docs: add soxr documentation

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg-resampler.texi

@@ -106,29 +106,54 @@ select triangular dither
 select triangular dither with high pass
 @end table
 
+@item resampler
+Set resampling engine. Default value is swr.
+
+Supported values:
+@table @samp
+@item swr
+select the native SW Resampler; filter options precision and cheby are not
+applicable in this case.
+@item soxr
+select the SoX Resampler (where available); compensation, and filter options
+filter_size, phase_shift, filter_type & kaiser_beta, are not applicable in this
+case.
+@end table
+
 @item filter_size
-Set resampling filter size, default value is 16.
+For swr only, set resampling filter size, default value is 16.
 
 @item phase_shift
-Set resampling phase shift, default value is 10, must be included
+For swr only, set resampling phase shift, default value is 10, must be included
 between 0 and 30.
 
 @item linear_interp
 Use Linear Interpolation if set to 1, default value is 0.
 
 @item cutoff
-Set cutoff frequency ratio. Must be a float value between 0 and 1,
-default value is 0.8.
+Set cutoff frequency (swr: 6dB point; soxr: 0dB point) ratio; must be a float
+value between 0 and 1.  Default value is 0.8 with swr, and 0.91 with soxr
+(which, with a sample-rate of 44100, preserves the entire audio band to 20kHz).
+
+@item precision
+For soxr only, the precision in bits to which the resampled signal will be
+calculated.  The default value of 20 (which, with suitable dithering, is
+appropriate for a destination bit-depth of 16) gives SoX's 'High Quality'; a
+value of 28 gives SoX's 'Very High Quality'.
+
+@item cheby
+For soxr only, selects passband rolloff none (Chebyshev) & higher-precision
+approximation for 'irrational' ratios. Default value is 0.
 
 @item min_comp
-Set the minimum difference between timestamps and audio data (in
+For swr only, set the minimum difference between timestamps and audio data (in
 seconds) to trigger stretching/squeezing/filling or trimming of the
 data to make it match the timestamps. The default is that
 stretching/squeezing/filling and trimming is disabled
 (@option{min_comp} = @code{FLT_MAX}).
 
 @item min_hard_comp
-Set the minimum difference between timestamps and audio data (in
+For swr only, set the minimum difference between timestamps and audio data (in
 seconds) to trigger adding/dropping samples to make it match the
 timestamps.  This option effectively is a threshold to select between
 hard (trim/fill) and soft (squeeze/stretch) compensation. Note that
@@ -136,14 +161,14 @@ all compensation is by default disabled through @option{min_comp}.
 The default is 0.1.
 
 @item comp_duration
-Set duration (in seconds) over which data is stretched/squeezed to
-make it match the timestamps. Must be a non-negative double float
-value, default value is 1.0.
+For swr only, set duration (in seconds) over which data is stretched/squeezed
+to make it match the timestamps. Must be a non-negative double float value,
+default value is 1.0.
 
 @item max_soft_comp
-Set maximum factor by which data is stretched/squeezed to make it
-match the timestamps. Must be a non-negative double float value,
-default value is 0.
+For swr only, set maximum factor by which data is stretched/squeezed to make it
+match the timestamps. Must be a non-negative double float value, default value
+is 0.
 
 @item matrix_encoding
 Select matrixed stereo encoding.
@@ -161,7 +186,7 @@ select Dolby Pro Logic II
 Default value is @code{none}.
 
 @item filter_type
-Select resampling filter type. This only affects resampling
+For swr only, select resampling filter type. This only affects resampling
 operations.
 
 It accepts the following values:
@@ -175,8 +200,8 @@ select Kaiser Windowed Sinc
 @end table
 
 @item kaiser_beta
-Set Kaiser Window Beta value. Must be an integer included between 2
-and 16, default value is 9.
+For swr only, set Kaiser Window Beta value. Must be an integer included between
+2 and 16, default value is 9.
 
