Ubuntu Manpage: bluealsa-plugins - Bluetooth Audio ALSA Plugins

name
description
pcm plugin
ctl plugin
notes
files
copyright
see also

Provided by: libasound2-plugin-bluez_4.3.1-3_amd64

NAME

       bluealsa-plugins - Bluetooth Audio ALSA Plugins

DESCRIPTION

       BlueALSA  permits  applications  to  access Bluetooth audio devices using the ALSA alsa-lib API. Users of
       those applications can then use Bluetooth speakers, headphones, headsets and hands-free devices  much  as
       if  they were local devices.  This integration is achieved by two ALSA plugins, one for PCM audio streams
       and one for CTL volume controls.

PCM PLUGIN

The BlueALSA ALSA PCM plugin communicates with the bluealsa(8) service. It can be used to define ALSA
PCMs in your own configuration file (e.g. ~/.asoundrc), or you can use the predefined bluealsa PCM.

The Predefined bluealsa PCM
The simplest way to use the PCM plugin is with the predefined ALSA PCM device bluealsa. The definition of
this PCM device is of type plug so audio format conversion, if required, is done automatically by the
PCM. It has parameters DEV, PROFILE, CODEC, VOL, SOFTVOL, DELAY, and SRV. All these parameters have
defaults. Parameter values in an ALSA PCM name are specified using the syntax:

bluealsa:DEV=01:23:45:67:89:AB,PROFILE=a2dp,CODEC=aac,VOL=60,SOFTVOL=no,DELAY=0,SRV=org.bluealsa

PCM Parameters
DEV The device Bluetooth address in the form XX:XX:XX:XX:XX:XX. Device names or aliases are not
valid here. The default value is 00:00:00:00:00:00 which selects the most recently connected
device of the chosen profile.

PROFILE
May be either a2dp or sco. sco selects either Hands-Free (HFP) or Headset (HSP) profile,
whichever is connected on the selected device. The default is a2dp.

CODEC Specifies the codec to be used by the profile. When a connection is established between a
device and a host, BlueALSA negotiates the best available codec with the device; this parameter
allows the ALSA configuration to override that selection. The default value is unchanged which
causes the PCM to use its existing codec setting. The codec name is case insensitive; so for
example aptX, aptx, and APTX are all accepted. If the specified codec is not available the
plugin issues a warning and uses the default value instead.

BlueALSA does not support changing the HFP codec from a HFP-HF node, only the HFP-AG node can
change the HFP codec.

oFono does not permit the audio agent to select the codec, so this parameter has no effect when
BlueALSA is used with oFono for HFP support.

For the A2DP profile it is possible to also specify a "configuration" for the codec by
appending the configuration as a hex string separated from the codec name by a colon. The bits
responsible for the number of channels and the sampling frequency are set by the plugin with
the respect to options provided by the user (channel mode and sampling frequency bits act as a
mask). For example:

CODEC=SBC:FC450240

This SBC configuration limits the channel mode options to mono and dual channel. So, in case of
2 channel audio stream, the plugin will negotiate the dual channel mode instead of default (if
supported) joint stereo mode.

VOL Specifies the initial volume for the PCM when opened. The default value is unchanged which
causes the PCM to use its existing volume setting. The value is an integer percentage of the
maximum volume [0-100]. The mute status can also be set by appending the character '-' to mute
the sound or '+' to unmute it. The volume is not restored to its original value when the PCM is
closed. For example to set the initial volume to 80% and ensure that mute is disabled for this
PCM:

VOL=80+

SOFTVOL
Enables or disables BlueALSA's software volume feature for this PCM. See the bluealsa(8) manual
page for more information on software volume. This is a boolean option (values on or off), but
also accepts the special value unchanged which causes the PCM to use its existing softvol
value. The default value is unchanged.

DELAY An integer number which is added to the reported delay (latency) value in order to manually
adjust the audio synchronization. It is not normally required and defaults to 0. See the EXT
parameter of the CTL plugin in the CTL Parameters section below for a more flexible and
convenient method of manually adjusting the reported delay by using a mixer control.

SRV The D-Bus service name of the BlueALSA daemon. Defaults to org.bluealsa. See bluealsa(8) for
more information. Not normally required.

Setting Different Defaults
The defaults can be overridden by defining the ones you want to change in your own configuration (e.g. in
~/.asoundrc.conf) for example:

defaults.bluealsa.device "00:11:22:33:44:55"
defaults.bluealsa.profile "sco"
defaults.bluealsa.codec "cvsd"
defaults.bluealsa.volume "50+"
defaults.bluealsa.softvol off
defaults.bluealsa.delay 5000
defaults.bluealsa.service "org.bluealsa.source"

Note that volume takes a string value and so the default must be enclosed in quotation marks.

Positional Parameters
ALSA permits arguments to be given as positional parameters as an alternative to explicitly naming them.
When using positional parameters it is important that the values are given in the correct sequence - DEV,
PROFILE, CODEC, VOL, SOFTVOL, DELAY, SRV. For example:

bluealsa:01:23:45:67:89:AB,a2dp,unchanged,unchanged,unchanged,0,org.bluealsa

When using positional parameters defaults can only be implied at the end of the id string, so

bluealsa:01:23:45:67:89:AB

is equivalent to the full form above, but

bluealsa:01:23:45:67:89:AB,a2dp,,80+

is not permitted.

Defining BlueALSA PCMs
You can define your own ALSA PCM in the ALSA configuration. To do this, create an ALSA configuration node
defining a PCM with type bluealsa. The configuration node has the following fields:

pcm.name {
type bluealsa # Bluetooth PCM
device STR # Device address in format XX:XX:XX:XX:XX:XX
profile STR # Profile type (a2dp or sco)
[codec STR] # Preferred codec
[volume STR] # Initial volume for this PCM
[softvol BOOLEAN] # Enable/disable BlueALSA's software volume
[delay INT] # Extra delay (frames) to be reported (default 0)
[service STR] # DBus name of service (default org.bluealsa)
}

The device and profile fields must be specified so that the plugin can select the correct Bluetooth
transport; the other fields are optional. Note that the default values for the optional fields are not
overridden automatically by the configuration defaults.bluealsa.* in a PCM defined this way; however the
configuration defaults can be referenced by use of @func refer (see the ALSA configuration file syntax
documentation for more information).

When choosing a name for your PCM definition, the name pcm.bluealsa is predefined by the bluez-alsa
installation (see section The Predefined bluealsa PCM above), so it should not be used as a name for your
own PCM devices as doing so will most likely have unexpected or undesirable results.

Note that the volume field is of type string, so the value must be enclosed in double-quotes. See the PCM
Parameters section above for more information on each field.

Do not confuse the PCM type bluealsa with the PCM named bluealsa. The type does not perform any audio
conversions, you will have to wrap your own defined PCMs with type plug to achieve that; whereas the
predefined PCM pcm.bluealsa is of type plug.

Name Hints
Applications that follow ALSA guidelines will obtain the list of defined PCMs by using the alsa-lib
namehints API. To make BlueALSA PCMs visible via that API it is necessary to add a "hint" section to the
ALSA configuration. If you have defined a new PCM, then the hint goes into the PCM configuration entry as
follows:

pcm.bt-headphones {
type plug
slave.pcm {
type bluealsa
device "00:11:22:33:44:55"
profile "a2dp"
}
hint {
show on
description "My Bluetooth headphones"
}
}

Now using aplay -L will include the following in its output:

# aplay -L
bt-headphones
My Bluetooth headphones
#

If you are using the predefined bluealsa PCM, then you can create a "namehint" entry in your ~/.asoundrc
file like this:

namehint.pcm {
mybluealsadevice "bluealsa:DEV=00:11:22:33:44:55,PROFILE=a2dp|My Bluetooth headphones"
}

Then aplay -L shows

# aplay -L
bluealsa:DEV=00:11:22:33:44:55,PROFILE=a2dp
My Bluetooth headphones

For alsa-lib versions before v1.2.3.2, a bug in the namehint parser means that a namehint.pcm entry has
to be written as

namehint.pcm {
mybluealsadevice "bluealsa:DEV=00:11:22:33:44:55,PROFILE=a2dp|DESCMy Bluetooth headphones"
}

(note the keyword DESC after the pipe symbol and before the description text.)

