Advanced Linux Sound Architecture
The Advanced Linux Sound Architecture (ALSA) provides kernel driven sound card drivers. It replaces the original Open Sound System (OSS).
Besides the sound device drivers, ALSA also bundles a user space driven library for application developers. They can then use those ALSA drivers for high level API development. This enables direct (kernel) interaction with sound devices through ALSA libraries.
Installation
ALSA is a set of built-in Linux kernel modules. Therefore, manual installation is not necessary.
udev will automatically detect your hardware and select needed drivers at boot time, therefore, your sound should already be working. However, your sound may be initially muted. If it is, see #Unmuting the channels.
User privileges
Usually, local users have permission to play audio and change mixer levels.
To allow remote users to use ALSA, you need to add those users to the audio group; however, this is not recommended by default (see note below).
audio group allows direct access to devices. Keep in mind that this allows applications to exclusively reserve output devices. This may break software mixing or fast-user-switching on multi-seat systems. Therefore, adding a user to the audio group is not recommended by default unless you specifically need to .ALSA utilities
Install the package. This contains (among other utilities) the and utilities. amixer is a shell command to change audio settings, while alsamixer provides a more intuitive ncurses based interface for audio device configuration.
If you need high quality resampling, install the package to enable upmixing/downmixing and other advanced features.
OSS compatibility
/dev/dsp or /dev/snd/seq.ALSA has some ability to intercept OSS calls and re-route them through ALSA instead. This emulation layer is useful e.g. for legacy applications which try to open /dev/dsp and write sound data to them directly. Without OSS or the emulation library, /dev/dsp will be missing, and the application will not produce any sound.
If you want OSS applications to work with dmix, install the package as well. Then load the , and kernel modules to enable OSS emulation.
PulseAudio compatibility
lets you use ALSA for applications that support only PulseAudio for sound. Usage is simply
$ apulse yourapplication
ALSA and systemd
The package comes with systemd unit configuration files and by default.
These are automatically installed and activated during installation (via package provided symlink to sound.target). The options are as follows:
- Reads on boot and writes updated values on shutdown, provided
/etc/alsa/state-daemon.confdoes not exist. As/etc/alsa/state-daemon.confis not created without a conscious action of the user, it is the default method. - (Re-)Starts alsactl in daemon mode to continuously keep track of, and persist, volume changes, under the condition that the user has consciously created
/etc/alsa/state-daemon.conf.
Evidently, both methods are mutually exclusive. You can decide for one of the two approaches depending on your requirements. To edit these units, see systemd#Editing provided units. You can check their status using systemctl.
For further information, see alsactl(1).
ALSA firmware
The package contains firmware that may be required for certain sound cards (e.g. Creative SB0400 Audigy2).
and/or are required for some newer laptop models (mainly since 2019) because they implement their drivers with firmware provided by the Sound Open Firmware project.
Unmuting the channels
By default, ALSA has all channels muted. Those have to be unmuted manually.
Unmute with amixer
Unmuting the sound card's master volume can be done by using amixer:
$ amixer sset Master unmute $ amixer sset Speaker unmute $ amixer sset Headphone unmute
Unmute with alsamixer
Unmuting the sound card can be done using alsamixer:
$ alsamixer
The label below a channel indicates that the channel is muted, and indicates that it is open.
Scroll to the and channels with the and Right keys and unmute them by pressing the m key.
Use the key to increase the volume and obtain a value of dB gain. The gain can be found in the upper left next to the field.
Unmute 5.1/7.1 sound
To get full 5.1 or 7.1 surround sound, you will likely need to unmute other channels such as , , , (subwoofer) and Side. (Those are channel names with Intel HD Audio; they may vary with different hardware)
Enable the microphone
To enable your microphone, switch to the Capture tab with and enable a channel with Space. See /Troubleshooting#Microphone if microphone does not work.
Test your changes
Next, test to see if sound works:
$ speaker-test -c 2
Change to fit your speaker setup. Use for 7.1, for instance:
$ speaker-test -c 8
If audio is being outputted to the wrong device, try manually specifying it with the argument .
