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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. Guest

    I would like to know why a CDDA/MP3 FM radio transmission at 88-108 MHz
    results in degradation of sound quality. As I understand it, standard
    CDDA/MP3 audio is sampled at 44.1 KHz. 88-108 MHz is well above this
    threshold, and I don't see how the transmitted audio "data" can't
    travel along this invisible pipe at an adequate enough speed to result
    in perfect end-to-end transmission (assuming no interference/loss). I
    have heard that USA public FM radio transmissions could be compared to
    64 Kbps MP3 audio. I guess that I am just interested in the physics of
    how this works, so don't be afraid to hit me with some math :)

    Kristian Hermansen
     
  2. On 23 Jun 2005 10:56:42 -0700, in sci.electronics.design
    Are they transmitting digital signals in theFM band?
    This is confusing me. If so I have no information

    Or are they transmitting mp3 in analogue on FM (Zenith GE)stereo.
    If this is the case they are processing the sound at the radio station
    with compression and dynamic equalisation, to give the station sound
    " a personality"
    This is a nasty habit that stations do to make them sound louder than
    the next station in the FM band.

    They use mp3 for storage of the music to save disc space. The
    conversion from linear encoding (uncompressed to mp3 is not perfect

    88/108 MHz has nothing to do with the encoding, it is just the
    carrier. It is generally transparent to what is conveyed on it.

    sorry, no maths involved




    martin
     
  3. Ian Stirling

    Ian Stirling Guest

    Err, no.
    Brimming over with various sorts of wrongness...

    For a good signal, pretty much the only thing that you could notice is that
    the bandwidth is a bit lower, as the FM signals bandwidth is about 15Khz,
    compared to the CD bandwidth of about 20Khz.

    This is due to the way the signal is fitted into one FM channel, of 75Khz
    (0.075Mhz) width, so you can have more than one channel on the FM band.
    FM radio is much, much higher quality than 64K, if you're in decent signal
    area, I'd go so far as to say that a good quality signal is usually better
    quality than 128K.

    As to "degradation of sound quality", you're not using a FM transmitter
    plugged into a MP3 player are you?
    These are generally designed down to a price, not up to best audio quality.
     
  4. Pooh Bear

    Pooh Bear Guest

    Do you want the long answer or the short answer ?

    Graham
     
  5. Long + incomprehensible maths please


    martin
     
  6. Jon

    Jon Guest

     
  7. Jon

    Jon Guest

    Kristian,
    Refer to:
    h**p://www.radio-electronics.com/info/broadcast/vhffm/vhf_fm.php,
    Figure 3.
    The allocated modulating bandwidth for the L+R (Mono) signal is 15KHz.
    Any modulating signal above 19KHz would interfere with the 19KHz pilot
    signal, which is necessary for the receiver to reconstruct the 38KHz
    stereo carrier. Sorry about the earlier "Blank Post". I inadvertently
    hit the "Post message" button.
    Regards,
    Jon
     
  8. Guest

    Will I lose the low or high frequency bands? I mean, will I get less
    bass or less treble? What is the frequency window?
    Why can it not be CDDA quality? For instance, why can't a CD be
    streamed over the radio without loss in quality? The frequency of the
    music is 44.1 KHz and the FM frequency is between 88-108 Mhz -- which
    is a magnitude of 10^3 apart. I'm confused :-(
    Yes, I am. Got me :)
    What if I had a high-quality transmitter?

    Kristian Hermansen
     
  9. Ian Stirling

    Ian Stirling Guest

    Treble.
    There is no real reason why you should lose bass, though it may decay under
    10Hz.
    Irrelevant.
    It's transmitted in a 75Khz bandwidth, about 32Khz above and below the
    nominal frequency, and would have exactly the same information content
    if it was centered on 88,108, or even 1Mhz.
     
  10. Guest

    Long.........

    Kristian Hermansen
     
  11. Pooh Bear

    Pooh Bear Guest

    Now look here Martin. Do we really want to go into frequency deviation
    and error correction protocols vs S/N ratio - line of sight signal
    strength - loss and reflection via obstacles - multi-path distortion
    and all that other shit ? Not to mention weather-related signal
    transmission / reception obstacles.

    Graham

    and that's just the beginning......

    Oh god and it's digital *too* - ok add some more crap !
     
  12. Pooh Bear

    Pooh Bear Guest

    I sense this immeasurable force pulling tight on my leg......

    Graham ;-)
     
  13. Pooh Bear

    Pooh Bear Guest

    That's standard *analogue* transmission. With all its attendant faults.

    Now pls try doing the same with a bigger bandwidth digital signal with error
    correction !

    Don't bother ringing me back !

