[SciPy-User] Low pass filter

[Ivo Maljevic]

Kaan,
If you have two consecutive plot calls, you need to call figure in between,
or you will plot over the first one. See the modified code below (blue).
Also, put the time on the proper place (red).
I cannot comment on the frequency comment of the signal you are trying to
LPF.

Notes:

1. Your butterworth filter is too short: N=2?
2. Your Wn should be normalized and < 1, I think.

Ivo


#!/usr/bin/python
# -*- coding: utf-8 -*-
import time, array, datetime
import matplotlib
import matplotlib.pyplot as plt
from scipy import *
from scipy.signal import butter, lfilter

input = 0
timing = []
voltage  = []
values = []

input = 0
infile = open('measurements/' + "data.csv", "r")
for line in infile.readlines():
    values = line.split(',')
    voltage.insert(input,float(
>
> values[0]))
>     timing.insert(input,float(values[1]))
>     input = input + 1
> plt.xlabel('Time steps')
> plt.ylabel('Voltage')
> plt.title('Measurments from Oscilloscope')
> plt.grid(True)
> plt.plot(timing, voltage)
>

   plt.figure()

[b,a]=butter(2,5)
voltagelowp=lfilter(b,a,
>
> voltage)
> plt.plot(timing, voltagelowp)
> plt.show()
> plt.close()
>



2009/9/17 Kaan AKŞİT <kunguz at gmail.com>

> Thanks Ivo, The problem is after filtering I got the same signal. I wanted
> to clear the frequencies higher then 5 Hz. My code is similar to yours but
> the output doesn't seem logical:
>
> #!/usr/bin/python
> # -*- coding: utf-8 -*-
> import time, array, datetime
> import matplotlib
> import matplotlib.pyplot as plt
> from scipy import *
> from scipy.signal import butter, lfilter
>
> input = 0
> timing = []
> voltage  = []
> values = []
>
> input = 0
> infile = open('measurements/' + "data.csv", "r")
> for line in infile.readlines():
>     values = line.split(',')
>     voltage.insert(input,float(values[0]))
>     timing.insert(input,float(values[1]))
>     input = input + 1
> plt.xlabel('Time steps')
> plt.ylabel('Voltage')
> plt.title('Measurments from Oscilloscope')
> plt.grid(True)
> plt.plot(timing, voltage)
>

   plt.figure()


> [b,a]=butter(2,5)
> voltagelowp=lfilter(b,a,voltage)
> plt.plot(timing, voltagelowp)
> plt.show()
> plt.close()
>
> 2009/9/17 Ivo Maljevic <ivo.maljevic at gmail.com>
>
> Does the following:
>>
>> plt.plot(timing, voltage)
>> plt.show()
>>
>> work before filtering? (Suggestion, you may want to put time on x-axis,
>> see the line above)
>>
>> I just wrote a small test program, and it works:
>>
>>
>> #!/usr/bin/python
>>
>> # import
>> import pylab as plt
>> from scipy import *
>> from scipy.signal import butter, lfilter
>>
>> t=linspace(0,1,100)
>> v=sin(2*pi*t)
>>
>> (b,a)=butter(2,5,btype='low')
>> v=lfilter(b,a,v)
>> plt.plot(t,v)
>> plt.show()
>>
>>
>> 2009/9/17 Kaan AKŞİT <kunguz at gmail.com>
>>
>>>  Hi all,
>>>
>>> This is my first time that I use Scipy. I have two arrays both of them
>>> one dimensional. One of them is filled with voltage values and one of them
>>> is filled with time values. I want to apply low pass filter to clear the
>>> noise from the data. Can you please help me? I already wrote something but
>>> it is not working:
>>>
>>> (b,a)=butter(2,5,btype='low')
>>> voltage=lfilter(b,a,voltage)
>>> plt.plot(voltage,timing)
>>>
>>> Best regards,
>>> Kaan
>>>
>>> _______________________________________________
>>> SciPy-User mailing list
>>> SciPy-User at scipy.org
>>> http://mail.scipy.org/mailman/listinfo/scipy-user
>>>
>>>
>>
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>
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