Error concealment techniques for digital TV

Compressed video bitstreams are intended for real-time transmission over communication networks. Most of the video coding standards employ the temporal and spatial prediction structure to reduce the transmitted video data. Therefore, the coded video bitstreams are highly sensitive to information loss and channel errors. Even a single bit error can lead to disastrous quality degradation in both time and space. This quality deterioration is exacerbated when no error resilient coding mechanism is employed to protect coded video data against the error prone environments. Error concealment is a data recovery technique that enables the decoder to conceal effects of transmission errors by predicting the lost or corrupted video data from the previously reconstructed error-free information. Motion vector recovery and motion compensation with the estimated motion vector is a good approach to conceal the corrupted macroblock data. In this paper, we develop various error concealment algorithms based on motion vector recovery, and compare their performances to those of conventional error concealment methods.     

An error concealment algorithm for images subject to channel errors

We present an algorithm to conceal bit errors in still images and image sequences that are coded using the discrete cosine transform (DCT) and variable length codes (VLCs). No modification is necessary to an existing encoder, and no additional bit rate is required. The concealment algorithm is kept simple so that real-time decoding and concealment is possible. A single bit error in these images can cause a block to split into several blocks or several blocks to merge into one. This causes the DCT coefficients of all subsequent blocks to be correctly decoded but stored in the wrong location in the image. Furthermore, the DC coefficient of all subsequent blocks may be incorrect. The error concealment algorithm uses transform domain information to identify the location of the affected blocks and to correct errors. The image quality after error concealment is shown to be significantly improved.     

Soft‐output MMSE V‐BLAST receiver with MMSE channel estimation under correlated Rician fading MIMO channels

For wireless multiple‐input multiple‐output (MIMO) communications systems, both channel estimation error and spatial channel correlation should be considered when designing an effective signal detection system. In this paper, we propose a new soft‐output MMSE based Vertical Bell Laboratories Layered Space‐Time (V‐BLAST) receiver for spatially‐correlated Rician fading MIMO channels. In this novel receiver, not only the channel estimation errors and channel correlation but also the residual interference cancellation errors are taken into consideration in the computation of the MMSE filter and the log‐likelihood ratio (LLR) of each coded bit. More importantly, our proposed receiver generalizes all existing soft‐output MMSE V‐BLAST receivers, in the sense that, previously proposed soft‐output MMSE V‐BLAST receivers can be derived as the reduced forms of our receiver when the above three considered factors are partially or fully simplified. Simulation results show that the proposed soft‐output MMSE V‐BLAST receiver outperforms the existing receivers with a considerable gain in terms of bit‐error‐rate (BER) performance. Copyright © 2011 John Wiley & Sons, Ltd.     

Efficient error localization and temporal concealment based on motion estimation of enlarged block

We propose a modified motion estimation algorithm that is adequate for error localization and temporal error concealment in transmitting videos over unreliable channels. In order to achieve good error concealment performance, the proposed algorithm implicitly imposes spatial correlations on motion vectors by extending the block size and overlapping blocks in motion estimation. Thereby, the obtained motion vectors can be used to improve error concealment performance while keeping the encoding efficiency with negligible overhead. In addition, the proposed motion estimation can provide a new error detection measure so that we can maximally utilize uncorrupted data rather than simply discarding all data in a defected packet. Simulation results show that the proposed motion estimation scheme provides significant improvements in error concealment performance over the existing schemes and improves the bit utility over a wide range of error conditions.     

Multiframe error concealment for MPEG-coded video delivery over error-prone networks

Compressed video sequences are very vulnerable to channel disturbances when they are transmitted through an unreliable medium such as a wireless channel. Transmission errors not only corrupt the current decoded frame, but they may also propagate to succeeding frames. A number of post-processing error concealment (ECN) methods that exploit the spatial and/or temporal redundancy in the video signal have been proposed to combat channel disturbances. Although these approaches can effectively conceal lost or erroneous macroblocks (MBs), all of them only consider spatial and/or temporal correlation in a single frame (the corrupted one), which limits their ability to obtain an optimal recovery. Since the error propagates to the next few motion-compensated frames in the presence of lost MBs in an I or P frame, error concealment should simultaneously minimize the errors not only in the current decoded frame but also in the succeeding B and P frames that depend on the corrupted frame. We propose a novel multiframe recovery principle which analyzes the propagation of a lost MB into succeeding frames. Then, MPEG-compatible spatial and temporal error concealment approaches using this multiframe recovery principle are proposed, where the lost MBs are recovered in such a way that the error propagation is minimized.     

