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

[Tanu Kaskinen]

On Mon, 2015-04-20 at 16:56 +0200, Wilck, Martin wrote:
> Hello,
> 
> there has been some discussion about Mic Boost  / Dock Mic Boost in the
> past already. Some have reported no Mic output at all without Boost,
> other say that enabling Boost will cause high noise on their systems.
> 
> I have another point here that AFAICS hasn't been reported yet.
> 
> It has been caused by the commit e6051cdf "alsa-mixer: Prefer moving
> "Capture" before moving boosts"
> (http://cgit.freedesktop.org/pulseaudio/pulseaudio/commit/?id=e6051cdf8db554c0bbd4257959c37a7ecc9c10c5)
> (see also
> https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/1085402).
>  
> In short, on my system, input volume control is hardly possible above
> 25% because of this patch.
> 
> The effect that I am seeing is as follows: I have two merged volume
> controls for my Mic, "Capture" and "Dock Mic Boost". They are applied by
> PA in the order encountered in the path conf file. On my system (Fujitsu
> Lifebook E744 with Realtek ALC282 Codec), the "Capture" Amp has a range
> of -18..30 dB in 0.75 dB steps, and "Dock Mic Boost" has 4 12dB steps
> (0, 12, 24, 36). PA merges these two controls to a single volume control
> with an overall range of -18..66dB. Normalizing at the max, PA converts
> this to -84..0dB, where the Boost comes in at -36dB, or 25% of the
> scale. The "base volume", calculated from the "offset" of the ALSA amp,
> is at -66dB or 8%.
> 
> Because of the above patch, the "Capture" control is used first. Thus
> the range from -84...-36 dB (or 0..25%, because of the non-linearity of
> PA's volume control) is covered with "Capture". Above that level,
> "Capture" is always held at 100% and only "Doc Mic Boost" changes. This
> means that above 25%, there are only 3 steps that are actually possible:
> -24dB / 40%, -12dB / 63%, and 0dB / 100%. 
> => There is no "smooth" volume control above 25%.
> The graphical volume controls don't reflect this; thus user control
> input volume is almost impossible at higher levels than 25%. Volume will
> appear to change non-predictably to users.

PulseAudio compensates coarse hardware volume by applying software
volume when necessary. This means that users get a smooth volume curve.
For example, you presented a nice table of different volume levels, and
there was this line:

PA(%)   PA(dB)  Captu   Boost   Capture+Boost-66dB
56%     -15.00  30.00   12.00   -24.000000

The ideal situation would be where the second and the last column would
be equal, but in this case there's a 9dB difference. But no worries,
PulseAudio compensates that by amplifying the signal by 9dB in software.

Did you observe some volume problems by ear, or did you only notice
"weird behaviour" by looking at the alsa mixer?

-- 
Tanu