[alsa-devel] Mixer volume/decibel mapping

[Raymond Yau]

2010/6/7 Sebastian H. <vand2 at gmx.de>

> Am 07.06.2010 05:37, schrieb Raymond Yau:
> >>> 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.
> >>
> >>
>
> > The main difference between those volume controls using TLV_DB_SCALE and
> > TLV_DB_LINEAR is -6dB is at the middle of TLV_DB_LINEAR and TLV_DB_SCALE
> has
> > fixed dB value between any two steps.
>
> What's TLV_DB_SCALE and TLV_DB_LINEAR for?
> A grep in the alsa-lib sources did not return anything.
>
> > https://bugs.launchpad.net/ubuntu/+source/linux/+bug/332631
> >
> > if the user has both hda onboard sound and emu10k1, he may notice that
> there
> > is big difference if he set volume at percentage instead of dB
>
> Interesting, that's my setup. I have an onboard HDA Intel and a
> pci SBLive! Card and some el cheapo USB sound cards to experiment
> with. Thanks I'll give the volume listening test a try at some point.
>


https://bugtrack.alsa-project.org/alsa-bug/view.php?id=4883

Pekka Lampila reported that when he set -40dB and only get back -9999999 by
his program alsa_mixer_test.c

I don't have emu10k1 so don't know whether this is a bug of alsa-lib or not