a decoding device reduces abrupt changes in the number of channels in a decoded signal when transmission errors occur as a result of lost frames in an encoding / decoding system for multichannel signals . in the device , a demultiplexer receives an encoded monaural signal and an encoded differential signal and detects change over time in the received encoded differential signal . an m signal decoder decodes the encoded monaural signal and obtains a decoded monaural signal . an s signal decoder decodes the encoded differential signal and obtains a decoded differential signal . a smoothing unit performs smoothing on the decoded differential signal by means of a computation involving the decoded differential signal and coefficients corresponding to the change over time detected by the demultiplexer . an l / r signal computation unit computes a decoded stereo signal from the decoded monaural signal and the smoothed decoded differential signal .