ADAPTIVE SPEECH CODER HAVING CODE EXCITED LINEAR PREDICTION WITH MULTIPLE CODEBOOK SEARCHES

摘要

Methods and apparatus determining codevectors in response to a speech signal include a first codebook member which stochastically determines the characteristics of a bi-pulse codevector representative of a target vector associated with the speech signal and for removing the bi-pulse codevector from the target vector thereby forming an intermediate target vector. A second codebook member stochastically determines the characteristics of a second bi-pulse codevector in response to the intermediate target vector. In one embodiment of the invention a third codebook member adaptively determines a first codeword in response to the target signal and forms another intermediate target signal and a fourth codebook member stochastically determines a second codeword in response to the intermediate target signal. Synthesized speech signals are determined from the first and second codevectors and from the first and second codewords formed from either the first and second codebook pairing or from the third and fourth codebook pairing. A comparator determines and chooses the synthesized speech signal having the least difference with the speech signal. The codewords or codevectors associated with the chosen synthesized speech signal are selected for transmission. In most cases, the speech signal is divided into frames and each frame is divided into subframes. In such situations, codebook searches may be determined for each subframe. In a further embodiment, an additional codebook search, a single pulse codebook search, is performed over a plurality of subframes, preferably two. In another embodiment, remainder signal are formed by removing the codewords and codevectors from the speech signal. In such an embodiment, a weighting filter is provided for weighting predetermined portions of the remainder signals prior to determining which remainder signal is representative of the synthesized speech signal having the least difference with the original speech signal.