Patent Publication Number: US-9426499-B2

Title: System and method for scalable and low-delay videoconferencing using scalable video coding

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of Ser. No. 12/015,956, filed Jan. 17, 2008, which is a is a continuation of PCT International Application No. PCT/US06/028365 filed Jul. 20, 2006 which claims the benefit of U.S. provisional patent application Ser. Nos. 60/701,108 filed Jul. 20, 2005, 60/714,741 filed Sep. 7, 2005 60/723,392 filed Oct. 4, 2005, and 60/775,100 filed Feb. 21, 2006. Further, this application is related to International Application Nos. PCT/US2006/028366 filed Jul. 20, 2006, PCT/US2006/028367 filed Jul. 20, 2006, and PCT/US2006/028368 filed Jul. 20, 2006. All of the aforementioned priority and related applications are hereby incorporated by reference herein in their entireties, and from which priority is claimed. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to multimedia and telecommunications technology. In particular, the invention relates to systems and methods for videoconferencing between user endpoints with diverse access equipment or terminals, and over inhomogeneous network links. 
     BACKGROUND OF THE INVENTION 
     Videoconferencing systems allow two or more remote participants/endpoints to communicate video and audio with each other in real-time using both audio and video. When only two remote participants are involved, direct transmission of communications over suitable electronic networks between the two endpoints can be used. When more than two participants/endpoints are involved, a Multipoint Conferencing Unit (MCU), or bridge, is commonly used to connect to all the participants/endpoints. The MCU mediates communications between the multiple participants/endpoints, which may be connected, for example, in a star configuration. 
     For a videoconference, the participants/endpoints or terminals are equipped with suitable encoding and decoding devices. An encoder formats local audio and video output at a transmitting endpoint into a coded form suitable for signal transmission over the electronic communication network. A decoder, in contrast, processes a received signal, which has encoded audio and video information, into a decoded form suitable for audio playback or image display at a receiving endpoint. 
     Traditionally, an end-user&#39;s own image is also displayed on his/her screen to provide feedback (to ensure, for example, proper positioning of the person within the video window). 
     In practical videoconferencing system implementations over communication networks, the quality of an interactive videoconference between remote participants is determined by end-to-end signal delays. End-to-end delays of greater than 200 ms prevent realistic live or natural interactions between the conferencing participants. Such long end-to-end delays cause the videoconferencing participants to unnaturally restrain themselves from actively participating or responding in order to allow in-transit video and audio data from other participants to arrive at their endpoints. 
     The end-to-end signal delays include acquisition delays (e.g., the time it takes to fill up a buffer in an A/D converter), coding delays, transmission delays (the time it takes to submit a packet-full of data to the network interface controller of an endpoint), and transport delays (the time a packet travels in a communication network from endpoint to endpoint). Additionally, signal-processing times through mediating MCUs contribute to the total end-to-end delay in the given system. 
     An MCU&#39;s primary tasks are to mix the incoming audio signals so that a single audio stream is transmitted to all participants, and to mix video frames or pictures transmitted by individual participants/endpoints into a common composite video frame stream, which includes a picture of each participant. It is noted that the terms frame and picture are used interchangeably herein, and further that coding of interlaced frames as individual fields or as combined frames (field-based or frame-based picture coding) can be incorporated as is obvious to persons skilled in the art. The MCUs, which are deployed in conventional communication networks systems, only offer a single common resolution (e.g., CIF or QCIF resolution) for all the individual pictures mixed into the common composite video frame distributed to all participants in a videoconferencing session. Thus, conventional communication networks systems do not readily provide customized videoconferencing functionality by which a participant can view other participants at different resolutions. Such desirable functionality allows the participant, for example, to view another specific participant (e.g., a speaking participant) in CIF resolution and view other, silent participants in QCIF resolution. MCUs can be configured to provide this desirable functionality by repeating the video mixing operation, as many times as the number of participants in a videoconference. However, in such configurations, the MCU operations introduce considerable end-to-end delay. Further, the MCU must have sufficient digital signal processing capability to decode multiple audio streams, mix, and re-encode them, and also to decode multiple video streams, composite them into a single frame (with appropriate scaling as needed), and re-encode them again into a single stream. Video conferencing solutions (such as the systems commercially marketed by Polycom Inc., 4750 Willow Road, Pleasanton, Calif. 94588, and Tandberg, 200 Park Avenue, New York, N.Y. 10166) must use dedicated hardware components to provide acceptable quality and performance levels. 
     The performance levels of and the quality delivered by a videoconferencing solution are also a strong function of the underlying communication network over which it operates. Videoconferencing solutions, which use ITU H.261, H.263, and H.264 standard video codecs, require a robust communication channel with little or no loss for delivering acceptable quality. The required communication channel transmission speeds or bitrates can range from 64 Kbps up to several Mbps. Early videoconferencing solutions used dedicated ISDN lines, and newer systems often utilize high-speed Internet connections (e.g., fractional T1, T1, T3, etc.) for high-speed transmission. Further, some videoconferencing solutions exploit Internet Protocol (“IP”) communications, but these are implemented in a private network environment to ensure bandwidth availability. In any case, conventional videoconferencing solutions incur substantial costs associated with implementing and maintaining the dedicated high-speed networking infrastructure needed for quality transmissions. 
     The costs of implementing and maintaining a dedicated videoconferencing network are avoided by recent “desktop videoconferencing” systems, which exploit high bandwidth corporate data network connections (e.g., 100 Mbit, Ethernet). In these desktop videoconferencing solutions, common personal computers (PCs), which are equipped with USB-based digital video cameras and appropriate software applications for performing encoding/decoding and network transmission, are used as the participant/endpoint terminals. 
     Recent advances in multimedia and telecommunications technology involve integration of video communication and conferencing capabilities with Internet Protocol (“IP”) communication systems such as IP PBX, instant messaging, web conferencing, etc. In order to effectively integrate video conferencing into such systems, both point-to-point and multipoint communications must be supported. However, the available network bandwidth in IP communication systems can fluctuate widely (e.g., depending on time of day and overall network load), making these systems unreliable for the high bandwidth transmissions required for video communications. Further, videoconferencing solutions implemented on IP communication systems must accommodate both network channel heterogeneity and endpoint equipment diversity associated with the Internet system. For example, participants may access videoconferencing services over IP channels having very different bandwidths (e.g., DSL vs. Ethernet) using a diverse variety of personal computing devices. 
     The communication networks on which videoconferencing solutions are implemented can be categorized as providing two basic communication channel architectures. In one basic architecture, a guaranteed quality of service (QoS) channel is provided via a dedicated direct or switched connection between two points (e.g., ISDN connections, T1 lines, and the like). Conversely, in the second basic architecture, the communication channels do not guarantee QoS, but are only “best-effort” packet delivery channels such as those used in Internet Protocol (IP)-based networks (e.g., Ethernet LANs). 
     Implementing video conferencing solutions on IP-based networks may be desirable, at least due to the low cost, high total bandwidth, and widespread availability of access to the Internet. As noted previously, IP-based networks typically operate on a best-effort basis, i.e., there is no guarantee that packets will reach their destination, or that they will arrive in the order they were transmitted. However, techniques have been developed to provide different levels of quality of service (QoS) over the putatively best-effort channels. The techniques may include protocols such as DiffSery for specifying and controlling network traffic by class so that certain types of traffic get precedence and RSVP. These protocols can ensure certain bandwidth and/or delays for portions of the available bandwidth. Techniques such as forward error correction (FEC) and automatic repeat request (ARQ) mechanisms may also be used to improve recovery mechanisms for lost packet transmissions and to mitigate the effects of packet loss. 
     Implementing video conferencing solutions on IP-based networks requires consideration of the video codecs used. Standard video codecs such as the standard H.261, H.263 codecs designated for videoconferencing and standard MPEG-1 and MPEG-2 Main Profile codecs designated for Video CDs and DVDs, respectively, are designed to provide a single bitstream (“single-layer”) at a fixed bitrate. Some of these codecs may be deployed without rate control to provide a variable bitrate stream (e.g., MPEG-2, as used in DVDs). However, in practice, even without rate control, a target operating bitrate is established depending on the specific infrastructure. These video codecs designs are based on the assumption that the network is able to provide a constant bitrate, and a practically error-free channel between the sender and the receiver. The H-series Standard codecs, which are designed specifically for person-to-person communication applications, offer some additional features to increase robustness in the presence of channel errors, but are still only tolerant to a very small percentage of packet losses (typically only up to 2-3%). 
