avfilter: add high-order parametric multiband equalizer filter

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

doc/filters.texi

@@ -992,6 +992,100 @@ stream ends. The default value is 2 seconds.
 
 @end table
 
+@section anequalizer
+
+High-order parametric multiband equalizer for each channel.
+
+It accepts the following parameters:
+@table @option
+@item params
+
+This option string is in format:
+"c@var{chn} f=@var{cf} w=@var{w} g=@var{g} t=@var{f} | ..."
+Each equalizer band is separated by '|'.
+
+@table @option
+@item chn
+Set channel number to which equalization will be applied.
+If input doesn't have that channel the entry is ignored.
+
+@item cf
+Set central frequency for band.
+If input doesn't have that frequency the entry is ignored.
+
+@item w
+Set band width in hertz.
+
+@item g
+Set band gain in dB.
+
+@item f
+Set filter type for band, optional, can be:
+
+@table @samp
+@item 0
+Butterworth, this is default.
+
+@item 1
+Chebyshev type 1.
+
+@item 2
+Chebyshev type 2.
+@end table
+@end table
+
+@item curves
+With this option activated frequency response of anequalizer is displayed
+in video stream.
+
+@item size
+Set video stream size. Only useful if curves option is activated.
+
+@item mgain
+Set max gain that will be displayed. Only useful if curves option is activated.
+Setting this to reasonable value allows to display gain which is derived from
+neighbour bands which are too close to each other and thus produce higher gain
+when both are activated.
+
+@item fscale
+Set frequency scale used to draw frequency response in video output.
+Can be linear or logarithmic. Default is logarithmic.
+
+@item colors
+Set color for each channel curve which is going to be displayed in video stream.
+This is list of color names separated by space or by '|'.
+Unrecognised or missing colors will be replaced by white color.
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+Lower gain by 10 of central frequency 200Hz and width 100 Hz
+for first 2 channels using Chebyshev type 1 filter:
+@example
+anequalizer=c0 f=200 w=100 g=-10 t=1|c1 f=200 w=100 g=-10 t=1
+@end example
+@end itemize
+
+@subsection Commands
+
+This filter supports the following commands:
+@table @option
+@item change
+Alter existing filter parameters.
+Syntax for the commands is : "@var{fN}|f=@var{freq}|w=@var{width}|g=@var{gain}"
+
+@var{fN} is existing filter number, starting from 0, if no such filter is available
+error is returned.
+@var{freq} set new frequency parameter.
+@var{width} set new width parameter in herz.
+@var{gain} set new gain parameter in dB.
+
+Full filter invocation with asendcmd may look like this:
+asendcmd=c='4.0 anequalizer change 0|f=200|w=50|g=1',anequalizer=...
+@end table
+
 @section anull
 
 Pass the audio source unchanged to the output.