Algebraic code-excited linear prediction (ACELP) is a
speech coding algorithm in which a limited set of pulses is distributed as excitation to a
linear prediction filter. It is a
linear predictive coding (LPC) algorithm that is based on the
code-excited linear prediction (CELP) method and has an
algebraic structure. ACELP was developed in 1989 by the researchers at the
Université de Sherbrooke in
Canada.
The ACELP method is widely employed in current speech coding standards such as
AMR,
EFR,
AMR-WB (G.722.2),
VMR-WB,
EVRC,
EVRC-B,
SMV SMV may refer to:
People
* Sir Mokshagundam Visvesvaraya, Indian engineer, politician and Diwan of Mysore
In computer science
* Symbolic model verification
* SMV modelling language, used in model checking by the CMU SMV and NuSMV model checkers
P ...
,
TETRA, PCS 1900,
MPEG-4
MPEG-4 is a group of international standards for the compression of digital audio and visual data, multimedia systems, and file storage formats. It was originally introduced in late 1998 as a group of audio and video coding formats and related tec ...
CELP and
ITU-T G-series standards
G.729,
G.729.1
G.729.1 is an 8-32 kbit/s embedded speech and audio codec providing bitstream interoperability with G.729, G.729 Annex A and G.729 Annex B. Its official name is ''G.729-based embedded variable bit rate codec: An 8-32 kbit/s scalable wideband code ...
(first coding stage) and
G.723.1
G.723.1 is an audio codec for voice that compresses voice audio in frames. An algorithmic look-ahead of duration means that total algorithmic delay is . Its official name is ''Dual rate speech coder for multimedia communications transmitting at ...
.
The ACELP algorithm is also used in the proprietary ACELP.net codec.
Audible Inc. use a modified version for their speaking books. It is also used in conference-calling software, speech compression tools and has become one of the
3GPP formats.
The ACELP patent expired in 2018 and is now royalty-free.
Features
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
Rom, or ROM may refer to:
Biomechanics and medicine
* Risk of mortality, a medical classification to estimate the likelihood of death for a patient
* Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac
* R ...
) or complexity (
CPU
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...
time) problems.
Technology
The ACELP algorithm is based on that used in
code-excited linear prediction (CELP), 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 (pitch) synthesis filter is given by:
:
The short-term synthesis filter is given by:
:
References
{{Compression Methods
Speech codecs