With that hint in place, the PCM will be listed as both a Capture and Playback device. So arecord -L will
also list it. That is generally OK for HFP/HSP devices, but an A2DP device most often offers only Capture
(e.g. a mobile phone) or only Playback (e.g. a Bluetooth speaker). It is possible to use the hint
description to limit the listing to only one direction using an undocumented syntax of ALSA configuration
files.

If the hint.description value ends with |IOIDInput the PCM will only show in listings of Capture devices;
if it ends with |IOIDOutput the PCM will only show in listings of Playback devices.

So we can modify our example above to:

pcm.bt-headphones {
type plug
slave.pcm {
type bluealsa
device "00:11:22:33:44:55"
profile "a2dp"
}
hint {
show on
description "My Bluetooth headphones|IOIDOutput"
}
}

namehint.pcm {
mybluealsadevice "bluealsa:DEV=00:11:22:33:44:55,PROFILE=a2dp|My Bluetooth headphones|IOIDOutput"
}

Now the aplay -L output will be exactly the same as before, but arecord -L will not include bt-headphones
in its output.

When using the namehint.pcm method, the key (mybluealsadevice in the above example) must be unique but
otherwise is not used. The first part of the value string, before the pipe | symbol, is the string that
is to be passed to ALSA applications to identify the PCM (e.g. with aplay -D ...). The next section,
after the pipe symbol, is the description that will be presented to the user. The optional |IOID section
is not included in the description given to the application.

CTL PLUGIN

The BlueALSA ALSA CTL plugin can be used to define ALSA CTLs (mixer devices) in your own configuration
file (e.g. ~/.asoundrc), or you can use the predefined configuration that is included in the bluez-alsa
project.

A BlueALSA CTL device has no associated soundcard, so alsamixer will not list it in its F6 menu. It can
be selected either by starting alsamixer with

alsamixer -D bluealsa

or by selecting "enter device name .." on the F6 menu then typing out "bluealsa" in the "Device Name"
box.

The CTL has two operating modes, Default mode and Single Device mode.

Default Mode
In this mode when a device connects, the mixer will create new controls for it, and when a device
disconnects, the mixer will remove its controls. alsamixer(1) will show these changes dynamically.

Control names are constructed by combining the device Bluetooth alias with either the profile type
('A2DP' or 'SCO') of the controlled PCM or the word "Battery" for battery level indicators. If two or
more connected devices have the same alias then an index number is added to the name to make it unique.

The Bluetooth "alias" of a device is by default the same as its "name". The name is a string defined by
the device manufacturer and embedded in its firmware. Typically two identical devices will have identical
names. The "alias" is created by BlueZ and stored locally on the host computer. So the alias can be
changed using a tool such as bluetoothctl(1) to make it unique if desired. As manufacturers tend to use
long names for their devices the alias can also be useful to give a short "nickname" to a device.

Although this default mode works well with alsamixer, there are some limitations that may make it
unsuitable for some applications. In particular:

• If device aliases are not unique then the index number associated with each is not easily predictable
in advance; so it can be difficult to programmatically associate a PCM with its volume control.

• A consequence of the alsa-lib implementation of controls is that when one Bluetooth device connects or
disconnects it is necessary to remove all controls from all devices in the mixer and create a new set.
This invalidates pointers held by applications and can cause application crashes. (Hardware sound cards
do not have randomly appearing and disappearing controls, so many, or even most, applications are not
programmed correctly to deal with it.)

Single Device Mode
The BlueALSA CTL also implements an alternative mode that presents controls only for one specified
device. In this case the control names are simply the profile type of the controlled PCM ('A2DP' or
'SCO') or the word "Battery". There is never any need for index suffixes or device alias. Immediately
this overcomes the two main issues of the default mode.

