PDA

View Full Version : Re: EMG filtering and sampling rates



ywu89
10-31-2006, 02:30 PM
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
>
>---------------------------------------------------------------
>Information about BIOMCH-L: http://www.Biomch-L.org
>Archives: http://listserv.surfnet.nl/archives/Biomch-L.html
>---------------------------------------------------------------