Vulnérabilité aux erreurs de transmission de signaux sonores de haute qualité et méthodes de protection

The present paper deals with protection methods of high quality digital sound signals in presence of transmission errors. They are strongly related to the coding and the bit rate reduction methods used in the digital treatment of sound signals. Therefore, the first part is assigned to these aspects. Part 2 describes a procedure for a subjective evaluation of the impairments caused on digital sound signals by transmission errors. This procedure was used during tests realised in the ccett.Part 3 summarizes the main results of these tests and proposes a strategy for the protection which ensures a satisfying compromise between quality and simplicity.     

Le programme d’admissibilité avec contraintes de sécurité et coûts de mutations: application aux réseaux de transmission mixtes (analogiques et numériques)

This paper presents a second version of the «feasibility program» (securad)developed by the cnet.This program enables the routing at minimal cost of several thousands of requirements (expressed in circuits or groups of circuits) on very large mixed (analogue and digital) transmission networks, while respecting given capacities and taking into account security constraints and network-rearrangement costs. The authors also describe a procedure for determining lower bounds for the cost of the optimal solution by solving a dual problem, and thus checking the quality of the approximate solutions obtained. This program has already been widely applied to the long-distance transmission network and to large digital metropolitan networks for both short term and medium to long term planning purposes.     

Analyse de signaux à composantes quasi sinusoïdales

Estimation of spectral lines of a finite duration observation can be viewed as an application for linear prediction models. The authors discuss the feasibility in describing a quasisinusoidal signal with noise, either as a sum of spectral lines in additive noise or as anarma noise with few poles near the unit circle. To both of these models, correspond two problems of estimation which aims are two different characterizations of the observation. In these two cases, is given the form of the maximum likelihood estimator, and the consequences of the use of one estimator instead of the other are examined.     

Détermination des directions de normale d’onde de signaux variant dans le temps. Application aux émissions naturelles TBF détectées à bord de satellites

A method for studying the polarization of an electromagnetic plane wave, the characteristics of which vary rapidly in time, is described. This method is based upon the concept of the (instantaneous) complex representation of a polarized signal. It is possible in particular to yield the wave normal direction as a function of both time and frequency. The accuracy of the method is studied, as a function of both the signal-to-noise ratio and of the eccentricity of the polarization ellipse. This method is applied to the determination of wave normal directions of natural Vlf signals propagating in the whistler mode within the magneto sphere. Important variations in the propagation characteristics of the signals are evidenced.     

MP4与MPEG-2AAC技术

介绍了MP4的由来及新特点;并介绍了MP4基于的核心音频压缩技术MPEG-2 AAC(Advanced Audio Coding)技术;最后对MP4和MPEG-2 AAC技术的应用推广前景进行了简单总结。     

Modification des facteurs de diffraction d’un dièdre pour tenir compte de sa conductivité finie - application aux communications radiomobiles

A formulation of the diffraction coefficients of a high frequency wave under any incidence by a lossy wedge is presented. These results are then extended to determine the field doubly diffracted by two successive wedges having a common face.     

MPEG-2 AAC预测音调算法的改进

随着数字音频发展,心理声学原理被应用到音频压缩技术中,以获得低比特传输速率和透明音质。分析先进音频编码器(AAC)的编码原理,主要介绍心理声学模型的改进音调探测方法,并给出了其实验结果。此方法对实时实现音频编码器的研究具有重要意义。     

用定点DSP实现MPEG-2 AAC实时解码器

MPEG－2AAC（ISO／IEC13818－7）是一种高效多声道Hi-Fi音频编码标准,在每声道64kbps时重建音质优于MP3和AC－3。文中在介绍其解码流程的基础上,讨论了对其关键模块的优化措施,研究了基于定点DSP的AAS实时解码器的实现方法。实现了TMS320C549EVM板上对AAC（LC）码流的实时解码播放。     

MPEG-2 AAC与杜比AC-3音频编码

介绍了现有音频编码格式，重点分析了杜比AC-3格式立体环绕，全音频范围的宽频带，各声道分离，动态范围广以及兼容性好的优点。     

MPEG-2AAC与杜比AC-33音频编码

介绍了现有音频编码格式,重点分析了杜比AC-3格式立体环绕、全音频范围的宽频带、各声道分离、动态范围广以及兼容性好的优点。     

心理声学模型及其在MPEG-2 AAC中的应用

心理声学模型是宽带音频压缩中的核心技术。较为详细地介绍了心理声学的基本原理,指出先进音频编码(AAC)的新特色,并对心理声学模型1和模型2作了比较,详细阐述了心理声学模型2在AAC中的应用,为进一步研究指明了心理声学模型的改进方向。     

Conception et performances d’un système de stockage vidéo distribué et tolérant aux pannes

We describe the design and the implementation of the CFS (Cluster File System) storage system which is dedicated to video streams. Our goal is to provide a system with the following features: 1) High number of supported steams at low cost. 2) Transparent management with respect to the clients. 3) Reliability regarding data storage and service continuity. The CFS is implemented on a cluster of PCS connected with a high speed internal network. Its management is fully distributed among the cluster nodes so that there is no central component. Data is stored and retrieved among the cluster nodes using a « StreamingRaid strategy », to enhance reliability; We experimentally measured the CFS performance in terms of number of supported clients. We also evaluated the cost of the fault tolerance strategy.     

高效实时MPEG-2/4 AAC编码器的实现

介绍了一种高效的MPEG-2/4AAC低复杂度编码器的实现。由于MPEG-2/4AAC算法计算量相当大,为了达到实时性,对窗切换方法、心理声学模型、比特分配、滤波器等模块进行了优化,使其在Pentium41.8GHz处理器上达到比实时编码快4倍的编码速度。而且利用M/S编码和Huffman码表的有效选择,提高了声音质量。