Post Syndicated from Lennart Poettering original https://0pointer.net/blog/projects/decibel-data.html

#nocomments y

In recent versions PulseAudio
integrates the numerous mixer
elements ALSA exposes into one single powerful slider which tries to make
the best of the granularity and range of the hardware and extends that in
software so that we can provide an equally powerful slider on all systems.
That means if your hardware only supports a limited volume range (many
integrated USB speakers for example cannot be completely muted with the
hardware volume slider), limited granularity (some hardware sliders only have 8
steps or so), or no per-channel volumes (many sound cards have a single slider
that covers all channels), then PulseAudio tries its best to make use of the
next hardware volume slider in the pipeline to compensate for that, and so on,
finally falling back to software for everything that cannot be done in
hardware. This is
explained in more detail here.

Now this algorithm depends on that we know the actual attenuation factors
(factors like that are usually written in units of dB which is why I will call
this the “dB data” from now on) of the hardware volume controls. Thankfully
ALSA includes that information in its driver interfaces. However for some
hardware this data is not reliable. For example, one of my own cards (a
Terratec Aureon 5.1 MkII USB) contains invalid dB data in its USB descriptor
and ALSA passes that on to PulseAudio. The effect of that is that the
PulseAudio volume control behaves very weirdly for this card, in a way that the
volume “jumps” and changes in unexpected ways (or doesn’t change at all in some
ranges!) when you slowly move the slider, or that the volume is completely
muted over large ranges of the slider where it should not be. Also this breaks the
flat volume logic in PulseAudo, to the result that playing one stream
(let’s say a music stream) and then adding a second one (let’s say an event
sound) might incorrectly attenuate the first one (i.e. whenever you play an
event sound the music changes in volume).

Incorrect dB data is not a new problem. However PulseAudio is the first
application that actually depends on the correctness of this data. Previously
the dB info was shown as auxiliary information in some volume controls, and
only noticed and understood by very few, technical people. It was not used for
further calculations.

Now, the reasons I am writing this blog posting are firstly to inform you
about this type of bug and the results it has on the logic PulseAudio
implements, and secondly (and more importantly) to point you to this little Wiki page I wrote
that explains how to verify if this is indeed a problem on your card (in case
you are experiencing any of the symptoms mentioned above) and secondly what to
do to improve the situation, and how to get correct dB data that can be
included as quirk in your driver.

Thank you for your attention.