FFmpeg 4.0.6
Since* 1.2
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Apply a two-pole all-pass filter with central frequency (in Hz) frequency, and filter-width width. An all-pass filter changes the audio’s frequency to phase relationship without changing its frequency to amplitude relationship.

The filter accepts the following options:

frequency, f

Set frequency in Hz.

width_type, t

Set method to specify band-width of filter.

h

Hz

q

Q-Factor

o

octave

s

slope

k

kHz

width, w

Specify the band-width of a filter in width_type units.

channels, c

Specify which channels to filter, by default all available are filtered.

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Commands

This filter supports the following commands:

frequency, f

Change allpass frequency. Syntax for the command is : "frequency"

width_type, t

Change allpass width_type. Syntax for the command is : "width_type"

width, w

Change allpass width. Syntax for the command is : "width"