Interconnection mechanism for multiple data streams

Video data streams are routed between capture nodes and display nodes connected to one another through a high-bandwidth network switch and controlled by a stream manager connected to the switch. The stream manager negotiates a highest quality stream that can be processed by both the capture node and the one or more display nodes receiving the stream and that does not exceed available bandwidth through the switch. The stream manager includes a user interface by which a user can specify which of a number of video signals is to be displayed in what position within a display wall.

FIELD OF THE INVENTION

This invention relates to the field of audiovisual display systems, and more specifically to efficient movement of multiple audio and/or video data streams from multiple sources to multiple destinations.

BACKGROUND

Ever increasing demand for more content displayed in a single display has led to an increase in production and sales of display walls. Display walls generally include multiple display monitors positioned next to one another in a tiled fashion to present a single large image. Using relatively large flat-panel display monitors and/or rear-projection monitors and tiling them in grid sizes of two-by-two, three-by-three, and three-by-four, among others, an integrated displayed image can occupy an entire wall of a room. Display walls are frequently used in large system command and control centers such as war rooms, utility management centers, and traffic monitoring centers.

Frequently, the content displayed in a display wall includes motion video from multiple sources such as highway-mounted traffic monitoring cameras. For flexibility in viewing these multiple video sources, it is desirable to see each one in a “window,” which can be made any size (regardless of the native resolution of the source) and can be positioned at any location within the display wall regardless of the boundaries of the individual constituent display devices. Windows are known and conventional elements of a graphical display and are not described further herein. The transfer of high data-rate digital video streams from multiple sources to the multiple displays of a tiled display wall represents a particularly difficult problem. Since the tiled display monitors represent an integrated display, there is generally a central controller for the various display monitors. The multiple video streams are typically routed through the central controller. The central controller typically includes a number of video capture devices which receive and encode video streams into digital data streams and a number of video controller devices to drive the individual pixels of the respective tiled display monitors of the display wall. Such central controllers typically experience debilitating data bottlenecks at the bus or buses through which the respective video data streams travel within the central controller.

The buses represent a bottleneck because the aggregate bandwidth required to handle all incoming signals and distribute those signals at high update rates to any combination of the display devices far exceeds the capacity of a single bus. A bus-oriented architecture may suffice in a small system with five to ten input and output devices, but bus-oriented architectures cannot be effectively scaled up to handle larger systems. In other words, doubling the number of video sources and destinations requires much more than a doubling of the number of buses to handle the requisite data transfer bandwidth increase resulting from such a doubling. Multiple buses can be used, each bus connecting a subset of input devices to a subset of output devices, but this leads to a loss in flexibility as to where each incoming video signal can be displayed. Specifically, a video stream from a particular input device would be displayable only on a selected few of the output display devices. Thus, the end user could not have free reign in determining where a particular video stream is to be displayed in a display wall.

What is needed is a more efficient interconnection mechanism between the capture devices and the display devices.

SUMMARY OF THE INVENTION

In accordance with the present invention, video data streams are routed between capture nodes and display nodes connected to one another through a high-bandwidth network switch and controlled by a stream manager connected to the switch. The stream manager selects an interchange format in which the video signal can be transported through the switch and communicates this interchange format to the involved nodes. The interchange format is a format that is both producible by the involved capture nodes and displayable by the involved display nodes and that is optimized to provide a desired display quality without unnecessarily consuming, or exceeding, available bandwidth. Briefly stated, the preferred interchange format is the format that delivers the most fidelity that the source signal (as represented by the native interchange format) offers or the display device can effectively use (as represented by the displayable interchange format) without exceeding bandwidth limitations.

The result is that many, high-quality video streams can be routed through the switch at or near the full point-to-point bandwidth provided by the switch. In other words, the switch can handle multiple streams simultaneously whereas a bus is a shared resource that can process only a single stream at a time.

The stream manager controls the various streams between the capture nodes and the display nodes and includes a user interface by which a user can specify which of a number of video signals are to be displayed in what position within a display wall. To manage transportation of the various video streams through the switch, the stream manager identifies the involved capture and display nodes, determines an interchange format of the video stream to be delivered between the capture and display nodes, and instructs the capture node to deliver the video stream in the interchange format to the display nodes. The stream manager instructs the capture and display nodes to send and receive, respectively, the subject video stream in the interchange format. Thereafter, the capture and display nodes cooperate to transport the video stream through the switch.

The capture and display nodes can be relatively simple devices that receive and serve requests from the stream manager and send and receive, respectively, video signals of various forms that can be selected by the stream manager. Accordingly, capture and display nodes can be implemented as relatively small electronic appliances. In addition, the use of a network switch as an interconnect between the capture and display nodes provides high bandwidth at low cost yet provides the flexible routing required in a display wall application.

Some video streams are sent to multiple display nodes because the video window is to span multiple tiled displays of a display wall. To improve the quality of a video signal that can be transported without exceeding available bandwidth, such video streams are divided such that the capture node avoids sending the same portion of a video stream to more than one display node. In other words, the stream manager instructs the capture node to send to each display node only that portion of the video stream that is displayable by that display node.

The display of multiple parts of a video stream is synchronized on a frame-by-frame basis by the capture node by broadcasting a synchronization packet which indicates that a particular frame of the video stream is ready for display. Display nodes delay display of each frame of a received video stream until a synchronization packet for the frame is received. Thus, all display nodes displaying respective parts of one video stream will show their respective parts of each frame of the video stream at the same time.

The stream manager is responsive to a graphical user interface and controls the various video streams through a network switch in accordance with user-generated signals, providing a user's experience that is indistinguishable from a user controlling a single integrated display. The stream manager allows capture and display nodes to cooperate directly with one another to transfer video streams, satisfactorily processing the extraordinary bandwidth required of a large display wall displaying many video streams in a scalable manner.

DETAILED DESCRIPTION

In accordance with the present invention, a stream manager102(FIG. 1) controls audiovisual data streams through a switch104from a number of capture nodes106A-C to a number of display nodes108A-D. In this illustrative embodiment, switch104is a gigabit (Gb) Ethernet switch capable of point-to-point data throughput of about one billion bits per second through each port. Switch104overcomes the disadvantages of traditional bus interconnections in which only one device can write through the bus at any given time. Switches such as switch104can achieve maximum rated throughput for multiple channels simultaneously. As used herein, an audiovisual data stream can include image streams and/or audio streams, and “video” is used to refer to any sequence of images that are temporally related to one another.

Each of capture nodes106A-C has access to an audio and/or video data stream and makes that stream available to display nodes108A-D through switch104. As used herein, a “node” is any device or logic which can communicate through a network. Capture nodes106A-C are analogous to one another and the following description of capture node106A is equally applicable to capture nodes106B-C. Similarly, display nodes108A-D are analogous to one another and the following description of display node108A is equally applicable to display nodes108B-D.

It should also be appreciated that, while three capture nodes106A-C and four display nodes108A-D are described herein, more or fewer capture and display nodes can be used in the audiovisual distribution system ofFIG. 1. Similarly, while the network topology shown inFIG. 1is particularly simple—e.g., involving a single network switch such as switch104, more complex topologies can be used to distribute video signals in accordance with the present invention. In particular, switch104can be replaced with multiple interconnected switches, each of which can have many capture and display nodes attached. These interconnected switches can be in the same room or can be separated by miles or thousands of miles to provide a truly global means for acquisition, distribution, and display of audiovisual signals.

In addition, for capture and display nodes needing greater bandwidth, one or two additional links can be added to double or triple the amount of data that can be handled using link aggregation. For example, to double the bandwidth between switch104(FIG. 1) and display node108A, two (2) 1.0-gigabit connections can couple switch104and display node108A to one another. Similarly, capture node106A can be coupled to switch104using two (2) 1.0-gigabit connections to effectively double the available bandwidth between capture node106A and switch104. Link aggregation—e.g., Cisco Fast EtherChannel, Gigabit EtherChannel, and 802.3AD—is known and is not described further herein.

To facilitate appreciation and understanding of the following description, the various audiovisual signal formats referred to herein are briefly described and summarized in a table below. Capture node106A receives an audiovisual signal in a “native format” from a video source such as a DVD player or video camera or a computer. The native format can be analog or digital. The audiovisual signal can include video and/or audio signals, each of which is processed separately in this illustrative embodiment.

In a particularly simple example, the video source captured by capture node106A is to be displayed, by itself, on display monitor202A. In this example, display node108A produces, and display monitor202A receives and displays, a video signal in a “displayable format.” The displayable format can be analog or digital and can be the same as the native format or different from the native format. The native format of the source and the display format of the display monitor are external constraints and define the task to be performed by capture node106A and display node108A. In particular, a video signal that is received by capture node106A and that is to be displayed by display monitor202A is to be displayed with as little loss of signal fidelity as possible—that is the task of capture node106A and display node108A.

Capture node106A sends, and display node108A receives, the video signal in an “interchange format” through switch104. Interchange formats are digital. Capture node106A and display node108A can each support multiple interchange formats. In a manner described below, stream manager102selects a particular interchange format, referred to as the “selected interchange format” by which capture node106A and display node108A transport the video signal through switch104. Stream manager102selects the selected interchange format according to the capabilities of the involved nodes, e.g., capture node106A and display node108A and the user's wishes as to how the video signal should be displayed.

As described above, capture node106A captures the video signal from a video source. Capture node106A represents the captured video signal in a digital form that is an interchange format that most accurately represents the video signal in the native format. The format of the captured video signal is sometimes referred to as the “native interchange format.”

As described above, display node108A produces the video signal in the displayable format for display by display monitor202A. Display node108A produces the video signal in the displayable format from an interchange format which most accurately represents the displayable format, and this interchange format is sometimes referred to as the “displayable interchange format.”

It should be appreciated that the video distribution system described herein is not limited to this simple example of an entire single display monitor for displaying a single captured video signal. Instead, any video signal, regardless of its native format, can be displayed anywhere on the display wall in a window of any size. This allows multiple signals to be displayed on a single display monitor, one signal to be expanded to fill the entire display wall, or any combination of signals to be display in any configuration of any collection of display walls. Accordingly, the definition of the displayable interchange format changes somewhat in this distributed embodiment.

The displayable interchange format does not necessarily map to an entire display frame of display monitor202A. Instead, the displayable interchange format represents only pixels of that portion of a window used to represent the video signal. For example, if a video signal is to be displayed in a 640×480-pixel window within a 1600×1200 display monitor, the frame size of the displayable interchange format is 640 pixels by 480 pixels, i.e., the size of the display window within the display monitor. A video compositor, which is described more completely below, composites full frames for display by a display monitor to include any windows displaying video signals. Other parameters of the displayable interchange format are the same as in the single video signal in a single display monitor example—e.g., frame rate, color depth, and color model. It should be appreciated that the displayable interchange format is based on the size of the window displaying the video signal, even if the window spans multiple display monitors.

Thus, the overall video signal flow is as follows: Capture node106A captures a video signal in the native format into the native interchange format and sends the video signal through switch104in the selected interchange format, converting the video signal from the native interchange format to the selected interchange format if the two are different from one another. Display node108A receives the video signal and converts it from the selected interchange format to the displayable interchange format if the displayable interchange format is different from the selected interchange format. Display node108A converts the video signal from the displayable interchange format to the displayable format, compositing full frames as required, for playback by display monitor202A.

The various video format terms used herein are summarized in the Table below.

TermIn the context of FIG. 1Native formatReceived by capture nodes 106A-C.Native interchangePreferred digital representation of the nativeformatformat within capture nodes 106A-C.Displayable formatDriven by display nodes 108A-D to send todisplay monitors 202A-D, respectively.DisplayablePreferred digital format for compositing intointerchange formatframes for conversion to displayable format.SelectedThe particular interchange format in which theinterchange formatvideo signal travels from the capture node,through switch 104, and to the display node.Interchange formatA superset of the native interchange format, thedisplayable interchange format, and the selectedinterchange format - and including anycandidates for the selected interchange format.