     Further, the standard video codecs are based on “single-layer” coding techniques, which are inherently incapable of exploiting the differentiated QoS capabilities provided by modern communication networks. An additional limitation of the single-layer coding techniques for video communications is that even if a lower spatial resolution display is required or desired in an application, a full resolution signal must be received and decoded with downscaling performed at a receiving endpoint or MCU. This wastes bandwidth and computational resources. 
     In contrast to the aforementioned single-layer video codecs, in “scalable” video codecs based on “multi-layer” coding techniques, two or more bitstreams are generated for a given source video signal: a base layer and one or more enhancement layers. The base layer may be a basic representation of the source signal at a minimum quality level. The minimum quality representation may be reduced in the SNR (quality), spatial, or temporal resolution aspects or a combination of these aspects of the given source video signal. The one or more enhancement layers correspond to information for increasing the quality of the SNR (quality), spatial, or temporal resolution aspects of the base layer. Scalable video codecs have been developed in view of heterogeneous network environments and/or heterogeneous receivers. The base layer can be transmitted using a reliable channel, i.e., a channel with guaranteed Quality of Service (QoS). Enhancement layers can be transmitted with reduced or no QoS. The effect is that recipients are guaranteed to receive a signal with at least a minimum level of quality (the base layer signal). Similarly, with heterogeneous receivers that may have different screen sizes, a small picture size signal may be transmitted to, e.g., a portable device, and a full size picture may be transmitted to a system equipped with a large display. 
     Standards such as MPEG-2 specify a number of techniques for performing scalable coding. However, practical use of “scalable” video codecs has been hampered by the increased cost and complexity associated with scalable coding, and the lack of widespread availability of high bandwidth IP-based communication channels suitable for video. 
     Consideration is now being given to developing improved scalable codec solutions for video conferencing and other applications. Desirable scalable codec solutions will offer improved bandwidth, temporal resolution, spatial quality, spatial resolution, and computational power scalability. Attention is in particular directed to developing scalable video codecs that are consistent with simplified MCU architectures for versatile videoconferencing applications. Desirable scalable codec solutions will enable zero-delay MCU architectures that allow cascading of MCUs in electronic networks with no or minimal end-to-end delay penalties. 
     SUMMARY OF THE INVENTION 
     The present invention provides scalable video coding (SVC) systems and methods (collectively, “solutions”) for point-to-point and multipoint conferencing applications. The SVC solutions provide a coded “layered” representation of a source video signal at multiple temporal, quality, and spatial resolutions. These resolutions are represented by distinct layer/bitstream components that are created by endpoint/terminal encoders. 
     The SVC solutions are designed to accommodate diversity in endpoint/receivers devices and in heterogeneous network characteristics, including, for example, the best-effort nature of networks such as those based on the Internet Protocol. The scalable aspects of the video coding techniques employed allow conferencing applications to adapt to different network conditions, and also accommodate different end-user requirements (e.g., a user may elect to view another user at a high or low spatial resolution). 
     Scalable video codec designs allow error-resilient transmission of video in point-to-point and multipoint scenarios, and allow a conferencing bridge to provide continuous presence, rate matching, error localization, random entry and personal layout conferencing features, without decoding or recoding in-transit video streams and without any decrease in the error resilience of the stream. 
     An endpoint terminal, which is designed for video communication with other endpoints, includes video encoders/decoders that can encode a video signal into one or more layers of a multi-layer scalable video fat mat for transmission. The video encoders/decoders can correspondingly decode received video signal layers, simultaneously or sequentially, in as many video streams as the number of participants in a videoconference. The terminal maybe implemented in hardware, software, or a combination thereof in a general-purpose PC or other network access device. The scalable video codecs incorporated in the terminal may be based on coding methods and techniques that are consistent with or based on industry standard encoding methods such as H.264. 
     In an H.264 based SVC solution, a scalable video codec creates a base layer that is based on standard H.264 AVC encoding. The scalable video codec further creates a series of SNR enhancement layers by successively encoding, using again H.264 AVC, the difference between the original signal and the one coded at the previous layer with an appropriate offset. In a version of this scalable video codec, DC values of the direct cosine transform (DCT) coefficients are not coded in the enhancement layers, and further, a conventional deblocking filter is not used. 
     In an SVC solution, which is designed to use SNR scalability as a means of implementing spatial scalability, different quantization parameters (QP) are selected for the base and enhancement layers. The base layer, which is encoded at higher QP, is optionally low-pass filtered and downsampled for display at receiving endpoints/terminals. 
     In another SVC solution, the scalable video codec is designed as a spatially scalable encoder in which a reconstructed base layer H.264 low-resolution signal is upsampled at the encoder and subtracted from the original signal. The difference is fed to the standard encoder operating at high resolution, after being offset by a set value. In another version, the upsampled H.264 low-resolution signal is used as an additional possible reference frame in the motion estimation process of a standards-based high-resolution encoder. 
     The SVC solutions may involve adjusting or changing threading modes or spatial scalability modes to dynamically respond to network conditions and participants&#39; display preferences. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features of the invention, its nature, and various advantages will be more apparent from the following detailed description of the preferred embodiments and the accompanying drawing in which: 
         FIGS. 1A and 1B  are schematic diagrams illustrating exemplary architectures of a videoconferencing system, in accordance with the principles of the present invention. 
         FIG. 2  is a block diagram illustrating an exemplary end-user terminal, in accordance with the principles of the present invention. 
         FIG. 3  is a block diagram illustrating an exemplary architecture of an encoder for the base and temporal enhancement layers (i.e., layers 0 though 2), in accordance with the principles of the present invention. 
         FIG. 4  is a block diagram illustrating an exemplary layered picture coding structure for the base, temporal enhancement, and SNR or spatial enhancement layers, in accordance with the principles of the present invention. 
         FIG. 5  is a block diagram illustrating the structure of an exemplary SNR enhancement layer encoder, in accordance with the principles of the present invention. 
         FIG. 6  is a block diagram illustrating the structure of an exemplary single-loop SNR video encoder, in accordance with the principles of the present invention. 
         FIG. 7  is a block diagram illustrating an exemplary structure of a base layer for a spatial scalability video encoder, in accordance with the principles of the present invention. 
         FIG. 8  is a block diagram illustrating an exemplary structure of a spatial scalability enhancement layer video encoder, in accordance with the principles of the present invention. 
         FIG. 9  is a block diagram illustrating an exemplary structure of a spatial scalability enhancement layer video encoder with inter-layer motion prediction, in accordance with the principles of the present invention. 
         FIGS. 10 and 11  are block diagrams illustrating exemplary base layer and SNR enhancement layer video decoders, respectively, in accordance with the principles of the present invention. 
         FIG. 12  is a block diagram illustrating an exemplary SNR enhancement layer, single-loop video decoder, in accordance with the principles of the present invention. 
         FIG. 13  is block diagram illustrating an exemplary spatial scalability enhancement layer video decoder, in accordance with the principles of the present invention. 
         FIG. 14  is block diagram illustrating an exemplary video decoder for spatial scalability enhancement layers with inter-layer motion prediction, in accordance with the principles of the present invention. 
         FIGS. 15 and 16  are block diagrams illustrating exemplary alternative layered picture coding structures and threading architectures, in accordance with the principles of the present invention. 
         FIG. 17  is a block diagram illustrating an exemplary Scalable Video Coding Server (SVCS), in accordance with the principles of the present invention. 
         FIG. 18  is a schematic diagram illustrating the operation of an SVCS switch, in accordance with the principles of the present invention. 
         FIGS. 19 and 20  are illustrations of exemplary SVCS Switch Layer and Network Layer Configuration Matrices, in accordance with the principles of the present invention. 
     
