PURPOSE: To reduce a grating noise generated in the course of running in a tunnel by applying a mute to a receiving signal by a muting control means when running in a tunnel is detected.
CONSTITUTION: When electric field strength which is detected by an electric field strength detecting means (tunnel running detecting means) 9 is, for instance, ≤5dB, it is decided that an automobile is running in a tunnel, or in an area in which radio wave circumstances are extremely bad, and an AGC cut-off command signal is outputted from a muting control means 11. As a result, by a muting means (AGC) 12, a mute is applied to a receiving signal, an output voltage from a loudspeaker 8 is dropped remarkably, and a grating non- modulation noise is cut. Especially, when an antenna amplifier is provided in order to improve the electric field strength of a received radio wave, a noise in a tunnel becomes grating greatly, but by performing this AGC cut-off processing, the noise can be lowered effectively.
HASHIMOTO MASUMI
OKA TOMIHIKO
