Real time low bit rate speech transmission through underwater acoustic channel

Current needs in underwater acoustic communication, both for civil and military applications, are continuously increasing. During these last years, GESMA (Groupe d´Etudes Sous-Marines de l´Atlantique) and ENST-Bretagne developed the TRIDENT system (TRIDENT : TRansmission de Donne/spl acute/es EN Temps re/spl acute/el). This is the first real time acoustic link able to transmit images, text and data based on multichannel blind equalization. In 1994, a low bit rate speech encoder (5.45 kbit/s) was designed and validated for speech acoustic wave transmission in Brest bay (A. Goalic, et al., 1994). More recently, a speech transmission was also successfully realized in July 2003 at sea, using the new GESMA-ENST-Bretagne acoustic link. This paper provides an overview of this project. The context of the study and the implementation of a very low speech acoustic link are presented. Finally, Brest harbor speech transmission experiments (2003) are presented. The real time horizontal acoustic link performances are evaluated. Two carriers frequencies are available (20, 35 kHz). Acoustic communications for bit rate ranging from 6 to 20 kbps and range from 500 to 4000 m have been conducted successfully over several hours in shallow water environments.