AUCAT(1)

NAME
aucat, sndiod - audio/MIDI server and stream manipulation tool

SYNOPSIS
aucat [-dn] [-b nframes] [-C min:max] [-c min:max] [-e enc] [-f device] [-h fmt] [-i file] [-j flag] [-m mode] [-o file] [-q port] [-r rate] [-t mode] [-v volume] [-w flag] [-x policy] [-z nframes]
sndiod [-dM] [-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] [-x policy] [-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 were 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 can expose a MIDI port that can be used as a MIDI thru box (aka a “hub” for MIDI messages), allowing any program to send MIDI messages to MIDI hardware or to another program in a uniform way. Hardware ports can be subscribed to such MIDI thru boxes, allowing multiple programs to share the MIDI port.

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 aucat utility can play, record, mix, and convert regular audio files. It has the same processing capabilities as sndiod. Both operate the same way, except that the former processes audio data stored in files, while the latter processes audio data provided or consumed by programs. Instead of a list of sub-devices, aucat is given a list of files to play or record.

The options are as follows:

-a flag
    Control whether sndiod opens the audio device only when needed or keeps it open all the time. This applies to MIDI ports controlling the device as well. If the flag is on then the device 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 device, or the device to be disconnected. The default is off, except for the default device.

-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 stream channel numbers for recording and playback directions, respectively. The default is 0:1, i.e. stereo.

-d  Increase log verbosity. sndiod logs on stderr until it daemonizes.

-e enc
    Encoding of the playback or recording stream (see below). The default is signed, 16-bit, native byte order.

-f device
    Add this sndio(7) audio device to devices used for playing and/or recording. Preceding per-device options (-abwz) apply to this device. Streams (-ios) and control MIDI ports (-q) that are applied after will be attached to this device. Device mode and parameters are determined from streams attached to it.

-h fmt
    File format of the playback or record stream (see below). The default is auto.

-i file
    Add this file to the list of streams to play. If the option argument is ‘-’ then standard input will be used.

-j flag
    Control whether stream channels are joined or expanded if the stream number of channels is not equal to the device number of channels. If the flag is off then stream channels are routed to the corresponding device channel, possibly discarding channels not present in the device. If the flag is on, then a single stream channel may be sent on multiple device channels, or multiple stream channels may be sent to a single device channel. For instance, this feature could be used to request mono streams to be sent on multiple outputs or to record a stereo input into a mono stream. The default is on.

-L addr
    Specify a local network address sndiod should listen; 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.

-M  Create a MIDI thru box (i.e. MIDI-only pseudo device). It merges any number of MIDI inputs and broadcasts the result to any number of MIDI outputs, similarly to a hardware MIDI thru box. Only MIDI ports (-q) and MIDI files ( -io preceded by -m midi) can be attached to it. Exposed sub-devices by sndiod behave like software MIDI ports, allowing any MIDI-capable application to send MIDI messages to MIDI hardware or to another application in a uniform way.

-m mode
    Set the stream mode. Valid modes are play, rec, mon, and midi, corresponding to playback, recording, monitoring and MIDI control. 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 stream cannot be used for both recording and monitoring. The default is play,rec,midi (i.e. full-duplex with MIDI control enabled).

-n  Create a loopback pseudo audio device. Send input streams to the output, processing them on the fly. This pseudo-device is useful to mix, demultiplex, resample or re-encode audio files offline. It requires at least one input (-i) and one output (-o).

-o file
    Add this file to the list of recording streams. If the option argument is ‘-’ then standard output will be used.

-q port
    Expose the audio device clock on this sndio(7) MIDI port and allow audio device properties to be controlled through MIDI. This includes per-stream volumes and the ability to synchronously start, stop and relocate streams created in MIDI Machine Control (MMC) slave mode (-t).

-r rate
    Sample rate in Hertz of the stream. 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 logical 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 streams are controlled by MIDI Machine Control (MMC) messages. If the mode is off (the default), then streams are not affected by MMC messages. If the mode is slave, then streams 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 streams.

-U unit
    Unit number to use when running in server mode. Each sndiod server instance has an unique unit number, used in sndio(7) device names. The default is 0. The unit number must be set before any -L is used.

-v volume
    Software volume attenuation of the playback stream. The value must be between 1 and 127, corresponding to -42dB and -0dB attenuation in 1/3dB steps. In server mode, 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 streams to be doubled. The default is 127 i.e. no attenuation.

-w flag
    Control aucat and sndiod behaviour when the maximum volume of the hardware is reached and a new stream is connected. This happens only when stream volumes are not properly set using the -v option. If the flag is on, then the master volume (corresponding to the mix of all playback streams) is automatically adjusted to avoid clipping. Using off makes sense when all streams are recorded or produced with properly lowered volumes. The default is on.

-x policy
    Action when the output stream cannot accept recorded data fast enough or the input stream cannot provide data to play fast enough. If the policy is “ignore” (the default) then samples that cannot be written are discarded and samples that cannot be read are replaced by silence. If the policy is “sync” then recorded samples are discarded, but the same amount of silence will be written once the stream 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 stream is unblocked. If the policy is “error” then the stream is closed permanently.

    If a stream is created with the -t option, the “ignore” action is disabled for any stream connected to it to ensure proper synchronization.

