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for a Project I need to build an Audio Modem in GNU Radio.

I decided, that it would be the best to use DBPSK Modulation, because that is fast and relatively noise resistant.

But as it seems, the DBSK Modulators are just modulating the Signal on the Baseband.

That's not what I want. Would it be enough, to just multiply the signal with a signal source? And when I do that, do I have to meet any preconditions so that the demodulator doesnt expect a Baseband?

PS: If you have any recommendations what would be a good filter, to get the best out of it, I would like to hear that to.

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  • $\begingroup$ For filters: You can use a Root Raised Cosine (RRC) filters to pulse shape the baseband signal before modulating it and another RRC to filter it after demodulating it to increase the signal to noise ratio. $\endgroup$ – taffer Jun 29 '16 at 20:25
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Short answer: yes; digitally mixing this up to a carrier frequency and just using the real part will suffice. For more info, refer to the usual "complex baseband" introduction.

Your filter question is completely impossible to answer – it depends on the characteristics of your transmission and your audio channel, and you mention nothing of that.

Also, you say DBPSK is fast. I don't know what you really mean with "fast", here, but I assure you that a 1bit/symbol modulation isn't per se a very efficient one. Unless you're in a very special case, you typically stay far away from being a good modulation – it's easy to demodulate, indeed, because you don't need knowledge about the initial state and hence can vastly simplify synchronization, but that makes it an especially spectrally inefficient modulation. That means you waste bandwidth that you could use for more bit/second in favour of a simpler system.

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