Radio - Normal Conditions of FM & AM Reception
87mazda04Category Applicable Model/s Subject Bulletin No.. 050/87
15 All Models NORMAL CONDITIONS OF FM & AM Issued 9/10/87
RADIO RECEPTION Revised
DESCRIPTION
Radio reception in a moving vehicle is more difficult to control than in a fixed location such as a home. Often times when customers have a complaint about the radio in their vehicle, the problem is simply one of normal operation.
Careful explanation of normal radio operation and limits of the audio system will increase consumer awareness and satisfaction. Such explanations can also reduce unnecessary repairs and inconvenience to the customer.
The following are brief explanations of conditions encountered with radios in vehicles.
FM RECEPTION IN VEHICLES
FM broadcast range is generally about 25-30 miles from the source. Due to extra coding needed to break the sound into two channels, stereo FM has even less range than monaural (non-stereo) FM. Signals from an FM transmitter are similar to beams of light because they don't bend around corners, but they do reflect. Unlike AM signals, FM signals cannot travel beyond the horizon. Therefore FM stations cannot be received at the great distances possible with AM reception.
Imagine an FM transmitter as a long pole with a light bulb mounted at the top. Anything (such as buildings, landscape, or even the horizon) coming between the bulb and person viewing blocks out the light just as the FM signal would be blocked from directly reaching the radio.
Atmospheric conditions can also affect FM reception. High humidity will cause poor reception. However, cloudy days provide better reception than clear days.
Different characteristics of FM and AM broadcasts. Note how AM signal can travel beyond the horizon while FM cannot.
UNAVOIDABLE NOISES
1. Multipath Noise:
Since FM signals can be reflected by obstructions, it is possible to receive both the direct signal and reflected signal at the same time. This causes a slight delay in reception and may be heard as a broken sound or distortion. This problem can be encountered even at close proximity to the transmitter.
2. Flutter/Skip Noise:
Signals from the FM transmitter move in straight lines and become weak in "valleys" between tall buildings, mountains, and other obstacles. When a vehicle passes through such an area, the reception conditions change suddenly, resulting in annoying noise.
3. Weak Signal Noise:
In suburban areas, etc. broadcast signals become weak due to distance from the transmitter. Reception in such "fringe" areas is characterized by sound break-up.
4. Strong Signal Noise:
Occurs very close to transmitter tower. The broadcast signals are extremely strong so the result is noise and sound break-up at the radio receiver.
5. Station Drift Noise:
Occurs when radio is tuned to FM station and vehicle reaches strong area of another station broadcasting at similar frequency. The original station may be temporarily lost and the radio will begin to pick up the second station. At this time there will be some noise due to this disturbance.
NOTE:
Mazda has also developed a special "Diversity FM Reception System" featured on certain vehicles (see Owner's Manuals). This system compares signals (FM stations only) received by two separate antennas and chooses the best signal for reception.
AM RECEPTION IN VEHICLES
AM signals can bend around obstacles and skip along the ground. AM signals can even travel upwards, bounce off of the ionosphere and then come back to earth. Therefore, AM has a much longer range of reception than FM and can extend beyond the horizon.
However, AM signals can fade while passing under bridges, in tunnels, and in downtown areas. AM reception is also affected by electrical power lines and other sources of electrical static including traffic lights, signs, and thunderstorms.
Stereo AM is now available on some vehicles and can be affected by the same conditions as monaural (nonstereo) AM.