AU2007223300A1

AU2007223300A1 - System and method for providing error resilience, random access and rate control in scalable video communications

Info

Publication number: AU2007223300A1
Application number: AU2007223300A
Authority: AU
Inventors: Alexandros Eleftheriadis; Danny Hong; Ofer Shapiro; Thomas Wiegand
Original assignee: Vidyo Inc
Current assignee: Vidyo Inc
Priority date: 2006-01-27
Filing date: 2007-03-05
Publication date: 2007-09-13
Also published as: WO2007103889A3; WO2007103889A2; EP1997236A4; EP1997236A2; WO2007103889A8

Description

WO 2007/103889 PCT/US2007/063335 SYSTEM AND METHOD FOR PROVIDING ERROR RESILIENCE, RANDOM ACCESS AND RATE CONTROL IN SCALABLE VIDEO COMMUNICATIONS 5 CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of United States provisional patent application Serial No 60/778,760, filed March 3, 2006, of provisional patent application Serial No. 60/787,031, filed March 29, 2006, and of provisional patent 10 application Serial No. 60/862,510 filed October 23, 2006. Further, this application is claims the benefit of related International patent application Nos. PCT/US06/28365, PCT/US06/028366, PCT/US06/028367, PCT/US06/028368, PCT/US06/061815, PCT/US06/62569, and PCT/US07/62357, and U.S. provisional patent application Nos. 60/884,148, 60/786,997, and 60/829,609. All of the aforementioned priority and 15 related applications, which are commonly assigned, are hereby incorporated by reference herein in their entireties. FIELD OF THE INVENTION [0002] The present invention relates to video data communication systems. The invention specifically relates to simultaneously providing error resilience, random 20 access, and rate control capabilities in video communication systems utilizing scalable video coding techniques. BACKGROUND OF TE INVENTION [0003] Transmission of digital video on packet-based networks such as those based 25 on the Internet Protocol (IP) is extremely challenging, at least due to the fact that data transport is typically done on a best-effort basis. In modern packet-based communication systems errors typically exhibit themselves as packet losses and not 1 WO 2007/103889 PCT/US2007/063335 bit errors. Furthermore, such packet losses are typically the result of congestion in intermediary routers, and not the result of physical layer errors (one exception to this is wireless and cellular networks). When an error in transmission or receipt of a video signal occurs, it is important to ensure that the receiver can quickly recover from the 5 error and return to an error-free display of the incoming video signal. However, in typical digital video communication systems, the receiver's robustness is reduced by the fact that the incoming data is heavily compressed in order to conserve bandwidth. Further, the video compression techniques employed in the communication systems (e.g., state-of-the-art codecs ITU-T H.264 and H.263 or ISO MPEG-2 and MPEG-4 10 codecs) can create a very strong temporal dependency between sequential video packets or frames. In particular, use of motion compensated prediction (e.g., involving the use of P or B frames) codecs creates a chain of frame dependencies in which a displayed frame depends on past frame(s). The chain of dependencies can extend all the way to the beginning of the video sequence. As a result of the chain of 15 dependencies, the loss of a given packet can affect the decoding of a number of the subsequent packets at the receiver. Error propagation due to the loss of the given packet terminates only at an "intra" (I) refresh point, or at a frame that does not use any temporal prediction at all. [0004] Error resilience in digital video communication systems requires having at 20 least some level of redundancy in the transmitted signals. However, this requirement is contrary to the goals of video compression techniques, which strive to eliminate or minimize redundancy in the transmitted signals. [0005] On a network that offers differentiated services (e.g., DiffServ IP-based networks, private networks over leased lines, etc.), a video data communication 25 application may exploit network features to deliver some or all of video signal data in 2 WO 2007/103889 PCT/US2007/063335 a lossless or nearly lossless manner to a receiver. However, in an arbitrary best-effort network (such as the Internet) that has no provision for differentiated services, a data communication application has to rely on its own features for achieving error resilience. Known techniques (e.g., the Transmission Control Protocol - TCP) that are 5 useful in generic data communications are not appropriate for video or audio communications, which have the added constraint of low end-to-end delay arising out of human interface requirements. For example, TCP techniques may be used for error resilience in data transport using the File Transfer Protocol. TCP keeps on retransmitting data until confirmation that all data is received, even if it involves a 10 delay is several seconds. However, TCP is inappropriate for video data transport in a live or interactive videoconferencing application because the end-to-end delay, which is unbounded, would be unacceptable to participants. [0006] A related problem is that of random access. Assume that a receiver joins an existing transmission of a video signal. Typical instances are when a user who 15 joins a videoconference, or a user who tunes in to a broadcast. Such a user would have to find a point in the incoming bitstream where he/she can start decoding and be in synchronization with the encoder. Providing such random access points, however, has a considerable impact on compression efficiency. Note that a random access point is, by definition, an error resilience feature since at that point any error 20 propagation terminates (i.e., it is an error recovery point). Hence, the better the random access support provided by a particular coding scheme, the faster error recovery the coding scheme can provide. The converse may not always be true; it depends on the assumptions made about the duration and extent of the errors that the error resilience technique has been designed to address. For error resilience, some 3 WO 2007/103889 PCT/US2007/063335 state information could be assumed to be available at the receiver at the time the error occurred. [0007] As an example, in MPEG-2 video codecs for digital television systems (digital cable TV or satellite TV), I pictures are used at periodic intervals (typically 5 0.5 sec) to enable fast switching into a stream. The I pictures, however, are considerably larger than their P or B counterparts (typically by 3-6 times) and are thus to be avoided, especially in low bandwidth and/or low delay applications. [0008] In interactive applications such as videoconferencing, the concept of requesting an intra update is often used for error resilience. In operation, the update 10 involves a request from the receiver to the sender for an intra picture transmission, which enables the decoder to be synchronized. The bandwidth overhead of this operation is significant. Additionally, this overhead is also incurred when packet errors occur. If the packet losses are caused by congestion, then the use of the intra pictures only exacerbates the congestion problem. 15 [0009] Another traditional technique for error resilience, which has been used in the past (e.g., in the H.261 standard) to mitigate drift caused by mismatch in IDCT implementations, is to periodically code each macroblock in intra mode. The H.261 standard requires forced intra coding every 132 times a macroblock is transmitted. [0010] The coding efficiency decreases with increasing percentage of 20 macroblocks that are forced to be coded as intra in a given frame. Conversely, when this percentage is low, the time to recover from a packet loss increases. The forced intra coding process requires extra care to avoid motion-related drift, which further limits the encoder's performance since some motion vector values have to be avoided, even if they are the most effective. 4 WO 2007/103889 PCT/US2007/063335 [0011] In addition to traditional, single-layer codecs, layered or scalable coding is a well-known technique in multimedia data encoding. Scalable coding is used to generate two or more "scaled" bitstreams collectively representing a given medium in a bandwidth-efficient manner. Scalability can be provided in a number of different 5 dimensions, namely temporally, spatially, and quality (also referred to as SNR "Signal-to-Noise Ratio" scalability or fidelity scalability). For example, a video signal may be scalably coded in different layers at CIF and QCIF resolutions, and at frame rates of 7.5, 15, and 30 frames per second (fps). Depending on the codec's structure, any combination of spatial resolutions and frame rates may be obtainable 10 from the codec bitstream. The bits corresponding to the different layers can be transmitted as separate bitstreams (i.e., one stream per layer) or they can be multiplexed together in one or more bitstreams. For convenience in description herein, the coded bits corresponding to a given layer may be referred to as that layer's bitstream, even if the various layers are multiplexed and transmitted in a single 15 bitstream. Codecs specifically designed to offer scalability features include, for example, MPEG-2 (ISO/IEC 13818-2, also known as ITU-T H.262) and the currently developed SVC (known as ITU-T H.264 Annex G or MPEG-4 Part 10 SVC). Scalable coding techniques specifically designed for video communication are described in commonly assigned international patent application No. 20 PCT/US06/028365, "SYSTEM AND METHOD FOR SCALABLE AND LOW DELAY VIDEOCONFERENCING USING SCALABLE VIDEO CODING". It is noted that even codecs that are not specifically designed to be scalable can exhibit scalability characteristics in the temporal dimension. For example, consider an MPEG-2 Main Profile codec, a non-scalable codec, which is used in DVDs and 25 digital TV environments. Further, assume that the codec is operated at 30 fps and that 5 WO 2007/103889 PCT/US2007/063335 a group of pictures (GOP) structure of IBBPBBPBBPBBPBB (period N= 15 frames) is used. By sequential elimination of the B pictures, followed by elimination of the P pictures, it is possible to derive a total of three temporal resolutions: 30 fps (all picture types included), 10 fps (I and P only), and 2 fps (I only). The sequential elimination 5 process results in a decodable bitstream because the MPEG-2 Main Profile codec is designed so that coding of the P pictures does not rely on the B pictures, and similarly coding of the I pictures does not rely on other P or B pictures. In the following, single-layer codecs with temporal scalability features are considered to be a special case of scalable video coding, and are thus included in the term scalable video coding, 10 unless explicitly indicated otherwise. [0012] Scalable codecs typically have a pyramidal bitstream structure in which one of the constituent bitstreams (called the "base layer") is essential in recovering the original medium at some basic quality. Use of one or more the remaining bitstream(s) (hereinafter called "the enhancement layer(s)") along with the base layer increases the 15 quality of the recovered medium. Data losses in the enhancement layers may be tolerable, but data losses in the base layer can cause significant distortions or complete loss of the recovered medium. [0013] Scalable codecs pose challenges similar to those posed by single layer codecs for error resilience and random access. However, the coding structures of the 20 scalable codecs have unique characteristics that are not present