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Improving CELP coders by backward adaptive non‐linear prediction
Author(s) -
DíazdeMaría Fernando,
FigueirasVidal Aníbal R.
Publication year - 1997
Publication title -
international journal of adaptive control and signal processing
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.73
H-Index - 66
eISSN - 1099-1115
pISSN - 0890-6327
DOI - 10.1002/(sici)1099-1115(199711)11:7<585::aid-acs454>3.0.co;2-9
Subject(s) - code excited linear prediction , linear prediction , linear predictive coding , computer science , speech recognition , autoregressive model , artificial neural network , speech coding , cascade , vector sum excited linear prediction , filter (signal processing) , artificial intelligence , mathematics , statistics , chemistry , chromatography , computer vision
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.