# Removing noise from a signal

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MINA on 20 Sep 2016
Commented: Star Strider on 7 Apr 2021
I have a following signal, and I want to correct the part which has a sharp jump, based on the value of the signal sometimes before the jump and its future values.
Instead of that jump I want something like a red line or green line, or something in between (sth that is most likely to be the signal instead of that noise based on the behavior of the signal in the pas and in the future.) Any suggestions?

Image Analyst on 20 Sep 2016
You can use a modified median filter. Basically compute the median. Then find any points where the actual signal is some factor higher than the median signal. Replace those points with the median. Something like
medianSignal = medfilt1(signal, 5); % Smooth signal
badIndexes = abs(signal-medianSignal) > someValue; % Find bad ones
Christine Tobler on 11 Feb 2020
The function smoothdata in basic MATLAB (introduced in R2017a) has a median filter.

Star Strider on 20 Sep 2016
The ‘spike’ creates broadband, high-frequency noise. I would first do a fft of your signal to understand the frequency composition, then design a lowpass or bandpass filter to keep your frequencies of interest, and use it to filter your signal.
There are many ways to design filters in MATLAB, probably the easiest being designfilt. My IIR filter design procedure is outlined in: How to design a lowpass filter for ocean wave data in Matlab? Remember to do the actual filtering with the filtfilt function.
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Star Strider on 7 Apr 2021
I would use the lowpass function (introduced in R2018a) if you have it. Otherwise, designing the filter from command-line functions is the only option.

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