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I'm looking to transfer very simple information using audio waves. One of the approaches I'm looking into is using different frequencies. For example, "command 1" will be transmitted using 500Hz wave, where "command 2" using 700Hz wave. On the other side, the decoder will look if we have a wave with that frequency and then interpret the command.

The challenge appears when I'd like to transmit 2 (or more) commands simultaneously - I get a new wave form. If I remember right from my physics studies - it's a "Wave package", so I get new frequencies, additional to the original ones.

My question is - Is there a way to receive this "wave package" and interpret the original commands from it?

Many thanks!

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Actually, when you combine (superpose) waves with different frequencies, they don't produce new frequencies. You can perform a Fourier transform on the combined wave to get the two original frequencies that went into it. This is a very common operation in signal processing and it can easily be implemented by a computer, or by certain kinds of purely physical systems.

Just to give an example, this is how multiple radio stations can be broadcasting at once. Each station has a different frequency, and their signals add together in the air, but your radio is easily able to pick out the individual signal from just one station.

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I thought so as well - but when we tried to analyze the frequencies we get using FFT, we did get much more frequencies. The original ones were stronger - but we did have several new frequencies. –  Bob Jul 19 '11 at 7:37
    
Sorry. you were right! It was a problem with the speaker. When sounding 2 sounds - it created more frequencies. –  Bob Jul 19 '11 at 8:03
    
The keywords are superposition against mixing or linear in contrast to nonlinear mixing. Did You take up the sound with some microphone and feed the output to a FFT analyzer? –  Georg Jul 19 '11 at 9:15
    
@Georg - I've used a simple mic and fed the input stream to a FFT analyzer. –  Bob Jul 19 '11 at 11:52
    
I think the problem was with the speaker which is very cheap one (the simple 8ohm) –  Bob Jul 19 '11 at 11:52
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