 @end table
 

libswresample/swresample.c

@@ -80,15 +80,17 @@ static const AVOption options[]={
 {"triangular"           , "select triangular dither"    , 0                      , AV_OPT_TYPE_CONST, {.i64=SWR_DITHER_TRIANGULAR }, INT_MIN, INT_MAX   , PARAM, "dither_method"},
 {"triangular_hp"        , "select triangular dither with high pass" , 0                 , AV_OPT_TYPE_CONST, {.i64=SWR_DITHER_TRIANGULAR_HIGHPASS }, INT_MIN, INT_MAX, PARAM, "dither_method"},
 
-{"filter_size"          , "set resampling filter size"  , OFFSET(filter_size)    , AV_OPT_TYPE_INT  , {.i64=16                    }, 0      , INT_MAX   , PARAM },
-{"phase_shift"          , "set resampling phase shift"  , OFFSET(phase_shift)    , AV_OPT_TYPE_INT  , {.i64=10                    }, 0      , 30        , PARAM },
+{"filter_size"          , "set swr resampling filter size", OFFSET(filter_size)  , AV_OPT_TYPE_INT  , {.i64=16                    }, 0      , INT_MAX   , PARAM },
+{"phase_shift"          , "set swr resampling phase shift", OFFSET(phase_shift)  , AV_OPT_TYPE_INT  , {.i64=10                    }, 0      , 30        , PARAM },
 {"linear_interp"        , "enable linear interpolation" , OFFSET(linear_interp)  , AV_OPT_TYPE_INT  , {.i64=0                     }, 0      , 1         , PARAM },
 {"cutoff"               , "set cutoff frequency ratio"  , OFFSET(cutoff)         , AV_OPT_TYPE_DOUBLE,{.dbl=0.                    }, 0      , 1         , PARAM },
 {"resampler"            , "set resampling Engine"       , OFFSET(engine)         , AV_OPT_TYPE_INT  , {.i64=0                     }, 0      , SWR_ENGINE_NB-1, PARAM, "resampler"},
 {"swr"                  , "select SW Resampler"         , 0                      , AV_OPT_TYPE_CONST, {.i64=SWR_ENGINE_SWR        }, INT_MIN, INT_MAX   , PARAM, "resampler"},
 {"soxr"                 , "select SoX Resampler"        , 0                      , AV_OPT_TYPE_CONST, {.i64=SWR_ENGINE_SOXR       }, INT_MIN, INT_MAX   , PARAM, "resampler"},
-{"precision"            , "set resampling precision"    , OFFSET(precision)      , AV_OPT_TYPE_DOUBLE,{.dbl=20.0                  }, 15.0   , 33.0      , PARAM },
-{"cheby"                , "enable Chebyshev passband"   , OFFSET(cheby)          , AV_OPT_TYPE_INT  , {.i64=0                     }, 0      , 1         , PARAM },
+{"precision"            , "set soxr resampling precision (in bits)"
+                                                        , OFFSET(precision)      , AV_OPT_TYPE_DOUBLE,{.dbl=20.0                  }, 15.0   , 33.0      , PARAM },
+{"cheby"                , "enable soxr Chebyshev passband & higher-precision irrational ratio approximation"
+                                                        , OFFSET(cheby)          , AV_OPT_TYPE_INT  , {.i64=0                     }, 0      , 1         , PARAM },
 {"min_comp"             , "set minimum difference between timestamps and audio data (in seconds) below which no timestamp compensation of either kind is applied"
                                                         , OFFSET(min_compensation),AV_OPT_TYPE_FLOAT ,{.dbl=FLT_MAX               }, 0      , FLT_MAX   , PARAM },
 {"min_hard_comp"        , "set minimum difference between timestamps and audio data (in seconds) to trigger padding/trimming the data."
@@ -105,12 +107,12 @@ static const AVOption options[]={
     { "dolby", "select Dolby",              0, AV_OPT_TYPE_CONST, { .i64 = AV_MATRIX_ENCODING_DOLBY }, INT_MIN, INT_MAX, PARAM, "matrix_encoding" },
     { "dplii", "select Dolby Pro Logic II", 0, AV_OPT_TYPE_CONST, { .i64 = AV_MATRIX_ENCODING_DPLII }, INT_MIN, INT_MAX, PARAM, "matrix_encoding" },
 
