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Re: EMG filtering and sampling rates

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  • Re: EMG filtering and sampling rates

    Hi, Dr Kieran Moran,

    Let me summarize your questions.

    1. sampling rate?? more than twice the "highest amplifer bandwidth"
    or the "max. frequency. we interest"
    =========================
    Usually, thtere is a lowpass filter(Fc) before ADC( analog/ditital
    conveter),and then you could set the sampling rate(Fs) to more than
    twice the Fc.

    Why twice?
    Briefly speaking, There are 2 samples to express a cycle of sine wave.

    Fs >= 2 Fc. (for more info., you could search keywords,
    Shannon's sampling therom, Nyquist Frequency . )


    2. band setting?? Could we use a band setting more much more than
    twice we interest?
    =========================
    Inappropriate bandwidth setting would result in unnecessary signals(noise).

    Bandwidth & filter setting are different things.

    Bandwidth: this would relate to the momory for data capturing.
    Filter: the accuracy to your data.

    I tried to attch a file, but it was rejected by the server.

    Yuchin
    =========================


    On Tue, 31 Oct 2006 18:31:22 +0000, Kieran Moran wrote:

    >Hi
    >
    >On the issue of sampling and filtering, Peter Konrad (The ABC of EMG
    >published by Noraxon) states (p13)
    >
    >"In order to accurately “translate” the complete frequency spectrum of a
    >signal, the sampling rate at which the A/D board determines the voltage
    >of the input signal must be at least twice as high as the maximum
    >expected frequency of the signal. This relationship is described by the
    >sampling theorem of Nyquist: sampling a signal at a frequency which is
    >too low results in aliasing effects (Fig. 18). For EMG almost all of the
    >signal power is located between 10 and 250 Hz and scientific
    >recommendations (SENIAM, ISEK) require an amplifier band setting of 10
    >to 500 Hz. This would result in a sampling frequency of at least 1000 Hz
    >*(double band of EMG)* or even 1500 Hz to avoid signal loss".
    >
    >This is also suggested by ISEK
    >(http://isek-online.org/pdf/ISEK_EMG-Standards.pdf) on page 2.
    >
    >However, I am unsure why the sampling rate must be more than twice the
    >highest amplifier *band width* setting and not simply more than twice
    >the *"maximum frequency of interest in the signal"*. In other words, if
    >we use a band setting of 10 to 1000 Hz, and we are interested only in
    >signals up to 400 Hz, would it not be appropriate to sample at 1000Hz.
    >The recommendations from Peter Konrad would suggest the sampling rate
    >should be more than 2000Hz (twice the highest *band width*).
    >
    >A number of published experimental studies would seem to agree that the
    >sampling rate does not have to be twice the highest band width, but
    >twice the highest maximum frequency of interest
    >
    >J.R. Potvin & S. Brown (2004) Less is more: high pass filtering, to
    >remove up to 99% of the surface EMG signal power, improves EMG-based
    >biceps brachii muscle
    >force estimates. Journal of Electromyography and Kinesiology 14 (2004)
    >389–399:
    >(gain = 1000, input impedance = 10 GX, 10–1000 Hz, CMRR = 115 dB at 60
    >Hz, Bortec, Octopus AMT-8, Calgary, Canada) and sampling rate was 1024 Hz
    >
    >
    >M.G. Feltham et al. (2006) Changes in joint stability with muscle
    >contraction measured from
    >transmission of mechanical vibration
    >Journal of Biomechanics 39 (2006) 2850–2856
    >(gain = 1000, band-pass filtered between 10 Hz and 1000 Hz (NL820,
    >Neurolog System, Welwyn Garden City, UK), and sampled at 1000 Hz
    >
    >Does any one have any thoughts on this issue?
    >
    >Many thanks
    >
    >Kieran
    >
    >--
    >Dr Kieran Moran
    >Biomechanics Research Group
    >School of Health and Human Performance
    >Faculty of Science and Health
    >Dublin City University
    >Collins Avenue
    >Dublin 9
    >
    >tel; 00 353 1 700 8011
    >fax: 00 353 1 700 8888
    >
    >kieran.moran@dcu.ie
    >
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