Harmonic pre-emphasis for improving the performance of FS-1016 CELP

Selective harmonic pre-emphasis, a new algorithmic enhancement for FS-1016 CELP, is described. Selective harmonic pre-emphasis helps correct occasional errors with CELP due to unreliable estimation of the adaptive codebook delay, particularly at transitions from unvoiced to voiced such as at word onsets. Errors of this type result in audible roughness and distortion in initial voiced areas. Selective harmonic pre-emphasis is applied in the analysis-by-synthesis loop of the analyzer to strengthen selectively the harmonic structure in voiced areas so that adaptive codebook delay values can be estimated more reliably and smoothly. The procedure is adaptive so that pre-emphasis becomes more pronounced as voicing increases and is disabled completely when the current frame is unvoiced. Selective harmonic pre-emphasis is shown to produce an objective improvement of from 0.8 to 1.5 DRT points and from 0.9 to 1.2 DAM points in quiet and office environments, respectively. Full interoperability with the FS-1016 standard is preserved.