From 5b968e8250d83ff42b282adf14976ee8425907b0 Mon Sep 17 00:00:00 2001
From: Steve Daulton <Stevethefiddle@users.noreply.github.com>
Date: Tue, 12 Apr 2016 21:58:08 +0100
Subject: [PATCH] Fix bug 1374

---
 src/export/Export.cpp       | 28 +++++++++++++++++++++++-----
 src/export/Export.h         |  3 +++
 src/export/ExportMP3.cpp    | 10 ++++++++++
 src/prefs/WarningsPrefs.cpp |  7 +++++--
 4 files changed, 41 insertions(+), 7 deletions(-)

diff --git a/src/export/Export.cpp b/src/export/Export.cpp
index 92cdc69f9..e525f580a 100644
--- a/src/export/Export.cpp
+++ b/src/export/Export.cpp
@@ -748,6 +748,7 @@ bool Exporter::CheckMix()
    // and if mixing will occur.
 
    int downMix = gPrefs->Read(wxT("/FileFormats/ExportDownMix"), true);
+   int exportedChannels = mPlugins[mFormat]->SetNumExportChannels();
 
    if (downMix) {
       if (mNumRight > 0 || mNumLeft > 0) {
@@ -763,7 +764,18 @@ bool Exporter::CheckMix()
       int numRight = mNumRight + mNumMono;
 
       if (numLeft > 1 || numRight > 1 || mNumLeft + mNumRight + mNumMono > mChannels) {
-         if (mChannels == 2) {
+         wxString exportFormat = mPlugins[mFormat]->GetFormat(mSubFormat);
+         if (exportFormat != wxT("CL") && exportFormat != wxT("FFMPEG") && exportedChannels == -1)
+            exportedChannels = mChannels;
+
+         if (exportedChannels == 1) {
+            if (ShowWarningDialog(mProject,
+                                  wxT("MixMono"),
+                                  _("Your tracks will be mixed down to a single mono channel in the exported file."),
+                                  true) == wxID_CANCEL)
+               return false;
+         }
+         else if (exportedChannels == 2) {
             if (ShowWarningDialog(mProject,
                                   wxT("MixStereo"),
                                   _("Your tracks will be mixed down to two stereo channels in the exported file."),
@@ -772,8 +784,8 @@ bool Exporter::CheckMix()
          }
          else {
             if (ShowWarningDialog(mProject,
-                                  wxT("MixMono"),
-                                  _("Your tracks will be mixed down to a single mono channel in the exported file."),
+                                  wxT("MixUnknownChannels"),
+                                  _("Your tracks will be mixed down to one exported file according to the encoder settings."),
                                   true) == wxID_CANCEL)
                return false;
          }
@@ -781,9 +793,12 @@ bool Exporter::CheckMix()
    }
    else
    {
+      if (exportedChannels == -1)
+         exportedChannels = mPlugins[mFormat]->GetMaxChannels(mSubFormat);
+
       ExportMixerDialog md(mProject->GetTracks(),
                            mSelectedOnly,
-                           mPlugins[mFormat]->GetMaxChannels(mSubFormat),
+                           exportedChannels,
                            NULL,
                            1,
                            _("Advanced Mixing Options"));
@@ -1259,8 +1274,11 @@ ExportMixerDialog::ExportMixerDialog( const TrackList *tracks, bool selectedOnly
    // state the number of channels - so we assume there are always at least two.
    // The downside is that if someone is exporting to a mono device, the dialog
    // will allow them to output to two channels. Hmm.  We may need to revisit this.
+
    if (maxNumChannels < 2 )
-      maxNumChannels = 2;
+      // STF (April 2016): AMR (narrowband) and MP3 may export 1 channel.
+      // maxNumChannels = 2;
+      maxNumChannels = 1;
    if (maxNumChannels > 32)
       maxNumChannels = 32;
 
diff --git a/src/export/Export.h b/src/export/Export.h
index 3244366ee..ff130b2fc 100644
--- a/src/export/Export.h
+++ b/src/export/Export.h
@@ -90,6 +90,9 @@ public:
    virtual wxWindow *OptionsCreate(wxWindow *parent, int format) = 0;
 
    virtual bool CheckFileName(wxFileName &filename, int format = 0);
+   /** @brief Exporter plug-ins may override this to specify the number
+    * of channels in exported file. -1 for unspecified */
+   virtual int SetNumExportChannels() { return -1; }
 
    /** \brief called to export audio into a file.
     *
diff --git a/src/export/ExportMP3.cpp b/src/export/ExportMP3.cpp
index d692afb02..bf5249396 100644
--- a/src/export/ExportMP3.cpp
+++ b/src/export/ExportMP3.cpp
@@ -1605,6 +1605,7 @@ private:
 #ifdef USE_LIBID3TAG
    void AddFrame(struct id3_tag *tp, const wxString & n, const wxString & v, const char *name);
 #endif
+   int SetNumExportChannels() override;
 };
 
 ExportMP3::ExportMP3()
@@ -1636,6 +1637,15 @@ bool ExportMP3::CheckFileName(wxFileName & WXUNUSED(filename), int WXUNUSED(form
    return true;
 }
 
+int ExportMP3::SetNumExportChannels()
+{
+   bool mono;
+   gPrefs->Read(wxT("/FileFormats/MP3ForceMono"), &mono, 0);
+
+   return (mono)? 1 : -1;
+}
+
+
 int ExportMP3::Export(AudacityProject *project,
                        int channels,
                        const wxString &fName,
diff --git a/src/prefs/WarningsPrefs.cpp b/src/prefs/WarningsPrefs.cpp
index cb459028e..d6058fa86 100644
--- a/src/prefs/WarningsPrefs.cpp
+++ b/src/prefs/WarningsPrefs.cpp
@@ -63,11 +63,14 @@ void WarningsPrefs::PopulateOrExchange(ShuttleGui & S)
       S.TieCheckBox(_("&Low disk space at program start up"),
                     wxT("/Warnings/DiskSpaceWarning"),
                     true);
+      S.TieCheckBox(_("Mixing down to &mono during export"),
+                    wxT("/Warnings/MixMono"),
+                    true);
       S.TieCheckBox(_("Mixing down to &stereo during export"),
                     wxT("/Warnings/MixStereo"),
                     true);
-      S.TieCheckBox(_("Mixing down to &mono during export"),
-                    wxT("/Warnings/MixMono"),
+      S.TieCheckBox(_("Mixing down on export (Custom FFmpeg or external program)"),
+                    wxT("/Warnings/MixUnknownChannels"),
                     true);
       S.TieCheckBox(_("&Importing uncompressed audio files"),
                     wxT("/Warnings/CopyOrEditUncompressedDataAsk"),