Code-excited Linear Prediction Speech Coder Research Articles

Switched orthogonalization of fixed-codebook search in code-excited linear-predictive speech coder: Derivation of conditions for switching

Switched orthogonalization of fixed-codebook search in code-excited linear-predictive speech coder: Derivation of conditions for switching

Algebraic codebook search strategy for an algebraic code‐excited linear‐prediction speech coder by means of reduced candidate mechanism and iteration‐free pulse replacement

This work aims to present a combined version of reduced candidate mechanism (RCM) and iteration-free pulse replacement (IFPR) as a novel and efficient way to enhance the performance of algebraic codebook search in an algebraic code-excited linear-prediction speech coder. As the first step, individual pulse contribution in each track is given by RCM, and the value of N is then specified. Subsequently, the replacement of a pulse is performed through the search over the sorted top N pulses by IFPR, and those of 2–4 pulses are carried out by a standard IFPR. Implemented on a G.729A speech codec, this proposal requires as few as 20 searches, a search load tantamount to 6.25% of G.729A, 31.25% of the global pulse replacement method (iteration = 2), 41.67% of IFPR, but still provides a comparable speech quality in any case. The aim of significant search performance improvement is hence achieved in this work.

A non bit-exact approach for implementation verification of the CCITT LD-CELP speech coder

This paper presents the design methodology behind the floating point verification procedure for the Low-Delay Code-Excited Linear Prediction speech coder, recently selected by the CCITT as Recommendation G.728. This procedure is based on a non bit-exact specification, which is different from previous CCITT speech coder verification procedures. This approach gives additional freedom for the implementor of the algorithm, and will allow for more efficient implementations on various kinds of hardware. However, this flexibility also means that different implementations will respond slightly different to verification test signals. To cope with this, explicit objective measurements of such deviations are used in the verification process. These measurements are simple weighted and unweighted signal-to-noise ratios (WSNR and SNR). In addition to the objective measurements, certain restrictions had to be placed on the test sequence design. In spite of the input restrictions, a set of test sequences giving reasonably good coverage of the LD-CELP algorithm and state space has been found. Evaluation experiments are reported, showing that these sequences have a satisfactory error detecting capability. A final discussion concludes that the chosen verification approach is indeed feasible as an implementor s tool.