1.Outline
Since the car radio depends on the antenna length and vehicle noise, the broadcast service area in which the vehicle can receive programs clearly becomes small.
The noise is called impulse noise.
Generally, the concept that FM broadcasts can be heard in good sound quality is fixed. However, since the vehicle also drives in places where the radio waves are weak, this concept is not always applicable.
2.AM broadcast
(1)Description of noise
AM broadcasts are easily affected by ambient noise. If lightning strikes at the area where the radio wave is weak, or at the place such as near the traffic signal, power line, or rail track, noise is easily produced.
In addition, a car audio system is easily influenced by electrical noise generated by the automotive components of the vehicle in which the audio system is installed, such as the spark plugs, ignition coil and alternator.
Natural phenomena and noise from sources other than own vehicle itself cannot be avoided.
(2)Generation of electrical noise
When the current flowing to electrical components (particularly coils) is switched on or off by a switch or relay, sparks are generated between the contact points.
This causes an unwanted voltage component, called “noise” or “interference,” to be added to the current flowing through the wires connected to the points of the switch or relay.
This in turn causes noise to be generated by the wires.
Other possible sources of noise are the AC component from the alternator, the pulsed current generated by the engine ECU, etc.
This noise has an adverse effect on the car audio system, causing static and other noise to be output by the speakers.
(3)Countermeasures for noise
<1>Ignition system
High voltage generated at the ignition coil is transmitted to the spark plug via the high-tension cords.
This high voltage produces very strong noise at the high-tension cords and spark plugs.
This noise radiates to the engine hood and from there enters the radio antenna.
To prevent this noise generation, the following action is done.
High-tension cords
Either resistive cord or wound cord is used for the core wire of high-tension cords to convert the noise component of the current to thermal energy.
Resistor type spark plugs
A resistor is inserted in the center electrode of the spark plugs to suppress the noise component.
Grounding the engine hood
Engine hood cushions made of conductive rubber are used to link the engine hood electrically to the body.
Some models have an ignition coil noise filter on the engine.
<2>Horn
When the horn operates, electrical noise is generated at the on/off point of the horn.
To decrease this noise, a varistor is connected at the horn points in parallel.
<3>Wiper motor
While the wiper motor is operating, electrical noise is generated at the motor brush.
Since electrical noise is generated at the brush, a capacitor is installed in the circuit. This electrical noise is absorbed by the resistor by converting it into thermal energy.
In some models, a choke coil is connected to the outside of the motor.
<4>Turn signal flasher
When the turn signal flasher operates, the relay point in the turn signal flasher repeatedly turns on and off. As a result, electrical noise is produced at the relay point and the coil.
A capacitor is connected so that the noise is not produced on the power source line.
(4)Fading
During night, when the reflection of the ionosphere becomes stronger, the reflection wave and direct wave from the antenna of the broadcast interfere with each other and the sound voice may change. This phenomenon is called fading.
Since the reflection of the ionosphere makes the service area of the AM broadcast wide, the signals from distant broadcasts may interfere.
3.FM broadcast
FM broadcast is different from AM broadcast. The effect of the ambient noise in the service area is low and there is no reflection of the ionosphere.
As a result, fading does not occur but the noise such as fade-out or multi-path occurs.
(1)Fade-out
Since the frequency of the radio wave of FM broadcasts is high, the radio waves are reflected by hills or concrete structures. When the vehicle moves in the shade of one of these obstructions, the radio waves become extremely weak, suddenly the sound could stop, and white noise occurs. This phenomenon is called fade-out.
(2)Multi-path
When receiving an FM broadcast, the radio wave directly transmitted from the antenna of the broadcast and that reflected by obstructions are received.
The timing of the direct wave and the reflection wave is off. They interfere with each other and noise or distortion is generated. This phenomenon is called multi-path.