[SciPy-User] Low pass filter

[Ivo Maljevic]

Kaan,
Maybe this little example will help you better understand how this filtering
business works (run the attached file).
Ivo


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

> May be butterworth is not the correct way to make it. Can it be like that?
>
> 17 Eylül 2009 17:17 tarihinde Kaan AKŞİT <kunguz at gmail.com> yazdı:
>
> 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(voltage,timing)
>> [b,a]=butter(2,5)
>> voltagelowp=lfilter(b,a,voltage)
>> plt.plot(voltagelowp,timing)
>> 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
>>>>
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>>>> SciPy-User mailing list
>>>> SciPy-User at scipy.org
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>>>>
>>>>
>>>
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>>>
>>
>
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