NAME¶
sndiod
—
audio/MIDI server
SYNOPSIS¶
sndiod |
[ -d ]
[-a flag ]
[-b nframes ]
[-C min:max ]
[-c min:max ]
[-e enc ]
[-f device ]
[-j flag ]
[-L addr ]
[-m mode ]
[-q port ]
[-r rate ]
[-s name ]
[-t mode ]
[-U unit ]
[-v volume ]
[-w flag ]
[-z nframes ] |
DESCRIPTION¶
The
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:
- Change the sound encoding to overcome incompatibilities between software
and hardware.
- Route the sound from one channel to another, join stereo or split
mono.
- Control the per-application playback volume as well as the master
volume.
- Monitor the sound being played, allowing one program to record what other
programs play.
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:
- Volume control.
- Common clock source for audio and MIDI programs.
- Start, stop and relocate groups of audio programs.
The options are as follows:
-a
flag
- Control whether
sndiod
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
- The buffer size of the audio device in frames. A frame consists of one
sample for each channel in the stream. This is the number of frames that
will be buffered before being played and thus controls the playback
latency. The default is 7680 or twice the block size
(
-z
), if the block size is set.
-C
min:max,
-c
min:max
- The range of channel numbers for recording and playback directions,
respectively any client is allowed to use. This is a subset of the audio
device channels. The default is 0:1, i.e. stereo.
-d
- Enable debugging to standard error, and do not disassociate from the
controlling terminal. Can be specified multiple times to further increase
log verbosity.
-e
enc
- Attempt to configure the device to use this encoding. The default is
s16. Encoding names use the following
scheme: signedness (s or
u) followed by the precision in bits, the
byte-order (le or
be), the number of bytes per sample, and
the alignment (msb or
lsb). Only the signedness and the
precision are mandatory. Examples: u8,
s16le,
s24le3,
s24le4lsb.
-f
device
- Add this sndio(7) audio device to devices
used for playing and/or recording. Preceding per-device options
(
-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.
-j
flag
- Control whether program channels are joined or expanded if the number of
channels requested by a program is not equal to the device number of
channels. If the flag is off then client
channels are routed to the corresponding device channel, possibly
discarding channels not present in the device. If the flag is
on, then a single client channel may be
sent on multiple device channels, or multiple client channels may be sent
to a single device channel. For instance, this feature could be used for
mono to stereo conversions. The default is
on.
-L
addr
- Specify a local network address
sndiod
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
- Set the sub-device mode. Valid modes are
play,
rec, and
mon, corresponding to playback, recording
and monitoring. A monitoring stream is a fake recording stream
corresponding to the mix of all playback streams. Multiple modes can be
specified, separated by commas, but the same sub-device cannot be used for
both recording and monitoring. The default is
play,rec
(i.e. full-duplex).
-q
port
- Expose the given MIDI port. This allows multiple programs to share the
port.
-r
rate
- Attempt to force the device to use this sample rate in Hertz. The default
is 48000.
-s
name
- Add name to the list of sub-devices to
expose. This allows clients to use
sndiod
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
mode
- Select the way clients are controlled by MIDI Machine Control (MMC)
messages received by
sndiod
. 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
unit
- Unit number. Each
sndiod
server
instance has an unique unit number, used in
sndio(7) device names. The default is 0.
-v
volume
- Software volume attenuation of playback. The value must be between 1 and
127, corresponding to -42dB and -0dB attenuation in 1/3dB steps. Clients
inherit this parameter. Reducing the volume in advance allows a client's
volume to stay independent from the number of clients as long as their
number is small enough. 18 volume units (i.e. -6dB attenuation) allows the
number of playback programs to be doubled. The default is 118 i.e.
-3dB.
-w
flag
- Control
sndiod
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
- The audio device block size in frames. This is the number of frames
between audio clock ticks, i.e. the clock resolution. If a sub-device is
created with the
-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
SIGHUP
,
SIGINT
or
SIGTERM
, it terminates.
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.
MIDI CONTROL¶
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:
- relocate
- This message is ignored by audio
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).
- start
- Put all streams in starting mode. In this mode,
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.
- stop
- Put all streams in stopped mode (the default). In this mode, any stream
attempting to start playback or recording is paused. Client streams that
are already started are not affected until they stop and try to start
again.
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:
- 44100Hz, 441 frames (MTC rate is 100Hz)
- 48000Hz, 400 frames (MTC rate is 120Hz)
- 48000Hz, 480 frames (MTC rate is 100Hz)
- 48000Hz, 500 frames (MTC rate is 96Hz)
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.
EXAMPLES¶
Start server using default parameters, creating an additional sub-device for
output to channels 2:3 only (rear speakers on most cards), exposing the
snd/0 and
snd/0.rear devices:
$ 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
SEE ALSO¶
sndio(7)
BUGS¶
Resampling is low quality; down-sampling especially should be avoided when
recording.
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.