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you could do FDM within the audio range using Gnuradio... for the basic FDM you could run 4 different streams of data using 4 PSK. QPSK, or even FSK carriers. the oldest method is running 4 FSK (RTTY) modems simultaneously. very sharp filters are required to keep the modems from interfering with each other. Gnuradio is definitely the best tool to use for this. i've done other "tricks" using Gnuradio, including an old fashioned frequency inversion voice scrambler. to do the FDM as audio only doesn't require the use of any SDR equipment, just the computer and sound card. https://www.gnuradio.org/