-{ "filter_type"         , "select filter type"          , OFFSET(filter_type)    , AV_OPT_TYPE_INT  , { .i64 = SWR_FILTER_TYPE_KAISER }, SWR_FILTER_TYPE_CUBIC, SWR_FILTER_TYPE_KAISER, PARAM, "filter_type" },
+{ "filter_type"         , "select swr filter type"      , OFFSET(filter_type)    , AV_OPT_TYPE_INT  , { .i64 = SWR_FILTER_TYPE_KAISER }, SWR_FILTER_TYPE_CUBIC, SWR_FILTER_TYPE_KAISER, PARAM, "filter_type" },
     { "cubic"           , "select cubic"                , 0                      , AV_OPT_TYPE_CONST, { .i64 = SWR_FILTER_TYPE_CUBIC            }, INT_MIN, INT_MAX, PARAM, "filter_type" },
     { "blackman_nuttall", "select Blackman Nuttall Windowed Sinc", 0             , AV_OPT_TYPE_CONST, { .i64 = SWR_FILTER_TYPE_BLACKMAN_NUTTALL }, INT_MIN, INT_MAX, PARAM, "filter_type" },
     { "kaiser"          , "select Kaiser Windowed Sinc" , 0                      , AV_OPT_TYPE_CONST, { .i64 = SWR_FILTER_TYPE_KAISER           }, INT_MIN, INT_MAX, PARAM, "filter_type" },
 
-{ "kaiser_beta"         , "set Kaiser Window Beta"      , OFFSET(kaiser_beta)    , AV_OPT_TYPE_INT  , {.i64=9                     }, 2      , 16        , PARAM },
+{ "kaiser_beta"         , "set swr Kaiser Window Beta"  , OFFSET(kaiser_beta)    , AV_OPT_TYPE_INT  , {.i64=9                     }, 2      , 16        , PARAM },
 
 {0}
 };

libswresample/swresample_internal.h

@@ -74,17 +74,17 @@ struct SwrContext {
     int filter_size;                                /**< length of each FIR filter in the resampling filterbank relative to the cutoff frequency */
     int phase_shift;                                /**< log2 of the number of entries in the resampling polyphase filterbank */
     int linear_interp;                              /**< if 1 then the resampling FIR filter will be linearly interpolated */
-    double cutoff;                                  /**< resampling cutoff frequency. 1.0 corresponds to half the output sample rate */
-    enum SwrFilterType filter_type;                 /**< resampling filter type */
-    int kaiser_beta;                                /**< beta value for Kaiser window (only applicable if filter_type == AV_FILTER_TYPE_KAISER) */
-    double precision;                               /**< resampling precision (in bits) */
-    int cheby;                                      /**< if 1 then the resampling FIR filter will be configured for maximal passband flatness */
-
-    float min_compensation;                         ///< minimum below which no compensation will happen
-    float min_hard_compensation;                    ///< minimum below which no silence inject / sample drop will happen
-    float soft_compensation_duration;               ///< duration over which soft compensation is applied
-    float max_soft_compensation;                    ///< maximum soft compensation in seconds over soft_compensation_duration
-    float async;                                    ///< simple 1 parameter async, similar to ffmpegs -async
+    double cutoff;                                  /**< resampling cutoff frequency (swr: 6dB point; soxr: 0dB point). 1.0 corresponds to half the output sample rate */
+    enum SwrFilterType filter_type;                 /**< swr resampling filter type */
+    int kaiser_beta;                                /**< swr beta value for Kaiser window (only applicable if filter_type == AV_FILTER_TYPE_KAISER) */
+    double precision;                               /**< soxr resampling precision (in bits) */
+    int cheby;                                      /**< soxr: if 1 then passband rolloff will be none (Chebyshev) & irrational ratio approximation precision will be higher */
+
+    float min_compensation;                         ///< swr minimum below which no compensation will happen
+    float min_hard_compensation;                    ///< swr minimum below which no silence inject / sample drop will happen
+    float soft_compensation_duration;               ///< swr duration over which soft compensation is applied
+    float max_soft_compensation;                    ///< swr maximum soft compensation in seconds over soft_compensation_duration
+    float async;                                    ///< swr simple 1 parameter async, similar to ffmpegs -async
 
     int resample_first;                             ///< 1 if resampling must come first, 0 if rematrixing
     int rematrix;                                   ///< flag to indicate if rematrixing is needed (basically if input and output layouts mismatch)