François-Xavier Coudoux

Reduction of blocking effect in DCT-coded images based on a visual perception criterion

Abstract Blocking effect constitutes one of the main drawbacks of the actual DCT-based compression methods. We propose in this paper a new block reduction technique; it is based on a space-variant non-linear filtering operation of the blocking artifacts present in the image to be reconstructed. To account for the perceptual importance of the distortion, the amount of smoothing is adapted to the visibility of the blocking effect. A visibility parameter is computed for each artifact using the psychovisual properties of the human visual system (HVS). The postprocessing algorithm is in conformity with actual existing compression standards; it provides a way to greatly reduce the artifacts without degrading high-frequency information of the original image. First the proposed method is described and then experimental results are presented, showing the effectiveness of the correction.

A post-processor for reducing temporal busyness in low-bit-rate video applications

Abstract This paper presents an original post-processing algorithm to reduce the so-called temporal busyness phenomenon in low-bit-rate video coded sequences. The post-processor is designed to be implemented in digital video receivers; in this case, only reconstructed video is available with no reference to coding parameters. The proposed technique first identifies the distortion in decoded video, then reduces both mosquito noise and discrete cosine transform (DCT) basis images affect by appropriate 3D (2D+ t ) adaptive filtering, while preserving the sharpness and naturalness of the reconstructed video signals. The mosquito noise is reduced by 2D signal-adaptive filtering, and DCT basis image effects are reduced by temporal DCT median filtering. Experimental results show that the proposed post-filtering process improves the visual quality of video significantly.

Performance indicator for DMT systems with non perfect channel estimation

In this letter, we analyse the bit error rate (BER) of a discrete multitone modulation system considering the channel estimation error for the frequency equalisation. After having established an approximate relation for the BER, we propose a simple performance indicator, which accounts for the channel estimation error, the power emitted and the noise level. In order to check the validity of the indicator, simulation has been performed. The results prove that it is more adapted to discrete multi-tone modulation (DMT) systems than the classical mean squared channel estimation error (MSE), above all when the channel additive noise is coloured. Copyright

A Perceptual Approach to the Reduction of Blocking Effect in DCT-Coded Images

It is well known that blocking effects constitute one of the main drawbacks of discrete-cosine-transform-based lossy coding schemes. In this paper, we first propose a distortion measure for blocking artifacts in images. The visibility of blocking error is computed based on several characteristics of the observers visual perception, and an image quality factor is derived based on the artifact visibilities. We then develop a novel approach for removing blocking effects in transform-coded images. The technique is based on a nonlinear, spatially variant adaptive smoothing operation which exploits the visibility measure of blocking artifacts present in the decoded image. It introduces no modification on actual image compression standards and allows one to reduce the blockiness without significantly blurring the image. Experimental results obtained at low bit rate show that both objective and subjective quality of the reconstructed images are greatly improved.

Reduced Complexity Power Minimization Algorithm for DMT Transmission - Application to Layered Multimedia Services over DSL

This paper presents an original bit and power loading algorithm for discrete multi tone data transmission of layered multimedia services. The proposed method, based on the waterfilling principle, realizes the bit allocation directly from a non iterative LUT-based solution compared to classical loading algorithms. First, the method is described in the single layer case. Then, the method is extended for multiple services transmission. Each service is characterized by its own quality of service (QoS) requirement and the bit and power allocation minimizes the total power consumption under bit rates and bit error rates constraints. In this case, the simultaneous transmission of the multiple services is achieved using a frequency division multiplexing (FDM) scheme, and the method used to assign different layers to subchannels is detailed. Experimental results of video delivery over ADSL show that the proposed algorithm comes near the optimal solution (the increase of power is lower than 0.2 dB) whereas its computational complexity is lower than traditional loading algorithms.

An accurate performance analysis of an FFR scheme in the downlink of cellular systems under large-shadowing effect

We address the problem of analyzing the performances of interference-limited cellular networks in large-scale shadowing environments. Focusing on the fractional frequency reuse (FFR) framework, we examine how to optimally assign mobile users in a cell either to the full frequency reuse part or to the orthogonal part of the FFR band. Instead of using traditional Monte Carlo simulations, which do not provide sufficiently accurate results under important shadowing, we propose a fast and accurate numerical method. We consider a fast-fading environment and we use the ergodic capacity as the performance measure. Applying a distributed power control and scheduling strategy, we examine both cases where access points have knowledge of partial- or full-channel state information (CSI); for the latter, we also propose an approximated waterfilling strategy. The interest of our method lies in the fact that it allows for a fast and accurate analysis of the performances of FFR. In addition, it takes into account a broad range of shadowing environments.

