Sphere Packings, Lattices and Groups

Distributed coding is a new paradigm for video compression, based on Slepian and Wolf's and Wyner and Ziv's information-theoretic results from the 1970s. This paper reviews the recent development of practical distributed video coding schemes. Wyner-Ziv coding, i.e., lossy compression with receiver side information, enables low-complexity video encoding where the bulk of the computation is shifted to the decoder. Since the interframe dependence of the video sequence is exploited only at the decoder, an intraframe encoder can be combined with an interframe decoder. The rate-distortion performance is superior to conventional intraframe coding, but there is still a gap relative to conventional motion-compensated interframe coding. Wyner-Ziv coding is naturally robust against transmission errors and can be used for joint source-channel coding. A Wyner-Ziv MPEG encoder that protects the video waveform rather than the compressed bit stream achieves graceful degradation under deteriorating channel conditions without a layered signal representation.

Distributed Video Coding

A theoretical analysis of the overall mean squared error (MSE) in hybrid video coding is presented for the case of error prone transmission. Our model covers the complete transmission system including the rate-distortion performance of the video encoder, forward error correction, interleaving, and the effect of error concealment and interframe error propagation at the video decoder. The channel model used is a 2-state Markov model describing burst errors on the symbol level. Reed-Solomon codes are used for forward error correction. Extensive simulation results using an H.263 video codec are provided for verification. Using the model, the optimal tradeoff between INTRA and INTER coding as well as the optimal channel code rate can be determined for given channel parameters by minimizing the expected MSE at the decoder. The main focus of this paper is to show the accuracy of the derived analytical model and its applicability to the analysis and optimization of an entire video transmission system.

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Analysis of video transmission over lossy channels

This paper addresses the problem of streaming packetized media over a lossy packet network in a rate-distortion optimized way. We show that although the data units in a media presentation generally depend on each other according to a directed acyclic graph, the problem of rate-distortion optimized streaming of an entire presentation can be reduced to the problem of error-cost optimized transmission of an isolated data unit. We show how to solve the latter problem in a variety of scenarios, including the important common scenario of sender-driven streaming with feedback over a best-effort network, which we couch in the framework of Markov decision processes. We derive a fast practical algorithm for nearly optimal streaming in this scenario, and we derive a general purpose iterative descent algorithm for locally optimal streaming in arbitrary scenarios. Experimental results show that systems based on our algorithms have steady-state gains of 2-6 dB or more over systems that are not rate-distortion optimized. Furthermore, our systems essentially achieve the best possible performance: the operational distortion-rate function of the source at the capacity of the packet erasure channel.

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Rate-distortion optimized streaming of packetized media

Throughout this article, we concentrate on the transcoding of block-based video coding schemes that use hybrid discrete cosine transform (DCT) and motion compensation (MC). In such schemes, the frames of the video sequence are divided into macroblocks (MBs), where each MB typically consists of a luminance block (e.g., of size 16 /spl times/ 16, or alternatively, four 8 /spl times/ 8 blocks) along with corresponding chrominance blocks (e.g., 8 /spl times/ 8 Cb and 8 /spl times/ 8 Cr). This article emphasizes the processing that is done on the luminance components of the video. In general, the chrominance components can be handled similarly and will not be discussed in this article. We first provide an overview of the techniques used for bit-rate reduction and the corresponding architectures that have been proposed. Then, we describe the advances regarding spatial and temporal resolution reduction techniques and architectures. Additionally, an overview of error resilient transcoding is also provided, as well as a discussion of scalable coding techniques and how they relate to video transcoding. Finally, the article ends with concluding remarks, including pointers to other works on video transcoding that have not been covered in this article, as well as some future directions.

Video transcoding architectures and techniques: an overview

In this article we describe and investigate an Internet video streaming system based on a scalable video coder combined with unequal error protection that maintains an acceptable picture quality over a wide range of connection qualities. The proposed approach does not require any specific support from the network layer and is especially suited for Internet multicast applications where different users are perceiving different transmission conditions and no feedback channel can be employed. We derive a theoretical framework for the overall system by which the Internet packet loss behavior can be directly related to the picture quality perceived at the receiver. We demonstrate how this framework can be used to select appropriate parameter values for the overall system design. Experimental results show how the presented system achieves a gracefully degrading picture quality for packet losses up to 30%.

Robust Internet video transmission based on scalable coding and unequal error protection

Long-term memory prediction extends the spatial displacement vector utilized in hybrid video coding by a variable time delay, permitting the use of more than one reference frame for motion compensation. This extension leads to improved rate-distortion performance. However, motion compensation in combination with transmission errors leads to temporal error propagation that occurs when the reference frames at the coder and decoder differ. In this paper, we present a framework that incorporates an estimated error into rate-constrained motion estimation and mode decision. Experimental results with a Rayleigh fading channel show that long-term memory prediction significantly outperforms the single-frame prediction H.263-based anchor. When a feedback channel is available, the decoder can inform the encoder about successful or unsuccessful transmission events by sending positive (ACK) or negative (NACK) acknowledgments. This information is utilized for updating the error estimates at the encoder. Similar concepts, such as the ACK and NACK mode known from the H.263 standard, are unified into a general framework providing superior transmission performance.

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Error-resilient video transmission using long-term memory motion-compensated prediction

A theoretical analysis of the overall mean squared error (MSE) in hybrid video coding is presented for the case of error prone transmission The derived model for interframe error propagation includes the effects of INTRA coding and spatial loop filtering and corresponds to simulation results very accurately For a given target bit rate, only four parameters are necessary to describe the overall distortion behavior of the decoder Using the model, the optimal trade-off between INTRA and INTER coding can be determined for a given packet loss probability by minimizing the expected MSE at the decoder

Analysis of error propagation in hybrid video coding with application to error resilience

A new content-dependent fast discrete cosine transform (DCT) algorithm is introduced. Since it requires less than half of the samples of an 8/spl times/8 block as input and produces only the first three DCT coefficients, it is far less complex than conventional full fast DCT algorithms. It is shown that these three coefficients are sufficient for the majority of the blocks for motion-compensated low bit-rate video coding. For other blocks, a conventional full-coefficient DCT is employed. Experiments with both low and high complexity sequences show that the content-dependent algorithm reduces the computational load of the DCT by a factor >2/spl times/ without loss in image quality and a factor 4/spl times/, if a slight PSNR loss is acceptable.

K. Stuhlmuller

Papers

Analysis of video transmission over lossy channels

Robust Internet video transmission based on scalable coding and unequal error protection

Error-resilient video transmission using long-term memory motion-compensated prediction

Analysis of error propagation in hybrid video coding with application to error resilience

A content-dependent fast DCT for low bit-rate video coding