$ speaker-test -D default:PCH -c 8
accepts PCM channel names as values, which can be retrieved by running the following:
If that does not work, consult the #Configuration section or the /Troubleshooting page.
Additional notes
- If your system has more than one soundcard, then you can switch between them by pressing
- Some cards need to have digital output muted or disabled in order to hear analog sound. For the Soundblaster Audigy LS, mute the channel labeled .
- Some machines, like the Thinkpad T61, have a
Speakerchannel which must be unmuted and adjusted as well.
- Some machines, like the Dell E6400, may also require the and channels to be unmuted and adjusted.
- If your volume adjustments seem to be lost after you reboot, try running alsamixer as root.
Configuration
The system configuration file is , and the per-user configuration file is .
Basic syntax
ALSA configuration files follow a simple syntax consisting of hierarchical value to parameter (key) assignments. The syntax is also shown in .
Assignments and Separators
Assignments define a value of a given key. There are different assignment types and styles available.
Separators are used to indicate the start and end of an assignment, but using commas or whitespace is also possible.
Compound assignments use braces as separators.
For easier reading, it is recommended to use first style for definitions including more than three keys.
Array definition is an alternative syntax to define compound statements. It uses brackets as separators. The keys are automatically generated as numbers, starting with zero.
Single array
key [ "value0"; "value1"; "value''N''"; ] key.0 "value0"; # Equivalent to the example above. key.1 "value1"; key.''N'' "value''N''";
Everything depends on user preferences when it comes to different styles of configuration; however, one should avoid mixing different styles. Further information on basic configuration can be found in .
Data types
ALSA uses different data types for parameter values, which must be set in the users respective configuration file. Some keys accept multiple data types, while most do not. A list of configuration options and their respective type requirements for PCM plugins can be found in .
Operation modes
There are different operation modes for parsing nodes, the default mode is merge and create. If operation mode is either merge/create or merge, type checking is done. Only same type assignments can be merged, so strings cannot be merged with integers. Trying to define a simple assignment in default operation mode to a compound (and vice versa) will also not work.
Prefixes of operation modes:
- "+" -- merge and create
- "-" -- merge
- "?" -- do not override
- "!" -- override
Using override operation mode, when done correctly, is usually safe; however, one should bear in mind that there might be other necessary keys in a node for proper functioning.
An example of setting default device using "defaults" node
Assuming that "defaults" node is set in , where "defaults.pcm.card" and its "ctl" counterpart have assignment values "0" (type integer), user wants to set default pcm and control device to (third) sound card "2" or "SB" for an Azalia sound card.
Defaults node
defaults.ctl.card 2; # Sets default device and control to third card (counting begins with 0). defaults.pcm.card 2; # This does not change the data type. defaults.ctl.+card 2; # Equivalent to above. defaults.pcm.+card 2; defaults.ctl.-card 2; # Same effect on a default setup, however if defaults node was removed or defaults.pcm.-card 2; # type has been changed, merge operation mode will result in error. defaults.pcm.?card 2; # This does nothing, since this assignment already exists. defaults.ctl.?card 2; defaults.pcm.!card "SB"; # The override operation mode is necessary here, because of defaults.ctl.!card "SB"; # different value types.
Using double quotes here automatically sets values data type to string, so in the above example, setting defaults.pcm.!card "2" would result in retaining last default device, in this case card 0. Using double quotes for strings is not mandatory as long as no special characters are used, which ideally should never be the case. This may be irrelevant in other assignments.
Nesting
Sometimes, it may be useful and even easier to read using nesting in configuration.
Setting the default sound card via defaults node
Putting the previous example regarding and into practice, assuming we have 2 cards with index 0 and 1 respectively and wish to simply change the default card to index 1, would lead to the following configuration in or the user-specific to change both the playback and the mixer control card.
Configuring the index order via kernel module options
If your sound card order changes on boot, you can specify their order in any file ending with in /etc/modprobe.d ( is suggested).
