Code-excited linear prediction: Difference between revisions

Revision as of 20:30, 27 October 2011

Revision as of 20:31, 27 October 2011

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where <math>e_{a}[n]</math> is the adaptive ([[Pitch (music)|pitch]]) codebook contribution and <math>e_{f}[n]</math> is the stochastic (innovation or fixed) codebook contribution. The fixed codebook is a [[vector quantization]] dictionary that is (implicitly or explicitly) hard-coded into the codec. This codebook can be algebraic ([[ACELP]]) or be stored explicitly (e.g. [[Speex]]). The entries in the adaptive codebook consist of delayed versions of the excitation. This makes it possible to efficiently code periodic signals, such as voiced sounds.

The filter that shapes the excitation has an all-pole model of the form <math>1/A(z)</math>, where <math>A(z)</math> is called the prediction filter and is obtained using linear prediction ([[Levinson recursion|Levinson&ndash;DurbinLevinson–Durbin algorithm]]). An all-pole filter is used because it is a good representation of the human vocal tract and because it is easy to compute.

==CELP encoder==

The main principle behind CELP is called [[Analysis-by-Synthesis|Analysis-by-Synthesis (AbS)]] and means that the encoding (analysis) is performed by perceptually optimizing the decoded (synthesis) signal in a closed loop. In theory, the best CELP stream would be produced by trying all possible bit combinations and selecting the one that produces the best-sounding decoded signal. This is obviously not possible in practice for two reasons: the required complexity is beyond any currently available hardware and the “best sounding” selection criterion implies a human listener.

In order to achieve real-time encoding using limited computing resources, the CELP search is broken down into smaller, more manageable, sequential searches using a simple perceptual weighting function. Typically, the encoding is performed in the following order:

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==References==

* B.S. Atal, "The History of Linear Prediction," ''IEEE Signal Processing Magazine'', vol. 23, no. 2, March 2006, pp.&nbsp;154&ndash;161154–161.

* M. R. Schroeder and B. S. Atal, "Code-excited linear prediction (CELP): high-quality speech at very low bit rates," in ''Proceedings of the IEEE [[International Conference on Acoustics, Speech, and Signal Processing]]'' (ICASSP), vol. 10, pp.&nbsp;937&ndash;940937–940, 1985.

{{Compression Methods}}