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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.


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

    >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
    >( 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)
    >(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
    >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
    >Information about BIOMCH-L: