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Re: Fourrier Analysis of EMG

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  • Re: Fourrier Analysis of EMG

    > Nathan,
    >
    > How to choose the sampling frequency fs ?
    > The Nyquist Theorem says, that only the signal whose frequency components
    > are smaller than fs/2 is correctly sampled. Every signals whose freq is
    > above fs/2 will be sampled also, but its frequency component is translated
    > to another frequency in the band DC-fs/2.
    >
    > For example, if you sample at fs=3KHz a sinusoid of 1V at 1KHz, after
    > sampling its Fourier Transform will be a peak of 1V at 1KHz (that is OK).
    > But if you sample at 3KHz a sinusoid of 1V at 2KHz, after sampling its
    > Fourier Transform will be a peak of 1V at 1KHz also ! That is false, and
    > you cannot remove it from your signal (aliasing).
    >
    > So, if you want to sample a signal at fs, you need to pass your analog
    > signal through an anti-aliasing filter, in order to remove any component
    > (signal and noise) higher than fs/2.
    > For example, an 8th-order elliptic filter with a cutoff frequency at 1KHz
    > will remove any signal whose freq component is at and above 1.5KHz.
    > (between 1KHz and 1.5KHz the signal will be attenuated). Therefore you can
    > sample it at 3KHz.
    > Maybe your acquisition system has already such a filter build-in it, maybe
    > not. You should verify that very important point.
    >
    > In http://www.teleemg.com/Chapters/jbr011.htm you can read: "Sensory
    > studies are performed with the low frequency setting between 32 and 50 Hz
    > and the high frequencies between 1.6-2 and 3 KHz."
    > Therefore, if you consider that the signal you want to measure reaches
    > 1.6KHz, you need to put your anti-aliasing filter cutoff frequency at
    > 1.6KHz. This filter will remove any signal above 2.4KHz then you can
    > sample at 5KHz.
    >
    > If you want to sample at 3KHz, then put your filter at 1KHz. But check
    > carefully the quality of the EMG after filtering.
    >
    > Patrick
    >
    >
    > Patrick Maillard
    > Institut de Recherches Robert BOSCH SA
    > PO BOX 12, Route de Denges 2
    > CH-1027 LONAY, SWITZERLAND
    > phone: ++41 21 804 72 17
    > fax: ++41 21 804 72 05
    > e-mail: Patrick.Maillard@ch.bosch.com
    >

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