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-b option for the norm effect; can be used to fix stereo imbalance

--- a/soxeffect.7

+++ b/soxeffect.7

@@ -804,8 +804,9 @@

 	sox noisy.au -n trim 0 1 noiseprof | play noisy.au noisered

 .EE

 .TP

-\fBnorm\fR [\fB\-i\fR] [\fIlevel\fR]

-Normalise audio to 0dB FSD or to a given level relative to 0dB.

+\fBnorm\fR [\fB\-i\fR\^|\^\fB\-b\fR] [\fIlevel\fR]

+Normalise audio to 0dB FSD, to a given level relative to 0dB, or normalise

+the balance of multi-channel audio.

 Requires temporary file space to store the audio to be normalised.

 .SP

 To create a normalised copy of an audio file,

@@ -836,6 +837,15 @@

 .B \-i

 option is given, then each channel is treated individually and

 will attain the normalised level.

+.SP

+If the

+.B \-b

+option is given (with a multi-channel audio file), then the audio

+channels will be balanced; i.e. the RMS level of each channel will be

+normalised to that of the channel with the highest RMS level.  This can

+be used, for example, to correct stereo imbalance.  Note that

+.B \-b

+can cause clipping.

 .SP

 In most cases,

 .B norm \-3

--- a/src/normalise.c

+++ b/src/normalise.c

@@ -19,9 +19,11 @@

 #include <string.h>

 typedef struct {

-  sox_bool      individual;

+  sox_bool      individual, balance;

   double        norm0;    /* Multiplier to take to 0dB FSD */

   double        level;    /* Multiplier to take to 'level' */

+  double        rms;

+  off_t         num_samples;

   sox_sample_t  min, max;

   FILE          * tmp_file;

 } priv_t;

@@ -30,6 +32,7 @@

 {

   priv_t * p = (priv_t *)effp->priv;

   if (argc && !strcmp(*argv, "-i")) p->individual = sox_true, ++argv, --argc;

+  if (argc && !strcmp(*argv, "-b")) p->balance = sox_true, ++argv, --argc;

   do {NUMERIC_PARAMETER(level, -100, 0)} while (0);

   p->level = dB_to_linear(p->level);

   return argc?  lsx_usage(effp) : SOX_SUCCESS;

@@ -38,7 +41,7 @@

 static int start(sox_effect_t * effp)

 {

   priv_t * p = (priv_t *)effp->priv;

-  if (!p->individual)

+  if (!(p->individual || p->balance))

     effp->flows = 1;

   p->norm0 = p->max = p->min = 0;

   p->tmp_file = tmpfile();

@@ -59,7 +62,13 @@

     sox_fail("error writing temporary file: %s", strerror(errno));

     return SOX_EOF;

   }

-  for (len = *osamp; len; --len, ++ibuf) {

+  if (p->balance) for (len = *osamp; len; --len, ++ibuf) {

+    size_t dummy = 0;

+    double d = SOX_SAMPLE_TO_FLOAT_64BIT(*ibuf, dummy);

+    p->rms += sqr(d);

+    ++p->num_samples;

+  }

+  else for (len = *osamp; len; --len, ++ibuf) {

     p->max = max(p->max, *ibuf);

     p->min = min(p->min, *ibuf);

   }

@@ -70,12 +79,23 @@

 static int drain(sox_effect_t * effp, sox_sample_t * obuf, size_t * osamp)

 {

   priv_t * p = (priv_t *)effp->priv;

-  size_t len;

+  size_t len, i;

   int result = SOX_SUCCESS;

   if (!p->norm0) {

-    double max = lsx_sample_max(effp->out_encoding);

-    p->norm0 = p->level * min(max / p->max, (double)SOX_SAMPLE_MIN / p->min);

+    if (p->balance) {

+      double max_rms = 0, this_rms = sqrt(p->rms / p->num_samples);

+      sox_effect_t * effects = effp - effp->flow;

+      for (i = 0; i < effp->flows; ++i) {

+        priv_t * q = (priv_t *)(effects + i)->priv;

+        max_rms = max(max_rms, sqrt(q->rms / q->num_samples));

+      }

+      p->norm0 = p->level * (this_rms != 0? max_rms / this_rms : 1);

+    }

+    else {

+      double max = lsx_sample_max(effp->out_encoding);

+      p->norm0 = p->level * min(max / p->max, (double)SOX_SAMPLE_MIN / p->min);

+    }

     rewind(p->tmp_file);

   }

   len = fread(obuf, sizeof(*obuf), *osamp, p->tmp_file);

@@ -83,7 +103,9 @@

     sox_fail("error reading temporary file: %s", strerror(errno));

     result = SOX_EOF;

   }

-  for (*osamp = len; len; --len, ++obuf)

+  if (p->balance) for (*osamp = len; len; --len, ++obuf)

+    *obuf = SOX_ROUND_CLIP_COUNT(*obuf * p->norm0, effp->clips);

+  else for (*osamp = len; len; --len, ++obuf)

     *obuf = floor(*obuf * p->norm0 + .5);

   return result;

 }

@@ -97,7 +119,7 @@

 sox_effect_handler_t const * sox_norm_effect_fn(void)

 {

-  static sox_effect_handler_t handler = {"norm", "[-i] [level]", 0,

+  static sox_effect_handler_t handler = {"norm", "[-i|-b] [level]", 0,

     create, start, flow, drain, stop, NULL, sizeof(priv_t)};

   return &handler;

 }