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ladspa: support M:N channel mapping in plugins

This allows plugins such as the LADSPA vocoder to work.

Using the formant.wav and carrier.wav samples from
http://www.sirlab.de/linux/descr_vocoder.html the following
command-line is successful:

sox -M formant.wav carrier.wav \
  -b 16 result.wav rate 44100 \
  ladspa vocoder 16 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Using 16-bit (or higher) output reduces the noise-floor hiss
greatly since LADSPA processing is done as 32-bit float.

--- a/src/ladspa.c

+++ b/src/ladspa.c

@@ -195,18 +195,6 @@

     }

   }

-  /*

-   * very few LADSPA plugins support changing the number of channels,

-   * so we won't support it unless somebody has the time and resources

-   * for it.  The "mixer" plugin in cmt is one example that would fail here,

-   * but SoX already has the native capability to remix channels.

-   */

-  if (l_st->input_count != l_st->output_count) {

-    lsx_fail("mismatched input/output port count (%u != %u)",

-             (unsigned)l_st->input_count, (unsigned)l_st->output_count);

-    return SOX_EOF;

-  }

-

   /* Stop if we have any unused arguments */

   return argc? lsx_usage(effp) : SOX_SUCCESS;

 }

@@ -226,6 +214,7 @@

   if (l_st->input_count == 1 && l_st->output_count == 1 &&

       effp->in_signal.channels == effp->out_signal.channels) {

+    /* for mono plugins, they are common */

     if (!l_st->clone && effp->in_signal.channels > 1) {

       lsx_fail("expected 1 input channel(s), found %u; consider using -r",

@@ -255,20 +244,22 @@

                (unsigned)l_st->input_count, effp->in_signal.channels);

       return SOX_EOF;

     }

-    if (l_st->output_count < effp->out_signal.channels) {

-      lsx_fail("fewer plugin output ports than output channels (%u < %u)",

-               (unsigned)l_st->output_count, effp->out_signal.channels);

-      return SOX_EOF;

-    }

     /* warn if LADSPA audio ports are unused.  ecasound does this, too */

     if (l_st->input_count > effp->in_signal.channels)

       lsx_warn("more plugin input ports than input channels (%u > %u)",

                (unsigned)l_st->input_count, effp->in_signal.channels);

-    if (l_st->output_count > effp->out_signal.channels)

-      lsx_warn("more plugin output ports than input channels (%u > %u)",

-               (unsigned)l_st->output_count, effp->out_signal.channels);

+    /*

+     * some LADSPA plugins increase/decrease the channel count

+     * (e.g. "mixer" in cmt or vocoder):

+     */

+    if (l_st->output_count != effp->out_signal.channels) {

+      lsx_debug("changing output channels to match plugin output ports (%u => %u)",

+               effp->out_signal.channels, (unsigned)l_st->output_count);

+      effp->out_signal.channels = l_st->output_count;

+    }

+

     l_st->handle_count = 1;

     l_st->handles = lsx_malloc(sizeof(LADSPA_Handle *));

     l_st->handles[0] = l_st->desc->instantiate(l_st->desc, rate);

@@ -321,12 +312,17 @@

   const size_t total_input_count = l_st->input_count * l_st->handle_count;

   const size_t total_output_count = l_st->output_count * l_st->handle_count;

   const size_t input_len = len / total_input_count;

-  const size_t output_len = len / total_output_count;

+  size_t output_len = len / total_output_count;

-  *osamp = *isamp = len;

+  if (total_output_count < total_input_count)

+    output_len = input_len;

+  *isamp = len;

+  *osamp = 0;

+

   if (len) {

-    LADSPA_Data *buf = lsx_malloc(sizeof(LADSPA_Data) * len);

+    LADSPA_Data *buf = lsx_calloc(len, sizeof(LADSPA_Data));

+    LADSPA_Data *outbuf = lsx_calloc(len, sizeof(LADSPA_Data));

     LADSPA_Handle handle;

     unsigned long port;

     SOX_SAMPLE_LOCALS;

@@ -354,7 +350,7 @@

     for (j = 0; j < total_output_count; j++) {

       handle = l_st->handles[j / l_st->output_count];

       port = l_st->outputs[j / l_st->handle_count];

-      l_st->desc->connect_port(handle, port, buf + j * output_len);

+      l_st->desc->connect_port(handle, port, outbuf + j * output_len);

     }

     /* Run the plugin for each handle */

@@ -364,11 +360,13 @@

     /* Grab output if effect produces it, re-interleaving it */

     for (i = 0; i < output_len; i++) {

       for (j = 0; j < total_output_count; j++) {

-        LADSPA_Data d = buf[j * output_len + i];

+        LADSPA_Data d = outbuf[j * output_len + i];

         *obuf++ = LADSPA_DATA_TO_SOX_SAMPLE(d, effp->clips);

+        (*osamp)++;

       }

     }

+    free(outbuf);

     free(buf);

   }

@@ -421,7 +419,7 @@

 static sox_effect_handler_t sox_ladspa_effect = {

   "ladspa",

   "MODULE [PLUGIN] [ARGUMENT...]",

-  SOX_EFF_MCHAN | SOX_EFF_GAIN,

+  SOX_EFF_MCHAN | SOX_EFF_CHAN | SOX_EFF_GAIN,

   sox_ladspa_getopts,

   sox_ladspa_start,

   sox_ladspa_flow,