in single layer video codecs. Further, unlike single layer coding, scalable coding may involve switching from one scalability layer to another (e.g., switching back and forth between CIF and QCIF resolutions). Instantaneous layer switching when switching between different resolutions with very little bit rate overhead is desirable for random access in scalable 6 WO 2007/103889 PCT/US2007/063335 coding systems in which multiple signal resolutions (spatial/temporal/quality) may be available from the encoder. [0014] A problem related to those of error resilience and random access is that of rate control. The output of a typical video encoder has a variable bit rate, due to the 5 extensive use of prediction, transform and entropy coding techniques. In order to construct a constant bit rate stream, buffer-constrained rate control is typically employed in a video communication system. In such a system, an output buffer at the encoder is assumed, which is emptied at a constant rate (the channel rate); the encoder monitors the buffer's occupancy and makes parameter selections (e.g., quantizer step 10 size) in order to avoid buffer overflow or underflow. Such a rate control mechanism, however, can only be applied at the encoder, and further assumes that the desired output rate is known. In some video communication applications, including videoconferencing, it is desirable that such rate control decisions are made at an intermediate gateway (e.g., at a Multipoint Control Unit - MCU), which is situated 15 between the sender and the receiver. Bitstream-level manipulation, or transcoding, can be used at the gateway, but at considerable processing and complexity cost. It is therefore desirable to employ a technique that achieves rate control without requiring any additional processing at the intermediate gateway. [0015] Consideration is now being given to improving error resilience and 20 capabilities for random access to the coded bitstreams, and rate control in video communications systems. Attention is directed developing error resilience, rate control, and random access techniques, which have a minimal impact on end-to-end delay and the bandwidth used by the system. 25 7 WO 2007/103889 PCT/US2007/063335 SUMMARY OF THE INVENTION [0016] The present invention provides systems and methods to increase error resilience and provide random access and rate control capabilities in video communication systems that use scalable video coding. The systems and methods 5 also allow the derivation of an output signal at a resolution different than the coded resolutions, with excellent rate-distortion performance. [0017] In one embodiment, the present invention provides a mechanism to recover from loss of packets of a high resolution spatially scalable layer by using information from the low resolution spatial layer. In another embodiment, the present invention 10 provides a mechanism to switch from a low spatial or SNR resolution to a high spatial or SNR resolution with little or no delay. In yet another embodiment, the present invention provides a mechanism for performing rate control, in which the encoder or an intermediate gateway (e.g., an MCU) selectively eliminates packets from the high resolution spatial layer, anticipating the use of appropriate error recovery mechanisms 15 at the receiver that minimize the impact of the lost packets on the quality of the received signal. In yet another embodiment, the encoder or an intermediate gateway selectively replaces packets from the high resolution spatial layer with information that effectively instructs the encoder to reconstruct an approximation to the high resolution data being replaced using information from the base layer and past frames 20 of the enhancement layer. In another embodiment, the present invention describes a mechanism for deriving an output video signal at a resolution different than the coded resolutions, and specifically an intermediate resolution between those used for spatially scalable coding. These embodiments, either alone or in combination, allow the construction of video communication systems with significant rate control and 25 resolution flexibility as well as error resilience and random access. 8 WO 2007/103889 PCT/US2007/063335 [0018] The inventive systems and methods are based on "error concealment" techniques in conjunction with scalable coding techniques. The techniques simultaneously achieve error resilience and rate control for a particular family of video encoders referred to as scalable video encoders. The rate-distortion 5 performance of the error concealment techniques is such that it matches or exceeds that of coding at the effective transfer rate (total transmitted minus the rate of the lost packets). By appropriate selection of picture coding structures and transport modes the techniques allow nearly instantaneous layer switching with very little bit rate overhead. 10 [0019] Further, the techniques can be used to derive a decoded version of the received signal at a resolution different than the coded resolution(s). This allows, for example, the creation of a 12 CIF (HCIF) signal out of a spatially scalable coded signal at QCIF and CIF resolutions. In contrast with typical scalable coding, the receiver would either have to use the QCIF signal and upsample it (with poor quality), or use the CIF 15 signal and downsample it (with good quality but high bit rate utilization). The same problem also exists if the QCIF and CIF are simulcast as single-layer streams. [0020] The techniques also provide rate control with minimal processing of the encoded video bitstream without adversely affecting picture quality. 9 WO 2007/103889 PCT/US2007/063335 BRIEF DESCRIPTION OF THE DRAWINGS [0021] Further features, the nature, and various advantages of the invention will be more apparent from the following detailed description of the preferred embodiments 5 and the accompanying drawings in which: [0022] FIG. 1 is a block diagram illustrating the overall architecture of a videoconferencing system in accordance with the principles of the present invention; [0023] FIG. 2 is a block diagram illustrating an exemplary end-user terminal in accordance with the principles of the present invention; 10 [0024] FIG. 3 is a block diagram illustrating an exemplary architecture of a video encoder (base and temporal enhancement layers) in accordance with the principles of the present invention; [0025] FIG. 4 is a diagram illustrating an exemplary picture coding structure in accordance with the principles of the present invention; FIG. 5 is a diagram 15 illustrating an example of an alternative picture coding structure in accordance with the principles of the present invention; [0026] FIG. 6 is a block diagram illustrating an exemplary architecture of a video encoder for a spatial enhancement layer in accordance with the principles of the present invention; 20 [0027] FIG. 7 is a diagram illustrating an exemplary picture coding structure when spatial scalability is used in accordance with the principles of the present invention; [0028] FIG. 8 is a diagram illustrating an exemplary decoding process with concealment of enhancement layer pictures in accordance with the principles of the present invention; 10 WO 2007/103889 PCT/US2007/063335 [0029] FIG. 9 is a diagram illustrating exemplary R-D curves of the concealment process when applied to the 'Foreman' sequence in accordance with the principles of the present invention; [0030] FIG. 10 is a diagram illustrating an exemplary picture coding structure when 5 spatial scalability with SR pictures is used in accordance with the principles of the present invention. [0031] Throughout the Figures the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the present invention will now be 10 described in detail with reference to the Figures, it is done so in connection with the illustrative embodiments. DETAILED DESCRIPTION OF THE INVENTION [0032] Systems and methods are provided for error resilient transmission, random 15 access and rate control in video communication systems. The systems and methods exploit error concealment techniques based on features of scalable video coding, which may be used in the video communication systems. [0033] In a preferred embodiment, an exemplary video communication system may be a multi-point videoconferencing system 10 operated over a packet-based 20 network. (See e.g., FIG. 1). Multi-point videoconferencing system may include optional bridges 120a and 120b (e.g., Multipoint Control Unit (MCU) or Scalable Video Communication Server (SVCS)) to mediate scalable multilayer or single layer video communications between endpoints (e.g., users 1-k and 1-m) over the network. The operation of the exemplary video communication system is the same and as 25 advantageous for a point-to-point connection with or without the use of optional 11 WO 2007/103889 PCT/US2007/063335 bridges 120a and 120b. The techniques described in this invention can be applied directly to all other video communication applications, including point-to-point streaming, broadcasting, multicasting, etc. [0034] A detailed description of scalable video coding techniques and 5 videoconferencing systems based on scalable video coding is provided, for example, in commonly assigned International patent application Nos. PCT/US06/28365 and PCT/US06/28366. Further, descriptions of scalable video coding techniques and videoconferencing systems based on scalable video coding are provided in commonly assigned International patent application Nos. PCT/US06/62569 and 10 PCT/USO6/061815. [0035] FIG. 1 shows the general structure of a videoconferencing system 10. Videoconferencing system 10 includes a plurality of end-user terminals (e.g., users 1 k and users 1 -m) that are linked over a network 100 via LANs 1 and 2 and servers 120a and 120b. The servers may be traditional MCUs, or Scalable Video Coding 15 servers (SVCS) or Compositing Scalable Video Coding servers (CSVCS). The latter servers have the same purpose as traditional MCUs, but with significantly reduced complexity and improved functionality. (See e.g., International patent application Nos. PCT/US06/28366 and PCT/US06/62569). In the description herein, the term "server" may be used generically to refer to either an SVCS or an CSVCS. 