A 600 bps MELP vocoder for use on HF channels

The US government has developed and adopted a new military standard vocoder (MIL-STD-3005) algorithm called mixed excitation linear prediction (MELP) which operates at 2.4 kbps. The vocoder has good voice quality under benign error channels. However, when the vocoder is subjected to a HF channel with typical power output of a manpack radio (MPR), the vocoder speech quality is severely degraded. Harris has found that a 600 bps vocoder provides significant increase in secure voice availability relative to the 2.4 kbps vocoder. This paper describes a 600 bps MELP vocoder algorithm that takes advantage of the inherent inter-frame redundancy of the MELP parameters. Data is presented showing the advantage in both diagnostic acceptability measure (DAM) and diagnostic rhyme test (DRT) with respect to SNR on a typical HF channel when using the vocoder with a MIL-STD-188-110B waveform.