VOICED SPEECH CODING AT VERY-LOW BIT RATES BASED ON FORWARD-BACKWARD WAVE-FORM PREDICTION

Techniques for coding voiced speech at very low bit rates are investigated and a new algorithm, designed to produce high quality speech with low complexity, is proposed. This algorithm encodes and transmits partial representative waveforms (RW's) from which the complete speech waveforms are reconstructed by using a method called forward-backward waveform prediction (FBWP). The RW is encoded at 20-30 ms intervals with a low complexity approach, taking into account the special initial conditions of short- and long-term biters. The basic idea of FBWP is essentially consistent with that of the PWI algorithm, which was reported to be capable of producing high-quality voiced speech at a bit rate of between 3.0 and 4.0 kb/s. By implementing the FBWP in the time domain, fast computation is thereby made possible while high-quality speech can be obtained at bit rate of about 3 kb/s. As in the PWI method, the proposed algorithm may be combined with an LP-based speech coder which uses a noise-like excitation to reproduce unvoiced speech.