Single device mode is achieved by including the device Bluetooth address as an argument to the ALSA
device id, for example:

alsamixer -D bluealsa:00:11:22:33:44:55

A notable difference between single-device mode and the default mode is in the cases of the device not
being connected when the mixer is opened, and when the device disconnects while the mixer is open.

For the default mode, the mixer will still open, even if no devices are connected, but will display no
controls. In single device mode the open request will fail with an error message.

Similarly, in default mode when a device disconnects the mixer remains open but removes the set of
controls and creates a new control set without the disconnected device. That new set will be empty if no
devices remain. If the device then re-connects the mixer will again create a new set of controls with the
newly connected device included.

In single device mode when its device disconnects then the mixer will close. The alsamixer application
will continue running with no associated device or controls, but will not automatically re-open the mixer
if the device re-connects. The user can use F6 to open a new device.

As a special case, a single device mixer can be opened with the address 00:00:00:00:00:00. This will
create a mixer with controls for the most recently connected device at the time the mixer is opened. Once
created, that mixer behaves the same as if it had been opened with the actual address of the device: it
does not change to a new device if another is subsequently connected.

The Predefined bluealsa CTL
The bluealsa CTL has parameters DEV, EXT, BTT, DYN, and SRV. All the parameters have defaults.

CTL Parameters
DEV The device Bluetooth address in the form XX:XX:XX:XX:XX:XX. Device names or aliases are not
valid here. The default value is FF:FF:FF:FF:FF:FF which selects controls from all connected
devices (see Default Mode above). Also accepts the special address 00:00:00:00:00:00 which
selects the most recently connected device.

EXT Causes the plugin to include extra controls. These are the controls for Bluetooth codec
selection, volume mode selection, delay adjustment (sync) and/or battery level indicator. If
the value is yes then all of these additional controls are included; if the value is no then
none of them are included. The default is no.

This parameter can also select individual controls by using a colon (':') separated list of
control names. The control names are codec, mode, sync and battery. For example:

EXT=codec
EXT=mode:battery

See Codec switching in the NOTES section below for more information on the codec selection
control.

The volume mode controls take values "software" and "pass-through"; the playback control has
index 0 and capture control has index 1. See the Volume control section in the bluealsa(8) for
more information on the software volume setting.

The delay adjustment controls are called "Sync". They can be used to apply a fixed adjustment
to the delay reported by the associated PCM to the application, and may be useful with
applications that need to synchronize the bluetooth audio stream with some some other stream,
such as a video. The values are in milliseconds from -3275 ms to +3275 ms in steps of 25 ms.
The playback control has index 0 and the capture control has index 1. Each codec supported by a
PCM has its own delay adjustment value. Note that this control changes only the delay value
reported to the application by ALSA, it does not affect the actual delay (latency) of the PCM
stream. Values set by this control type are saved in the BlueALSA persistent state files, and
so are remembered and automatically applied each time the PCM is used.

The read-only battery level indicator will be shown only if the device supports battery level
reporting.

BTT Appends Bluetooth transport type (e.g. "-SNK" or "-HFP-AG") to the control element names. When
using with the Default Mode this will reduce the number of available characters for Bluetooth
device name, so the default value is no.

In some rare circumstances, when more than one A2DP or HFP/HSP profile is connected with a
single Bluetooth device, it might happen that the control element names for such device will
not be unique. This might be problematic for control applications which use ALSA High Level
Control Interface, e.g. amixer or alsamixer. Such applications will report error or simply
crash. This can be avoided by setting the BTT parameter to yes.

DYN Enables "dynamic" operation. The plugin will add and remove controls as profiles are connected
or disconnected. This is the normal behavior, so the default value is "yes". This argument is
ignored in default mode; in that mode operation is always dynamic. There are some applications
that are not programmed to handle dynamic addition or removal of controls, and can fail when
such events occur. Setting this argument to no in single device mode with such applications can
protect them from such failures. When dynamic operation is disabled, the plugin never adds or
removes any controls. If a single profile is disconnected, then its associated volume control
is put into an inactive state, i.e.: read-only with its value and playback/capture switch set
to 0.

