Sakari Himanen

A 4 kb/s hybrid MELP/CELP speech coding candidate for ITU standardization

This paper presents an improved 4 kb/s hybrid MELP/CELP speech coder submitted as a candidate for ITU standardization. The coder uses three modes: a high-quality MELP coder for strongly voiced speech frames, an ACELP coder with pitch prediction for weakly voiced frames, and a stochastic CELP coder for unvoiced frames. We present recent enhancements to this coder, both to improve speech quality and to reduce coder complexity. Previous lTU Selection Testing results on an earlier version of this coder showed that it met nearly all requirements for toll-quality speech, more than any other candidate. Our internal testing shows that the current reduced-complexity fixed-point coder maintains this high performance.

Hybrid MELP/CELP coding at bit rates from 6.4 to 2.4 kb/s

This paper describes extensions of the 4 kb/s hybrid MELP/CELP coder, up to 6.4 kb/s and down to 2.4 kb/s. The baseline 4 kb/s coder uses three coding modes: MELP in strongly voiced speech frames, CELP with pitch prediction in weakly voiced frames, and CELP with stochastic excitation in unvoiced frames. To minimize switching artifacts between parametric MELP and waveform CELP coding, an alignment phase is encoded in MELP and zero-phase equalization is applied to the CELP target signal. The 6.4 kb/s extension uses the same three modes as the 4 kb/s coder, with improved MELP and CELP coders. The 2.4 kb/s extension uses only two modes: MELP for voiced frames and CELP synthesis with random excitation for unvoiced frames. The alignment phase is encoded in MELP frames for all bit rates so that time synchrony with input speech is always maintained. Alignment phase and zero-phase equalization enable smooth switching between coders at different bit rates. The hybrid MELP/CELP coding structure leads to coders that perform better at a given bit rate than MELP or CELP separately, and better than or equivalent to higher bit-rate ITU standards. Formal subjective tests show that for all-but-one tested conditions, the 6.4 kb/s hybrid coder is better than 8 kb/s G.729 and the 2.4 kb/s coder is equivalent to, or better than, 6.4 kb/s G.729 Annex D.