The capture, conversion, sending, receipt, conversion, and display of the video signal all happen in real time. As used herein, “real time” means that an insubstantial amount of time is required for the video signal to travel from the video source through capture node106A to display monitor202through display node108A from a human user's perspective—e.g., no more than a few seconds. It is generally preferred that this amount of time is minimized, e.g., to no more than a small fraction of a second, but the term “real time” is considered applicable so long as the video signal presented by display monitor202A appears to be reasonably immediately responsive to a human user's control inputs into the video source attached to capture node106A. To transport the video signal in real time, the capture, conversion, sending, receipt, conversion, and display of the audiovisual signal all happen concurrently.

Capture node106A is shown in greater detail inFIG. 3. Capture node106A includes audiovisual capture logic302A that receives an audiovisual signal in the native format from a video source, e.g., an external device such as a computer, a video camera, a prerecorded video player, etc. The audiovisual signal captured by audiovisual capture logic302A can be a video signal or an audio signal or a combination of video and audio signals. Processing of audio signals is described below. For simplicity, only video signals are described first. Audiovisual capture logic302A converts the video signal from the native format to the native interchange format, which is typically a digital representation of the native format in which pixels are organized as frames with a frame size and frame rate specific to the native format. Audiovisual capture logic is generally known and can include signal conditioning elements to capture the signal in its best possible form.

Capture node106A includes a signal format converter304A that receives the captured video signal from audiovisual capture logic302A and converts the video signal from the native interchange format to the selected interchange format. Such conversion can require changes to various parameters of the native interchange format, including frame size (i.e., the number of pixels per line and the number of lines per frame), frame rate, color depth, and aspect ratio, for example. Changes to the frame size by capture logic302A is typically downscaling.

In one embodiment, signal format converter304A performs operations to produce frame sizes and frame rates within a continuous range. Thus, the particular video interchange formats supported by signal format converter304A can be expressed within capabilities314A as including ranges of properties. In an alternative embodiment, signal format converter304A performs only highly efficient operations such as omitting every other pixel and every other scanline to reduce frame sizes by integer ratios such as 2:1, 3:1, etc. In such an alternative embodiment of signal format converter304A, supported video interchange formats are expressed within capabilities314A as including individual, discrete values of supported properties. As described below in greater detail, capabilities314A influence selection of the interchange format such that the selected interchange format is ensured to be one that capture node106A supports.

Capture node106A includes an area of interest processor306A that forms one or more streams of data representing one or more corresponding areas of interest of the video signal in the selected interchange format produced by signal format converter304A. As can be seen in display wall200(FIG. 2), a given video signal can be displayed across multiple display monitors. In addition, portions of a display of a video signal can be obscured by other windows of display wall200and/or can be not visible for other reasons. One such reason is the dragging of a window such that part of the window reaches beyond the outer edges of display wall200. Each area of interest of the video signal is that portion of the video signal which is viewable within a single display monitor, and area of interest processor306A (FIG. 3) forms a separate stream for each area of interest of the video signal received from signal format converter304A. Stream manager102(FIG. 1), in conjunction with user interface110, maintains data representing which portions of which video signals are displayed and visible on each display monitor and uses that data to control the areas of interest processed by area of interest processor306A (FIG. 3).

Area of interest processor306A divides the video signal received from signal format converter304A in the selected interchange format into portions of the video signal destined for different display nodes. In one embodiment, area of interest processor306A receives data from stream manager102indicating which portions of the video signal are to be displayed by each respective display node, and area of interest processor306A determines to what extent each display node needs pixels beyond those of the portion. For example, upscaling a frame may involve pixel and/or scan-line interpolation, requiring access of pixels just outside the particular portion displayed by a particular display node. Area of interest processor102can determine that, to properly up-scale the portion of window204A (FIG. 2), display node108A requires additional pixels that are not within the portion of window204A but that are within a predetermined distance of the portion of window204A. In an alternative embodiment, stream manager102determines which pixels each of the respective display nodes needs to accurately represent pixels displayed thereby. In particular, stream manager102determines which extra pixels, if any, are needed by each of the display nodes in this alternative embodiment and communicates to capture node106A exactly which pixels to send to each of the involved display nodes.

In an alternative embodiment, the entirety of the video signal can be sent to all involved display nodes simultaneous. For example, capture node106A can use multicast addressing to send the same video signal to multiple display nodes. Multicast addressing is conventional and known and is not described further herein.

In this illustrative embodiment, the areas of interest processed by area of interest processor306A also exclude those portions of the video signal captured by capture node106A that are occluded or otherwise not visible, except that the edges of the areas of interest are similarly extended when the receiving display node is expected to perform upscaling or other processing that requires such extra pixels.

Area of interest processor306A forms one video stream for each of the portions resulting from division of the video signal. For example, in processing the video signal of window204A, which spans four (4) displays as shown inFIG. 2, area of interest processor306A produces four (4) separate video streams, each representing a respective area of interest of the video signal, and each of which is destined for a different display node in this example.

It should be observed that, in this illustrative embodiment, signal format converter304A precedes area of interest processor306A in the sequence shown inFIG. 3. Accordingly, a single selected interchange format applies to the entirety of the video signal processed by capture node106A.

A data rate reducer308A of capture node106A receives multiple video streams in the selected interchange format, each representing a portion of the video stream destined for a particular one of the involved display nodes, and data rate reducer308A processes each portion independently. Data rate reducer308A can apply any of a number of data rate reduction techniques to each video stream, such as redundancy avoidance, lossless compression, and lossy compression, for example. Redundancy avoidance can be spatial, as in selective use of run-length encoding, or temporal, as in avoiding sending pixels which are not changed from the preceding frame of the video stream.

Redundancy avoidance, lossless compression, and lossy compression can require access to pixel data of one or more previous frames of a video signal. Accordingly, data rate reducer308A stores one or more previous scans of each area of interest. As used herein, a scan is an area of interest of a single frame, and a stream of area of interest scans collectively represents the area of interest of a video stream. The previous scans stored by data rate reducer308A include, for example, (i) previous scans316A of the area of interest of window204A (FIG. 2) for display node108A; (ii) previous scans316B (FIG. 3) of the area of interest of window204A (FIG. 2) for display node108B; (iii) previous scans316C (FIG. 3) of the area of interest of window204A (FIG. 2) for display node108C; and (iv) previous scans316D (FIG. 3) of the area of interest of window204A (FIG. 2) for display node108D. In an alternative embodiment, area of interest processor306A stores these previous scans for subsequent use by data rate reducer308A. In either case, data rate reducer308A has access to such previous scans for redundancy avoidance and compression, whether lossy or lossless.

Capture node106A includes a packetizer310A that forms packets of data that serialize and collectively represent each of the streams processed by data rate reducer308A. As used herein, a “packet” is any collection of data to be transported together and that includes data specifying an intended destination. The video signals represented in the packets include data marking the end of each scan line and the end of each frame portion to assist in display of the video signal by display nodes108A-D. In addition, packetizer310A cooperates with network controller312A to maximize packet sizes, up to a predetermined maximum, to thereby minimize the overhead of sending many small packets.

Capture node106A includes a network controller312A that interacts with stream manager102(FIG. 1) and any of display nodes108A-D to control the series of packets and to send the series of packets through switch104. Capabilities314A identify the interchange formats that are supported by capture node106A by storing data identifying ranges of values and/or individual discrete values of various properties of the supported interchange formats. Capabilities314A also identify characteristics of the native interchange format and supported flow control methods implemented by network controller312A in this illustrative embodiment. The cooperation between network controller312A, stream manager102, and any involved display nodes to agree upon the selected interchange format and to effect transfer of the audiovisual signal in the selected interchange format is described below.

B. Display Nodes

Display nodes108A-D receive the video streams through switch104and display video signals represented in the various streams on respective ones of display monitors202A-D. As used herein, display monitors include generally any device for displaying images including, for example, LCD displays, CRT displays, plasma displays, and display projectors. Display monitors202A-D are placed next to one another to collectively form a tiled display wall200as shown inFIG. 2. Of course, it should be appreciated that display walls can have other configurations of display monitors, including more or fewer than the four display monitors shown inFIG. 2. In addition, it should be appreciated that, in addition to those display nodes which collectively provide display wall200, other additional display nodes can be coupled to switch104. In particular, the display nodes coupled to switch104, or a multitude of interconnected switches, can represent one or more display walls and/or one or more discrete, individual displays. For simplicity and clarity of description, the illustrative example described herein includes four display nodes108A-D (FIG. 1) which collectively control a single display wall200(FIG. 2).

In display wall200ofFIG. 2, three video signals are displayed, each of which is displayed in a respective window in the collective display of display wall200. Windows204A-C can span multiple display monitors202A-D and can overlap one another as shown. While only three video windows are shown, it should be appreciated that more or fewer windows can be displayed in display wall200. In addition, other items can be displayed in the collective display of display wall200such as graphical user interface components, for example.

The organization of content of display wall200is specified by a user through a user interface110(FIG. 1). In this illustrative embodiment, user interface110implements a window manager with a graphical user interface. Window managers are known components of graphics-based operating systems and are not described herein. Let it suffice to say that the user is provided with conventional user interface techniques to open, close, move, and resize windows such as windows204A-C (FIG. 2). User interface110is responsive to signals generated by a user through physical manipulation of one or more user input devices in a known and conventional manner.

Stream manager102controls the location of windows204A-C in accordance with window content and location information received from user interface110by sending commands through switch104to capture nodes106A-C and display nodes108A-D.

Display node108A is shown in greater detail inFIG. 4. Display node108A includes a network controller402A and display logic412A. Network controller402A cooperates with stream manager102(FIG. 1) and any of capture nodes106A-C to select an interchange format and to effect receipt of each of one or more video data streams through switch104. Network controller402A sends the received video data streams to depacketizer404A.

Depacketizer404A reconstructs individual data streams representing respective portions of video signals from capture nodes106A-C from the packets received through switch104. In particular, depacketizer404A extracts the substantive content (sometimes referred to as data payload) from the packets and sequences the content if any packets are received out of order. In some embodiments, display node108A can be coupled to switch104through more than one network port, e.g., using link aggregation. In such circumstances, packets can be received out of order if the packets travel through multiple ports of switch104. Depacketizer404A re-sequences the content of the packets such that the order is preserved. Packets can each include a packet sequence number to facilitate the re-sequencing by depacketizer404A.

From the substantive content extracted from the packets, depacketizer404A forms multiple data streams, each of which represents an individual video or audio signal. Within each data stream, data marking beginning and/or ending of scan lines and frames is included. It should be appreciated that, from the perspective of a display node, a portion of a video signal that is less than the entirety of the video signal appears to be, and is treated as, an entire complete video signal.

Display node108A includes a signal restorer406A that receives the data streams from depacketizer404A and restores full video signals represented by the respective data streams. In particular, signal restorer406A applies the inverse of any data rate reduction techniques applied by data rate reducer308A (FIG. 3), including restoration of redundancies and reversal of any applied lossless and/or lossy compression techniques.

To restore redundant data that may have been removed by data rate reducer308A from an area of interest stream, signal restorer406A (FIG. 4) maintains one or more previous scans, e.g., previous scan416A, which represent the most recently reconstructed scans of each received area of interest stream for display node108A. Each previous scan represents a previously received scan of the area of interest of display node108A that has been restored by signal restorer406A. Thus, if a scan line is omitted as redundant in view of the corresponding scan line of the previous corresponding area of interest scan, signal restorer406A substitutes the corresponding scan line of previous scan416A for the omitted scan line.

In addition, signal restorer406A applies the reverse of any compression techniques applied by data rate reducer308A (FIG. 3). Such decompression can require reference to pixels of previous scan416A (FIG. 4) and/or reference to near pixels in the same area of interest.

Signal restorer406A produces one or more separate data streams, each of which represents a single area of interest stream or audio signal. Each area of interest stream is complete in that all pixels of each scan line and all scan lines of each scan of each area of interest are represented completely and independently, i.e., without requiring reference to another pixel. In addition, data marking beginning and/or ending of scan lines and scans of the area of interest is included in each area of interest stream.

