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Stuffing a Wave

  1. Dec 9, 2004 #1
    Is there a graph or chart somewhere that would show how much bandwidth can be handled at certain frequencies?
     
  2. jcsd
  3. Dec 9, 2004 #2
    Without getting into anything too complex, I believe the maximum comfortable bit-rate will be one-half of whatever your carrier frequency is. But, be careful to check the FCC guidelines if this is something you're planning to transmit on the open airwaves. They get a little touchy about these things.
     
  4. Dec 9, 2004 #3
    So, as an example only; if I was transmitting on 50MHz, I could stuff 25MBPs of information into it?

    I think I just confused myself... :confused: Help?
     
  5. Dec 10, 2004 #4
    Essentially, yes.

    As you exceed the 50% rate you start to get artifacting interference between the carrier frequency and the data modulation frequency.

    If you want to see it in "slow motion", just set your computer sound card to record at a low rate, like 8000hz. Then record an upsweeping tone from 0hz up to 8000hz.

    Everything plays back pretty well until you go above 4000hz. Then it starts demonstrating some audio artifacting.

    Is this for some sort of site-to-site link you're working on?
     
  6. Dec 11, 2004 #5
    I'll have to test that "slow-motion" test, sonds like I could get a lit of information playing around with that...


    Well, sort of. I'm thinking of trying out an experiment with long-distance radio communications. I'm now just trying to get a feel for things before I go and spend money on this. :)
     
  7. Dec 16, 2004 #6
    Another question;

    Why is AM so shoddy at musical frequencies? 690kHz on the AM should be able to handle the 20Hz-20kHz of audio, shouldn't it?
     
  8. Dec 16, 2004 #7

    Cliff_J

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    Whether or not the radio broadcasts SHOULD be able to handle hi-fi sound and if the standard was written to accomodate it are two different things. :smile:
     
  9. Dec 16, 2004 #8
    You've lost me on that one...


    Is it possible to broadcast a CD on AM and be able to get the whole range of musical frequencies (5Hz-20kHz)? Are you saying it wasn't possible before because of records?
     
  10. Dec 17, 2004 #9

    Cliff_J

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    No, I'm saying something about the tranmission standard, not the source media. I know there are some guys here who know far more about radio than I do and may add more info beyond my basic understanding but here are my points.

    In FM the bandwidth is limited to something like 15KHz and even this seems high when considering the standard for FM transmission was created a few decades ago and AM even before that. What is the bandwidth on a telephone limited to, under 4KHz?

    The dynamic range is another consideration and with the low noise floor a CD offers the usable range is very large compared to analog sources. Radio stations setup compressors to lower the dynamic range of the signal for transmission even when analog tapes were used, even more important with the CD as a source.

    In short, the equipment sold isn't setup for it. If you were designing and building a transmitter and receiver from scratch you could obviously improve the quality of the signal but could you keep yours compatible with existing equipment in the process?

    I think any market for such a product is being targeted by the satellite radio people and the quality of the sound from sat radio doesn't impress me. Its not like its the best content to judge hi-fi sound with but it seems to sound very processed and not as good as a CD which is disappointing. Especially when an iPod is so cheap and portable and remarkably good sounding for lossy compression.

    Cliff
     
    Last edited: Dec 17, 2004
  11. Dec 25, 2004 #10
    Ok, what do you consider "long distance", and is it specifically a high-speed data link you're hoping for?

    If so, there might be some relatively cheap components that you can string together to get your project up and running.
     
  12. Dec 26, 2004 #11
    Start from FCC: http://wireless.fcc.gov/ Seriously, thats the main thing that limits you.

    Technically, useful bandwidth handled by radio frequency depends heavily on modulation techniques. Its possible to stuff way more signal bw into radio wave than the wave frequency itself. But it all gets down to what is allowed without getting in trouble with law.
     
  13. Jul 2, 2005 #12
    LOL. That only applies to transmitters within the US. For places like Antartica or in 'international waters', it's not a problem.
     
  14. Jul 2, 2005 #13
    Yeah, whatever. You can't just transmit on any frequency or modulation technique from "international waters" and be safe from regulation. Why? Because it affects the rest of the world.
     
  15. Jul 2, 2005 #14
    So you think the FCC controls what is transmitted anywhere?
     
  16. Jul 3, 2005 #15
    There are some other sides to it as well:

    (1) Specifically, the amount of modulation practical (how saturated the carrier is with the audio signal) and the noise floor (the volume range and headroom possible for the audio signal) determine both the sound quality and s/n ratio. Consider also that in most commercial AM broadcasting, to get around this somewhat, extra techniques are applied to the signal both before and after AM modulation, like 'compression', a method of active gain control that prevents loud peaks from occuring and being 'clipped', and boosts low volume sounds so they are not lost in the 'noise-floor'. Or some patented system like 'dolby' or DBX noise reduction to maximize the amount and quality of the encoded information in the signal.

    (2) (And this should be critically obvious) The receiving radio has to be set up to accept the same size bandwidth around the carrier frequency that the broadcaster is using to send: The information encoded (using Fourier Analysis) can be viewed as a carrier and accompanying upper and lower sidebands encoded with the audio signal. If the 'doorway' (frequency range) of the tuning dial (bandwidth filter) is too narrow, information and sound quality will be lost anyway, broadcast or not. If your receiver's tuning circuit is too wide, other noise (sidebands from other stations) or industrial transmissions bleed into your signal and add more noise and distortion.
     
  17. Jul 3, 2005 #16
    By no means. But FCC guidelines should be respected. After all, how would feel about my having a 1,000 watt Jacobs Ladder in your area?
    For those that don't know, a "Jacob's Ladder" emits broad spectrum noise, and will cause any local rf receiving device(radio, T.V. etc...) to incessently "crackle" as if near a thunderstorm.
    BTW, large-scale Jacobs Ladder's are very, very easy to make and are illegal.
     
  18. Jul 5, 2005 #17
    If you are not constrained by the laws of this planet, then yeah, its not a problem. If you haven't heard yet, the each country has some equivalent of FCC, and so does international waters.

    Anyway, in Antarctica or places like that, you are constrained by people there depending on radio not hunting you down and plainly *killing* you for interfering with their lifeline. If thats not a problem too, and you don't care of any military dudes around your area, then you are dandy.

    And as I've said, technically possible bandwidth at any frequency depends on modulation techniques. If you have no idea of whats that, then go google. Things to look for for example 256QAM and "bandwidth efficiency" for starters.

    You can pack many gigahertz of bandwidth somewhere, consuming all available bandwidth for anything else radio related. You can, technically. If you're an arse enough to do that in a stupid way, then I'm sorry for you. If not, then your first point of checking how to be nice to the world you live in, is places like FCC. There you at least can see on what frequencies what is working, so can pick a "free slot" for your pleasure.

    What you failed to see is that its not FCC that rules your actions, but *people* that use radio spectrum for something actually useful, that have received part of that spectrum, and expect that part to be free from dudes for whom bodies like FCC "are not a problem". FCC and alike bodies are coordinating radio spectrum usage to avoid trouble.

    AM modulation is very nice, if you don't need stereo, don't need large distance, don't need interference resistence, etc.
    If you want to make dual-channel AM for CD-quality inhouse distribution - no problem.
     
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