20 [0036] FIG. 2 shows the architecture of an end-user terminal 140, which is designed for use with videoconferencing systems (e.g., system 100) based on multi layer coding. Terminal 140 includes human interface input/output devices (e.g., a camera 210A, a microphone 210B, a video display 250C, a speaker 250D), and one or more network interface controller cards (NICs) 230 coupled to input and output signal 25 multiplexer and demultiplexer units (e.g., packet MUX 220A and packet DMUX 12 WO 2007/103889 PCT/US2007/063335 220B). NIC 230 may be a standard hardware component, such as an Ethernet LAN adapter, or any other suitable network interface device, or a combination thereof [0037] Camera 210A and microphone 210B are designed to capture participant video and audio signals, respectively, for transmission to other conferencing 5 participants. Conversely, video display 250C and speaker 250D are designed to display and play back video and audio signals received from other participants, respectively. Video display 250C may also be configured to optionally display participant/terminal 140's own video. Camera 210A and microphone 210B outputs are coupled to video and audio encoders 210 OG and 21 OH via analog-to-digital 10 converters 210 OE and 21 OF, respectively. Video and audio encoders 210 OG and 21 OH 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. The encoders compress the local digital signals in order to 15 minimize the bandwidth necessary for transmission of the signals. [0038] In an exemplary embodiment of the present invention, the audio signal may be encoded using any suitable technique known in the art (e.g., G.711, G.729, G.729EV, MPEG-1, etc.). In a preferred embodiment of the present invention, the scalable audio codec G.729EV is employed by audio encoder 210G to encode audio 20 signals. The output of audio encoder 210G is sent to multiplexer MUX 220A for transmission over network 100 via NIC 230. [0039] Packet MUX 220A may perform traditional multiplexing using the RTP protocol. Packet MUX 220A may also perform any related Quality of Service (QoS) processing that may be offered by network 100 or directly by a video communication 25 application (see e.g. International patent application No. PCT/USO6/061815). Each 13 WO 2007/103889 PCT/US2007/063335 stream of data from terminal 140 is transmitted in its own virtual channel or "port number" in IP terminology. [0040] Video encoder 210G is a scalable video encoder that has multiple outputs, corresponding to the various layers (here labeled "base" and "enhancement"). It is 5 noted that simulcasting is a special case of scalable coding, where no inter layer prediction takes place. In the following, when the term scalable coding is used, it includes the simulcasting case. The operation of the video encoder and the nature of the multiple outputs are described in more detail herein below. [0041] In the H.264 standard specification, it is possible to combine views of 10 multiple participants in a single coded picture by using a flexible macroblock ordering (FMO) scheme. In this scheme, each participant occupies a portion of the coded image corresponding to one of its slices. Conceptually, a single decoder can be used to decode all participant signals. However, from a practical view, the receiver/terminal will have to decode several smaller independently coded slices. 15 Thus, terminal 140 shown in FIG. 2 with decoders 230A may be used in applications of the H.264 specification. It is noted that the server for forwarding slices is a CSVCS. [0042] In terminal 140, demultiplexer DMUX 220B receives packets from NIC 320 and redirects them to the appropriate decoder unit 230A. 20 [0043] The SERVER CONTROL block in terminal 140 coordinates the interaction between the server (SVCS/CSVCS) and the end-user terminals as described in International patent applications Nos. PCT/US06/028366 and PCT/US06/62569. In a point-to-point communication system without intermediate servers, the SERVER CONTROL block is not needed. Similarly, in non 25 conferencing applications, point-to-point conferencing applications, or when a 14 WO 2007/103889 PCT/US2007/063335 CSVCS is used, only a single decoder may be needed at a receiving end-user terminal. For applications involving stored video (e.g., broadcast of pre-recorded, pre-coded material, the transmitting end-user terminal may not involve the entire functionality of the audio and video encoding blocks and all blocks preceding them (camera, 5 microphone, etc.). Specifically, only the portions related to selective transmission of video packets, as explained below, need to be provided. [0044] Although the word "terminal" is used in this context, the various components of the terminal may be separate devices that are interconnected to each other, they may be integrated in a personal computer in software or hardware, or they 10 could be combinations thereof [0045] FIG. 3 shows an exemplary base layer video encoder 300. Encoder 300 includes a FRAME BUFFERS block 310 and an Encoder Reference Control (ENC REF CONTROL) block 320 in addition to conventional "text-book" variety video coding process blocks 330 for motion estimation (ME), motion compensation (MC), 15 and other encoding functions. Video encoder 300 may be designed, for example, according to the H.264/MPEG-4 AVC (ITU-T and ISO/IEC JTC 1, "Advanced video coding for generic audiovisual services," ITU-T Recommendation H.264 and ISO/IEC 14496-10 (MPEG4-AVC)) or SVC (J. Reichel, H. Schwarz, and M. Wien, "Joint Scalable Video Model JSVM 4," JVT-Q202, Document of Joint Video Team 20 (JVT) of ITU T SG16/Q.6 and ISO/IEC JTC 1/SC 29/WG 11, October 2005). It will be understood that any other suitable codecs or designs can be used for the video encoder, including, for example, the designs disclosed in International patent applications Nos. PCT/US06/28365 and PCT/US06/62569. If spatial scalability is used, then a DOWNSAMPLER is optionally used at the input to reduce the input 25 resolution (e.g., from CIF to QCIF). 15 WO 2007/103889 PCT/US2007/063335 [0046] ENC REF CONTROL block 300 is used to create a "threaded" coding structure. (See e.g., International patent application No. PCT/US06/28365). Standard block-based motion-compensated codecs have a regular structure of I, P, and B frames. For example, in a picture sequence (in display order) such as IBBPBBP, the 5 'P' frames are predicted from the previous P or I frame in the sequence, whereas the B pictures are predicted using both the previous and next P or I frame. Although the number of B pictures between successive I or P pictures can vary, as can the rate at 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 10 one. The H.264 coding standard advantageously provides an exception in that two reference picture lists are maintained by the encoder and decoder, respectively, with appropriate signaling information that provide for reordering and selective use of pictures from within those lists. This exception can be exploited to select which pictures are used as references and also which references are used for a particular 15 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 320 is designed to determine which reference picture is to be used for the current picture at the encoder side. [0047] The operation of ENC REF CONTROL block 320 is placed in further context 20 with reference to an exemplary layered picture coding "threading" or "prediction chain" structure 400 shown in FIG. 4, in which the letter 'L' is used to indicate an arbitrary scalability layer, followed by a number to indicate the temporal layer (0 being the lowest, or coarsest). The arrows indicate the direction, source, and target of prediction. LO is simply a series of regular P pictures spaced four pictures apart. L1 25 has the same frame rate, but prediction is only allowed from the previous LO frame. 16 WO 2007/103889 PCT/US2007/063335 L2 frames are predicted from the most recent LO or L1 frame. LO provides one fourth (1:4) of the full temporal resolution, L1 doubles the LO frame rate (1:2), and L2 doubles the LO+L1 frame rate (1:1). [0048] Additional or fewer layers can be similarly constructed to accommodate 5 different bit rate/scalability requirements, depending on the requirements of the specific implementation of the present invention. A simple example is shown in FIG. 5 where a traditional prediction series of IPPP... frames is converted to two layers. [0049] 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 10 410-430) 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. 15 Threads 410-420 have a common source LO but different targets and paths (e.g., targets L2, L2, and LO, 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. [0050] It is noted that in encoder 300, ENC REF CONTROL block may use only P 20 pictures as reference pictures. The use of B pictures with both forward and backward prediction increases the coding delay by the time it takes to capture and encode the reference pictures used for the B pictures. In traditional interactive communications, the use of B pictures with prediction from future pictures increases the coding delay and is therefore avoided. However, B pictures also may be used with accompanying 25 gains in overall compression efficiency. Using even a single B picture in the set of 17 WO 2007/103889 PCT/US2007/063335 threads (e.g., by having L2 be coded as a B picture) can improve compression efficiency. For applications that are not delay-sensitive, some or all pictures (with the possible exception of LO) can be B pictures with bi-directional prediction. It is noted that specifically with the H.264 standard, it is possible to use B pictures without 5 incurring extra delay, as the standard allows the use of two motion vectors that both use reference pictures that are in the past in display order. In this case, such B pictures can be used without increasing the coding delay compared with P picture coding. Similarly, the LO pictures could be I pictures, forming traditional groups of pictures (GOPs). 10 [0051] With renewed reference to FIG. 3, base layer encoder 300 can be augmented to create spatial and/or quality enhancement layers, as described, for example in the H.264 SVC Standard draft and in International patent application No. PCT/US06/28365. FIG. 6 shows the structure of an exemplary encoder 600 for creating the spatial enhancement layer. The structure of encoder 600 is similar to that 15 of base layer codec 300, with the additional feature that the base layer information is also made available to encoder 600. This information may include motion vector data, macroblock mode data, coded prediction error data, and reconstructed pixel data. Encoder 600 can re-use some or all of this information in order to make coding decisions for the enhancement layer. For this purpose, the base layer data has to be 20 scaled to the target resolution of the enhancement layer (e.g., by factor of 2 if the base layer is QCIF and the enhancement layer is CIF). Although spatial scalability usually requires two coding loops to be maintained, it is possible (e.g., under the H.264 SVC draft standard) to perform single-loop decoding by limiting the base layer data that is used for enhancement layer coding to only values that are computable from the 25 information encoded in the current picture's base layer. For example, if a base layer 18 WO 2007/103889 PCT/US2007/063335 macroblock is inter-coded, then the enhancement layer cannot use the reconstructed pixels of that macroblock as a basis for prediction. It can, however, use its motion vectors and the prediction error values since they are obtainable by just decoding the information contained in the current base layer picture. Single-loop decoding is 5 desirable since the complexity of the decoder is significantly decreased. [0052] The threading structure can be utilized for the enhancement layer frames in the same manner as for the base layer frames. FIG. 7 shows an exemplary threading structure 700 for the enhancement layer frames following the design shown in FIG. 4. In FIG. 7, the enhancement layer blocks in structure 700 are indicated by the letter 10 'S'. It is noted that threading structures for the enhancement layer frames and the base layer can be different, as explained in International patent application No. PCT/US06/28365. [0053] Further, similar enhancement layer codecs for quality scalability can be constructed, for example, as described in the SVC draft standard and described in 15 International patent application No. PCT/USO6/28365. In such codecs for quality scalability, instead of building the enhancement layer on a higher resolution version of the input, the enhancement layer is built by coding the residual prediction error at the same spatial resolution as the input. As with spatial scalability, all the macroblock data of the base layer can be re-used at the enhancement layer for quality scalability, 20 in either single- or dual-loop coding configurations. [0054] For brevity, the following description is limited to spatial scalability, but