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Transmitting and receiving data underwater

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Bazza175

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I am looking at possible ways to communicate data underwater, so far i have come across VLF and ultrasonics.

With reference to VLF does anyone have any experience in building transceivers of this kind and what would there range and capabilities be?
 
I have read both these, the problem with such transmitters is the antenna length. I only need to achieve 5-10 metres tho.
 
For that kind of range I'd be having a play with ultrasonics. Maybe even piezoelectric modules etc
 
i am looking into ultrasonics now, the thing im having dificulty figuring out is how itransmit and receive the data. Is this something i just make up or is there a way to do it... There seems to be very little data on the net about it.
 
I have read both these, the problem with such transmitters is the antenna length. I only need to achieve 5-10 metres tho.

Hate to state the obvious, but with 5-10 meters, why not just use cable?
 
the thing im having dificulty figuring out is how itransmit and receive the data
What data transmission rate is necessary? That may limit your choice of transmission medium.
 
Hate to state the obvious, but with 5-10 meters, why not just use cable?

Cable is not something i want to use, this is for a diving project. The diver will likely get tangled by the cable so therefore wireless options are much preffered.

What data transmission rate is necessary? That may limit your choice of transmission medium.

In terms of transmittion rates, it doesnt have to be all that fast. I dont yet know how i will transmit the data back and forth using ultrasonics, from what i can see the use of ultrasonics is used mostly for distance measure. You send a pulse and listen for an echo. In my case i want to send data and have a different device pick it up and decode it.
Data to be sent is depth and somehow the distance from the transmitter.

There will also be a distress code that will need to be sent.
 
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You could send the data as Morse code, using ultrasonic bursts.
 
Ok, looks like im going to hit a small snag here, i will have a problem with echoing sound etc, does anyone have any idea how this could be stopped? Would i have to send and confirm each character in turn or is there another way?
 
Make the bursts small and short enough and at a high enough frequency so you sample the first train of pulses and ignore the echoes for a preset time ?

You could also timestamp the signals so you could tell if you received a duplicate echo.

I'm not an expert in any shape or form for this kind of communication but it's a thought.
 
You can buy depth gauges for under a $100.0 bucks, why would you want to build this thing?
https://www.scuba.com/scuba-gear-51...icator..html?gclid=CM7e9NKUz7ICFeaDQgod0RIALw

Thanks for the link but its not exactly a depth guage im building, oh and im aware they exist im a divemaster, However the problem is none of them can send out there depth to another device.
I know my post seems vague as to what i am doing but thats just because the overall thing i want to build hasnt been built yet, so its a secret lol.

The major hurdle is this communication underwater hurdle.

I am currently reading through a long pdf file on optical transmitters. this coupled with ultrasound should see me right.
 
Radio wave attenuation in water is directly proportional to frequency. So the lower in frequency you go the less attenuation. You may be able to get away with 100KHz at about 5-10 meters but the power source would need to be high. With acoustics you will encounter a multitude of issues, echo, multipath, and delay to name a few. You would need some kind of DSP to handle the echo cancellation and this is not trivial. You may look into echo canceller IC chips used in the hands free market, but they might not have the delay you need.
 
Ok, looks like im going to hit a small snag here, i will have a problem with echoing sound etc, does anyone have any idea how this could be stopped? Would i have to send and confirm each character in turn or is there another way?

Also ultrasonic energy is quickly absorbed and won't travel very well, and tends to be directional.

I would use a much lower frequency. If you use DTMF encoder/decoder chips they detect the percentage of two frequency components and will be pretty immune to echo. Chips are readily available and give you 12 to 16 codes.
 
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