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Re: Linear enveloping of EMG

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  • Re: Linear enveloping of EMG

    Dear Tony,

    Yes - I was confused by this for a while, too!

    The answer is in the rectifier. This is the component that is
    responsible for the linear envelope - not the low-pass filter. By
    rectifying the signal, you are, in effect, moving everything downwards
    in the frequency domain. This is also how a radio set works - the high
    frequency carrier wave is removed (demodulated) by the "detector" (which
    these days is a diode, but used to be a crystal in crystal sets). When
    you remove the negative going part of the signal the shape of the top
    (positive) half is the linear envelope. Of course, a bit of smoothing
    helps, but all is does really is fill in the bits between the
    positive-going pulses.

    Here's a nice page I found that describes it in radio terms:

    Hope this helps!

    Dr. Chris Kirtley MD PhD
    Associate Professor
    HomeCare Technologies for the 21st Century (Whitaker Foundation)
    NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
    Dept. of Biomedical Engineering, Pangborn 105B
    Catholic University of America
    620 Michigan Ave NE
    Washington, DC 20064
    Tel. 202-319-6247, fax 202-319-4287

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