Étude de l’erreur introduite par la numérisation du signal dans un problème de reconnaissance des formes: Application a la parole

Decision in pattern recognition according to the likelihood theory leads to the determination of a threshold S.Classification of an unknown signal Icaracterized by one parameter x,is made according to the following rule: I = Ω₁ if x ≤Sand I = Ω₂ if x > S (Ibelongs to one of the two classes Ω₁ or Ω₂ ). When xis fed into an analog to digital converter, one gets a quantized value x₀ ; sometimes it is not possible to fix the relative position of xwith regard to Swhen x₀ is known. The distorsion increase is studied in this paper, a suitable determination of Scan reduce the increase. An experimental illustration is given. Speech recognition is made on data coming from the CNET vocoder in Lannion. The distorsion introduced by quantizing is studied when several thresholds sets are proposed. The influence of numérisation is pointed out; a high accuracy on the thresholds values is not needed and this allows wide applications.     

Étude du champ créé par un simulateur d’impulsions électromagnétiques : influence d’une déformation doublet-cornet et étude numérique des interactions dans un simulateur du type cornet

The first part of this paper compares the frequency responses of a horn type and of a dipole type simulators. The field in the test volume is computed using a wire model for the simulators, and its values over a wide frequencies range are compared. The radiation of the horn type simulator is more homogeneous with regard to frequency variations. This is due to the compact shape of this type of simulator. This compactness can lead to a strong and unwanted coupling between the object and the structures of the simulator. In the second part of the paper, the interaction between the simulator and the object is investigated by means of an integral representation of the field, applied to a two dimensional model of the simulator. The latter is considered as a non-uniform guiding structure. The currents on the object along with their rise time and their shape are then used to probe the coupling phenomena.     

Influence des effets de contrainte induits par la température sur la conversion TE-TM dans des guides planaires de YIG substitué

We have demonstrated the tuning of linear birefringence in magnetooptic planar thin film waveguides by temperature adjustment. Using this method, phase-matching between coupled te₀and tm₀ modes, a necessary condition for the integrated optical isolator based on mode conversion, has been obtained at λ = 1.52 µm in bilayer gadolinium gallium substituted yig films. The waveguide temperature was 85°C. Moreover, phase-matching temperature can be modified by a slight variation of gadolinium and gallium substitutions as shown by a phase-matching observed in a waveguide at room temperature.     

Application des décisions markoviennes dans un problème de reconnaissance séquentielle de la parole

The sequential organisation of speech syntax can be used in recognition: to limitate, at each utterance, the number of possible words, to find out the most likely archetypes series. The principle of the sequential decision which is analyzed holds in four points: syntax is used for next occurrence prediction, a fast comparison between an utterance and the words in a lexicon is made through hypothesis testing, a probabilistic distance is computed between the utterance and the archetypes, the similitude between a sequence of archetypes and a serie of utterances is evaluated from markovian chains properties. Automata theory provides a good way to handle sequential connexions. The authors indicate mathematical techniques which are used and results obtained by a simulation made on a computer. Then, a generalization is given to handle the insufficiency of the above results; speech decomposition must be more thoroughly investigated.     

Illumination d’un réseau de lignes ou de câbles par une onde de foudre Modélisation par des générateurs de courant

In this paper we propose a new formalism for the analysis of electromagnetic coupling between the lightning wave and a network of lines or cables. This approach allows obtaining currents and induced voltages directly on the set of lines or cables extremities. For this objective we use the current generators representation of the lightning wave. Then by a mathematical formulation based on the topological analysis of the network, we deduce a linear equation system which resolution leads directly to the currents and induced voltages. In order to confirm our theoretical work, we present a set of applications that allows validating this formalism.     

Application de l’étude de la diffraction par un réseau à la propagation dans les guides périodiques

The authors propose a method for the study of the propagation of an electromagnetic wave in a periodical guide, which follows that of the plane wave diffraction by a grating. The solution of the Maxwell’s equations under covariant form is calculated in a non-orthogonal system of coordinates, following the geometry of the problem. It leads to the search for eigenvalues and eigenvectors of a matrix which is characteristic of the coordinates and of the medium. For gratings, one has to solve an inhomogeneous linear system. In the case of a rectangular waveguide with meanders, the system becomes homogeneous, which allows the calculation of the dispersion condition.     

Quelques applications des boucles à verrouillage de phase aux problèmes de synchronisation dans les liaisons spatiales

This paper deals with a presentation of carrier and bit synchronizers used for low signal-to-noise ratio space digital communications. Such synchronizers are closed loop phase estimators and carry out a mechanization of the maximum likelihood criterium. The paper presents well known carrier synchronizers such as the decision feedback synchronizer, the Costas loop and the x² non-linearity synchronizer. Bit synchronizers such as the early late gate, the data transition tracking loop and others synchronizers associated with a non-linearity are also introduced.     

Caractérisation des composants de protection soumis à des impulsions électromagnétiques à pente raide de I’ordre de 1 kV/ns

This paper presents the experimental and theoretical methods to characterize protective devices under fast transient perturbations with rising fronts of 1 kV/ns. This study concerns the main parallel protective components used in telecommunications equipments. Under high voltage rise, these components have a dynamic behaviour due to their intrinsic capacitance and inductance. The influence of these elements on component responses is analysed numerically with a parametric study by varying simultaneously their value. The comparison of these responses with experimental results permits to fix the values of capacitance and inductance.     

Détermination directe des vitesses de phase et de groupe dans les lignes microondes par la méthode TLM (Direct determination of phase and group velocities in microlines by the TLM method)

La méthode TLM permet d'agir sur l'excitation dans lea lignes. Les vitesses de phase sont déterminées en s'approchant du synchronisme entre l'excitation et les modes existants. La vitesse de groupe est déduite de la modulation de fréquence créée par variation des dimensions de la structure. L'accès à ces grandeurs réduit notablement les temps calcul. The TLM method permits modification of the excitation of modes in microlines. This property gives us access to phase velocities by synchronisation of excitation and propagating modes. Producing frequency modulation by variation of geometrical dimensions, the group velocities are deduced. Theoretical aspects and numerical results are given.     

Réception des messages biphase dans les systémes de télécommunications par fibres optiques

The author considers the problems relevant to the detection of unipolar split-phase signals in digital optical fiber communication systems. By use of simplifications concerning the fiber frequency response and the noise power spectrum, the error rate is computed in different kinds of signal processing. Compared to the theoretical optimum detection, the results are presented as a function of the practical receiver bandwidth.