Code-excited linear prediction: Difference between revisions

Revision as of 02:56, 30 January 2011

Revision as of 17:24, 28 April 2011

Line 3:

== Introduction ==

The CELP algorithm is based on four main ideas:

* Using the [[source-filter model of speech production]] through [[linear prediction]] (LP)(see the textbook "speech coding algorithm");

* Using an adaptive and a fixed codebook as the input (excitation) of the LP model;

Line 12 ⟶ 11:

== CELP decoder ==

[[File:Celp_decoderCelp decoder.pngsvg|300px|thumb|Figure 1: CELP decoder]]

Before exploring the complex encoding process of CELP we introduce the decoder here. Figure 1 describes a generic CELP decoder. The excitation is produced by summing the contributions from an adaptive (aka pitch) codebook and a stochastic (aka innovation or fixed) codebook:

Line 23 ⟶ 22:

== 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“best sounding"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: