Low rate speech coders for robust VHF radiocommunications

The authors describe the use of low rate vocoders (4800 b/s, 2400 b/s and 800 b/s) as an alternative to CVSD 16 kb/s coder. The available bit rate on the global 16 kb/s channel can be used to improve robustness of the communication or to offer new services (multiplexing speech and data or proposing a duplex service). The authors report the theoretical studies that took into account both the speech coder and the channel characteristics to derive an error protection scheme. A real-time implementation has been carried out. It uses a vehicular radio set type of the PR4G VHF/frequency hopping system. Performance has been assessed during field experiments and comparisons have been drawn with the standard CVSD coder. Finally, perspectives of industrialization are mentioned