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Audio quality degradation over FM transmission

Discussion in 'Electronic Design' started by [email protected], Jun 23, 2005.

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  1. Tam/WB2TT

    Tam/WB2TT Guest

    Actually, it gets worse than that. In that 75 KHz channel, you have 15 KHz
    of LEFT+RIGHT, another 15 KHz of LEFT-RIGHT (or is it RIGHT-LEFT), plus
    MUSAC, or whatever. The LEFT-RIGHT channel is transmitted as a double
    sideband suppressed carrier AM signal; so, it actually takes up 30 KHz. I
    don't know this for a fact, but I suspect that if an FM station is
    transmitting digital music, it is in the slot formerly used for MUSAC.

    To transmit a 75 KHz bandwidth of signal, the FM transmitter actually uses
    about 200 KHz of spectrum. That is just the way FM works. Defined by Bessel

    The 88 to 108 is immaterial, as has been pointed out. In the FM receiver,
    the first thing that happens is that whatever station the radio is tuned to
    gets converted to a narrow slot at 10.7 MHz. You can't make the slot wider,
    or you would get interference from the adjacent channel.

  2. Pooh Bear

    Pooh Bear Guest

    Isn't it fairly flat ( topologically ) where you are ?

    We ( with all our hills and valleys ) get all this atmospherically introduced
    multi-path trouble ( at least I think that's the cause ). Never actually got
    into really examining the fine detail of RF transmission and reception. I
    reckoned I'd leave the finer points to someone else and concentrate on what I
    seemed to be best at.

  3. Pooh Bear

    Pooh Bear Guest

    Yes but....

    My car's stereo has an auxilliary input for such things.

    Might it not be simpler just to connect it in some such way ?

    I've never come across an mp3 player with an FM transmitter. Is it legal ?
    Does the Chinese company making it care ?
    In a car.... for sure !

  4. Pooh Bear

    Pooh Bear Guest

    Errr what ?

    If you're sending an analogue signal it doesn't matter how it originated. Or
    how good it is / isn't. The transmission medium ( unless digital ) doesn't
    care how the signal was encoded. Totally irrelevant.

  5. Jim Thompson

    Jim Thompson Guest

    I'm in somewhat mountainous terrain here. But about the only
    multi-path stuff I ever see is in the downtown "canyons"... probably
    all that steel.

    ...Jim Thompson
  6. John Perry

    John Perry Guest

    Kristian, you seem to have missed the repeated statement that there is
    no 108Mhz bandwidth -- it's _always_ a 100KHz bandwidth centered around
    88.1 MHz, 88.3 Mhz, 88.5 MHz, ..., 107.9 MHz.

    The FM modulation technique restricts the audio bandwidth to 15KHz for
    reasons others have stated. FM trades off extravagant bandwidth for
    excellent resistance to interference and good audio fidelity.

    MP3 will be the limitation, even if its parameters are adjusted for
    maximum fidelity (if I understand the MP3 paradigm correctly, which I
    may well not). But a standard CD will have better fidelity than the FM
    channel because the CD is limited to 20KHz, and the FM channel is
    limited to 15KHz.

    John Perry
  7. Firstly, FM involves some distortion even at the theoretical level, because
    you receiver has to discard some of the ("infinite") FM sidebands. I don't
    know how much distortion. The distortion will be worse at high deviation.
    Your car radio will have narrow band filters, which increase distortion.

    Secondly, most FM radios are not kind to audio. Phase shifts in the IF, and
    non-linearity of tuned circuit discriminators add distortion. Especially at
    full deviation, distortion is not negligable. With a clean music source, I
    would expect to easily hear the degradation caused by a car radio. FM
    receivers do have decent treble, but it I think it is often audibly
    distorted. High end MP3 should easily beat FM radio. Just what bit rate
    you need for "FM" levels of degradation, I don't know.

    Thirdly, your transmitter will have distortion. 2% THD or more would be not
    surprise me.

    Your whole transmitter - receiver chain at full deviation could do anything
    from 7% THD down to about 1% THD.

  8. Ian Stirling

    Ian Stirling Guest

    Which is usually unnoticable, except for the lowest bitrates, and the
    best listening conditions.
    I've measured the outputs of my cheap MP3 player with single tone outputs.
    It vastly outperforms my ears, and shows good results on test equipment,
    coming up with a dynamic range of about the same as my CD player.

    Many/most low-power FM transmitters are of very bad quality, and come
    nowhere near what FM can achieve.
  9. gwhite

    gwhite Guest

    I doubt many, if any, car radios have bandwidth switches anymore.
    But that is not such a big concern, as they are only brief transients.
    Besides, the car environment is loaded with other noise.
    For S/N relative to some distortion level ("big signal ref"), you can
    probably expect > 65 dB even in car radios.
    The cheezy low powered TX'ers are the weaker part of the link, from my

    Example of what a decent FM RX'er can do:

    I don't know about car RX'ers -- I just thought the linked one might be
    an easy reference.
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