A MIXED EXCITATION LPC VOCODER MODEL FOR LOW BIT-RATE SPEECH CODING

Traditional pitch-excited linear predictive coding (LPC) vocoders use a fully parametric model to efficiently encode the important informati on in human speech. These vocoders can produce intelligible speech at low data rates (800-2400 b/s), but they often sound synthetic and gene rate annoying artifacts such as buzzes, thumps, and tonal noises. Thes e problems increase dramatically if acoustic background noise is prese nt at the speech input. This paper presents a new mixed excitation LPC vocoder model that preserves the low bit rate of a fully parametric m odel but adds more free parameters to the excitation signal so that th e synthesizer can mimic more characteristics of natural human speech. The new model also eliminates the traditional requirement for a binary voicing decision so that the vocoder performs well even in the presen ce of acoustic background noise. A 2400-b/s LPC vocoder based on this model has been developed and implemented in simulations and in a real- time system. Formal subjective testing of this coder confirms that it produces natural sounding speech even in a difficult noise environment . In fact, diagnostic acceptibility measure (DAM) test scores show tha t the performance of the 2400-b/s mixed excitation LPC vocoder is clos e to that of the government standard 4800-b/s CELP coder.