it will be understood that the described techniques also can be applied to quality or fidelity scalability. [0055] It is noted that due to the inherent temporal dependency arising from motion 25 compensated prediction in state-of-the-art video codecs, any packet losses at a given 19 WO 2007/103889 PCT/US2007/063335 picture will not only affect the quality of that particular picture, but will also affect all future pictures for which the given picture acts as a reference, either directly or indirectly. This is because the reference frame that the decoder can construct for future predictions will not be the same as the one used at the encoder. The ensuing 5 difference, or drift, can have tremendous impact on the visual quality of the decoded video signals. However, as described in International patent application Nos. PCT/USO6/28365 and PCT/USO6/061815, structure 400 (FIG. 4) has distinct advantages in terms of robustness in the presence of transmission errors. [0056] As shown in FIG. 4, threading structure 400 creates three self-contained 10 chains of dependencies. A packet loss occurring at an L2 picture will only affect L2 pictures; LO and L1 pictures can still be decoded and displayed. Similarly, a packet loss occurring at an L1 picture will only affect L1 and L2 pictures; LO pictures can still be decoded and displayed. [0057] The same error containment properties of the threads extend to S packets. For 15 example, with structure 700 (FIG. 7) a loss occurring at an S2 picture only affects the particular picture, whereas a loss at an S 1 picture will also affect the following S2 picture. In either case, drift will terminate upon decoding of the next SO picture. [0058] With the use of threaded structures, if the base layer and some enhancement layer pictures are transmitted in such a way that their delivery is guaranteed, the 20 remaining layers can be transmitted on a best-effort basis without catastrophic results in the case of a packet loss. The required guaranteed transmissions can be performed using DiffServ, FEC techniques, or other suitable techniques known in the art. For the description herein it is assumed that the guaranteed and best effort transmissions occur over the two actual or virtual channels (e.g. a High Reliability Channel (HRC) 25 and Low Reliability Channel (LRC), respectively) that offer such differentiated 20 WO 2007/103889 PCT/US2007/063335 quality of service. (See e.g., International patent application Nos. PCT/US06/028366 and PCT/US06/061815). [0059] Consider, for example, that layers LO-L2 and SO are transmitted on the HRC, and that S 1 and S2 are transmitted on the LRC. Although the loss of an S 1 or S2 5 packet would cause limited drift, it would still be desirable to be able to conceal as much as possible the loss of information. The concealment of a lost S 1 or S2 picture can only use information available to the decoder, namely past S pictures, and also the coded information of the current picture's base layer. [0060] An exemplary concealment technique according to the present invention 10 utilizes the base layer information of the lost enhancement layer frame, and applies it in the decoding loop of the enhancement layer. The base layer information that can be used includes motion vector data (appropriately scaled for the target layer resolution), coded prediction error difference (upsampled for the enhancement layer resolution, if necessary), and intra data (upsampled for the enhancement layer 15 resolution, if necessary). Prediction references from prior pictures are taken, when needed, from the enhancement layer resolution pictures rather than the corresponding base layer pictures. This data allows the decoder to reconstruct a very close approximation of the missing frame, thus minimizing the actual and perceived distortions on the missing frame. Furthermore, decoding of any dependent frames is 20 now also possible since a good approximation of the missing frame is available. [0061] FIG. 8 shows exemplary steps 810-840 of a concealment decoding process 800, using an example of a two-layer spatial scalability encoded signal with resolutions QCIF and CIF and two prediction threads (LO/SO and L1/S1). It will be understood that process 800 is applicable to other resolutions and to different numbers 25 of threads than shown. In the example, it is assumed that at coded data arrival step 21 WO 2007/103889 PCT/US2007/063335 810 the coded data for LO, SO, and L1 arrive intact at the receiving terminal, but the coded data for S 1 are lost. Further, it is assumed that all coded data for pictures prior to the picture corresponding to time tO also have been received at the receiving terminal. The decoder is thus able to properly decode both a QCIF and a CIF picture 5 at time tO. The decoder can further use the information contained in LO and L1 to reconstruct the correct decoded L1 picture corresponding to time t l. [0062] FIG. 8 shows a particular example, in which a block of the L1 picture at time tl, LB 1 is encoded at base layer decoding step 820 by using motion-compensated prediction with a motion vector LMV1 and a residual LRES 1 that is to be added to the 10 motion-compensated prediction. The data for LMV1 and LRES1 are contained in the L1 data received by the receiving terminal. The decoding process requires block LB0 from the prior base layer picture (the LO picture), which is available at the decoder as a result of the normal decoding process. Since the S1 data assumed to be lost in this example, the decoder cannot use the corresponding information to decode the 15 enhancement layer picture. [0063] Concealment decoding process 800, constructs an approximation for an enhancement layer block SB i. At concealment data generation step 830, process 800 generates concealment data by obtaining the coded data of the corresponding base layer block LB 1, in this example LMV1 and LRES 1. It then scales the motion vector 20 to the resolution of the enhancement layer, to construct an enhancement layer motion vector SMV1. For the two-layer video signal example considered, SMV1 is equal to two times LMV1 since the ratio of resolutions of the scalable signal is 2. Further, the concealment decoding process 800 upsamples the base layer residual signal to the resolution of the enhancement layer, by a factor of 2 in each dimension, and then 25 optionally low-pass filters the result with the filter LPF, in accordance with well 22 WO 2007/103889 PCT/US2007/063335 known principles of sample rate conversion processes. The further result of concealment data generation step 830 is a residual signal SRES1. Next step 840 (Decoding process for the enhancement layer with concealment) uses the constructed concealment data SMV1 and SRES1 to approximate block SB 1. It is noted that the 5 approximation requires the block SB0 from the previous enhancement layer picture, which is assumed to be available at the decoder as a result of the regular decoding process of the enhancement layer. Different encoding modes may operate in the same or similar way. [0064] A further illustrative application of the inventive concealment technique 10 relates to the example of high resolution images. In high resolution images (e.g., greater than CIF) often more then one MTU (maximum transmission unit) is required to transmit a frame of the enhancement layer. If the chance of successful transmission of a single MTU sized packet is p, the chance of successful transmission of a frame comprised of n MTUs is pn. Traditionally, in order to display such a frame, all n 15 packets have to be successfully delivered. [0065] In the application of the inventive concealment technique, an S layer frame is broken into MTU size slices at the encoder for transmission. On the decoder side whatever slices are available from the S picture as received are used. Missing slices are compensated for using the concealment method (e.g., process 800), thus reducing 20 the overall distortion. [0066] In a laboratory experiment, this concealment technique provided similar or better performance when compared with direct coding at the effective communication rate (total rate minus loss rate). For the experiment, it was assumed that layers L0-L2 are reliably transmitted on the HRC, while layers S 1 and S2 are transmitted on the 25 LRC. Actual quality losses, in terms of Y-PSNR, were in the range of 0.2-0.3 dB per 23 WO 2007/103889 PCT/US2007/063335 5% of packet loss, clearly outperforming other known concealment techniques such as frame copy or motion-compensated frame copy. (See e.g., S. Bandyopadhyay, Z. Wu, P. Pandit, and J. Boyce, "Frame Loss Error Concealment for H.264/AVC," Doc. JVT-P072, Poznan, Poland, July 2005, who report several dBs of loss with loss rates 5 of even 5% in evaluations of single-layer AVC coding with an IPP... PI structure, and an I period of 1 sec.) The laboratory experiment results demonstrate that the technique is effective for providing error resilience in scalable codecs. [0067] FIG. 9 shows rate-distortion curves obtained using the standard "foreman" video test sequence with different QPs. For each QP, rate-distortion values were 10 obtained by dropping different amount of S 1 and S2 frames, while applying the inventive error concealment technique described above. As seen in FIG. 9, the right most points for each QP curve correspond to no loss, and then (in a right-to-left direction), 50% of S2 dropped, 100% of S2 dropped, 100% of S2 and 50% of S1 dropped, and 100% of S1 and S2 dropped. The R-D curve of the codec, which is 15 obtained by connecting the zero-loss points for the different QPs, is overlayed. It will be seen from FIG. 9 that various curves particularly for QPs smaller than 30 are close to the R-D curve but in some case are higher. It is expected that the difference will be eliminated with further optimization of the basic codec used. [0068] The laboratory experiment results show that Y-PSNR is similar to the Y 20 PSNR of the same encoder operating at the effective transmission rate. This suggests that the concealment technique can be advantageously used for rate control purposes. The effective transmission rate is defined as the transmission rate minus the loss rate, i.e., the rate calculated based on the packets that actually arrive at the destination. The bit rate corresponding to S1 and S2 frames is typically 30% of the total for the 25 specific coding structure, which implies that any bit rate between 7 0% and 100% may 24 WO 2007/103889 PCT/US2007/063335 be achieved by eliminating a selected number of S 1 and S2 frames for rate control. Bit rates between 7 0% and 100% may be achieved by selecting the number of S2 or S 1 and S2 frames that are dropped in a given time period. [0069] An even wider range for rate control may be obtained for picture coding 5 structure using LR/SR pictures, which are described, for example, in International patent application No. PCT/US06/061815. With such picture structures, it possible not to transmit the SO in the HRC, but to only include the lower temporal resolution SR in the HRC. This feature enables a wider range for rate control. [0070] Table I summarizes the rate percentage of the different frame types for a 10 typical video sequences (e.g., spatial scalability, QCIF-CIF resolution, three-layer threading, 380 Kbps). Table I Frame Type Rate (%) Cumulative Rate (%) LO 15 15 L1 7 22 L2 4 26 SO 46 72 S1 18 90 S2 10 100 15 [0071] By combining different frame types, the concealment technique can achieve practically any desired rate. For example, when all of the LO-L2 and SO pictures are included, and only 1 out of 10 S 1 pictures are dropped, a rate which is approximately 72 +1.