Display node108A includes a signal format converter408A that converts each of the one or more video data streams received from signal restorer406A from the respective selected interchange format into a form suitable for processing by video compositor410A, i.e., the respective displayable interchange format. Such conversion can require modification of such parameters as frame size, frame rate, color depth, and aspect ratio, for example. It should be appreciated that some parameters corresponding to the captured image stream as a whole, e.g., frame size, have a different meaning in the context of an area of interest. For example, when processing an area of interest that is less than the entirety of a frame, image stream format converter408A does not modify the frame size of an entire captured frame of a video signal but instead adjusts the scale of scans of the area of interest to match a displayed frame size. Signal format converter408A can support ranges of values of various properties of the audiovisual data stream and/or can be limited to specific discrete values of such properties in the manner described above with respect to signal format converter304A. Those supported values are represented in capabilities414A.

To convert the video signals, signal format converter408A stores data representing the selected interchange format and the display interchange format for each video stream received by display node108A (FIG. 4). Network controller402A forwards information regarding the selected interchange format to signal format converter408A at the start of each new stream at the request of stream controller102(FIG. 1). The displayable interchange format for a particular video stream depends on the size of the window in which the stream is to be displayed. The number of physical scan lines and the number of pixels across each scan line within the area of interest of display monitor202A (FIG. 2) defined by window204A correspond to the frame size of the displayable interchange format for the video stream of window204A. Other aspects of the displayable format are defined by the particular type or types of video signals displayable by display monitor202A, such as color space, color depth, interlaced or progressively scanned frames, and frame rate, for example.

Conversion from the selected interchange format to the displayable interchange format by signal format converter408A (FIG. 12) for a particular video signal can require access to spatially or temporally near pixels. For example, de-interlacing can require access to previous fields, e.g., for application of motion compensation. Similarly, upscaling a scan of an area of interest can require interpolation between adjacent pixels. Accordingly, signal format converter408A has access to previous scan416A stored by signal restorer406A.

Signal format converter408A produces area of interest streams each of which is in a respective displayable interchange format. The area of interest streams in the respective displayable interchange formats match, pixel-for-pixel, with a portion of the frame layout of display monitor202A and that portion is defined by stream manager102as representing a corresponding window, e.g., window202A (FIG. 2).

Display node108A includes a video compositor410A that assembles respective portions of respective area of interest streams processed by signal format converter408A into individual frames of a complete video display, e.g., a complete frame for display by display monitor202A (FIG. 1). Such assembly—sometimes referred to as compositing—is described more completely in the co-pending and commonly owned U.S. patent application Ser. No. 10/795,088 filed Mar. 4, 2004 by Eric Wogsberg and entitled “Compositing Multiple Full-Motion Video Streams for Display on a Video Monitor” and published as USPTO Publication Number 2005/0195206, and that publication is incorporated herein in its entirety by reference.

Video compositor410A receives these area of interest streams from signal format converter408A. In addition, video compositor410A stores data representing corresponding locations within display monitor202A (FIG. 2) at which specific areas of interest are to be displayed. Such provides the necessary mapping of pixels of each area of interest stream to a location within display monitor202A. Video compositor410A (FIG. 12) composites scans of the one or more areas of interest into complete frames for display in display monitor202A. The result of such compositing is a complete frame ready for display in display monitor202A by display logic412A (FIG. 4).

Display node108A includes display logic412A that drives the video signal in the displayable video format to display monitor202A for display. Such driving may require conversion of the video signal in the displayable interchange format to the displayable format, which can be an analog format, much like video circuitry in personal computers drives display of analog and/or digital computer display monitors.

Display node108A includes capabilities414A that represent the ranges and/or discrete values of various properties of interchange formats supported by signal format converter408A and used by network controller402A to select an interchange format for transport of video signals through switch104. Capabilities414A also represent the displayable format produced by display node108A. Capabilities414A can be static and established during initial configuration of display node108A or can be discovered, at least in part, from display monitor202A using a conventional plug-and-play device discovery process such as the use of VESA's DDC/EDID (Display Data Channel/Extended Display Identification Data) to obtain operational limits of a display device. Display node108A determines the best supported characteristics—i.e., video format and timing—of display monitor202A that display node108A can drive and selects a displayable format according to those characteristics. In addition, capabilities414A identify any higher-level signal processing capabilities of display node108A such as de-interlacing, for example.

C. Interface Between Stream Manager and Capture and Display Nodes

To implement the distribution of video data streams in the manner described herein, capture nodes106A-D implement a number of operations that can be requested by stream manager102. Collectively, these operations allow stream manager102to gather information regarding capture nodes106A-C, specify characteristics of video streams sent by capture nodes106A-C, specify to which display nodes such streams are sent, and to start and stop sending of such streams. These operations are briefly introduced here and described in more detail below.

Network controller312A (FIG. 3) implements operations (i) to register capture node106A; (ii) to report and accept various parameters of audio and video data processed through signal format converter304A, area of interest processor306A, and data rate reducer308A; (iii) to report and accept a capture node identifier; (iv) to report and accept positioning information and a clock signal; (v) to report and accept information about display nodes to which video data streams are being sent; (vi) to open and close video and/or audio data streams; and (vii) to initiate or terminate a video or audio data stream.

Network controller402A (FIG. 4) of display node108A similarly implements operations (i) to register display node108A, (ii) to report and accept various parameters of audio and video data processed by display node108A, (iii) to report and accept a display node identifier, (iv) to report and accept positioning information and a clock signal, (v) to report and accept information about capture nodes from which video and/or audio data streams are being received, (vi) to open and close video data streams, (vii) to initiate or terminate receipt of a video or audio data stream, and (vii) to suggest to a capture node changes to the selected interchange format and/or changes in applied data rate reduction techniques. Collectively, these operations allow stream manager102to gather information regarding display nodes108A-D, specify characteristics of video and/or audio data streams received by display nodes108A-D, and to start and stop receiving of such streams. In addition, once a stream is started between a capture node and a display node, the display node is able to suggest, and the capture node is able to accept, changes in the selected interchange format and/or applied data rate reduction techniques.

Registration by a capture node or a display node is illustrated as logic flow diagram500(FIG. 5). Logic flow diagram500is described in the context of capture node106A and is equally applicable to registration by capture nodes106B-C and display nodes108A-D.

Initially, i.e., when capture node106A is first operational and in communication with switch104, stream manager102is unaware of the presence of capture node106A and network controller312A has no network address at which to register with stream manager102. In step502, network controller312A broadcasts a request for an address of stream manager102through switch104. Stream manager102receives the broadcast request and responds to such a request by returning its address to the node from which the request was broadcast. In step504, network controller312A receives the address of stream manager102. Thus, after step504, capture node106A and stream manager102are aware of each other and can send messages between one another. In this illustrative embodiment, the address of stream manager102is the MAC (Media Access Controller) address of network access circuitry of stream manager102. Of course, other address schemes can be used, such as Internet Protocol (IP) addresses.

The request broadcast of step502and the response of step504collectively represent one way by which network controller312A can discover the address of stream manager102. Of course, network controller312A can discover the address of stream manager102in other ways, such as manual programming of the address into network controller312A. However, one advantage of the broadcast request is that network controller312A can provide what is commonly referred to as plug-and-play functionality—i.e., requiring no configuration for initial functionality other than coupling to a network to which stream manager102is also coupled.

In step506, network controller312A forms and sends a registration packet to stream manager102through switch104using the address received in step504. Network controller312A forms the registration packet by including data to inform stream manager102of the identity, properties, and capabilities of capture node106A. The identity of capture node106A can be indicated by the MAC or other address of capture node106A, can include an identifier unique among capture nodes106A-C, and/or can include a brief description of capture node106A such as “Interstate 5 traffic camera number 15.” The properties of capture node106A can include geolocation and/or positioning information related to the captured video signal. For example, the geolocation information can identify a location of a camera in latitude and longitude coordinates. Similarly, the positioning information can identify the direction and elevation angle of the camera.

The capabilities information included in the registration packet from capture node106A (FIG. 1) informs stream manager102of the various capabilities of capture node106A, e.g., as represented in capabilities314A (FIG. 3). For each of a number of properties of a video stream, network controller312A sends data representing one or more values for that property supported by capture node106A. For example, the capabilities information can indicate which video frame resolutions, frame rates, color depths, color models, audio encoding parameters, video encoding parameters, compression techniques, etc. are supported by capture node106A. As described above, such capabilities can be expressed as ranges of values and/or discrete values. Since capture nodes can also process audio streams as described above, the capabilities information can indicate similar capabilities with respect to audio data streams. In addition, network controller312A specifies the native interchange format in the registration packet sent in step506.

Registration by network controller402A (FIG. 4) of display node108A is generally analogous to registration by network controller312A (FIG. 3) except that geolocation information is omitted in some embodiments and positioning information indicates a position, e.g., row and column, of associated display monitor202A within display wall200(FIG. 2) and can identify display wall200uniquely within a collection of two or more display walls served by stream manager102. In addition, network controller402A specifies the displayable interchange format rather than a native interchange format.

To report position information of each display node, such information is programmed into each display node at the time of network configuration in this illustrative embodiment. In an alternative embodiment, such position information is programmed into stream manager102, e.g., as a pairing of unique display node identifiers (such as MAC addresses) to corresponding position information. In either case, stream manager102is able to determine which display nodes are to display which parts of a video window spanning multiple displays of a display wall.

In this illustrative embodiment, capture nodes106A-C and display nodes108A-D are configured to execute the registration operation upon power-up and detection of a good connection to switch104. In addition, the broadcast of step502is repeated periodically if no response is received from stream manager102. At power-up, stream manager102is configured to respond to requests for the address of stream manager102and to receive registrations from capture nodes and display nodes. Thus, initialization of the audiovisual stream distribution system ofFIG. 1can be as simple as plugging in connecting cables between—and powering on—capture nodes106A-C, display nodes108A-D, stream manager102, and switch104.

After registration of capture nodes106A-C and display nodes108A-D, stream manager102stores identifiers, descriptions, addresses, and capabilities of capture nodes106A-C and display nodes108A-D in a registration table604(FIG. 6). Stream manager102includes stream management logic602that uses the information of registration table604to manage audiovisual data streams between capture nodes106A-C and display nodes108A-D.

D. Movement of a Video Stream from a Capture Node to One or More Display Nodes

As described above, the entirety of display wall200(FIG. 2) is controlled by user interface110(FIG. 1), which implements a window manager or other graphics-based operating system. User interface110includes a conventional interface by which the user directs that a video signal be displayed in a specified location within display wall200. For example, the user can initiate execution of a process that presents a video stream selection user-interface dialog and displays the selected video stream in a window that the user can later move and/or resize within display wall200. For the purposes of illustration, consider that the user has directed the display of window204A (FIG. 2) in the position shown. In this illustrative example, the video stream representing the video content of window204A is sometimes referred to as the selected video stream.

Initiation of display of the selected video stream by stream manager102is illustrated by logic flow diagram700(FIG. 7). In step702, stream manager102receives the request to start the selected video stream at the location shown inFIG. 2. In step704(FIG. 7), stream manager102determines which capture node and which one or more display nodes are involved in the selected video stream. Generally, one of capture nodes106A-C (FIG. 1) will be involved since generally each video stream is displayed in its own window within display wall200. The involved one of capture nodes106A-C is identified as the one providing the selected video stream. For example, if the user had selected a video stream with the description “Interstate 5 traffic camera number 15,” the identifier and address associated with that description within registration table604(FIG. 6) identifies the involved one of capture nodes106A-C (FIG. 1). In this illustrative example, the involved capture node is capture node106A.

Stream manager102determines the one or more involved display nodes108A-D by determining the entire display position of the specified display window, e.g., window204A (FIG. 2) in this illustrative example. Using simple pixel arithmetic, stream manager102(FIG. 1) determines that all of display nodes108A-D are involved in this illustrative example since each of display nodes108A-D displays a respective portion of window204A as shown inFIG. 2.

1. Selection of the Interchange Format

In step706(FIG. 7), stream manager102selects an interchange format of the selected video stream to represent the video of window204A. The primary concern in selecting an interchange format is the preservation of the quality of the video signal to the greatest extent possible. To the extent any characteristic of the video signal is modified to reduce data rate (e.g., down-scaling the frame size), it is preferred that such conversion is performed by the involved capture node, e.g., capture node106A. Such reduces the amount of data bandwidth required at the capture node, through switch104, and at the display node without any degradation of the video signal beyond that required by the native interchange format and the displayable interchange format. Conversely, to the extent any characteristic of the audiovisual signal is modified to increase data rate (e.g., up-scaling the frame size), it is preferred that such conversion is performed by the involved display node, e.g., display node108A. Such avoids excessive consumption of bandwidth through switch104. However, it should be noted that, unlike most other systems, avoiding excessive consumption of bandwidth is not the primary concern. Bandwidth is generally only a concern (i) if the audiovisual signal in the selected interchange format would exceed available bandwidth or (ii) when selecting whether a capture node or a display node is to perform a particular component of video signal processing.

