lavfi: add atempo filter

Add atempo audio filter for adjusting audio tempo without affecting
pitch. This filter implements WSOLA algorithm with fast cross
correlation calculation in frequency domain.
Signed-off-by: NPavel Koshevoy <pavel@homestead.aragog.com>
Signed-off-by: NStefano Sabatini <stefasab@gmail.com>

Changelog

@@ -7,6 +7,7 @@ version next:
 - Indeo Audio decoder
 - channelsplit audio filter
 - setnsamples audio filter
+- atempo filter
 
 
 version 0.11:

MAINTAINERS

@@ -275,6 +275,7 @@ Video filters:
   graphdump.c                           Nicolas George
   af_amerge.c                           Nicolas George
   af_astreamsync.c                      Nicolas George
+  af_atempo.c                           Pavel Koshevoy
   af_pan.c                              Nicolas George
   vsrc_mandelbrot.c                     Michael Niedermayer
   vf_yadif.c                            Michael Niedermayer

configure

@@ -1702,6 +1702,7 @@ amovie_filter_deps="avcodec avformat"
 aresample_filter_deps="swresample"
 ass_filter_deps="libass"
 asyncts_filter_deps="avresample"
+atempo_filter_deps="avcodec"
 blackframe_filter_deps="gpl"
 boxblur_filter_deps="gpl"
 colormatrix_filter_deps="gpl"

doc/filters.texi

@@ -406,6 +406,24 @@ amovie=file.ogg [a] ; amovie=file.mp3 [b] ;
 [a2] [b2] amerge
 @end example
 
+@section atempo
+
+Adjust audio tempo.
+
+The filter accepts exactly one parameter, the audio tempo. If not
+specified then the filter will assume nominal 1.0 tempo. Tempo must
+be in the [0.5, 2.0] range.
+
+For example, to slow down audio to 80% tempo:
+@example
+atempo=0.8
+@end example
+
+For example, to speed up audio to 125% tempo:
+@example
+atempo=1.25
+@end example
+
 @section earwax
 
 Make audio easier to listen to on headphones.

libavfilter/Makefile

@@ -9,6 +9,7 @@ FFLIBS-$(CONFIG_SCALE_FILTER)   += swscale
 FFLIBS-$(CONFIG_ACONVERT_FILTER)             += swresample
 FFLIBS-$(CONFIG_AMOVIE_FILTER)               += avformat avcodec
 FFLIBS-$(CONFIG_ARESAMPLE_FILTER)            += swresample
+FFLIBS-$(CONFIG_ATEMPO_FILTER)               += avcodec
 FFLIBS-$(CONFIG_MOVIE_FILTER)                += avformat avcodec
 FFLIBS-$(CONFIG_PAN_FILTER)                  += swresample
 FFLIBS-$(CONFIG_REMOVELOGO_FILTER)           += avformat avcodec
@@ -56,6 +57,7 @@ OBJS-$(CONFIG_ASHOWINFO_FILTER)              += af_ashowinfo.o
 OBJS-$(CONFIG_ASPLIT_FILTER)                 += split.o
 OBJS-$(CONFIG_ASTREAMSYNC_FILTER)            += af_astreamsync.o
 OBJS-$(CONFIG_ASYNCTS_FILTER)                += af_asyncts.o
+OBJS-$(CONFIG_ATEMPO_FILTER)                 += af_atempo.o
 OBJS-$(CONFIG_CHANNELSPLIT_FILTER)           += af_channelsplit.o
 OBJS-$(CONFIG_EARWAX_FILTER)                 += af_earwax.o
 OBJS-$(CONFIG_PAN_FILTER)                    += af_pan.o

libavfilter/allfilters.c

@@ -45,6 +45,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER (ASPLIT,      asplit,      af);
     REGISTER_FILTER (ASTREAMSYNC, astreamsync, af);
     REGISTER_FILTER (ASYNCTS,     asyncts,     af);
+    REGISTER_FILTER (ATEMPO,      atempo,      af);
     REGISTER_FILTER (CHANNELSPLIT,channelsplit,af);
     REGISTER_FILTER (EARWAX,      earwax,      af);
     REGISTER_FILTER (PAN,         pan,         af);

libavfilter/version.h

@@ -29,7 +29,7 @@
 #include "libavutil/avutil.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  2
-#define LIBAVFILTER_VERSION_MINOR 80
+#define LIBAVFILTER_VERSION_MINOR 81
 #define LIBAVFILTER_VERSION_MICRO 100
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \