to the MAC module for transmission over the wireless medium.

Tx_frame module 707 then checks during step 835 whether last processed frame was the last frame of the current image. If it was the last frame of the current image, the process goes back to step 838. Alternatively, the process goes back to step 836.

During step 836, Tx_frame module 707 waits for an indication 712 of an SBF from the MAC module.

During step 837, Tx_frame module 707 reads pixel data from the dual port memory at the memory address pointed to by the pointer obtained during step 832. The pixel data are packed into a frame, for example as provided in the IEEE 802.15.3 TM standard specification (part 15.3), and sent to the MAC module for transmission over the wireless medium. The process then goes back to test 835.

Step 838 is a test during which Tx_frame module 707 checks whether the CPU has sent a stop signal 714 to stop the video streaming over the wireless network.

If the stop signal has been sent, the process goes back to the initial step 830. Alternatively1 the process goes back to step 831 in order to send a new image.

Figure 11 is a detailed view of display module 205 of video controller 204 (see Figure 4).

Display module 205 comprises an HDMI transmitter module 212, a video packet receiver module 214 and an RX synchronization manager module 213.

Video packet receiver module 214 comprises an "RX from network" module 900 in charge of packet receipt management (as described hereinafter with reference to Figure 12). It extracts pixels from video packets and stores them into a video FIFO buffer 901. It receives ISB (Image Start Beacon) frame notifications from MAC module 208 and sends these notifications to a drift computer module 904 (part of RX synchronization manager 213). A Playback manager module 907 (part of RX synchronization manager 213) reads data from the video FIFO buffer, and its output is connected to the pixel bus of the HDMI transmitter module.

RX synchronization manager module 213 comprises the drift computer module. It is in charge of computing the drift between local Vsync signal 908 and the reference Vsync signal as reflected by the ISB (Image start beacon) notification frequency. The algorithm carried out by this module is described with reference to Figure 13.

The local Vsync signal is obtained as a sum of clock rising edges. A local clock signal 909, a counter 902, a register 903 and a comparator 905 are used.

The nominal value of the Vsync signal for a given image frequency is computed as a number of local clock rising edges. For example, an image frequency of 60 Hz can be obtained from a local clock running at 25 Mhz by counting 416666 times (rising edges) with a precision of +1-40 nanoseconds.

Thus, when counter 902 reaches the value of the nominal value register, comparator 905 asserts the local Vsync signal.

As represented in Figure 13, drift computer 904 increases or decreases the nominal value register 903 depending on the drift computed in order to make the local Vsync signal frequency as close as possible to the ISB notification frequency.

Local Vsync signal 908 is fed to a PLL module 906 that is in charge of generating a pixel clock signal Pix_clk from the local Vsync signal. PLL module 906 divides the local Vsync signal by the constant NB_pixel_img (representing the number of pixels in the images) that is set by the CPU at startup or when the system changes the image resolution to be displayed.

Another PLL module 910 multiplies the Pix_clk signal by the constant NB_pixel_line (representing the number of lines in the images) to generate the horizontal synchronization signal.

Playback manager 907 receives the resulting Pix_clk, local Vsync and Hsync signals. Based on these video synchronization signals, the playback manager 907 reads Video FIFO buffer 901 in order to generate the pixels along with the video synchronization signals and sends them to HDMI transmitter module 212. For example, the playback manager follows the video format timing as defined in document CEA-861-E of the Consumer Electronics Association. The video format information is given by the CPU module.

Figure 12 is a flowchart iltustrating an algorithm performed by the "Rx from network module".

During an initial step 1000, it is waited for the receipt of an ISB (Image Start Beacon) frame notification from network controller 207. Once an ISB frame notification is received, the process goes to step 1003.

During step 1003, an ISB frame notification is sent to the drift computer module 904. Next, during step 1007, it is waited for the next video frame receipt from network controller 207.

Upon video frame receipt, step 1004 is performed. During step 1004, the video line from the video frame payload is stored into video FIFO buffer 901.

