circuit that amplifies the audio signals rin and lin and generates the stereo composite signal corresponding to the audio signals rin and lin and outputs the signals to the fm modulating ic 41 the fm modulating ic 41 audio signal processing circuit is a circuit that generates the carrier of the frequency based on the serial data sda from the microcomputer 21 to be modulated with the stereo composite signal the modulated signal is output as the fm signal to the power amplifier 42 for wireless transmission to an fm radio receiver not shown of the car stereo not shown the fm modulating ic 41 according to a first embodiment of the present invention includes an idc instantaneous deviation control circuit 50 a crystal oscillator circuit 51 a setting register 52 and a frequency modulating circuit 53 the idc circuit 50 is a circuit that limits amplitude of the stereo composite signal amplified by the audio amplifier circuit 40 and outputs the signal of limited amplitude to the frequency modulating circuit 53 the crystal oscillating circuit 51 is a circuit that generates a reference frequency which serves as a reference of the fm modulating ic 51 together with a quarts crystal not shown connected thereto the setting register 52 is a circuit that holds the serial data sda input from the microcomputer 21 in synchronization with a clock signal scl to be output to the frequency modulating circuit 53 as setting data set in the setting register 52 according to an embodiment of the present invention the setting data.