Patent US 5,651,093

An audio signal is coded by converting the audio signal into a signal in a frequency domain and effecting a bit allocation on the converted audio signal. A masking threshold level for reducing aurally recognized noise due to a masking effect is determined using a signal spectral distribution in a present frame and a signal spectral distribution in a past frame. If the difference between the determined masking threshold level and a masking threshold level in the past frame is equal to or greater than a predetermined level, then a level limited to the difference corresponding to the predetermined level is regarded as a masking threshold level in the present frame.