Supposing a computer gets data over telephone wire, it has to send/recieve signals, but how do things work?
Does it use analog signal or a digital bits?
How does it send/recieve "1"s and "0"s?
Electronic – How does a signal travel down a telephone wire
signaltelephonetransmission
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I suppose really I don't understand how the line termination works and so therefore how to effectively remove the send signal from the transmit path.
Basically, a telephone handset works like this: -
simulate this circuit – Schematic created using CircuitLab
As can be seen the microphone amplified signal is split into two paths via 2 resistors R1 and R2. If R3 matches the real network line impedance at all relevant frequencies, then amplifier OA1 will only amplify the signal from the network and there will be minimal "sidetone" from microphone to handset earpiece.
Your idea of using an ultrasonic signal to the audio channel seems doable. Basically you can implement something like RTTY (http://en.wikipedia.org/wiki/Radioteletype) or one of those old 300 bps modem using FSK modulation (Bell 103A or V.21 standards -- see http://en.wikipedia.org/wiki/Bell_103_modem), just using very high frequencies. Modulating the signal with an Arduino might be doable (e.g., generating square waves and filtering them to something closer to a sine wave), but the demodulation part might be trickier (possibly filtering the audio signal to only include your ultrasound FSK tones, and then detecting and differentiating them: you will need a pretty high sampling rate on your ADC).
Best Answer
Over several decades of time analog telephony modem technology has evolved quite a bit. To understand the basics it is essential to understand Frequency Shift Keying, (in my opinion) the mother of all modulation schemes for digital signals.
Essentially what is happening is that a byte is broken up into separate bits, which will be sent one after the other. Next a couple of extra bits are introduced to simplify error detection and synchronization to the signal. If you miss a single bit, you don't want to corrupt the rest of the data stream too.
Unfortunately a telephone line cannot transmit logic levels on its own, the frequency band that it allows is too small (I believe 300-4000Hz) and thus you need a way to change the bits into something that can be transmitted across the telephone line => tones. This is just what FSK does, it changes a logic 0 into a tone of a certain frequency and a logic 1 into a tone of a different frequency.
For example a simple modulation scheme would work like this:
Notice how 1200Hz and 2400Hz are right in the supported bandwidth of the telephone line (300-4000Hz).
At the receiving end it is you do the reverse, if you detect a 1200Hz wave, you shift out a 0 and if you detect two 2400Hz wave, you shift out a 1.
Now you can pick two other frequencies for the communication in the other direction.
To increase bitrate there are many options. You can use multiple frequencies, multiple amplitudes and some schemes can even synchronize on the clock noise of the remote modem.