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Re: EMG filtering and sampling rates

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  • Re: EMG filtering and sampling rates

    It is the maximum frequency of the signal (not the maximum frequency
    of interest) that dictates sampling requirements. The reason for
    this is noise, which is often broad-spectrum (e.g. white or pink).
    Any noise in the signal above the Nyquist frequency (1/2 the sampling
    frequency) will be aliased back to lower frequencies, probably
    overlapping with the signal of interest and leading to signal
    distortion. Once aliasing has occurred, there is no way to remove
    the noise with digital filtering.

    So, the proper way to match sampling frequency to desired bandwidth
    is to limit signal bandwidth to the frequencies of interest via
    analog "antialiasing" filters, and then sample the data at a minimum
    of 3-5 times the filter cutoff frequency. It is important to
    remember that the "2X" sampling rule is a theoretical ideal, assuming
    a perfect filter. In real life, analog filters have gradual roll-
    offs, so a higher oversampling rate is required.

    The observation that these guidelines have not been followed in some
    published work (e.g. 1 kHz filter with 1 kHz sampling) does not
    necessarily imply that undersampling is generally acceptable or that
    any published data is erroneous. It is often possible to get by
    without "proper" antialiasing filters if you have very clean signals,
    or if your application is such that the effects of high-frequency
    noise do not alter data interpretation significantly. But, it is
    always preferable to know the effects of noise in your signals. A
    good way to characterize the frequency content of your signal is to
    acquire some data under realistic conditions using a very high
    sampling rate (several times the expected highest frequency, e.g.
    10X), and check for the presence of high-frequency noise using an FFT
    power spectral analysis. Most signal processing software (e.g.
    Matlab) has tools for this kind of analysis.

    Scott Tashman, Ph.D.
    Associate Professor
    Dept. of Orthopaedics
    University of Pittsburgh

    phone: (412) 260-7102