Early, unregulated phone access

pots
texas
phone
decentralised

(Full Snack Developer) #1

Way back in the day (about 1902, actually) ranches in rural Texas had a rather ingenious way of calling each other on the telephone: They used their barbed wire.

Anyone who has ever played with a POTS (Plain Old Telephone System) phone probably knows that a phone is little more than a 2-wire circuit. What happens when you send current through hundreds of miles of connected barbed wires? Ad-hoc telephone networks!

This is an excellent read, highly recommended.

http://www.texasescapes.com/CFEckhardt/Rural-Telephone-Systems-in-the-West.htm


#2

Wow! That’s some neat stuff.


#3

It’s very interesting how analog technology works.

Did you know that the phone line still works even when there is no electricity?


(Full Snack Developer) #5

That’s a bit of a misconception. The phone line’s voltage doesn’t come from the same source as the electricity in your house, but it still requires electricity to run.


(Command-Line Ninja) #6

Do you know anything about earthing systems? In the UK, there is an earthing system called Tera-Tera for rural properties, you put an earth stake into the ground, and it can use the earth to transfer earth current in the event of a fault.

I have often wondered if we could use this earth transfer as a sort of conductor, for things like phone lines. Could this work? I know the can get a resistance of <200 ohms at 230 volts. What would you need for an earth transfer communications network?

As a side idea, could you use low frequencies to send data through the ground? The ground is everywhere, so could be invaluable for communications.


(Full Snack Developer) #7

Elephants do that, ya know.

I think the biggest obstacle to something like that would be the interference from so many other seismic sources. Also, the difference in density of the soil and obstructing objects may make for very messy signals. Not saying it’s impossible, but those are the two biggest obstacles I can see right away.


(Command-Line Ninja) #8

How do elephants do it? Perhaps we can learn from the way they do it.


(Full Snack Developer) #9

They detect infrasonic vibrations primarily using their feet. Oh, another thing to consider on top of noise is how informationally dense the signal can be and how it would be modulated to encode information. Lower frequencies do not tolerate FM very well, and AM would be hard to detect. Some sort of analog pulse encoding would probably be useful for transmitting digital information, but the bitrate would be painfully slow–slower than a modem most likely.

Here’s some literature on sampling rate and why it’s important


(Command-Line Ninja) #10

On my research for this, I came across this video.

It has helped me to understand Nyquist sampling so easily.


(pico) #11

@fraq has dropped a bunch of concepts here that may be confusing for people not familiar with communications systems. So I will try, with @fraq’s permission, to expand a bit further. Sure. I like these topics :slight_smile:

As @fraq said, that elephant thingy is related to mechanical waves, like sound, not electromagnetic waves. You can always use transducer to convert one type of signal into another tho. Both are waves, but one moves some physical thing (air for sound) and the others “moves” electric and magnetic fields.

Your best option to transmit signals over long distances is to use longwaves (see also VLF). Or short waves bouncing your signal into the ionosphere… but in that case it will be a kindof point to point link. The best you can do is probably trying a Moon Bounce.

FM (and PM) and AM are analogue modulation techniques. You can decide to use some analogue representation of a digital signal using these modulation, but it is better to use a digital modulation. Their counterparts are called FSK and ASK, being those the simplest digital modulation you could use. But probably you will find more often a QAM with some complex symbol constellation. Anyway, at the end, you will get an analogue signal at a certain frequency (that is how nature works :slight_smile: ).

Finally, the Nyquist theorem is related to sampling, not even to communications. Sampling is the process of converting an analogue signal into a digital signal. For doing that you have to take samples of the continuous analogue signal every certain time (the sample frequency/rate). The Nyquist theorem basically tells you that you need to sample as fast as twice the higher frequency in your analogue signal, to be able to capture all the information it contains.

If you sample at a lower frequency, you may need an antialiasing filter (that will limit the higher frequency in your analogue signal), if you sample faster you are wasting bandwidth.

However, note that sampling is just the first step. You do not get a digital signal after sampling. You actually get a discrete-time signal. To make it digital, you need to quantize the analogue value you have recorded for each sample. In other words, you have to map each value to a combination of bits. This two process usually happen together tho… your ADC circuitry will allow you to specify the sample rate and the number of bits to use (that may be fixed in some cases).

So, for instance your sound card have an ADC. If you record an audio file at 16KHz 16bits, you are sampling at 16KHz (so you are capturing signals up to 8KHz) and quantizing the samples using 16bits.

When you signal is already digital, as it happen with a computer generated value, you do not need to sample it, and the Nyquist theorem does not apply. The value you have is what you have…


(Full Snack Developer) #12

Be still, my heart, for I have found another waterwalker in the wild.

It’s not often I get to talk about signal sampling, multiplexing, digital/analog encoding, or wave propagation with people. It’s a niche subject.


(pico) #13

We should talk about this stuff more often :wink:


(Full Snack Developer) #14

You’ve got me thinking about writing an article on tropo-scatter radio shots now.


(pico) #15

I will read such an article for sure!


(Trey Ramey) #16

Fraq may I personally ask you a question that I’ve asked my pin testing friend that you might have more info on


(Full Snack Developer) #17