Evaluation of the distance spectrum of variable-length finite-state codes

International audienceThe class of variable-length finite-state joint source-channel codes is defined and a polynomial complexity algorithm for the evaluation of their distance spectrum presented. Issues in truncating the spectrum to a finite number of (possibly approximate) terms are discussed and illustrated by experimental results

New Free Distance Bounds and Design Techniques for Joint Source-Channel Variable-Length Codes

International audienceThis paper proposes branch-and-prune algorithms for searching prefix-free joint source-channel codebooks with maximal free distance for given codeword lengths. For that purpose, it introduces improved techniques to bound the free distance of variable-length codes

Practical distributed source coding with impulse-noise degraded side information at the decoder

International audienceThis paper introduces a practical method for distributed lossy compression (Wyner-Ziv quantization) with side information available only at the decoder, where the side information is equal to the signal affected by background noise and additional impulse noise. At the core of the method is an LDPC-based lossless distributed (Slepian-Wolf) source code for q-ary alphabets, which is matched to the impulse probability and allows to remove the scalar-quantized impulse noise. Applications of this method to distributed compressed sensing of signals that differ in a sparse set of locations is also discussed, as well as some differences and similarities of variable- and fixed-length coding of sparse signals

Central stars of planetary nebulae : II. New OB-type and emission-line stars

Context. There are more than 3000 confirmed and probably known Galactic planetary nebulae (PNe), but central star spectroscopic information is available for only 13% of them. Aims. We have undertaken a spectroscopic survey of the central stars in PNe to identify their spectral types. Methods. We performed spectroscopic observations at low resolution with the 2-m telescope at CASLEO, Argentina. Results. We present the spectra of 46 central stars of PNe, most of them are OB-type and emission-line stars.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Central stars of planetary nebulae : II. New OB-type and emission-line stars

Context. There are more than 3000 confirmed and probably known Galactic planetary nebulae (PNe), but central star spectroscopic information is available for only 13% of them. Aims. We have undertaken a spectroscopic survey of the central stars in PNe to identify their spectral types. Methods. We performed spectroscopic observations at low resolution with the 2-m telescope at CASLEO, Argentina. Results. We present the spectra of 46 central stars of PNe, most of them are OB-type and emission-line stars.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Central stars of planetary nebulae: New spectral classifications and catalogue

Context. There are more than 3000 confirmed and probable known Galactic planetary nebulae (PNe), but central star spectroscopic information is available for only 13% of them. Aims. We undertook a spectroscopic survey of central stars of PNe at low resolution and compiled a large list of central stars for which information was dispersed in the literature. Methods. We observed 45 PNs using the 2.15 m telescope at Casleo, Argentina. Results. We present a catalogue of 492 confirmed and probable CSPN and provide a preliminary spectral classification for 45 central star of PNe. This revises previous values of the proportion of CSPN with atmospheres poor in hydrogen in at least 30% of cases and provide statistical information that allows us to infer the origin of H-poor stars.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

Analytical tools for optimizing the error correction performance of arithmetic codes

International audienceIn joint source-channel arithmetic coding (JSCAC) schemes, additional redundancy may be introduced into an arithmetic source code in order to be more robust against transmission errors. The purpose of this work is to provide analytical tools to predict and evaluate the effectiveness of that redundancy. Integer binary Arithmetic Coding (AC) is modeled by a reduced-state automaton in order to obtain a bit-clock trellis describing the encoding process. Considering AC as a trellis code, distance spectra are then derived. In particular, an algorithm to compute the free distance of an arithmetic code is proposed. The obtained code properties allow to compute upper bounds on both bit error and symbol error probabilities and thus provide an objective criterion to analyze the behavior of JSCAC schemes when used on noisy channels. This criterion is then exploited to design efficient error-correcting arithmetic codes. Simulation results highlight the validity of the theoretical error bounds and show that for equivalent rate and complexity, a simple optimization yields JSCACs that outperform classical tandem schemes at low to medium SNR

Rate Distortion Behavior of Sparse Sources

This paper studies the rate distortion behavior of sparse memoryless sources that serve as models of sparse signal representations. For the Hamming distortion criterion, $R(D)$ is shown to be essentially linear. For the mean squared error measure, two models are analyzed: the mixed discrete/continuous spike processes and Gaussian mixtures. The latter are shown to be a better model for ``natural'' data such as sparse wavelet coefficients. Finally, the geometric mean of a continuous random variable is introduced as a sparseness measure. It yields upper and lower bounds on the entropy and thus characterizes high-rate $R(D)$