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 da ...

algorithm in which a limited set of pulses is distributed as excitation to a

linear prediction Linear prediction is a mathematical operation where future values of a discrete-time signal are estimated as a linear function of previous samples. In digital signal processing, linear prediction is often called linear predictive coding (LPC) and ...

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 Code-excited linear prediction (CELP) is a linear predictive speech coding algorithm originally proposed by Manfred R. Schroeder and Bishnu S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algori ...

(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 a ...

ic structure. ACELP was developed in 1989 by the researchers at the

Université de Sherbrooke The University of Sherbrooke ( French: Université de Sherbrooke) (UdS) is a large public French-language university in Quebec, Canada with campuses located in Sherbrooke and Longueuil, a suburb of Montreal approximately west of Sherbrooke. It ...

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 tot ...

. 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 sp ...

(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 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 cap ...

, 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 go ...

, 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 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 Commu ...

G-series standards

G.729 G.729 is a royalty-free narrow-band vocoder-based audio data compression algorithm using a frame length of 10 milliseconds. It is officially described as ''Coding of speech at 8 kbit/s using code-excited linear prediction'' speech coding (CS-ACELP ...

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. Audible is an American online audiobook and podcast service that allows users to purchase and stream audiobooks and other forms of spoken word content. This content can be purchased individually or under a subscription model where the user receiv ...

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

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 * Ra ...

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

(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: :

\frac1 = \frac1

The short-term synthesis filter is given by: :

\frac1 = \frac1

References

{{Compression Methods Speech codecs