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Speech Codec Intelligibility Testing in Support of Mission-Critical Voice Applications for LTE

Report ID
Technical Report TR-15-520
September 01, 2015
Stephen D. Voran; Andrew A. Catellier

We describe a major effort to quantify the speech intelligibility associated with a range of narrowband, wideband, and fullband digital audio coding algorithms in various acoustic noise environments. The work emphasizes the relationship between these intelligibility results and analogous ones for an analog FM land-mobile radio reference. The initial phase of this project includes 54 noise environments and 83 audio codec modes. We use an objective intelligibility estimator to narrow the scope and then design a practically sized modified rhyme test (MRT) covering 6 challenging yet relevant noise environments and 28 codec modes for a total of 168 conditions. The MRT used 36 subjects to produce 432 trials for each condition. Results show that intelligibility depends strongly on noise environment, data rate, and audio bandwidth. For each noise environment we identify codec modes that produce MRT intelligibility values that meet or exceed those of analog FM. We expect that these results can inform some of the design and provisioning decisions required in the development of mission-critical voice applications for LTE.

Keywords: background noise; speech coding; modified rhyme test (MRT); speech intelligibility; audio coding; acoustic noise; ABC-MRT