| 摘要 |
A speech decoder capable of auditorially reducing the quantization noise superimposed on the synthesized signal and improving a speech quality at lower bit rates is disclosed. A de-multiplexer unit 100 receives and separates an index concerning spectrum parameter, an index concerning amplitude, an index concerning pitch and an index concerning excitation signal. A synthesis filter unit 140 restores a synthesis filter drive signal based on the index concerning pitch, the index concerning excitation signal and the index concerning amplitude, forming the synthesis filter based on the index concerning spectrum parameter and obtaining a synthesized signal by driving the synthesis filter with the synthesis filter drive signal. A postfilter unit 200 receives the output signal of the synthesis filter and controls the spectrum of the synthesized signal. A filter coefficient calculation unit 210 derives an auditory masking threshold value from the synthesized signal and derives postfilter coefficients corresponding to the masking threshold value.
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