Low-frequency components (S2) are selected from the outputted analog sound signal (S1) so that a low-pitched sound signal is derived from the analog sound signal. A key of the low-pitched sound signal is lowered so that a very-low-pitched sound signal (S3) is derived from the low-pitched sound signal (S2). The analog sound signal (S1) and the very-low-pitched sound signal (S3) may be converted into corresponding sounds respectively (S4).