sndio/sndiod/sndiod.1

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.\" Copyright (c) 2006 Alexandre Ratchov <alex@caoua.org>
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.Dd $Mdocdate$
.Dt SNDIOD 1
.Os
.Sh NAME
.Nm sndiod
.Nd audio/MIDI server
.Sh SYNOPSIS
.Nm sndiod
.Bk -words
.Op Fl d
.Op Fl a Ar flag
.Op Fl b Ar nframes
.Op Fl C Ar min : Ns Ar max
.Op Fl c Ar min : Ns Ar max
.Op Fl e Ar enc
.Op Fl f Ar device
.Op Fl j Ar flag
.Op Fl L Ar addr
.Op Fl m Ar mode
.Op Fl q Ar port
.Op Fl r Ar rate
.Op Fl s Ar name
.Op Fl t Ar mode
.Op Fl U Ar unit
.Op Fl v Ar volume
.Op Fl w Ar flag
.Op Fl x Ar policy
.Op Fl z Ar nframes
.Ek
.Sh DESCRIPTION
The
.Nm
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,
.Nm
accepts connections from programs
running on the same system only;
it initializes only when programs are using its services,
allowing
.Nm
to consume a negligible amount of system resources the rest of the time.
Systems with no audio hardware can use
.Nm
to keep hot-pluggable devices usable by default at
virtually no cost.
.Pp
.Nm
operates as follows: it exposes at least one
.Em sub-device
that any number of audio programs can connect to and use as if it was
audio hardware.
During playback,
.Nm
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
.Nm
process, it has the opportunity to process audio data on the fly:
.Pp
.Bl -bullet -offset indent -compact
.It
Change the sound encoding to overcome incompatibilities between
software and hardware.
.It
Route the sound from one channel to another,
join stereo or split mono.
.It
Control the per-application playback volume as well as the
master volume.
.It
Monitor the sound being played, allowing one program to record
what other programs play.
.El
.Pp
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.
.Pp
.Nm
can expose a MIDI port that can be used as a MIDI thru box
(aka a
.Dq 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.
.Pp
Finally,
.Nm
exposes a control MIDI port usable for:
.Pp
.Bl -bullet -offset indent -compact
.It
Volume control.
.It
Common clock source for audio and MIDI programs.
.It
Start, stop and relocate groups of audio programs.
.El
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl a Ar flag
Control whether
.Nm
opens the audio device or the MIDI port only when needed or keeps
it open all the time.
If the flag is
.Va 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
.Va 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
.Va off .
.It Fl b Ar 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
.Pq Fl z ,
if the block size is set.
.It Xo
.Fl C Ar min : Ns Ar max ,
.Fl c Ar min : Ns Ar max
.Xc
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.
.It Fl d
Increase log verbosity.
.Nm
logs on
.Em stderr .
.It Fl e Ar enc
Audio device encoding (see below).
The default is signed, 16-bit, native byte order.
.It Fl f Ar device
Add this
.Xr sndio 7
audio device to devices used for playing and/or recording.
Preceding per-device options
.Pq Fl aberwz
apply to this device.
Sub-devices
.Pq Fl s
that are applied after will be attached to this device.
Device mode and parameters are determined from sub-devices
attached to it.
.It Fl j Ar 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
.Va off
then client channels are routed to the corresponding
device channel, possibly discarding channels not present in the device.
If the flag is
.Va 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
.Ar on .
.It Fl L Ar addr
Specify a local network address
.Nm
should listen;
.Nm
will listen on TCP port 11025+n, where n is the unit number
specified with
.Fl U .
Without this option,
.Nm
listens on the
.Ux Ns -domain
socket only, and is not reachable from any network.
If the option argument is
.Sq -
then
.Nm
will accept connections from any address.
.It Fl m Ar mode
Set the sub-device mode.
Valid modes are
.Ar play ,
.Ar rec ,
and
.Ar 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
.Ar play , Ns Ar rec
(i.e. full-duplex).