Next, during step 1005, it is tested whether the end of an image has been reached. The end of the image is reached when all the lines of the image have been stored.

The number of lines per image depends on the video format. The video format information is sent by HDMI receiver 215 to the sending node's CPU. Next, the CPU sends the information to the receiving node's CPU. For example, the information is contained in the control data of network controller 207.

The receiver's CPU sends back the information relating to the number of lines per video frame to "RX from network" module 900. Thus, if the end of the image has not been reached, the process goes to step 1006 in order to wait for the next video frame. In case the end of the image has been reached, the process goes back to initial step 1000.

During step 1006 it is waited for the next video frame receipt from network controller 207. Upon video frame receipt, the process goes back to step 1004.

Figure 13 is a flowchart illustrating an algorithm executed by the Drift computer module.

During an initial step 1100, the nominal Vsync register is loaded with a value representing the targeted image frequency and a value representing the local clock frequency. These two values are set by the CPU. For example, for a local clock frequency of 25 MHz and for a target image frequency of 60 0Hz, the nominal Vsync register is set to ((1/60/ 1/25)*106) = 416666, Next, during step 1101, it is waited for an ISB (Image start Beacon) frame notification from the "Rx from network" module.

Upon receipt of the notification, counter 902 is started during step 1102. Thus, counter 902 is synchronized with the reference start of image as reflected by the ISB (Image start beacon).

Next, during step 1104, it is waited for the occurrence of both the next ISB notification from RX network module 900 and for the occurrence of the local Vsync signal.

It is then tested during step 1105 whether the ISB frequency is slower than the local Vsync signal. For example, it may be checked which one of the two signals has the earliest rising edge. If the local Vsync signal rises first, then the ISB frame frequency is the slowest and the process goes to step 1106 to slowdown the local Vsync signal. Alternatively, the process goes to step 1107.

During step 1106, the local Vsync is slowed down by one local clock increment (40 nanoseconds) by activating once the increment command of the nominal Vsync register.

Next, the process goes back to step 1104 in order to wait for the next ISB frame and next Vsync signal sample.

During step 1107, it is tested whether the ISB frame frequency is faster than the local Vsync signal frequency. For example, it is checked which of the two signals rises first. In case the ISB signal rises first, the ISB frames are faster than the Vsync signal arid the process goes to step 1108 in order to speed up the local Vsync signal. Alternatively, the process goes to step 1104.

During step 1108, the local Vsync signal is accelerated by one local clock increment (40 nanoseconds) by activating once the decrement command of the nominal Vsync register.

Next, the process goes back to step 1104.

Figure 14 schematically illustrates a video switching protocol according to embodiments.

In the figure, there are represented message exchanges between a master node 1201, a first sending node 1202, a second sending node 1203 and a receiving node 1204. The messages implement a gapless video source switching protocol.

The first sending node 1202 streams video 1205 that is received by receiving node 1204. Video is streamed at every new superframe in the corresponding video time slot (for example timeslot 403 described with reference to Figure 6). For the sake of conciseness, only one stream is represented.

A switch 1206 can be requested by a user from the first sending node to the second sending node on the display device attached to the receiving node 1204. The request can be made through the user interface of the master node.

Upon receipt of the request, the master node waits for the occurrence of the next ISB frame to send a "start stream" control packet to the second sending node. The "start stream" control packet includes the identification of the target image communication time slot in the superframe.

Upon receipt of a start stream control packet, the CPU of the second sending node checks whether it will be ready to start streaming video at the next ISB notification. To do so, the CPU reads the last pointer value. If it is null then the node is not ready to stream the video and the CPU sends a "too fast" control packet 1208 to the master node. lithe value of the last pointer is not null then the CPU sends an acknowledgement control packet 1211 to the master node and sends a start command to its Tx_frame module.

If the master node receives a "too fast" control packet from the sending node it waits for next ISB 1209 and sends again a start stream control packet 1210 to the sending node.

