avfilter/af_silenceremove: add standard deviation detector

Useful in cases audio samples DC offset is not ~0.0, where
other detectors will fail to detect silence.

doc/filters.texi

@@ -6474,6 +6474,8 @@ Maximum of absolute values of samples in moving window.
 Median of absolute values of samples in moving window.
 @item ptp
 Absolute of max peak to min peak difference of samples in moving window.
+@item dev
+Standard deviation of values of samples in moving window.
 @end table
 Default value is @code{rms}.
 

libavfilter/af_silenceremove.c

@@ -38,6 +38,7 @@ enum SilenceDetect {
     D_PEAK,
     D_MEDIAN,
     D_PTP,
+    D_DEV,
     D_NB
 };
 
@@ -146,6 +147,7 @@ static const AVOption silenceremove_options[] = {
     {   "peak",          "use max absolute values of samples",                 0,                           AV_OPT_TYPE_CONST,    {.i64=D_PEAK},0,         0, AF, "detection" },
     {   "median",        "use median of absolute values of samples",           0,                           AV_OPT_TYPE_CONST,    {.i64=D_MEDIAN},0,       0, AF, "detection" },
     {   "ptp",           "use absolute of max peak to min peak difference",    0,                           AV_OPT_TYPE_CONST,    {.i64=D_PTP}, 0,         0, AF, "detection" },
+    {   "dev",           "use standard deviation from values of samples",      0,                           AV_OPT_TYPE_CONST,    {.i64=D_DEV}, 0,         0, AF, "detection" },
     { "window",          "set duration of window for silence detection",       OFFSET(window_duration_opt), AV_OPT_TYPE_DURATION, {.i64=20000}, 0, 100000000, AF },
     { "timestamp",       "set how every output frame timestamp is processed",  OFFSET(timestamp_mode),      AV_OPT_TYPE_INT,      {.i64=TS_WRITE}, 0, TS_NB-1, AF, "timestamp" },
     {   "write",         "full timestamps rewrite, keep only the start time",  0,                           AV_OPT_TYPE_CONST,    {.i64=TS_WRITE}, 0,       0, AF, "timestamp" },
@@ -230,6 +232,9 @@ static int config_output(AVFilterLink *outlink)
     case D_RMS:
         s->cache_size = 1;
         break;
+    case D_DEV:
+        s->cache_size = 2;
+        break;
     case D_MEDIAN:
     case D_PEAK:
     case D_PTP:
@@ -263,6 +268,10 @@ static int config_output(AVFilterLink *outlink)
         s->compute_flt = compute_avg_flt;
         s->compute_dbl = compute_avg_dbl;
         break;
+    case D_DEV:
+        s->compute_flt = compute_dev_flt;
+        s->compute_dbl = compute_dev_dbl;
+        break;
     case D_PTP:
         s->compute_flt = compute_ptp_flt;
         s->compute_dbl = compute_ptp_dbl;

libavfilter/silenceremove_template.c

@@ -307,6 +307,23 @@ static ftype fn(compute_rms)(ftype *cache, ftype sample, ftype wsample,
     return SQRT(r / window_size);
 }
 
+static ftype fn(compute_dev)(ftype *ss, ftype x, ftype px,
+                             int n, int *unused, int *unused2)
+{
+    ftype r;
+
+    ss[0] += x;
+    ss[0] -= px;
+
+    ss[1] += x * x;
+    ss[1] -= px * px;
+    ss[1] = FMAX(ss[1], ZERO);
+
+    r = FMAX(ss[1] - ss[0] * ss[0] / n, ZERO) / n;
+
+    return SQRT(r);
+}
+
 static void fn(filter_start)(AVFilterContext *ctx,
                              const ftype *src, ftype *dst,
                              int *nb_out_samples,