Caustic Song file (optional):
Here is a Waveform Generator. It's running through a state variable filter. I'm using high pass mode. Feel free to play it. The next machine down is, pretty much, the same thing. If you play with the (right side) filter controls, you'll hear the same thing. But, I'm using low pass mode.
If you mix an audio signal with a low pass filtered, phase inverted version of itself, the low frequencies cancel out, leaving the high frequencies. And vice versa. If I'd used the high pass mode here, you'd hear a low pass filter. There's not a lot of point doing this with a state variable filter, but it's not the only filter in the toolbox.
The fat filters. Just something to call them, really. To put it, very simplistically, LPFs are delay effects. As you increase the delay time, more high frequencies go out of phase with each other and get quieter. And you can muck around with phase offsets by 'upstreaming' the audio. If you mix and match SVF outputs, with different phase inversions, then you can create different filter responses. Note. The band pass output of the SVF already is phase inverted.