If the master node 1201 receives an acknowledgement control packet 1211 from the sending node, then it sends a stop stream control packet 1212 to the sending node and the master node changes the allocation of the video time slot used by the first sending node. At the next lSB frame, the time slot will be allocated to the second sending node (see step 603 described with reference to Figure 8).

Upon receipt of a "stop stream" control packet 1212, the CPU of the sending node sends a stop command to its Tx_frame module.

As a result, in the superirame following the next ISB frame, the first sending node will stop streaming video in the video time slot and the second sending node will start streaming video in the video time slot.

The receiving node has nothing to change, the video change is seamless.

A computer program according to embodiments may be designed based on the flowcharts of Figures 7a, 7b, 8, lOa, lob, 12, 13, 14 and the

present description.

Such computer program may be stored in a ROM memory of a device as described with reference to Figure 4. It may then be loaded into and executed by a processor of such device for implementing steps of a method according to the invention.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention being not restricted to the disclosed embodiment. Other variations to the disclosed embodiment can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims (95)

1. CLAIMS1. A method of transmitting image data of a video stream from a sending node of a communication network comprising the following steps: -receiving from a master node of the communication network, a synchronization signal comprising synchronization elements, -transmitting image data of the video stream to a receiving node of the network according to first synchronization elements of the synchronization signal, said first synchronization elements having a frequency above the highest image frequency of the video streams transmitted by the sending nodes of the communication network, and -maintaining synchronization with the master node using second synchronization elements of the synchronization signal.
2. 2. A method according to claim 1, further comprising the following steps: -determining whether image data of a current image of the video stream is ready for transmission when a current first synchronization element is received1 and -if said image data of said current image is ready for transmission, transmitting said image data, or -if said image data of said current image is not ready for transmission, transmitting image data of a previous image of the video stream.
3. 3. A method according to claim 2, further comprising buffering said image data of said current image, when said current image data is not ready for transmission, in order to transmit the buffered data upon receipt of another first synchronization element.
4. 4. A method according to any one of the preceding claims, wherein the image data of a given image of the video stream is transmitted between receipts of two successive first synchronization elements.
5. 5. A method according to any one of the preceding claims, further comprising the following steps: -receiving a start signal for starting transmitting image data of the video stream, said start signal identifying a synchronization element upon receipt of which image data of a first image of the video stream should be transmitted, -determining whether said image data of said first image can be ready for transmission upon receipt of said synchronization element identified, and -outputting a response to said start signal, according to a result of the determination step, said response comprising an indication as whether image data transmission can be started upon receipt of the synchronization element identified.
6. 6. A method according to claim 5, wherein the start signal is received from the master node and the response is transmitted to said master node.
7. 7. A method according to any one of the preceding claims, further comprising receiving a stop signal for stopping transmitting image data of the video stream to a receiving node, said stop signal identifying a first synchronization element after receipt of which image data transmission should be stopped.
8. 8. A method according to claim 7, wherein the stop signal is received from the master node.
9. 9. A method according to any one of the preceding claims, wherein the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second elements.
10. 10. A method according to any one of the preceding claims, wherein the transmission of the image data of the video stream is performed according to a TDMA (Time Division Multiple Access) scheme.
11. 11. A method according to any one of the preceding claims, wherein the synchronization elements are transmitted as part of beacon frames.
12. 12. A method according to claim 11, wherein the beacon frames comprise a field indicating a frequency of receipt of synchronization elements.
13. 13. A method according to any one of claims 11 and 12, wherein the beacon frames comprise a field indicating a type of synchronization element they comprise.
14. 14. A method according to claim 13, wherein a first type, corresponding to the first elements, is image synchronization.
15. 15. A method according to any one of claims 13 and 14, wherein a second type, corresponding to the second elements, is frequency synchronization.
16. 16. A method according to any one of the preceding claims, further comprising transmitting to the master node a message indicating an image frequency of the video stream.
