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 investig ated and a new algorithm, designed to produce high quality speech with low complexity, is proposed. This algorithm encodes and transmits par tial representative waveforms (RW's) from which the complete speech wa veforms are reconstructed by using a method called forward-backward wa veform prediction (FBWP). The RW is encoded at 20-30 ms intervals with a low complexity approach, taking into account the special initial co nditions of short- and long-term biters. The basic idea of FBWP is ess entially consistent with that of the PWI algorithm, which was reported to be capable of producing high-quality voiced speech at a bit rate o f between 3.0 and 4.0 kb/s. By implementing the FBWP in the time domai n, 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, th e proposed algorithm may be combined with an LP-based speech coder whi ch uses a noise-like excitation to reproduce unvoiced speech.