Thèse soutenue

Codage imbriqué pour la parole à 8-32 KBIT/S combinant techniques CELP, ondelettes et extension de bande
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Auteur / Autrice : Mickaël De Meuleneire
Direction : Samir Saoudi
Type : Thèse de doctorat
Discipline(s) : Sciences pour l'ingénieur
Date : Soutenance en 2007
Etablissement(s) : Télécom Bretagne

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Résumé

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The constraints of quality of service related to Voice over IP applications have made necessary the development of a new class of codecs, called embedded, or scalable, codecs, able to decode a part of the generated bitstream. The wideband speech codec developed during this thesis provides an embedded bitstream that can be decoded at bitrates ranging from 8 to 32 kbit/s. To do so, the codec structure comprises three layers. First, a split band structure separates the narrowband component and wideband component of the input signal. Then, the first layer, called core layer, encodes the narrow band component of the input signal. This layer makes use of the ITU-T G. 729 coder. Afterwards, the second layer, called first enhancement layer, utilizes bandwidth extensio techniques relying on a wavelet filter bank to reproduce artificially the wideband component, with an additional bitrate of 2 kbit/s. Finally, the second and last enhancement layer, progressively encodes the wavelet coefficients of the difference between the original signal and the G. 729 output in the narrowband part, and encodes the wavelet coefficients of the original signal in the wideband part. Hence, the decoder ensures a narrowband signal at 8 kbit/s, enables wideband rendering at 10 kbit/s and improves the quality up to 32 kbit/s. Listening tests have shown that the quality of the codec improves gracefully as the bitrate increases. For speech signals the codec at 24 kbit/s and 32 kbit/s is shown to be equivalent to the ITU-T G. 722 codec at 56 and 64 kbit/s, respectively. Moreover, the codec at 32 kbit/s is assessed to be equivalent to the recently standardized embedded codec ITU-T G. 729. 1 at the same bitrate. .