    
    
     Throughout the figures, unless otherwise stated, the same reference numerals and characters are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the present invention will now be described in detail with reference to the figures, it is done so in connection with the illustrative embodiments. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides systems and techniques for scalable video coding (SVC) of video data signals for multipoint and point-to-point video conferencing applications. The SVC systems and techniques (collectively “solutions”) are designed to allow the tailoring or customization of delivered video data in response to different user participants/endpoints, network transmission capabilities, environments, or other requirements in a videoconference. The inventive SVC solutions provide compressed video data in a multi-layer format, which can be readily switched layer-by-layer between conferencing participants using convenient zero- or low-algorithmic delay switching mechanisms. Exemplary zero- or low-algorithmic delay switching mechanisms—Scalable Video Coding Servers (SVCS), are described in co-filed International Patent application No. PCT/US2006/028366. 
       FIGS. 1A and 1B  show exemplary videoconferencing system  100  arrangements based on the inventive SVC solutions. Videoconferencing system  100  may be implemented in a heterogeneous electronic or computer network environment, for multipoint and point-to-point client conferencing applications. System  100  uses one or more networked servers (e.g., an SVCS or MCU  110 ), to coordinate the delivery of customized data to conferencing participants or clients  120 ,  130 , and  140 . As described in co-pending International Patent Application PCT/US2006/028366, MCU  110  may coordinate the delivery of a video stream  150  generated by endpoint  140  for transmission to other conference participants. In system  100 , a video stream is first suitably coded or scaled down using the inventive SVC techniques into a multiplicity of data components or layers. The multiple data layers may have differing characteristics or features (e.g., spatial resolutions, frame rates, picture quality, signal-to-noise ratio qualities (SNR), etc.). The differing characteristics or features of the data layers may be suitably selected in consideration, for example, of the varying individual user requirements and infrastructure specifications in the electronic network environment (e.g., CPU capabilities, display size, user preferences, and bandwidths). MCU  110  is suitably configured to select an appropriate amount of information (i.e., SVC layers) for each particular participant/recipient in the conference from a received data stream (e.g., SVC video stream  150 ), and to forward only the selected or requested amounts of information/layers to the respective participants/recipients  120 - 130 . MCU  110  may be configured to make the suitable selections in response to receiving-endpoint requests (e.g., the picture quality requested by individual conference participants) and upon consideration of network conditions and policies. 
     This customized data selection and forwarding scheme exploits the internal structure of the SVC video stream, which allows clear division of the video stream into multiple layers having different resolutions, frame rates, and/or bandwidths, etc.  FIG. 1B , which is reproduced from the referenced patent application PCT/US2006/028366, shows an exemplary internal structure of SVC video stream  150  that represents a media input of endpoint  140  to the conference. The exemplary internal structure of SVC video stream  150  includes a “base” layer  150   b , and one or more distinct “enhancement” layers  150   a.    
       FIG. 2  shows an exemplary participant/endpoint terminal  140 , which is designed for use with SVC-based videoconferencing systems (e.g., system  100 ). Terminal  140  includes human interface input/output devices (e.g., a camera  210 A, a microphone  210 B, a video display  250 C, a speaker  250 D), and a network interface controller card (NIC)  230  coupled to input and output signal multiplexer and demultiplexer units (e.g., packet MUX  220 A and packet DMUX  220 B). NIC  230  may be a standard hardware component, such as an Ethernet LAN adapter, or any other suitable network interface device. 
     Camera  210 A and microphone  210 B are designed to capture participant video and audio signals, respectively, for transmission to other conferencing participants. Conversely, video display  250 C and speaker  250 D are designed to display and play back video and audio signals received from other participants, respectively. Video display  250 C may also be configured to optionally display participant/terminal  140 &#39;s own video. Camera  210 A and microphone  210 B outputs are coupled to video and audio encoders  210 G and  210 H via analog-to-digital converters  210 E and  210 F, respectively. Video and audio encoders  210 G and  210 H are designed to compress input video and audio digital signals in order to reduce the bandwidths necessary for transmission of the signals over the electronic communications network. The input video signal may be live, or pre-recorded and stored video signals. 
     Video encoder  210 G has multiple outputs connected directly to packet MUX  220 A. Audio encoders  210 H output is also connected directly to packet MUX  220 A. The compressed and layered video and audio digital signals from encoders  210 G and  210 H are multiplexed by packet MUX  220 A for transmission over the communications network via NIC  230 . Conversely, compressed video and audio digital signals received over the communications network by NIC  230  are forwarded to packet DMUX  220 B for demultiplexing and further processing in terminal  140  for display and playback over video display  250 C and speaker  250 D. 
     Captured audio signals may be encoded by audio encoder  210 H using any suitable encoding techniques including known techniques, for example, G.711 and MPEG-1. In an implementation of videoconferencing system  100  and terminal  140 , G.711 encoding is preferred for audio encoding. Captured video signals are encoded in a layered coding format by video encoder  210 G using the SVC techniques described herein. Packet MUX  220 A may be configured to multiplex the input video and audio signals using, for example, the RTP protocol or other suitable protocols. Packet MUX  220 A also may be configured to implement any needed QoS-related protocol processing. 
