CODEWITHSUNDEEP
c#c++signal-processingfft
JustAMartin
JustAMartin

Reputation: 13723

Calculating FIR coefficients from frequency response

I have a list of amplitude frequency response points. The amplitudes are given in decibels.

The task is to export this AFR list to a set of coefficients for some hardware DSP device. I know the file format for this device. The specification for the device says that it needs 12288 coefficients and the DSP has 2 FIR filter blocks (6144 taps each). The idea is that after loading those coefficients, this device should work as an equalizer which transforms the signal according to the initial amplitue-frequency point list.

I have found out that the coefficients for a FIR filter can be calculated by taking the inverse Fourier transform of the desired frequency response and also using some windowing function (preferably not rectangle).

The problem is that I am not good at signal processing, I do not know much about FIRs, I have used FFT to get the frequency response from audio data, but I still have a pretty vague idea about how FFT and related stuff works.

The calculation of points should be done in C# or C++ (I am good at creating C++/CLI wrappers) so I can integrate it into already existing application. The exporting process is not time critical, so I can use simple and slow algorithms, but anyway it should not take more than a minute on a mid-range computer.

Is there any free library or code to get FIR coefficients from the amplitude-frequency response data?

The best solution would be something like a "black box" so I can just feed the list of AFR points and get out 12288 coefficients, but I would be grateful also for multiple libraries/pieces of code if only it can be put together easily.

Additional info:

the points were calculated using the following formula:

       float x;
       for(int i = 0; i<bandPoints; i++){
               x = (((float)i + 1) / ((float)(bandPoints + 2)));
               bandsHz[i] = ((x*x)*(22000-20))+20; // 20...22000
       }

Upvotes: 1

Views: 3753

Answers (1)

Jens
Jens

Reputation: 429

The standard way of finding good FIR coefficients is using the "Remez exchange" algorithm. I found a link to some code (haven't tried it myself) that you may find useful: http://www.janovetz.com/jake/remez/remez-19980711.zip. Another key word to search for is "Parks-McClellan".

Input to the algorithm is a description of the amplitude versus frequency and a set of weight factors versus frequency that describes the relative importance of meeting the amplitude requirement at that frequency. The algorithm then finds the optimum filter in the mini-max sense (minimizing the maximum error).

Upvotes: 5

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