On optimal analysis/synthesis filters for coding gain maximization

We consider the use of pre and postfilters in conjunction with M-channel, uniform-band paraunitary (orthonormal) filter banks. We show that given any orthonormal filter bank, the pre and postfilters that maximize the coding gain are determined entirely by the power spectrum of the input process regardless of the details of the orthonormal filter blank (which could be FIR, IIR, or even the ideal brickwall filter bank). The optimized coding gain, however, depends on the prefilter as well as the sandwiched orthonormal filter bank. The coding gain improvement due to pre and postfiltering is often significant as we demonstrate with numerical examples and comparisons. The validity of our results depends strongly on the orthonormality property of the filter bank in between the pre and postfilters. In the nonorthonormal case, most of these results are not true, as is demonstrated     

A digital alternative to a video decoder

New video transmission systems such as the satellite D2-MAC/packet system and second-generation local area networks for video-communications will use digital coding and decoding of video signals. To develop reliable and low-cost receivers, it appears necessary to implement the main functions using CMOS technology. Therefore many analog functions must be replaced by the equivalent digital ones before implementation. Digital circuits are described for the final video postprocessing, i.e. video postfiltering and dematrixing. An NMOS digital filter for Y, CR, and CB oversampling is presented. R, G, and B components are obtained using a CMOS matrix operator, also described. These two circuits are used in a 108-Mb/s experimental video decoder.     

Correlation based universal image/video coding loss recovery

Coding artifacts are annoying in highly compressed signals. Most of the existing artifact reduction methods are designed for one specific type of artifacts, codecs, and bitrates, which are complex and exclusive for one type of artifact reduction. Since both the compressed image/video and the coding error contain information of the original signal, they are highly correlated. Therefore, we try to recover some lost data based on the correlation between the compressed signal and the coding error, and introduce a novel and universal artifact reduction method. Firstly, according to the spatial correlation among pixels, a pixel-adaptive anisotropic filter is designed to reconstruct the distorted signal. Next, a globally optimal filter is designed to further recover the coding loss. Experimental results demonstrate that within an extensive range of bitrates, the proposed method achieves about 0.8 dB, 0.45 dB, 0.3 dB, and 0.2 dB on average of PSNR improvement for JPEG, MPEG4, H.264/AVC, and HEVC compressed signals, respectively.     

Undersampled boundary pre-/postfilters for low bit-rate DCT-based block coders.

It has been well established that critically sampled boundary pre-/postfiltering operators can improve the coding efficiency and mitigate blocking artifacts in traditional discrete cosine transform-based block coders at low bit rates. In these systems, both the prefilter and the postfilter are square matrices. This paper proposes to use undersampled boundary pre- and postfiltering modules, where the pre-/postfilters are rectangular matrices. Specifically, the prefilter is a "fat" matrix, while the postfilter is a "tall" one. In this way, the size of the prefiltered image is smaller than that of the original input image, which leads to improved compression performance and reduced computational complexities at low bit rates. The design and VLSI-friendly implementation of the undersampled pre-/postfilters are derived. Their relations to lapped transforms and filter banks are also presented. Two design examples are also included to demonstrate the validity of the theory. Furthermore, image coding results indicate that the proposed undersampled pre-/postfiltering systems yield excellent and stable performance in low bit-rate image coding.     

新一代视频编码标准HEVC中的环路滤波技术分析

新一代视频编码标准HEVC中增加了环路滤波技术来提高编码图像质量。本文研究了滤波过程中的两种算法：去块滤波和自适应采样点滤波。去块滤波可以消除块效应,自适应采样点滤波可以减小图像失真。通过实验仿真,分析了滤波技术的有效性。结果表明该滤波技术提高了编码性能,改善了图像质量。     

