Code-excited Linear Prediction Speech 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.

Glottal-Shape Codebook to Improve Robustness of CELP Codecs

This paper presents a new technique for the class of code-excited linear prediction speech codecs designed to reduce error propagation after lost frames. Its principle consists in replacing the interframe long-term prediction with a glottal-shape codebook in the subframe containing the first glottal impulse in a given frame. This technique, independent of previous frames, is of particular interest in voiced speech frames following transitions as these frames are the most sensitive to frame erasures. It is a basis of a structured coding scheme called <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">transition coding</i> (TC). The TC greatly improves codec performance in noisy channels while maintaining clean channel performance. It is a part of the new embedded speech and audio codec recently standardized as Recommendation G.718 by ITU-T.

Watermarking Combined with CELP Speech Coding for Authentication

This letter presents a speech watermarking scheme that is combined with CELP (Code Excited Linear Prediction) speech coding for speech authentication. The excitation codebook of CELP is partitioned into three parts and labeled '0', '1' and 'any' according to the private key. Watermark embedding process chooses the codebook whose label is the same as the watermark bit and combines it with the codebook labeled 'any' for CELP coding. A statistical method is employed to detect the watermark, and the watermark length for authentication and detection threshold are determined by false alarm probability and missed detection probability. The new codebook partition technique produces less distortion, and the statistical detection method guarantees that the error probability can be controlledunder prescribed level.

Pitch-controlled variable bitrate CELP speech coding

Most research in efficient speech coding concentrated for many years on algorithms which produced a fixed bit rate. Fixed bit rates are, however, not a requirement for modern packet-based telecommunications and computer networks as well as voice storage applications. This makes the implementation of speech coders feasible, which adapt to the changing properties of speech for improved efficiency or quality. Variable bit-rate coding has therefore become the focus of considerable research activity in recent years [A. Gersho and E. Paksoy, ‘‘Variable Bit Rate Coding,’’ Signal Processing VII, Theories and Applications, EUSIPCO 1994, pp. 1169–1173; L. Zhang et al., ‘‘A CELP Variable Rate Speech Codec with Low Average Rate,’’ ICASSP 1997, pp. 735–738; B. C. Xydeas, ‘‘Source Driven Variable Bit Rate Prototype Interpolation Coding,’’ ICASSP 1996, p. 220; B. Shen et al., ‘‘A Robust Variable-Rate Speech Coder,’’ ICASSP 1995, p. 249]. While all of these coders focus on classification procedures for efficient bit allocation, the analysis frame size remains mainly static. The coder featured in this paper adapts the analysis frame size and bit allocation according to the pitch of the signal. A pitch frame extractor and classifier at the front end feeds the detected frame into the speech coding back end, which uses traditional CELP-based techniques [B. S. Schroeder et al., ‘‘CodeExcited Linear Prediction (CELP): High Quality Speech at Very Low Bit Rates,’’ ICASSP 1985, pp. 937–940]. This approach allows the reduction of the bit rate from a constant 4800 bps (bits per second) with original CELP to typically 3200–4000 bps, depending on the speech situation. This figure is even reduced to typically 2600–3200 with the inclusion of Voice Activity Detection. The speech quality equals the quality of the original CELP coder.

Efficient codebook structure for code excited linear prediction coding

A speech communication system using a code excited linear prediction speech decoder. The decoder using a first codebook containing a first digital value sequence selected from the set of binary values {0, 1}. The decoder also using a second codebook containing a second digital value sequence having values selected from the set of binary values {−1, 0}. The first digital value sequence and the second digital value sequence are combined to become a third digital value sequence having a set of ternary values from the set of {−1, 0, 1}.

Generating stressed speech from neutral speech using a modified CELP vocoder

The problem of speech modeling for generating stressed speech using a source generator framework is addressed in this paper. In general, stress in this context refers to emotional or task induced speaking conditions. Throughout this particular study, the focus will be limited to speech under angry, loud and Lombard effect (i.e., speech produced in noise) speaking conditions. Source generator theory was originally developed for equalization of speech under stress for robust recognition (Hansen, 1993, 1994). It was later used for simulated stressed training token generation for improved recognition (Bou-Ghazale, 1993; Bou-Ghazale and Hansen, 1994). The objective here is to generate stressed perturbed speech from neutral speech using a source generator framework previously employed for stressed speech recognition. The approach is based on (i) developing a mathematical model that provides a means for representing the change in speech production under stressed conditions for perturbation, and (ii) employing this framework in an isolated word speech processing system to produce emotional/stressed perturbed speech from neutral speech. A stress perturbation algorithm is formulated based on a CELP (code-excited linear prediction) speech synthesis structure. The algorithm is evaluated using four different speech feature perturbation sets. The stressed speech parameter evaluations from this study revealed that pitch is capable of reflecting the emotional state of the speaker, while formant information alone is not as good a correlate of stress. However, the combination of formant location, pitch and gain information proved to be the most reliable indicator of emotional stress under a CELP speech model. Results from formal listener evaluations of the generated stressed speech show successful classification rates of 87% for angry speech, 75% for Lombard effect speech and 92% for loud speech.

Method for improving speech quality in code excited linear predictive speech coding

By reconciling differences between the estimator and the filter of a code excited linear predictive (CELP) voice coder, higher quality is achieved in the output speech. The pulse amplitudes and pitch tap gain are solved for simultaneously to minimize the estimator bias in the CELP excitation. Increased signal to noise ratio is accomplished by modifying the pitch predictor such that the pitch synthesis filter accurately reflects the estimation procedure used to find the pitch tap gain, and by improving the excitation analysis technique such that the pitch predictor tap gain and codeword gain are solved for simultaneously, rather than sequentially. These modifications do not result in an increased transmission rate or significant increase in complexity of the CELP coding algorithm.