[FFmpeg-cvslog] avfilter/af_acrossover: document roll-off of each filter order

[Paul B Mahol]

ffmpeg | branch: master | Paul B Mahol <onemda at gmail.com> | Wed Dec  2 12:28:01 2020 +0100| [0a5ff1964355f6d288071b7c0bc4fb24f658c9fc] | committer: Paul B Mahol

avfilter/af_acrossover: document roll-off of each filter order

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=0a5ff1964355f6d288071b7c0bc4fb24f658c9fc
---

 doc/filters.texi            | 14 +++++++++++++-
 libavfilter/af_acrossover.c | 22 +++++++++++-----------
 2 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/doc/filters.texi b/doc/filters.texi
index d2afc7115d..f7b8f92629 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -523,19 +523,31 @@ The filter accepts the following options:
 Set split frequencies. Those must be positive and increasing.
 
 @item order
-Set filter order. Available values are:
+Set filter order for each band split. This controls filter roll-off or steepness
+of filter transfer function.
+Available values are:
 
 @table @samp
 @item 2nd
+12 dB per octave.
 @item 4th
+24 dB per octave.
 @item 6th
+36 dB per octave.
 @item 8th
+48 dB per octave.
 @item 10th
+60 dB per octave.
 @item 12th
+72 dB per octave.
 @item 14th
+84 dB per octave.
 @item 16th
+96 dB per octave.
 @item 18th
+108 dB per octave.
 @item 20th
+120 dB per octave.
 @end table
 
 Default is @var{4th}.
diff --git a/libavfilter/af_acrossover.c b/libavfilter/af_acrossover.c
index 0be2f3a76f..241f1dbb77 100644
--- a/libavfilter/af_acrossover.c
+++ b/libavfilter/af_acrossover.c
@@ -83,17 +83,17 @@ typedef struct AudioCrossoverContext {
 
 static const AVOption acrossover_options[] = {
     { "split", "set split frequencies", OFFSET(splits_str), AV_OPT_TYPE_STRING, {.str="500"}, 0, 0, AF },
-    { "order", "set order",             OFFSET(order_opt),  AV_OPT_TYPE_INT,    {.i64=1},     0, 9, AF, "m" },
-    { "2nd",   "2nd order",             0,                  AV_OPT_TYPE_CONST,  {.i64=0},     0, 0, AF, "m" },
-    { "4th",   "4th order",             0,                  AV_OPT_TYPE_CONST,  {.i64=1},     0, 0, AF, "m" },
-    { "6th",   "6th order",             0,                  AV_OPT_TYPE_CONST,  {.i64=2},     0, 0, AF, "m" },
-    { "8th",   "8th order",             0,                  AV_OPT_TYPE_CONST,  {.i64=3},     0, 0, AF, "m" },
-    { "10th",  "10th order",            0,                  AV_OPT_TYPE_CONST,  {.i64=4},     0, 0, AF, "m" },
-    { "12th",  "12th order",            0,                  AV_OPT_TYPE_CONST,  {.i64=5},     0, 0, AF, "m" },
-    { "14th",  "14th order",            0,                  AV_OPT_TYPE_CONST,  {.i64=6},     0, 0, AF, "m" },
-    { "16th",  "16th order",            0,                  AV_OPT_TYPE_CONST,  {.i64=7},     0, 0, AF, "m" },
-    { "18th",  "18th order",            0,                  AV_OPT_TYPE_CONST,  {.i64=8},     0, 0, AF, "m" },
-    { "20th",  "20th order",            0,                  AV_OPT_TYPE_CONST,  {.i64=9},     0, 0, AF, "m" },
+    { "order", "set filter order",      OFFSET(order_opt),  AV_OPT_TYPE_INT,    {.i64=1},     0, 9, AF, "m" },
+    { "2nd",   "2nd order (12 dB/8ve)", 0,                  AV_OPT_TYPE_CONST,  {.i64=0},     0, 0, AF, "m" },
+    { "4th",   "4th order (24 dB/8ve)", 0,                  AV_OPT_TYPE_CONST,  {.i64=1},     0, 0, AF, "m" },
+    { "6th",   "6th order (36 dB/8ve)", 0,                  AV_OPT_TYPE_CONST,  {.i64=2},     0, 0, AF, "m" },
+    { "8th",   "8th order (48 dB/8ve)", 0,                  AV_OPT_TYPE_CONST,  {.i64=3},     0, 0, AF, "m" },
+    { "10th",  "10th order (60 dB/8ve)",0,                  AV_OPT_TYPE_CONST,  {.i64=4},     0, 0, AF, "m" },
+    { "12th",  "12th order (72 dB/8ve)",0,                  AV_OPT_TYPE_CONST,  {.i64=5},     0, 0, AF, "m" },
+    { "14th",  "14th order (84 dB/8ve)",0,                  AV_OPT_TYPE_CONST,  {.i64=6},     0, 0, AF, "m" },
+    { "16th",  "16th order (96 dB/8ve)",0,                  AV_OPT_TYPE_CONST,  {.i64=7},     0, 0, AF, "m" },
+    { "18th",  "18th order (108 dB/8ve)",0,                 AV_OPT_TYPE_CONST,  {.i64=8},     0, 0, AF, "m" },
+    { "20th",  "20th order (120 dB/8ve)",0,                 AV_OPT_TYPE_CONST,  {.i64=9},     0, 0, AF, "m" },
     { "level", "set input gain",        OFFSET(level_in),   AV_OPT_TYPE_FLOAT,  {.dbl=1},     0, 1, AF },
     { NULL }
 };