Filter tremolo
  Apply tremolo effect.
    Inputs:
       #0: default (audio)
    Outputs:
       #0: default (audio)
tremolo AVOptions:
  f                 <double>     ..F.A.... set frequency in hertz (from 0.1 to 20000) (default 5)
  d                 <double>     ..F.A.... set depth as percentage (from 0 to 1) (default 0.5)