Thus, as a general rule, any required down-scaling is performed by a capture node and any required up-scaling is performed by a display node. One way to implement this general rule is to limit characteristics of the selected interchange format to the lesser of the characteristics of the native interchange format and the displayable interchange format. By not exceeding characteristics of the native interchange format, any modifications of the audiovisual signal that increase the data rate of the audiovisual signal are performed by the display node after the signal has been transported through switch104, thereby avoiding unnecessary use of data bandwidth through switch104. Use of bandwidth is unnecessary when such use does not serve to maximize fidelity to the video stream in the native interchange format. By not exceeding characteristics of the displayable interchange format, any modifications of the audiovisual signal that reduce the data rate of the audiovisual signal are performed by the capture node, before the signal has been transported through switch104, thereby similarly avoiding unnecessary use of data bandwidth through switch104. In such cases, the saved bandwidth is unnecessary since the excess data represents more signal than the display node can fully utilize.

Under some circumstances, some of which are described below, the interchange format selected in the manner described above is estimated to exceed the available bandwidth of a port of switch104, thereby likely to result in failure to deliver the video signal through switch104. If the selected interchange format is estimated to exceed available bandwidth through switch104to the intended display node, the selected interchange format is modified by application of data rate reduction techniques that are described in greater detail below. In this illustrative embodiment, the available bandwidth of a single port of switch104for data payload is a predetermined proportion (e.g., 90%) of the total available bandwidth of that port. For example, if a data connection through a particular port of switch104, e.g., the port of switch104to which capture node106A is connected, is established at 1 gigabit per second, the available bandwidth of that port to capture node106A is 900 megabits per second.

In addition, as described more completely below, stream manager102can limit available bandwidth to even less of the total bandwidth between capture node106A and switch104—particularly when capture node106A is sending more than a single video data stream through switch104.

Step706is shown in greater detail as logic flow diagram706(FIG. 8). Loop step802and next step808define a loop in which stream manager102processes each of a number of various characteristics specified in capabilities314A and414A according to step804. Such characteristics include frame size, frame rate, color depth, color space, etc. For each such characteristic, processing transfers from loop step802to step804.

In step804, stream manager102determines the value of the subject characteristic for the selected interchange format. As described briefly above, the selected interchange format is the interchange format that delivers the most fidelity that the native interchange format offers or the displayable interchange format can effectively use without exceeding bandwidth limitations. In this illustrative embodiment, stream manager102defers bandwidth considerations until steps812-814, which are described below. Thus, the immediate concern in step804is the particular value of the characteristic that delivers the most fidelity that the native interchange format offers and the displayable interchange format can effectively use.

This determination depends largely on the nature of the characteristic under consideration. Some characteristics are fairly straightforward. For example, frame size represents a number of scanlines and a number of pixels per scanline. The greatest fidelity of the native interchange format is a frame size of exactly the same dimensions. If the displayable interchange format is capable of including each and every pixel of each frame of this size, the dimensions of the native interchange format are used for the selected interchange format. Conversely, if the displayable interchange format cannot display all pixels of frames of that size, the frame size of the selected interchange format is one that does not include pixels that cannot be represented in the displayable interchange format. Specifically, if the frame size of the displayable interchange format is smaller than the frame size of the native interchange format, the selected interchange format uses the frame size of the displayable interchange format. Other straightforward characteristics include such things as frame rates and color depth.

Other characteristics are not so straightforward. For example, the color model can be RGB or YCrCb, among others. If the native interchange format represents colors using the YCrCb model and the displayable interchange format represents colors using the RGB color model, the audiovisual signal undergoes color model conversion. However, it's less clear whether such color model conversion is best performed by capture node106A or display node108A. This issue can be resolved in any of a number of ways. For example, capabilities314A and414A can indicate that only display node108A is capable of such color model conversion. In this case, the selected interchange format represents pixels in the YCrCb color model since capture node106A is not capable of converting the color model to RGB. One feature that tends to require significant processing is de-interlacing. For cost reduction, it is useful to implement de-interlacing in only one of capture node106A and display node108A. Whether the selected interchange format includes interlaced or progressive scan video depends upon the native interchange format, the displayable interchange format, and which of capture node106A and display node108A can perform de-interlacing.

These same principles of preserving the most fidelity of the native interchange format to the extent the displayable interchange format can effectively use that fidelity are applied across each characteristic of the selected interchange format in the loop of steps802-808.

When stream manager102has processed all characteristics of the selected interchange format according to the loop of steps802-808, processing according to the loop of steps802-808completes. At this point, stream manager102has determined a selected interchange format such that each selected characteristic is an optimum selection for preservation of audiovisual signal quality without unnecessary use of bandwidth through switch104to represent data that can't be effectively used by the involved display nodes.

2. Selection of Data-Rate Reduction Techniques

After the loop of steps802-808(FIG. 8), processing according to logic flow diagram706transfers to step810. In step810, stream manager102estimates the data rate associated with the selected interchange format selected according to the loop of steps802-808. Data rate estimation can be as simple as the product of (i) the frame rate (frames per second), (ii) the resolution (pixels per frame), and (iii) the pixel depth (bits per pixel)—plus any data overhead such as time-stamps, frame-start, and scanline-start markers and packet data overhead. The result is an estimated data rate in bits per second.

In test step812, stream manager102determines whether the estimated data rate exceeds the available bandwidth through switch104. In this illustrative embodiment, switch104supports 1000BaseT connections and can support up to one gigabit per second data throughput. However, actual available bandwidth through a single port of switch104can be a bit less than one gigabit per second.

In addition, the available bandwidth between capture node106A and an involved display node, e.g., display node108A, can be even less if display node108A receives video and/or audio data streams from multiple capture nodes. In such cases, stream manager102allocates a data rate associated with display node108A to capture node106A. In addition, capture node106A and/or display node108A can effectively double their respective available bandwidth using link aggregation. The bandwidth available to the various nodes in the system ofFIG. 1are directly and easily scalable by merely connecting any node to multiple ports of switch104.

If the estimated data rate of the selected interchange format exceeds the bandwidth of switch104that is available for the audiovisual data stream, processing transfers to step814. In step814, stream manager102adjusts the constituent characteristics of the selected interchange format to reduce the bandwidth required by the selected interchange format. In one embodiment, stream manager102reduces the frame rate of the selected interchange format by one-half to reduce the estimated data rate of the selected interchange format. Of course, much more complex mechanisms can be used to reduce the data rate of the selected interchange format. In an alternative embodiment, data rate reduction is accomplished according to a predetermined default policy that can be specified according to the particular preferences of a given implementation. For example, image clarity may be paramount for a particular implementation and the default policy can prefer frame rate reduction over resolution reduction and lossy compression. In another implementation, smoothness of motion video may be paramount and the default policy can prefer resolution reduction and/or lossy compression over frame rate reduction. Other data rate reduction techniques can use lossless compression (e.g., run-length encoding) and frame-to-frame redundancy avoidance to reduce the data rate of the video interchange format without reducing quality of the transmitted audiovisual signal and without requiring particularly sophisticated logic in either capture node106A or display node108A.

If, in test step812, stream manager102determines that the estimated bit-rate does not exceed the available bandwidth through switch104, step814is skipped since bit-rate reduction is unnecessary. After steps812-814, processing by stream manager102according to logic flow diagram706, and therefore step706(FIG. 7) completes.

Thus, in step706, stream manager102selects an interchange format that is mutually supported by all involved nodes and that maximizes quality of the displayed audiovisual signal and that avoids unnecessary use of, or exceeding, available bandwidth through switch104. In step708, stream manager102establishes the video data stream from capture node106A to display nodes108A-D.

In this illustrative embodiment, stream manager102causes data rate reduction techniques to be applied equally to all portions of the same video signal. Accordingly, all portions of a video signal displayed in a window spanning multiple display monitors have a uniform appearance to a viewer. In an alternative embodiment, stream manager102causes data rate reduction techniques to be applied only as needed and allows different portions of the same video signal to have different applied data rate reduction techniques. This approach maximizes video signal quality of the various portions at the risk of a slightly different appearance between the various portions of the video signal corresponding to different display monitors.

In some instances, stream manager102is not able to effectively predict the data rate of a video signal after application of data rate reduction techniques. Such is particularly true if the amount of data rate reduction actually achieved depends upon the substantive content of the video signal, e.g., in such data rate reduction techniques as intra- and inter-frame redundancy avoidance as well as lossy and lossless compression. Accordingly, stream manager102can authorize capture node106A to apply data rate reduction techniques on its own initiative within specified parameters. The parameters can include a maximum permissible data rate for the particular video signal and can include a policy which specifies preferences for various types of data rate reduction techniques. Stream manager102can communicate the authorization and the parameters to capture node106A during initiation of the data stream representing the video signal.

To facilitate self-directed application of data rate reduction techniques by capture node106A, data rate reducer308A includes a number of data rate reduction modules1602A-E as shown inFIG. 16. A data rate reduction technique selector1604selects a data rate reduction technique according to the results of data rate reduction modules1602A-E, a data rate reduction policy1606, and maximum permissible area of interest size(s)1608.

To make this selection, data rate reducer308A receives corresponding area of interest scans that collectively represent a frame of the video signal from area of interest processor306A. Of course, when the entirety of a video stream is destined for a single display node and is fully visible, there will likely be only one area of interest, namely, the entire frame. Data rate reducer308A stores the scans in respective previous scans316A-D and submits the scans to data rate reduction modules1602A-E. Data rate reduction modules1602A-E each apply a respective data rate reduction technique concurrently with the others of data rate reduction modules1602A-E.

In this illustrative embodiment, (i) data rate reduction module1602A applies frame size down-scaling to one-quarter size (half height and half width); (ii) data rate reduction module1602B applies run-length encoding for intra-scan redundancy avoidance; (iii) data rate reduction module1602C applies selective update encoding for inter-scan redundancy avoidance; (iv) data rate reduction module1602D applies lossless compression; and (v) data rate reduction module1602E applies lossy compression. Some data rate reduction modules, e.g., data rate reduction modules1602C-E, require access to previous scans316A-D to properly apply their respective data rate reduction techniques. It should be appreciated that different and more (or fewer) data rate reduction modules can be implemented by data rate reducer308A, including modules implementing combinations of data rate reduction techniques.

Each of data rate reduction modules1602A-E produces a data rate reduced scan of each respective area of interest as a candidate scan for consideration by data rate reduction technique selector1604. Thus, for each area of interest scan submitted to data rate reduction modules1602A-E, five (5) candidate scans representing the same area of interest with different data rate reduction techniques applied are produced. In this illustrative embodiment, each area of interest scan also bypasses data rate reduction modules1602A-E such that each area of interest has a sixth candidate scan with no data rate reduction applied at all. For each area of interest, data rate reduction technique selector1604receives all six (6) candidate scans and selects one for submission to packetizer310A. By selecting one of the candidate scans of an area of interest for submission to packetizer310A and eventually for transportation to a display node, data rate reduction technique selection1604selects the particular data rate reduction techniques to be applied to each area of interest scan.

Data rate reduction policy1606implements a policy received from stream manager102in selecting one of the candidate scans for submission to packetizer310A. Maximum permissible scan size(s)1608represents a constraint on the maximum data size of respective scans of the areas of interest that can be sent through switch104to respective display nodes.