Analysis of the power line channel estimation error based on comb-type pilot arrangement

In this paper, we analyze the error of channel estimation using the pilot tones associated with interpolation technique. Our method is based on a unified mathematical decomposition of the channel estimation error, which takes both the signal-to-noise ratio at pilot sub-carrier locations and the interpolation kernel used into account. This method has been recently proposed by the authors and illustrated over a channel model. We complete here the previous analysis and provide news results over a power line severe real channel.

Optimal Bit Rate Adaptation for Layered Video Transmission over Spectrally Shaped Channels Using Multicarrier Modulation

In this paper, we consider video transmission over spectrally shaped channels using multicarrier modulation (MCM). Here, the system is first composed of a two layer scalable video transcoding scheme, which adapts the total bit rate and splits the video bitstream into a base layer (BL) and an enhancement layer (EL). It is followed by a hierarchical bit and power loading algorithm to optimize the transmission over the MCM channel. For this whole system, we propose an original method to automatically determine both optimal BL and EL bit rates corresponding to the highest visual quality at the receiver side. Then we illustrate the proposed method in the case of video delivery over ADSL channels.

Picture quality measurement based on block visibility in discrete cosine transform coded video sequences

In this paper, we present a perceptual measure that pre- dicts the visibility of the well-known blocking effect in discrete cosine transform (DCT) coded image sequences. The main objective of this work is to use the results of the measure derived for adaptive video postprocessing, in order to significantly improve the visual quality of the video decoded sequences at the receiver. The proposed mea- sure is based on a visual model accounting for both the spatial and temporal properties of the human visual system. The input of the visual model is the distorted sequence only. Psycho-visual experi- ments have been carried out to determine the eye sensitivity to blocking artifacts, by varying a number of visually significant param- eters: background level, spatial, and temporal activities in the sur- rounding image. Results obtained for the measurement of the vis- ibility thresholds enable us to estimate the model parameters. The visual model is finally applied to real coded video sequences. The comparison of measurement results with subjective tests shows that the proposed perceptual measure has a good correlation with sub- jective evaluation.

New adaptive filters as perceptual preprocessing for rate-quality performance optimization of video coding

In this paper, we introduce two perceptual filters as pre-processing techniques to reduce the bitrate of compressed high-definition (HD) video sequences at constant visual quality. The goal of these perceptual filters is to remove spurious noise and insignificant details from the original video prior to encoding. The proposed perceptual filters rely on two novel adaptive filters (called BilAWA and TBil) which combine the good properties of the bilateral and Adaptive Weighting Average (AWA) filters. The bilateral and AWA filters being initially dedicated to denoising, the behaviour of the proposed BilAWA and TBil adaptive filters is first analyzed in the context of noise removal on HD test images. The first set of experimental results demonstrates their effectiveness in terms of noise removal while preserving image sharpness. A just noticeable distortion (JND) model is then introduced in the novel BilAWA and TBil filters to adaptively control the strength of the filtering process, taking into account the human visual sensitivity to signal distortion. Visual details which cannot be perceived are smoothed, hence saving bitrate without compromising perceived quality. A thorough experimental analysis of the perceptual JND-guided filters is conducted when using these filters as a pre-processing step prior to MPEG-4/AVC encoding. Psychovisual evaluation tests show that the proposed BilAWA pre-processing filter leads to an average bitrate saving of about 19.3% (up to 28.7%) for the same perceived visual quality. The proposed new pre-filtering approach has been also tested with the new state-of-the-art HEVC standard and has given similar efficiency in terms of bitrate savings for constant visual quality. Display Omitted Perceptual pre-filtering for rate-quality optimization of video codecs.High bit rate saving for same subjective quality demonstrated with an H.264/AVC and an HEVC codec.Perceptually -guided filters offering a good trade-off between noise removal, blurring and complexity.