Information about Algebraic Code Excited Linear Prediction

Algebraic Code Excited Linear Prediction (ACELP) is a speech encoding algorithm where a limited set of pulses is distributed as excitation to linear prediction filter.

The ACELP method is widely employed in current speech coding standards such as AMR, EFR, AMR-WB and ITU-T G-series standards G.729 and G.723.1.

ACELP is a registered trademark of VoiceAge Corporation^[1] in Canada and/or other countries and was developed in 1989 by the Université de Sherbrooke in Canada^[2].

Features

The ACELP vocoder has a bit rate of 7.2 kbit/s including forward error correction (FEC).

The main advantage of ACELP is that the algebraic codebook it uses can be made very large (> 50 bits) without running into storage (RAM/ROM) or complexity (CPU time) problems.

Technology

The ACELP vocoder algorithm is based on the CELP coding model, but ACELP codebooks have a specific algebraic structure imposed upon them.

A 16-bit algebraic codebook shall be used in the innovative codebook search, the aim of which is to find the best innovation and gain parameters. The innovation vector contains, at most, four non-zero pulses.

In ACELP a block of N speech samples is synthesized by filtering an appropriate innovation sequence from a codebook, scaled by a gain factor g c, through two time varying filters.

The long-term or pitch, synthesis filter is given by: 1/B(z) = 1/(1 - g_pz^-T)

The short-term synthesis filter is given by: H(z) = 1/A(z) = 1/(1 + ZIGMA_{i = 1 to P}a_iz^-i)

References