The following example is helpful in understanding the manner in which a maximum permissible scan size is determined for each area of interest scan. Consider hypothetically that full bandwidth between capture node106A and the destination display node is 1 gigabit per second and that overhead is estimated to be 10%, leaving 900 megabits per second for video signal payload. In this example, the captured video signal is allocated 100% of available bandwidth, i.e., the full 900 megabits. Consider further that the video signal captured by capture node106A has a resolution of 1600×1200 with 24 bits of color per pixel and 60 frames per second. For simplicity in this example, the whole video signal is one whole area of interest and no part of the video signal is destined for a different display node. In one-sixtieth of a second, the maximum data capacity from capture node106A to the destination display node is 15 megabits. The maximum permissible area of interest scan size generalizes to the total bandwidth available to the video signal in bits per second divided by the frame rate in frames per second to provide a maximum size of a scan of the area of interest in bits per scan.

If the respective sizes of the incoming scans of the respective areas of interest are no greater than the respective maximum permissible sizes, no data rate reduction techniques are required and data rate reducer308A can by-pass data rate reduction modules1602A-E altogether. In this example, a single frame of the captured video signal is 46 megabits of data, more than three times the maximum permissible frame size. Accordingly, some data rate reduction is required.

For a given area of interest, data rate reduction technique selector1604discards all candidate scans that exceed the maximum permissible scan size of the area of interest. Thus, any data rate reduction technique, including application of no data rate reduction at all, that results in an area of interest scan that exceeds the maximum permissible scan size is rejected by data rate reduction technique selection1604. Of those candidate scans of the given area of interest that are not discarded and are therefore within the maximum permissible scan size, data rate reduction technique selector1604selects one candidate scan of the given area of interest according to data rate reduction policy1606. Data rate reduction policy1606specifies respective relative priorities for data rate reduction modules1602A-E and indicates which of data rate reduction modules1602A-E are supported by the involved display nodes, i.e., apply data rate reduction techniques that can be reversed by the involved display nodes. Data rate reduction technique selector1604selects the candidate scan of the given area of interest from the data rate reduction module that has the highest relative priority from among all data rate reduction modules supported by the involved display nodes. In other words, data rate reduction technique selector1604also discards all candidate scans of the given area of interest produced by data rate reduction modules not supported by the involved display nodes and selects the remaining candidate scan of the given area of interest with the highest relative priority.

In this illustrative embodiment, the highest priority is always the by-pass candidate scan of a given area of interest, i.e., the candidate scan for which no data rate reduction technique is applied. The involved display nodes always support application of no data rate reduction technique. Accordingly, data rate reduction is only applied when the incoming scan of a give area of interest exceeds the maximum permissible scan size the given area of interest. Generally, lossless techniques such as redundancy avoidance and lossless compression are preferred, and therefore have higher priority, to lossy techniques such as frame size downscaling and lossy compression.

Continuing in the example above, data rate reduction module1602A performs frame size downscaling of the 1600×1200 frames to 800×600 frames, each of which is 11.5 megabits in size. Run-length encoding performed by data rate reduction module1602B reduces data rate by an amount that depends on the substantive content of the area of interest scan and can produce data rate reductions of 1:1 (no reduction), 3:1, 5:1, or even 10:1. Selective updating (inter-scan redundancy avoidance) also reduces data rate by an amount that depends on the substantive content of the area of interest scan and can reduce data rate to near zero if the current scan is identical to the previous scan of the area of interest or not at all if the current scan is not at all the same as the previous scan.

By selecting the candidate scan according to data rate reduction policy1606and maximum permissible scan size(s)1608, data rate reduction technique selector1604selects the data rate reduction technique producing the highest quality video signal that will not exceed the maximum permissible size. Having made the selection, data rate reduction technique selector1604passes the selected candidate scan to packetizer310A along with data identifying the particular data rate reduction technique(s) applied to the selected candidate scan such that the packets ultimately received by the display nodes include sufficient information to reverse the data rate reduction techniques.

In this illustrative embodiment, data rate reducer308A processes individual areas of interest of the subject video signal independently of one another. In particular, data rate reduction technique select1604selects from candidate scans of each area of interest independently of all other areas of interest of the same video signal. Accordingly, each area of interest of each frame can have a different data rate reduction technique applied. For example, in various areas of interest of a given frame of a video signal, one area of interest can require lossless compression while a second area of interest can be best transported with selective updating, a third area of interest can be best transported with run-length encoding, and a fourth area of interest can be transported with no data rate reduction at all.

In an alternative embodiment, data rate reduction technique selector1604ensures that the same data rate reduction techniques are applied to all areas of interest of a given frame. For example, data rate reduction technique selector1604discards all candidate frames of which at least one constituent area of interest scan exceeds the corresponding maximum permissible scan size. In other words, if a particular data rate reduction technique cannot be applied to any area of interest of a given frame (e.g., for exceeding the maximum permissible scan size for that area of interest), that particular data rate reduction technique is not applied to any other area of interest of the given frame. In this alternative embodiment, data rate reduction technique selector1604selects candidates scans for all areas of interest of a given frame from the same one of data rate reduction modules1602A-E.

In another alternative embodiment, data rate reducer308A selects data rate reduction techniques for scanlines rather than for the area of interest as a whole. In other words, each scanline of an area of interest scan is considered independently when selecting a data rate reduction technique.

3. Establishing the Video Data Stream from a Capture Node to One or More Display Nodes

Step708is shown in greater detail as logic flow diagram708(FIG. 9). Loop step902and next step906define a loop in which stream manager102processes each of the involved display nodes according to step904. In step904, stream manager102requests that the subject display node open a video stream. The request identifies the involved capture node from which the video stream should be received and specifies the interchange format of the video stream. The request can be sent as a single data packet or as multiple data packets. For example, one data packet can identify the involved capture node, one or more other data packets can each specify one or more properties of the interchange format, and a last data packet can instruct the display node to expect and process the subject stream in accordance with the earlier data packets. In addition, the request specifies a location within the respective display monitor the video component of the audiovisual stream is to be displayed. Once stream manager102performs step904for all involved display nodes, processing according to the loop of steps904-906completes.

In step908, stream manager102sends a request to the involved capture node, e.g., capture node106A in this illustrative example, to open a video stream. The request specifies both the interchange format for the subject video stream and one or more display nodes to which the subject video stream is to be sent. As described above with respect to step904, the request can be a single packet or multiple packets. In this embodiment, the request—whether in one or multiple packets—specifies which parts of the video stream are to be sent to which recipient display nodes. In addition, the request can authorize capture node106A to apply data rate reduction techniques at will and can specify parameters of such data rate reduction techniques as described more completely below. Since window204A occupies all of display monitors202A-D, each of display nodes108A-D receives at least part of the video stream associated with window204A.

After step908, processing according to logic flow diagram708, and therefore step708(FIG. 7), completes. After step708, processing by stream manager102according to logic flow diagram700completes. Once the video stream is initiated between the involved capture node and the involved display node(s), the involved nodes cooperate with one another through switch104without requiring direct involvement by stream manager102. Stream manager102stops the video stream in a directly analogous manner, requesting that the involved capture and display nodes terminate the previously initiated stream. Stream manager102stops the video stream in response to signals received from user interface110representing a user's request to stop display of the video stream in window204A, e.g., in response to closing of window204A by the user. If user interface110sends signals representing a change in the display of window204A to stream manager102, e.g., in response to a moving or resizing of windows204A, stream manager102stops the video stream and then subsequently re-starts the audiovisual stream in the manner described above with the respective locations in the involved display node(s) updated to reflect the new display location of window204A.

In response to the request from stream manager102sent in step908, capture node106A initiates sending of the video stream in a manner described above with respect toFIG. 3. In response to the request from stream manager102sent in step904, each of the involved display nodes, e.g., display node108A, receives and processes the video stream in a manner described above with respect toFIG. 4.

Capture node106A and display node108A cooperate to transport a video stream through switch104without requiring direct involvement of stream manager102. In short, the video stream simply runs from capture node106A to display node108A at up to the full bandwidth of port-to-port connections through switch104. With the current availability of one-gigabit network switches, high quality video streams can be routed through switch104from as many capture nodes and to as many display nodes as can be coupled to switch104.

4. Synchronization of Monitors in a Display Wall

As shown inFIGS. 1 and 2, display monitors202A-D are arranged in a tiled fashion to provide a display wall200(FIG. 2) as shown. One of the difficulties of display walls is that constituent display monitors each have their own independent display timing generator and, accordingly, refresh asynchronously with respect to one another. Such asynchronous refreshing of display monitors in a display wall can cause some displeasing visual artifacts, an example of which is referred to as “frame tearing.” A conventional approach to synchronization of multiple display monitors is commonly referred to as “genlock” in which all display monitors to be synchronized receive a hard-wired signal representing vertical and horizontal synchronization signals. This genlock technique is well-known in the broadcast industry.

Genlock provides very tight synchronization, e.g., within a few nanoseconds. However, genlock requires coordination of hard-wired signals and limits flexibility otherwise provided by the video distribution system described herein. In addition, frame tearing can be avoided with less tightly coupled synchronization between display monitors of a display wall. If display monitors202A-D are synchronized to within a few tens of microseconds, no frame tearing should be visible. Accordingly, a more flexible synchronization mechanism is used in the video distribution system ofFIG. 1to avoid visual artifacts such as frame tearing without requiring hard-wired synchronization.

To synchronize all display nodes, stream manager102periodically broadcasts a timing synchronization packet. Ordinarily, broadcasting a timing synchronization packet would not be an effective synchronization mechanism since traffic within the network can vary the arrival time of the timing synchronization packet at various destinations in unpredictable ways. However, a few characteristics of the video distribution system described herein enables effective use of timing synchronization packets for synchronization of display monitors202A-D.

First, the network shown inFIG. 1is closed, meaning that stream manager102, capture nodes106A-C, and display nodes108A-D account for nearly all traffic through switch104. As shown inFIG. 14, other nodes can send data through switch104; however, all the nodes attached to switch104are components of the video distribution system described herein.

Second, all components of the video distribution system can predict, with a fair degree of accuracy, when the next timing synchronization packet is to be sent by stream manager102. To enable such prediction, stream manager102sends the timing synchronization packet at a regular interval, e.g., once per second, and this regular interval is known by all components of the video distribution system, e.g., by manual programming of the node or by notification during the registration process described above. In addition, each component, e.g., each of capture nodes106A-C, includes an internal clock that is sufficiently accurate to determine when the stream manager102is about to send a timing synchronization packet.

To avoid unpredictable delay in propagation of the timing synchronization packet through switch104, all nodes, especially capture nodes106A-C, voluntarily stop transmitting data for a predetermined period of time prior to expected transmission of the next timing synchronization packet. The predetermined period of time is selected such that all previously transmitted data has time to travel through switch104to respective destination nodes, e.g., display nodes108A-D, and depends upon the particular topology of the network through which video signals are distributed. In this illustrative embodiment, the predetermined period of time is 15 microseconds. In embodiments with multiple switches and/or particularly large packet sizes, the predetermined period of time can be 100 microseconds.

During this predetermined period of time, no new data is being transmitted through switch104and all data currently stored in buffers within switch104is allowed to make its way to its various destination nodes. Thus, when stream manager102broadcasts the next timing synchronization packet, the buffers of switch104are empty and no traffic interferes with propagation of the timing synchronization packet to display nodes108A-D. Accordingly, the timing synchronization packet propagates through the network with fixed delays, thereby enabling effective synchronization of display nodes108A-D to within a few tens of microseconds. Once the timing synchronization packet is received, all nodes can resume transmission of data until a short time before the next synchronization packet is expected.

In this illustrative embodiment in which stream manager102sends a timing synchronization packet once per second, the overhead imposed by the cessation of transmission immediately prior to receipt of the timing synchronization packet is less than 0.1%. If stream manager102sends a timing synchronization packet much more frequently, e.g., once per video frame, the overhead is still less than 2%. In addition, while it is described that stream manager102periodically sends the timing synchronization packet, it should be appreciated that another node, such as capture node106A or a timer node1406(FIG. 14) can periodically send the timing synchronization packet. All that is needed for accurate synchronization in the manner described herein is that capture node106A can predict the time of each timing synchronization packet with sufficient accuracy to suspend transmission and allow clearing of the data path ahead of the timing synchronization packet.

5. Per-Stream Frame Synchronization Across Multiple Display Nodes

Synchronizing display monitors in a display wall as described above is not always sufficient to avoid undesirable visual artifacts such as frame tearing. Consider that, at the time of a frame refresh of display monitors202A-D, display nodes108A-B (FIG. 1) have completely received their respective portions of the video stream of window204A (FIG. 2) but display nodes108C-D have not. Display monitors202A-B could display their respective portions of the newly received frame while display monitors202C-D can display their respective portions of a prior and previously received frame. Such would result in frame tearing at the horizontal boundary between the tiled display monitors.