     In system  100 , each stream of data from terminal  140  is transmitted in its own virtual channel (or port number in IP terminology) over the electronics communication network. In an exemplary network configuration, QoS may be provided via Differentiated Services (DiffServ) for specific virtual channels or by any other similar QoS-enabling technique. The required QoS setups are performed prior to use of the systems described herein. DiffSery (or the similar QoS-enabling technique used) creates two different categories of channels implemented via or in network routers (not shown). For convenience in description, the two different categories of channels are referred to herein as “high reliability” (HRC) and “low reliability” (LRC) channels, respectively. In the absence of an explicit method for establishing an HRC or if the HRC itself is not reliable enough, the endpoint (or the MCU  110  on behalf of the endpoint) may (i) proactively transmit the information over the HRC repeatedly (the actual number of repeated transmissions may depend on channel error conditions), or (ii) cache and retransmit information upon the request of a receiving endpoint or SVCS, for example, in instances where information loss in transmission is detected and reported immediately. These methods of establishing an HRC can be applied in the client-to-MCU, MCU-to-client, or MCU-to-MCU connections individually or in any combination, depending on the available channel type and conditions. 
     For use in a multi-participant videoconferencing system, terminal  140  is configured with one or more pairs of video and audio decoders (e.g., decoders  230 A and  230 B) designed to decode signals received from the conferencing participants who are to be seen or heard at terminal  140 . The pairs of decoders  230 A and  230 B may be designed to process signals individually participant-by-participant or to sequentially process a number of participant signals. The configuration or combinations of pairs of video and audio decoders  230 A and  230 B included in terminal  140  may be suitably selected to process all participant signals received at terminal  140  with consideration of the parallel and/or sequential processing design features of the encoders. Further, packet DMUX  220 B may be configured to receive packetized signals from the conferencing participants via NIC  230 , and to forward the signals to appropriate pairs of video and audio decoders  230 A and  230 B for parallel and/or sequential processing. 
     Further in terminal  140 , audio decoder  230 B outputs are connected to audio mixer  240  and a digital-to-analog converter (DA/C)  250 B, which drives speaker  250 D to play back received audio signals. Audio mixer  240  is designed to combine individual audio signals into a single signal for playback. Similarly, video decoder  230 A outputs are combined in frame buffer  250 A by a compositor  260 . A combined or composite video picture from frame buffer  250 A is displayed on monitor  250 C. 
     Compositor  260  may be suitably designed to position each decoded video picture at a corresponding designated position in the composite frame or displayed picture. For example, monitor  250 C display may be split into four smaller areas. Compositor  260  may obtain pixel data from each of video decoders  230 A in terminal  140  and place the pixel data in an appropriate frame buffer  250 A position (e.g., filling up the lower right picture). To avoid double buffering (e.g., once at the output of decoder  230 B and once at frame buffer  250 A), compositor  260  may, for example, be configured as an address generator that drives the placement of output pixels of decoder  230 B. Alternative techniques for optimizing the placement of individual video decoder  230 A outputs on display  210 C may also be used to similar effect. 
     It will be understood that the various terminal  140  components shown in  FIG. 2  may be implemented in any suitable combination of hardware and/or software components, which are suitably interfaced with each other. The components may be distinct stand-alone units or integrated with a personal computer or other device having network access capabilities. 
     With reference to video encoders used in terminal  140  for scalable video coding,  FIGS. 3-9  respectively show various scalable video encoders or codecs  300 - 900  that may be deployed in terminal  140 . 
       FIG. 3  shows exemplary encoder architecture  300  for compressing input video signals in a layered coding format (e.g., layers L0, L1, and L2 in SVC terminology, where L0 is the lowest frame rate). Encoder architecture  300  represents a motion-compensated, block-based transform codec based, for example, on a standard H.264/MPEG-4 AVC design or other suitable codec designs. Encoder architecture  300  includes a FRAME BUFFERS block  310 , an ENC REF CONTROL block  320 , and a DeBlocking Filter block  360  in addition to conventional “text-book” variety video coding process blocks  330  for motion estimation (ME), motion compensation (MC), and other encoding functions. The motion-compensated, block-based codec used in system  100 /terminal  140  may be a single-layer temporally predictive codec, which has a regular structure of I, P, and B pictures. A picture sequence (in display order) may, for example, be “IBBPBBP”. In the picture sequence, the ‘P’ pictures are predicted from the previous P or I picture, whereas the B pictures are predicted using both the previous and next P or I picture. Although the number of B pictures between successive I or P pictures can vary, as can the rate in which I pictures appear, it is not possible, for example, for a P picture to use as a reference for prediction another P picture that is earlier in time than the most recent one. Standard H.264 coding advantageously provides an exception in that two reference picture lists are maintained by the encoder and decoder, respectively. This exception is exploited by the present invention to select which pictures are used as references and also which references are used for a particular picture that is to be coded. In  FIG. 3 , FRAME BUFFERS block  310  represents memory for storing the reference picture list(s). ENC REF CONTROL block  310  is designed to determine which reference picture is to be used for the current picture at the encoder side. 
     The operation of ENC REF CONTROL block  310  is placed in context further with reference to an exemplary layered picture coding “threading” or “prediction chain” structure shown in  FIG. 4 . ( FIGS. 8-9  show alternative threading structures). Codecs  300  utilized in implementations of the present invention may be configured to generate a set of separate picture “threads” (e.g., a set of three threads  410 - 430 ) in order to enable multiple levels of temporal scalability resolutions (e.g., L0-L2) and other enhancement resolutions (e.g., S0-S2). A thread or prediction chain is defined as a sequence of pictures that are motion-compensated using pictures either from the same thread, or pictures from a lower level thread. The arrows in  FIG. 4  indicate the direction, source, and target of prediction for three threads  410 - 430 . Threads  410 - 420  have a common source L0 but different targets and paths (e.g., targets L2, L2, and L0, respectively). The use of threads allows the implementation of temporal scalability, since any number of top-level threads can be eliminated without affecting the decoding process of the remaining threads. 
     It will be noted that in encoder  300 , according to H.264, ENC REF CONTROL block may use only P pictures as reference pictures. However, B pictures also may be used with accompanying gains in overall compression efficiency. Using even a single B picture in the set of threads (e.g., by having L2 be coded as a B picture) can improve compression efficiency. In traditional interactive communications, the use of B pictures with prediction from future pictures increases the coding delay and is therefore avoided. However, the present invention allows the design of MCUs with practically zero processing delay. (See co-filed U.S. patent application Ser. No. SCVS). With such MCUs, it is possible to utilize B pictures and still operate with an end-to-end delay that is lower than state-of-the-art traditional systems. 
     In operation, encoder  300  output L0 is simply a set of P pictures spaced four pictures apart. Output L1 has the same frame rate as L0, but only prediction based on the previous L0 picture is allowed. Output L2 pictures are predicted from the most recent L0 or L1 picture. Output L0 provides one fourth (1:4) of the full temporal resolution, L1 doubles the L0 frame rate (1:2), and L2 doubles the L0+L1 frame rate (1:1). A lesser number (e.g., less than 3, L0-L2) or an additional number of layers may be similarly constructed by encoder  300  to accommodate different bandwidth/scalability requirements or different specifications of implementations of the present invention. 
     In accordance with the present invention, for additional scalability, each compressed temporal video layer (e.g., L0-L1) may include or be associated with one or more additional components related to SNR quality scalability and/or spatial scalability.  FIG. 4  shows one additional enhancement layer (SNR or spatial). Note that this additional enhancement layer will have three different components (S0-S2), each corresponding to the three different temporal layers (L0-L2). 
       FIGS. 5 and 6  show SNR scalability encoders  500  and  600 , respectively.  FIGS. 7-9  show spatial scalability encoders  700 - 900 , respectively. It will be understood that SNR scalability encoders  500  and  600  and spatial scalability encoders  700 - 900  are based on and may use the same processing blocks (e.g., blocks  330 ,  310  and  320 ) as encoder  300  ( FIG. 3 ). 
     It is recognized that for the base layer of an SNR scalable codec, the input to the base layer codec is a full resolution signal ( FIGS. 5-6 ). In contrast, for the base layer of a spatial scalability codec, the input to the base layer codec is a downsampled version of the input signal  FIGS. 7-9 . It is also noted that the SNR/spatial quality enhancement layers S0-S2 may be coded according to the forthcoming ITU-T H.264 Annex F standard or other suitable technique. 
       FIG. 5  shows the structure of an exemplary SNR enhancement encoder  500 , which is similar to the structure of layered encoder  300  based on H.264 shown in  FIG. 3 . It will, however, be noted that the input to the SNR enhancement layer coder  500  is the difference between the original picture (INPUT,  FIG. 3 ) and the reconstructed coded picture (REF,  FIG. 3 ) as recreated at the encoder. 
       FIG. 5  also shows use of encoder  500  based on H.264 for encoding the coding error of the previous layers. Non-negative inputs are required for such encoding. To ensure this, the input (INPUT-REF) to encoder  500  is offset by a positive bias (e.g., by OFFSET  340 ). The positive bias is removed after decoding and prior to the addition of the enhancement layer to the base layer. A deblocking filter that is typically used in H.264 codec implementations (e.g., Deblocking filter  360 ,  FIG. 3 ) is not used in encoder  500 . Further, to improve subjective coding efficiency, DC direct cosine transform (DCT) coefficients in the enhancement layer may be optionally ignored or eliminated in encoder  500 . Experimental results indicate that the elimination of the DC values in an SNR enhancement layer (S0-S2) does not adversely impact picture quality, possibly due to the already fine quantization performed at the base layer. A benefit of this design is that the exactly same encoding/decoding hardware or software can be used both for the base and SNR enhancement layers. In a similar fashion—spatial scalability (at any ratio) may be introduced by applying the H.264 base layer coding to a downsampled image and upsampling the reconstructed image before calculating the residual. Further, standards other than H.264 can be used for compressing both layers. 
     In the codecs of the present invention, in order to decouple the SNR and temporal scalabilities, all motion prediction within a temporal layer and across temporal layers may be performed using the base layer streams only. This feature is shown in  FIG. 4  by the open arrowheads  415  indicating temporal prediction in the base layer block (L) rather than in the combination of L and S blocks. For this feature, all layers may be coded at CIF resolutions. Then, QCIF resolution pictures may be derived by decoding the base layer stream having a certain temporal resolution, and downsampling in each spatial dimension by a dyadic factor (2), using appropriate low-pass filtering. In this manner, SNR scalability can be used to also provide spatial scalability. It will be understood that CIF/QCIF resolutions are referred to only for purposes of illustration. Other resolutions (e.g., VGA/QVGA) can be supported by the inventive codecs without any change in codec design. The codecs may also include traditional spatial scalability features in the same or similar manner as described above for the inclusion of the SNR scalability feature. Techniques provided by MPEG-2 or H.264 Annex F may be used for including traditional spatial scalability features. 
     The architecture of codecs designed to decouple the SNR and temporal scalabilities described above, allows frame rates in ratios of 1:4 (L0 only), 1:2 (L0 and L1), or 1:1 (all three layers). A 100% bitrate increase is assumed for doubling the frame rate (base is 50% of total), and a 150% increase for adding the S layer at its scalability point (base is 40% of total). In a preferred implementation, the total stream may, for example, operate at 500 Kbps, with the base layer operating at 200 Kbps. A rate load of 200/4=50 Kbps per frame may be assumed for the base layer, and (500−200)/4=75 Kbps for each frame. It will be understood that the aforementioned target bitrates and layer bitrate ratio values are exemplary and have been specified only for purposes of illustrating the features of the present invention, and that the inventive codecs can be easily adapted to other target bitrates, or layer bitrate ratios. 
     Theoretically, up to 1:10 scalability (total vs. base) is available when the total stream and the base layer operate at 500 Kbps and 200 Kbps, respectively. TABLE I shows examples of the different scalability options available when SNR scalability is used to provide spatial scalability. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Scalability Options 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 QCIF* (Kbps) 
                 CIF (Kbps) 
               
