Mono.EffectSourceA compander following the mu-law (see http://en.wikipedia.org/wiki/Mu-law).
class biquad_filter : int -> [ `Band_pass
| `High_pass
| `Low_pass
| `Notch
| `All_pass
| `Peaking
| `Low_shelf
| `High_shelf ] -> ?gain:float -> float -> float -> t