FFmpeg 3.1.11
Since* 3.0
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Linearly increases the difference between left and right channels which adds some sort of "live" effect to playback.

The filter accepts the following option:

m

Sets the difference coefficient (default: 2.5). 0.0 means mono sound (average of both channels), with 1.0 sound will be unchanged, with -1.0 left and right channels will be swapped.

c

Enable clipping. By default is enabled.