[alsa-devel] wrong decibel data?

Raymond Yau superquad.vortex2 at gmail.com
Mon Jun 28 03:47:20 CEST 2010

2010/6/23 Colin Guthrie <gmane at colin.guthr.ie>

> 'Twas brillig, and Raymond Yau at 23/06/10 02:15 did gyre and gimble:
> > why PA server still insist to open front device for capturing ?
> Couldn't tell you. Perhaps Lennart is unaware that it is considered
> invalid? I'll ask him.
> > http://thread.gmane.org/gmane.linux.alsa.devel/67912/focus=68248
> >
> > As Takashi had already mention that
> >
> > "I agree that the capture from "front" PCM isn't considered as valid.
> > The "front", "rear", "center_lfe" definitions are rather for
> multi-channel
> > playbacks. The capture on these channels aren't useful in most cases."
> >
> > you can perform an experiement
> >
> > pcm.test {
> >     type softvol
> >     slave.pcm "hw:0,0"
> >     control {
> >         name "PA Playback Volume"
> >         card 0
> >     }
> >
> > arecord -D test -f CD -d 10 -v test.wav
> >
> > you will find "PA Playback Volume" in playback screen of alsamixer
> >
> > I am not sure the softvol control created when PA open front device
> > for playback and capture is used for playback or capture
> >
> > BTW , PA still using "front" to open CTL device too
> You'll have to ask Lennart directly about that bit.

arecord -Dfront:0 -v -f cd | aplay -Dfront:0 -v -f cd

the softvol control was seem to used by arecord and aplay concurrently

Using front device for capture
1)  HDA, the problem is softvol plugin,
2)  emu10k1 , the problem is the hook controls in front playback device
which change the route of volume of the DSP hardware mixer

> I can't speak for him with regards to the full setup of the mixer path
> probing stuff, so perhaps "front" should be edited out of this for
> paths. It would likely be a fairly trivial change to configuration files
> rather than any code changes.
> Col
Well , He had already hardcoded to use "hw:card_number" in dbverify.c , it
is only PA server still try to open mixer device "front"

The tooltip of gnome volume control in Fedora 13 clearly indiacte that 100%
is 0dB for intel8x0 which use ac97 codec  when you put the cursor on top of
the speaker icon at the system bar


according to gnome-media-developer  Bastien Nocera

In any case, we only display what PulseAudio tells us, so you should poke the
PulseAudio devs about this.

This is the code in question:
gvc_mixer_stream_get_decibel (GvcMixerStream *stream)
        g_return_val_if_fail (GVC_IS_MIXER_STREAM (stream), 0);

        return pa_sw_volume_to_dB(

if you don't have any ac97 sound card , you can user virtualbox which has an
emulated intel8x0 sound card

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