Avoiding slow band-edge convergence in subband echo cancelers

Subband echo cancelers decompose their input signals into frequency bands and then do echo canceling on a per-band basis. Such structures have both computational and algorithmic advantages over conventional full-band structures. However, many implementations have been found to suffer from slow convergence at band edges, that is, at frequencies carried roughly equally by two adjacent subbands. A poorly conditioned correlation matrix with eigenvalues approaching zero is generally cited as the culprit. One possible solution to the poor band-edge convergence problem is a scheme dubbed postfiltering by Morgan (1995) and De Leon (1995). Under postfiltering, the slowly converging eigenmodes continue to be excited, but their energy is removed from the final output. An alternative solution developed here makes the subband analysis filter for the reference signal slightly broader in spectrum than the analysis filter for the echo. With unequal analysis filtering, it is possible to avoid eliciting the bad eigenmodes in the first place. There are important practical advantages for unequal analysis filtering. Extreme care must be exercised in filter design. However, given a proper understanding of requirements, it is possible to synthesize reasonable-length FIR filters with entirely satisfactory performance     

Adaptive deblocking method for low bitrate coded HEVC video

HEVC in-loop deblocking filter significantly improves the subjective quality of coded video by removing blocking artifact. However, there are still visible blocking artifacts in the complex videos with fast and chaotic motions coded at a low bitrate. In this paper, we propose a three-step deblocking filter scheme, which pre-processes video to remove undesired noise, next removes the corner outliers, and then suppresses the normal blocking artifacts with adaptive deblocking filters. The whole deblocking filtering process is applied on both luma and chroma components. Experimental results show that the proposed method could effectively improve the subjective quality for various videos, and outperform other typical post-processing deblocking methods.     

Morphological pseudoconvolutions: One-parameter families of derived filters with increased invariant classes

The present paper introduces a digital gray-scale morphological filtering technique that is based on postfiltering a given filter so the invariant class of the new filter is larger than the invariant class of the original filter. More specifically, the invariant class of the new filter contains the original invariant class together with all signals whose variation is below a chosen threshold. The postfiltering includes a single erosion and a single dilation by a one-parameter structuring element, the choice of parameter determining the resulting invariant class. While the methodology is quite general, the two applications considered pertain to moving averages with nonnegative weights and moving medians, both of which are morphological filters. Both suppress noise in a signal and each possesses specific advantages and disadvantages. In brief, means tend to give better noise suppression while blurring edges, whereas medians preserve edges, while at the same time flattening small background variation in the underlying signal. The new one-parameter family of filters derived from moving averages are called pseudomeans. The filters derived from medians are called pseudomedians. Both preserve uncorrupted low background variation, as well as steps. Because they preserve small variation while at the same time behaving like the original filters, both filters are especially effective when the noise occurs in bursts, rather than uniformly across the signal.     

Computation of the Para-Pseudoinverse for Oversampled Filter Banks: Forward and Backward Greville Formulas

Frames and oversampled filter banks have been extensively studied over the past few years due to their increased design freedom and improved error resilience. In frame expansions, the least square signal reconstruction operator is called the dual frame, which can be obtained by choosing the synthesis filter bank as the para-pseudoinverse of the analysis bank. In this paper, we study the computation of the dual frame by exploiting the Greville formula, which was originally derived in 1960 to compute the pseudoinverse of a matrix when a new row is appended. Here, we first develop the backward Greville formula to handle the case of row deletion. Based on the forward Greville formula, we then study the computation of para-pseudoinverse for extended filter banks and Laplacian pyramids. Through the backward Greville formula, we investigate the frame-based error resilient transmission over erasure channels. The necessary and sufficient condition for an oversampled filter bank to be robust to one erasure channel is derived. A postfiltering structure is also presented to implement the para-pseudoinverse when the transform coefficients in one subband are completely lost.      

Adaptive Fuzzy Filtering for Artifact Reduction in Compressed Images and Videos

A fuzzy filter adaptive to both sample's activity and the relative position between samples is proposed to reduce the artifacts in compressed multidimensional signals. For JPEG images, the fuzzy spatial filter is based on the directional characteristics of ringing artifacts along the strong edges. For compressed video sequences, the motion compensated spatiotemporal filter (MCSTF) is applied to intraframe and interframe pixels to deal with both spatial and temporal artifacts. A new metric which considers the tracking characteristic of human eyes is proposed to evaluate the flickering artifacts. Simulations on compressed images and videos show improvement in artifact reduction of the proposed adaptive fuzzy filter over other conventional spatial or temporal filtering approaches.      

