SNDIOD(8) | System Manager's Manual | SNDIOD(8) |
sndiod
—
sndiod |
[-d ]
[-a flag]
[-b nframes]
[-C min:max]
[-c min:max]
[-e enc]
[-F device]
[-f device]
[-j flag]
[-L addr]
[-m mode]
[-Q port]
[-q port]
[-r rate]
[-s name]
[-t mode]
[-U unit]
[-v volume]
[-w flag]
[-z nframes] |
sndiod
daemon is an intermediate layer between audio
or MIDI programs and the hardware. It performs the necessary audio processing
to allow any program to work on any supported hardware. By default,
sndiod
accepts connections from programs running on
the same system only; it initializes only when programs are using its
services, allowing sndiod
to consume a negligible
amount of system resources the rest of the time. Systems with no audio
hardware can use sndiod
to keep hot-pluggable devices
usable by default at virtually no cost.
sndiod
operates as follows: it exposes at
least one sub-device that any number of audio programs can
connect to and use as if it was audio hardware. During playback,
sndiod
receives audio data concurrently from all
programs, mixes it and sends the result to the hardware device. Similarly,
during recording it duplicates audio data recorded from the device and sends
it to all programs. Since audio data flows through the
sndiod
process, it has the opportunity to process
audio data on the fly:
Processing is configured on a per sub-device basis, meaning that the sound of all programs connected to the same sub-device will be processed according to the same configuration. Multiple sub-devices can be defined, allowing multiple configurations to coexist. The user selects the configuration a given program will use by selecting the sub-device the program uses.
sndiod
exposes MIDI thru boxes (hubs),
allowing programs to send MIDI messages to each other or to hardware MIDI
ports in a uniform way.
Finally, sndiod
exposes a control MIDI
port usable for:
The options are as follows:
-a
flagsndiod
opens the audio device or
the MIDI port only when needed or keeps it open all the time. If the flag
is on then the audio device or MIDI port is kept
open all the time, ensuring no other program can steal it. If the flag is
off, then it's automatically closed, allowing other
programs to have direct access to the audio device, or the device to be
disconnected. The default is off.-b
nframes-z
), if the block size is set.-C
min:max,
-c
min:max-d
-e
enc-F
device-f
or -F
options
will be used. For instance, specifying a USB device following a PCI device
allows sndiod
to use the USB one preferably when
it's connected and to fall back to the PCI one when it's disconnected.
Alternate devices may be switched with the
server.device control of the
sndioctl(1) utility.-f
device-aberwz
) apply to this device. Sub-devices
(-s
) that are applied after will be attached to
this device. Device mode and parameters are determined from sub-devices
attached to it. If no -f
option is used,
sndiod
will use rsnd/0,
rsnd/1, ...,
rsnd/3.-j
flag-L
addrsndiod
should
listen on; sndiod
will listen on TCP port 11025+n,
where n is the unit number specified with -U
.
Without this option, sndiod
listens on the
UNIX-domain socket only, and is not reachable from
any network. If the option argument is ‘-’ then
sndiod
will accept connections from any address.
As the communication is not secure, this option is only suitable for local
networks where all hosts and users are trusted.-m
mode-Q
port-Q
or -q
options will be used. For instance, this allows a USB MIDI controller to
be replaced without the need to restart programs using it.-q
port-q
option is used,
sndiod
will use rmidi/0,
rmidi/1, ...,
rmidi/7.-r
rate-s
namesndiod
instead of the
physical audio device for audio input and output in order to share the
physical device with other clients. Defining multiple sub-devices allows
splitting a physical audio device into sub-devices having different
properties (e.g. channel ranges). The given name
corresponds to the “option” part of the
sndio(7) device name string.-t
modesndiod
. If the mode is
off (the default), then programs are not affected by
MMC messages. If the mode is slave, then programs
are started synchronously by MMC start messages; additionally, the server
clock is exposed as MIDI Time Code (MTC) messages allowing MTC-capable
software or hardware to be synchronized to audio programs.-U
unitsndiod
server instance has an
unique unit number, used in sndio(7)
device names. The default is 0.-v
volume-w
flagsndiod
behaviour when the maximum volume
of the hardware is reached and a new program starts playing. This happens
only when volumes are not properly set using the
-v
option. If the flag is
on, then the master volume is automatically adjusted
to avoid clipping. Using off makes sense in the rare
situation where all programs lower their volumes. The default is
on.-z
nframes-t
option, and MTC is used for
synchronization, the clock resolution must be 96, 100 or 120 ticks per
second for maximum accuracy. For instance, 100 ticks per second at 48000Hz
corresponds to a 480 frame block size. The default is 960 or half of the
buffer size (-b
), if the buffer size is set.On the command line, per-device parameters
(-aberwz
) must precede the device definition
(-f
), and per-sub-device parameters
(-Ccjmtvx
) must precede the sub-device definition
(-s
). Sub-device definitions
(-s
) must follow the definition of the device
(-f
) to which they are attached.
