[pitivi] waveform: Make sure to downscale to mono before generating waveforms

[Thibault Saunier <tsaunier src gnome org>]

commit e5d33d16919a3350e89305b282ded29a3a9b8007
Author: Thibault Saunier <tsaunier gnome org>
Date:   Thu Nov 2 16:35:30 2017 -0300

    waveform: Make sure to downscale to mono before generating waveforms
    
    Otherwise only right/left channels are taken into account by the level element
    
    Reviewed-by: Alex Băluț <<alexandru balut gmail com>>
    Differential Revision: https://phabricator.freedesktop.org/D1876

 pitivi/timeline/previewers.py |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/pitivi/timeline/previewers.py b/pitivi/timeline/previewers.py
index 02d5041..b40f7f7 100644
--- a/pitivi/timeline/previewers.py
+++ b/pitivi/timeline/previewers.py
@@ -186,7 +186,8 @@ class WaveformPreviewer(PreviewerBin):
 
     def __init__(self):
         PreviewerBin.__init__(self,
-                              "audioconvert ! audioresample ! level name=level"
+                              "audioconvert ! audioresample ! "
+                              "audio/x-raw,channels=1 ! level name=level"
                               " ! audioconvert ! audioresample")
         self.level = self.internal_bin.get_by_name("level")
         self.debug("Creating waveforms!!")