Code excited linear prediction

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.

Introduction

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 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.

CELP decoder

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External links

• This is based on a paper presented at Linux.Conf.Au
• Some parts based on the Speex codec manual
• reference implementations of CELP 1016A (CELP 3.2a) and LPC 10e.
• Linear Predictive Coding (LPC)

References

• B.S. Atal, "The History of Linear Prediction," IEEE Signal Processing Magazine, vol. 23, no. 2, March 2006, pp. 154–161.

• 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.