.It Fl q Ar port
Expose the given MIDI port.
This allows multiple programs
.Nm
instead of the given physical MIDI port which can be used
by only one program.
.It Fl r Ar rate
Audio device sample rate in Hertz of the stream.
The default is 48000.
.It Fl s Ar name
Add
.Ar name
to the list of sub-devices to expose.
This allows clients to use
.Nm
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
.Ar name
corresponds to the
.Dq option
part of the
.Xr sndio 7
device name string.
.It Fl t Ar mode
Select the way clients are controlled by MIDI Machine Control (MMC)
messages received by
.Nm .
If the mode is
.Va off
(the default), then programs are not affected by MMC messages.
If the mode is
.Va 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.
.It Fl U Ar unit
Unit number to use when running in server mode.
Each
.Nm
server instance has an unique unit number,
used in
.Xr sndio 7
device names.
The default is 0.
The unit number must be set before any
.Fl L
is used.
.It Fl v Ar volume
Software volume attenuation of playback.
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 programs to be doubled.
The default is 127 i.e. no attenuation.
.It Fl w Ar flag
Control
.Nm
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
.Fl v
option.
If the flag is
.Va on ,
then the master volume is automatically adjusted to avoid clipping.
Using
.Va off
makes sense in the rare situation where all programs lower their volumes.
The default is
.Va on .
.It Fl x Ar policy
Action when the program cannot accept
recorded data fast enough or cannot provide data to play fast enough.
If the policy is
.Dq 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
.Dq sync
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.
If the policy is
.Dq error
then the program is disconnected.
.Pp
If a sub-device is created with the
.Fl t
option,
the
.Dq ignore
action is disabled for any program connected to it
to ensure proper synchronization.
.It Fl z Ar 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
.Fl 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
.Pq Fl b ,
if the buffer size is set.
.El
.Pp
On the command line,
per-device parameters
.Pq Fl aberwz
must precede the device definition
.Pq Fl f ,
and per-sub-device parameters
.Pq Fl Ccjmtvx
must precede the sub-device definition
.Pq Fl s .
Sub-device definitions
.Pq Fl s
must follow the definition of the device
.Pq Fl f
to which they are attached.
.Pp
If no audio devices
.Pq Fl f
are specified,
settings are applied as if
the default device is specified.
If no
.Nm
sub-devices
.Pq Fl s
are specified for a device, a default server sub-device is
created attached to it.
If a device
.Pq Fl f
is defined twice, both definitions are merged:
parameters of the first one are used but sub-devices
.Pq Fl s
of both definitions are created.
The default
.Xr sndio 7
device used by
.Nm
is
.Pa rsnd/0 ,
and the default sub-device exposed by
.Nm
is
.Pa snd/0 .
.Pp
If
.Nm
is sent
.Dv SIGHUP ,
.Dv SIGINT
or
.Dv SIGTERM ,
it terminates.
.Pp
Encodings are specified using the
.Fl e
option.
The following encodings are supported:
.Pp
.Bl -tag -width s32lexxx -offset indent -compact
.It s8
signed 8-bit
.It u8
unsigned 8-bit
.It s16le
signed 16-bit, little endian
.It u16le
unsigned 16-bit, little endian
.It s16be
signed 16-bit, big endian
.It u16be
unsigned 16-bit, big endian
.It s24le
signed 24-bit, stored in 4 bytes, little endian
.It u24le
unsigned 24-bit, stored in 4 bytes, little endian
.It s24be
signed 24-bit, stored in 4 bytes, big endian
.It u24be
unsigned 24-bit, stored in 4 bytes, big endian
.It s32le
signed 32-bit, little endian
.It u32le
unsigned 32-bit, little endian
.It s32be
signed 32-bit, big endian
.It u32be
unsigned 32-bit, big endian
.It s24le3
signed 24-bit, packed in 3 bytes, little endian
.It u24le3
unsigned 24-bit, packed in 3 bytes, big endian
.It s24be3
signed 24-bit, packed in 3 bytes, little endian
.It u24be3
unsigned 24-bit, packed in 3 bytes, big endian
.It s20le3
signed 20-bit, packed in 3 bytes, little endian
.It u20le3
unsigned 20-bit, packed in 3 bytes, big endian
.It s20be3
signed 20-bit, packed in 3 bytes, little endian
.It u20be3
unsigned 20-bit, packed in 3 bytes, big endian
.It s18le3
signed 18-bit, packed in 3 bytes, little endian
.It u18le3
unsigned 18-bit, packed in 3 bytes, big endian
.It s18be3
signed 18-bit, packed in 3 bytes, little endian
.It u18be3
unsigned 18-bit, packed in 3 bytes, big endian
.El
.Sh SERVER MODE
.Nm
can be used
to overcome hardware limitations and allow applications
to run on fixed sample rate devices or on devices
supporting only unusual encodings.
