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sndiod.manpage
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SNDIOD(8) BSD System Manager's Manual SNDIOD(8)
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 hard‐
ware. During playback, sndiod receives audio data concurrently from all
programs, mixes it and sends the result to the hardware device. Simi‐
larly, 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 mas‐
ter 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 con‐
trols 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 direc‐
tions, respectively any client is allowed to use. This is a sub‐
set 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: signed‐
ness (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 manda‐
tory. 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 chan‐
nels 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 spec‐
ified 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 monitor‐
ing 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 proper‐
ties (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 hard‐
ware to be synchronized to audio programs.
-U unit
Unit number. Each sndiod server instance has an unique unit num‐
ber, 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 vol‐
ume 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. sam‐
ples 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 motor‐
ized 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 recom‐
mended:
· 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. Regard‐
less 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 addi‐
tional 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 fre‐
quency, 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, cre‐
ating a deadlock. There's nothing to prevent the user from shooting him‐
self in the foot by creating such a deadlock.
BSD January 1, 2018 BSD