When receiving a frequency modulation (FM) broadcast, conventional vehicle radios normally perform signal processes in accordance with a voltage of a received radio signal. Examples of such signal processes include a muting process for decreasing an output voltage of an audio signal (audio output voltage) and a high-frequency-component cutoff process for decreasing voltages of high-frequency components in sound. The reason why those processes are required is to mitigate the effects of noise that may be unpleasant sound under a weak electric field environment.
Right after a running vehicle enters a tunnel, for example, strength of an electric field around an antenna of a vehicle radio becomes weak. If an audio output voltage is unchanged after the vehicle has entered the tunnel, the vehicle radio may emit much louder noises inside the tunnel than before the vehicle enters the tunnel. This makes a driver feel unpleasant. For this reason, in response to the detection of a decreased voltage of a radio signal, the vehicle radio performs the muting process for decreasing an audio output voltage. For the same purpose, the vehicle radio also performs the high-frequency-component cutoff process for decreasing voltages of high-frequency components in sound.
Conventional vehicle radios have an antenna, called a rod antenna (with about 75 cm in length, for example). However, some modern vehicle radios have a more compact antenna, called a shark fin antenna (with about 10 cm in length, for example).
The above compact antenna is equipped with an antenna amplifier that amplifies a received signal in order to compensate for a low reception gain of a miniaturized antenna element. Such an antenna equipped with an antenna amplifier is referred to as an “active antenna”. Unfortunately, the antenna amplifier of an active antenna amplifies not only a radio signal containing broadcast waves but also noise. As a result, compared to the rod antenna, the noise floor of active antennas becomes higher, and a radio signal voltage of active antennas does not decrease sufficiently even under a weak electric field environment.
In some cases, a radio signal voltage is used as a criterion (an operating point) for determining whether to perform the above audio signal processes. One index for indicating a radio signal voltage is an “S meter”. In this description, the “S meter” represents a radio signal voltage at a reception channel which is to be tuned and demodulated by a radio receiver.
Comparing the cases of radio receivers with and without an antenna amplifier, there is a difference in the voltages (the S meters) of the received radio signals due to the gain of the antenna amplifier even when the input levels of the received signals are substantially same. Thus, if software programs for an audio signal process installed in both the radio receivers are configured such that an audio signal is muted based on an operating point related to an S meter preset without an antenna amplifier, there are cases where the radio receiver without an antenna amplifier does not mute the audio signal but the radio receiver with an antenna amplifier mutes the audio signal.
Thus, for example, when the vehicle is running inside a tunnel, the active antenna (in the case with an antenna amplifier) may fail to perform the signal process appropriately, and thus disadvantageously emits loud noise, for example.
To address the above disadvantages, an operating point may be varied for a signal process in accordance with a gain of an antenna amplifier when the antenna amplifier is used. It is more preferable to control an audio signal process by using a carrier to noise (C/N) ratio as an index representing the quality of a received radio signal, instead of an S meter, which is an index purely indicating a voltage of the radio signal.
A method has been proposed in Patent Literature 1, for example, as a conventional technique for detecting a C/N ratio. In this method, digital broadcast signals are demodulated and measured constellations are obtained. Then, errors (power ratios) between the measured constellations and theoretical constellations are measured as modulation error ratios (MERs) related one-to-one to C/N ratios.
A method has been proposed in Patent Literature 2, for example, as another conventional technique for detecting a C/N ratio. In this method, an input signal is demodulated and a baseband signal is obtained. Then, noise components that fall outside a signal bandwidth are extracted from the baseband signal. Based on levels of the extracted noise, a reception level of an input radio wave related to a C/N ratio is displayed.