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Code Excited Linear Prediction (CELP) is a speech coding algorithm originally proposed by M.R. Schroeder and B.S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algorithms, such as RELP and LPC vocoders (e.g. FS-1015). Along with its variants, such as ACELP, RCELP, LD-CELP and VSELP, it is currently the most widely used speech coding algorithm. CELP is now used as a generic term for a class of algorithms and not for a particular codec. Speech coding is the compression of speech (into a code) for transmission with speech codecs that use audio signal processing and speech processing techniques. ...
RELP stands for Residual Excited Linear Prediction and is an old (no longer used) speech coding algorithm. ...
It has been suggested that this article or section be merged with Code Excited Linear Prediction. ...
A vocoder (name derived from voice coder, formerly also called voder) is a speech analyser and synthesizer. ...
FS-1015 is a secure telephony speech encoding standard developed by the United States Department of Defense and later by NATO. It is also known as LPC-10 and STANAG 4198. ...
Algebraic Code Excited Linear Prediction or ACELP is a speech encoding algorithm where a limited set of pulses is distributed as excitation to linear prediction filter. ...
RCELP stands for Relaxed Code Excited Linear Prediction. ...
G.728 is a ITU-T standard for speech coding operating at 16 kbit/s. ...
VSELP stands for Vector Sum Excited Linear Prediction and is a speech coding method used in the IS-54 standard. ...
Introduction
The CELP algorithm is based on four main ideas: - Using the source-filter model of speech production through linear prediction (LP);
- 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 proposed by Schroeder and Atal required 100 seconds to encode 1 second of speech when run on a Cray I 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. The source-filter model of speech production assumes that the vocal cords are the source of spectrally flat sound (the excitation signal), and that the vocal tract acts as a filter to spectrally shape the various sounds of speech. ...
Linear prediction is a mathematical operation where future values of a discrete-time signal are estimated as a linear function of previous samples. ...
Vector quantization is a classical technique from signal processing, originally used for data compression, which provides a method for modeling probability density functions by the distribution of prototype vectors. ...
CRAY-1 at the EPFL in Switzerland. ...
CELP Decoder Before exploring the complex encoding process of CELP we introduce the Speex decoder here. Figure 1 describes a generic CELP decoder. The excitation is produced by summing the contributions from an adaptive (aka pitch) codebook and a fixed (aka innovation) codebook: Image File history File links Celp_decoder. ...
Image File history File links Celp_decoder. ...
- e[n] = ea[n] + ef[n]
where ea[n] is the adaptive (pitch) codebook contribution and ef[n] is the fixed (innovation) codebook contribution. The fixed codebook is a vector quantization dictionary that is (implicitly or explicitly) hard-coded into the codec. This codebook can be algebraic (ACELP) or be stored explicitly (e.g. Speex). The entries in the adaptive codebook consist of delayed versions of the excitation. This makes it possible to efficiently code periodic signals, such as voiced sounds. Pitch is the perceived fundamental frequency of a sound. ...
Vector quantization is a classical technique from signal processing, originally used for data compression, which provides a method for modeling probability density functions by the distribution of prototype vectors. ...
Algebraic Code Excited Linear Prediction or ACELP is a speech encoding algorithm where a limited set of pulses is distributed as excitation to linear prediction filter. ...
Speex is a free software speech codec that may be used on VoIP applications and podcasts. ...
The filter that shapes the excitation has an all-pole model of the form 1 / A(z), where A(z) is called the prediction filter and is obtained using linear prediction (Levinson-Durbin algorithm). An all-pole filter is used because it is a good representation of the human vocal tract and because it is easy to compute. Levinson recursion is a mathematical procedure which recursively calculates the solution to a Toeplitz matrix. ...
CELP Encoder The main principle behind CELP is called Analysis-by-Synthesis (AbS) and means that the encoding (analysis) is performed by perceptually optimising the decoded (synthesis) signal in a closed loop. In theory, the best CELP stream would be produced by trying all possible bit combinations and selecting the one that produces the best-sounding decoded signal. This is obviously not possible in practice for two reasons: the required complexity is beyond any currently available hardware and the "best sounding" selection criterion implies a human listener.
In order to achieve real-time encoding using limited computing resources, the CELP search is broken down into smaller, more manageable, sequential searches using a simple perceptual weighting function. Typically, the encoding is performed in the following order:
- LPC coefficients are computed and quantized, usually as LSPs
- The adaptive (pitch) codebook is searched and its contribution removed
- The fixed (innovation) codebook is searched
Line Spectral Pairs (LSP) are used to represent Linear Prediction Coefficients (LPC) for transmission over a channel. ...
Noise Weighting Most (if not all) modern audio codecs attempt to shape the coding noise so that it appears mostly in the frequency regions where the ear cannot detect it. For example, the ear is more tolerant to noise in parts of the spectrum that are louder and vice versa. That's why instead of minimizing the simple quadratic error, CELP minimizes the error for the perceptually weighted domain. The weighting filter W(z) is typically derived from the LPC filter by the use of bandwidth expansion: Psychoacoustics is the study of subjective human perception of sounds. ...
Bandwidth expansion is a technique for widening the bandwidth or the resonances in an LPC filter. ...
where γ1 > γ2.
External links Speex is a free software speech codec that may be used on VoIP applications and podcasts. ...
References M. R. Schroeder and B. S. Atal, "Code-excited linear prediction (CELP): high-quality speech at very low bit rates," in Proceedings of the IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP), vol. 10, pp. 937-940, 1985. | 
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