[alsa-devel] Mixer volume/decibel mapping

Sebastian H. vand2 at gmx.de
Fri May 21 10:18:41 CEST 2010

Am Freitag, 21. Mai 2010 03:30:20 schrieb Raymond Yau:
> 2010/5/21 James Courtier-Dutton <james.dutton at gmail.com>
> > On 20 May 2010 10:41, Sebastian H. <vand2 at gmx.de> wrote:
> > > That's clear.
> > > I imagined that maybe the dB scale could be the "real thing" and the
> >
> > volume
> >
> > > steps would be a convenient but possibly coarse mapping into a denser
> > > stepped dB space provided by the hardware/driver.
> > > This was unlikely but I wanted to ask anyway ;-).
> >
> > I believe it could be extended to support a continuous range.
> > For example, some hardware uses a 32bit value to control the gain.
> > The mix operation is simply "the_sample" * "the_control_value".
> > So in this case the hardware has a linear control that can be
> > considered continuous for our purposes.
> > The conversion from the control value to a usable value would have to
> > include a user land log operation.
> > It was never implemented due to lack of demand.
> > For the cards that do have this 32bit linear value, a simple lookup
> > table is used in the kernel code to convert it to 256 step values.
> > If implemented, one could then introduce professional gain controls
> > that do not produce "clicks" in the output when adjusted.
> > gain controls with steps in them produce clicks when the gain control
> > is adjusted.
> I thinks Sebastian's question is whether he can use
> snd_mixer_selem_set_playback_
> dB() set any values in between the volume steps
> e.g. Master volume control of ac97 is 1.5 dB per step,
> when using snd_mixer_selem_set_playback_dB() to set -0.9dB , how can the
> mixer application know what dB value has been set since the value is in
> between -1.5dB and 0dB without snd_mixer_selem_get_playbackdB()

Indeed I quite happy with what ALSA already provides (I'm just an interested 
amateur). My main problem is that there is no documentation for the Mixer API
and how things are related to each other. Like CTL, HCTL, Mixer and 
SimpleMixer and of course volume and dB in the snd_mixer_selem_ functions.

The root issue is that I wondered if the mixer application should offer a
separate text input for decibel values besides the text input for volume 
values. As pointed out this would not allow for finer grained volume control
therefore for the moment I'll probably stick with a mere text label for dB.

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