A decoder for processing an audio signal receives an audio bitstream, decodes the bitstream to obtain a set of spectral frequency parameters that are arranged in an order of frequencies, determines a minimum spectral frequency parameter difference from a plurality of calculated spectral frequency parameter differences, determines a start frequency bin for predicting a high band excitation signal according to the minimum spectral frequency parameter difference, generates the high band excitation signal by selecting a frequency band with a preset bandwidth selected from a low band excitation signal according to the start frequency bin, and synthesizes a wideband signal based on the generated high band excitation signal.