FFT? Digital Filter? Problem Exactly Defined

AI Thread Summary
A user seeks assistance with applying a lowpass filter to a measured signal stored in a data file, specifically using C or C++. They inquire about the calculation of filter coefficients, particularly for a Butterworth filter, and request recommendations for user-friendly C/C++ libraries to implement the filtering. The user later finds a helpful resource online that addresses their needs. The discussion highlights the importance of accessible tools and methods for signal processing in programming.
jensel
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Hello,

Please help me with the following problem.

Given a measured signal which has to be smoothed. A lowpass filter has to be applied. The signal is available as measured data (a data file on the hard disk, numerical, not real-time) ( it has to be applied in C or C++). The filter frequency where "the cut-off" should apply is well-known. It is a real signal and has constant delta t.
I have no serious background in frequency data analysis so I will order my questions in a way of most urgency (sorry if one of the points is "easy to see" or wrong):

- if the signal is given as a series f(n), the converted signal is (depending from the order of the filter)
f_filtered(n)=f(n-k)*a_(-k)+f(n-k+1)*a_(-k+1)+...+f(n)*a_0+...+f(n+k)*a_(k+1)
under the condition that we have a kernel in the integration which is, let us say Butterworth (as far I understand the problem).
If this is right, how are the coefficients calculated? (for example with Matlab or in a different way)
- which free C/C++ library can be included to be more flexible to exactly solve this problem (it is not needed to be the universal library with all sorts of filtering - easy to use is priority)

Thanks a lot for helping me with this issue.


Jens
 
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Hi,

Problem seems to be solved. I found this brilliant website (rest in peace Tony, you genius, you helped me a lot!):
http://www-users.cs.york.ac.uk/~fisher/mkfilter/

So great...


Jens
 
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