SRV The D-Bus service name of the BlueALSA daemon. Defaults to org.bluealsa. See bluealsa(8) for
more information.

The default values can be overridden in the ALSA configuration, for example:

defaults.bluealsa.ctl.device "00:11:22:33:44:55"
defaults.bluealsa.ctl.bttransport "no"
defaults.bluealsa.ctl.dynamic "yes"
defaults.bluealsa.ctl.extended "no"

Defining BlueALSA CTLs
You can define your own ALSA CTL in the ALSA configuration. To do this, create an ALSA configuration node
defining a CTL with type bluealsa. The configuration node has the following fields:

ctl.name {
type bluealsa # Bluetooth PCM
[device STR] # Device address (default "FF:FF:FF:FF:FF:FF")
[extended STR] # Include additional controls (default no)
[bttransport STR] # Append BT transport to element names (yes/no, default no)
[dynamic STR] # Enable dynamic operation (yes/no, default yes)
[service STR] # D-Bus name of service (default "org.bluealsa")
}

All the fields (except type) are optional. See the CTL Parameters section above for more information on
each field. As for PCM definitions above, the default values for the optional fields are hard-coded into
the plugin; they are not overridden by the configuration defaults.bluealsa. settings.

NOTES

Codec selection
When used on a HFP gateway node, there may be a brief delay with HFP PCMs after connection until the
codec is selected. This delay is typically less than two seconds. During this time interval it is not
possible to open the PCM plugin, it will fail with "Resource temporarily unavailable" (EAGAIN).

Codec switching
Changing the codec used by a BlueALSA transport causes the PCM(s) running on that transport to terminate.
Therefore using a Codec control can have undesirable consequences. Unfortunately the alsamixer(1) UI does
not present a separate pick-list for enumerated types, so merely browsing the list of codecs using this
control actually issues a Codec change request every time a different codec is displayed. This is not
ideal, so the use of this control type with alsamixer(1) is not recommended. The control type does
however work well with other mixer applications such as amixer(1).

Note that BlueALSA does not support changing the HFP codec from a HFP-HF node, only the HFP-AG node can
change the HFP codec.

Transport acquisition
The audio connection of a profile is not established immediately that a device connects. The A2DP source
device, or HFP/HSP gateway device, must first "acquire" the profile transport.

When the BlueALSA PCM plugin is used on a source A2DP or gateway HFP/HSP node, then bluealsa(8) will
automatically acquire the transport and begin audio transfer when the plugin starts the PCM.

When used on an A2DP sink or HFP/HSP HF/HS node then bluealsa(8) must wait for the remote device to
acquire the transport. During this waiting time the PCM plugin behaves as if the device "clock" is
stopped, it does not generate any poll() events, and the application will be blocked when writing or
reading to/from the PCM. For applications playing audio from a file or recording audio to a file this is
not normally an issue; but when streaming between some other device and a BlueALSA device this may lead
to very large latency (delay) or trigger underruns or overruns in the other device.

PCM drain and non-blocking operation
The BlueALSA PCM plugin does not support draining of capture PCMs. For a capture PCM snd_pcm_drain() has
the same effect as snd_pcm_drop(). This is a limitation of the ALSA ioplug external plugin API.

For playback PCMs, BlueALSA has support for the drain operation in both blocking and non-blocking modes.
In blocking mode the drain operation will wait until the BlueALSA server has played out the final audio
frame. In non-blocking mode the plugin will inform the application of drain completion as soon as the
ALSA ring buffer has been flushed; this means that some audio frames at the end of the stream may be lost
in non-blocking mode as the PCM may stop before the server has had time to encode and play out all the
frames.

FILES

       /etc/alsa/conf.d/20-bluealsa.conf
              BlueALSA  device  configuration file.  ALSA additional configuration, defines the bluealsa PCM and
              CTL devices.

COPYRIGHT

       Copyright (c) 2016-2023 Arkadiusz Bokowy.

       The bluez-alsa project is licensed under the terms of the MIT license.