From help-octave-request at bevo dot che dot wisc dot edu Wed Dec  5 14:30:06 2001
Subject: using conv vs. filter
From: "E. Joshua Rigler" <Jrigler at colorado dot edu>
To: Help-Octave <help-octave at bevo dot che dot wisc dot edu>
Date: 05 Dec 2001 13:29:09 -0700

I'm looking at the code for conv.m in the octave libraries.  It does
some sort of zero-padding, then calls the "filter" function.  The output
is a vector that is length(a)+lenght)b)+1.

Now, I am trying to convolve an filter with both negative, zero, and
positively lagged coefficients with an input time series.  In order for
my time-steps to be synchronized between the input time series, and the
output from conv.m, how would I index the output?  I guess I just don't
really understand how the conv.m function works in octave.

At the moment, I use a filter that has an equal number of positive and
negatively lagged coefficients, and the output seems reasonable when I
do something like:

output = conv (filter, input);
output = output (length(filter)/2 : length(output)- \
                                    length(filter)/2);

1) Does what I have seem right?
2) What do I do if my filter has fewer/more negatively lagged
coefficients than positively lagged coefficients?

Thanks,

-EJR



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