Filter afade
  Fade in/out input audio.
    Inputs:
       #0: default (audio)
    Outputs:
       #0: default (audio)
afade AVOptions:
  type              <int>        ..F.A.... set the fade direction (from 0 to 1) (default in)
     in                           ..F.A.... fade-in
     out                          ..F.A.... fade-out
  t                 <int>        ..F.A.... set the fade direction (from 0 to 1) (default in)
     in                           ..F.A.... fade-in
     out                          ..F.A.... fade-out
  start_sample      <int64>      ..F.A.... set number of first sample to start fading (from 0 to I64_MAX) (default 0)
  ss                <int64>      ..F.A.... set number of first sample to start fading (from 0 to I64_MAX) (default 0)
  nb_samples        <int64>      ..F.A.... set number of samples for fade duration (from 1 to I64_MAX) (default 44100)
  ns                <int64>      ..F.A.... set number of samples for fade duration (from 1 to I64_MAX) (default 44100)
  start_time        <duration>   ..F.A.... set time to start fading (default 0)
  st                <duration>   ..F.A.... set time to start fading (default 0)
  duration          <duration>   ..F.A.... set fade duration (default 0)
  d                 <duration>   ..F.A.... set fade duration (default 0)
  curve             <int>        ..F.A.... set fade curve type (from 0 to 15) (default tri)
     tri                          ..F.A.... linear slope
     qsin                         ..F.A.... quarter of sine wave
     esin                         ..F.A.... exponential sine wave
     hsin                         ..F.A.... half of sine wave
     log                          ..F.A.... logarithmic
     ipar                         ..F.A.... inverted parabola
     qua                          ..F.A.... quadratic
     cub                          ..F.A.... cubic
     squ                          ..F.A.... square root
     cbr                          ..F.A.... cubic root
     par                          ..F.A.... parabola
     exp                          ..F.A.... exponential
     iqsin                        ..F.A.... inverted quarter of sine wave
     ihsin                        ..F.A.... inverted half of sine wave
     dese                         ..F.A.... double-exponential seat
     desi                         ..F.A.... double-exponential sigmoid
  c                 <int>        ..F.A.... set fade curve type (from 0 to 15) (default tri)
     tri                          ..F.A.... linear slope
     qsin                         ..F.A.... quarter of sine wave
     esin                         ..F.A.... exponential sine wave
     hsin                         ..F.A.... half of sine wave
     log                          ..F.A.... logarithmic
     ipar                         ..F.A.... inverted parabola
     qua                          ..F.A.... quadratic
     cub                          ..F.A.... cubic
     squ                          ..F.A.... square root
     cbr                          ..F.A.... cubic root
     par                          ..F.A.... parabola
     exp                          ..F.A.... exponential
     iqsin                        ..F.A.... inverted quarter of sine wave
     ihsin                        ..F.A.... inverted half of sine wave
     dese                         ..F.A.... double-exponential seat
     desi                         ..F.A.... double-exponential sigmoid

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

