I'm trying to do some simple filtering for an audio signal using a window-sinc low pass filter. Supposing that my buffer has M values and the filter kernel size is N, after convolving these two arrays I would obtain an output of m+n-1 values. Which part should I take into consideration? I'm asking this because, if I read a M values array I should write a M values array. I have tried to take the first M or the last M values for output, but the result is not too good. I was wondering if I have to use a circular buffer in which I put every time M+N-1 values, but I read the first M values.

  • $\begingroup$ Are you performing one-shot filtering of single sample buffers, or are you going to be filtering an arbitrarily long signal one block at a time? $\endgroup$ – Jason R Mar 26 '13 at 13:02
  • $\begingroup$ Practically, I read a .wav or a .mp3 chunk by chunk, I do some filtering (like in winamp or in other player) and after that I play it. $\endgroup$ – Victor Mar 26 '13 at 20:44
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    $\begingroup$ OK, then in that case you need to make sure you maintain the state of the filter between blocks, as Hilmar's answer below suggests. $\endgroup$ – Jason R Mar 26 '13 at 20:54
  • Output the first M samples of your convolution result
  • Keep the remaining samples and ADD those to the result next buffer

Google "overlap add" for more information. While overlap add is a frequency domain method, it explains the framing and buffer handling well.

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  • $\begingroup$ It worked! Thanks a lot. I have another question regarding recursive biquad filters. In this case, I need to store only out[i-1],out[i-2],in[i-1],in[i-2]. I am asking this because convolution is practically hidden behind the recursive formula. The problem is that I hear some "high frequency cracks". I've also created a post with this issue, but their solution didn't work. dsp.stackexchange.com/questions/8272/… I know that I was making a big mistake: the for loop should be placed after computing out[0] and out[1], but this change doesn't help me. $\endgroup$ – Victor Mar 27 '13 at 15:56

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