17. 17. A method according to claim 18, further comprising measuring a duration of a current image of the video stream to transmit and transmitting the duration measured as part of the message indicating the image frequency of the video stream.
18. 18. A method according to claim 17, further comprising comparing the duration measured to an image duration of a previous image of the video stream and wherein the message indicating the image frequency of the video stream is transmitted only in case the durations of the current and the previous images are different.
19. 19. A method according to any one of the preceding claims, further comprising locking in an image frequency of the video stream to the first synchronization elements frequency.
20. 20. A method of controlling synchronization between sending nodes of a communication network, for transmission by said sending nodes of image data of respective video streams, the method comprising the following steps: -determining the lowest image frequency of the video streams, and -transmitting by a master node of the communication system, a synchronization signal comprising first synchronization elements enabling the sending nodes to synchronize image data transmission, said first synchronization elements having a frequency above the highest image frequency of the video streams, the synchronization signal further comprising second synchronization elements enabling the sending nodes to maintain synchronization with the master node.
21. 21. A method according to claim 20, further comprising transmitting a start signal to a sending node, said start signal identifying a first synchronization element upon receipt of which the sending node should start transmitting image data of its video stream.
22. 22. A method according to claim 21, further comprising the following steps: "3 -receiving a response to said start signal, said response comprising an indication as whether image data transmission can be started upon receipt of the synchronization element identified, -if image data transmission cannot be started, determining a new synchronization element upon receipt of which the sending node should start transmitting image data, and -transmitting a message identifying said new synchronization element identified.
23. 23. A method according to any one of the claims 20 to 22, wherein the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second synchronization elements.
24. 24. A method according to any one of claims 20 to 23, wherein the control of the synchronization is performed according to a TDMA (Time Division Multiple Access) scheme.
25. 25. A method according to any one of claims 20 to 24, wherein the synchronization elements are transmitted as part of respective beacon frames.
26. 26. A method according to claim 25, wherein the beacon frames comprise a field indicating a frequency of transmission of synchronization elements.
27. 27. A method according to any one of claims 25 to 26, wherein the beacon frames comprise a field indicating a type of synchronization element they comprise.
28. 28. A method according to claim 27, wherein a first type, corresponding to the first elements, is image synchronization.
29. 29. A method according to any one of claims 27 and 28, wherein a second type, corresponding to the second elements, is frequency synchronization.
30. 30. A method according to any one of claims 20 to 29, further comprising receiving from a sending node a message indicating an image frequency of the video stream it transmits.
31. 31. A method according to any one of claims 20 to 30, further comprising transmitting a stop signal to a sending node for stopping transmitting a video stream.
32. 32. A method according to any one of claims 20 to 31, further comprising the following steps: -determining an image duration of a video stream transmitted by a sending node of the network, -comparing said image duration received to the smallest image duration among the image durations previously received, and -modifying a frequency of the synchronization signal, if said image duration received is below said smallest image duration.
33. 33. A method according to any one of claims 20 to 32, further comprising the following steps: -performing a first integer division of an image duration IMG_D of a video stream transmitted by a sending node of the communication network by a duration SF_DURATION of a superframe comprising a synchronization element, -performing a second integer division of the remainder Ri of the first integer division by the quotient 01 of the first integer division, and -defining a new period for the synchronization signal as SF_DURATION + 02 ÷1, Q2 being the quotient of the second integer division.
34. 34. A method according to claim 33, further comprising the following steps: comparing the new period defined to the current period of the synchronization signal, and -setting the period of the synchronization signal to the new period defined, if the new period is different from the current period.
35. 35. A method of controlling video stream transmissions in a communication network comprising the following steps: -transmitting, by a master node of the communication network to sending nodes of the communication network, a synchronization signal comprising first synchronization elements having a frequency above the highest image frequency of the video streams transmitted by the sending nodes of the communication network, -transmitting, by a sending node to a receiving node of the communication network, image data of a video stream according to said first synchronization elements, -receiving, by a receiving node of the communication network, said image data, and -maintaining, by the sending node, synchronization with the master node using second synchronization elements of the synchronization signal.
