Showing papers on "Code-excited linear prediction published in 1988"

Linear predictive coding

Abstract: The basic principles of linear predictive coding (LPC) are presented. Least-squares methods for obtaining the LPC coefficients characterizing the all-pole filter are described. Computational factors, instantaneous updating, and spectral estimation are discussed. >

Digital speech coder having improved vector excitation source

Abstract: An improved excitation vector generation and search technique (FIG. 1) is described for a code-excited linear prediction (CELP) speech coder (100) using a codebook of excitation code vectors. A set of M basis vectors Vm (n) are used along with the excitation signal codewords (i) to generate the codebook of excitation vectors ui (n) according to a "vector sum" technique (120) of converting the selector codewords into a plurality of interim data signals, multiplying the set of M basis vectors by the interim data signals, and summing the resultant vectors to produce the set of 2M codebook vectors. The entire codebook of 2M possible excitation vectors is efficiently searched by using the vector sum generation technique with the M basis vectors--without ever having to generate and evaluate each of the 2M code vectors themselves. Furthermore, only M basis vectors need to be stored in memory (114), as opposed to all 2M code vectors.

On robust linear prediction of speech

Abstract: A robust linear prediction (LP) algorithms is proposed that minimizes the sum of appropriately weighted residuals. The weight is a function of the prediction residual, and the cost function is selected to give more weight to the bulk of small residuals while deemphasizing the small portion of large residuals. In contrast, the conventional LP procedure weights all prediction residuals equally. The robust algorithm takes into account the non-Gaussian nature of the excitations for voiced speech and gives a more efficient (less variance) and less biased estimate for the prediction coefficients than conventional methods. The algorithm can be used in the front-end features extractor for a speech recognition system and as an analyzer for a speech coding system. Testing on synthetic vowel data demonstrates that the robust LP procedure is able to reduce the formant and bandwidth error rate by more than an order of magnitude compared to the conventional LP procedures and is relatively insensitive to the placement of the LPC (LP coding) analysis window and to the value of the pitch period, for a given section of speech signal. >

Strategies for improving the performance of CELP coders at low bit rates (speech analysis)

Abstract: Some of the distortions produced by CELP (code-excited linear prediction) coders are characterized. It is found that the coder does not reproduce high frequencies well and that rapid changes in the speech signal are not adequately tracked. Within the framework of the current CELP concept, strategies are discussed that can reduce these distortions. Nonstationarities in the speech signal can be better followed by allowing a flexible allocation of the bits used for the excitation. However, the bit allocation procedures and the way the bits are used need further improvement. The reproduction of higher frequencies can be improved by changing the error-weighting procedure or by shaping the code-book excitation functions. >

Synthesis filter optimization and coding: applications to celp

Abstract: The success of Code-Exclted Llnear Predlctron (CELP) for codlng speech signals depends on the accurate representatlon of the pltch structure and the formant structure of the Input speech In thls type of coder an excltatton waveform chosen from a dlctlonary of waveforms drlves a cascade of a pltch and a formant synthesis fllter Thls paper develops the methodology to allow for a Joint optlmlzatlon of the waveform selection process waveform scallng. and the pltch filter determlnatlon Methods to accommodate hlgh-pitch speakers (pltch lag smaller than the analysls frame slze) are given Addltlonally. the requlrements for codlng the synthesls parameters Into a bit stream at 4 8 kbis are discussed

Code excited linear predictive vocoder and method of operation

Abstract: Apparatus (101-112) for encoding speech using an improved code excited linear predictive (CELP) encoder (106, 104) using a virtual searching technique (708-712) to improve performance during speech transitions such as from unvoiced to voiced regions of speech. The encoder compares candidate excitation vectors stored in a codebook with a target excitation vector representing a frame of speech to determine the candidate vector that best matches the target vector by repeating a first portion of each candidate vector into a second portion of each candidate vector. For increased performance, a stochastically excited linear predictive (SELP) encoder (105, 107) is used in series with the adaptive CELP encoder. The SELP encoder is responsive to the difference between the target vector and the best matched candidate vector to search its own overlapping codebook in a recursive manner to determine a candidate vector that provides the best match. Both of the best matched candidate vectors are used in speech synthesis.

A 4.8 kbps code-excited linear predictive coder

Abstract: A secure voice system STU-3 capable of providing end-to-end secure voice communications (1984) was developed The terminal for the new system will be built around the standard LPC-10 voice processor algorithm The performance of the present STU-3 processor is considered to be good, its response to nonspeech sounds such as whistles, coughs and impulse-like noises may not be completely acceptable Speech in noisy environments also causes problems with the LPC-10 voice algorithm In addition, there is always a demand for something better It is hoped that LPC-10's 24 kbps voice performance will be complemented with a very high quality speech coder operating at a higher data rate This new coder is one of a number of candidate algorithms being considered for an upgraded version of the STU-3 in late 1989 The problems of designing a code-excited linear predictive (CELP) coder to provide very high quality speech at a 48 kbps data rate that can be implemented on today's hardware are considered

Synthesis filter optimization and coding: Applications to CELP (speech analysis)

Abstract: Code-excited linear prediction (CELP) is examined for the coding of speech signals. The methodology is developed to allow for a joint optimization of waveform selection, waveform scaling, and pitch filter determination. Methods to accommodate high-pitch speakers (pitch lag smaller than the analysis frame size) are given. The requirements for coding the synthesis parameters into a bit stream at 4.8 kb/s are discussed. The coder has been tested at channel error rates of 0.001, with only minor degradations in the resulting speech. An adaptive postfilter has been added to achieve a small increase in perceived speech quality. >

Multimode coding: A novel approach to narrow‐ and medium‐band coding

Abstract: Most research on narrow‐band coding is concentrated on how to transmit excitation parameters efficiently. However, the important thing in obtaining a good reproduced speech quality is how to control the balance of the transmission bit rate between the excitation and the LPC parameters. Multimode coding, which is proposed here, has two coding modes: One is a mode that transmits the LPC parameters in every frame, as conventional coders (A mode). The other is a mode that avoids the transmission of the LPC parameters by using the same coefficients as the previous frame and increases the bits allocated to the residual quantization instead of to the LPC transmission (B mode). In each frame, the mode selection takes place based on an evaluation of the reproduced speech quality, and the assignment of transmission information is dynamically controlled by switching between the two modes. This coding algorithm is applied to a 7.2 kb/s CELP coder, and approximately 3 dB of improvement is achieved in SNR compared with a conventional CELP coder. The B mode was used 78%–82% of the time.

Experiments with a high quality, low complexity 4800 bps residual excited LPC (RELP) vocoder

Abstract: The authors present an approach for achieving high quality speech coding at 4800 bps using a residual excited LPC (RELP) coder. Numerous experiments were performed to identify and code the significant parameters in the spectral, pitch, and residual models. Major principles for high quality residual coding are discussed. >

RPCELP: a high quality and low complexity scheme for narrow band coding of speech

Abstract: Code-excited linear prediction (CELP), a powerful technique for low-bit-rate speech coding, leads to a huge computational load. A related scheme that replaces the conventional stochastic excitation by an efficient regular pulse excitation is introduced. This method, called regular pulse CELP (RPCELP), takes advantage of the codebook structure together with a convenient perceptual criterion to achieve a very low complexity while maintaining high-quality output speech. The algorithm can be implemented easily in real-time and can provide good communication quality at 6 Kb/s, even over noisy transmission channels. Objective performance results are reported for several configurations. >

2.4 kbps pitch prediction multi-pulse speech coding

Abstract: 2.4-kb/s speech coding based on pitch-prediction multipulse speech coding is described. An adaptive segmentation procedure to effectively control the renewal rate for synthesis model parameters, a linear time-varying pitch-synthesis filter, a model for multipulse excitation during one pitch period, and a vector quantization for linear predictive coding parameters are introduced. Subjective evaluation demonstrates that pitch-prediction multiphase speech coding can provide much more natural-sounding synthetic speech than single-pulse-excitation speech coding (as with a vocoder) or pitch-interpolation multipulse speech coding at 2.4-kb/s. >

Speaker adaptive vector quantisation of LPC parameters of speech

Abstract: Recently, it has been shown that good quality speech at rates as low as 6 Kbit/s can be achieved with CELP and its derivatives. However, in bringing down the bit rate even further these coding schemes have resorted to allocating fewer bits to the quantisation of the LPC parameters. It is known that CELP and its derivatives are sensitive to LPC parameters quantisation errors, and recently various schemes have been proposed to overcome this degradation. In the paper we describe how speaker adaptive vector quantisation (SAVQ) can be applied to the quantisation of LPC parameters and assess its performance when incorporated into a low bit rate coding scheme.

Rule-based speech analysis and application of CELP coding

Abstract: An approach is presented for efficiently encoding speech signals at low bit rates, by exploiting a combination of various speech analysis and compression techniques cooperating via a rule-based reasoning system. A front-end analyzer compresses speech events at nonuniformly spaced time intervals, by resorting to dynamic and static variable-frame-rate methods relevant to perception models. Then a codebook-excited-linear-predictive (CELP) coder performs a perceptually meaningful identification and quantization of the excitation parameters, to provide an optimal rendition of the original signal. This coding scheme can reduce the transmission rate down to 2.4-2.8 kb/s, while retaining a very good quality. The main applications are in voice response systems and voice mail. >

CELP base-band coder for high quality speech coding at 9.6 to 2.4 kbps

Abstract: A baseband coder is discussed and its subjective quality compared with code-excited linear prediction (CELP) and vector-quantized transform coding at various bit rates from 9.6 to 4.8 kb/s. It is also with that CELP-BB coding is capable of producing natural, intelligible and smooth speech at 2.4 kb/s. The code-book search in CELP-BB requires no convolution. The pitch filter memory is subtracted from the original baseband signal to form the reference signal, which is directly matched by the code-book sequences. Because most of the computation is performed on the baseband, which is decimated, and during code-book search only the pitch filter response is considered, the complexity of CELP-BB is very much less than that of CELP and its speech quality is not affected by LPC quantization error to such an extent as in CELP. >

CELP Base-Band Coder For High Quality Speech

Abstract: y CELP [l] has proved that it is possible to use vector ions as large as 40 or more samples long and still maingh quality. This is achieved by error minimization n the original and the synthesized vectors, rather than inimization between the residual vectors and code-book in earlier VQ designs. The CELP design however, is ex and its quality is affected very much by the n error of the LPC parameters [2] making it difficult new vector quantized and easily implementer (CELP-BB) which was originally proposed below 4.8 Kb/s in [3]. Because most of the rformed on the base-band which is decimated, and that during code-book search only the pitch filter response -BB is very much less affected by LPC quantLP. CELP-BB can proo the original) down to ty starts to deteriorate natural and intelligible

Analysis of the self-excited subband coder: a new approach to medium band speech coding

Abstract: A speech coding technique is introduced for bit rates below 10 kb/s. The coder exploits the frequency-domain properties of aural perception that are associated with subband coding and uses the block coding techniques which have recently been used to great advantage in low-rate full-band coders such as the self-excited vocoder and the code-excited linear predictive coder. The coder contains an analysis/synthesis system for decomposing the speech into contiguous frequency bands and merging the bands in reconstruction. The coding of the individual subbands is accomplished by using long-term predictors to represent channel outputs. Various configurations of the analysis/synthesis structure and the self-excitation model are considered. Issues related to the analysis procedure, the performance and the system efficiency are discussed. >

Improvement in method for compression of speech digitally coded

Abstract: PURPOSE: To obtain a high quality speech with sufficiently low complexity for real-time coding by connecting vector quantization(VQ) to advantages of both adaptive prediction coding (ASO) and code excited linear prediction(CELP). CONSTITUTION: The VQ is connected to the advantages of both the APC and CELP. Namely, respective vectors of K speech samples are roughly calculated by using M fixed vectors stored in a VQ code book and selecting an optimum synthesized vector minimizing the size of perceptually meaningful distortion to excite a composite filter for time variation. Consequently, an analog speech or audio waveform can be coded into a bit stream which is compressed for storage and/or transmission in real time, so the waveform can be reconstitution later for reproduction and the adaptive postfiltering of a speech or audio signal which is disordered with a noise generated owing to deterioration of a coding system or other sources can be enabled so as to improve the perceptual quality of the speech or audio signal.

High-quality vector adaptive transform coding at 4.8 kb/s

Abstract: A frequency-domain search procedure is presented that requires a computational load of approximately 1.5 MFLOPS, reduces the storage cost, and maintains the speech quality. A vector-excitation-coding (VXC) method in the transform domain called vector adaptive transform coding (VATC) is proposed. VATC provides a general framework for VXC in the transform domain, and the quality for the synthetic speech is close to that obtained in code-excited line prediction (CELP). The major advantages of VATC are the different ways of including information from previous frames, different perceptual distortion measurements, different transformations, efficient search algorithms, and the use of vector predictive quantization techniques so as to have a fully quantized coder at 4.8 kb/s. >

A comparison between CELP and MPLPC at 8 kbit/s for mobile communications

Abstract: Two analysis-by-synthesis schemes, with a different excitation structure, have been optimized for a bit-rate of 8 kb/s. The codebook excited linear predictive (CELP) coder and the multi-pulse linear predictive coder (MPLPC) were considered. Some improvements over conventional coders have been achieved by computing the long-term parameters by a closed-loop procedure and using a split vector quantization of line spectrum pair coefficients for the short-term spectrum representation. The complexity of the two schemes has been considered as a design constraint and the result is the implementation feasibility of both algorithms with standard floating-point digital signal processors. Both coders are shown to provide very high speech quality without transmission errors. The present study is pertinent to the development of the pan-European digital mobile radio system. >

Speech coding at 9.6 Kb/s and below using vector quantized transform coder

Abstract: The vector-quantized transform coder (VQTC) is proposed as an alternative to a code-excited linear predicting (CELP) coder. The consideration of more efficient and subjectively meaningful residual quantization allows the VQTC to operate at rates as low as 6 kb/s without significant degradations in speech quality. Below 6 kb/s the quality of the speech was good and did not contain any background noise peculiar to transform coders. Simulations of the VQTC and standard CELP were performed to assess the performance of the VQTC as an alternative coding scheme for low bit rates. In general, the performance of the VQTC was as good as CELP at rates of 9 to 4.8 kb/s. However, the VQTC requires substantially fewer computations than CELP, which makes the VQTC attractive for real-time applications. >

Medium band speech coding for mobile radio communications

Abstract: The authors propose a low cost 6 kbps multipulse-excited linear prediction coder for 8 kbps transmissions on 12.5 kHz radio channels. The main purpose of this system is to give good intelligibility and good speaker recognition. Against transmission errors, the authors present a correcting strategy and an extrapolation method that hides lost speech frames. >

Multipulse excitation codebook design and fast search methods for CELP speech coding

Abstract: Presents an efficient method for multipurpose excitation coding using direct vector quantization. This method is then extended to the design of the excitation codebook for a CELP (code excited linear predictive) coder. For the best excitation-vector search at low data rates, an approach combining direct vector quantization and an analysis-by-synthesis procedure is used. Computer simulation results for a 4.8 kb/s speech coder based on the new excitation codebook are presented. >

Improvement to vector quantizing coder

Abstract: A signal vector quantizing coder (CELP) is provided with an adaptive codebook originally loaded with preselected codewords. The codebook is split into a fixed contents portion and a fixed length adaptive contents portion. During coding operations, the codewords dynamically selected for coding the coder input signal are shifted into the fixed length adaptive codebook section for codebook contents updating purposes.

Signal Prediction by Pattern Search

Abstract: Prediction of a signal from earlier samples is widely used to reduce the information that must be transmitted for a suitably accurate reconstruction at a receiver. Linear prediction is a common means of effecting the prediction, but it does not accommodate well signals that include dominant innovations from time to time, as in the case of speech, or signals that are not well suited to modeling as the output of a linear dynamical system. We have investigated a scheme in which a set of recent samples is matched to an earlier set of samples and then the sample following the earlier set is used as the prediction. Error behavior is compared with linear prediction, theoretically and experimentally, and found to be reasonably similar on a mean square basis.

Low bit-rate image coding using 2-D linear prediction and 2-D stochastic excitation

Abstract: The use of stochastic excited linear predictive coding method is investigated for image coding and its parameters are studied. It is shown that it is not necessary to transmit local bias values of the image frames. It is also shown that the stochastic excitation is not adequate to represent the prediction residual signal. In order to get good performance from this coder, it is necessary to generate the codebook from the actual prediction residual signal. >

Real-time speech encoding based on Code-Excited Linear Prediction (CELP)

Abstract: This paper reports on the work proceeding with regard to the development of a real-time voice codec for the terrestrial and satellite mobile radio environments. The codec is based on a complexity reduced version of code-excited linear prediction (CELP). The codebook search complexity was reduced to only 0.5 million floating point operations per second (MFLOPS) while maintaining excellent speech quality. Novel methods to quantize the residual and the long and short term model filters are presented.

Fast CELP coder using Givens bidiagonalisation

Abstract: CELP speech coders give good performance but require considerable computational power. Earlier work showed how singular value decomposition can be used to speed up the calculations required for codebook search in the CELP. The bidiagonal decomposition proposed by the author is simpler and results in a fast algorithm with fixed running time. >