Code-excited linear prediction (CELP) is a
linear predictive 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 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 algorithms, such as
residual-excited linear prediction Residual-excited linear prediction (RELP) is an obsolete speech coding algorithm. It was originally proposed in the 1970s and can be seen as an ancestor of code-excited linear prediction (CELP). Unlike CELP however, RELP directly transmits the resid ...
(RELP) and
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)
vocoders
A vocoder (, a portmanteau of ''voice'' and ''encoder'') is a category of speech coding that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption or voice transformation.
The vocoder was i ...
(e.g.,
FS-1015 FIPS 137, originally issued as FED-STD-1015, is a secure telephony speech encoding standard for Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on November 28, 1984.
It was based on the earlier STA ...
). Along with its variants, such as
algebraic CELP
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- ...
,
relaxed CELP,
low-delay CELP G.728 is an ITU-T standard for speech coding operating at 16 kbit/s. It is officially described as ''Coding of speech at 16 kbit/s using low-delay code excited linear prediction''.
Technology used is LD-CELP, low-delay code excited linear pre ...
and
vector sum excited linear prediction
Vector sum excited linear prediction (VSELP) is a speech coding method used in several cellular standards. The VSELP algorithm is an analysis-by-synthesis coding technique and belongs to the class of speech coding algorithms known as CELP (Code Exc ...
, it is currently the most widely used speech coding algorithm. It is also used in
MPEG-4 Audio
MPEG-4 Part 3 or MPEG-4 Audio (formally International Organization for Standardization, ISO/International Electrotechnical Commission, IEC 14496-3) is the third part of the International Organization for Standardization, ISO/International Electrote ...
speech coding. CELP is commonly used as a generic term for a class of algorithms and not for a particular codec.
Background
The CELP algorithm is based on four main ideas:
* Using the
source-filter model of speech production through
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 ...
(LP) (see the textbook "speech coding algorithm");
* Using an adaptive and a fixed codebook as the input (excitation) of the LP model;
* Performing a search in closed-loop in a "perceptually weighted domain".
* Applying
vector quantization
Vector quantization (VQ) is a classical quantization technique from signal processing that allows the modeling of probability density functions by the distribution of prototype vectors. It was originally used for data compression. It works by di ...
(VQ)
The original algorithm as simulated in 1983 by Schroeder and Atal required 150 seconds to encode 1 second of speech when run on a
Cray-1 supercomputer. Since then, more efficient ways of implementing the codebooks and improvements in computing capabilities have made it possible to run the algorithm in embedded devices, such as mobile phones.
CELP decoder
![Celp decoder](https://upload.wikimedia.org/wikipedia/commons/c/c8/Celp_decoder.svg)
Before exploring the complex encoding process of CELP we introduce the decoder here. Figure 1 describes a generic CELP decoder. The excitation is produced by summing the contributions from fixed (a.k.a. stochastic or innovation) and adaptive (a.k.a. pitch) codebooks:
:
where