Statistical Properties of Timing Jitter in a PAM Timing Recovery Scheme

A new technique is presented for evaluating the performance of a popular type of timing recovery circuit for baseband synchronous pulse amplitude modulation (PAM) data signals. The timing circuit consists of a square-law device followed by a narrowband filter tuned to the pulse repetition frequency along with provision for reshaping the pulses entering the timing path (prefiltering). The output of the timing circuit is a nearly sinusoidal timing wave whose zero crossings indicate the appropriate sampling instants for demodulation of the PAM signal. For a random data sequence, the timing wave exhibits phase fluctuations which are strongly dependent on the pulse shapes entering the timing path and the passband shape of the narrow-band filter. Expressions for rms phase fluctuation in the timing wave as a function of the prefiltering and postfiltering characteristics of the filters preceding and following the square-law device are presented. These expressions have a form which is especially suitable for studying the case where the baseband PAM signal is band-limited to frequencies less than the pulse repetition frequency. A condition on prefiltering and postfiltering which gives error-free timing recovery is presented. Results obtained from some specific examples serve to illustrate several aspects of the timing recovery problem.     

H.265解码器去块滤波并行化设计与性能优化

H.265继续沿用H.264编码架构,去方块滤波器也是H.265视频编码标准的一个重要选项,去除混合编码带来的块效应极大改善了视频的质量,但由于H.265超级宏块的存在,去方块效应滤波相关参数层层嵌入在每个小的处理单元中,这种结构不利于实现基于宏块行间的并行化,同时也很难高效地利用Cortex-A9架构SIMD优化性能.首先详细分析H.265标准去块滤波器的处理过程以及并行处理的困难,进而提出一种便于实现基于宏块行间的并行去块滤波结构,然后进行Cortex-A9汇编优化.基于HM14.0实验,改进去方块效应滤波器计算复杂度从占整个解码器25％降至14％,大大提升了解码器性能,为移动设备上实现H.265大分辨率视频实时播放奠定基础.     

Detection and measurement of the blocking artifact in decoded video frames

A method for measuring the blocking artifact in video frames is presented, which is capable of detecting this artifact even outside a regular geometric structure, that is, on moving objects in frames encoded with prediction-based techniques. Information-theoretic measures are integrated with models of the human perception, to account for the visibility of the artifact on different image content. A final metric is produced that yields coherent values on variously degraded versions derived from different originals. No information is required on the encoding procedure that originated the artifact; this makes this method suitable to operate on the decoded version of the frames, at the final stage of the video processing chain. Experiments show that the metric has a good correlation with subjective scores and possesses some additional desirable properties.     

针对MPEG的去块效应技术的研究及其VLSI实现

现今随着数据压缩的广泛应用,以及MPEG视频编解码的普遍使用,块效应噪声也随之而来。为了消除块效应、提高视频质量,许多去块效应的算法[2-12]被人们所提出,然而大部分算法的运算量都很大,不适合应用在视频处理芯片上。针对这个问题,H.-S. Kong等人[13]提出了一种新的自适应后处理算法,该算法既能有效消除块效应又大幅度地降低了运算量,然而对图像边缘信息保护不够。本文在H.-S. Kong等人[13]的算法的基础上,提出了一种针对MPEG视频流的去方块后处理方法。该方法以方块边缘（水平或垂直）附近各两个4×4区域作为滤波处理的分类依据,通过对该两个区域的像素点的值进行计算,将块边缘分为平滑渐变区域、亮度阶跃区域以及复杂纹理区域,并根据这三种区域对图像边缘信息影响程度的大小,而决定采取强滤波、弱滤波或者不滤波等处理方式。这种方法一方面较好地提取了块效应噪声,一方面又保护了图像的边缘信号,而且计算量较小,满足视频处理芯片低复杂度的去噪要求,通过VLSI实现后有着很好的应用前景。      

A pixel-wise directional intra prediction method

In a decade where the prominence of video applications has become increasingly prevalent, the optimization of video coding processes remains as important as ever. The directional intra prediction has been proved as an effective tool for intra picture coding. However, in a directional intra prediction based coding structure, there is a considerable gap between the transform efficiency of DCT and the optimal transform KLT. In this paper, we analyze these problems and propose a pixel-wise directional intra prediction (PDIP) method to solve this problem. This method exploits the reconstruction value of the adjacent pixels to predict the current pixel, and keeps the block-based lossy coding structure of H.264/AVC. The proposed method can significantly improve the coding efficiency with little decoding complexity increment.     

