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Very low frequency data - thoughts?

  1. Dec 7, 2003 #1
    Hi everybody,

    I'm working on a concept for a movie that I'm making. It's a fictional movie, but the concepts in it are definitely grounded in current and theoretical science. Coming from a big science background, I of course want the fictional technologies in the movie to be somewhat technologically feasible.

    So for the first concept:

    Could you transmit data from one point to another, or to one main base station of some sort, over a very low frequency such as 12Hz?

    * If not, why not? Is it just a horrible frequency to transmit at? What would be the reasons it would not work?

    * If not, are there any technologies, whether current or theoretical, that could make a low frequency like 12hz viable for data transmission?

    * If so, how efficient would it be in comparison to (for example), transmitting at 1900MHz (a frequency used for US GSM signals)?

    * Any thoughts overall about transmitting data at very low frequencies?

    If anyone interested could post their thoughts on this, I'd really appreciate it. Once I get some feedback I'll of course post what the conept actually is. :)

    Thanks in advance everyone!
  2. jcsd
  3. Dec 7, 2003 #2
  4. Dec 8, 2003 #3
    There is one principal limit to it: you can't transfer higher information rate than the rate of carrier. Thus at 12Hz, your information rate is about 12 bits per second. Roughly. There are few tricks to increase that alittle, and quite many issues to reduce it alot.

    Roughly, with carrier, information rate bandwidth is both added and subtracted with the carrier frequency. for eg. if you have 4Hz information rate, then with 12Hz carrier, it would fill 12-4..12+4 or 8Hz..16Hz. In such low end, limiters of frequency itself appear. for eg, at 12Hz, widest bandwidth meaningful can be 6Hz. That 6bits per sec.

    So low frequency radio is totally pain, especially for transmitters. not sure why would it be useful apart from its ability to penetrate substance, or more correctly, "shake" the substance. It takes enormous amounts of energy to create VLF waves.

    As to efficiency, compared to 1.9GHz gsm, 12Hz radio would be trillions times less efficient per same amount of information transmitted.

    Imo, quite abit more interesting is techniques of spread spectrum, especially ultrawideband, where there is no carrier as such, but information rate is multiplied by huge number and then transmitted in parallel. Eg for 100Hz information, multiplication of 1M and being spread over 200MHz bw. This allows to send information at strength so low at each frequency that its physically undetectable inside ambient noise.

    Then there are perhaps interesting ideas regarding, hmm, how to call it, "volumetric" or "holographic" transmissions. As example, consider ocean waves: there are zillions of them, each VLF. If you combine them into single spatial point, they mixup, but if you look at the area, they each can carry some separate information. To keep them separate, you need to create "parallel channels" of similar frequency, or basically holographic patterns at both transmitter and receiver. Unsure how to do that though. I think I've seen papers about antenna grids of few tens of transmitters where spatial coordination was part of the "code" to transmit and receive. There is a race for "bits-per-hertz". It was useful on that front.

    Abit similarily, deep earth core probing works: they dig into ground tens of probes, and then blow up some explosives. Then they catch the echoes in several points on ground, and later after processing and correlation they can have idea of inner structure of given area. This is rudimentally holographic reception. On same note, networked seismic stations can give alot of information about what happens in the ground.
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