A 2000 BPS LPC vocoder based on multiband excitation

Abstract

This paper presents an improved mixed LPC vocoder at 2000 bps using Multi-Band Excitation analysis by a synthesis algorithm. The new vocoder determines the voiced/unvoiced characteristics harmonic by harmonic in a frame, and finds the first voiced/unvoiced transition as the cut-off frequency, which is more accurate and efficient than traditional cut-off frequency detection. The synthetic speech below the cut-off frequency is excited by a series of voiced harmonics, while the signal above the cut-off frequency is simulated by a noise source. The final output speech is the sum of these two outputs. To increase the naturalness and clearness of the synthesized speech, this model applies phase prediction and spectral enhancement in the synthesizer. It is also possible to reduce the bit rate to 1200 bps. Informal listening tests indicate that the output speech possesses higher intelligibility and quality than that of the 2.4 kbps LPC-10e standard, and is comparable with the 4.8 kbps FS1016 CELP vocoder.