1.Outline
The radio receiver chooses a desired program from many radio waves transmitted from broadcast stations.
In the radio broadcast, there are AM and FM. The radio receiver that receives these broadcasts differs between AM and FM. The radio receiver that can receive both AM and FM has two tuners for AM and FM. These are switched by button operation.
Since the vehicle goes many places such as cities, suburbs, and mountain areas, the strength of radio waves that the radio receiver receives via the antenna widely changes.
Therefore, the vehicle radio has sensitivity so that extremely weak radio wave in the shade of buildings or between mountains can be received.
The noise is reduced using AGC circuit, ATC circuit, ASC circuit, etc.
2.Function of radio
Besides the choice of received broadcasts via the antenna, the radio eliminates the carrier wave from electrical signal (carrier wave + sound signal) to produce the sound signal.
This operation is called demodulation.
The sound signals of music and voice transmitted from the broadcast station are synthesized with the carrier wave and become modulated signals.
Therefore, to convert this signal into music or voice, it is necessary to eliminate the carrier wave and obtain only the sound signal. Since FM broadcast uses the stereo method, the different signal for right and left is transmitted.
Therefore, FM radio has also a function to separate the synthesized signal into right and left.
Since the sound signal received by the radio receiver is extremely weak, the amplifier amplifies the signal sufficiently so that the speaker sounds.
This amplifier may be built in the radio receiver or may be independent, like a component stereo set.
3.AM and FM
AM and FM broadcasts differ in the modulation method (the method of synthesizing the sound signal with the carrier wave). AM is the abbreviation of Amplitude Modulation, which converts the amplitude of carrier wave according to the sound signal. FM is the abbreviation of Frequency Modulation, which converts the frequency of carrier wave according to the sound signal.
Comparing AM signal with FM signal, the following differences are seen.
FM broadcast has a good quality in sound and less noise compared to AM broadcast. All FM broadcasts are stereophonic broadcasts, but AM broadcasts are monophonic broadcasts except for some broadcast stations (or programs).
AM broadcasts use medium wave and FM broadcasts use ultrahigh frequency waves. The service area of AM broadcasts is greater than that of FM broadcasts.
There are various types of broadcast stations depending on the countries or areas as shown in the left figure.
The service area varies depending on the frequency.