PURPOSE: To reduce the number of bits per one word of a memory and improve response, by storing change components of an input signal so that their range may be changed in respect to exponential function.

CONSTITUTION: An inputted signal is subjected to A/D conversion by A/D converter 1 to become a digital value and is given to CPU2. In CPU2, A register 21 stores temporarily total sum V'n of coded change components, and B register 22 stores temporarily input voltage Vn converted to the digital value by converter 1. C register 23 stores the difference between the output of converter 1 and total sum V'n of coded change components, and D register 24 designates addresses of ROM31 and ROM32, and the address of waveform storage memory 41 is designated by E register 25. Thus, change components of the input signal are so stored that their range may be changed in respect to exponential function, so that the number of bits per one word of the memory can be reduced to improve response.