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10.6. Summary

This chapter has described the MPEG-4 tools for coding of natural speech—that is, representations derived from sampled speech signals. The required flexibility with respect to bit-rate and quality ranges, speech bandwidth, and other functionalities lead to the support of two coding algorithms: CELP and HVXC.

The results of the verification tests presented in Chapter 16 show that MPEG-4 CELP at 6 kbit/s provides competitive quality to ITU-T G723.1 [G723] at 6.3 kbit/s, and higher quality at higher bit rates that is competitive to the existing standards for the same bit-rate range of telephone band speech, allowing flexible bit-rate controllability and scalability up to 12 kbit/s. Wideband speech coding with 16-kHz sampling rate is supported at 10 to 24 kbit/s, in which quality comparable to ITU T G.722 [G722] 48/56 kbit/s is obtained at around 18 kbit/s [N2424]. Bandwidth scalability (in addition to bit-rate scalability) is supported, in which telephone band decoding is possible using a subset of the bitstream for wideband speech. The operating bit-rate range of the CELP coder, however, starts at about 4 kbit/s—as, for lower bit rates, the quality deteriorates because of the nature of the waveform matching used in the CELP algorithm.


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