Ubuntu Manpage: sndio — interface to audio and MIDI

NAME

     sndio — interface to audio and MIDI

DESCRIPTION

The sndio audio and MIDI system provides access to audio and MIDI hardware and to services
provided by sndiod(8), summarized below.

Hardware audio(4) devices correspond to peripherals. Only one application may use any
device at a given time. Generally a limited number of encodings, sample rates and channel
numbers are supported by the hardware, which may not meet the requirements of audio
programs.

To overcome hardware limitations and to allow multiple applications to share the hardware,
sndiod(8) can be used. It exposes one or more software sub-devices backed by the underlying
hardware, while doing all necessary conversions on the fly. It can mix multiple streams or
split the hardware into multiple sub-devices, to allow programs to use the hardware
concurrently.

Hardware MIDI ports correspond to serial connectors provided by the midi(4) driver. They
are typically used to access MIDI hardware (synthesizers, keyboards, control surfaces,
etc.), but they do not allow applications to exchange information using the MIDI protocol.

Software MIDI thru boxes allow one application to send MIDI data to other applications
connected to the thru box (for instance a software sequencer can send events to multiple
software synthesizers). There's no hardware involved: thru boxes are created by sndiod(8).

Additionally, sndiod(8) exposes a MIDI port used to control and monitor audio streams in
real time using MIDI.

DEVICE NAMES

     From the user's perspective every audio interface, MIDI port, and sndiod(8) service has a
     name of the form:

                            type[@hostname][,unit]/devnum[.option]

     This information is used by audio and MIDI applications to determine how to access the audio
     device or MIDI port.

     type      The type of the audio device or MIDI port.  Possible values are:

                  rsnd      Raw audio(4) device.
                  rmidi     Raw midi(4) port.
                  snd       Audio device exposed by sndiod(8).
                  midithru  MIDI thru box created with sndiod(8).
                  midi      MIDI port exposed by sndiod(8).
                  default   Default audio device or MIDI port (see below).

     hostname  The hostname or address where the remote sndiod(8) server to connect to is
               running.

     unit      The number of the sndiod(8) server to connect to, corresponding to the integer
               specified using the -U option of sndiod(8).  Useful only if multiple sndiod(8)
               servers are running on the same system.

     devnum    Device number.  For hardware audio or MIDI ports, this corresponds to the
               character device minor number.  For audio devices or MIDI ports created with
               sndiod(8) it corresponds to the number of the corresponding -fq option on the
               command line.

     option    Corresponds to the sub-device string registered using the -s option of sndiod(8).

     For example:

        rsnd/0      First hardware audio device.
        rmidi/5     Hardware MIDI port number 5.
        snd/0       First audio device exposed by sndiod(8).
        snd/0.rear  Sub-device registered with -s rear.
        midithru/0  First MIDI thru box created with sndiod(8).

DEFAULTS

     If default is used as the audio device, the program will use the one specified in the
     AUDIODEVICE environment variable.  If it is not set, the program first tries to connect to
     snd/0.  If that fails, it then tries to use rsnd/0.  This allows the sndiod(8) audio server
     to be used by default and the bare hardware as fallback; programs don't have to be
     reconfigured when sndiod(8) is started or stopped.

     If default is used as the MIDI port, the program will use the one specified in the
     MIDIDEVICE environment variable.  If it is not set, the program first tries to connect to
     midithru/0.  If that fails, it then tries to use rmidi/0.  As long as sndiod(8) is running,
     this allows programs to exchange MIDI data on machines with no MIDI hardware by default,
     e.g. a MIDI player could use a software synthesizer with no manual configuration required.

AUTHENTICATION

     If a shared sndiod(8) server is running, for privacy reasons only one user may have
     connections to it at a given time (though the same user could have multiple connections to
     it).  Users are identified by their session cookie, which is automatically generated by
     audio or MIDI applications upon the first connection to the server.  The cookie is stored in
     $HOME/.aucat_cookie and contains 128 bits of raw random data.

     If a session needs to be shared between multiple users, they can connect to the server using
     the same cookie.

ENVIRONMENT

     AUDIODEVICE     Audio device to use if the application provides no device chooser.
     MIDIDEVICE      MIDI port to use if the application provides no MIDI port chooser.

     Environment variables are ignored by programs with the set-user-ID or set-group-ID bits set.

FILES

     /dev/audioN     Audio devices.
     /dev/rmidiN     MIDI ports.