Frame Error Concealment Using Pixel Correlation in Overlapped Motion Compensation Regions

In low bit‐rate video transmission, the payload of a single packet can often contain a whole coded frame due to the high compression ratio in both spatial and temporal domains of most modern video coders. Thus, the loss of a single packet not only causes the loss of a whole frame, but also produces error propagation into subsequent frames. In this paper, we propose a novel whole frame error concealment algorithm which reconstructs the first of the subsequent frames instead of the current lost frame to suppress the effects of error propagation. In the proposed algorithm, we impose a constraint which uses side match distortion (SMD) and overlapped region difference (ORD) to estimate motion vectors between the target reconstructed frame and its reference frame. SMD measures the spatial smoothness connection between a block and its neighboring blocks. ORD is defined as the difference between the correlated pixels which are predicted from one reference pixel. Experimental results show that the proposed algorithm effectively suppresses error propagation and significantly outperforms other conventional techniques in terms of both peak signal‐to‐noise ratio performance and subjective visual quality.     

基于H.263视频码流的差错掩蔽算法

在H．263视频信号的传输过程中,由于采用了可变长编码,当发生比特错误时,很容易造成错误的扩散传播及图像质量下降。由于H．263为低速率传输模式,在进行错误恢复时不适于采用网络开销较大的前向纠错方式,所以采用了基于解码器端的差错掩蔽方法来消除错误的影响。使用的是自适应分类差错掩蔽方法,通过分析图像的运动剧烈程度,将图像分为三类进行掩蔽,既降低了运算复杂度,又取得了较好的掩蔽效果。     

Effects of pointing errors and channel fading on the performance of free‐space optically‐preamplified PPM systems

In this paper, we study the impact of pointing errors and channel fading on the performance of free‐space, optically preamplified, M ‐ary PPM systems. We consider two types of free‐space optical links: (i) inter‐satellite links and (ii) inter‐building links. For inter‐satellite links, only pointing error is considered. Starting with a Rayleigh model for the pointing error angle, we derive analytically the PDF for the pointing error parameter and for the signal‐to‐noise ratio (SNR) per bit. For inter‐building links, we derive the density function for the SNR per bit that includes the combined effects of pointing errors and channel fading, assuming Rayleigh‐distributed pointing errors. The channel fading models considered in this study for inter‐buildings links are the log‐normal and gamma–gamma models. We provide the error probability as a function of the average SNR per bit for both types of links. To cover systems with and without forward error correction, we compute the average SNR per bit required to achieve a bit error rate of 10^?4 and 10^?9. The corresponding power penalties are computed for different symbol sizes, scintillation indexes, and pointing jitters. Copyright © 2016 John Wiley & Sons, Ltd.     

An Error Concealment Approach Based on H.263 Video Decoding in Pixel Domain

1　ＩｎｔｒｏｄｕｃｔｉｏｎＷｉｔｈｔｈｅｒａｐｉｄｌｙｄｅｖｅｌｏｐｍｅｎｔｏｆｍｕｌｔｉｍｅｄｉａｃｏｍｍｕｎｉｃａｔｉｏｎｔｅｃｈｎｏｌｏｇｙ ,ｔｈｅＨ .2 6 1 ,ＭＰＥＧ 1ａｎｄＭＰＥＧ 2ｒｅｃｏｍｍｅｎｄａｔｉｏｎｓｈａｖｅｂｅｅｎｅｓｔａｂ ｌｉｓｈｅｄｂｙｔｈｅｒｅｌｅｖａｎｔｏｒｇａｎｉｚａｔｉｏｎｓｏｆＩＴＵ ＴｏｒＩＳＯｉｎｔｈｅｐａｓｔｙｅａｒｓ.Ｔｈｅｓｅｓｔａｎｄａｒｄｓｐｒｏｖｉｄｅｔｈｅｔｏｏｌｓｆｏｒｉｍａｇｅｓｔｏｒｅｏｒｔｒａｎｓｍｉｓｓｉｏｎａｔｔｈｅｈｉｇｈｅｒ…     

