Design a low pass filter for time domain data

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Shawn Chang
Shawn Chang 2012-11-22
评论： nam bui 2021-2-12
Hi, I wish to design a low pass filter for the time domain response graph and hence filter out the noise. So that, the noise area can be smoothed out but I has no idea on how to design the low pass filter because I don't know what is my sampling frequency. I reads most of the threads are about how to designing the low pass filter with a user-defined sampling frequency, sampling interval and cut-off frequency.
Can I know how to determine the sampling frequency for my time domain data? Link below show the time domain response graph which need to be filter using low pass filter.
https://www.dropbox.com/s/e614o5edodauumr/example.png
Any helps is much appreciate. Thanks you.

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Pedro Villena
Pedro Villena 2012-11-22
编辑：Pedro Villena 2012-11-28
  • You need the sampling rate to design a digital filter, but if you want to design an analog filter, you don't need the sampling rate, instead you only need the cut-off frequency.
fc = 25; %%cut-off frequency [Hz]
sys = tf([1],[1/fc 1]); %%transfer function in continuous time
velocity = ?; %%your data array in m/s
time = ?; %%your time array in second
lsimplot(sys,velocity,time,0,'zoh'); %%discretization of SYS using ZOH method
Your sampling frequency is around 70Hz
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Shawn Chang
Shawn Chang 2012-11-26
编辑：Shawn Chang 2012-11-26
May I know how to determine the 70Hz?
I try to design it with 1st order butterworth,with -3dB cut-off freqyency 700Hz and sampling frequency 8kHz using the the below code.
dr=fdesign.lowpass('N,F3dB',1,700,8000);
Hdr = design(dr,'butter');
right_f = filter(Hdr,right_end);
The filter response graph I obtain is the one with green color, does it fine to have the filter output like that? Because that was the smoother filter graph I can obtained and with the least phase shift.
https://www.dropbox.com/s/t37sbb6abls703p/left_right_encoder.png

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iyad salameh
iyad salameh 2015-1-31
did you solve your problem ? because am doing the exact same thing , i need to filter it for system identification toolbox. please let me know iyad.salameh.44@gmail.com

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