lavfi: add compand audio filter

Signed-off-by: Anton Khirnov <anton@khirnov.net>

Changelog

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 releases are sorted from youngest to oldest.
 
+version <next>:
+- compand audio filter
+
+
 version 10:
 - av_strnstr
 - support ID3v2 tags in ASF files

doc/filters.texi

@@ -467,6 +467,79 @@ To fix a 5.1 WAV improperly encoded in AAC's native channel order
 avconv -i in.wav -filter 'channelmap=1|2|0|5|3|4:channel_layout=5.1' out.wav
 @end example
 
+@section compand
+Compress or expand audio dynamic range.
+
+A description of the accepted options follows.
+
+@table @option
+
+@item attacks
+@item decays
+Set list of times in seconds for each channel over which the instantaneous level
+of the input signal is averaged to determine its volume. @var{attacks} refers to
+increase of volume and @var{decays} refers to decrease of volume. For most
+situations, the attack time (response to the audio getting louder) should be
+shorter than the decay time because the human ear is more sensitive to sudden
+loud audio than sudden soft audio. A typical value for attack is 0.3 seconds and
+a typical value for decay is 0.8 seconds.
+
+@item points
+Set list of points for the transfer function, specified in dB relative to the
+maximum possible signal amplitude. Each key points list must be defined using
+the following syntax: @code{x0/y0|x1/y1|x2/y2|....}
+
+The input values must be in strictly increasing order but the transfer function
+does not have to be monotonically rising. The point @code{0/0} is assumed but
+may be overridden (by @code{0/out-dBn}). Typical values for the transfer
+function are @code{-70/-70|-60/-20}.
+
+@item soft-knee
+Set the curve radius in dB for all joints. Defaults to 0.01.
+
+@item gain
+Set additional gain in dB to be applied at all points on the transfer function.
+This allows easy adjustment of the overall gain. Defaults to 0.
+
+@item volume
+Set initial volume in dB to be assumed for each channel when filtering starts.
+This permits the user to supply a nominal level initially, so that, for
+example, a very large gain is not applied to initial signal levels before the
+companding has begun to operate. A typical value for audio which is initially
+quiet is -90 dB. Defaults to 0.
+
+@item delay
+Set delay in seconds. The input audio is analyzed immediately, but audio is
+delayed before being fed to the volume adjuster. Specifying a delay
+approximately equal to the attack/decay times allows the filter to effectively
+operate in predictive rather than reactive mode. Defaults to 0.
+
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+Make music with both quiet and loud passages suitable for listening in a noisy
+environment:
+@example
+compand=.3|.3:1|1:-90/-60|-60/-40|-40/-30|-20/-20:6:0:-90:0.2
+@end example
+
+@item
+Noise gate for when the noise is at a lower level than the signal:
+@example
+compand=.1|.1:.2|.2:-900/-900|-50.1/-900|-50/-50:.01:0:-90:.1
+@end example
+
+@item
+Here is another noise gate, this time for when the noise is at a higher level
+than the signal (making it, in some ways, similar to squelch):
+@example
+compand=.1|.1:.1|.1:-45.1/-45.1|-45/-900|0/-900:.01:45:-90:.1
+@end example
+@end itemize
+
 @section join
 Join multiple input streams into one multi-channel stream.
 

libavfilter/Makefile

@@ -34,6 +34,7 @@ OBJS-$(CONFIG_ASYNCTS_FILTER)                += af_asyncts.o
 OBJS-$(CONFIG_ATRIM_FILTER)                  += trim.o
 OBJS-$(CONFIG_CHANNELMAP_FILTER)             += af_channelmap.o
 OBJS-$(CONFIG_CHANNELSPLIT_FILTER)           += af_channelsplit.o
+OBJS-$(CONFIG_COMPAND_FILTER)                += af_compand.o
 OBJS-$(CONFIG_JOIN_FILTER)                   += af_join.o
 OBJS-$(CONFIG_RESAMPLE_FILTER)               += af_resample.o
 OBJS-$(CONFIG_VOLUME_FILTER)                 += af_volume.o

libavfilter/allfilters.c

@@ -54,6 +54,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER(ATRIM,          atrim,          af);
     REGISTER_FILTER(CHANNELMAP,     channelmap,     af);
     REGISTER_FILTER(CHANNELSPLIT,   channelsplit,   af);
+    REGISTER_FILTER(COMPAND,        compand,        af);
     REGISTER_FILTER(JOIN,           join,           af);
     REGISTER_FILTER(RESAMPLE,       resample,       af);
     REGISTER_FILTER(VOLUME,         volume,         af);

libavfilter/version.h

@@ -30,8 +30,8 @@
 #include "libavutil/version.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  4
-#define LIBAVFILTER_VERSION_MINOR  1
-#define LIBAVFILTER_VERSION_MICRO  1
+#define LIBAVFILTER_VERSION_MINOR  2
+#define LIBAVFILTER_VERSION_MICRO  0
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \