[pulseaudio-discuss] Questions about setting volume over "base volume"

[Colin Guthrie]

'Twas brillig, and fykcee1 at gmail.com at 09/04/12 15:05 did gyre and gimble:
> Hi all,
> 
> I found the sound playback quality will become bad if I set volume
> over "base volume" in gnome-volume-control.
> Why? Does it boost volume by software way in the case of "over base
> volume"? If yes, who do the job? PA or alsa?

Basically the "Base Volume" is what ALSA reports to us as it's 0dB
point. Going above the base volume may very well case clipping and
quality loss. This is why we show it clearly.

We include this such that we can provide a full range to the underlying
h/w controls is exposed between the 0% and 100% range in PA.

> 1. Is base volume calculated from max dB?

No, base volume is the 0dB point. 100% aka PA_VOLUME_NORM is mapped to
the max dB in ALSA.

> 2. Is most drivers' playback max dB 0?

Yeah, for playback, most drivers have the max dB in alsa of 0dB. This
means that Base Volume == 100% == PA_VOLUME_NORM and is thus not displayed.

For recording, many drivers will provide a "mic boost" or similar. This
can quite frequently mean that capture volumes will have a base volume
whereas playback will not.

> 3. What is the difference between snd_mixer_selem_set_playback_volume and snd_mixer_selem_set_playback_dB?

Can't recall off hand (would have to look at the code) but one
presumably sets the volume in volume steps (e.g. the driver will say "i
have 10 steps" and thus you can give values 0-9), whereas the other will
set the appropriate step for a supplied dB value (with appropriate
rounding).

HTHs

Col



-- 

Colin Guthrie
gmane(at)colin.guthr.ie
http://colin.guthr.ie/

Day Job:
  Tribalogic Limited http://www.tribalogic.net/
Open Source:
  Mageia Contributor http://www.mageia.org/
  PulseAudio Hacker http://www.pulseaudio.org/
  Trac Hacker http://trac.edgewall.org/