Richard Harry Ketchum

Improved speech quality and efficient vector quantization in SELP

A SELP (stochastically excited linear prediction) algorithm consisting of a two-stage vector quantization using an adaptive codebook and a stochastic codebook is described. The adaptive codebook quantization is similar to a closed-loop long-term prediction filter if the predictor delay is more than one frame length. The performance of the adaptive codebook procedure is improved by extending its codebook to include candidate vectors constructed from past synthetic excitation displaying a high level of periodicity. An algorithm is introduced which through increased symmetry of the error criterion significantly reduces the computational effort required for the search through the adaptive codebook. The algorithm can also be used for stochastic codebooks consisting of overlapping candidate vectors. It is shown that for stochastic codebooks in which neighboring candidates overlap for all but two samples, the quantization performance is as high as for codebooks containing fully independent candidate vectors.<<ETX>>

Fast methods for the CELP speech coding algorithm

Special fast procedures for the code excited linear predictive coding (CELP) algorithm have been developed to make implementation on modest hardware possible. The advantages, as well as the disadvantages, of the various fast procedures are discussed. A general formalism for the algorithm is developed, followed by the discussion of the individual procedures which are grouped according to their features. Along with the computational complexity of each procedure, its storage requirement and numerical accuracy are discussed. A large number of the fast procedures are designed to search through a particular type of codebook (most of the codebooks are stochastic in character, while a few are deterministic). Other fast procedures can be used for arbitrary codebooks and are thus also applicable to trained codebooks. Some of the fast procedures designed for stochastic codebooks can also be used for the computation of the closed pitch loop parameters, which can be interpreted as a search through a time-dependent codebook. >

An efficient stochastically excited linear predictive algorithm for high quality low bit rate transmission of speech

Abstract The Stochastically Excited Linear Prediction (SELP) algorithm for speech coding offers good performance at bit rates as low as 4.8 kbit/s. Linear Predictive Coding (LPC) techniques remove the short-term correlation from the speech. A pitch loop removes long-term correlation, producing a noise-like residual, which is vector quantized. Information describing the LPC filter coefficients, the long-term predictor, and the vector quantization is transmitted. In this paper, we describe improvements to the SELP algorithm which result in better speech quality and higher computational efficiency. In its closed-loop form, the pitch loop can be interpreted as a vector quantization of the desired excitation signal with an adaptive codebook populated by previous excitation sequences. To better model the non-stationarity of speech we extend this adaptive codebook with a special set of candidate vectors which are transform of other codebook entries. The second stage vector quantization is performed using a fixed stochastic codebook. In its original form, the SELP algorithm requires excessive computational effort. We employ a new recursive algorithm which performs a very fast search through the adaptive codebook. In this method, we modify the error criterion, and exploit the resulting symmetries. The same fast vector quantization procedure is applied to the stochastic codebook.

Backward gain adaptation method in code excited linear prediction coders

An exemplary CELP coder where gain adaptation is performed using previous gain values in conjunction with an entry in a table comprising the logarithms of the root-mean-squared values of the codebook vectors, to predict the next gain value. Not only is this method less complex because the table entries are determined off-line, but in addition the use of a table at both the encoder and the decoder allows fixed-point/floating-point interoperability requirements to be met.