most appropriate data of a plurality of level - converted digital data obtained by level conversion of the same input digital data by different conversion factors is selected based on a signal quality of each level - converted digital data . the other level - converted digital data is attenuated to or below a predetermined noise level . switching between previously selected data and newly selected data is effected by cross - fading . the level - converted digital data are d / a converted to respective analog signals . level conversion of the analog signals are carried out again based on respective corresponding ones of the conversion factors to restore a level of the input digital data , and all the level - converted analog signals are then added together . when an amplitude level of the input digital data exceeds a predetermined value , the cross - fading is started between the previously selected data and the newly selected data which are delayed by the predetermined amount of delay , and when during the cross - fading a larger rate of change in the amplitude level of the input digital data than a predetermined value is detected , the cross - fading is carried out at a rate larger than a predetermined rate in dependence on the rate of change in the amplitude level . in another form , the cross - fading is started at a first rate when the amplitude level exceeds a first predetermined value , and when during the cross - fading the amplitude level exceeds a second predetermined value larger than the first one , the cross - fading is carried out at a second rate larger than the first rate .