               
                   
                 Temporal (fps) 
                 L only 
                 L to L + S 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 7.5 (L0) 
                 50 
                  50-125 
               
               
                   
                  15 (L0 + L1) 
                 100 
                 100-250 
               
               
                   
                  30 (L0 + L1 + L2) 
                 200 
                 200-500 
               
               
                   
                   
               
               
                   
                 *Although no QCIF component is present in the bitstreams, it can be provided by scaling down the CIF image by a factor of 2. In this example, the lower resolution of QCIF presumably allows this operation to be performed from the base CIF layer without noticeable effect on quality. 
               
            
           
         
       
     
       FIG. 6  shows alternate SNR scalable encoder  600 , which is based on a single encoding loop scheme. The structure and operation of SNR scalable encoder  600  is based on that of encoder  300  ( FIG. 3 ). Additionally in encoder  600 , DCT coefficients that are quantized by Q0 are inverse-quantized and subtracted from the original unquantized coefficients to obtain the residual quantization error (QDIFF  610 ) of the DCT coefficients. The residual quantization error information (QDIFF  610 ) is further quantized with a finer quantizer Q1 (Block  620 ), entropy coded (VLC/BAC), and output as the SNR enhancement layer S. It is noted that there is a single coding loop in operation, i.e., the one operating at the base layer. 
     Terminal  140 /video  230  encoders may be configured to provide spatial scalability enhancement layers in addition to or instead of the SNR quality enhancement layers. For encoding spatial scalability enhancement layers, the input to the encoder is the difference between the original high-resolution picture and the upsampled reconstructed coded picture as created at the encoder. The encoder operates on a downsampled version of the input signal.  FIG. 7  shows exemplary encoder  700  for encoding the base layer for spatial scalability. Encoder  700  includes a downsampler  710  at the input of low-resolution base layer encoder  720 . For a full resolution input signal at CIF resolution, base layer encoder  720  may with suitable downsampling operate at QCIF, HCIF (half CIF), or any other resolution lower than CIF. In an exemplary mode, base layer encoder  720  may operate at HCIF. HCIF-mode operation requires downsampling of a CIF resolution input signal by about a factor of √2 in each dimension, which reduces the total number of pixels in a picture by about one-half of the original input. It is noted that in a video conferencing application, if a QCIF resolution is desired for display purposes, then the decoded base layer will have to be further downsampled from HCIF to QCIF. 
     It is recognized that an inherent difficulty in optimizing the scalable video encoding process for video conferencing applications is that there are two or more resolutions of the video signal being transmitted. Improving the quality of one of the resolutions may result in corresponding degradation of the quality of the other resolution(s). This difficulty is particularly pronounced for spatially scalable coding, and in current art video conferencing systems in which the coded resolution and the display resolutions are identical. The inventive technique of decoupling the coded signal resolution from the intended display resolution provides yet another tool in a codec designer&#39;s arsenal to achieve a better balance between the quality and bitrates associated with each of the resolutions. According to the present invention, the choice of coded resolution for a particular codec may be obtained by considering the rate-distortion (R-D) performance of the codec across different spatial resolutions, taking into account the total bandwidth available, the desired bandwidth partition across the different resolutions, and the desired quality difference differential that each additional layer should provide. 
     Under such a scheme, a signal may be coded at CIF and one-third CIF (⅓CIF) resolutions. Both CIF and HCIF resolution signals may be derived for display from the CIF-coded signal. Further, both ⅓CIF and QCIF resolution signals may similarly be derived for display from the ⅓CIF-coded signal. The CIF and ⅓CIF resolution signals are available directly from the decoded signals, whereas the latter HCIF and QCIF resolution signals may be obtained upon appropriate downsampling of the decoded signals. Similar schemes may also be applied in the case of other target resolutions (e.g., VGA and one-third VGA, from which half VGA and quarter VGA can be derived). 
     The schemes of decoupling the coded signal resolution from the intended display resolution, together with the schemes for threading video signal layers ( FIG. 4 , and  FIGS. 15 and 16 ), provide additional possibilities for obtaining target spatial resolutions with different bitrates, in accordance with the present invention. For example, in a video signal coding scheme, spatial scalability may be used to encode the source signal at CIF and ⅓CIF resolutions. SNR and temporal scalabilities may be applied to the video signal as shown in  FIG. 4 . Further, the SNR encoding used may be a single loop or a double loop encoder (e.g., encoder  600   FIG. 6  or encoder  500   FIG. 5 ), or may be obtained by data partitioning (DP). The double loop or DP encoding schemes will likely introduce drift whenever data is lost or removed. However, the use of the layering structure will limit the propagation of the drift error until the next L0 picture, as long as the lost or removed data belongs to the L1, L2, S1, or S2 layers. Further taking into account the fact that the perception of errors is reduced when the spatial resolution of the displayed video signal is reduced, it is possible to obtain a low bandwidth signal by eliminating or removing data from the L1, L2, S1, and S2 layers, decoding the ⅓CIF resolution, and displaying it downsampled at a QCIF resolution. The loss of data because of downsampling will cause errors in the corresponding L1/S1 and L2/S2 pictures, and will also propagate errors to future pictures (until the next L0 picture), but the fact that the display resolution is reduced makes the quality degradation less visible to a human observer. Similar schemes may be applied to the CIF signal, for display at HCIF, ⅔ CIF or at any other desired resolution. These schemes advantageously allow the use of quality scalability to effect spatial scalability at various resolutions, and at various bitrates. 
       FIG. 8  shows the structure of an exemplary spatially scalable enhancement layer encoder  800 , which, like encoder  500 , uses the same H.264 encoder structure for encoding the coding error of the previous layers but includes an upsampler block  810  on the reference (REF) signal. Since non-negative input is assumed for such an encoder, the input values are offset (e.g., by offset  340 ) prior to coding. Values that still remain negative are clipped to zero. The offset is removed after decoding and prior to the addition of the enhancement layer to the upsampled base layer. 
     For the spatial enhancement layer encoding, like for the SNR layer encoding ( FIG. 6 ), it may be advantageous to use frequency weighting in the quantizers (Q) of the DCT coefficients. Specifically, coarser quantization can be used for the DC and its surrounding AC coefficients. For example, a doubling of the quantizer step size for the DC coefficient may be very effective. 
       FIG. 9  shows the exemplary structure of another spatially scalable video encoder  900 . In encoder  900 , unlike in encoder  800 , the upsampled reconstructed base layer picture (REF) is not subtracted from the input, but instead serves as an additional possible reference picture in the motion estimation and mode selection blocks  330  of the enhancement layer encoder. Encoder  900  can accordingly be configured to predict the current full resolution picture either from a previous coded full resolution picture (or future picture, for B pictures), or an upsampled version of the same picture coded at the lower spatial resolution (inter-layer prediction). It should be noted that, whereas encoder  800  can be implemented using the same codec for the base and enhancement layers with only the addition of downsampler  710 , upsampler  810 , and offset  340  blocks, encoder  900  requires that the enhancement layer encoder&#39;s motion estimation (ME) block  330 * is modified. It is also noted that enhancement layer encoder  900  operates on the regular pixel domain, rather than a differential domain. 
     It is also possible to combine the predictions from a previous high resolution picture and the upsampled base layer picture by using the B picture prediction logic of a standard single-layer encoder, such as an H.264 encoder. This can be accomplished by modifying the B picture prediction reference for the high resolution signal so that the first picture is the regular or standard prior high resolution picture, and the second picture is the upsampled version of the base layer picture. The encoder then performs prediction as if the second picture is a regular B picture, thus utilizing all the high-efficiency motion vector prediction and coding modes (e.g., spatial and temporal direct modes) of the encoder. Note that in H.264, “B” picture coding stands for ‘bi-predictive’ rather than ‘bi-directional’, in the sense that the two reference pictures could both be past or future pictures of the picture being coded, whereas in traditional ‘bi-directional’ B picture coding (e.g., MPEG-2) one of the two reference pictures is a past picture and the other is a future picture. This embodiment allows the use of a standard encoder design, with minimal changes that are limited to the picture reference control logic and the upsampling module. 
