lavfi: add aecho filter

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

Changelog

@@ -3,6 +3,8 @@ releases are sorted from youngest to oldest.
 
 version <next>
 
+- aecho filter
+
 
 version 2.0:
 

doc/filters.texi

@@ -347,6 +347,66 @@ aconvert=u8:auto
 @end example
 @end itemize
 
+@section aecho
+
+Apply echoing to the input audio.
+
+Echoes are reflected sound and can occur naturally amongst mountains
+(and sometimes large buildings) when talking or shouting; digital echo
+effects emulate this behaviour and are often used to help fill out the
+sound of a single instrument or vocal. The time difference between the
+original signal and the reflection is the @code{delay}, and the
+loudness of the reflected signal is the @code{decay}.
+Multiple echoes can have different delays and decays.
+
+A description of the accepted parameters follows.
+
+@table @option
+@item in_gain
+Set input gain of reflected signal. Default is @code{0.6}.
+
+@item out_gain
+Set output gain of reflected signal. Default is @code{0.3}.
+
+@item delays
+Set list of time intervals in milliseconds between original signal and reflections
+separated by '|'. Allowed range for each @code{delay} is @code{(0 - 90000.0]}.
+Default is @code{1000}.
+
+@item decays
+Set list of loudnesses of reflected signals separated by '|'.
+Allowed range for each @code{decay} is @code{(0 - 1.0]}.
+Default is @code{0.5}.
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+Make it sound as if there are twice as many instruments as are actually playing:
+@example
+aecho=0.8:0.88:60:0.4
+@end example
+
+@item
+If delay is very short, then it sound like a (metallic) robot playing music:
+@example
+aecho=0.8:0.88:6:0.4
+@end example
+
+@item
+A longer delay will sound like an open air concert in the mountains:
+@example
+aecho=0.8:0.9:1000:0.3
+@end example
+
+@item
+Same as above but with one more mountain:
+@example
+aecho=0.8:0.9:1000|1800:0.3|0.25
+@end example
+@end itemize
+
 @section afade
 
 Apply fade-in/out effect to input audio.

libavfilter/Makefile

@@ -52,6 +52,7 @@ OBJS-$(CONFIG_AVFORMAT)                      += lavfutils.o
 OBJS-$(CONFIG_SWSCALE)                       += lswsutils.o
 
 OBJS-$(CONFIG_ACONVERT_FILTER)               += af_aconvert.o
+OBJS-$(CONFIG_AECHO_FILTER)                  += af_aecho.o
 OBJS-$(CONFIG_AFADE_FILTER)                  += af_afade.o
 OBJS-$(CONFIG_AFORMAT_FILTER)                += af_aformat.o
 OBJS-$(CONFIG_AINTERLEAVE_FILTER)            += f_interleave.o

libavfilter/allfilters.c

@@ -48,6 +48,7 @@ void avfilter_register_all(void)
 #if FF_API_ACONVERT_FILTER
     REGISTER_FILTER(ACONVERT,       aconvert,       af);
 #endif
+    REGISTER_FILTER(AECHO,          aecho,          af);
     REGISTER_FILTER(AFADE,          afade,          af);
     REGISTER_FILTER(AFORMAT,        aformat,        af);
     REGISTER_FILTER(AINTERLEAVE,    ainterleave,    af);

libavfilter/version.h

@@ -30,8 +30,8 @@
 #include "libavutil/avutil.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  3
-#define LIBAVFILTER_VERSION_MINOR  79
-#define LIBAVFILTER_VERSION_MICRO 101
+#define LIBAVFILTER_VERSION_MINOR  80
+#define LIBAVFILTER_VERSION_MICRO 100
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \