Extended linear prediction tools for lossless audio coding
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Two extension tools for enhancing the compression performance of prediction-based lossless audio coding are proposed. One is progressive-order prediction of the starting samples at the random access points, where the information of previous samples is not available. The first sample is coded as is, the second is predicted by first-order prediction, the third is predicted by second-order prediction, and so on. This can be efficiently carried out with PAR-COR (PARtial autoCORrelation) coefficients. The second tool is interchannel joint coding. Both predictive coefficients and prediction error signals are efficiently coded by interchannel differential or three-tap adaptive prediction. These new prediction tools lead to a steady reduction in bit rate when random access is activated and the interchannel correlation is strong.Keywords:
Linear prediction
Random access
A new analysis-by-synthesis speech coding structure is presented for high-quality speech coding in the 4 to 8 kb/s range. CELP with generalized pitch prediction (GPP-CELP) differs from classical code-excited linear prediction (CELP) in that for voiced segments it is the speech signal that is decomposed into a component predictable with the aid of the adaptive codebook (ACB) and a nonpredictable aperiodic component, not the LPC residual. The spectrum of the aperiodic component is estimated by linear-prediction analysis. An approximation to the aperiodic component is synthesized from a stochastic codebook of sparse pulse sequences and its spectrum is shaped by the LPC synthesis filter. The ACB contains samples of the past reconstructed signal, low-passed to increase the pitch prediction gain. For voiced segments the new structure yields higher pitch prediction gain and lower linear-prediction gain than classical CELP. Subjective and objective comparisons reveal significant advantages for GPP-CELP over classical CELP.
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Linear predictive coding of speech has been widely used at 16 kb/s in the form of adaptive predictive coding (APC) down to 4.8 kb/s in the form of code-excited linear prediction (CELP). Since its invention in 1984 there have been many variations of CELP which differ mainly in the way the final excitation signal (codebook) is produced and quantised. These variations either produce better speech quality or lower complexity. Three new excitation types, all of which are based on a pulsed residual, are proposed. The new pulsed residual excitations improve the speech quality significantly. In addition a novel mathematically equivalent codebook search method which reduces the search complexity significantly is described.
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A novel variable-rate linear predictive coding (LPC) parameter quantization scheme is proposed, in which linear prediction is done by using either the current (forward LPC) or previously decoded (backward LPC) speech blocks. The proposed LPC quantization scheme was integrated into the FS1016 Federal Standard CELP coder. Significant LPC bit rate reduction is achieved without compromising the decoded speech quality.
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The code excited linear prediction coder (CELP) makes it possible to synthesize good quality speech at low bit rates. In such a case, speech quality mainly depends on spectral envelope design accuracy. Different kinds of parameters belonging to the parametrical domain (linear prediction coefficients.
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Envelope (radar)
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A new speech coding technique at low bit-rate is presented in this paper. The coder is based upon a novel speech production model, independently developed by the authors [1,2] and by Atal and Schroeder [3,4], called CELP (Codebook Excited Linear Prediction). Differences exist between the two approaches, both in the strategy chosen to construct codebooks, and in the method to generate the innovation sequence. In this scheme, we split the incoming speech signal into two frequency bands in order to gain the benefits of the piecewise LP (Linear Prediction) approximation. Then, each residual signal is coded in blocks of 5-ms duration through an adaptive vector quantizer incorporating a noise shaping filter. Our results show that good quality speech can be obtained at 8 kbit/s.
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We present an approach to low-rate speech coding based on multistage self-excited linear prediction (SELP). The coder has much lower complexity than a code-excited linear prediction (CELP) coder, but yields comparable output speech quality at the same transmission rate. In the proposed multistage SELP coder, the long-term predictor is modified to model better the onset of a voiced section of speech or voice with high pitch.
Linear prediction
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Predictive coding
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Non-linear prediction is a natural way to increase the quality of speech coders. In particular, low-delay CELP-type coders can incorporate this improvement because the predictor adaptation is backward. Consequently, there is the possibility of using neural networks as predictors, since their weights (usually a larger number than required in the linear approach) do not have to be transmitted. We apply a radial basis function (RBF) network for this purpose since it computes a regularized solution to the prediction problem. As a result, the stability of the non-linear autoregressive synthesis system can be guaranteed. Investigations of how to combine non-linear predictors with linear predictors indicate that a cascade of an RBF network and a linear filter is a suitable selection since it provides good results and its application to analysis-by-synthesis coders results in large computational advantages with respect to the parallel configuration. This hybrid predictor has been tested for a low-delay code-excited predictive coder, providing an average improvement of 0·4 dB with respect to a CELP coder. Additionally, subjective listening tests give the proposed coder a slight preference over the CELP coder. These results are encouraging because we consider that the proposed coder can be implemented in real time after some improvements, which are detailed as the subject of further work. © 1997 John Wiley & Sons. Ltd.
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Perceptual linear prediction (PLP) is widely used in speech recognition systems as a feature extraction method. Also code-exited linear prediction coder (CELP) is one of the well known speech coders which widely used in communication systems. In this paper the application of PLP in speech coding has been discussed. In the first stage the parameters of formant synthesis filter are determined by applying PLP algorithms. Then these parameters are used in coder, code-exited linear prediction coder, to improve the efficiency of this kind of coder. The experiments show promising result in some cases.
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