A low bit rate speech codec using multi band excitation and LPC modeling of harmonic magnitudes for private mobile radio

摘要

A new designed low bit rate speech codec for next generation Japanese private mobile radio is proposed. This codec can be used on narrow air channel separation system of 3.125 kHz almost half frequency separation of current narrowest private mobile radio system. The speech codec is based on Multi-band Excitation (MBE), and speech analysis frame period is 20 msec composed of two sub-frames. Speech synthesis parameters, pitch period, voiced/unvoiced decision of each spectrum band (V/UV) and harmonic amplitude, are extracted by novel methods from each sub-frame. The speech parameters are quantized at 1.6kbit/sec, by differential log of pitch period, representative V/UV pattern and LPC (Linear Predictive Coding) parameters converted from harmonic amplitude using LPC modeling. Error correction uses 1.6kbit/s and its scheme is composed of CRC, RCPC (Rate Compatible Punctured Convolutional) codes for classified speech parameters according to error sensitivity scores. Speech quality by MOS (Mean Opinion Score) and articulation tests under noisy channel up-to 7% of BER (bit error rate) are performed, The MOS of proposed codec is slightly lower than FS1016 (CELP, 4.8kbit/s) but quality degradation is small up-to 5% of BER, and sound articulations are over 80%, expected value for communication without problem, up-to 7% of BER.