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FM Transmitter

  1. Jul 10, 2008 #1

    FredGarvin

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    Science Advisor

    A quick one for you sparkies...

    There are a ton of FM transmitters you can use to pipe your iPod or other device over your FM stereo. There is one thing in common to the three units I have tried that is very annoying. I have noticed that there is very poor sound quality for the higher frequency content. All three units I have tried have had such poor sound quality that I can't play a fair amount of the music in my collection. Admittedly, these units are the cheaper models that you can get anywhere, in the $10-20 price range. I can definitely point the problem to the transmitter because I don't have the issue with a direct hard line from the player to the same stereo.

    My question is, is (are) there a specification/parameter(s) that I should keep an eye out for that would help me avoid waisting money on other units that will repeat this problem?

    Thanks!
     
  2. jcsd
  3. Jul 10, 2008 #2

    berkeman

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    Staff: Mentor

    Can you list the models you have that are not working well? There should be an audio bandwidth spec somewhere, at least on the better models.

    Yoiks! Here's one for $250 with good audio bandwidth: http://www4.shopping.com/xPO-AKG-WMS40-Progb-60-A-B

    The Monster Cable units look like they may be good candidates:

    $70 http://www.monstercable.com/mpc/productPageMPC.asp?pin=3232&section=ipod

    $85 http://www.monstercable.com/mpc/productPageMPC.asp?pin=3233&section=ipod

    I don't find audio bandwidth specs there, though -- you might see if you can find them. Maybe you'll just have to go to a store that has them on display and try them...presumably some stores would be set up to let you do that. Maybe like a Circuit City or a Frys?
     
  4. Jul 10, 2008 #3

    NoTime

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    Ignoring the transmitter aspect for the moment

    Even a good FM receiver has a bandwidth limit of 15khz.
    If you have good hearing that in itself is quite noticeable.

    Another parameter that is much more difficult to find is dynamic range.
    IIRC a CD or high quality MP3 can have a dynamic range of 90db, while the FM transmission max falls around 60db.

    While a good receiver has a harmonic distortion close to 0, this parameter also applies to the transmitter.

    Edit: There is also channel separation.

    I have no idea how close your FM transmitters come to the allowable limits.

    Since you say that the result is horrible (I agree).
    What do you think of the same song picked up from a commercial transmission?
    This could give you a good idea if it's just a lousy transmitter or if you are never going to be happy with the results.
     
  5. Jul 11, 2008 #4

    FredGarvin

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    Unfortunately, my musical tastes do not coincide with what is played over the radio stations these days. However, you do bring up something that I can try. I'll download a popular song and do the back to back compare. Perhaps there is something in the receiver...

    Thanks to both of you guys for the info!
     
  6. Jul 17, 2008 #5
    I suspect the problem lies in the stereo encoder having a crude 15 kHz low pass filters with a much more gradual fall-off than a broadcast transmitter.
     
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