Improving the performance of the 16 kb/s LD-CELP speech coder

The CCITT is in the process of standardizing a 16 kb/s speech coder submitted by AT&T called low-delay code excited linear prediction (LD-CELP). In the first phase of CCITT testing, the coder met all performance requirements except for the tandeming condition. To improve the tandeming performance, the authors first tuned the perceptual weighting filter to optimize the coder´s performance after three tandems. They then added an adaptive postfilter which was also tuned for three tandems. The excitation codebook was also reoptimized using a multiple-language IRS-weighted training database. With these changes, in the second phase of CCITT testing, the coder significantly exceeded the tandeming performance requirement and also met all other requirements. In fact, the speech quality of LD-CELP was equivalent to or better than that of 32 kb/s ADPCM for all conditions tested. It is anticipated that the version of LD-CELP will be formally ratified as a CCITT standard in 1992. The author describes the modifications made to improve the performance and discusses the CCITT test results