SNDIOD(8)               FreeBSD System Manager's Manual              SNDIOD(8)

NAME
     sndiod â< 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:

           â<¢<¢   Change the sound encoding to overcome incompatibilities between
               software and hardware.
           â<¢<¢   Route the sound from one channel to another, join stereo or
               split mono.
           â<¢<¢   Control the per-application playback volume as well as the
               master volume.
           â<¢<¢   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:

           â<¢<¢   Volume control.
           â<¢<¢   Common clock source for audio and MIDI programs.
           â<¢<¢   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
             Specify an alternate device to use.  If it doesn't work, the one
             given with the last -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
             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 â<-â< 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 â<optionâ< 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 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 â<full frameâ< 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:

           â<¢<¢   44100Hz, 441 frames (MTC rate is 100Hz)
           â<¢<¢   48000Hz, 400 frames (MTC rate is 120Hz)
           â<¢<¢   48000Hz, 480 frames (MTC rate is 100Hz)
           â<¢<¢   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.

FreeBSD 13.0-RELEASE             June 15, 2021            FreeBSD 13.0-RELEASE