Error-resilient transcoding for video over wireless channels

We describe a method to maintain quality for video transported over wireless channels. The method is built on three fundamental blocks. First, we use a transcoder that injects spatial and temporal resilience into an encoded bitstream. The amount of resilience is tailored to the content of the video and the prevailing error conditions, as characterized by bit error rate. Second, we derive analytical models that characterize how corruption propagates in a video that is compressed using motion-compensated encoding and subjected to bit errors. Third, we use rate distortion theory to compute the optimal allocation of bit rate among spatial resilience, temporal resilience, and source rate. Furthermore, we use the analytical models to generate the resilience rate distortion functions that are used to compute the optimal resilience. The transcoder then injects this optimal resilience into the bitstream. Simulation results show that using a transcoder to optimally adjust the resilience improves video quality in the presence of errors while maintaining the same input bit rate     

Error Concealment Using Inter‐layer Correlation for Scalable Video Coding

In this paper, we propose a new error concealment (EC) method using inter‐layer correlation for scalable video coding. In the proposed method, the auxiliary motion vector (MV) and the auxiliary mode number (MN) of intra prediction are interleaved into the bitstream to recover the corrupted frame. In order to reduce the bit rate, the proposed method encodes the difference between the original and the predicted values of the MV and MN instead of the original values. Experimental results show that the proposed EC outperforms the conventional EC by 2.8 dB to 6.7 dB.     

整帧丢失时基于多帧参考的差错掩盖算法

压缩视频对于信道误码十分敏感,可导致重组视频的质量严重下降。在低码率的情况下,当网络传输中出现数据包丢失的情况时,通常对应于一整帧图像内容的丢失。为此本文提出了两种针对整帧图像丢失的差错掩盖算法一基于多帧参考的差错掩盖算法。研究结果表明,该算法不仅能够恢复整个丢失帧,而且其掩盖效果比传统的掩盖算法更好。     

Spatial error concealment with edge related perceptual considerations

Video transmission over error-prone networks can suffer from packet erasures which can greatly reduce the quality of the received video. Error concealment methods reduce the perceived quality degradation at the receiving end by masking the effects of such errors. They accomplish this by exploiting temporal and spatial correlations that exist in image sequences. Spatial error concealment approaches conceal errors by making use of spatial information only which is necessary in cases where motion information is not available or reliable. The performance of such methods can be greatly increased if perceptual considerations are taken into account as, e.g., the preservation of edge information. This paper proposes a spatial error concealment method that uses edge-related information in order not only to preserve existing edges but also to avoid introducing new strong ones by switching to a smooth approximation of missing information where necessary. A novel switching algorithm which uses the directional entropy of neighbouring edges chooses between two interpolation methods, a directional along detected edges or a bilinear using the nearest neighbouring pixels. Results show that the performance of the proposed method is better compared to both ‘single interpolation’ and to edge strength-based switching methods.     

On cracking direct‐sequence spread‐spectrum systems

Secure transmission of information over hostile wireless environments is desired by both military and civilian parties. Direct‐sequence spread‐spectrum (DS‐SS) is such a covert technique resistant to interference, interception, and multipath fading. Identifying spread‐spectrum signals or cracking DS‐SS systems by an unintended receiver (or eavesdropper) without a priori knowledge is a challenging problem. To address this problem, we first search for the start position of data symbols in the spread signal (for symbol synchronization); our method is based on maximizing the spectral norm of a sample covariance matrix, which achieves smaller estimation error than the existing method of maximizing the Frobenius norm. After synchronization, we remove a spread sequence by a cross‐correlation based method, and identify the spread sequence by a matched filter. The proposed identification method is less expensive and more accurate than the existing methods. We also propose a zigzag searching method to identify a generator polynomial that reduces memory requirement and is capable of correcting polarity errors existing in the previous methods. In addition, we analyze the bit error performance of our proposed method. The simulation results agree well with our analytical results, indicating the accuracy of our analysis in additive white Gaussian noise (AWGN) channel. By simulation, we also demonstrate the performance improvement of our proposed schemes over the existing methods. Copyright © 2009 John Wiley & Sons, Ltd.     

