A fixed-point 16 kb/s LD-CELP algorithm

The authors describe the algorithm modifications they have made in order to make the LD-CELP (low-delay code-excited linear prediction) algorithm suitable for 16 bit fixed-point implementation. They replaced T.P Barnwell´s recursive windowing method (1981) by a novel hybrid which is partially recursive and partially nonrecursive. This method avoided the dynamic range problem and the double-precision arithmetic that would otherwise have been required. A fourth-order LPC filter was cascaded at the output of the original 50th-order LPC filter. This filter effectively reduced the spectral dynamic range of the output of the 50th-order LPC filter, therefore alleviating the ill-conditioning problem of the 50th-order LPC analysis. Although the algorithm of J. LeRoux and C. Gueguen (1979) was believed to be better suited for fixed-point arithmetic than Durbin´s recursion (see L.R. Rabiner and R.W. Schafer, 1978), this was not necessarily true for the two-stage cascaded LPC filter used. Fixed-point simulation showed that the speech quality of the modified fixed-point LD-CELP algorithm was essentially the same as that of the original floating-point LD-CELP algorithm