Above filter_biquad(), I'd add what these xs and ys mean. Yes, you
do follow the established standard naming convention when dealing with
IIR filters, but not all pulseaudio hackers are DSP specialists. I.e.
please add something like:
"Here x0, x1 and x2 are the most recent input sample, the previous one
and the second-previous one. y0, y1 and y2 are the output samples
corresponding to x0, x1 and x2."
Ok.