If no audio devices (-f
) are specified,
settings are applied as if the default device is specified. If no
sub-devices (-s
) are specified for a device, a
default sub-device is created attached to it. If a device
(-f
) is defined twice, both definitions are merged:
parameters of the first one are used but sub-devices
(-s
) of both definitions are created. The default
sndio(7) device used by
sndiod
is rsnd/0, and the
default sub-device exposed by sndiod
is
snd/0.
If sndiod
is sent
SIGINT
or SIGTERM
, it
terminates. If sndiod
is sent
SIGHUP
, it reopens all audio devices and MIDI
ports.
By default, when the program cannot accept recorded data fast
enough or cannot provide data to play fast enough, the program is paused,
i.e. samples that cannot be written are discarded and samples that cannot be
read are replaced by silence. If a sub-device is created with the
-t
option, then recorded samples are discarded, but
the same amount of silence will be written once the program is unblocked, in
order to reach the right position in time. Similarly silence is played, but
the same amount of samples will be discarded once the program is unblocked.
This ensures proper synchronization between programs.
sndiod
creates a MIDI port with the same name as the
exposed audio sub-device to which MIDI programs can connect.
sndiod
exposes the audio device clock and allows audio
device properties to be controlled through MIDI.
A MIDI channel is assigned to each stream, and the volume is changed using the standard volume controller (number 7). Similarly, when the audio client changes its volume, the same MIDI controller message is sent out; it can be used for instance for monitoring or as feedback for motorized faders.
The master volume can be changed using the standard master volume system exclusive message.
Streams created with the -t
option are
controlled by the following MMC messages:
sndiod
clients,
but the given time position is sent to MIDI ports as an MTC “full
frame” message forcing all MTC-slaves to relocate to the given
position (see below).sndiod
waits for all streams to become ready to
start, and then starts them synchronously. Once started, new streams can
be created (sndiod
) but they will be blocked until
the next stop-to-start transition.Streams created with the -t
option export
the sndiod
device clock using MTC, allowing
non-audio software or hardware to be synchronized to the audio stream.
Maximum accuracy is achieved when the number of blocks per second is equal
to one of the standard MTC clock rates (96, 100 and 120Hz). The following
sample rates (-r
) and block sizes
(-z
) are recommended:
For instance, the following command will create two devices: the default snd/0 and a MIDI-controlled snd/0.mmc:
$ sndiod -r 48000 -z 400 -s default -t slave -s mmc
Streams connected to snd/0 behave normally, while streams connected to snd/0.mmc wait for the MMC start signal and start synchronously. Regardless of which device a stream is connected to, its playback volume knob is exposed.
$ sndiod -s default -c 2:3 -s rear
Start server creating the default sub-device with low volume and an additional sub-device for high volume output, exposing the snd/0 and snd/0.max devices:
$ sndiod -v 65 -s default -v 127 -s max
Start server configuring the audio device to use a 48kHz sample frequency, 240-frame block size, and 2-block buffers. The corresponding latency is 10ms, which is the time it takes the sound to propagate 3.5 meters.
$ sndiod -r 48000 -b 480 -z 240
Processing is done using 16-bit arithmetic, thus samples with more than 16 bits are rounded. 16 bits (i.e. 97dB dynamic) are largely enough for most applications though. Processing precision can be increased to 24-bit at compilation time though.
If -a
off is used,
sndiod
creates sub-devices to expose first and then
opens the audio hardware on demand. Technically, this allows
sndiod
to attempt to use one of the sub-devices it
exposes as an audio device, creating a deadlock. There's nothing to prevent
the user from shooting himself in the foot by creating such a deadlock.
March 8, 2021 | x86_64 |