avcodec/libopusenc: change default frame duration to 20 ms

20 ms is used by libopus encoder.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/encoders.texi

@@ -1063,7 +1063,7 @@ Set maximum frame size, or duration of a frame in milliseconds. The
 argument must be exactly the following: 2.5, 5, 10, 20, 40, 60. Smaller
 frame sizes achieve lower latency but less quality at a given bitrate.
 Sizes greater than 20ms are only interesting at fairly low bitrates.
-The default of FFmpeg is 10ms, but is 20ms in @command{opusenc}.
+The default is 20ms.
 
 @item packet_loss (@emph{expect-loss})
 Set expected packet loss percentage. The default is 0.

libavcodec/libopusenc.c

@@ -400,7 +400,7 @@ static const AVOption libopus_options[] = {
         { "voip",           "Favor improved speech intelligibility",   0, AV_OPT_TYPE_CONST, { .i64 = OPUS_APPLICATION_VOIP },                0, 0, FLAGS, "application" },
         { "audio",          "Favor faithfulness to the input",         0, AV_OPT_TYPE_CONST, { .i64 = OPUS_APPLICATION_AUDIO },               0, 0, FLAGS, "application" },
         { "lowdelay",       "Restrict to only the lowest delay modes", 0, AV_OPT_TYPE_CONST, { .i64 = OPUS_APPLICATION_RESTRICTED_LOWDELAY }, 0, 0, FLAGS, "application" },
-    { "frame_duration", "Duration of a frame in milliseconds", OFFSET(frame_duration), AV_OPT_TYPE_FLOAT, { .dbl = 10.0 }, 2.5, 60.0, FLAGS },
+    { "frame_duration", "Duration of a frame in milliseconds", OFFSET(frame_duration), AV_OPT_TYPE_FLOAT, { .dbl = 20.0 }, 2.5, 60.0, FLAGS },
     { "packet_loss",    "Expected packet loss percentage",     OFFSET(packet_loss),    AV_OPT_TYPE_INT,   { .i64 = 0 },    0,   100,  FLAGS },
     { "vbr",            "Variable bit rate mode",              OFFSET(vbr),            AV_OPT_TYPE_INT,   { .i64 = 1 },    0,   2,    FLAGS, "vbr" },
         { "off",            "Use constant bit rate", 0, AV_OPT_TYPE_CONST, { .i64 = 0 }, 0, 0, FLAGS, "vbr" },