Second report of the multirate processor (MRP) for digital voice communications

This report documents recent developments of the Navy-developed voice processor that provides compatible low-data-rate and high-data-rate voice communication: 2.4 kilobits per second (kb/s) for narrowband channels; 9.6 and 16 kb/s for wideband channels. The compatibility of the narrowband and wideband channels arises from the unique characteristic of the voice processor that the transmitted bit-stream of the 16-kb/s data contains the bit-stream of the 9.6-kb/s data as a subset. Likewise, the bit-stream of the 9.6-kb/s data also contains the bit-stream of the 2.4-kb/s data as a subset. This embedded data structure makes it possible to interconnect, without user intervention, narrowband and wideband systems via a digital rate-converter located somewhere along the link. An advantage of this method of rate conversion is that it permits end-to-end encryption of speech data. Also during overloaded or disrupted channel conditions, communication survivability may be increased by rate reduction and/or rerouting through other available narrowband communication links. This report includes results of intelligibility and conversation tests which indicate the embedded 9.6-kb/s rate compares well with continuously variable slope delta modulation (CVSD) at 16 kb/s, and the embedded 16 kb/s performs comparably to CVSD at 32 kb/s.