usr.bin/sndiod/sndiod.8

.\"	$OpenBSD: sndiod.8,v 1.11 2021/07/16 15:05:58 sthen Exp $
.\"
.\" Copyright (c) 2006-2012 Alexandre Ratchov <alex@caoua.org>
.\"
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.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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.Dd $Mdocdate: July 16 2021 $
.Dt SNDIOD 8
.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 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 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 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
exposes MIDI thru boxes (hubs),
allowing programs to send MIDI messages to each other
or to hardware MIDI ports in a uniform way.
.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
Enable debugging to standard error, and do not disassociate from the
controlling terminal.
Can be specified multiple times to further increase log verbosity.
.It Fl e Ar enc
Attempt to configure the device to use this encoding.
The default is
.Va s16 .
Encoding names use the following scheme: signedness
.Po
.Va s
or
.Va u
.Pc
followed
by the precision in bits, the byte-order
.Po
.Va le
or
.Va be
.Pc ,
the number of
bytes per sample, and the alignment
.Po
.Va msb
or
.Va lsb
.Pc .
Only the signedness and the precision are mandatory.
Examples:
.Va u8 , s16le , s24le3 , s24le4lsb .
.It Fl F Ar device
Specify an alternate device to use.
If it doesn't work, the one given with the last
.Fl f
or
.Fl F
options will be used.
For instance, specifying a USB device following a
PCI device allows
.Nm
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
.Va server.device
control of the
.Xr sndioctl 1
utility.
.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.
If no
.Fl f
option is used,
.Nm
will use
.Pa rsnd/0 , rsnd/1 ,
.No ... ,
.Pa rsnd/3 .
.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 on;
.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.
As the communication is not secure, this
option is only suitable for local networks where all hosts
and users are trusted.
.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
Specify an alternate MIDI port to use.
If it doesn't work, the one given with the last
.Fl Q
or
.Fl q
options will be used.
For instance, this allows a USB MIDI controller to be replaced without
the need to restart programs using it.
.It Fl q Ar port
Expose the given MIDI port.
This allows multiple programs to share the port.
If no
.Fl q
option is used,
.Nm
will use
.Pa rmidi/0 , rmidi/1 ,
.No ... ,
.Pa rmidi/7 .
.It Fl r Ar rate
Attempt to force the device to use this sample rate in Hertz.
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.
Each
.Nm
server instance has a unique unit number,
used in
.Xr sndio 7
device names.
The default is 0.
.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.
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.
.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.
The default is
.Va off .
.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 480 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 sub-devices
.Pq Fl s
are specified for a device, a default 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 SIGINT
or
.Dv SIGTERM ,
it terminates.
If
.Nm
is sent
.Dv SIGHUP ,
it reopens all audio devices and MIDI ports.
.Pp
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
.Fl 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.
.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 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.
Processing precision can be increased to 24-bit at compilation time 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.
There's nothing to prevent the user
from shooting himself in the foot by creating such a deadlock.