[gstreamer-bugs] [Bug 567660] [API] need a stream volume interface for sinks that do volume control

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=567660
  GStreamer | gst-plugins-base | git

--- Comment #12 from Lennart Poettering <lennart at poettering.net> 2009-09-11 13:57:05 UTC ---
One thing to keep in mind: if you define the interface like this it is only
implementable by sinks that output to some software sound service, or to very
few hardware devices. The reason for that is that not for all hardware we
actually know the volume factor each possible volume step refers to. For
example Bluetooth audio devices generally don't communicate which volume step
means which amplification. They just offer an opaque integer in a range 0..15
where 0 is somehow less loud than 15, but that's all we know. Something similar
is true for many USB devices and so on.

Of course, this interface is about stream, not device volumes, so this probably
isn't a real problem. And if some day we want to support 'opaque' stream
volumes then we could still add another interface for that. But all I wanted to
make sure is that this is kept in mind.

Two comments though: it might be worth adding comments that the dB scale you
chose is for amplitude, not power. (i.e. 20*log10, not 10*log10). Also, you
might want to add a comment making clear that the cubic scale should be used
for presentation in the UI. And referring to
http://www.robotplanet.dk/audio/audio_gui_design/ or
http://lists.linuxaudio.org/pipermail/linux-audio-dev/2009-May/thread.html#23151
might be a good idea, too.

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