Algebraic code-excited linear prediction (ACELP) is a
speech coding
Speech coding is an application of data compression of 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 d ...
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 mod ...
(LPC) algorithm that is based on the
code-excited linear prediction (CELP) method and has an
algebra
Algebra () is one of the broad areas of mathematics. Roughly speaking, algebra is the study of mathematical symbols and the rules for manipulating these symbols in formulas; it is a unifying thread of almost all of mathematics.
Elementary ...
ic structure. ACELP was developed in 1989 by the researchers at the
Université de Sherbrooke in
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
.
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 speech audio coding standard developed based on Adaptive Multi-Rate encoding, using a similar methodology to algebraic code-excited linear prediction (ACELP). AMR-WB provides improved ...
(G.722.2),
VMR-WB
Variable-Rate Multimode Wideband (VMR-WB) is a source-controlled variable-rate multimode codec designed for robust encoding/decoding of wideband/narrowband speech. The operation of VMR-WB is controlled by speech signal characteristics (i.e., source ...
,
EVRC,
EVRC-B,
SMV,
TETRA
Terrestrial Trunked Radio (TETRA; formerly known as Trans-European Trunked Radio), a European standard for a trunked radio system, is a professional mobile radio and two-way transceiver specification. TETRA was specifically designed for use by ...
, 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 t ...
CELP and
ITU-T
The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Co ...
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
algebra
Algebra () is one of the broad areas of mathematics. Roughly speaking, algebra is the study of mathematical symbols and the rules for manipulating these symbols in formulas; it is a unifying thread of almost all of mathematics.
Elementary ...
ic codebook it uses can be made very large (> 50 bits) without running into storage (
RAM
Ram, ram, or RAM may refer to:
Animals
* A male sheep
* Ram cichlid, a freshwater tropical fish
People
* Ram (given name)
* Ram (surname)
* Ram (director) (Ramsubramaniam), an Indian Tamil film director
* RAM (musician) (born 1974), Dutch
* ...
/
ROM) 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, a ...
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