FIR Filtering (Correctly)



I’m trying to use the FIR filter class (nothing more than that right now, just testing some stuff). I’m not sure how to do a couple of things:

a) Running processSample on each sample in the buffer in a loop works (in so far as it compiles), but doesn’t seem to actually do anything to the audio. I’m generating low pass filter coefficients for the time being. I think I wanna use fir.process() instead, but I’m not exactly sure what I’m supposed to pass into that (the buffer writepointer is apparently not it!). Here’s what I have so far within the processBlock:

    float* channelData = buffer.getWritePointer (channel);
	for (int sample = 0; sample < buffer.getNumSamples(); ++sample)


b) I want to use an array of pre-designed coefficients for the filter, i.e literally have a bunch of numbers written into the code, but I’m not sure what format it should be in/how exactly to do that. Currently I’m generating the coefficients as:

fir.coefficients = dsp::FilterDesign<float>::designFIRLowpassWindowMethod(440.0f, sampleRate, 21, dsp::WindowingFunction<float>::blackman);

But I’m looking to do something like:

fir.coefficients = [0,1,2,3,4 <whatever>]

Just, y’know, correct!

c) This method is clearly not going to work for multiple channels. I presume I can just hard-code definitions for a filter for the left/right channels, but I image there’s a better way to do it (one that can scale to more channels etc automatically?).

d) More of an add-on question, but I’m intending to operate on the incoming audio buffer and not necessarily have that as the output. In other words, I don’t wanna operate directly on the buffer - can I copy the input audio buffer and operate on that, or is there a more elegant way to do that?

Thanks for any help, much appreciated!


Hello,I also try to implement basic filterig with FIR filters. I can try and give you only answer for a) part.
Someone correct me if I am wrong but you do all the processing to channelData which is copied from buffer.I think buffer doesnt suffer any changes.You shall do processing directly on buffer or if not,after processing give new results to real buffer.
Sorry if I am wrong,


processSample returns a value, but you’re not doing anything with it!