I am using in my signal processing code (for EMG signal), the function 'butter' to specify IIR filter of a specific order, n. Then to nullify the effect of non-linear phase shift, I am using 'filtfilt' function. Does the final output I get is an output coming from a filter of order n multiplied by 2? If I need to use a specific order of the filter for my signal processing shall I have to specify inside the butter function in a way that it acts as n/2 nd order filter?
How should I report in research paper about the filter order then?
filtfilt takes your filter and processes your data in the forward direction and then in the backward direction.
Assuming that you use a filter of order n, you should report that you process your data by two passes of the n-th order filter. This technique is also sometimes called forward-backward filtering, see, e.g., Article Determining the Initial States in Forward-Backward Filtering
Ranajay Samanta
The final output you get from using the 'butter' and 'filtfilt' functions is not equivalent to a filter of order n multiplied by 2. 'butter' function is used to specify the design of an IIR filter with a specified order n. 'filtfilt' function is used to zero-phase filter the signal, which involves applying the filter forward and then backward, effectively doubling the effective filter length and effectively providing a filter of order 2n. However, it doesn't mean that the filter has an order of 2n, but only the length of impulse response has increased to 2n, which can result in increased smoothing of the signal.
To report the filter order in a research paper, you can specify the order of the filter specified in the 'butter' function, and also mention the use of 'filtfilt' function for zero-phase filtering. It would be a good idea to describe the implications of zero-phase filtering on the filter's effective order, if necessary.
The butter function in MATLAB and Octave designs an IIR filter of a specific order n. When you use the filtfilt function to nullify the non-linear phase shift, it applies the filter twice, which effectively doubles the order of the filter. Therefore, the effective order of the filter is 2n.
If you need to use a specific order n for your signal processing, you should still specify n as the desired filter order in the butter function. The filtfilt function will then apply the filter twice, resulting in an effective order of 2n. When reporting the filter order in your research paper, you should report the order n that you specified in the butter function. If you used filtfilt to nullify the non-linear phase shift, you should also mention that the effective order of the filter is 2n. This is important for other researchers to understand the characteristics of the filter that you used in your signal processing.
- Badges
- Science topic
- Similar topics
- Control Systems Engineering
- Filtering