Code-excited linear prediction (CELP) is a
linear predictive speech coding algorithm originally proposed by
Manfred R. Schroeder and
Bishnu S. Atal
Bishnu S. Atal (born 1933) is an Indian physicist and engineer. He is a noted researcher in acoustics, and is best known for developments in speech coding. He advanced linear predictive coding (LPC) during the late 1960s to 1970s, and developed ...
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 mod ...
(LPC)
vocoders (e.g.,
FS-1015). 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, it is currently the most widely used speech coding algorithm. It is also used in
MPEG-4 Audio 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 (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 (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
The Cray-1 was a supercomputer designed, manufactured and marketed by Cray Research. Announced in 1975, the first Cray-1 system was installed at Los Alamos National Laboratory in 1976. Eventually, over 100 Cray-1s were sold, making it one of the ...
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
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