Method and apparatus for calculating downmixed signal and residual signal

A method for calculating a downmixed signal and a residual signal, the method comprising:

• obtaining, by an audio encoder comprising a processor, an audio signal including a plurality of frames;
• obtaining, by the audio encoder, an initial downmixed signal and an initial residual signal of a subband corresponding to a preset frequency band in a current frame of the audio signal;
• determining, by the audio encoder, whether a first target frame of the audio signal is a switching frame, wherein the first target frame is the current frame or a previous frame of the current frame;
• if the first target frame is a switching frame, calculating, by the audio encoder and based on a switch fade-in/fade-out factor of a second target frame, the initial downmixed signal, and the initial residual signal, a to-be-encoded downmixed signal and a to-be-encoded residual signal of the subband corresponding to the preset frequency band in the current frame;
• and
• encoding, by the audio encoder, the to-be-encoded downmixed signal and the to-be-encoded residual signal to obtain an encoded bitstream;
• wherein the second target frame is the current frame or the previous frame of the current frame, and the switch fade-in/fade-out factor is determined based on a residual signal coding parameter of the second target frame and at least one of an inter-frame energy fluctuation parameter or an inter-frame amplitude fluctuation parameter of the second target frame, and
• wherein the residual signal coding parameter represents an energy relationship between a first downmixed signal and a first residual signal of the second target frame, and the inter-frame energy fluctuation parameter or the inter-frame amplitude fluctuation parameter represents an energy or amplitude relationship between the second target frame and M frames previous to the second target frame, wherein M is a positive integer.