SNDIOD(8)		    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] [-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]

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
	     Same as -f except that if the device is disconnected, the one
	     given with the previous -f or -F option will be used.

     -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.  If no -f option is used, sndiod will
	     use rsnd/0, rsnd/1, ..., rsnd/3.

     -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
	     Specify an alternate MIDI port to use.  If it doesn't work, the
	     one given with the last -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
	     Expose the given MIDI port.  This allows multiple programs to
	     share the port.  If no -q option is used, sndiod will use
	     rmidi/0, rmidi/1, ..., rmidi/7.

     -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 a 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 127.

     -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.  The default is off.

     -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 480 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/default.

     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.

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/default and a MIDI-controlled snd/mmc:

	   $ sndiod -r 48000 -z 400 -s default -t slave -s mmc

     Streams connected to snd/default behave normally, while streams connected
     to snd/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.

HOT PLUGGING
     If devices specified with -F are unavailable when needed or unplugged at
     runtime, sndiod will attempt to seamlessly fall back to the last device
     specified.

     sndiod will not automatically switch to specified device that is plugged
     at runtime.  Instead, sndioctl(1) must be used to change the
     server.device control.

     For instance, specifying a USB device with -F following a PCI device with
     -f allows sndiod to use the USB one preferably when it's connected and to
     fall back to the PCI one when it's disconnected.

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/default and snd/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/default
     and snd/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.

     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
     themselves in the foot by creating such a deadlock.

Debian			       October 19, 2025				Debian