Filter lowpass
  Apply a low-pass filter with 3dB point frequency.
    slice threading supported
    Inputs:
       #0: default (audio)
    Outputs:
       #0: default (audio)
lowpass AVOptions:
  frequency         <double>     ..F.A.... set frequency (from 0 to 999999) (default 500)
  f                 <double>     ..F.A.... set frequency (from 0 to 999999) (default 500)
  width_type        <int>        ..F.A.... set filter-width type (from 1 to 5) (default q)
     h               1            ..F.A.... Hz
     q               3            ..F.A.... Q-Factor
     o               2            ..F.A.... octave
     s               4            ..F.A.... slope
     k               5            ..F.A.... kHz
  t                 <int>        ..F.A.... set filter-width type (from 1 to 5) (default q)
     h               1            ..F.A.... Hz
     q               3            ..F.A.... Q-Factor
     o               2            ..F.A.... octave
     s               4            ..F.A.... slope
     k               5            ..F.A.... kHz
  width             <double>     ..F.A.... set width (from 0 to 99999) (default 0.707)
  w                 <double>     ..F.A.... set width (from 0 to 99999) (default 0.707)
  poles             <int>        ..F.A.... set number of poles (from 1 to 2) (default 2)
  p                 <int>        ..F.A.... set number of poles (from 1 to 2) (default 2)
  mix               <double>     ..F.A.... set mix (from 0 to 1) (default 1)
  m                 <double>     ..F.A.... set mix (from 0 to 1) (default 1)
  channels          <channel_layout> ..F.A.... set channels to filter (default 0xffffffffffffffff)
  c                 <channel_layout> ..F.A.... set channels to filter (default 0xffffffffffffffff)

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

