Military Speech Communications over Vocoders in Tandem

Speech intelligibility of two types of vocoders was measured using the modified rhyme test. One type of vocoder, a continuous variable slope delta CVSD, was a waveform encoder. The other type, an advanced multi-band excitation AMBE, was a parametric encoder. In the first experiment, clear speech was processed through the vocoders. Intelligibility was measured in a control condition, i.e. without vocoding, with each type alone and with two vocoders in tandem. AMBE and CVSD performed similarly, 92.6 and 90.4, respectively. CVSD-to-AMBE had little effect on intelligibility, measured at 89.2. However, AMBEto- CVSD had a large degrading effect on intelligibility. The AMBE-to-CVSD direction scored about 81.7 intelligibility with clear, unaltered speech signals. The asymmetry between waveform-to-parametric and parametric-to-waveform encoders underscores the non-linear nature of tandem vocoders on intelligibility. When vocoders of the same type were in tandem, there was no additional effect on intelligibility. The double CVSD condition yielded 92.2 intelligibility and the double AMBE condition yielded 91. The deleterious effects of speech clipping were measured in a second experiment, as these are ubiquitous in military radio transmission systems. The AMBE parametric vocoder performed at the 88 level in isolation and at 84 when tandemed with the CVSD waveform vocoder. Alternative methods of encoding speech signals are being explored to improve speech intelligibility performance in military communication systems.