man/man4/pcm.4

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.Dd February 15, 2025
.Dt SOUND 4
.Os
.Sh NAME
.Nm sound ,
.Nm pcm ,
.Nm snd
.Nd
.Fx
PCM audio device infrastructure
.Sh SYNOPSIS
To compile this driver into the kernel, place the following line in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "device sound"
.Ed
.Sh DESCRIPTION
The
.Nm
driver is the main component of the
.Fx
sound system.
It works in conjunction with a bridge device driver on supported devices
and provides PCM audio record and playback once it attaches.
Each bridge device driver supports a specific set of audio chipsets and
needs to be enabled together with the
.Nm
driver.
PCI and ISA PnP audio devices identify themselves so users are usually not
required to add anything to
.Pa /boot/device.hints .
.Pp
Some of the main features of the
.Nm
driver are: multichannel audio, per-application
volume control, dynamic mixing through virtual sound channels, true full
duplex operation, bit perfect audio, rate conversion and low latency
modes.
.Pp
The
.Nm
driver is enabled by default, along with several bridge device drivers.
Those not enabled by default can be loaded during runtime with
.Xr kldload 8
or during boot via
.Xr loader.conf 5 .
The following bridge device drivers are available:
.Pp
.Bl -bullet -compact
.It
.Xr snd_ai2s 4 (enabled by default on powerpc)
.It
.Xr snd_als4000 4
.It
.Xr snd_atiixp 4
.It
.Xr snd_cmi 4 (enabled by default on amd64, i386)
.It
.Xr snd_cs4281 4
.It
.Xr snd_csa 4 (enabled by default on amd64, i386)
.It
.Xr snd_davbus 4 (enabled by default on powerpc)
.It
.Xr snd_emu10k1 4
.It
.Xr snd_emu10kx 4 (enabled by default on amd64, i386)
.It
.Xr snd_envy24 4
.It
.Xr snd_envy24ht 4
.It
.Xr snd_es137x 4 (enabled by default on amd64, i386)
.It
.Xr snd_fm801 4
.It
.Xr snd_hda 4 (enabled by default on amd64, i386)
.It
.Xr snd_hdsp 4
.It
.Xr snd_hdspe 4
.It
.Xr snd_ich 4 (enabled by default on amd64, i386)
.It
.Xr snd_maestro3 4
.It
.Xr snd_neomagic 4
.It
.Xr snd_solo 4
.It
.Xr snd_spicds 4
.It
.Xr snd_uaudio 4 (auto-loaded on device plug)
.It
.Xr snd_via8233 4 (enabled by default on amd64, i386)
.It
.Xr snd_via82c686 4
.It
.Xr snd_vibes 4
.El
.Pp
Refer to the manual page for each bridge device driver for driver specific
settings and information.
.Ss Boot Variables
In general, the module
.Pa snd_foo
corresponds to
.Cd "device snd_foo"
and can be
loaded by the boot
.Xr loader 8
via
.Xr loader.conf 5
or from the command line using the
.Xr kldload 8
utility.
Options which can be specified in
.Pa /boot/loader.conf
include:
.Bl -tag -width ".Va snd_driver_load" -offset indent
.It Va snd_driver_load
.Pq Dq Li NO
If set to
.Dq Li YES ,
this option loads all available drivers.
.It Va snd_hda_load
.Pq Dq Li NO
If set to
.Dq Li YES ,
only the Intel High Definition Audio bridge device driver and dependent
modules will be loaded.
.It Va snd_foo_load
.Pq Dq Li NO
If set to
.Dq Li YES ,
load driver for card/chipset foo.
.El
.Pp
To define default values for the different mixer channels,
set the channel to the preferred value using hints, e.g.:
.Va hint.pcm.0.line Ns = Ns Qq Li 0 .
This will mute the input channel per default.
.Ss Multichannel Audio
Multichannel audio, popularly referred to as
.Dq surround sound
is supported and enabled by default.
The
.Fx
multichannel matrix processor supports up to 18 interleaved channels, but the
limit is currently set to 8 channels (as commonly used for 7.1 surround sound).
The internal matrix mapping can handle reduction, expansion or
re-routing of channels.
This provides a base interface for related multichannel
.Fn ioctl
support.
Multichannel audio works both with and without VCHANs.
.Pp
Most bridge device drivers are still missing multichannel matrixing
support, but in most cases this should be trivial to implement.
Use the
.Va dev.pcm.%d.[play|rec].vchanformat
.Xr sysctl 8
to adjust the number of channels used.
The current multichannel interleaved structure and arrangement was
implemented by inspecting various popular UNIX applications.
There were no single standard, so much care has been taken to try
to satisfy each possible scenario, despite the fact that each
application has its own conflicting standard.
.Ss EQ
The Parametric Software Equalizer (EQ) enables the use of
.Dq tone
controls (bass and treble).
Commonly used for ear-candy or frequency compensation due to the vast
difference in hardware quality.
EQ is disabled by default, but can be enabled with the
.Va hint.pcm.%d.eq
tunable.
.Ss VCHANs
Each device can optionally support more playback and recording channels
than physical hardware provides by using
.Dq virtual channels
or VCHANs.
VCHAN options can be configured via the
.Xr sysctl 8
interface but can only be manipulated while the device is inactive.
.Ss VPC
.Fx
supports independent and individual volume controls for each active
application, without touching the master
.Nm
volume.
This is sometimes referred to as Volume Per Channel (VPC).
The VPC feature is enabled by default.
