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Why do FM stations always end in odd tenths of a MHz?


Why do FM stations end in .1, .3, .5, etc?

4450 day(s) ago

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cabbagehead
In the U.S. and Canada FM radio is broadcast between 87.9MHz and 107.9MHz. This range is divided into channels 200 kHz apart for a total of 101 channels.

Why 200 kHz? FM stands for "frequency modulation." While a station might say they broadcast at 91.7MHz this is actually the center frequency for their transmission. The frequency of the broadcast goes up and down, relaying information to the radio. Radio stations are given 150kHz of deviation around their channel frequency, so the 91.7MHz station is actually broadcasting between 91.625MHz and 91.775MHz. This is why you can tune slightly above or below a station's broadcast frequency and still hear at least part of the broadcast. The extra 50kHz of space ensures stations won't interfere with one another. The FCC further separates channels in area markets using a complicated formula based on frequency strengths to further reduce interference.

Why odd?

Countries mostly* adhere to rules set down by the ITU (International Telecommunication Union) in 1984. At the time some countries used stations with as little as 50kHz in differentiation. In other words, a station could broadcast at 91.25 or 105.75. Digital tuners, which were just coming to market, couldn't tune to these small frequencies so channels were set to 100kHz intervals. At this point almost all countries adopted odd-numbered frequencies. This standardization makes it easy to manufacture radios that will work world-wide. FM frequency ranges may differ, but they can be adjusted with a minor modification. Taking a North American market radio to use Japan's 76-90 MHz band requires the addition of a single diode. Europe uses 87.5 to 104 MHz, so most radios are designed to handle both Europe and North American bands.

*Italy still allows 50Khz channels.

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