8

=

7 3

.

8 % of the total can be achieved. Alternative techniques known in the art such as Fine Granularity Scalability (FGS) attempt to achieve similar rate flexibility, 20 but with very poor rate-distortion performance and significant computational 25 WO 2007/103889 PCT/US2007/063335 overhead. The concealment technique of the present invention offers the rate scalability associated with FGS, but without the coding efficiency penalty associated with such techniques. [0072] The intentional elimination of S 1 and S2 frames from the video transmission 5 may be performed either at the encoder or at an available intermediate gateway (e.g., a SVCS/CSVCS). [0073] Further, it will be understood that the application of the concealment technique of the present invention for achieving rate control has been described herein with the loss of S 1 frames in a two-layer structure, only for purposes of illustration. In 10 practice, the technique is not limited to a particular threading structure, but can be applied to any spatially-scalable codec that uses a pyramidal temporal structure (e.g., structures including more than two quality or spatial levels, different temporal structures, etc.). [0074] A further use of the inventive concealment technique is to display the video 15 signal at a resolution in between the two coded resolutions. For example, assume a video signal is coded at QCIF and CIF resolution using a spatially scalable codec. If a user wants to display the output in 12 CIF resolution (HCIF), a traditional decoder would follow one of two approaches: 1) decode the QCIF signal and upsample to HCIF, or 2) decode the CIF signal and downsample to HCIF. In the first case, the 20 HCIF picture quality will not be good, but the bitrate used will be low. In the second case, the quality can be very good, but the bitrate used will also be nearly double that required in the first approach. These disadvantages of traditional decoders are overcome by the inventive error concealment techniques. [0075] For example, intentionally discarding all S 1 and S2 frames can result in a 25 significant bandwidth reduction with very little drop in quality by applying the S 1/S2 26 WO 2007/103889 PCT/US2007/063335 error concealment technique described herein. By downsampling the resulting decoded CIF signal, a very good rendition of the HCIF signal is obtained. It is noted that conventional simulcast techniques in which separate single-layer streams are transmitted at QCIF and CIF resolutions, do not allow such derivation of the signal at 5 an intermediate resolution at a usable bit rate unless the frame rate is also dropped. The inventive concealment technique exploits spatially scalable coding for deriving intermediate resolution signals at a usable bit rate. [0076] In practice, application of the inventive concealment technique for deriving an intermediate resolution requires operation of the enhancement layer decoding loop for 10 SO at full resolution. The decoding involves both the generation of the decoded prediction error, as well as the application of motion compensation at full resolution. In order to reduce the computational requirements only the decoded prediction error may be generated in full resolution, followed by downsampling to the target resolution (e.g., HCIF). The reduced resolution signal may then be motion 15 compensated using appropriately scaled motion vectors and residual information. This technique can also be used on any portion of the 'S' layer that is retained for transmission to the receiver. As there will be drift introduced in the enhancement layer decoding loop, a mechanism to periodically eliminate drift may be required. In addition to standard techniques such as I frames, the periodic use of the INTRA_BL 20 mode of spatial scalability for each enhancement layer macroblock may be employed, where only information from the base layer is used for prediction. (See e.g., PCT/US06/28365). Since no temporal information is used, the drift for that particular macroblock is eliminated. If SR pictures are used, drift can also be eliminated by decoding all SR pictures at full resolution. Since SR pictures are far apart, there can 25 still be considerable gain in computational complexity. In some cases, the technique 27 WO 2007/103889 PCT/US2007/063335 for deriving an intermediate resolution signal may be modified by operating the enhancement layer decoder loop in reduced resolution. In cases, where CPU resources are not a limiting factor and faster switching than the SR separation is required or desired, the same (i.e., operating the decoder loop at full resolution) can be 5 applied to higher temporal level (e.g., SO) as needed. [0077] Another exemplary application of the inventive concealment technique is to a video conferencing system in which spatial or quality levels are achieved via simulcast. In this case, concealment is performed using base layer information as described above. The enhancement layer's drift can be eliminated via any one of a) 10 threading, b) standard SVC temporal scalability, c) periodic I frames, and d) periodic intra macroblocks. [0078] An SVCS/CSVCS that is utilizing simulcast to provide spatial scalability, and is only transmitting the higher resolution information for a particular destination for a particular stream (for example if it assumes no or almost no errors), may replace a 15 missing frame of the high resolution with a low resolution one, anticipating such error concealment mechanism on the decoder, and relying on temporal scalability to eliminate drift as discussed above. It will be understood that the concealment process described can be readily adapted to create an effective rate control on such a system. [0079] In the event that the SVCS, CSVCS or the encoder responsible for discarding 20 the higher resolution frames or detecting its loss, cannot assume that the decoder receiving such frames is equipped with the concealment method described herein, such entity may create a replacement high resolution frame that will achieve a similar functionality by one of following methods: a) for error resilience in spatial scalability coding, create a synthetic 25 frame, based on parsing of the lower resolution frame that will include only the 28 WO 2007/103889 PCT/US2007/063335 appropriate signaling to use upsampled base layer information without any additional residuals or motion vector refinement; b) for rate control in a system using spatial scalability, the combination of the method described in (a) with the addition that some macroblocks (MBs) 5 containing significant information from the original high resolution frame are retained; c) for an error resilient system using simulcast for spatial scalability, create a replacement high resolution frame that will include synthetic MBs that will include upsampled motion vectors and residual information; d) for rate control in a 10 system using simulcast for spatial scalability, the method described in (c) with the addition that some MBs containing significant information from the original high resolution frame are retained. [0080] In the cases a) and b) above, the signaling to use only an upsampled version of the base layer picture can be performed either in-band through the coded video 15 bitstream or through out-of-band information that is sent from the encoder or SVCS/CSVCS to the receiving terminal. For the in-band signaling case, specific syntax elements in the coded video bitstream must be present in order to instruct the decoder to use only the base layer information for some or all enhancement layer MBs. In an exemplary codec of the present invention, which is based on the JD7 20 version of the SVC specification (see T. Wiegand, G. Sullivan, J. Reichel, H. Schwarz, M. Wien, eds., "Joint Draft 7, Rev. 2: Scalable Video Coding," Joint Video Team, Doc. JVT-T201, Klagenfurt, July 2006, incorporated herein by reference in its entirety) and described in provisional U.S. patent application Serial No. 60/862,510, a set of flags can be introduced at the slice header, to indicate that when a macroblock 25 is not coded, specific prediction modes that utilize the base layer data are to be used. 29 WO 2007/103889 PCT/US2007/063335 By skipping all enhancement layer macroblocks, the encoder or SVCS/CSVCS will practically eliminate the S 1 or S2 frames, but replace them with very small data packets that only contain the few bytes necessary to indicate the default prediction modes and the fact that all macroblocks are skipped. Similarly, for performing rate 5 control, the encoder or SVCS/SVCS may selectively eliminate some information from enhancement layer MBs. For example, the encoder or SVCS/SVCS may selectively maintain motion vector refinements, but eliminate residual prediction, or keep residual prediction, but eliminate motion vector refinements. [0081] With continued reference to the SVC JD7 specification, there are several flags 10 in the MB layer (in scalable extension) that are used for predicting information from the base layer, if the base layer exists. They are base_mode_flag, motionpredictionflag and residualpredictionflag. Similarly, there already exists a flag in the slice header, adaptiveprediction_flag, which is used to indicate the presence of base_mode_flag in the MB layer. To trigger the concealment operation, 15 one needs to set base_modeflag to 1 for every MB, which can be done using the already existing adaptiveprediction_flag. By setting the slice header flag adaptivepredictionflag to 0, and taking into account that the default value for the residuepredictionflag in inter MBs is 1. we can indicate that all MBs in a slice are skipped (using mb_skiprun or mb_skip_flag signaling) and thus direct the decoder to 20 essentially perform the concealment operation disclosed herein. [0082] It is recognized that a potential drawback of the concealment technique is that the bitrate of the coded stream without the S 1 and S2 frames may be very uneven or "bursty," since the SO frames are typically quite large (e.g., as high as 45% of the total bandwidth. To mitigate this behavior, in a modification (hereinafter "progressive 25 concealment") the SO packets may be transmitted by splitting them into smaller 30 WO 2007/103889 PCT/US2007/063335 packets and/or slices and spreading their transmission over the time interval between successive SO pictures. The entire SO picture will not be available for the first S2 picture, but information that has been received by the first S2 picture (i.e., portions of SO and the entire LO and L2) can be used for concealment purposes. In this manner 5 the decoder can also recover an appropriate reference frame in time to display the L1/S 1 picture, which would further help in creating decoded version of both the L1/S 1 picture and the second L2/S2. Otherwise, as they are further apart from the LO frame, they may show more concealment artifacts due to motion. [0083] Another alternative solution to mitigate the effects of bursty SO transmissions 10 is to smooth out the variable bit-rate (VBR) traffic by additional buffering at the cost of increased end-to-end delay. It is noted that in multipoint conferencing applications, there is inherent statistical multiplexing at the server. Therefore, the VBR behavior of the traffic originating from the server will be naturally smoothed. [0084] International patent application No. PCT/USO6/061815 describes the problems 15 of error resilience and random access and provides solutions appropriate for different application scenarios. [0085] The progressive concealment technique provides a further solution for performing video switching. The progressive concealment technique described above also may be used for video switching. An exemplary switching application is to a 20 single-loop, spatially scalable signal coded at QCIF and CIF resolutions with a three layer threading structure, with the three-layer threading structure shown in FIG. 7. As described in International patent application No. PCT/USO6/061815, increased error resilience can be achieved by ensuring reliable transmission of some of the LO pictures. The LO pictures that are reliably transmitted are referred to as LR pictures. 25 The same threading pattern can be extended to the S pictures, as shown in FIG. 10. 31 WO 2007/103889 PCT/US2007/063335 The temporal prediction paths for the S pictures are identical to those of the L pictures. FIG. 10 shows an exemplary SR period of 1/3 (one out of every 3 SO pictures is SR) for purposes of illustration. In practice, different periods and different threading patterns can be used in accordance with the principles of the present 5 invention. Further, different paths in the S and L pictures could also be used, but with a reduction in coding efficiency for the S pictures. As with LR pictures, the SR pictures are assumed to be transmitted reliably. As described in International patent application No. PCT/USO6/061815, this can be accomplished using a number of techniques, such as DiffServ coding (where LR and SR are in the HRC), FEC or 10 ARQ. [0086] In the exemplary switching application of the progressive concealment technique, the progressive concealment technique, an end-user at terminal receiving a QCIF signal may desire to switch to a CIF signal. In order to be able to start decoding the enhancement layer CIF signal, the terminal must acquire at least one correct CIF 15 reference picture. A technique disclosed in International patent application No. PCT/USO6/061815 involves using periodic intra macroblocks, so that within a period of time all macroblocks of the CIF picture will be intra coded. A drawback is that it will take a significant amount of time to do so, if the percentage of intra macroblocks is kept low (to minimize their impact on the total bandwidth). In contrast, the 20 switching application of the progressive concealment technique exploits the reliable transmission of the SR pictures in order to be able to start decoding the enhancement layer CIF signal. [0087] The SR pictures can be transmitted to the receiver and be decoded even if it operates at a QCIF level. Since they are infrequent, their overall effect on the bit rate 25 can be minimal. When a user switches to the CIF resolution, the decoder can utilize 32 WO 2007/103889 PCT/US2007/063335 the most recent SR frame, and proceed as if intermediate S pictures until the first S picture received were lost. If additional bit rate is available, the sender or server can also forward cached copies of all intermediate SO pictures to further aid the receiver in constructing a reference picture as close to the starting frame of CIF playback as 5 possible. The rate-distortion performance of the S 1/S2 concealment technique will ensure that the impact on quality is minimized. [0088] The inventive technique can also be used advantageously when the end-user decodes at an intermediate output resolution, e.g., HCIF, and desires to switch to CIF. An HCIF signal can be effectively derived from the LO-L2 and portion of the SO-S2 10 pictures (e.g., only SO), coupled with concealment for dropped S frames. In this case, the decoder, which receives at least a portion of the SO pictures, can immediately switch to CIF resolution with very small PSNR penalty. Further, this penalty will be eliminated as soon as the next SO/SR picture arrives. Thus, in this case, there is practically no overhead and almost instantaneous switching can be achieved. 15 [0089] It is noted that although typical spatial coding structures employ 1:4 picture area ratios, some users are more comfortable with resolution changes of 1:2. Therefore, in practice HCIF to CIF switching transitions are much more likely than QCIF to CIF switching transitions, for example, in desktop communication applications. A common scenario in video conferencing is that the screen real estate 20 is split into a large picture of the active speaker surrounded by smaller pictures of the other participants, and where the active speaker image automatically occupies the larger image. In the case where the smaller images where created using the rate control methods described herein, such a switch can be done frequently without any overhead. The switching of participant images can be done frequently in an "active" 25 layout without any overhead. This feature is desirebable for accommodating both 33 WO 2007/103889 PCT/US2007/063335 conference participants who prefer to view such an active layout, and other conference participants who prefer a static view. Since the switching-by-concealment method does not require any additional information to be sent by the encoder, the choice of layout by one receiver does not impact the bandwidth received by others. 5 [0090] The foregoing description refers to creating effective rendering for intermediate resolutions and bit rates that span the range between resolutions/bit rates directly provided by the encoder. It will be understood that other methods that are known to decrease the bit rate (e.g., by introducing drift) such as data partitioning or re-quantization can be employed by the SVCS/CSVCS in conjunction with inventive 10 methods described herein to provide a more detailed manipulation of the bit stream. For example, assume that a resolution of 1/3 CIF is desired when only QCIF and CIF are available, and that the SR, SO-S2 coding structure is used. Eliminating S1 and S2 only may result in a bit rate that is too high to effectively be used as 1/3 CIF. Further, eliminating SO may result in a bit rate that is too low and/or be visually unacceptable 15 due to motion-related artifacts. In such a case, reducing the amounts of bits of the SO frames using known methods as data partitioning or re-quantization may be useful in conjunction with the SR transmission (either in VBR mode or using the progressive concealment) to provide a more optimized result. It will be understood that these methods may be applied to the S 1 and S2 levels to achieve more fine-tuned rate 20 control. [0091] Although the preferred embodiments described herein use the H.264 SVC draft standard, as is obvious to persons skilled in the art the techniques can be directly applied to any coding structure that allows multiple spatial/quality, and temporal levels. 34 WO 2007/103889 PCT/US2007/063335 [0092] It also will be understood that in accordance with the present invention, the scalable codecs and concealment techniques 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 5 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. 35