When network controller312A (FIG. 3) determines that the entirety of all scans of constituent areas of interest of a particular frame has been sent through switch104and before network controller312A begins to process the next frame of the subject video stream, network controller312A sends a frame synchronization packet to all display nodes to which an area of interest of the particular frame have been sent. The frame synchronization packet can be a simulated vertical synchronization signal, except that the frame synchronization packet identifies a specific frame of the video signal and is sent by network controller312A separately from pixel data to all display nodes receiving at least a portion of the video signal at about the same time, e.g., as a multicast packet. Sending the frame synchronization packet separately from pixel data ensures a relatively small packet that is less likely to arrive at the various display nodes at significantly different times.

The frame synchronization packet is received by all involved display nodes. In this embodiment in which frame display is synchronized among multiple display nodes and as incorporated by reference above with respect to video compositor410A, the writing of pixels by video compositor410A is to a temporary buffer and not directly to a frame buffer. The display node, e.g., display node108A, copies that portion of the temporary buffer to the frame buffer upon receipt of the frame synchronization packet. In this illustrative embodiment, the received frame synchronization packet is processed as a vertical synchronization signal, thus effecting very quick transfer of the received pixel data to a frame buffer of display node108A. Capture node106A sends the frame synchronization packet to all involved display nodes at about the same time, and components402A-408A of display node108A pass the frame synchronization packet straight through to video compositor410A for quick and immediate processing. Thus, nearly contemporaneously, all display nodes involved in the display of the subject video stream write their respective portions to their respective frame buffers. Such prevents frame tearing at boundaries between display monitors.

While stream manager102limits the bandwidth available to each video data stream sent by the capture nodes, a concern is that simultaneous bursts from more than one of the capture nodes can overflow an outbound buffer in the port of switch104connected to display node108C so as to cause loss of pixel data. Accordingly, congestion avoidance improves performance of the video distribution system described herein.

In one embodiment, capture nodes and display nodes cooperatively implement a metered approach to avoid congestion. In this metered approach, each of capture nodes106A-C is configured to meter transmission of data through switch104to avoid such bursts. Since the capture nodes meter their own respective transmission rates, this approach follows a push paradigm.

In another embodiment, capture nodes and display nodes cooperatively implement a burst approach to avoid congestion. In the burst mode described more completely below, display nodes request video data to be sent at full data rate from each capture node in turn in a pull paradigm.

In a third embodiment, capture nodes and display nodes cooperatively implement a hybrid “metered burst” approach which uses both the metered and burst approaches. This metered burst congestion avoidance approach is described more completely below.

In the metered approach, the metering of data transmission is controlled, at least in part, according to a smallest buffer size encountered en route from a given capture node to the destination display node, and the size of this smallest buffer is sometimes referred to as a minimum buffer size for the corresponding display node. Determining the minimum buffer size requires some information regarding the topology of the network through which audiovisual data streams pass in various selected interchange formats. In one embodiment, a minimum buffer size can be directly and manually programmed into each capture node. For example, a number of jumpers can be made user-accessible and various combinations of jumper locations can select a nearest minimum buffer size. Alternatively, capture nodes106A-C can include embedded web servers and implement an SNMP configuration tool to allow user specification of the minimum buffer size. However, in this illustrative embodiment, capture nodes106A-C are agnostic with respect to the topology of the network to which they are connected and receive information regarding the minimum buffer size associated with each respective one of display nodes108A-D from stream manager102. Stream manager102can be programmed with data representing the overall topology of the network interconnecting capture nodes106A-C and display nodes108A-D and the buffer sizes of respective switches, e.g., switch104, such that stream manager102can determine the smallest buffer size that will be encountered in the paths from capture nodes106A-C to any of display nodes108A-D.

Bandwidth allocated to each of the capture nodes for delivering data streams to a particular display node is limited in that their summed bandwidth must be no greater than the total bandwidth available to the display node. Briefly stated, the metering by each of capture nodes106A-C means that, for a time interval determined according to the minimum buffer size, each capture node maintains a ratio of transmit time to idle time where the ratio is related to the allocated bandwidth of the capture node. To facilitate understanding and appreciation of this point, the illustrative example of display node108C, driving display monitor202C (FIG. 2) of display wall200, is described. Display node108C receives a portion of the video signal of window204A from capture node106A, the entirety of the video signal of window204B from capture node106B, and a portion of the video signal of window204C from capture node106C.

In this illustrative example, stream manager102has allocated 30% of the available bandwidth received by display node108C to capture node106A. Thus, in the maximum time interval, capture node106A maintains a ratio of 30% transmit time to 70% idle time with respect to display node108C. During this idle time, capture node106A can send data streams to other display nodes. Similarly, capture nodes106B-C maintain respective ratios of (i) 60% transmit time to 40% idle time and (ii) 10% transmit time to 90% idle time. As a result, the outbound buffer of switch104to display node108C is never overwhelmed by simultaneous bursts from two or more of capture nodes106A-C.

The time interval is the amount of time the smallest buffer in the path from capture node106A to display node108C can be filled at the connection data rate. In this illustrative embodiment, the smallest buffer is 16 kB and the connection data rate is 1 Gb/s. Thus, the time interval is 128 microseconds. To maintain the proper ratio, capture node106A transmits to display node108C no more than 30% of any 128-microsecond interval to maintain a ratio of 30% transmit time to 70% idle time with respect to the video data stream transmitted to display node108C. That results in generally 38 microseconds of transmit time and 90 microseconds during which transmission from capture node106A to display node108C is suspended during any 128-microsecond interval. It should be noted that the idle time of capture node106A with respect to display node108C pertains only to display node108C; capture node106A is free to continue transmission of other audiovisual data streams to other display nodes during that idle time.

In metering audiovisual stream transmission in this manner, capture nodes106A-C avoid exceeding the available bandwidth to display node108C, even for short bursts which might overflow buffers in display node108C or in intermediate network devices between capture nodes106A-C and display node108C.

In the burst approach, the display node selects one of a number of capture nodes to send video data at full data rate. In the same example involving display node108C, display node108C sends a packet to only one of capture nodes106A-C authorizing the capture node to send a data stream at full data rate. While one capture node sends a data stream to display node108C, other capture nodes are idle with respect to display node108C but can send data streams to other display nodes. To change sending authorization from one capture node to another, display node108C sends a stop packet to the previously authorized capture node and sends an authorization packet to a new capture node.

In this example, display node108C can authorize capture node106A to send a data stream at full data rate for a predetermined duration or until a complete frame of a portion has been received by display node108C. Then, display node108C can send a stop packet to capture node106A and authorize capture node106B to send a data stream at full data rate for a predetermined duration or until the current frame of the portion has been received by display node108C. Capture node108C can do the same with respect to capture node106C.

Both the metered approach and the burst approach have difficulties in specific circumstances. For example, if a display node receives many small video signals from many capture nodes, the metered approach allocates to each capture node a very small portion of the available bandwidth to the display node. This situation is illustrated inFIG. 11. In addition to a portion of window204A, display monitor202B also includes small windows204D-O, each of which can show a reduced-size version of a video signal received from respective capture nodes (not shown). In this illustrative example, stream manager102can allocate 40% of available bandwidth to the portion of window204A displayed by display node108B and 5% of available bandwidth to each of windows204D-O, i.e., 60% to windows204D-O collectively. Thus, in the metered approach, the respective capture node for each of windows204D-O would transmit packets 5% of relatively short periods of time and remain idle with respect to display node108B 95% of the relatively short periods of time. Accordingly, packet sizes can get quite small, leading to relatively high overhead in the form of packet headers and the requisite processing of those packet headers by depacketizer404A, for example.

The burst approach requires coordination between display nodes. For example, if display node108B authorizes capture node106A to send a data stream representing the video of window204A at full data rate and capture node106A complies, capture node106A cannot simultaneously send a data stream representing another portion of the video of window204A to a different display node, e.g., display node108A. In a small display wall with relatively few display nodes, such coordination such that full data rate to all display nodes is fully utilized may be reasonably feasible. However, as the number of display nodes in a display wall increase and the number of capture nodes driving the display wall increase, the coordination effort grows substantially and can easily become impractical.

In the third embodiment, stream manager102implements the metered burst approach to avoid congestion as illustrated by logic flow diagram1000(FIG. 10). Before processing according to logic flow diagram1000, stream manager102is presumed to have already determined what share of the total available bandwidth is allocated to each of the data streams received by a particular display node. Logic flow diagram1000illustrates the process by which stream manager102informs the display node as to the particular manner in which such allocations are to be enforced.

Loop step1002and next step1010define a loop in which stream manager102processes all data streams received by a particular display node, e.g., display node108B in this illustrative example, accordingly to steps1004-1008. For each iteration of the loop of steps1002-1010, the particular data stream processed by stream manager102is sometimes referred to as the subject data stream.

In test step1004, stream manager102determines whether the subject data stream is exclusive, i.e., whether the subject data stream represents the entirety of a video signal and no other portions of the video signal are received by any other display nodes. If the subject data stream is not exclusive, processing transfers to step1006and stream manager102sends data to display node108B instructing display node108B to use the metered approach described above with respect to the subject stream and informing display node108B of the allocated share of bandwidth for the subject data stream. Such data can be included in the request of step904(FIG. 9) described above. By using the metered approach with non-exclusive data streams, no coordination is required between multiple display nodes receiving data streams from the same capture node.

Conversely, if stream manager102determines in test step1004(FIG. 10) that the subject data stream to display node108B is exclusive, processing transfers to step1008. In step1008, stream manager102determines that a metered burst approach should be implemented by display node108B. However, stream manager102postpones informing display node108B of the use of the metered burst mode until the relative bandwidth allocation for the metered burst mode has been determined in step1012below.

After step1006or step1008, processing by stream manager102transfers through next step1010to loop step1002in which stream manager102processes the next data stream received by display node108B according to steps1004-1008. When stream manager102has processed all data streams received by display node108B according to the loop of steps1002-1010, processing transfers from loop step1002to step1012.

In step1012, stream manager102determines the burst bandwidth. The burst bandwidth is the total available bandwidth en route to display node108B less any allocated bandwidth using the metered approach. Alternatively, the burst bandwidth is the sum of the allocated bandwidth for the data streams for which the metered burst approach is to be used. In either case, in this illustrative embodiment, the burst bandwidth is 60% since display node108B uses the metered approach for window204A (allocated 40%) and the metered burst approach for windows204D-O (allocated 5% each, 60% collectively). Stream manager102instructs display node108B to use the metered burst approach for the applicable data streams as determined in step1008and communicates the burst bandwidth. Such instruction can be included in the request sent in step904(FIG. 9) as described above.

In the metered burst approach, display node108B uses the pull paradigm described above with respect to the burst approach but also includes data representing a bandwidth allocation in the instructions to start sending data. Capture nodes respond by starting transmission that is metered in the manner described above with respect to the metered approach to no more than the burst bandwidth. Since display node108B uses the metered approach with respect to the area of interest of window204A, display node108B instructs capture node106A to send video data metered at 40%, the allocated bandwidth for window204A with respect to display node108B. Display node108B does not instruct capture node106A to stop sending video data for the duration of the display of window204A.

With respect to windows204D-O for which display node108B uses the metered burst approach, display node108B authorizes one associated capture node at a time to send data metered at the burst bandwidth, e.g., 60%. Thus, display node108B receives video data for only one of windows204D-O at a time but in bursts of up to 60% of the total available bandwidth received by display node108B. Accordingly, video data for windows204D-O can be received in relatively large packets, thereby avoiding the inefficiencies associated with small packets.

