Filter lowpass
  Apply a low-pass filter with 3dB point frequency.
    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                            ..F.A... Hz
     q                            ..F.A... Q-Factor
     o                            ..F.A... octave
     s                            ..F.A... slope
     k                            ..F.A... kHz
  t                 <int>        ..F.A... set filter-width type (from 1 to 5) (default q)
     h                            ..F.A... Hz
     q                            ..F.A... Q-Factor
     o                            ..F.A... octave
     s                            ..F.A... slope
     k                            ..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)
  channels          <channel_layout> ..F.A... set channels to filter (default 0xffffffffffffffff)
  c                 <channel_layout> ..F.A... set channels to filter (default 0xffffffffffffffff)


