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Thread: FSK Demodulation with ADSR

  1. #1
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    FSK Demodulation with ADSR

    Hi all,

    I have a project in which I need to implement FSK communication with hydrophones.
    Thus far, my plan is to implement noncoherent demodulation of the signal by passing the signal through bandpass filters and envelope detectors in a similar fashion to page 10.
    If I were to use this demodulation scheme, would I be able to use the ADSR envelope?

    I'm open to other demodulation schemes as well, bearing in mind that I shall implement chirp spread spectrum FSK in a high noise environment.

    Thanks

  2. #2
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    Quote Originally Posted by DBW View Post
    Hi all,

    I have a project in which I need to implement FSK communication with hydrophones.
    Thus far, my plan is to implement noncoherent demodulation of the signal by passing the signal through bandpass filters and envelope detectors in a similar fashion to page 10.
    If I were to use this demodulation scheme, would I be able to use the ADSR envelope?
    No, because that's a generated/sequenced envelope. An envelope detector measures the envelope of a signal,
    the AudioAnalyzePeak and AudioAnalyzeRMS classes in the Audio library each do a version of this, but at a granluarity
    of the audio block size, and this isn't tunable.

    I'm open to other demodulation schemes as well, bearing in mind that I shall implement chirp spread spectrum FSK in a high noise environment.

    Thanks
    Another approach is using FFT bins corresponding to the two frequencies, again issues of granularity need to be addressed.

    Or write your own envelope detector that's aware of the carrier and modulation frequencies and is tuned to them for best
    noise performance.

  3. #3
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    Thanks very much for your input! I'll look into using FFTs rather than envelope detection.

  4. #4
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    What you need is a peak detector. This is being brought into the audio library but isn't yet released (SFAIK). It's a full wave rectifier which, combined with a tunable lowpass filter gives you envelope demodulation.

    https://github.com/PaulStoffregen/Au...2000567a8bdc76

  5. #5
    Senior Member
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    The ultimate approach to envelope detection would be to use a discrete Hilbert transform direct into a complex absolute-value
    function, but that's overkill (although elegant and probably the "correct" way, mathematically)

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