For example, if you want your mia sound card to be #0:
/etc/modprobe.d/alsa-base.conf
options snd_mia index=0 options snd_hda_intel index=1
Use to get the loaded sound modules and their order. This list is usually all that is needed for the loading order. Use to get a devices & modules list. This configuration assumes you have one mia sound card using and one (e.g. onboard) card using .
You can also provide an index of to instruct ALSA to never use a card as the primary one. Distributions such as Linux Mint and Ubuntu use the following settings to avoid USB and other "abnormal" drivers from getting index :
These changes require a system reboot.
Select the default PCM via environment variable
Probably, it is enough to set ALSA_CARD to the name of the device. First, get the names with , then set ALSA_CARD to the name which comes after the colon and before the bracket; e.g. if you have
card 1: HDMI [HDA ATI HDMI], device 3: HDMI 0 [HDMI 0]
then set .
Other variables are also checked in the default global configuration . By looking there for constructs of the form vars [ ... ], the following table emerges:
| Variable name | Used by | |
|---|---|---|
| ALSA_CARD | pcm.default , pcm.hw , pcm.plughw , ctl.sysdefault , ctl.hw , rawmidi.default , rawmidi.hw , hwdep.hw | |
| ALSA_CTL_CARD | ctl.sysdefault , ctl.hw | |
| 3 | ALSA_HWDEP_CARD | hwdep.default , hwdep.hw |
| ALSA_HWDEP_DEVICE | hwdep.default , hwdep.hw | |
| ALSA_PCM_CARD | pcm.default , pcm.hw , pcm.plughw | |
| ALSA_PCM_DEVICE | pcm.hw , pcm.plughw | |
| ALSA_RAWMIDI_CARD | rawmidi.default , rawmidi.hw | |
| ALSA_RAWMIDI_DEVICE | rawmidi.default , rawmidi.hw |
Alternatively, you can override the behavior in your own configuration file, preferably the global one (). Add:
pcm.!default {
type plug
slave.pcm {
@func getenv
vars [ ALSAPCM ]
default "hw:Audigy2"
}
}
In this case as well, replace with the name of your device. You can get the names with or you can also use PCMs like surround51. But if you need to use the microphone, it is a good idea to select full-duplex PCM as default.
Now, you can select the sound card when starting programs by just changing the environment variable ALSAPCM. It works fine for all program that do not allow to select the card; for the others, ensure you keep the default card.
For example, assuming you wrote a downmix PCM called , you can use it with using the commandline
Alternative method
First, you will have to find out the card and device id that you want to set as the default:
type hw as default card is equivalent to addressing hardware directly, which leaves the device unavailable to other applications. This method is only recommended if it is a part of a more sophisticated setup ~/.asoundrc or if user deliberately wants to address sound card directly (digital output through eic958 or dedicated music server for example).For example, the last entry in this list has the card ID 2 and the device ID 0. To set this card as the default, you can either use the system-wide file or the user-specific file . You may have to create the file if it does not exist. Then insert the following options with the corresponding card.
pcm.!default {
type hw
card 2
}
ctl.!default {
type hw
card 2
}
In most cases, it is recommended to use sound card names instead of number references. Card names are easier to grasp, and also overcomes the boot order problem. Therefore, the following would be correct for the above example.
pcm.!default {
type hw
card Audio
}
ctl.!default {
type hw
card Audio
}
To get valid ALSA card names, use aplay:
Alternatively, use cat, which might return unused devices:
$ cat /proc/asound/card*/id
PCH ThinkPadEC
The 'pcm' options affect which card and device will be used for audio playback while the 'ctl' option affects which card is used by control utilities like alsamixer.
The changes should take effect as soon as you (re-)start an application (MPlayer etc.). You can also test with a command like aplay.
$ aplay -D default:PCH your_favourite_sound.wav
If you receive an error regarding your asound configuration, check the upstream documentation for possible changes to the configuration file format.