    Graham
     
  14. Guest

    Actually, these things won't be very relevant. I am using this in my
    car! The device will site less than 15 feet from the receiving antenna
    in the same vehicle. Can we make some assumptions that way?
    No, it's an analog signal propagating from my MP3 player's attached
    transmitter to my car's antenna on the back -- which hooks into my FM
    stereo :) Easier now?

    Anyways, I think someone answered that since it is encapsulated in a 75
    Khz band, I am not going to lose much quality at all -- except for the
    15 Khz band instead of 20 Khz. I don't think I can hear below 30 Hz or
    above 18.5 KHz anyways...so no problem losing the highs!!!

    Kristian Hermansen
     
  15. Nope, the MP3 player doesn't have that quality. The reason there are
    MP3s is because of compression, they eliminate some of the 'unneeded'
    data that is stored on the CD so that it will take up less space. This
    compression means that there is more noise and less dynamic range in an
    MP3 than the original. You also loose some of the highs.

    Next, you have your MP3 player. When he said designed down to a price,
    he was talking about the MP3 player, not the transmitter. They often do
    not have the 'best' audio stages, being just good enough to not sound
    too crappy, usually. Now, you take this already distorted, noisy source
    data, the MP3, put it out from the so-so audio out of an MP3 player,
    couple it (usually badly, with impedance mismatches and other signal
    degraders) into your FM transmitter with a MAXIMUM frequency range or
    probably 15K, and you will hear a lot poorer music than the original CD.
    Such is engineering.

    Charlie
     
  16. Digital = almost perfect, when it works
    analogue= graceful degradation


    martin
     
  17. Pooh Bear

    Pooh Bear Guest

    Has your car radio ( FM band ) been bad in this recent hot weather ?
    Mine's shocking. Thank goodness it's analogue !

    Graham
     
  18. Jim Thompson

    Jim Thompson Guest

    Well! In "this recent hot weather", 108°F/42°C, my car radio has been
    working just fine ;-)

    ...Jim Thompson
     
  19. Guest

    I know that MP3 is lossy. All I am asking is if the audio input can
    *physically* (due to transmission) match the audio heard over an FM
    receiver exactly. Just assume that you have the highest quality CD
    source media, CD player, FM transmitter, Antenna, FM receiver, and
    Speakers -- and all that is left to degrade the signal is the
    modulation of the input over the FM channel -- will the input match the
    output??? This is what I want to know. I mean, the act of taking a
    100% clean CD audio input -- piping it over FM (with no constraints but
    the theoretical math involved in modulating the signal over say 108
    MHz) -- will it end up 100% clean on the other side? I don't care
    about any other factors. Mathematically, does the transmission of CD
    audio input via FM have the effective output of about 256 Kbps MP3
    lossy audio on the receiving end? Is it a degrading function (forget
    the very low and very high frequencies)? Wow, I must have reworded
    this about 50 times now...lol Forgive my naivety!

    Kristian Hermansen
     
  20. Guest

    I know that MP3 is lossy. All I am asking is if the audio input can
    *physically* (due to transmission) match the audio heard over an FM
    receiver exactly. Just assume that you have the highest quality CD
    source media, CD player, FM transmitter, Antenna, FM receiver, and
    Speakers -- and all that is left to degrade the signal is the
    modulation of the input over the FM channel -- will the input match the
    output??? This is what I want to know. I mean, the act of taking a
    100% clean CD audio input -- piping it over FM (with no constraints but
    the theoretical math involved in modulating the signal over say 108
    MHz) -- will it end up 100% clean on the other side? I don't care
    about any other factors. Mathematically, does the transmission of CD
    audio input via FM have the effective output of about 256 Kbps MP3
    lossy audio on the receiving end? Is it a degrading function (forget
    the very low and very high frequencies)?

    And if not, what is the theoretical maximum source input before you
    start incurring loss on output? Obviously one could not send audio
    sampled at 20 Ghz (insane) and get all that information over to the
    receiver -- since 20 Ghz of information can not fit into that 108 Mhz
    frequency band. I synnonomize this with mapping a much larger data set
    onto a smaller data set (like an MP3 encoding). With two CD recordings
    in a studio done at the same time using slightly different audo
    equipment, it is mathematically possible for these two differing audio
    samples to end up as an identical MP3 file in the end -- even though
    there are subtleties in the original source audio, right? Would the
    same be true of FM transmission of two similar sources? If not, then
    you have a widening transformation of the original vector space.
    Otherwise, it is a narrowing transformation resulting in loss of input
    during mappings. Wow, I must have reworded this about 50 times
    now...lol Forgive my naivety! I know I must be confusing some of you,
    because I am confusing myself now...

    Kristian Hermansen
     
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