36. 36. A method according to claim 35, further comprising the following steps: -transmitting, by the master node to the sending node, a start signal for starting transmitting image data to the receiving node, said start signal identifying a synchronization element at which image data of a first image of the video stream should be transmitted, -starting transmitting, by the sending node, to the receiving node, said image data upon receipt of the synchronization element identified.
37. 37. A method according to any one of claims 35 and 36, further comprising the following steps: -transmitting, by the master node to the sending node, a stop signal for stopping transmitting image data to the receiving node, said stop signal identifying a first synchronization element after receipt of which image data transmission should be stopped, -stopping, by the sending node, transmitting data to the receiving node, when the stop signal is received, upon receipt of the first synchronization element identified.
38. 38. A method according to any one of claims 35 to 37, wherein image data is transmitted according to any one of claims ito 19.
39. 39. A method according to any one of claims 35 to 38, wherein synchronization between sending nodes of the communication network is controlled according to any one of claims 20 to 34.
40. 40. A method according to any one of claims 35 to 39, wherein the master node is a sending node or a receiving node.
41. 41. A method according to claim 40, wherein the master node is elected among the nodes of the network using an election protocol.
42. 42. A method according to claim 41, wherein the master node is elected using a Best Master Selection Algorithm.
43. 43. A method according to any one of claims 35 to 42, wherein the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second synchronization elements.
44. 44. A method according to any one of claim 35 to 43, wherein image data is transmitted according to a TDMA (Time Division Multiple Access) scheme.
45. 45. A method according to any one of claims 35 to 44, further comprising the following steps, performed by the receiving node: -comparing an image frequency of the video stream to the first synchronization elements frequency, and -decreasing the image frequency in case the image frequency is greater than the first synchronization elements frequency, or -increasing the image frequency in case the image frequency is lower than the first synchronization elements frequency.
46. 46. A computer program product comprising instructions for implementing a method according to any one of claims 1 to 45 when the program is loaded and executed by a programmable apparatus.
47. 47. An information storage means readable by a computer or a microprocessor storing instructions of a computer program, characterized in that it makes it possible to implement a method according to any one of claims 1 to 45.
48. 48. A sending node device for transmitting image data of a video stream over a communication network comprising: -a communication module configured to receive from a master node of the communication network, a synchronization signal comprising synchronization elements, -a communication module configured to transmit image data of the video stream to a receiving node of the network, and -a processing module configured to control transmission of the image data according to first synchronization elements of the synchronization signal, said first synchronization elements having a frequency above the highest image frequency of the video streams transmitted by the sending nodes of the communication network, and to maintain synchronization with the master node using second synchronization elements of the synchronization signal.
49. 49. A device according to claim 48 wherein the processing module is further configured to determine whether image data of a current image of the video stream is ready for transmission when a current first synchronization element is received, and if said image data of said current image is ready for transmission, to transmit said image data upon receipt of a next first synchronization element, or if said image data of said current image is not ready for transmission, to transmit image data of a previous image of the video stream upon receipt of said next first synchronization element.
50. 50. A device according to claim 49, further comprising a buffering module configured to buffer said image data of said current image when said current image data is not ready for transmission, for transmission of the buffered data upon receipt of another first synchronization element.
51. 51. A device according to any one of claims 48 to 50, wherein the processing unit is further configured to transmit the image data of a given image of the video stream between receipts of two successive first synchronization elements.
52. 52. A device according to any one of claims 48 to 51, further comprising: -a communication module configured to receive a start signal for starting transmitting image data of the video stream, said start signal identifying a synchronization element upon receipt of which image data of a first image of the video stream should be transmitted, -a communication module configured to output a response to said start signal, said response comprising an indication as whether image data transmission can be started upon receipt of the synchronization element identified, wherein the control unit is further configured to determine whether said image data of said first image can be ready for transmission upon receipt of said synchronization element identified, and wherein the response outputted by the communication module depends on the result of the determination.
