1. An audio signal processing method comprising: receiving a residual and long term prediction information; performing inverse frequency transforming with respect to the residual to generate a synthesized residual; and performing long term synthesis based on the synthesized residual and the long term prediction information to generate a synthesized audio signal of a current frame, wherein the long term prediction information comprises a final prediction gain and a final pitch lag, the final pitch lag has a range starting with 0, and the long term synthesis is performed based on a synthesized audio signal of a frame comprising a preceding frame.