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AUDIO AND VISUAL SYSTEM IDENTIFICATION OF NOISE SOURCE

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  1. RADIO DESCRIPTION


    1. Radio frequency band


      1. A017F9PE07

        Radio broadcasts use the radio frequency bands shown in the table below.

    2. A017DDFE01

      Service area


      1. The service areas of AM and FM broadcasts are vastly different. Sometimes an AM broadcast can be received very clearly but an FM stereo cannot. FM stereo has the smallest service area, and is prone to pick up static and other types of interference such as noise.

    3. Radio reception problems

      Tip:

      In addition to static, other problems such as "phasing", "multipath" and "fade out" exist. These problems are not caused by electrical noise, but by the radio signal propagation method itself.


      1. A017E6CE02

        Phasing

        AM broadcasts are susceptible to electrical interference and another kind of interference called phasing. Occurring only at night, phasing is the interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off the ionosphere and the other signal is received directly from the transmitter.

      2. A017BULE02

        Multipath

        Multipath is a type of interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off buildings or mountains and the other signal is received directly from the transmitter.

      3. A017G37E02

        Fade out

        Fade out is caused by objects (buildings, mountains and other such large obstacles) that deflect away part of a signal, resulting in a weaker signal when the object is between the transmitter and vehicle. High frequency radio waves, such as FM broadcasts, are easily deflected by obstructions. Low frequency radio waves, such as AM broadcasts, are less likely to deflect.

    4. Noise problem

      Technicians must have a clear understanding about each customer's noise complaint. Use the following table to diagnose noise problems.

      Radio Frequency Noise Occurrence Condition Presumable Cause
      AM Noise occurs in a specified area Foreign noise
      AM Noise occurs when listening to an intermittent broadcast An identical program transmitted from multiple towers can cause noise where the signals overlap
      AM Noise occurs only at night Signal phasing
      FM Noise occurs while driving in a specified area Multipath resulting from a change in FM frequency