Summary: I want to transmit an audio signal (300Hz-3kHz) without modulation over a 10 km cable.
I don't know how to.
I have looked for a similar post and couldn't find any I hope I am not creating a duplicate post.
I am an electronics and communications student and also a caver.
This summer in a cave I went, the communication system didn't work properly. I said to myself maybe I can make one that works better. The current system was a commercial intercom board fitted into a waterproof box. I suspect it performed poorly because it was designed for much shorter distances (and due to budget limitations it is very likely to be a cheap one.)
This particular cave is 1200m deep and the cable length is probably 6-7 km.
I want to design an intercom that could work reliably for 10 km.
I am having difficulties in analog audio transmission. I haven't taken analog electronics class yet. I could wait until I take it next semester but I am trying to make at least a version1 by the summer (in summer there is an expedition to the aforementioned cave.) I have kind of figured out the ringing circuitry etc. I need to figure out audio transmission circuitry.
My budget is limited, that is why I want to avoid modulating the signal which (might) increase the overall cost.
The cabling in the cave has approximately 100ohm/km and 120nF/km. The cables have no shielding, but there isn't much electromagnetic noise in caves.
So far I am thinking of driving the speaker using an IC like the LM386. I was thinking of amplifying the microphone output to around 5-6V and then reducing it to smaller than 0.4V (LM386 max input 0.4V) at the receiver side, nd use a some sort of common emitter or common collector amplifier (though there isn't much reasoning behind this.)
I haven't made much progress. My questions are:
- If you were to do it how would you do it ?
- What are the things I should consider regarding the 10 km cable ?
- Is there a particular type of amplifier circuit for this type of application (perhaps an amplifier which doesn't get affected by the output impedance very much?)
- Does it make sense to amplify the microphone output to 5-6V then reduce it at the receiver side? If it does or doesn't what are the reasons behind it?
- How should I pick parts for amplifier (transistors, etc,) what should I consider for device specifications?
If you can guide me by recommending sources or giving keywords so I can research I would be very happy.
Edit: I forget to mention that there will be multiple nodes in parallel. Transmitter will work in push-to-talk fashion and all nodes can listen to it simultaneously given they are in receive mode not standby mode. Privacy is not needed.
