Low-pass filtering in MATLAB - adds data to beginning of vector

Question

I am using a FIR filter in MATLAB to filter a signal. In the figure below, applying my filter to the top plot creates the bottom plot:

enter image description here

The effect this has is to successfully low-pass filter the data, but to shift everything along by 500 msec.

Here is the routine I use to low-pass filter the data:

% (Start with any vector called 'inputData')

samplingRate = 1000;
filterLength = 1000;
filterCutOff = 90;

filterType = fir1(filterLength , filterCutOff/(samplingRate/2), 'low'); % define the low pass filter
inputData = filter(filterType,1,inputData); % filter the data

I'm aware that the 500 msec shift of my data relates to half the filter length (1000ms), but why does this happen, and am I doing something fundamentally wrong? I know that I could just delete the first 500 msec of my filtered data, but I am also missing the last 500 msec of the data.

Note that this example requires the signal processing toolbox.

If you don't want the delay, you will need to use an IIR filter zero phase delay (filtfilt). You can't have a FIR without delay. That is, you will need to choose the filter which is best for your situation… — Werner
@Werner, thanks for this. If you add this as an answer I can accept it as correct. — CaptainProg
There you are, added a book reference that I used, but of course there are many books. — Werner

Werner Werner · Accepted Answer · 2013-07-05T18:37:42

If you don't want the delay, you will need to use an IIR filter zero phase delay (use filtfilt).

You can't have a FIR without delay. That is, you will need to choose the filter which is best for your situation…

For more information on filters, use:

Mitra, S. K. Digital Signal Processing: A Computer-Based Approach, 2nd Ed. Mcgraw-Hill College

Of course, you may choose the newest edition if you like.

Low-pass filtering in MATLAB - adds data to beginning of vector

2 Answers