.Pp
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.
.Sh MIDI CONTROL
.Nm
creates a MIDI port with the same name as the exposed audio
sub-device to which MIDI programs can connect.
.Nm
exposes the audio device clock
and allows audio device properties to be controlled
through MIDI.
.Pp
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.
.Pp
The master volume can be changed using the standard master volume
system exclusive message.
.Pp
Streams created with the
.Fl t
option are controlled by the following MMC messages:
.Bl -tag -width relocateXXX -offset indent
.It relocate
This message is ignored by audio
.Nm
clients, but the given time position is sent to MIDI ports as an MTC
.Dq "full frame"
message forcing all MTC-slaves to relocate to the given
position (see below).
.It start
Put all streams in starting mode.
In this mode,
.Nm
waits for all streams to become ready
to start, and then starts them synchronously.
Once started, new streams can be created
.Pq Nm sndiod
but they will be blocked
until the next stop-to-start transition.
.It 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.
.El
.Pp
Streams created with the
.Fl t
option export the
.Nm
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
.Pq Fl r
and block sizes
.Pq Fl z
are recommended:
.Pp
.Bl -bullet -offset indent -compact
.It
44100Hz, 441 frames (MTC rate is 100Hz)
.It
48000Hz, 400 frames (MTC rate is 120Hz)
.It
48000Hz, 480 frames (MTC rate is 100Hz)
.It
48000Hz, 500 frames (MTC rate is 96Hz)
.El
.Pp
For instance, the following command will create two devices:
the default
.Va snd/0
and a MIDI-controlled
.Va snd/0.mmc :
.Bd -literal -offset indent
$ sndiod -r 48000 -z 400 -s default -t slave -s mmc
.Ed
.Pp
Streams connected to
.Va snd/0
behave normally, while streams connected to
.Va 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.
.Sh 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
.Pa snd/0
and
.Pa snd/0.rear
devices:
.Bd -literal -offset indent
$ sndiod -s default -c 2:3 -s rear
.Ed
.Pp
Start server creating the default sub-device with low volume and
an additional sub-device for high volume output, exposing the
.Pa snd/0
and
.Pa snd/0.max
devices:
.Bd -literal -offset indent
$ sndiod -v 65 -s default -v 127 -s max
.Ed
.Pp
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.
.Bd -literal -offset indent
$ sndiod -r 48000 -b 480 -z 240
.Ed
.Sh SEE ALSO
.Xr aucat 1 ,
.Xr audioctl 1 ,
.Xr cdio 1 ,
.Xr mixerctl 1 ,
.Xr audio 4 ,
.Xr sndio 7
.Sh BUGS
Resampling is low quality; down-sampling especially should be avoided
when recording.
.Pp
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.
.Pp
If
.Fl a Ar off
is used,
.Nm
creates sub-devices to expose first
and then opens the audio hardware on demand.
Technically, this allows
.Nm
to attempt to use one of the sub-devices it exposes as an audio device,
creating a deadlock.
To avoid this,
.Fl a Ar off
is disabled for the default audio device, but nothing prevents the user
from shooting himself in the foot by creating a similar deadlock.
.Pp
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.