Algebraic code-excited linear prediction (ACELP) is a
speech coding
Speech coding is an application of data compression to digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic da ...
algorithm in which a limited set of pulses is distributed as excitation to a
linear prediction filter. It is a
linear predictive coding
Linear predictive coding (LPC) is a method used mostly in audio signal processing and speech processing for representing the spectral envelope of a digital signal of speech in compressed form, using the information of a linear predictive model ...
(LPC) algorithm that is based on the
code-excited linear prediction (CELP) method and has an
algebra
Algebra is a branch of mathematics that deals with abstract systems, known as algebraic structures, and the manipulation of expressions within those systems. It is a generalization of arithmetic that introduces variables and algebraic ope ...
ic structure. ACELP was developed in 1989 by the researchers at the
Université de Sherbrooke
The Université de Sherbrooke (UdeS; Quebec English, English: ''University of Sherbrooke'') is a French-language Public university, public research university in Sherbrooke, Quebec, Canada, with a second campus in Longueuil, a suburb on the Mont ...
in
Canada
Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
.
The ACELP method is widely employed in current speech coding standards such as
AMR,
EFR,
AMR-WB
Adaptive Multi-Rate Wideband (AMR-WB) is a patented Wideband audio, wideband speech coding, speech audio coding standard developed based on Adaptive Multi-Rate audio codec, Adaptive Multi-Rate encoding, using a similar methodology to algebraic cod ...
(G.722.2),
VMR-WB,
EVRC
Enhanced Variable Rate CODEC (EVRC) is a speech codec used in CDMA networks. It was developed in 1995 to replace the QCELP vocoder which used more bandwidth on the carrier's network, thus EVRC's primary goal was to offer the mobile carriers more ...
,
EVRC-B
Enhanced Variable Rate Codec B (EVRC-B) is a speech codec used by CDMA networks. EVRC-B is an enhancement to EVRC and compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speech
input into output frames of one of the four different sizes ...
,
SMV,
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 ...
CELP and
ITU-T
The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating Standardization, standards fo ...
G-series standards
G.729,
G.729.1 (first coding stage) and
G.723.1.
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
The 3rd Generation Partnership Project (3GPP) is an umbrella term for a number of standards organizations which develop protocols for mobile telecommunications. Its best known work is the development and maintenance of:
* GSM and related 2G and ...
formats.
The ACELP patent expired in 2018 and is now royalty-free.
Features
The main advantage of ACELP is that the
algebra
Algebra is a branch of mathematics that deals with abstract systems, known as algebraic structures, and the manipulation of expressions within those systems. It is a generalization of arithmetic that introduces variables and algebraic ope ...
ic codebook it uses can be made very large (> 50 bits) without running into storage (
RAM
Ram, ram, or RAM most commonly refers to:
* A male sheep
* Random-access memory, computer memory
* Ram Trucks, US, since 2009
** List of vehicles named Dodge Ram, trucks and vans
** Ram Pickup, produced by Ram Trucks
Ram, ram, or RAM may also ref ...
/
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
* ...
) or complexity (
CPU 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
Data compression