Digital Kazoo Filter

Question

I'm working on an iPhone app where I need to record audio, modify the audio to sound like a kazoo, and play it back. I don't need help with the coding aspect, but I do need help designing some sort of digital audio filter to produce a kazoo-like buzzing effect. I've tried ring modulators and reverb filters and nothing really seems to come close. I'm really in over my head and I could use any guidance anyone could give me.

I'm getting raw, linear PCM 32 bit floating point samples from the mic. After that, I can do pretty much anything to it. I was thinking of using some sort of low-pass filter to ignore higher-pitch sounds like "s" and then introducing some kind of noise.

Answer 1

A kazoo contains a resonator that vibrates at one (or perhaps a series of) fixed frequency, so you should look for an algorithm that will transform any incoming frequency to a fixed output frequency but otherwise match the input amplitude. This way any formants and transients will still be recognizable. A vocoder or phase vocoder algorithm might work.

You can often hear the original humming mixed with the sound of a real kazoo, so you'll probably want to mix the original sound with the effect for a more realistic result.

To test this (and, perhaps, to make this question & answer more relevant on this site), you should experiment with generic vocoder and pitch correction plugins in your favorite DAW-software before you start coding. That should give you a much better idea what algorithm to pick when it comes time to implement the iPhone app.