[alsa-devel] amixer: convert percentage into db wrongly

[Takashi Iwai]

At Fri, 9 Mar 2012 12:22:47 +0800,
Adam Lee wrote:
> 
> Add Vincent in cc, because conky read amixer's result.
> 
> On Thu, Mar 08, 2012 at 05:45:14PM +0100, Takashi Iwai wrote:
> > > * Adam Lee <adam8157 at gmail.com> [2012-03-08 20:36 +0800]:
> > > 
> > > Package: alsa-utils
> > > Version: 1.0.25-1
> > > Severity: important
> > > 
> > > 
> > > db is not linear, but amixer believe it is.
> > > 
> > > "amixer get Master" says "Limits: Playback 0 - 74", then everytime I run
> > > "amixer -q sset Master 10%-", there is 8db dec.
> > > 
> > > For example, at first Master is 100% and 0db, both alsamixer and amixer
> > > think it is, and after I run "amixer -q sset Master 10%-", both
> > > alsamixer and amixer says Master is -8.00db, but alsamixer says it is
> > > 72%, amixer says it is 89%.
> > > 
> > > alsamixer is right, amixer calc and set wrongly.
> > 
> > No, both are correct.  You are dreaming too much on the world unified
> > percentage representation :)
> > 
> > The percentage in amixer has nothing to do with dB level.
> > It's just the percentage of the raw value range of that mixer
> > element.  Thus showing 89% is correct.  It's 10% down from 100%
> > (1% is because of the resolution of the raw values).
> > 
> > Now, alsamixer shows the percentage in a different way.  It's
> > explained well in the source code (alsamixer/volume_mapping.c), but
> > not mentioned in the man page, unfortunately.
> > 
> >  * The mapping is designed so that the position in the interval is proportional
> >  * to the volume as a human ear would perceive it (i.e., the position is the
> >  * cubic root of the linear sample multiplication factor).  For controls with
> >  * a small range (24 dB or less), the mapping is linear in the dB values so
> >  * that each step has the same size visually.  Only for controls without dB
> >  * information, a linear mapping of the hardware volume register values is used
> >  * (this is the same algorithm as used in the old alsamixer).
> > 
> > The percentage representation in alsamixer corresponds to this
> > mapping, thus it's neither dB nor linear percent.
> > 
> 
> Hi, Takashi
> 
> Thank you for replying. But I still insist this is a bug. Three
> questions:
> 
> 1, several months ago, it's OK, both amixer and alsamixer use the human
> mapping(0-10% and 90%-100% are the same change by a human ear), why not
> now?

amixer hasn't been changed until yet.  It handles either in raw values
or in dB.  No human-ear mapping at all.  It's never changed since
years, and won't be changed.  If the volume mapping would be
implemented to amixer in future, it must be only optional.

Only the recent alsamixer introduced the volume mapping to visualize
the volumes reasonably.

> 2, conky(Vincent, I mean ${mixer}), some other software, lot of user's
> scripts use amixer to set or get volume, expecting the human mapping,
> why change the behavior?

You must be smoking something bad.  The behavior of amixer hasn't been
changed.

> 3, alsamixer and amixer use the same dB value, why there is difference
> in percentage? If alsa-utils developer think the human mapping sucks,
> why you guys still use it in alsamixer? There is no "both correct", the
> difference confuses user...

That's true.  alsamixer should have stopped showing the stupid
percentage.

The biggest understand is that people (including you) think there is
an absolutely perfect percentage definition for the sound level.  It's
an illusion.

> IMO: Any, any human says 10% plus, she or he definitely wants the human
> mapping. Maybe you developing guys think there is nothing wrong now, but
> how about think it from the perspective of user?
> 
> Please consider about fixing it, at least discuss it in alsa-utils mail
> list, thank you.

There is no such ML...


Takashi