CELP Codec

CELP is a hybrid codec using both waveform and source coding techniques. Thus, as expected, the process of determination of the filter coefficients introduces high delays (cf. PCM codecs). The delay of a speech codec is defined as the time from when a speech sample arrives at the input of its encoder to when the corresponding sample is produced at the output of its decoder, assuming the bit stream from the encoder is fed directly to the decoder. For a typical hybrid speech codec this delay will be of the order of 50 to 100 ms, and such a high delay can cause problems. Thus, many efforts have been focused in providing a standard codec that has as bit rate 16 Kbps, while providing a quality comparable to that provided by the ADPCM 32 Kbps. The major challenge is to reduce the delay to about 5 ms. This is satisfied by a backward adaptive CELP codec, standardized in 1992 as ITU-T G728 recommendation. Another CELP codec is standardized in 1991 by the American Department of Defense (DoD). It operates at a 4.8 Kbps.