Filter asoftclip
  Audio Soft Clipper.
    Inputs:
       #0: default (audio)
    Outputs:
       #0: default (audio)
asoftclip AVOptions:
  type              <int>        ..F.A.... set softclip type (from 0 to 6) (default tanh)
     tanh            0            ..F.A....
     atan            1            ..F.A....
     cubic           2            ..F.A....
     exp             3            ..F.A....
     alg             4            ..F.A....
     quintic         5            ..F.A....
     sin             6            ..F.A....
  param             <double>     ..F.A.... set softclip parameter (from 0.01 to 3) (default 1)

This filter has support for timeline through the 'enable' option.

