I don't understand concept mp3 FM

Discussion in 'Wireless & RF Design' started by Eng_Bandar, Feb 27, 2010.

  1. Eng_Bandar

    Thread Starter Active Member

    Feb 27, 2010
    50
    1
    Hi all,

    I don't understand concept mp3 FM ( in the car ), how is it work ? I will give example suppose find station A, its frequency 95 Mhz means can't any other station have same frequency of station A. Why ? to avoid interference until now OK. But when you play mp3 fm and turn it to frequency of station A works without any problems. The question now where did station A go ?

    I was trying this and I have seen station A took other frequency Why ? Why this station took other frequency and we know any station take its frequency from government that means have reserved frequency in order avoid interference

    Can any one explain this to me ?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If I get your question properly, you are feeding the MP3 output into your car's sound system. The MP3 audio source is just using the car radio amplifier to make the music come out the speakers. It is not broadcast as radion, unless you have a Bluetooth link to the car sound system.

    MP3 is a compression technique to reduce the size of audio files. FM (frequency modulation) is a means of modulating an RF carrier with an audio signal. Other than the outputs being music and speech, the two things are not related.
     
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  3. rebelde

    New Member

    Feb 28, 2010
    2
    1
    I guess you're using a FM-transmitter for your MP3 player? In this case the FM-transmitter signal in your car will be much stronger compared to the radio signal. Hence the radio station "disappears" below the MP3 player signal in case your receiver is near to your FM transmitter.
     
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  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    That is correct. Many devices do this. The 'Jupiter Jack' is a device that connects to the headphone jack of a Blackberry (or any 1/8" headphone jack) and transmits the output to a preset FM station. Some devices, like satellite radio receivers, can broadcast to any station between 89.9 and 107.9 (or right around there) to be picked up and played on your FM radio.

    Because the MP3 players signal is closer to the cars antenna than the radio station, it overwhelms the radio station and "copies" over it.
     
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  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Guess I'm behind the times - my car has a plug for the MP3 input.
     
  6. hwy101

    Active Member

    May 23, 2009
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    You're not behind the times.
    That's the best way to feed the audio signal to your car's sound system, FM transmitters are designed for car decks that don't have MP3 input.
     
  7. Eng_Bandar

    Thread Starter Active Member

    Feb 27, 2010
    50
    1
    thank you for your replying

    I don't understand your sentence " It is not broadcast as radion, unless you have a Bluetooth link to the car sound system. "

    Car MP3 Player works as any station in emitting FM signals but not to far distances this correct or no ? ( I mean it is work as source like any FM station )

    you can see this video that contains Car MP3 Player like my Car MP3 Player

    http://www.youtube.com/watch?v=_Vn8soPyTdw&feature=related
     
  8. Eng_Bandar

    Thread Starter Active Member

    Feb 27, 2010
    50
    1
    Yes this is what I mean.
    But I shall ask you if find two FM stations have same frequency but these station differ in amplitude of signals, Is this cause confusion ? because both have same frequency.
     
  9. Eng_Bandar

    Thread Starter Active Member

    Feb 27, 2010
    50
    1
    Thanks for
    beenthere
    rebelde
    retched
    hwy101

    to help me
     
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Radio signal strength obeys the 'inverse square' rule.

    This means that a radio station 1Km (1000m) away needs to be one Million times more powerful than an MP3 sender that is (for the sake of argument) about 1M away from the car radio antenna, just to match the signal strength.

    If it were 10Km away, it would need to be 100,000,000 times more powerful to get equal strength.

    (Think of a torch or flashlight, pointed at a wall from far enough away so the beam is 1m across. Double the distance from the wall it will be 2m across, which is lighting four times the area at one quarter the intensity. Triple distance would be 1/9th intensity and so on).

    FM receivers also show 'capture effect' - it only takes a fractional difference in signal strength for an FM receiver to lock onto the stronger signal and ignore the weaker one.

    More info here:

    http://en.wikipedia.org/wiki/Inverse-square_law

    http://en.wikipedia.org/wiki/Capture_effect
     
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