Abstract: A harmonic generation circuit compares audio data supplied from the outside every sample, detects a top-peak and an under-peak of the audio data on the basis of the comparison outputs, and detects patterns of the comparison output between continuous top-peak and under-peak. The harmonic generation circuit forms an addition-subtraction data corresponding to harmonic depending on the patterns, and supplies the addition-subtraction data to an adder at a timing depending on the patterns. The adder performs an addition-subtraction process for adding/subtracting the addition-subtraction data formed by the harmonic generation circuit to/from the audio data supplied from the outside. In this manner, with a compact, simple, and low-price circuit arrangement for performing only addition-subtraction, a harmonic component can be added to original audio data, and audio data having a wide frequency band can be formed.