基于HOG和粒子滤波的足球视频中球的跟踪方法

在足球视频中,由于球员和摄像机的运动使得基于背景的方法无法有效检测和跟踪足球;足球在视频中所占像素个数少、运动复杂,并且在球场中有相似目标如球袜等的干扰,使得目标难以准确跟踪。为改善足球的跟踪效果,文中提出了一种基于梯度向量直方图和粒子滤波的足球跟踪算法。算法以粒子滤波为跟踪框架,用HOG特征与颜色特征融合,结合足球的结构信息与颜色信息,实现了足球视频中足球的稳定跟踪。实验结果表明,文中算法跟踪足球更准确,并可在存在相似目标干扰的条件下有效地跟踪足球。     

Object-oriented H.263 compatible video coding platform forconferencing applications

This paper describes a prototype video coding platform meant for the conception and testing of multimedia products such as next-generation videophones. The platform is largely based on ITU-T Recommendation H.263, with a number of additional object-oriented quality enhancement features which make it especially well suited for very low bit-rate coding of “head-and-shoulders” video material typical of real-time multimedia applications, video teleconferencing, and video telephony. These features consist of: (1) segmentation into objects of interest, (2) segmentation-based prefiltering, (3) model-assisted rate control, (4) adaptive vector quantization, and finally (5) segmentation-based postfiltering. In the spirit of Recommendation H.263, these enhancements are modular and can be selectively turned on or off, thereby enabling a wide variety of coding modes     

Optimal resource allocation for wireless video over CDMA networks

We present a multiple-channel video transmission scheme in wireless CDMA networks over multipath fading channels. We map an embedded video bitstream, which is encoded into multiple independently decodable layers by 3D-ESCOT video coding technique, to multiple CDMA channels. One video source layer is transmitted over one CDMA channel. Each video source layer is protected by a product channel code structure. A product channel code is obtained by the combination of a row code based on rate compatible punctured convolutional code (RCPC) with cyclic redundancy check (CRC) error detection and a source-channel column code, i.e., systematic rate-compatible Reed-Solomon (RS) style erasure code. For a given budget on the available bandwidth and total transmit power, the transmitter determines the optimal power allocations and the optimal transmission rates among multiple CDMA channels, as well as the optimal product channel code rate allocation, i.e., the optimal unequal Reed-Solomon code source/parity rate allocations and the optimal RCPC rate protection for each channel. In formulating such an optimization problem, we make use of results on the large-system CDMA performance for various multiuser receivers in multipath fading channels. The channel is modeled as the concatenation of wireless BER channel and a wireline packet erasure channel with a fixed packet loss probability. By solving the optimization problem, we obtain the optimal power level allocation and the optimal transmission rate allocation over multiple CDMA channels. For each CDMA channel, we also employ a fast joint source-channel coding algorithm to obtain the optimal product channel code structure. Simulation results show that the proposed framework allows the video quality to degrade gracefully as the fading worsens or the bandwidth decreases, and it offers improved video quality at the receiver.     

基于传声器阵列语音增强方法研究

传声器阵列通过对拾取的多路语音信号进行分析与处理,能取得改进语音质量、消除背景噪声和提高语音可懂度等明显效果,现已成为语音信号增强的一个重要的研究领域。介绍了基于传声器阵列的自适应波束形成方法,该方法采用GSC结构基于TF-GSC的最优后置滤波算法。仿真实验结果表明,该自适应波束形成器对干扰有很好的消除作用,对阵元的增益误差、位置误差不敏感,可以取得较好的语音增强效果。