[pulseaudio-discuss] Mic Boost on AC97 audio causes bad behavior of mic volume control

Wilck, Martin martin.wilck at ts.fujitsu.com
Wed Apr 22 02:43:28 PDT 2015

On Mi, 2015-04-22 at 10:24 +0800, Raymond Yau wrote:
> The Capture Volume and Mic Boost are similar to fine adjustment and
> coarse adjustment of the microscope

Tank you for the excellent analogy. Microscope users do the adjustment
first and fine later to get a sharp picture. Commit e6051cdf inverts
that to "fine first, coarse later" - obtaining a good result would be
almost impossible with a microscope operated like that. The same holds
for the volume control.
This strengthens my point that the above commit should be reverted.

> It is not easy to combine the "fine" and "coarse" adjustment into a
> single volume slider which fit different scale in volume/boost 

Agreed. That's why I suggested not to combine them by default (but PA
does this).

> some user may prefer to adjust Boost first and Volume but other user
> may prefer adjust Volume and keep Boost fixed 

If PA applies "coarse first, fine later", the volume control behaves
smoothly over the entire range. The reason why users have preferred an
inverted order in the past is that on some systems, high Mic Boost
levels introduce a lot of noise. But (I think) the solution taken by
commit e6051cdf is wrong. It would have been better to fix the Boost at
a low level (0 or 1).

I agree that users may have different preferences in this area. Fixing
that would require to expose the choice to users in a user-friendly way.
Currently it is deeply hidden in the Alsa mixer path conf files.

> As pulseaudio is using floating point volume scale and combine volume
> and boost to a single slider, the optimal solution may need a very
> complex method of adjusting the fine and coarse adjustment 

Currently, PA adjusts the controls in the order specified in the path
conf file. In order to ensure "smooth" volume control over the entire
dynamic range, PA should instead figure out which controls are "coarse"
and which are "fine", and adjust the former before the latter.
Unfortunately, a proper destinction between "coarse" and "fine" is
difficult with PA's current "Mixer Element" abstraction, AFAICS.

However, to re-iterate my previous statement, the proper solution for
this problem is not to change the order of the merged mixer elements,
but not to merge them at all.


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