avfilter: add sidechain compress audio filter

Signed-off-by: Paul B Mahol <onemda@gmail.com>

Changelog

@@ -24,6 +24,7 @@ version <next>:
 - Random filter
 - deband filter
 - AAC fixed-point decoding
+- sidechaincompress audio filter
 
 
 version 2.7:

doc/filters.texi

@@ -622,6 +622,7 @@ slope
 Specify the band-width of a filter in width_type units.
 @end table
 
+@anchor{amerge}
 @section amerge
 
 Merge two or more audio streams into a single multi-channel stream.
@@ -2020,6 +2021,7 @@ Applies only to double-pole filter.
 The default is 0.707q and gives a Butterworth response.
 @end table
 
+@anchor{pan}
 @section pan
 
 Mix channels with specific gain levels. The filter accepts the output
@@ -2121,6 +2123,66 @@ At end of filtering it displays @code{track_gain} and @code{track_peak}.
 Convert the audio sample format, sample rate and channel layout. It is
 not meant to be used directly.
 
+@section sidechaincompress
+
+This filter acts like normal compressor but has the ability to compress
+detected signal using second input signal.
+It needs two input streams and returns one output stream.
+First input stream will be processed depending on second stream signal.
+The filtered signal then can be filtered with other filters in later stages of
+processing. See @ref{pan} and @ref{amerge} filter.
+
+The filter accepts the following options:
+
+@table @option
+@item threshold
+If a signal of second stream raises above this level it will affect the gain
+reduction of first stream.
+By default is 0.125. Range is between 0.00097563 and 1.
+
+@item ratio
+Set a ratio about which the signal is reduced. 1:2 means that if the level
+raised 4dB above the threshold, it will be only 2dB above after the reduction.
+Default is 2. Range is between 1 and 20.
+
+@item attack
+Amount of milliseconds the signal has to rise above the threshold before gain
+reduction starts. Default is 20. Range is between 0.01 and 2000.
+
+@item release
+Amount of milliseconds the signal has to fall bellow the threshold before
+reduction is decreased again. Default is 250. Range is between 0.01 and 9000.
+
+@item makeup
+Set the amount by how much signal will be amplified after processing.
+Default is 2. Range is from 1 and 64.
+
+@item knee
+Curve the sharp knee around the threshold to enter gain reduction more softly.
+Default is 2.82843. Range is between 1 and 8.
+
+@item link
+Choose if the @code{average} level between all channels of side-chain stream
+or the louder(@code{maximum}) channel of side-chain stream affects the
+reduction. Default is @code{average}.
+
+@item detection
+Should the exact signal be taken in case of @code{peak} or an RMS one in case
+of @code{rms}. Default is @code{rms} which is mainly smoother.
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+Full ffmpeg example taking 2 audio inputs, 1st input to be compressed
+depending on the signal of 2nd input and later compressed signal to be
+merged with 2nd input:
+@example
+ffmpeg -i main.flac -i sidechain.flac -filter_complex "[1:a]asplit=2[sc][mix];[0:a][sc]sidechaincompress[compr];[compr][mix]amerge"
+@end example
+@end itemize
+
 @section silencedetect
 
 Detect silence in an audio stream.

libavfilter/Makefile

@@ -80,6 +80,7 @@ OBJS-$(CONFIG_LOWPASS_FILTER)                += af_biquads.o
 OBJS-$(CONFIG_PAN_FILTER)                    += af_pan.o
 OBJS-$(CONFIG_REPLAYGAIN_FILTER)             += af_replaygain.o
 OBJS-$(CONFIG_RESAMPLE_FILTER)               += af_resample.o
+OBJS-$(CONFIG_SIDECHAINCOMPRESS_FILTER)      += af_sidechaincompress.o
 OBJS-$(CONFIG_SILENCEDETECT_FILTER)          += af_silencedetect.o
 OBJS-$(CONFIG_SILENCEREMOVE_FILTER)          += af_silenceremove.o
 OBJS-$(CONFIG_TREBLE_FILTER)                 += af_biquads.o

libavfilter/allfilters.c

@@ -96,6 +96,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER(PAN,            pan,            af);
     REGISTER_FILTER(REPLAYGAIN,     replaygain,     af);
     REGISTER_FILTER(RESAMPLE,       resample,       af);
+    REGISTER_FILTER(SIDECHAINCOMPRESS, sidechaincompress, af);
     REGISTER_FILTER(SILENCEDETECT,  silencedetect,  af);
     REGISTER_FILTER(SILENCEREMOVE,  silenceremove,  af);
     REGISTER_FILTER(TREBLE,         treble,         af);

libavfilter/version.h

@@ -30,7 +30,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  5
-#define LIBAVFILTER_VERSION_MINOR  28
+#define LIBAVFILTER_VERSION_MINOR  29
 #define LIBAVFILTER_VERSION_MICRO 100
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \