Ubuntu Manpage: bluealsa-plugins - Bluetooth Audio ALSA Plugins

Provided by: libasound2-plugin-bluez_4.2.0-1_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. For example:

CODEC=aptx:4f0000000100ff

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.