1. An audio signal processing method comprising: generating a first target signal based on a plurality of linear predictive conversion coefficients by performing linear predictive analysis on a current frame of an audio signal; performing vector quantization on a first stage based on the first target signal, the vector quantization including generating first candidate code vectors including a first initial best code vector having a smallest error based on the first target signal and outputting a first initial best error corresponding to the first initial best code vector as a second target signal which is a target signal of a second stage; repeatedly performing the vector quantization from the second stage to an Nth stage; determining a Kth stage (K=1,. .. , N) in which index update is to be performed among the first to Nth stages; correcting the Kth target signal using the first target signal and an Kth-excluded sum signal; determining a Kth optimal best code vector among Kth candidate code vectors based on the corrected Kth target signal; and selecting one of a Kth initial best code vector and the Kth optimal best code vector as a Kth final best code vector, wherein the Kth-excluded sum signal is a sum of first to Nth initial best code vectors excluding the Kth initial best code vector.