基于多方向边界匹配的视频误码掩盖算法

视频压缩码流对于信道误码十分敏感,可能导致重建图像质量严重下降.误码掩盖技术利用图像在时间和空间上的相关性,可以有效地降低误码对视频图像的影响.文中提出了一种基于多方向边界匹配的时域误码掩盖算法.该算法能更精确地恢复错误宏块的运动矢量,从而获得比传统的时域掩盖算法更好的视频质量.     

Combined and iterative form of spatial and temporal error concealment for video signals

Video transmission over unreliable channels may suffer from the data loss or corruption. To facilitate the transmission, video content is compressed to save the bandwidth. The compression algorithms remove the redundancies in the video signal, meanwhile increasing the dependencies among symbols in the compressed bit-stream. Thus, when errors occur, they may propagate in both space and time. Among various error control techniques, error concealment (EC) is an effective method that is performed at the decoder to mitigate the influence of errors on the quality of reconstructed images. In this paper, a joint spatial and temporal EC algorithm (JSTEC) is presented. First, several existing temporal EC algorithms are combined in an appropriate order. Then, the spatial correlation in the video is exploited in an iterative form to improve the performance of the temporal EC. Experimental results show that JSTEC performs better both in peak signal-to-noise ratio and subjective quality of images than the existing algorithms.     

Hardware‐Software Implementation of MPEG‐4 Video Codec

This paper presents an MPEG‐4 video codec, called MoVa, for video coding applications that adopts 3G‐324M. We designed MoVa to be optimal by embedding a cost‐effective ARM7TDMI core and partitioning it into hardwired blocks and firmware blocks to provide a reasonable tradeoff between computational requirements, power consumption, and programmability. Typical hardwired blocks are motion estimation and motion compensation, discrete cosine transform and quantization, and variable length coding and decoding, while intra refresh, rate control, error resilience, error concealment, etc. are implemented by software. MoVa has a pipeline structure and its operation is performed in four stages at encoding and in three stages at decoding. It meets the requirements of MPEG‐4 SP@L2 and can perform either 30 frames/s (fps) of QCIF or SQCIF, or 7.5 fps (in codec mode) to 15 fps (in encode/decode mode) of CIF at a maximum clock rate of 27 MHz for 128 kbps or 144 kbps. MoVa can be applied to many video systems requiring a high bit rate and various video formats, such as videophone, videoconferencing, surveillance, news, and entertainment.     

Robust image transmission using resynchronizing variable-lengthcodes and error concealment

Resynchronizing variable-length codes (RVLCs) for large alphabets are designed by first creating resynchronizing Huffman codes and then adding an extended synchronizing codeword, and the RVLCs are applied to both JPEG and wavelet-based image compression. The RVLCs demonstrate the desired resynchronization properties, both at a symbol level and structurally so that decoded data can be correctly placed within an image following errors. The encoded images, when subject to both structural and statistical error detection and concealment, can tolerate BERs of up to 10^-4 and are very tolerant of burst errors. The RVLC-JPEG images have negligible overhead at visually lossless bit rates, while the RVLC-wavelet overhead can be adjusted based on the desired tolerance to burst errors and typically ranges from 7 to 18%. The tolerance to both bit and burst errors demonstrates that images coded with such RVLCs can be transmitted over imperfect channels suffering bit errors or packet losses without channel coding for the image data, or with less channel coding than would be required if the encoded image data could tolerate no bit errors. While the overhead is nontrivial for the RVLC-wavelet images and the lower-rate RVLC-JPEG images, the encoded bitstreams do not have the firm restrictions on numbers or spacings of bit errors that some error correcting codes have, and hence provide more graceful degradation     

A Full Rate Quasi‐orthogonal STF‐OFDM with DAC‐ZF Decoder over Wireless Fading Channels

In this letter, we propose a quasi‐orthogonal space‐time‐frequency (QOSTF) block coded orthogonal frequency division multiplexing (OFDM) that can achieve full symbol rate with four transmit antennas. Since the proposed QOSTF‐OFDM cannot achieve full diversity, we use a diversity advantage collection with zero forcing (DAC‐ZF) decoder to compensate the diversity loss at the receiving side. Due to modulation advantage and collected diversity advantage, the proposed scheme exhibits a better bit‐error rate performance than other orthogonal schemes.