E. System Performance Regulation by Stream Manager102

Thus, an audiovisual signal in a native format is captured by capture node106A into a native interchange format, transported through switch104to one or more of display nodes108A-D in a selected interchange format, and converted from a displayable interchange format by the involved display nodes to a displayable format for display in one or more of display monitors202A-D. Since the audiovisual signal is converted to a digital, packetized format for transport through switch104, format conversion is supported by both capture nodes and display nodes. As a result, the native format and the displayable format can be different from one another and conversion from the native format to the displayable format is almost incidental. In fact, the entire system can be completely heterogeneous. Each of capture nodes106A-C can capture audiovisual signals in different native formats, and—while such is unlikely in any individual display wall—display nodes108A-D can drive respective displays requiring different displayable formats. In fact, the video distribution system described herein supports display of a single audiovisual signal across tiled display monitors requiring different respective displayable formats. However, it is appreciated that most display walls will include homogeneous display monitors to enhance the user's perception of the display wall as a single, integrated display.

Another advantage of the system ofFIG. 1is that stream manager102configures and initiates transportation of audiovisual data streams through switch104and thereafter allows capture nodes and display nodes to cooperate directly to transport such signals through switch104without ongoing intervention by stream manager102. Therefore, stream manager102does not represent a limitation on the throughput of audiovisual streams through switch104. Instead, audiovisual streams can move through switch104at nearly the full connection speed.

Accordingly, many more audiovisual streams can be transported from capture nodes106A-C to display nodes108A-D than can be transported in conventional bus-oriented architectures.

It should be appreciated that, while display monitors202A-D are shown to be arranged in a tiled display and are the only display monitors connected to switch104, other devices which are not part of the same tiled display can be connected to switch104. In addition, switch104can be multiple inter-connected switches.

The system ofFIG. 1can support multiple, distinct display walls and other independent display devices. The interaction of capture nodes with multiple independent display nodes is described more completely in the parent U.S. patent application Ser. No. 11/111,182 and that description is incorporated herein by reference in its entirety. One significant consequence is that, while capture node106A captures a single audiovisual signal, capture node106A can send multiple versions of the captured audiovisual signal to multiple respective display nodes, each of the versions in its respective interchange format.

1. Multiple Outgoing Streams from a Single Capture Node

It is possible that such multiple versions of the captured audiovisual signal in the respective interchange formats can exceed the available bandwidth from capture node106A to switch104. Stream manager102, in combination with user interface110, provides a mechanism by which a human user can weigh the various tradeoffs involved in reducing bandwidth for one or more of the versions of the audiovisual signal sent by capture node106A. For example, user interface110provides a graphical user interface by which the user can specify that a particular version of the audiovisual signal can have a reduced frame rate to preserve image clarity or that signal fidelity of one version of the audiovisual signal is to be preserved at the expense of significant signal fidelity loss in another version of the audiovisual signal. The role of stream manager102as a centralized controller of audiovisual signal data streams through switch104in combination with user interface110allows a user the opportunity to control some of the choices made by stream manager102.

Stream manager102selects interchange formats and can cause application of data rate reduction techniques to specify video and/or audio data streams which collectively remain within the available outbound bandwidth of a given capture node. In one embodiment, stream manager102allocates a percentage of such outbound bandwidth to each outgoing data stream, e.g., according to overall size (frame size and/or frame rate) of the content of the data stream, relative priority (e.g., window priority within a display), and/or relative desired quality. For example, stream manager102can allocate 92% of the outbound bandwidth of capture node106A to the video data streams for window204A (FIG. 2) and 8% for a highly reduced video signal to be sent to computer114(FIG. 1) for remote monitoring, which is described below. It should be noted that allocation of outbound bandwidth is not necessary for individual portions destined for different ones of display node108A-D since metered congestion avoidance resolves any outbound bandwidth problems.

Once stream manager102has allocated a share of outbound bandwidth to a data stream, stream manager102selects an interchange format, and perhaps data rate reduction techniques, to produce an estimated data rate within the allocated share. Thus, each data stream is configured to fit within its allocated share of the outbound bandwidth.

Alternatively, stream manager102selects interchange formats for the respective data streams, and can cause application of data rate reduction techniques to each data stream, in such a manner that the aggregate estimated data rate is within the limits of the outbound bandwidth.

2. Multiple Incoming Streams to a Single Display Node

As shown inFIG. 2, three different capture nodes are sending video data to display monitor202C, through display node108C. Accordingly, three ports of switch104, which are connected to respective capture nodes, send video data to a single port of switch104connected to display node108C. Since stream manager102participates in the initiation of each audiovisual stream transported through switch104, stream manager102is aware that not all three capture nodes can transmit at full bandwidth to the same display node as such would exceed the available inbound bandwidth of display node108C. Under the control of user interface110in the manner described above, stream manager102allocates portions of the available inbound bandwidth of display node108C to each of capture nodes106A-C. For example, stream manager102can allocate 30% of the available inbound bandwidth of capture node106A to deliver the lower left portion of window204A (FIG. 2); 60% to capture node106B to deliver the entirety of window204B; and 10% to capture node106C to deliver the left portion of window204C.

This allocation by stream manager102is possible because (i) stream manager102controls, in response to user interface110, the location of windows204A-C, and therefore knows what proportion of each video signal is to be received by each display node and (ii) stream manager102receives, through user interface110, information regarding user preferences with respect to relative priorities of different video signals and what aspects of each video signal are more valuable than others (e.g., image clarity vs. smooth motion). Once stream manager102allocates the inbound bandwidth of display node108C, stream manager102prevents congestion inbound to display node108C using congestion avoidance techniques such as those described above.

3. Reallocating Bandwidth for Adding a New Video Stream

As described above, user interface110(FIG. 1) allows a user to specify which video signals are to be displayed in which locations within display wall200(FIG. 2). The user can open a new window to add a displayed video signal to display wall200or close a window, e.g., window204C, to remove a displayed video signal from display wall200. In addition, the user can move and/or resize a window to alter the display size and/or location of a displayed video signal within display wall200. Each such change can overwhelm available bandwidth, outbound at one or more capture nodes or inbound at one or more display nodes or both.

FIG. 12illustrates a change to the state of display wall200. In particular, a new window204P (FIG. 15) is to be displayed by display monitor202D. For illustration purposes, consider that display by display monitor202D of the portions of windows204A and204C viewable within display monitor202D occupied all of the available inbound bandwidth (Figure of display node108D prior to the change in state. Thus, insufficient bandwidth remains for transportation of the video signal to be displayed in window204P from switch104to display node108D.

The general approach taken by stream manager102in adding a new video stream to those received by display node108D is this: determine whether adding the new video stream will exceed bandwidth to display node108D and, if so, adjust video streams to display node108D so as to not exceed bandwidth thereto. This is illustrated in logic flow diagram1200(FIG. 12).

Since stream manager102configures and initiates streams between capture nodes106A-C and display nodes108A-D in the manner described above, stream manager102knows the parameters of all video streams transported therebetween. In some cases, stream manager102can accurately determine the data rate of a particular stream of video data. However, other video streams, videos streams using redundancy avoidance and/or lossy or lossless compression in particular, cannot be accurately predicted since the data rate depends to some degree on the substantive content of the video signal. To determine the data rate of these video streams, stream manager102periodically receives information regarding data rates of video streams from capture nodes106A-C from each of capture nodes106A-C, sent either automatically and periodically or in response to polls received from stream manager102.

To determine whether the addition of a video stream for window204P (FIG. 15) will exceed bandwidth to display node108D, stream manager102estimates a data rate of that video stream in step1202(FIG. 12) and adds the estimated data rate to the data rates of the other video streams to display node108D in step1204to determine an estimated aggregate data rate and compares the aggregate data rate to the available bandwidth to display node108D in test step1206. To estimate the data rate required for the video stream of window204P, stream manager102selects an interchange format for the video stream in the manner described above.

If the estimated aggregate data rate is within the available bandwidth to display node108D, no adjustment to any of the video streams is necessary and stream manager102initiates the video stream of window204P in step1208. Conversely, if the estimated aggregate data rate exceeds the available bandwidth to display node108D, the video streams require adjustment to reduce the aggregate data rate.

Stream manager102reduces the aggregate data rate so as to not exceed the available bandwidth to display node108D in step1210. Step1210is shown in greater detail as logic flow diagram1210(FIG. 13). In step1302, stream manager102sorts all feasible data rate reduction techniques for all video streams received by display node108D according to relative respective priorities.

The sorting of feasible data rate reduction techniques according to relative priorities can vary widely from one embodiment to another. In one embodiment, more recently started video streams are given higher priority than less recently started video streams and a default data rate reduction policy is applied to each video stream. Alternatively, relative priorities of windows in display wall200(FIG. 15) are based on respective depths of the windows in display wall200as represented by user interface110(FIG. 1). In a graphical user interface having windows, windows typically have an associated depth. When two windows in a graphical user interface overlap, the relative depths of the windows determines which window obscures the other. In essence, a window's depth can be considered one representation of the window's relative priority in the graphical user interface. In sorting data rate reduction techniques, stream manager102can give higher priority to more aggressive data rate reduction techniques to windows of greater depths, i.e., that are more likely to be obscured by another window. The default policy can specify a series of data rate reduction techniques to be applied in sequence.

In a more complex embodiment, user interface110and stream manager102cooperate to allow a user to manually specify relative priorities for each video stream and, for each video stream, specify ranked preferences for application of data rate reduction techniques. For example, the user can rank preservation of frame rate relatively highly if smoothness of motion is particularly important for a given video stream or can rank preservation of image clarity if smoothness of motion is less important than image clarity in the video stream.

Data rate reduction techniques for a particular video stream are not necessarily contiguous within the sorted list of feasible data rate reduction techniques. For example, the sorted list might indicate that the first three data rate reduction techniques to apply in sequence are: reduction of the frame rate of the video stream of window204P, redundancy avoidance of the lower right portion of window204A, and color depth reduction of window204P.

Loop step1304and next step1308define a loop in which stream manager102performs step1306while the aggregate data rate exceeds the available bandwidth to display node108D. Thus, in loop step1304, stream manager102determines the aggregate data rate and compares the aggregate rate to the available bandwidth to display node108D, transferring processing to step1306if the aggregate data rate exceeds the available bandwidth to display node108D.

In step1306, stream manager102applies the next data rate reduction technique of the sorted list constructed in step1302. The list is sorted in ascending priority such that highest priority techniques are applied last. For example, assigning a high priority to the data rate of the video stream of window204A causes stream manager102to avoid reduction of the frame rate of that video stream. Stream manager102applies a data rate reduction technique by recording a change in the interchange format of the particular video stream to which the data rate reduction technique pertains and estimates a new aggregate data rate based on the newly modified interchange format. The following example is illustrative.

Consider that the data rate reduction technique to be applied in a given iteration of the loop of steps1304-1308is the reduction of frame rate in the video stream of window204P. Stream manager102has determined an interchange format in which display node108D is to receive the video stream to display in window204P. To apply the data rate reduction technique, stream manager102modifies the interchange format for the video stream of window204P, e.g., by reducing the frame rate by 50%. Knowing the other parameters of the interchange format, stream manager102can estimate a data rate for the video stream of window204P with the modified interchange format and can therefore estimate a new aggregate data rate reflecting the same change.

Processing transfers through next step1308to loop step1304in which stream manager102determines whether the most recently applied data rate reduction technique has sufficiently reduced the aggregate data rate to be within the available bandwidth to display node108D. If the aggregate data rate still exceeds the available bandwidth to display node108D, processing transfers to step1306and stream manager102applies the next data rate reduction technique in the manner described above. It should be noted that the data rate reduction techniques applied in repeated iterations of step1306accumulate. In an alternative embodiment, feasible combinations of data rate reduction techniques are listed and sorted in step1302and application of the combinations of data rate reduction techniques do not accumulate.

Once the aggregate data rate does not exceed the available bandwidth to display node108D, processing according to logic flow diagram1210, and therefore step1210(FIG. 12), completes. After either step1208or step1210, processing according to logic flow diagram1200completes. Thus, stream manager102applies data rate reduction techniques in order of increasing priority until all video streams can be delivered to display node108D within the available bandwidth.

It is also possible that addition of a new video stream would exceed available bandwidth from a particular capture node, e.g., capture node106A. Consider, for example, that the video stream to be displayed in window204P (FIG. 15) is a reduced-size version of the video stream produced by capture node106A, e.g., the video stream displayed in window204A. In another example, the reduced-size version of the same video stream produced by capture node106A can be directed to another display node which is not part of display wall200, either within a local area network accessible through switch104and perhaps one or more other switches or accessible through the Internet112.

