Weird crest of first wavelength when summing bandpass filter outputs

Reply to Weird crest of first wavelength when summing bandpass filter outputs on Fri, 09 Jun 2017 10:38:24 GMT

Kronax — Fri, 09 Jun 2017 10:38:24 GMT

I am creating sounds by exciting bandpass filters with a short impulse and summing the outputs together. The more filters I use the more there seems to be a clipping sound at the very beginning. When I visualize the sound wave I realize the gain of the first crest of the first wavelength is higher than it shoud be when compared to the next wavelenght. When I just have one filter the output is fine. I attach the patch and the sound wave with one filter and with four filters for illustration. How can I correct that first crest? Has it got anything to do with the phase? Cheers!

Reply to Weird crest of first wavelength when summing bandpass filter outputs on Sat, 10 Jun 2017 06:24:03 GMT

seb-harmonik.ar — Sat, 10 Jun 2017 06:24:03 GMT

band pass filters are kind of like exponentially dampened sine waves. keeping this in mind, try recording 4 sine waves of equal amplitudes added at those exact frequencies and look at the waveform, it's very similar. It's just a result of adding those frequencies together

in this case it is a phase issue because the band pass filters all have the same kind of phase response, and when the frequencies are that close together they will combine to create a gibbs-like effect in that first part

imagine adding a bunch of sine waves together: if they are positive, then the first bump will be tall because the first bump of each sine wave adds together. However, as they progress they interfere destructively after that more

as for how to change it, maybe you could try just inverting the polarity ([*~ -1]) of every other filter output in order of frequency?

Reply to Weird crest of first wavelength when summing bandpass filter outputs on Mon, 12 Jun 2017 18:26:28 GMT

Kronax — Mon, 12 Jun 2017 18:26:28 GMT

@seb-harmonik.ar Thanks for your answer! I tried your recommendation and it solves the problem despite my initial thought that the problem would appear with more than two filters, but it doesnt. Cheers!