53. 53. A device according to claim 52, wherein the start signal is received from the master node and the response is transmitted to said master node.
54. 54. A device according to any one of claims 48 to 53, further comprising a communication module configured to receive a stop signal and wherein the processing module is further configured to stop transmission of the image data of the video stream to a receiving node, said stop signal identifying a,first synthronization element after receipt of which image data transmission should be stopped.
55. 55. A device according to claim 54, wherein the stop signal is received from the master node.
56. 56. A device according to any one of claims 48 to 55, wherein the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second synchronization elements.
57. 57. A device according to any one of claims 48 to 56, wherein the transmission of the image data of the video stream is performed according to a TDMA (Time Division Multiple Access) scheme.
58. 58. A device according to any one of claims 48 to 57, wherein the synchronization elements are transmitted as part of beacon frames.
59. 59. A device according to claim 58, wherein the beacon frames comprise a field indicating a frequency of receipt of synchronization elements.
60. 60. A device according to any one of claims 58 and 59, wherein the beacon frames comprise a field indicating a type of synchronization element they comprise.
61. 61. A device according to claim 60, wherein a first type, corresponding to the first elements, is image synchronization.
62. 62. A device according to any one of claims 60 and 61, wherein a second type, corresponding to the second elements, is frequency synchronization.
63. 63. A device according to any one of claims 48 to 62, further comprising a communication module configured to transmit to the master node a message indicating an image frequency of the video stream.
64. 64. A device according to claim 63, wherein the processing module is further configured to measure a duration of a current image of the video stream to transmit and wherein the duration measured is transmitted as part of the message indicating the image frequency of the video stream.
65. 65. A device according to claim 64, wherein the processing unit is further configured to compare the duration measured to an image duration of a previous image of the video stream and wherein the message indicating the image frequency of the video stream is transmitted only in case the durations of the current and the previous images are different.
66. 66. A device according to any one of claims 48 to 65, wherein the processing unit is further configured to compare an image frequency of the video stream to the first synchronization elements frequency, and to decrease the image frequency in case the image frequency is greater than the first synchronization elements frequency, or to increase the image frequency in case the image frequency is lower than the first synchronization elements frequency.
67. 67. A device according to any one of claims 48 to 66, wherein the processing module is further configured to lock in an image frequency of the video stream to the first synchronization elements frequency.
68. 68. A master node device for controlling synchronization between sending nodes of a communication network, for transmission by said sending nodes of image data of respective video streams, the device comprising: -a processing unit configured to determine the lowest image frequency of the video streams, and -a communication unit configured to transmit a synchronization signal comprising first synchronization elements enabling the sending nodes to synchronize image data transmission, said first synchronization elements having a frequency above the highest image frequency of the video streams, the synchronization signal further comprising second synchronization elements enabling the sending nodes to maintain synchronization with the master node.
69. 69. A device according to claim 68, further comprising a communication module configured to transmit a start signal to a sending node, said start signal identifying a first synchronization element upon receipt of which the sending node should start transmitting image data of its video stream.
70. 70. A device according to claim 69 further comprising: -a communication module configured to receive a response to said start signal, said response comprising an indication as whether image data transmission can be started upon receipt the synchronization element identified, and -a communication module configured to transmit a message identifying a new synchronization element, wherein the processing module is further configured to determine said new synchronization element upon receipt of which the sending node should start transmitting image data, if image data transmission cannot be started.
71. 71. A device according to any one of claims 68 to 70, wherein the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second synchronization elements.
72. 72. A device according to any one of claims 68 to 71, wherein the control of the synchronization is performed according to a TDMA (Time Division Multiple Access) scheme.
73. 73. A device according to any one of claims 68 to 72, wherein the synchronization elements are transmitted as part of respective beacon frames.
74. 74. A device according to claim 73, wherein the beacon frames comprise a field indicating a frequency of transmission of synchronization elements.
