[solved] Anyone familiar with filters?

I am trying to filter accelerometer data using a Butterworth 4-Pole Phaseless Digital Filter from an SAE standard. To make it phaseless I have to run the data through forwards and backwards. The formula given is Y(n)=a*X(n)+b*X(n-1)+c*X(n-2)+d*Y(n-1)+e*Y(n-2)
where X(n) is an unfiltered data point and Y(n) is the corresponding filtered datapoint. and a,b,c,d, and e are constant coefficients.

Does forwards and backwards refer to replacing n-1 with n+1 etc? I can filter using LS-PrePost but I am using Octave (the LS-PrePost results are not quite the same which is why I ask).

Thanks,
- Matt Bondy

Edit: A professor cleared it up. Forward pass is per the given formula. Backwards pass is Z(k)=a*Y(k)+b*Y(k+1)+c*Y(k+2)+d*Z(k+1)+e*Z(k+2) where k runs from the last data point to the first and Y(k) is previously filtered (and phase shifted) data and Z(k) is the phaseless filtered data (n-1, n-1, k+1, k+2, etc. are vector indexes).<br /><br />Post edited by: Matthew Bondy, at: 2010/02/23 04:23