Pulseaudio uses the decibel levels reported by the different driver-level mixers to combine them into one big mixer. However, the driver tells dirty lies, causing pulseaudio to misbehave. The root of the problem is that when any relevant mixer is at zero percent, the sound will be completely muted, but the driver will report a dB gain value that indicates otherwise. For example, at zero percent, the Master mixer reports "-48.00" dB gain. Hence, if pulseaudio attempts to set any volume level below -48 dB, it will turn the master mixer all the way down and mute the sound. Hence, the bottom third portion of my volume slider is completely mute.
The current workaround for this is to use the ignore_db=1 trick in default.pa. However, maybe pulseaudio needs a config option to specify the allowed range of each hardware mixer. Then I could tell it to never set the Master mixer below 5%.
anyway, the bug is that when the hardware mixers are completely silent, they lie about their dB gain values and thereby confuse pulseaudio.
ProblemType: Bug
DistroRelease: Ubuntu 10.04
Package: alsa-base 1.0.22.1+dfsg-0ubuntu3
ProcVersionSignature: Ubuntu 2.6.32-24.38-preempt 2.6.32.15+drm33.5
Uname: Linux 2.6.32-24-preempt x86_64
NonfreeKernelModules: wl nvidia
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.21.
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/controlC0: ryan 23668 F.... pulseaudio
Card0.Amixer.info:
Card hw:0 'Intel'/'HDA Intel at 0xf6ffc000 irq 21'
Mixer name : 'SigmaTel STAC9228'
Components : 'HDA:83847616,10280209,00100402'
Controls : 28
Simple ctrls : 18
Date: Sat Jul 31 12:34:22 2010
PackageArchitecture: all
ProcEnviron:
LANGUAGE=
PATH=(custom, user)
LANG=en_US.utf8
SHELL=/bin/zsh
SelectedCard: 0 Intel HDA-Intel - HDA Intel
SourcePackage: alsa-driver
Symptom: audio
Title: [SigmaTel STAC9228] Playback problem
dmi.bios.date: 12/26/2008
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A15
dmi.board.name: 0U8042
dmi.board.vendor: Dell Inc.
dmi.chassis.type: 8
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA15:bd12/26/2008:svnDellInc.:pnXPSM1330:pvr:rvnDellInc.:rn0U8042:rvr:cvnDellInc.:ct8:cvr:
dmi.product.name: XPS M1330
dmi.sys.vendor: Dell Inc.
I'm attaching the output of amixer with all the volume mixers un-muted but at zero percent. Notice that dB gain values are reported, but putting either Master or PCM at zero percent results in silence. Similarly, putting any of the relevant recording mixers to zero results in recording silence, regardless of the bogus dB gain values reported.