Addition of a new video stream from capture node106A by stream manager102is generally analogous to the addition of a new stream to a given display node. While a single capture node typically captures and sends only a single video signal, it may be desirable to send the video signal of the capture node to multiple destination nodes simultaneously. Capture node106A can send the same video stream to multiple destinations using a multicast technique. However, sending full and complete video streams to multiple display nodes that display only a portion of that video signal reduces the ability of those display nodes to receive other video streams due to bandwidth limitations at the display nodes. In this illustrative example, display node108D cannot produce the reduced-size video stream of window204P from the full-size video stream of window204A for two reasons. First, display node108D receives only a portion of the video signal of window204A for display of the lower right corner and therefore does not already receive the entire full-size video stream. Second, in this illustrative example, receipt of the respective portions of the video signals of windows204A and204C already use most, if not all, of the available bandwidth to display node108D; insufficient bandwidth is available to display node108D to receive the full-size video stream of window204A. Thus, capture node106A must provide the reduced-size version of the video stream in addition to the full-size portions displayed in window204A.

In a manner analogous to that described above with respect to logic flow diagram1210, stream manager102applies data rate reduction techniques to the various video streams produced by capture node106A in order of ascending priority until the aggregate data rate of all video streams produced by capture node106A are within the available bandwidth from capture node106A. As described above with respect to prioritizing video streams received by display node108D, video streams produced by capture node106A can be prioritized in a wide variety of ways, including automated prioritization according to predetermined policies and manual prioritization by a user through user interface110.

F. Extensibility of the Video Distribution System

FIG. 14shows the video distribution system ofFIG. 1with some additional elements, including a data store1402, a digital signal processor1404, and a timer1406.

Data store1402functions generally as a display node as described above but archives video streams rather than displays them. If a video stream is to be archived, stream manager102causes the capture node producing the video stream, e.g., capture node106A, to send a highly condensed version of the video stream to data store1402. In an illustrative embodiment, stream manager102and user interface110allow a user to specify whether a particular video stream is to be archived. In another embodiment, all video streams are archived without requiring intervention by any user. In yet another embodiment, video streams from predetermined capture nodes are archived.

The highly condensed version of the video stream can be highly condensed in any of a number of ways. For example, the highly condensed version can be (i) reduced in size, e.g., in the number of scanlines and/or number of pixels per scanline; (ii) reduced in frame rate, e.g., as low as one frame per second or lower; (iii) reduced color depth; and/or (iv) aggressively compressed using lossy compression techniques. Of course, lossless compression techniques can also be used.

In receiving the highly condensed video stream, data store1402does not convert the video stream to any displayable interchange format in this illustrative embodiment since there is no particular interchange format preferred by data store1402. Instead, data store1402merely accumulates and stores the highly condensed video stream in the interchange format selected by stream manager102. In an alternative embodiment, data store1402converts the highly condensed video stream to a standard archival format such as the known MPEG-4 format, for example.

For playback of archived video streams, data store1402can also act as a capture node, sending the archived video stream in the interchange format in which the video stream was archived or, alternatively, converting the archived video stream from the archival format to an interchange format selected by stream manager102for delivery to a display node. In addition, using timestamps embedded in the video signal and in other video signals that were captured concurrently, data store1402can synchronize sending of multiple video signals such that the temporal relationships between the multiple video signals are preserved. Such allows recall of several cameras observing the same time space, such as an array of security cameras focused on different subject spaces and the same time, to recreate the video signal content of multiple windows that might have been viewed, for example in display wall200, in a given point in time.

Digital signal processor1404can perform such complex tasks as high-quality de-interlacing, edge detection, motion detection, and filtering such as sharpening, smoothing, and/or noise reduction on behalf of other nodes shown inFIG. 14. For illustration purposes, it is helpful to consider the example of an interlaced video signal captured by capture node106A and a de-interlaced video signal expected by display node108A. Consider also that capture node106A produces only interlaced signals and display node108A only accepts progressive scan signals. In determining a selected interchange format, stream manager102determines that no interchange formats are commonly supported by both capture node106A and display node108A. Rather than indicating a failure to select an acceptable interchange format, stream manager102can request de-interlacing service from digital signal processor1404. Thus, digital signal processor1404can receive a video signal in one interchange format and send the video signal in a different interchange format. In addition, digital signal processor1404can receive and send the video signal in the same interchange format, processing the video signal content, e.g., by applying edge detection, motion detection, and filtering such sharpening, smoothing, and/or noise reduction to the video signal itself. Edge detection, motion detection, and filtering are known and are not described herein.

Digital signal processor1404performs such a service by acting as both (i) a display node receiving an interlaced audiovisual signal from capture node106A and (ii) a capture node producing a de-interlaced audiovisual signal for display node108A.

Timer1406is attached to a port of switch104and provides a system-wide clock signal. In one embodiment, each of capture nodes106A-C is configured to discover the presence of timer1406and to synchronize internal clocks with timer1406when timer1406is present. By synchronizing internal clocks of multiple capture nodes, display nodes are able to synchronize multiple audiovisual signals from multiple capture nodes by comparison of timestamps that are included in the audiovisual streams in the manner described below. In addition, timer1406can periodically send the timing synchronization packet at predetermined intervals known to nodes relying on the timing synchronization packet. Furthermore, in a video distribution system including multiple display walls, multiple timers like timer1405can each serve display nodes of a respective display wall.

Data store1402, digital signal processor1404, and timer1406illustrate the modularity of the video distribution system described herein. Additional data stores and digital signal processors can be coupled to switch104to provide additional storage and processing capacity and/or to provide additional types of archival and/or digital signal processing. Furthermore, data stores, digital signal processors, and timers can serve subsets of the video distribution network providing services regionally. As an illustrative example of such regional service, a separate data store, digital signal processor and/or timer can serve each collection of display nodes collectively constituting a respective display wall in a video distribution having multiple such display walls.

G. Remote Monitoring

Stream manager102implements a remote monitoring system in this illustrative embodiment. In particular, stream manager102communicates with a remote client computer114through the Internet112. In this illustrative embodiment, stream manager102receives highly condensed versions of video streams passing through switch104in the manner described above with respect to data store1402and makes those highly condensed video streams available to a remotely located computer such as client computer114. For example, client computer114can receive a video stream representing a miniaturized view of the collective display of display monitors202A-D, i.e., of display wall200. In addition, a user interface similar to user interface110is provided within client computer114such that a remotely located user can configure a number of features of display wall200, such as locations and sizes of respective windows displayed in display wall200and various priorities for various respective video streams to optimize data rate reduction technique selection by stream manager102in the manner described above.

In addition to, or instead of, providing a miniaturized representation of display wall200through Internet112, stream manager102can also make highly condensed versions in individual video streams and/or the miniaturized representation of display wall200available to computer114directly through switch104or though a LAN rather than through Internet112.

H. Distribution of Audio Signals

While display of video signals side-by-side seems perfectly manageable for a viewer, sound from multiple sources is not so easily juxtaposed for listening. Accordingly, any audio signal received by any of capture nodes106A-C is treated as a separate signal for routing through switch104independently of any accompanying video signal received by the same capture node. Any of display monitors202A-D can play audio associated with the video of any of windows204A-C and still provide an integral audiovisual experience for a human viewer. For example, while window204B is displayed entirely within display monitor202C, display monitor202B can play audio associated with window204B. Since display monitors202A-D are in close physical proximity to each other, sound produced by any of display monitors202A-D can be perceived as produced by display wall200. In addition, an audio-only display device1408(FIG. 14) can receive audio streams for playback in conjunction with video displayed by display wall200.

Transporting an audio signal as a separate data stream from any video signal to which the audio signal corresponds raises a number of issues. These issues include correlation, synchronization, and mixing.

While audio streams are transported independently of any corresponding video streams through switch104, it is preferred that audio streams are sufficiently linked to corresponding video streams that the audio and video signals can be synchronized for playback to a human viewer for an integrated audiovisual experience. In one embodiment, stream manager102assumes that each of capture nodes106A-C captures a single signal from a single source. Thus, if a capture node produces both video and audio streams, it is presumed that the video and audio streams are captured from a single, integrated audiovisual signal. Accordingly, stream manager102assumes that all video streams sent by a single capture node are various representations of a single video signal and that any audio stream sent by the same capture node corresponds to that video signal. In an alternative embodiment, each capture node associates an identifier with each source device from which audiovisual signals are captured and reports that identifier to stream manager102for each video and/or audio stream started at the request of stream manager102. Thus, stream manager102can determine whether a particular audio stream corresponds to a particular video stream by comparison of respective source identifiers. Use of source identifiers enables proper tracking by a capture node of various signals captured from multiple sources in an embodiment in which capture nodes can capture signals of multiple source devices.

It is preferred that such source identifiers are unique within the video distribution system ofFIG. 14. In one embodiment, each capture node generates source identifiers unique to itself and concatenates the source identifier with a unique capture node identifier to ensure that the complete source identifier is unique within the video distribution system. In an alternative embodiment, each capture node requests a new source identifier from stream manager102when a new source identifier is needed, and stream manager102ensures that all issued source identifiers are unique with respect to one another.

Since separate audio and video streams can represent a single, integrated audiovisual signal, it is preferred that the audio and video streams are synchronized during playback to a viewer. It should be noted that human perception of sounds and sights are such that playback of audio can be delayed relative to playback of corresponding video but playback of sound should not be advanced relative to playback of corresponding video. Since light travels faster than sound, people are accustomed to hearing a distant event slightly after seeing the distant event and people can properly correlate early video and late audio that correspond to one another. However, the reverse is not true; people have substantial difficulty processing an audiovisual experience in which the sound is early relative to the video. Accordingly, some leeway can be allowed with respect to delaying playback of an audio stream relative to a corresponding video signal where no leeway should be allowed with respect to delaying playback of a video stream relative to a corresponding audio signal.

As described above, capture node106A includes frame numbers in video streams and broadcasts a synchronize packet such that all recipient display nodes, e.g., display nodes108A-D, display various portions of the same frame at the same time. Capture node106A also includes timestamps in the video signals and/or in the broadcast synchronize packets to identify a time at which a particular frame of the captured video signal was captured. When capturing an audio signal, capture node106A also inserts periodic timestamps into the audio stream, using the same clock according to which timestamps are included in the video streams of capture node106A.

To avoid allowing playback of the audio to advance before the corresponding playback of a video stream, the display node receiving the audio stream buffers the audio data and only plays back those parts of the audio signal associated with timestamps which are earlier than the timestamp of the most recently received synchronize packet. As described above, a frame synchronization packet indicates to all involved display nodes that a particular frame of a video signal is ready to be displayed. Audio data associated with timestamps equal to, or earlier than, the timestamp of the synchronize packet corresponds to the current frame or perhaps an earlier frame and is therefore not premature for playback.

As described above, multiple audio signals do not lend themselves to convenient juxtaposition. Multiple audio streams can be available through the video distribution system ofFIG. 14, and simultaneous playback of those multiple audio streams by display nodes108A-D and/or1408can result in a cacophony of discordant sounds. Accordingly, stream manager102allows a user, through user interface110, to control the volume of each of a multitude of audio streams individually, much like an audio mixer. In one embodiment, each of windows204A-D (FIG. 15) includes a graphical user interface object, such as a slider bar or twist knob, by which the user can control the volume at which an associated audio signal is played back. In an alternative embodiment, user interface110includes a virtual mixer display, e.g., within its own window within display wall200, in which textual identifiers of each audio stream are associated with a slider volume control graphical user interface object. It is preferred that both embodiments include mute controls for each audio stream such that the user can easily mute individual audio signals.

In response to a user command through user interface110to change the volume at which a particular audio stream is played back, stream manager102identifies the particular display device playing back the particular audio stream and issues a command to the display device to play the audio stream back at a specified volume. Thus, through display device1408and/or through the respective audio playback circuitry of display monitors202A-D, a single, integrated, and user-controlled audio mix of one or more audio streams from capture nodes106A-C can be played, all managed by stream manager102.

The above description is illustrative only and is not limiting. Instead, the present invention is defined solely by the claims which follow and their full range of equivalents.