-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 stream 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 (-abwz) must precede the device definition (-fMn), and per-stream parameters (-Ccehjmrtvx) must precede the stream definition (-ios). MIDI ports (-q) and stream definitions (-ios) must follow the definition of the device (-fMn) to which they are attached.

If no audio devices (-fMn) are specified, settings are applied as if the default device is specified. If no sndiod sub-devices (-s) are specified for a device, a default server sub-device is created attached to it. If a device (-fMn) is defined twice, both definitions are merged: parameters of the first one are used but streams (-ios) and MIDI control ports (-q) 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 or aucat is sent SIGHUP, SIGINT or SIGTERM, it terminates recording to files.

File formats are specified using the -h option. The following file formats are supported:

raw Headerless file. This format is recommended since it has no limitations.

wav Microsoft WAVE file format. There are limitations inherent to the file format itself: not all encodings are supported, file sizes are limited to 2GB, and the file must support the lseek(2) operation (e.g. pipes do not support it).

auto
    Try to guess, depending on the file name.

Encodings are specified using the -e option. The following encodings are supported:

s8  signed 8-bit
u8  unsigned 8-bit
s16le
    signed 16-bit, little endian
u16le
    unsigned 16-bit, little endian
s16be
    signed 16-bit, big endian
u16be
    unsigned 16-bit, big endian
s24le
    signed 24-bit, stored in 4 bytes, little endian
u24le
    unsigned 24-bit, stored in 4 bytes, little endian
s24be
    signed 24-bit, stored in 4 bytes, big endian
u24be
    unsigned 24-bit, stored in 4 bytes, big endian
s32le
    signed 32-bit, little endian
u32le
    unsigned 32-bit, little endian
s32be
    signed 32-bit, big endian
u32be
    unsigned 32-bit, big endian
s24le3
    signed 24-bit, packed in 3 bytes, little endian
u24le3
    unsigned 24-bit, packed in 3 bytes, big endian
s24be3
    signed 24-bit, packed in 3 bytes, little endian
u24be3
    unsigned 24-bit, packed in 3 bytes, big endian
s20le3
    signed 20-bit, packed in 3 bytes, little endian
u20le3
    unsigned 20-bit, packed in 3 bytes, big endian
s20be3
    signed 20-bit, packed in 3 bytes, little endian
u20be3
    unsigned 20-bit, packed in 3 bytes, big endian
s18le3
    signed 18-bit, packed in 3 bytes, little endian
u18le3
    unsigned 18-bit, packed in 3 bytes, big endian
s18be3
    signed 18-bit, packed in 3 bytes, little endian
u18be3
    unsigned 18-bit, packed in 3 bytes, big endian

SERVER MODE
sndiod can be used to overcome hardware limitations and allow applications to run on fixed sample rate devices or on devices supporting only unusual encodings.

Certain applications, such as synthesis software, require a low latency audio setup. To reduce the probability of buffer underruns or overruns, especially on busy machines, the server can be started by the super-user, in which case it will run with higher priority. Any user will still be able to connect to it, but for privacy reasons only one user may have connections to it at a given time.

MIDI CONTROL
aucat can expose the audio device clock on registered MIDI ports (-q) and allows audio device properties to be controlled through MIDI. Additionally, sndiod creates a MIDI port with the same name as the exposed audio sub-device to which MIDI programs can connect.

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
    Streams are relocated to the requested time position relative to the beginning of the stream, at which playback and recording must start. If the requested position is beyond the end of file, the stream is temporarly disabled until a valid position is requested. 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 or aucat 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. Files (aucat) are stopped and rewound back to the starting position, while client streams (sndiod) 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.

For instance, the following command will play a file on the snd/0.mmc audio device, and give full control to MIDI software or hardware connected to the snd/0.thru MIDI port:

    $ aucat -f snd/0.mmc -t slave -q midithru/0 -i file.wav

At this stage, aucat will start, stop and relocate automatically following all user actions in the MIDI sequencer. Note that the sequencer must use snd/0 as the MTC source, i.e. the audio server, not the audio player.

ENVIRONMENT
AUCAT_COOKIE
    File containing user's session cookie.
AUDIODEVICE
    sndio(7) audio device to use if the -f option is not specified.

EXAMPLES
Mix and play two stereo streams, the first at 48kHz and the second at 44.1kHz:

    $ aucat -r 48000 -i file1.raw -r 44100 -i file2.raw

Record channels 2 and 3 into one stereo file and channels 6 and 7 into another stereo file using a 96kHz sampling rate for both:

    $ aucat -j off -r 96000 -C 2:3 -o file1.raw -C 6:7 -o file2.raw

Split a stereo file into two mono files:

    $ aucat -n -j off -i stereo.wav -C 0:0 -o left.wav -C 1:1 \
    	-o right.wav

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
audioctl(1), cdio(1), mixerctl(1), audio(4), sndio(7)

BUGS
The aucat utility assumes non-blocking I/O for input and output streams. It will not work reliably on files that may block (ordinary files block, pipes don't). To avoid audio underruns/overruns or MIDI jitter caused by file I/O, it's recommended to use two processes: a sndiod server handling audio and MIDI I/O and a aucat client handling disk I/O.

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.

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. To avoid this, -a off is disabled for the default audio device, but nothing prevents the user from shooting himself in the foot by creating a similar deadlock.

The ability to merge multiple inputs is provided to allow multiple applications producing MIDI data to keep their connection open while idling; it does not replace a fully featured MIDI merger.