1. A method of processing an audio signal, comprising the steps of: determining bandwidth information indicating that a current frame corresponds to which one among a plurality of bands including a 1 st band and a 2 nd band by performing a spectrum analysis on the current frame of the audio signal; determining order information corresponding to the current frame based on the bandwidth information; generating a 1 st set linear-predictive transform coefficient of a 1 st order by performing a linear-predictive analysis on the current frame; generating a 1 st set index by vector-quantizing the 1 st set linear-predictive transform coefficient; generating a 2 nd set linear-predictive transform coefficient of a 2 nd order in accordance with the order information by performing the linear-predictive analysis on the current frame; and if the 2 nd set linear-predictive transform coefficient is generated, performing a vector-quantization on a 2 nd set difference using the 1 st set index and the 2 nd set linear-predictive transform coefficient.