Methods, apparatus and system for encoding and decoding a signal

A method for encoding an audio signal, comprising: performing a decision process classification of high frequency signals audio signal input, wherein performing the process of classification decision on the high frequency signals of the input signals comprises: calculating parameters of the high frequency signals; determining a current frame type of high frequency signals according to the parameters and decision mechanism; when it is determined that the current frame type of high frequency signals is a transient signal, encode four temporal envelopes and four spectral envelopes for transient signal; and output the encoded stream of bits low frequency signals, the adaptive encoded bitstream of high frequency signals and the result of the classification decision process.