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Acoustic Modem

  1. Jan 3, 2010 #1

    I am actually working on a project involving Acoustic Modem. However, we met up with some problems.

    We actually took the acoustic modem from another item. Thus there might have been some programming done to the modem which we do not know.

    The acoustic modem will work when its powered up but when we connect the TX & RX cables, it stops working.

    Can any one help me on this?
  2. jcsd
  3. Jan 4, 2010 #2


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    Staff: Mentor

    Welcome to the PF. Can you post some diagrams and maybe a picture or two? That would help us to figure out what is going on.
  4. Jan 5, 2010 #3

    Sorry I do not have any diagrams. I do have the pin configurations for the acoustic modem.

    Pin number Description

    1 External system reset line – optional feature
    2 External wake up line – optional feature
    3 Ground
    4 RS232 receive
    5 RS232 transmit
    6 DSP serial transmit – not used, do not connect
    7 DSP serial receive – not used, do not connect
    8 GND
    9 Power supply input 9V –5V DC
    10 Power supply input 9V –5V DC

    Connecting Pins 8-10 and 1-2 only will blink the LED which indicate normal operation mode. However, when we connect the Tx/Rx/GND cables,the LED stops blinking.

    These are taken from a information sheet they gave us.
    Last edited: Jan 5, 2010
  5. Jan 7, 2010 #4

    I went online and found the connections for straight DTE - DCE connection. Following the instructions, I can only receive some signs like triangle, brackets, square roots,etc..

    The setting was:
    Baud Rate: 9600
    Data Bit: 8
    Parity: None
    Stop Bit: 1
    Flow Control: None

    Do anyone know what's wrong here?
  6. Jan 7, 2010 #5


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    I remember acoustic modems that worked on 300 baud. I doubt if any of those went any higher than 300 baud.

    Access to the telephone line was via a speaker and microphone arranged so that the telephone handpiece could be laid into rubber pads and acoustically coupled to the modem.

    You would have to clarify the power supply pins.
    These modems used a AM 7910 "world Modem Chip" which did require a positive and negative 5 volt supply.
    It could do 1200 baud, but only on receive. It transmitted at 75 baud when receiving at 1200 baud.

    These days a 56000 bit /second (baud) modem is considered hopelessly slow so you will be shocked at how slow 300 baud is. It is OK for short text messages but painful for anything bigger.

    Here is a circuit using a AM79101 chip which is similar (I think) but it isn't acoustic so yours would have additional speaker microphone circuitry for that.
  7. Jan 7, 2010 #6
    Hi Vk6kro

    Thanks very much for your help. Just curious to know, is this a underwater communication model?
  8. Jan 8, 2010 #7


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    Science Advisor

    is this a underwater communication model?

    No, these were used 25 years ago, mainly if you were unable to access the phone line directly, like in a hotel room.
    It used to be illegal to plug anything into the phone line unless it was type approved, so acoustic modems were good for getting around this.

    If yours is for underwater use, none of the above probably applies.
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