     In an implementation of the present invention, the SNR and spatial scalability encoding modes may be combined in one encoder. For such an implementation, video-threading structures (e.g., shown in two dimensions in  FIG. 4 ) may be expanded in a third dimension, corresponding to the additional third scalability layer (SNR or spatial). An implementation in which SNR scalability is added on the full resolution signal of a spatially scalable codec may be attractive in terms of range of available qualities and bitrates. 
       FIGS. 10-14  show exemplary architectures for a base layer decoder  1000 , SNR enhancement layer decoder  1100 , a single-loop SNR enhancement layer decoder  1200 , a spatially scalable enhancement layer decoder  1300  and spatially scalable enhancement layer decoder  1400  with interlayer motion prediction, respectively. These decoders complement encoders  300 ,  500 ,  600 ,  700 ,  800  and  900 . Decoders  1000 ,  1100 ,  1200 ,  1300 , and  1400  may be included in terminal  140  decoders  230 A as appropriate or needed. 
     The scale video coding/decoding configurations of terminal  140  present a number of options for transmitting the resultant layers over the HRC and LRC in system  100 . For example, (L0 and S0) layers or (L0, S0 and L1) layers may be transmitted over HRC. Alternate combinations also may be used as desired, upon due consideration of network conditions, and the bandwidths of high and low reliability channels. For example, depending on network conditions, it may be desirable to code S0 intra-mode but not to transmit S0 in a protected HRC. In such case, the frequency of intra-mode coding, which does not involve prediction, may depend on network conditions or may be determined in response to losses reported by a receiving endpoint. The S0 prediction chain may be refreshed in this manner (i.e. if there was an error at the S0 level, any drift is eliminated). 
       FIGS. 15 and 16  show alternative threading or prediction chain architectures  1500  and  1600 , which may be used in video communication or conferencing applications, in accordance with the present invention. Implementations of threading structures or prediction chains  1500  and  1600  do not require any substantial changes to the codec designs described above with reference to  FIGS. 2-14 . 
     In architecture  1500 , an exemplary combination of layers (S0, L0, and L1) is transmitted over high reliability channel  170 . It is noted that, as shown, L1 is part of the L0 prediction chain  430 , but not for S1. Architecture  1600  shows further examples of threading configurations, which also can achieve non-dyadic frame rate resolutions. 
     System  100  and terminal  140  codec designs described above are flexible and can be readily extended to incorporate alternative SVC schemes. For example, coding of the S layer may be accomplished according to the forthcoming ITU-T H.264 SVC FGS specification. When FGS is used, the S layer coding may be able to utilize arbitrary portions of a ‘S’ packet due to the embedded property of the produced bitstream. It may be possible to use portions of the FGS component to create the reference picture for the higher layers. Loss of the FGS component information in transmission over the communications network may introduce drift in the decoder. However, the threading architecture employed in the present invention advantageously minimizes the effects of such loss. Error propagation may be limited to a small number of frames in a manner that is not noticeable to viewers. The amount of FGS to include for reference picture creation may change dynamically. 
     A proposed feature of the H.264 SVC FGS specification is a leaky prediction technique in the FGS layer. See Y. Bao et al., “FGS for Low Delay”, Joint Video Team (JVT) of ISO/IEC MPEG &amp; ITU-T VCEG, 15 th  meeting, Busan, Korea, 18-22 Apr., 2005. The leaky prediction technique consists of using a normalized weighted average of the previous FGS enhancement layer picture and the current base layer picture. The weighted average is controlled by a weight parameter alpha; if alpha is 1 then only the current base layer picture is used, whereas if it is 0 then only the previous FGS enhancement layer picture is used. The case where alpha is 0 is identical to the use of motion estimation (ME  330 ,  FIG. 5 ) for the SNR enhancement layer of the present invention, in the limiting case of using only zero motion vectors. The leaky prediction technique can be used in conjunction with regular ME as described in this invention. Further, it is possible to periodically switch the alpha value to 0, in order to break the prediction loop in the FGS layer and eliminate error drift. 
       FIG. 17  shows the switch structure of an exemplary MCU/SVCS  110  that is used in videoconferencing system  100  ( FIG. 1 ). MCU/SVCS  110  determines which packet from each of the possible sources (e.g., endpoints  120 - 140 ) is transmitted to which destination and over which channel (high reliability vs. low reliability) and switches signals accordingly. The designs and the switching functions MCU/SVCS  110  are described in co-filed International Patent Application No. PCT/US2006/028366, incorporated by reference herein. For brevity, only limited details of the switch structure and switching functions of MCU/SVCS  110  are described further herein. 
       FIG. 18  shows the operation of an exemplary embodiment of MCU/SVCS switch  110 . MCU/SVCS switch  110  maintains two data structures in its memory—an SVCS Switch Layer Configuration Matrix  110 A and an SVCS Network Configuration Matrix  110 , examples of which are shown in  FIGS. 19 and 20 , respectively. SVCS Switch Layer Configuration Matrix  110 A ( FIG. 19 ) provides information on how a particular data packet should be handled for each layer and for each pair of source and destination endpoints  120 - 140 . For example, a matrix  110 A element value of zero indicates that the packet should not be transmitted; a negative matrix element value indicates that the entire packet should be transmitted; and a positive matrix element value indicates that only the specified percentage of the packet&#39;s data should be transmitted. Transmission of a specified percentage of the packet&#39;s data may be relevant only when an FGS-type of technique is used to scalably code signals. 
       FIG. 18  also shows an algorithm  1800  in MCU/SVCS  110  for directing data packets utilizing Switch Layer Configuration Matrix  110 A information. At step  1802 , MCU/SVCS  110  may examine received packet headers (e.g., NAL headers, assuming use of H.264). At step  1804 , MCU/SVCS  110  evaluates the value of relevant matrix  110 A elements for source, destination, and layer combinations to establish processing instructions and designated destinations for the received packets. In applications using FGS coding, positive matrix element values indicate that the packet&#39;s payload must be reduced in size. Accordingly, at step  1806 , the relevant length entry of the packet is changed and no data is copied. At step  1808 , the relevant layers or combination of layers are switched to their designated destinations. 
     With reference to  FIGS. 18 and 20 , SVCS Network Configuration Matrix  110 B tracks the port numbers for each participating endpoint. MCU/SVCS  110  may use Matrix  110 B information to transmit and receive data for each of the layers. 
     The operation of MCU/SVCS  110  based on processing Matrices  110 A and  110 B allows signal switching to occur with zero or minimal internal algorithmic delay, in contrast to traditional MCU operations. Traditional MCUs have to compose incoming video to a new frame for transmission to the various participants. This composition requires full decoding of the incoming streams and recoding of the output stream. The decoding/recoding processing delay in such MCUs is significant, as is the computational power required. By using scalable bitstream architecture, and providing multiple instances of decoders  230 A in each endpoint terminal  140  receiver, MCU/SVCS  110  is required only to filter incoming packets to select the appropriate layer(s) for each recipient destination. The fact that no or minimal DSP processing is required can advantageously allow MCU/SVCS  110  to be implemented with very little cost, offer excellent scalability (in terms of numbers of sessions that can be hosted simultaneously on a given device), and with end-to-end delays which may be only slightly larger than the delays in a direct endpoint-to-endpoint connection. 
     Terminal  140  and MCU/SVCS  100  may be deployed in different network scenarios using different bitrates and stream combinations. TABLE II shows the possible bitrates and stream combinations in various exemplary network scenarios. It is noted that base bandwidth/total bandwidth&gt;=50% is the limit of DiffServ layering effectiveness, and further a temporal resolution of less than 15 fps is not useful. 
     
