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AM vs. FM quality

  1. Jul 19, 2007 #1


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    Correct me if I'm wrong.

    I've always taken it on faith that the switch to frequency modulation from amplitude modulation resulted in the much better quality signal and listening experience we have today. Modulation of the frequency is likely less susceptible to distortion than is modulation of the amplitude, meaning that, all other things being equal, FM will prodice a more faithful signal with less degradation.

    But ...

    I speculate that modulation is not the largest component of why FM is so much better quality radio. Surely a much larger component is the absolute frequency i.e. FM is in the Megahertz range whereas AM is merely the kilohertz range. MHz will penetrate better, support more more subtlety in the signal, have better redundancy (thousands of carrier cycles per signal cycle as opposed to dozens) and a host of other improved properties.

    So: It seems to me that AM radio in the Megahertz range would be much more nearly comparable to FM radio.

  2. jcsd
  3. Jul 19, 2007 #2


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    Interesting notion. Certainly, the late-night AM signals from Albany and Buffalo sounded great when I was about 12 or so (1964) and listened to my little radio at night. The switch to FM was fraught with signal-strength problems ans channel drop-outs.
  4. Jul 19, 2007 #3


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    Good question Dave. (and thanks again for your help in that vision physiology thread -- much appreciated)

    The best way to think about AM versus FM (for analog RF audio transmissions), is to think about the main impairments that the radio channel has to deal with. There are certainly noise impairments that affect both AM and FM, but the main impairment for moving radio receivers (like car radios) is multipath. The signal that you receive is the sum of the main signal (if you have a reasonable line-of-sight to the transmitting antenna) plus all of the reflected signals off buildings, metal towers, people, etc. The reflection amplitudes depend on the frequency of the radio channel obviously, but you will almost always be dealing with a composition of the multipath signals in your receive antenna and receive amp.

    So, multipath affects the amplitude of the received signal, and if you are driving around and the multipath composition is varying at audio frequencies, then your radio will sound awful. Luckily, the AM band is low enough in frequency (around 1MHz) that the variation due to multipath is at a low enough frequency that it doesn't bleed past the audio bandpass filters very much. But up in the broadcast FM band frequencies, if they tried to use analog AM, the multipath amplitude variations with time would destroy the audio.

    And no matter where you are in the spectrum, you will generally try to only use a bandwidth that is wide enough to transmit the data (analog or digital) that you want to transmit. Of course, the regulatory agencies know how wide each designated use band should be anyway, so that's what you get.

    Make sense?
  5. Jul 20, 2007 #4

    George Jones

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    For me, one of the biggest differences between the AM and FM broadcast bands is audio frequency response. If I remember correctly, AM stations are allocated 10 KHz of bandwidth, which means that audio frequency response has to be down substantially at 5 kHz. The high-frequency audio response for FM signals is much better, so sound from musical instruments that have high (audio) harmonic content (eg., cymbals) are reproduced more faithfully by FM signals.
  6. Aug 2, 2007 #5


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    That's 10khz between stations. In the USA adjacent stations aren't allowed, and some AM stations used 20khz of bandwidth. Dynamic range was about 50db (rate of change of volume impacts bandwidth). Later the FCC started enforcing the 10khz rule. Also because of the wave bouncing off the air layers at night, AM stations had to cut power, except for the few that had "clear" channels (no other station on that channel in the USA). This allowed stations like KOMA (Oklahoma) to be heard from just east of the Rockies to the Applachian Mountains) at night time. XERB, broadcast from Mexico, covered most of the south western states (Wolfman Jack was on XERB). There was a short period of time when AM stereo stations existed, apparently using "two channels" to get 15khz on both channels.

    FM stations are seperated by 100khz, and again in the USA, adjacent stations aren't allowed. However each 100khz is broken up into smaller bands. 15khz for mono (left + right), 15khz for each stereo channel, and then some other stuff like the digital stuff (like song names you can see on a radio). FM dynamic range is around 70db; this is a factor of the maximum frequency deviation from the "center" frequency (which represents 0 db). Having both mono and stereo signals, if the signals get weak, FM receivers can combine the signals, transitioning into full mono mode to compensate.

    Note also that modern receivers generally have terrible AM radios (reducing response to around 5khz, when 10khz is available), as there's no demand for maximizing the AM quality.

    Widipedia has a descent article on all this stuff.
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