This paper presents increased bit-rate waveform interpolation (WI) coders with rates between 2.4b kb/s and 3.6 kb/s. In particular, the performance of WI coders at bit rates higher than 2.4 kb/s is examined. It is found that by increasing the bit rate, and with no or little change in the coder structure, the perceptual quality of speech produced by WI coders is substantially improved. Both open-loop and closed-loop WI coders (the latter using an analysis-by-synthesis (A-by-S) technique) are tested. The potential of higher bit rates for each of the quantised parameters is discussed. Hence, suggested bit allocations for different increased bit rates are derived. Four coders, operating at bit rates of 2.4 kb/s, 2.72 kb/s, 3.24 kb/s and 3.6 kb/s, respectively are considered. Informal listening tests show successive improvement in speech quality.
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