(57) A mode discriminating circuit (300) extracts a feature quantity from a voice signal
and makes mode discrimination. When the output of the circuit (300) represents a predetermined
mode, a smoothing circuit (450) executes time-wise smoothing of at least either one
of excitation gain, adaptive codebook gain, spectral parameter and excitation signal
level. A multiplexer (400) locally reproduces a synthesized signal by using a signal
obtained through the smoothing, and feeds out a combination of the outputs of spectral
parameter calculating circuit, mode discriminating circuit, adaptive codebook circuit,
excitation quantizing circuit and gain quantizing circuit. Thus it makes possible
to provide a voice coding and a voice decoding apparatus, which permit good coded
sound quality to be obtained for voice with background noise superimposed thereon
even at a low bit rate.
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