75. 75. A device according to any one of claims 68 to 74, wherein the synchronization elements are transmitted as part of beacon frames.
76. 76. A device according to any one of claims 74 to 75, wherein the beacon frames comprise a field indicating a type of synchronization element they comprise.
77. 77, A device according to claim 76, wherein a first type, corresponding to the first elements, is image synchronization.
78. 78. A device according to any one of claims 76 and 77, wherein a second type, corresponding to the second elements, is frequency synchronization.
79. 79. A device according to any one of claims 68 to 78, further comprising a communication module configured to receive from a sending node a message indicating an image frequency of the video stream it transmits.
80. 80. A device according to any one of claims 68 to 79, further comprising a communication module configured to transmit a stop signal to a sending node for stopping transmitting a video stream.
81. 81. A device according to any one of claims 68 to 80, wherein the processing module is further configured to determine an image duration of a video stream transmitted by a sending node of the network, compare said image duration received to the smallest image duration among the image durations previously received, and to modify a frequency of the synchronization signal, if said image duration received is below said smallest image duration.
82. 82. A device according to any one of claims 68 to 81, wherein the processing module is further configured to perform a first integer division of an image duration IMG_D of a video stream transmitted by a sending node of the communication network by a duration SF_DURATION of a superframe comprising a synchronization element, to perform a second integer division of the remainder Ri of the first integer division by the quotient 01 of the first integer division, and to define a new period for the synchronization signal as SF_DURATION + 02 +1, Q2 being the quotient of the second integer division.
83. 83, A device according to claim 82, wherein the processing unit is further configured to compare the new period defined to the current period of the synchronization signal, and to set the period of the synchronization signal to the new period defined, if the new period is different from the current period.
84. 84. A system for controlling video stream transmissions in a communication network comprising: -a master node device configured to transmit to sending node devices of the communication network, a synchronization signal comprising first synchronization elements having a frequency above the highest image frequency of the video streams transmitted by the sending node devices of the communication network, -at least one sending node device configured to transmit to at least one receiving node device of the communication network, image data of a video stream according to said first synchronization elements, said sending node device being further configured to maintain synchronization with the master node device using second synchronization elements of the synchronization signal, and -a receiving node device configured to receive said image data.
85. 85. A system according to claim 84, wherein the master node device is further configured to transmit to the sending node device, a stop signal for stopping transmitting image data to the receiving node device, and wherein the sending node device is further configured to stop transmitting data to the receiving node device,
86. 86. A system according to any one of claims 84 and 85, wherein the master node device is further configured to transmit to the sending node device, a start signal for starting transmitting image data to the receiving node device, said start signal identifying a synchronization element at which image data of a first image of the video stream should be transmitted, and wherein the sending node device is further configured to start transmitting to the receiving node device, said image data upon receipt of the synchronization element identified.
87. 87. A system according to any one of claims 84 to 86, wherein the sending node device is a device according to claims 48 to 67,
88. 88. A system according to any one of claims 84 to 87, wherein the master node device is a device according to claims 68 to 83.
89. 89. A system according to any one of claims 84 to 88, wherein the master node device is a sending node device or a receiving node device.
90. 90. A system according to claim 89, wherein the master node device is elected among the node devices of the network using an election protocol.
91. 91. A system according to claim 90, wherein the master node device is elected using a Best Master Selection Algorithm.
92. 92. A method according to any one of claims 84 to 81, wherein the first synchronization elements and the second synchronization elements are regularly emitted by the master node according to respective frequencies, the first synchronization elements being a subset of the second synchronization elements, the first frequency of the first elements thereby being lower than the second frequency of the second synchronization elements.
93. 93. A system according to any one of claim 84 to 92, wherein image data is transmitted according to a TOMA (Time Division Multiple Access) scheme.
94. 94. A device substantially as hereinbefore described with reference to, and as shown in, Figures 4, 9 and 11 of the accompanying drawings.
95. 95. A method substantially as hereinbefore described with reference tot and as shown in, Figures 7a, 7b, 8, ba, lOb, 12, 13 and 14 of the accompanying drawings.