PURPOSE: To enable real-time encoding by switching and selecting the output from a system equipped with the second filter system by a switching means when encoding can not be executed in a system equipped with the first filter group.

CONSTITUTION: When overflow is generated, a noise shaping circuit 22 in the system equipped with a first filter group 2 supplies an overflow signal to a switch 24 so as to show the generation of overflow. When this overflow signal is supplied, the switch 24 is changed over from a terminal 24a to be selected to a terminal 24b to be selected. Therefore, the audio data to be supplied to the terminal 24b to be selected is passed through a second filter group 3 where encoding is hardly disabled. Accordingly, by changing over the switch when overflow is generated, the data not to generate the overflow can be selected while avoiding the data generating overflowing. Thus, re-encoding is not required and real-time encoding is enabled.