Sound signal refining method, sound signal decoding method, apparatus thereof, program, and storage medium

A sound signal purification device for obtaining, for each frame, an n-th channel purified decoded sound signal {tilde over ( )}Xn that is a sound signal of each channel of stereo by using at least an n-th channel decoded sound signal {circumflex over ( )}Xn (n is each integer of 1 or more and N or less) that is a decoded sound signal of the each channel of the stereo obtained by decoding a stereo code CS and a monaural decoded sound signal {circumflex over ( )}XM that is a monaural decoded sound signal obtained by decoding a monaural code CM that is a code different from the stereo code CS, wherein

• the n-th channel decoded sound signal {circumflex over ( )}Xn is obtained by decoding the stereo code CS without using either information obtained by decoding the monaural code CM or the monaural code CM, and
• the sound signal purification device further comprises
• a monaural decoded sound upmixing circuitry configured to obtain, for the each frame, an n-th channel upmixed monaural decoded sound signal {circumflex over ( )}XMn that is a signal obtained by upmixing the monaural decoded sound signal {circumflex over ( )}XM for the each channel by an upmixing process using the monaural decoded sound signal {circumflex over ( )}XM and inter-channel relationship information that is information indicating a relationship between the channels of the stereo, and
• an n-th channel signal purification circuitry configured to obtain, for the each frame and for each corresponding sample t with respect to the each channel n, a sequence based on a value {tilde over ( )}xn(t)=(1−αn)×{circumflex over ( )}xn(t)+αn×{circumflex over ( )}xMn(t) obtained by adding a value αn×{circumflex over ( )}xMn(t) obtained by multiplying an n-th channel purification weight αn by a sample value {circumflex over ( )}xMn(t) of the n-th channel upmixed monaural decoded sound signal {circumflex over ( )}XMn and a value (1−αn)×{circumflex over ( )}xn(t) obtained by multiplying a value (1−αn) obtained by subtracting the n-th channel purification weight αn from 1 by a sample value {circumflex over ( )}xn(t) of the n-th channel decoded sound signal {circumflex over ( )}Xn, as the n-th channel purified decoded sound signal {tilde over ( )}Xn.