FFmpeg 4.3.8
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.

mix, m

How much to use filtered signal in output. Default is 1. Range is between 0 and 1.

channels, c

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

normalize, n

Normalize biquad coefficients, by default is disabled. Enabling it will normalize magnitude response at DC to 0dB.

order, o

Set the filter order, can be 1 or 2. Default is 2.

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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"

mix, m

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