Abstract: An audio encoder includes a time-frequency mapping block, a psychoacoustic model block, a middle/side (M/S) encoding block, a parameter calculation block, a bit allocation and quantization block and a bitstream formatting block. The encoder is forced to operate in M/S mode for reducing the calculation time of the parameter used for bit allocation, quantization and encoding. In addition, the calculation of the parameter only needs to consider the middle and side channels but not the left and right channels, thus the complexity of the psychoacoustic model for analyzing the input audio signal can be reduced.