Claims

1. A digital video decoding system, the system comprising: 5 a decoder that is capable of decoding a received digital video signal, which is coded in a scalable video coding format supporting temporal scalability and at least one of spatial and quality scalability, wherein the scalable video coding format for spatial scalability includes a base spatial and at least one spatial enhancement layer, for quality scalability includes a base 10 quality layer and at least one quality enhancement layer, and for temporal scalability includes a base temporal layer and at least one temporal enhancement layer, wherein the base temporal layers and enhancement temporal layers are interlinked by a threaded picture prediction structure for at least one of the spatial or quality scalability layers, 15 and wherein, for decoding a picture at a target spatial or quality layer higher than the corresponding base layer, the decoder is configured to use coded information from a layer lower than the target layer when a portion of the target layer's coded information is lost or not available.

2. The system of claim 1, wherein the digital video decoding system is 20 disposed in a receiving endpoint, the system further comprising: a linking communication network; a conferencing server linked to the receiving endpoint and at least one transmitting endpoint by at least one communication channel each over the communication network, and 36 WO 2007/103889 PCT/US2007/063335 at least one endpoint that transmits the coded digital video that is coded in the scalable video coding format, wherein the conferencing server is configured to selectively eliminate portions of the input video signals received from transmitting endpoints that correspond to layers 5 higher than the base spatial or quality layer, prior to creating the output video signal that is forwarded to the receiving endpoint.

3. The system of claim 2 wherein the conferencing server linked to the receiving endpoint and at least one transmitting endpoint is one of a Transcoding Multipoint Control Unit using cascaded decoding and 10 encoding; a Switching Multipoint Control Unit by selecting which input to transmit as output; a Scalable Video Communication Server using selective multiplexing; and 15 a Compositing Scalable Video Communication Server using selective multiplexing and bitstream-level compositing.

4. The system of claim 2 wherein an encoder of the at least one transmitting endpoint is configured to encode transmitted media as frames in a threaded coding structure having a number of different temporal levels, wherein a 20 subset of the frames ("R") is particularly selected for reliable transport and includes at least the frames of the lowest temporal layer in the threaded coding structure so that the decoder can decode at least a portion of received media based on a reliably received frame of the type R after packet loss or error and thereafter is synchronized 37 WO 2007/103889 PCT/US2007/063335 with the encoder, and wherein the server selectively eliminates portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer in non-R frames only, prior to creating the output video signal that is forwarded to the receiving endpoint.

5 5. The system of claim 2, wherein the conferencing server is further configured to control the transmission rate of the output video signal that is forwarded to the at least one receiving endpoint so that the retained portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer do not adversely affect the smoothness of the output bit 10 rate.

6. The system of claim 2, wherein the selective elimination by the conferencing server is performed according to desired output bit rate requirements.

7. The system of claim 1, wherein the digital video decoding system is disposed in a receiving endpoint, the system further comprising: 15 a transmitting endpoint that transmits coded digital video using a scalable video coding format; a communication network that links the transmitting endpoint with the receiving endpoint, wherein the transmitting endpoint is configured to selectively not transmit portions of 20 its input video signal that correspond to layers higher than the base spatial or quality layer prior to creating the output video signal that is transmitted to the at least one receiving endpoint in order to achieve a desired output bit rate. 38 WO 2007/103889 PCT/US2007/063335

8. The system of claim 7 wherein the encoder of the transmitting endpoint is configured to encode transmitted media as frames in a threaded coding structure having a number of different temporal levels, wherein a subset of the frames ("R") is particularly selected for reliable transport and includes at least the frames of 5 the lowest temporal layer in the threaded coding structure and such that the decoder can decode at least a portion of received media based on a reliably received frame of the type R after packet loss or error and thereafter is synchronized with the encoder, and wherein the encoder selectively does not transmit to the at least one receiving endpoint portions of its input video signal that correspond to layers higher than the 10 base spatial or quality layer in non-R frames only.

9. The system of claim 7, wherein the transmitting endpoint is further configured to control the transmission rate of the output video signal that is forwarded to the at least one receiving endpoint so that the retained portions of its input video signal that corresponds to layers higher than the base spatial or quality layer do not 15 adversely affect the smoothness of the output bit rate.

10. The system of claim 7, wherein the decision for selective transmission by the transmitting endpoint is performed according to desired output bit rate requirements.

11. The system of claim 1, wherein the decoder is configured to display 20 the decoded output picture at a desired spatial resolution that falls in between an immediately lower and an immediately higher spatial layer provided by the coded video signal.

12. The system of claim 1, wherein the decoder is further configured to operate the decoding loop of the immediately higher spatial layer at the desired spatial 39 WO 2007/103889 PCT/US2007/063335 resolution by scaling all coded data of the immediately higher spatial layer to the desired spatial resolution, and wherein the resultant drift is eliminated by using at least one of periodic intra pictures; 5 periodic use of intra base layer mode; and full resolution decoding of at least the lowest temporal layer of the immediately higher spatial layer.

13. The system of claim 1, wherein the scalable video coding format is further configured with at least one of 10 periodic intra pictures, periodic intra macroblocks, and threaded picture prediction, in order to avoid drift when the target layer's coded information that is lost or is not available corresponds to the base temporal layer. 15

14. The system of claim 1, where the scalable video coding format is based on hybrid coding such as in H.264, VC-1 or AVS standards, wherein the coded information from a spatial or quality layer lower than the target layer used by the decoder when some or all of the target layer's coded information is lost or is not available comprises at least one of 20 motion vector data, appropriately scaled for the target layer's resolution; 40 WO 2007/103889 PCT/US2007/063335 coded prediction error difference, upsampled to the target layer's resolution; and intra data, upsampled to the target layer's resolution, and wherein the decoder is further configured to use the target layer's decoded 5 pictures as references in the decoding process in order to construct the decoded output picture, rather than the lower layer decoded reference pictures.

15. The system of claim 1, wherein the decoder is further configured to operate at least one decoding loop for spatial or quality layers higher than the target spatial or quality layer for at least the base temporal layer, so that when the decoder 10 switches target layers it can immediately display decoded pictures at the new target layer resolution.

16. A video communication system comprising: a communication network, a conferencing server disposed in the network and linked to at least one 15 receiving and at least one transmitting endpoint by at least one communication channel each over the communication network, at least one endpoint that transmits coded digital video using a scalable video coding format, and at least one receiving endpoint that is capable of decoding a digital 20 video signal coded in a scalable video coding format supporting temporal scalability and at least one of spatial and quality scalability, wherein the scalable video coding format for spatial scalability includes a base spatial and at least one spatial enhancement layer, for quality scalability includes a base 41 WO 2007/103889 PCT/US2007/063335 quality layer at least one quality enhancement layer, and for temporal scalability includes a base temporal layer and at least one temporal enhancement layer, wherein the base temporal layers and enhancement temporal layers are interlinked by a threaded picture prediction structure for at least one of the spatial or quality scalability 5 layers, and wherein the conferencing server is configured to selectively eliminate or modify portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer, prior to creating the output video signal that is forwarded to the at least one receiving endpoint, so that use 10 of lower spatial or quality layer data is signaled or explicitly coded in the output video signal for use in decoding pictures at resolutions higher than the base spatial or quality layer.

17. The system of claim 16, wherein the scalable video coding format where the scalable video coding format is based on hybrid coding such as in H.264, 15 VC-1 or AVS standards, and wherein the lower spatial or quality layer data that is signaled for use or explicitly coded in the output video signal forwarded to the at least one receiving endpoints is comprised of at least one of motion vector data, coded prediction error difference, 20 intra data, and reference picture indicators, 42 WO 2007/103889 PCT/US2007/063335 wherein the data is further appropriately scaled to the desired target resolution when explicitly coded in the output video signal that is transmitted to the one or more receiving endpoints.

