[alsa-devel] Mixer volume/decibel mapping

[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()