1. A digital-signal processing apparatus for converting an input digital signal, comprising: envelope calculation means for calculating the envelope of the input digital signal; class classification means for classifying the input digital signal into a class according to the calculated envelope; and prediction calculation means for prediction-calculating the input digital signal by a prediction method corresponding to the class to generate a digital signal converted from the input digital signal, wherein the digital signal is provided to an output device, and wherein the envelope calculation means calculates a positive envelope in a positive region of the input signal and a negative envelope in a negative region of the input signal.