Dan Major

07-26-1999, 02:40 AM

Thanks again to everyone that responded. My technical reference library

definitly needs some additions!

>From the replies I received, no one could tell me exactly what the effect

of a rolling average would be, except that it is a low-pass filter, and the

mathematical formula that describes it is called an FIR (Finite Impulse

Response) filter. An FIR filter is a polynomial that uses preceeding data

values to calculate the value of the current datum. A rolling average is a

special case of the FIR called a rectangular filter. By adjusting the

coeffecients of the polynomial, as well as the number of terms (order), a

low pass, band pass, or high pass filter may be created.

The math that goes into calculating the values of the coeffecents is well

beyond the scope of a single maillist posting, however I am told that

Matlab has features that make these calculations very easy.

In my specific case, I am sampling at 100Hz, and need a low pass filter to

eliminate everything above 10Hz. I used a 10 point (or 10 "tap") FIR

rectangular filter, each coeffecient having the value of 0.1 . Since I am

looking at postural sway, I think the filtering of data higher in frequency

that 10Hz will not introduce any errors.

Dan Major

Univ. of Okla. School of Industrial Engineering

major@ou.edu htp://www.ecn.ou.edu/~major

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definitly needs some additions!

>From the replies I received, no one could tell me exactly what the effect

of a rolling average would be, except that it is a low-pass filter, and the

mathematical formula that describes it is called an FIR (Finite Impulse

Response) filter. An FIR filter is a polynomial that uses preceeding data

values to calculate the value of the current datum. A rolling average is a

special case of the FIR called a rectangular filter. By adjusting the

coeffecients of the polynomial, as well as the number of terms (order), a

low pass, band pass, or high pass filter may be created.

The math that goes into calculating the values of the coeffecents is well

beyond the scope of a single maillist posting, however I am told that

Matlab has features that make these calculations very easy.

In my specific case, I am sampling at 100Hz, and need a low pass filter to

eliminate everything above 10Hz. I used a 10 point (or 10 "tap") FIR

rectangular filter, each coeffecient having the value of 0.1 . Since I am

looking at postural sway, I think the filtering of data higher in frequency

that 10Hz will not introduce any errors.

Dan Major

Univ. of Okla. School of Industrial Engineering

major@ou.edu htp://www.ecn.ou.edu/~major

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For information and archives: http://isb.ri.ccf.org/biomch-l

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