If the absolute value of the current sample is greater than or equal to an envelope value at the immediately preceding sample, an envelope value at the current sample is made greater than the envelope value at the immediately preceding sample. If the absolute value of the current sample is smaller than the envelope value at the immediately preceding sample and a count value C does not reach a predetermined number N, the count value C is incremented by one and the envelope value at the current sample is held at the envelope value at the immediately preceding sample. If the absolute value of the current sample is smaller than the envelope value at the immediately preceding sample and the count value C reaches the predetermined number N, the envelope value at the current sample is made smaller than the envelope value at the immediately preceding sample.