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Re: Linear enveloping of EMG

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  • Re: Linear enveloping of EMG

    >Dr. Chris Kirtley writes:
    >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.

    Actually, a rectifier is a non-linear device (or operation). In a AM
    demodulator the "signal" is removed from the carrier because the
    "signal" was added to the carrier by modifying the AMPLITUDE of the
    carrier. In a Series-Diode Detector (very simple AM radio receiver),
    the carrier which is Amplitude Modulated by the signal (this is what
    the radio station broadcasts) is passed through a very narrow band
    pass filter then half-wave rectified with a diode. The half-wave
    rectification is what extracts the "signal" from the the carrier -- a
    bit of smoothing then removes the carrier frequency.

    See figure 3-5A and discussion at
    http://www.tpub.com/neets/book12/51a.htm

    -Jason

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