18. The system of claim 16 wherein the server is further configured to 5 create the output video signal that is forwarded to the at least one receiving endpoint as one of a Transcoding Multipoint Control Unit using cascaded decoding and encoding; a Switching Multipoint Control Unit by selecting which input to 10 transmit as output; a Scalable Video Communication Server using selective multiplexing; and a Compositing Scalable Video Communication Server using selective multiplexing and bitstream-level compositing. 15

19. The system of claim 16 wherein an encoder of the at least one transmitting endpoint is configured to encode transmitted media as frames in a threaded coding structure having a number of different temporal levels, wherein a subset of the frames ("R") is particularly selected for reliable transport and includes at least the frames of the lowest temporal layer in the threaded coding structure and such 20 that the decoder can decode at least a portion of received media based on a reliably received frame of the type R after packet loss or error and thereafter is synchronized with the encoder, and wherein the server selectively eliminates portions of the input video signals received from transmitting endpoints that correspond to layers higher 43 WO 2007/103889 PCT/US2007/063335 than the base spatial or quality layer in non-R frames only, prior to creating the output video signal that is forwarded to the at least one receiving endpoint.

20. The system of claim 16 wherein the conferencing server is further configured to control the transmission rate of the output video signal that is forwarded 5 to the at least one receiving endpoint so that the retained portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer do not adversely affect the smoothness of the output bit rate.

21. The system of claim 16, wherein the selective elimination or 10 modification by the conferencing server is performed according to desired output bit rate requirements.

22. The system of claim 16, wherein the at least one receiving endpoint is configured to display the decoded output picture at a desired spatial resolution that falls in between an immediately lower and an immediately higher spatial layer 15 provided by the received coded video signal.

23. The system of claim 22, wherein the at least one receiving endpoint is further configured to operate the decoding loop of the immediately higher spatial layer at the desired spatial resolution by scaling all coded data of the immediately higher spatial layer to the desired spatial resolution, and wherein the resultant drift is 20 eliminated by using at least one of periodic intra pictures, periodic use of intra base layer mode, 44 WO 2007/103889 PCT/US2007/063335 full resolution decoding of at least the lowest temporal layer of the immediately higher spatial layer.

24. The system of claim 16, wherein the scalable video coding format is further configured with at least one of 5 periodic intra pictures; periodic intra macroblocks; and threaded picture prediction; in order to avoid drift when the higher than the base spatial or quality layer's coded information that is modified or eliminated corresponds to the 10 base temporal layer.

25. The system of claim 16, wherein the receiving endpoint is further configured to operate at least one decoding loop for spatial or quality layers higher than the target spatial or quality layer for at least the base temporal layer, so that when the at least one receiving endpoint switches target layers it can immediately display 15 decoded pictures at the new target layer resolution.

26. A video communication system comprising: a communication network, one endpoint that transmits coded digital video using a scalable video coding format, and 20 at least one receiving endpoint that is capable of decoding a digital video signal coded in a scalable video coding format supporting temporal scalability and at least one of spatial and quality scalability, 45 WO 2007/103889 PCT/US2007/063335 wherein the scalable video coding format for spatial scalability includes a base spatial and at least one spatial enhancement layer, for quality scalability includes a base quality layer at least one quality enhancement layer, and for temporal scalability includes a base temporal layer and at least one temporal enhancement layer, wherein 5 the base temporal layers and enhancement temporal layers are interlinked by a threaded picture prediction structure for at least one of the spatial or quality scalability layers, and wherein the transmitting endpoint is configured to selectively eliminate or modify portions of its coded video signal that correspond to layers higher than the base spatial 10 or quality layer, prior to creating the output video signal that is forwarded to the at least one receiving endpoint, so that use of lower spatial or quality layer data is signaled or explicitly coded in the output video signal for use in decoding pictures at resolutions higher than the base spatial or quality layer.

27. The system of claim 26, wherein the scalable video coding format is 15 based on hybrid coding such as in H.264, VC-1 or AVS standards, and wherein the lower spatial or quality layer data that is signaled for use or explicitly coded in the output video signal forwarded to the at least one receiving endpoints is comprised of at least one of motion vector data; 20 coded prediction error difference; intra data; and reference picture indicators, 46 WO 2007/103889 PCT/US2007/063335 wherein the data is further appropriately scaled to the desired target resolution when explicitly coded in the output video signal that is transmitted to the one or more receiving endpoints.

28. The system of claim 26 wherein the transmitting endpoint is 5 configured to encode transmitted media as frames in a threaded coding structure having a number of different temporal levels, wherein a subset of the frames ("R") is particularly selected for reliable transport and includes at least the frames of the lowest temporal layer in the threaded coding structure and such that the decoder can decode at least a portion of received media based on a reliably received frame of the 10 type R after packet loss or error and thereafter is synchronized with the encoder, and wherein the transmitting endpoint selectively eliminates portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer in non- R frames only, prior to creating the output video signal that is transmitted to the at least one receiving endpoint. 15

29. The system of claim 26, wherein the transmitting endpoint is further configured to control the transmission rate of the output video signal that is transmitted to the at least one receiving endpoint so that the retained portions of its input video signal that correspond to layers higher than the base spatial or quality layer do not adversely affect the smoothness of the output bit rate. 20

30. The system of claim 26, wherein the selective elimination or modification by the transmitting endpoint is performed according to desired output bit rate requirements.

31. The system of claim 26, wherein the at least one receiving endpoint is configured to display the decoded output picture at a desired spatial resolution that 47 WO 2007/103889 PCT/US2007/063335 falls in between an immediately lower and an immediately higher spatial layer provided by the received coded video signal.

32. The system of claim 26, wherein the at least one receiving endpoint is further configured to operate the decoding loop of the immediately higher spatial 5 layer at the desired spatial resolution by scaling all coded data of the immediately higher spatial layer to the desired spatial resolution, and wherein the resultant drift is eliminated by using at least one of periodic intra pictures, periodic use of intra base layer mode, 10 full resolution decoding of at least the lowest temporal layer of the immediately higher spatial layer.

33. The system of claim 26, wherein the scalable video coding format is further configured with at least one of periodic intra pictures; 15 periodic intra macroblocks; and threaded picture prediction, in order to avoid drift when the higher than the base spatial or quality layer's coded information that is modified or eliminated corresponds to the base temporal layer.

34. The system of claim 26, wherein the receiving endpoint is further 20 configured to operate at least one decoding loop for spatial or quality layers higher than the target spatial or quality layer for at least the base temporal layer, so that when the at least one receiving endpoint switches target layers it can immediately display decoded pictures at the new target layer resolution. 48 WO 2007/103889 PCT/US2007/063335

35. A method for decoding a digital video signal, the digital video signal coded in a scalable video coding format supporting temporal scalability and at least one of spatial and quality scalability, 5 wherein the scalable video coding format for spatial scalability includes a base spatial and at least one spatial enhancement layer, for quality scalability includes a base quality layer and at least one quality enhancement layer, and for temporal scalability includes a base temporal layer and at least one temporal enhancement layer, wherein the base temporal layers and enhancement temporal layers are interlinked by a 10 threaded picture prediction structure for at least one of the spatial or quality scalability layers, the method comprising: receiving the digital video signal at a decoder; and for decoding a picture at a target spatial or quality layer higher than the 15 corresponding base layer, using coded information from a spatial or quality layer lower than the target layer in the threaded prediction structure when a portion of the target layer's coded information is lost or not available.

36. The method of claim 35, wherein the decoder is disposed in a receiving endpoint in a linking communication network, 20 wherein a conferencing server is linked to the receiving endpoint and at least one transmitting endpoint by at least one communication channel each over the communication network, and 49 WO 2007/103889 PCT/US2007/063335 wherein the at least one transmitting endpoint transmits the coded digital video that is coded in the scalable video coding format, the method further comprising, at the conferencing server, selectively eliminating portions of the input video signals received from transmitting endpoints 5 that correspond to layers higher than the base spatial or quality layer, prior to creating the output video signal that is forwarded to the receiving endpoint.

37. The method of claim 36 wherein the conferencing server linked to the receiving endpoint and at least one transmitting endpoint is one of a Transcoding Multipoint Control Unit using cascaded decoding and 10 encoding; a Switching Multipoint Control Unit by selecting which input to transmit as output; a Scalable Video Communication Server using selective multiplexing; and 15 a Compositing Scalable Video Communication Server using selective multiplexing and bitstream-level compositing.

38. The method of claim 36, further comprising, at an encoder of the at least one transmitting endpoint, encoding transmitted media as frames in a threaded coding structure having a number of different temporal levels, wherein a subset of the 20 frames ("R") is particularly selected for reliable transport and includes at least the frames of the lowest temporal layer in the threaded coding structure so that the decoder can decode at least a portion of received media based on a reliably received frame of the type R after packet loss or error and thereafter is synchronized with the 50 WO 2007/103889 PCT/US2007/063335 encoder, and wherein the server selectively eliminates portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer in non-R frames only, prior to creating the output video signal that is forwarded to the receiving endpoint. 5

39. The method of claim 36, further comprising, at the conferencing server controlling the transmission rate of the output video signal that is forwarded to the at least one receiving endpoint so that the retained portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer do not adversely affect the smoothness of the output bit rate. 10

40. The method of claim 36, wherein the selective elimination by the conferencing server is performed according to desired output bit rate requirements.

41. The method of claim 35, wherein a transmitting endpoint transmits coded digital video using a scalable video coding format; 15 wherein a communication network links the transmitting endpoint with the receiving endpoint, the method further comprising, at the transmitting endpoint, selectively not transmitting portions of its input video signal that correspond to layers higher than the base spatial or quality layer, prior to creating the output video signal that is 20 transmitted to the at least one receiving endpoint in order to achieve a desired output bit rate.

42. The method of claim 41, further comprising, at the transmitting endpoint encoding transmitted media as frames in a threaded coding structure having 51 WO 2007/103889 PCT/US2007/063335 a number of different temporal levels, wherein a subset of the frames ("R") is particularly selected for reliable transport and includes at least the frames of the lowest temporal layer in the threaded coding structure and such that the decoder can decode at least a portion of received media based on a reliably received frame of the 5 type R after packet loss or error and thereafter is synchronized with the encoder, and wherein the encoder selectively does not transmit to the at least one receiving endpoint portions of its input video signal that correspond to layers higher than the base spatial or quality layer in non-R frames only.

43. The method of claim 41, further comprising, at the transmitting 10 endpoint controlling the transmission rate of the output video signal that is forwarded to the at least one receiving endpoint so that the retained portions of its input video signal that corresponds to layers higher than the base spatial or quality layer do not adversely affect the smoothness of the output bit rate.

44. The method of claim 41, wherein the decision for selective 15 transmission by the transmitting endpoint is performed according to desired output bit rate requirements.