       
         
           
               
             
               
                 TABLE II 
               
             
            
               
                   
               
               
                 Bitstream Components for Various Network Scenarios 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Total 
                 HRC vs. 
               
               
                   
                   
                   
                 line 
                 LRC 
               
               
                   
                 HRC 
                 LRC 
                 speed 
                 bandwidth 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Client transmits 
                 L0 + L1 = 
                 S0 + S1 + L2 + 
                 500 
                 1:4 
               
               
                   
                 100 
                 S2 = 150 + 
               
               
                   
                   
                 100 + 150 = 400 
               
               
                 SVCS reflects 
                 Same 
                 Same 
                 500 
                 1:4 
               
               
                 for CIF 
               
               
                 recipient 
               
               
                 SVCS for lower 
                 L0 + L1 = 100 
                 S0 + ½ × 
                 350 
                 1:2.5 
               
               
                 speed client 1 
                   
                 (S1 + S2) + 
               
               
                   
                   
                 L2 = 150 + 
               
               
                   
                   
                 100 = 250 
               
               
                 SVCS for lower 
                 L0 + L1 = 100 
                 L2 = 100 
                 200 
                 1:1 
               
               
                 speed client 2 
               
               
                 QCIF view at 30 
               
               
                 fps 
               
               
                 SVCS for lower 
                 L0 = 50 
                 L1 + S0 + S1 = 
                 200 
                 1:1 
               
               
                 speed client 3 
                   
                 50 + 150 
               
               
                 CIF view at 15 
               
               
                 fps 
               
               
                 SVCS for lower 
                 L0 = 50 
                 L1 = 50 
                 100 
                 1:1 
               
               
                 speed client 4 
               
               
                 QCIF at 15 fps 
               
               
                 SVCS for very 
                 L0 = 50 
                 S0 = 50 
                 100 
                 1:1 
               
               
                 low speed client 
               
               
                 CIF 7.5 fps 
               
               
                   
               
            
           
         
       
     
     Terminal  140  and like configurations of the present invention allow scalable coding techniques to be exploited in the context of point-to-point and multi-point videoconferencing systems deployed over channels that can provide different QoS guarantees. The selection of the scalable codecs described herein, the selection of a threading model, the choice of which layers to transmit over the high reliability or low reliability channel, and the selection of appropriate bitrates (or quantizer step sizes) for the various layers are relevant design parameters, which may vary with particular implementations of the present invention. Typically, such design choices may be made once and the parameters remain constant during the deployment of a videoconferencing system, or at least during a particular videoconferencing session. However, it will be understood that SVC configurations of the present invention offer the flexibility to dynamically adjust these parameters within a single videoconferencing session. Dynamic adjustment of the parameters may be desirable, taking into account a participant&#39;s/endpoint&#39;s requirements (e.g., which other participants should be received, at what resolutions, etc.) and network conditions (e.g., loss rates, jitter, bandwidth availability for each participant, bandwidth partitioning between high and low reliability channels, etc.). Under suitable dynamic adjustment schemes, individual participants/endpoints may interactively be able to switch between different threading patterns (e.g., between the threading patterns shown in  FIGS. 4, 8, and 9 ), elect to change how layers are assigned to the high and low reliability channels, elect to eliminate one or more layers, or change the bitrate of individual layers. Similarly, MCU/SVCS  110  may be configured to change how layers are assigned to the high and low reliability channels linking various participants, eliminate one or more layers, scale the FGS/SNR enhancement layer or some participants. 
     In an exemplary scenario, a videoconference may have three participants, A, B, and C. Participants A and B may have access to a high-speed 500 Kbps channel that can guarantee a continuous rate of 200 Kbps. Participant C may have access to a 200 Kbps channel that can guarantee 100 Kbps. Participant A may use a coding scheme that has the following layers: a base layer (“Base”), a temporal scalability layer (“Temporal”) that provides 7.5 fps, 15 fps, 30 fps video at CIF resolutions, and an SNR enhancement layer (“FPS”) that allows increase of the spatial resolution at either of the three temporal frame rates. The Base and Temporal components each require 100 Kbps, and FGS requires 300 Kbps for a total of 500 Kbps bandwidth. Participant A can transmit all three Base, Temporal, and FPS components to MCU  110 . Similarly, participant B can receive all three components. However, since only 200 Kbps are guaranteed to participant B in the scenario, FGS is transmitted through the non-guaranteed 300 Kbps channel segment. Participant C can receive only the Base and Temporal components with the Base component guaranteed at 100 Kbps. If the available bandwidth (either guaranteed or total) changes, then Participant A&#39;s encoder (e.g., Terminal  140 ) can in response dynamically change the target bitrate for any of the components. For example, if the guaranteed bandwidth is more than 200 Kbps, more bits may be allocated to the Base and Temporal components. Such changes can be implemented dynamically in real-time response since encoding occurs in real-time (i.e., the video is not pre-coded). 
     If both participants B and C are linked by channels with restricted capacity, e.g., 100 Kbps, then participant A may elect to only transmit the Base component. Similarly, if participants B and C select to view received video only at QCIF resolution, participant A can respond by not transmitting the FGS component since the additional quality enhancement offered by the FGS component will be lost by downsampling of the received CIF video to QCIF resolution. 
     It will be noted that in some scenarios, it may be appropriate to transmit a single-layer video stream (base layer or total video) and to completely avoid the use of scalability layers. 
     In transmitting scalable video layers over HRCs and LRCs, whenever information on the LRCs is lost, only the information transmitted on the HRC may be used for video reconstruction and display. In practice, some portions of the displayed video picture will include data produced by decoding the base layer and designated enhancement layers, but other portions will include data produced by decoding only the base layer. If the quality levels associated with the different base layer and enhancement layer combinations are significantly different, then the quality differences between the displayed video picture that include or do not include lost LRC data may become noticeable. The visual effect may be more pronounced in the temporal dimension, where repeated changes of the displayed picture from base layer to ‘base plus enhancement layer’ may be perceived as flickering. To mitigate this effect, it may be desirable to ensure that the quality difference (e.g., in terms of PSNR) between the base layer picture and ‘base plus enhancement layer’ picture is kept low, especially on static parts of the picture where flickering is visually more obvious. The quality difference between the base layer picture and ‘base plus enhancement layer’ picture may be deliberately kept low by using suitable rate control techniques to increase the quality of the base layer itself. One such rate control technique may be to encode all or some of the L0 pictures with a lower QP value (i.e., a finer quantization value). For example, every L0 picture may be encoded with a QP lowered by a factor of 3. Such finer quantization may increase the quality of the base layer, thus minimizing any flickering effect or equivalent spatial artifacts caused by the loss of enhancement layer information. The lower QP value may also be applied every other L0 picture, or every four L0 pictures, with similar effectiveness in mitigating flickering and like artifacts. The specific use of a combination of SNR and spatial scalability (e.g., using HCIF coding to represent the base layer carrying QCIF quality) allows proper rate control applied to the base layer to bring static objects close to HCIF resolution, and thus reduce flickering artifacts caused when an enhancement layer is lost. 
     While there have been described what are believed to be the preferred embodiments of the present invention, those skilled in the art will recognize that other and further changes and modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the true scope of the invention. 
     It also will be understood that in accordance with the present invention, the scalable codecs described herein may be implemented using any suitable combination of hardware and software. The software (i.e., instructions) for implementing and operating the aforementioned scalable codecs can be provided on computer-readable media, which can include without limitation, firmware, memory, storage devices, microcontrollers, microprocessors, integrated circuits, ASICS, on-line downloadable media, and other available media.