Add MIXER_MAX_VOLUME setting for capping the max volume

docs/changes.rst

@@ -34,6 +34,7 @@ greatly improved MPD client support.
   ``SPOTIFY_LIB_APPKEY`` setting is thus removed.
 - Added new :mod:`mopidy.mixers.GStreamerSoftwareMixer` which now is the
   default mixer on all platforms.
+- New setting ``MIXER_MAX_VOLUME`` for capping the maximum output volume.
 - MPD frontend:
 
   - Relocate from :mod:`mopidy.mpd` to :mod:`mopidy.frontends.mpd`.

mopidy/mixers/__init__.py

@@ -1,3 +1,5 @@
+from mopidy import settings
+
 class BaseMixer(object):
     """
     :param backend: a backend instance
@@ -6,6 +8,7 @@ class BaseMixer(object):
 
     def __init__(self, backend, *args, **kwargs):
         self.backend = backend
+        self.amplification_factor = settings.MIXER_MAX_VOLUME / 100.0
 
     @property
     def volume(self):
@@ -15,11 +18,11 @@ class BaseMixer(object):
         Integer in range [0, 100]. :class:`None` if unknown. Values below 0 is
         equal to 0. Values above 100 is equal to 100.
         """
-        return self._get_volume()
+        return self._get_volume() / self.amplification_factor
 
     @volume.setter
     def volume(self, volume):
-        volume = int(volume)
+        volume = int(volume) * self.amplification_factor
         if volume < 0:
             volume = 0
         elif volume > 100:

mopidy/settings.py

@@ -115,6 +115,16 @@ MIXER_EXT_SPEAKERS_A = None
 #: Default: :class:`None`.
 MIXER_EXT_SPEAKERS_B = None
 
+#: The maximum volume. Integer in the range 0 to 100.
+#:
+#: If this settings is set to 80, the mixer will set the actual volume to 80
+#: when asked to set it to 100.
+#:
+#: Default::
+#:
+#:     MIXER_MAX_VOLUME = 100
+MIXER_MAX_VOLUME = 100
+
 #: Audio output handler to use.
 #:
 #: Default::