45. The method of claim 35, further comprising, at the decoder, displaying the decoded output picture at a desired spatial resolution that falls in between an immediately lower and an immediately higher spatial layer provided by the coded 20 video signal.

46. The method of claim 35, further comprising, at the decoder, operating the decoding loop of the immediately higher spatial layer at the desired spatial resolution by scaling all coded data of the immediately higher spatial layer to the 52 WO 2007/103889 PCT/US2007/063335 desired spatial resolution, and wherein the resultant drift is eliminated by using at least one of periodic intra pictures; periodic use of intra base layer mode; and 5 full resolution decoding of at least the lowest temporal layer of the immediately higher spatial layer.

47. The method of claim 35, wherein the scalable video coding format is further configured with at least one of periodic intra pictures, 10 periodic intra macroblocks, and threaded picture prediction, in order to avoid drift when the target layer's coded information that is lost or is not available corresponds to the base temporal layer.

48. The method of claim 35, where the scalable video 15 coding format is based on hybrid coding such as in H.264, VC-1 or AVS standards, wherein the coded information from a spatial or quality layer lower than the target layer used by the decoder when some or all of the target layer's coded information is lost or is not available comprises at least one of motion vector data, appropriately scaled for the target layer's 20 resolution; coded prediction error difference, upsampled to the target layer's resolution; and 53 WO 2007/103889 PCT/US2007/063335 intra data, upsampled to the target layer's resolution, the method further comprising, at the decoder using the target layer's decoded pictures as references in the decoding process in order to construct the decoded output picture, rather that the lower layer decoded reference pictures. 5

49. The method of claim 35 further comprising, at the decoder operating at least one decoding loops for spatial or quality layers higher than the target spatial or quality layer for at least the base temporal layer, so that when the decoder switches target layers it can immediately display decoded pictures at the new target layer resolution. 10

50. A method for video communication over a communication network, having a conferencing server disposed therein and linked to at least one receiving and at least one transmitting endpoint by at least one communication channel each over the communication network, the at least one endpoint transmitting coded digital video using a scalable video coding format, and the at least one receiving endpoint capable 15 of decoding a digital video signal coded in a scalable video coding format supporting temporal scalability and at least one of spatial and quality scalability, wherein the scalable video coding format for spatial scalability includes a base spatial and at least one spatial enhancement layer, for quality scalability includes a base quality layer at least one quality enhancement layer, and for temporal scalability includes a base 20 temporal layer and at least one temporal enhancement layer, wherein the base temporal layers and enhancement temporal layers are interlinked by a threaded picture prediction structure for at least one of the spatial or quality scalability layers, the method comprising: 54 WO 2007/103889 PCT/US2007/063335 at the conferencing server, selectively eliminating or modifying modify portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer prior to creating the output video signal that is forwarded to the at least one receiving endpoint, so that use 5 of lower spatial or quality layer data is signaled or explicitly coded in the output video signal for use in decoding pictures at resolutions higher than the base spatial or quality layer.

51. The method of claim 50, wherein the scalable video coding format is based on hybrid coding such as in H.264, VC-1 or AVS standards, and wherein the 10 lower spatial or quality layer data that is signaled for use or explicitly coded in the output video signal forwarded to the at least one receiving endpoints is comprised of at least one of motion vector data, coded prediction error difference, 15 intra data, and reference picture indicators, wherein the data is further appropriately scaled to the desired target resolution when explicitly coded in the output video signal that is transmitted to the one or more receiving endpoints. 20

52. The method of claim 50 wherein the server is further configured to create the output video signal that is forwarded to the at least one receiving endpoint as one of 55 WO 2007/103889 PCT/US2007/063335 a Transcoding Multipoint Control Unit using cascaded decoding and encoding; a Switching Multipoint Control Unit by selecting which input to transmit as output; 5 a Scalable Video Communication Server using selective multiplexing; and a Compositing Scalable Video Communication Server using selective multiplexing and bitstream-level compositing.

53. The method of claim 50, further comprising, at an encoder of the at 10 least one transmitting endpoint, encoding transmitted media as frames in a threaded coding structure having a number of different temporal levels, wherein a subset of the frames ("R") is particularly selected for reliable transport and includes at least the frames of the lowest temporal layer in the threaded coding structure and such that the decoder can decode at least a portion of received media based on a reliably received 15 frame of the type R after packet loss or error and thereafter is synchronized with the encoder, and wherein the server selectively eliminates or modifies portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer in non-R frames only, prior to creating the output video signal that is forwarded to the at least one receiving endpoint. 20

54. The method of claim 50, further comprising, at the conferencing server controlling the transmission rate of the output video signal that is forwarded to the at least one receiving endpoint so that the retained portions of the input video signals received from transmitting endpoints that correspond to layers higher than the base spatial or quality layer do not adversely affect the smoothness of the output bit rate. 56 WO 2007/103889 PCT/US2007/063335

55. The method of claim 50, further comprising, at the conferencing server performing the selective elimination or modification according to desired output bit rate requirements.

56. The method of claim 50, further comprising, at the at least one 5 receiving endpoint displaying the decoded output picture at a desired spatial resolution that falls in between an immediately lower and an immediately higher spatial layer provided by the received coded video signal.

57. The method of claim 56, further comprising, at the at least one receiving endpoint, operating the decoding loop of the immediately higher spatial 10 layer at the desired spatial resolution by scaling all coded data of the immediately higher spatial layer to the desired spatial resolution, and wherein the resultant drift is eliminated by using at least one of periodic intra pictures, periodic use of intra base layer mode, 15 full resolution decoding of at least the lowest temporal layer of the immediately higher spatial layer.

58. The method of claim 50, wherein the scalable video coding format is further configured with at least one of periodic intra pictures; 20 periodic intra macroblocks; and threaded picture prediction; 57 WO 2007/103889 PCT/US2007/063335 in order to avoid drift when the higher than the base spatial or quality layer's coded information that is modified or eliminated corresponds to the base temporal layer.

59. The method of claim 50, further comprising, at the at least one 5 receiving endpoint operating at least one decoding loop for spatial or quality layers higher than the target spatial or quality layer for at least the base temporal layer, so that when the at least one receiving endpoint switches target layers it can immediately display decoded pictures at the new target layer resolution.

60. A video communication method comprising: 10 a communication network, one endpoint that transmits coded digital video using a scalable video coding format, and at least one receiving endpoint that is capable of decoding a digital video signal coded in a scalable video coding format supporting temporal 15 scalability and at least one of spatial and quality scalability, wherein the scalable video coding format for spatial scalability includes a base spatial and at least one spatial enhancement layer, for quality scalability includes a base quality layer at least one quality enhancement layer, and for temporal scalability includes a base temporal layer and at least one temporal enhancement layer, wherein 20 the base temporal layers and enhancement temporal layers are interlinked by a threaded picture prediction structure for at least one of the spatial or quality scalability layers, and 58 WO 2007/103889 PCT/US2007/063335 wherein the transmitting endpoint is configured to selectively eliminate or modify portions of its coded video signal that correspond to layers higher than the base spatial or quality layer, prior to creating the output video signal that is forwarded to the at least one receiving endpoint, so that use of lower spatial or quality layer data is 5 signaled or explicitly coded in the output video signal for use in decoding pictures at resolutions higher than the base spatial or quality layer.

61. The method of claim 60, wherein the scalable video coding format is based on hybrid coding such as in H.264, VC-1 or AVS standards, and wherein the lower spatial or quality layer data that is signaled for use or explicitly coded in the 10 output video signal forwarded to the at least one receiving endpoints is comprised of at least one of motion vector data; coded prediction error difference; intra data; and 15 reference picture indicators, wherein the data is further appropriately scaled to the desired target resolution when explicitly coded in the output video signal that is transmitted to the one or more receiving endpoints.

62. The method of claim 60, further comprising, at the transmitting 20 endpoint encoding transmitted media as frames in a threaded coding structure having a number of different temporal levels, wherein a subset of the frames ("R") is particularly selected for reliable transport and includes at least the frames of the lowest temporal layer in the threaded coding structure and such that the decoder can 59 WO 2007/103889 PCT/US2007/063335 decode at least a portion of received media based on a reliably received frame of the type R after packet loss or error and thereafter is synchronized with the encoder, and wherein the transmitting endpoint selectively eliminates or modifies portions of its input video signal that correspond to layers higher than the base spatial or quality 5 layer in non- R frames only, prior to creating the output video signal that is transmitted to the at least one receiving endpoint.

63. The method of claim 60, further comprising, at the transmitting endpoint controlling the transmission rate of the output video signal that is transmitted to the at least one receiving endpoint so that the retained portions of its input video 10 signal that correspond to layers higher than the base spatial or quality layer do not adversely affect the smoothness of the output bit rate.

64. The method of claim 60, further comprising, at the transmitting endpoint performing the selective elimination or modification according to desired output bit rate requirements. 15

65. The method of claim 60, further comprising, at the at least one receiving endpoint displaying the decoded output picture at a desired spatial resolution that falls in between an immediately lower and an immediately higher spatial layer provided by the received coded video signal.

66. The method of claim 65, further comprising, at the at least one 20 receiving endpoint operating the decoding loop of the immediately higher spatial layer at the desired spatial resolution by scaling all coded data of the immediately higher spatial layer to the desired spatial resolution, and wherein the resultant drift is eliminated by using at least one of periodic intra pictures, 60 WO 2007/103889 PCT/US2007/063335 periodic use of intra base layer mode, full resolution decoding of at least the lowest temporal layer of the immediately higher spatial layer.

67. The method of claim 60, wherein the scalable video coding format is 5 further configured with at least one of periodic intra pictures; periodic intra macroblocks; and threaded picture prediction, in order to avoid drift when the higher than the base spatial or quality layer's coded 10 information that is modified or eliminated corresponds to the base temporal layer.

68. The method of claim 60, further comprising, at the receiving endpoint operating at least one decoding loop for spatial or quality layers higher than the target spatial or quality layer for at least the base temporal layer, so that when the at least one receiving endpoint switches target layers it can immediately display decoded 15 pictures at the new target layer resolution.

69. Computer readable media comprising a set of instructions to perform the steps recited in at least one of the method claims 35-68. 61