Verifying correct sound modules are loaded
You can assume that udev will autodetect your sound properly. You can check this with the command:
If the output looks similar, your sound drivers have been successfully autodetected.
You might also want to check the directory for the right device files:
If you have at least the devices controlC0 and pcmC0D0p or similar, then your sound modules have been detected and loaded properly.
If this is not the case, your sound modules have not been detected properly. To solve this, you can try loading the modules manually:
- Locate the module for your sound card: ALSA Soundcard Matrix The module will be prefixed with 'snd-' (for example: ).
- Load the module.
- Check for the device files in (see above) and/or try if
alsamixeror have reasonable output. - Configure and
snd-pcm-ossto load at boot.
Getting S/PDIF output
S/PDIF is a digital audio interface often used to connect a computer to a digital amplifier (such as a home theatre with 5.1/7.1 surround sound).
Depending on what shell you use, add the following line to your shell's configuration file:
$ amixer -c 0 cset name='IEC958 Playback Switch' on
You can see the name of your card's digital output with:
$ amixer scontrols
ALSAEqual system-wide
Install the package.
After installing the package, add the following to your ALSA configuration file:
To change your equalizer settings, run
$ alsamixer -D equal
Note that the equalizer configuration is different for each user (until not specified else). It is saved in . So if you want to use ALSAEqual with mpd or another software running under different user, you can configure it using
$ su mpd -c 'alsamixer -D equal'
or for example, you can make a symlink to your in their home directory.
ALSAEqual for specific outputs only
If you wish to apply an equalizer to a specific output device only (for example your speakers connected to the S/PDIF output, but not your headphones connected to the headphone jack), but also want be able to output from multiple applications and to both output devices simultaneously, you need to create two devices that feed into their respective devices (slave.pcm) directly. The following works for stereo output and maintains a regular stereo input, applying the equalizer to the S/PDIF output only.
Managing ALSAEqual states
Install the package.
Configure the equalizer as usual with
$ alsamixer -D equal
When you are satisfied with the state, you may give it a name (foo in this example) and save it:
$ alsaequal-mgr save foo
The state "foo" can then be restored at a later time with
$ alsaequal-mgr load foo
This, however, only restores . You then have to update the equalizer by .
You can thus create different equalizer states for games, movies, music genres, VoIP apps, etc. and reload them as necessary.
See the project page and the help message for more options.
Using mbeq
mbeq is a fairly typical multi band graphical equalizer.
Install the , and packages if you do not already have them.
If you have not already created either an or a file, then create either one and insert the following:
High quality resampling
When software mixing is enabled, ALSA is forced to resample everything to the same frequency (48 kHz by default when supported). By default, it will try to use the speexrate converter to do so, and fallback to low-quality linear interpolation if it is not available. Thus, if you are getting poor sound quality due to bad resampling, the problem can be solved by simply installing the package.
For even higher quality resampling, you can change the default rate converter to or speexrate_best. Both perform well enough that in practice it does not matter which one you choose, so using the best converter is usually not worth the extra CPU cycles it requires.
To change the default converter, place the following contents in your or :
Upmixing/downmixing
Upmixing
In order for stereo sources like music to be able to saturate a 5.1 or 7.1 sound system, you need to use upmixing. In darker days, this used to be tricky and error prone, but nowadays, plugins exist to easily take care of this task. We will use the upmix plugin, included in the package.
Then add the following to your ALSA configuration file of choice (either or ):
pcm.upmix71 {
type upmix
slave.pcm "surround71"
delay 15
channels 8
}
You can easily change this example for 7.1 upmixing to 5.1 or 4.0.
The following example adds a new PCM channel that you can use for upmixing. If you want all sound sources to go through this channel, add it as a default below the previous definition like so:
pcm.!default "plug:upmix71"
The plugin automatically allows multiple sources to play through it without problems so setting is as a default is actually a safe choice. If this is not working, you have to setup your own dmixer for the upmixing PCM like this:
pcm.dmix6 {
type asym
playback.pcm {
type dmix
ipc_key 567829
slave {
pcm "hw:0,0"
channels 6
}
}
}
and use "dmix6" instead of "surround71". If you experience skipping or distorted sound, consider increasing the buffer_size (to 32768, for example) or use a high quality resampler.
Downmixing
If you want to downmix sources to stereo because you, for instance, want to watch a movie with 5.1 sound on a stereo system, use the vdownmix plugin, included in the package.
Again, in your configuration file, add this:
pcm.!surround51 {
type vdownmix
slave.pcm "default"
}
pcm.!surround40 {
type vdownmix
slave.pcm "default"
}
Dmix
Mixing enables multiple applications to output sound at the same time. Most discrete sound cards support hardware mixing, which is enabled by default if available. Integrated motherboard sound cards (such as Intel HD Audio), usually do not support hardware mixing. On such cards, software mixing is done by an ALSA plugin called . This feature is enabled automatically if hardware mixing is unavailable.
To manually enable dmix, add the following to your ALSA configuration file:
Tips and tricks
Disabling auto mute on startup
Auto-Mute Mode can be configured on startup with . For example, to disable it:
# amixer -c 0 sset "Auto-Mute Mode" Disabled
Alternatively, the graphical interface can be utilized through alsamixer. In order to save any modifications, use:
# alsactl store
or
# alsactl daemon
See ALSA and Systemd
Hot-plugging a USB sound card
Simultaneous output
You might want to play music via external speakers connected via mini jack and internal speakers simultaneously. This can be done by unmuting Auto-Mute item using alsamixer or :
$ amixer sset "Auto-Mute" unmute
and then unmuting other required items, such as Headphones, Speaker, Bass Speaker...
Keyboard volume control
Map the following commands to your volume keys: , , .
To raise the volume:
amixer set Master 5%+
To lower the volume:
amixer set Master 5%-
To toggle mute/unmute of the volume:
amixer set Master toggle
Virtual sound device using snd-aloop
You might want a jack alternative to create a virtual recording or play device in order to mix different sources, using the snd-aloop module:
modprobe snd-aloop
List your new virtual devices using:
aplay -l
now you can for example using ffmpeg:
ffmpeg -f alsa -i hw:1,1,0 -f alsa -i hw:1,1,1 -filter_complex amerge output.mp3
In the hw:R,W,N phrase, R is your virtual card device number. W should be set to 1 for recording devices, or 0 for playing. N is your sub device. You can use all the virtual devices available and play/stop using applications like mplayer:
mplayer -ao alsa:device=hw=1,0,0 fileA mplayer -ao alsa:device=hw=1,0,1 fileB
Another thing you could do with this approach is using festival to generate a voice into a recording stream using a script like this:
#!/bin/sh echo "$1" | iconv -f utf-8 -t iso-8859-1 | text2wave > "_tmp_.wav" mplayer -ao alsa:device=hw=2,0,0 "_tmp.wav" rm "_tmp.wav"
Retrieving the driver state
The alsa-utils package also contains the command, which can be used to gather detailed data on the ALSA driver and user-space state.
See ALSA bug tracking for more info.
Reconfiguring input/output ports
The package contains the tool, which can be used (on Intel HDA cards) to reconfigure the sound card input/output ports; for instance, to turn a microphone jack into a headphone jack.
Resetting codecs
The ALSA driver can fully reconfigure attached codecs (the parts of the sound system that actually process audio streams), by doing:
# echo 1 > /sys/class/sound/card/reconfig
Before doing this, all processes using the ALSA driver (such as Pulseaudio or JACK) must be stopped.
Correctly detect microphone plugged in a 4-pin 3.5mm (TRRS) jack
On some modern laptops, you may have a combined 3.5mm headset jack, instead of two separated ones, which may not be correctly detected by default. To make ALSA correctly detect plug-in status on your 3.5mm jack, you can put the following line into your :
options snd_hda_intel index=0 model=your_model_setting
For a complete list of options to put in , see HD-Audio Codec-Specific Models or its source located at (provided by the linux-docs package). A common model is , even if the hardware is not from Dell.