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Acoustic Modem

  1. Jul 7, 2006 #1

    I'm making an AUV and need to design my own acoustic modem. Now, after hours of research I found these PDFs (attached).

    In thoery, can I remove the signal generator from XXXXPinger.pdf, add a DTMF encoder and then use a build the transducer as described in bender2.pdf to transmit the sound? Then use XXXXReciever.pdf with a bender2.pdf transducer to receiver the sound then put it through a DTMF decoder?

    It can't really be that simple can it?



    Attached Files:

  2. jcsd
  3. Jul 7, 2006 #2
    You'd be best starting over completely. The transmitter link you have is not really suitable to send DTMF. The output stage is not at all linear. Why do you need DTMF? Can't you just go with something simpler like AFSK?
    At any rate, the whole project sounds kind of interesting.
    Last edited: Jul 7, 2006
  4. Jul 7, 2006 #3

    No, I don't need to use DTMF, and having just looked up AFSK (thanks Wikipedia :D ) I think that's a much better method.

    If I try to do it as I said in my first post, what problems would I get?

    Am I right in thinking piezoelectric discs for transducers?

  5. Jul 7, 2006 #4
    DTMF is an acronym for Dual Tone Multi Frequency which I'm sure you already knew. The frequencies are SINE waves. The schematic you posted does not have an amplifier capable of driving the transducer without distorting the sine waves.
    I cannot say whether any of what you are doing is right without knowing more about the project. All I know is you have something under water and need to send data either to or from (or both ways for that matter) this thing that is under water.
  6. Jul 7, 2006 #5
    I meant if I use AFSK with the first method would it work...

    The project needs a duplex comms link between the surface and AUV in order to program it in the water and relay sensor information back to the surface. It needs to have 32 on/off commands and 8 values 9 (0-255) and work over 100m max.

    Any thing else you need to know?

  7. Jul 7, 2006 #6


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    What data rate are you shooting for? Keep in mind that for reliable comm in real-world channels, you will need to do things like send a preamble to start each packet and include some level of error detection (like a CRC) or correction.
  8. Jul 7, 2006 #7
    I'd say <50bps, I just want to be able to get something working that I can improve upon as time progresses. Not looking for the ideal solution first time round.

  9. Jul 7, 2006 #8


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    I only skimmed the documents that you attached. Where does the 200kHz number come from? Is that optimal for underwater comm?

    Also, for the best chance at good communication, you will want to use a correlator in your receiver. Have you learned about correlators yet? How about error detection and correction algorithms? You will get extra credit in your project if you use real-world communication techniques like these.
  10. Jul 7, 2006 #9
    I start electronics classes in September, everything I know at the moment is self-taught. Although I'm very keen to see this project through as lot (both time and money) has been invested in it. Not looking for a detailed what-to-do, just areas I should look into/ investigate etc.

    Other forums I've been to suggested 40KHz but purely because they designed their own system to work with that (they did it 20 years ago and only remember it was 40KHz and used a variety of op-amps in the receiver).

    Navy submarines use somewhere in the region of 3-4KHz i believe for their comms, although they have to go alot further, so I guess that that is the optimal frequency.

    The site I pulled those schematics of ( http://www.dstl.gov.uk/news_events/competitions/sauce/index.php ) don't make any comment as to why 200KHz was used as you can see for yourself.

  11. Jul 7, 2006 #10


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    I'd suggest doing a little looking around the Internet to see if you can find out what the optimum short-range acoustic comm frequency is, and if there is a preferred modulation scheme. You will want to avoid frequency ranges where there is a lot of natural (and human-made) noise, for example. If you look at the spectra of typical noise sources in the ocean and from ships, then that may help point you to a range of quieter frequencies. And the correlator will help you reject noise as well, if you choose your modulation scheme carefully.

    Working on projects like these will help you go a long way in your upcoming electronics classes. What signal processing books and resources have you been studying from?
  12. Jul 7, 2006 #11


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  13. Jul 7, 2006 #12
    Only book I've looked at Physical Computing, a run though of circuit basics. And everything I've learnt for this topic has been in the last 24hours staying up well into the early hours reading all sorts of PDFs and reading through old forum posts etc. It was this thread (https://www.physicsforums.com/showthread.php?t=119489 ) that lead me to this forum...

    Your welcome to suggest books as the more I delve into the subject the more fascinated I get by it.

  14. Jul 7, 2006 #13
    Thanks for the link

  15. Jul 7, 2006 #14
    Oh, one point I ought to make is that the AUV will be run in a fresh water environment (local pond) and so traffic in the area will be minimal.

  16. Jul 7, 2006 #15


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    One basic electronics book that I really like is "The Art of Electronics" by Horowitz and Hill. You can get it through Amazon.com, or at your local university bookstore. It takes you from the basics up through opamps and some digital circuitry.

    BTW, you mentioned that the link will be duplex, but keep in mind that the packets that you send in each direction will have to be separated in time. I don't think you will be able to talk in both directions at exactly the same time, since you would not be able to hear over your own transmission. So you will need to work out a protocol based on packets. Since you have a specialized situation of one boat and one UAV, the addressing scheme can be simplified. But you will still need to come up with the format of the packets, including the preamble, the data unit and the CRC. You will also need to come up with a scheme for getting acknowledgements back for packets, and for doing retries if no ACK is received within some timeout period.

    Fun stuff.
  17. Jul 7, 2006 #16
    Thanks for the link, I'll have a look,

    I'm going to setup a tank in the back garden tomorrow. 10m long made up of drainage tubes and probably some sort of lining to adsorb some waves. I've got a piezo siren which I'll take apart and build an preamp to add to the rx piezo disc and see what happens. Just out of interest, don't know what will happen.

    Been doing research for long enough and want to build up some 'experience', I'll post here with my results.

  18. Jul 7, 2006 #17


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    BTW, I think you saw our warnings in the other thread that you referred to, but think about some hearing protection while you work with the ultrasonic power stuff. I don't know what-all precautions you should follow, though, so maybe google a little to see if just regular ear-muff protection works for ultrasound. Even though you won't be in the water, there may be safety issues. Come to think of it, that must be a general consideration for UAVs using ultrasonic communication. I wonder what kind of diver safety precautions should be taken at various power levels.... We didn't talk much about that in my SCUBA certification classes....
  19. Jul 7, 2006 #18


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  20. Jul 7, 2006 #19
    I'm guessing that 200 Khz is best suited for sonar. It may not necessarily be suitable for communications and it may be somewhat directional in the water. How many dollars are you willing to drop on this project? I'm also wondering what you are going with for a microprocessor/microcontroller. Have you looked into some of the equipment used in amateur radio? One of the protocols used is AX25. I think it may be what you are looking for.
  21. Jul 8, 2006 #20
    Well been and picked up the tubes, going out later to get some piezo transducers..

    At the moment I'm just going to try submerging a piezo siren that's designed for air, no plans to go ultrasonic just yet, want to build the RX and see how well that performs.

    In terms of money I'd say $200 (on electronic components) initially to get some results, then if it looks promising I'm happy to spend more on it.

    At the moment I've got some BS2 and PICAXE programming experience, although if I have to use a different chip then I'm happy to learn.

    I haven't looked into amateur radio as I didn't think it would suitable, I just want to see how well I can transmit/receive sounds first and then I'll start think about transfer data.

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