.Ss Loader Tunables
The following loader tunables are used to set driver configuration at the
.Xr loader 8
prompt before booting the kernel, or they can be stored in
.Pa /boot/loader.conf
in order to automatically set them before booting the kernel.
It is also possible to use
.Xr kenv 1
to change these tunables before loading the
.Nm
driver.
The following tunables can not be changed during runtime using
.Xr sysctl 8 .
.Bl -tag -width indent
.It Va hint.pcm.%d.eq
Set to 1 or 0 to explicitly enable (1) or disable (0) the equalizer.
Requires a driver reload if changed.
Enabling this will make bass and treble controls appear in mixer applications.
This tunable is undefined by default.
Equalizing is disabled by default.
.It Va hint.pcm.%d.vpc
Set to 1 or 0 to explicitly enable (1) or disable (0) the VPC feature.
This tunable is undefined by default.
VPC is however enabled by default.
.El
.Ss Runtime Configuration
There are a number of
.Xr sysctl 8
variables available which can be modified during runtime.
These values can also be stored in
.Pa /etc/sysctl.conf
in order to automatically set them during the boot process.
.Va hw.snd.*
are global settings and
.Va dev.pcm.*
are device specific.
.Bl -tag -width indent
.It Va hw.snd.compat_linux_mmap
Linux
.Xr mmap 2
compatibility.
The following values are supported (default is 0):
.Bl -tag -width 2n
.It -1
Force disabling/denying PROT_EXEC
.Xr mmap 2
requests.
.It 0
Auto detect proc/ABI type, allow
.Xr mmap 2
for Linux applications, and deny for everything else.
.It 1
Always allow PROT_EXEC page mappings.
.El
.It Va hw.snd.default_auto
Automatically assign the default sound unit.
The following values are supported (default is 1):
.Bl -tag -width 2n
.It 0
Do not assign the default sound unit automatically.
.It 1
Use the best available sound device based on playing and recording
capabilities of the device.
.It 2
Use the most recently attached device.
.El
.It Va hw.snd.default_unit
Default sound card for systems with multiple sound cards.
When using
.Xr devfs 4 ,
the default device for
.Pa /dev/dsp .
Equivalent to a symlink from
.Pa /dev/dsp
to
.Pa /dev/dsp Ns Va ${hw.snd.default_unit} .
.It Va hw.snd.feeder_eq_exact_rate
Only certain rates are allowed for precise processing.
The default behavior is however to allow sloppy processing for all rates,
even the unsupported ones.
Enable to toggle this requirement and only allow processing for supported
rates.
.It Va hw.snd.feeder_rate_max
Maximum allowable sample rate.
.It Va hw.snd.feeder_rate_min
Minimum allowable sample rate.
.It Va hw.snd.feeder_rate_polyphase_max
Adjust to set the maximum number of allowed polyphase entries during the
process of building resampling filters.
Disabling polyphase resampling has the benefit of reducing memory usage, at
the expense of slower and lower quality conversion.
Only applicable when the SINC interpolator is used.
Default value is 183040.
Set to 0 to disable polyphase resampling.
.It Va hw.snd.feeder_rate_quality
Sample rate converter quality.
Default value is 1, linear interpolation.
Available options include:
.Bl -tag -width 2n
.It 0
Zero Order Hold, ZOH.
Very fast, but with poor quality.
.It 1
Linear interpolation.
Fast, quality is subject to personal preference.
Technically the quality is poor however, due to the lack of anti-aliasing
filtering.
.It 2
Bandlimited SINC interpolator.
Implements polyphase banking to boost the conversion speed, at the cost of
memory usage, with multiple high quality polynomial interpolators to improve
the conversion accuracy.
100% fixed point, 64bit accumulator with 32bit coefficients and high precision
sample buffering.
Quality values are 100dB stopband, 8 taps and 85% bandwidth.
.It 3
Continuation of the bandlimited SINC interpolator, with 100dB stopband, 36
taps and 90% bandwidth as quality values.
.It 4
Continuation of the bandlimited SINC interprolator, with 100dB stopband, 164
taps and 97% bandwidth as quality values.
.El
.It Va hw.snd.feeder_rate_round
Sample rate rounding threshold, to avoid large prime division at the
cost of accuracy.
All requested sample rates will be rounded to the nearest threshold value.
Possible values range between 0 (disabled) and 500.
Default is 25.
.It Va hw.snd.latency
Configure the buffering latency.
Only affects applications that do not explicitly request
blocksize / fragments.
This tunable provides finer granularity than the
.Va hw.snd.latency_profile
tunable.
Possible values range between 0 (lowest latency) and 10 (highest latency).
.It Va hw.snd.latency_profile
Define sets of buffering latency conversion tables for the
.Va hw.snd.latency
tunable.
A value of 0 will use a low and aggressive latency profile which can result
in possible underruns if the application cannot keep up with a rapid irq
rate, especially during high workload.
The default value is 1, which is considered a moderate/safe latency profile.
.It Va hw.snd.vchans_enable
Global VCHAN setting to enable (1) or disable (0) VCHANs.
This setting can be overridden for an individual device by using the
.Va dev.pcm.%d.[play|rec].vchans
tunables.
Default is enabled.
.It Va hw.snd.report_soft_formats
Controls the internal format conversion if it is
available transparently to the application software.
When disabled or not available, the application will
only be able to select formats the device natively supports.
.It Va hw.snd.report_soft_matrix
Enable seamless channel matrixing even if the hardware does not support it.
Makes it possible to play multichannel streams even with a simple stereo
sound card.
.It Va hw.snd.verbose
Level of verbosity for the
.Pa /dev/sndstat
device.
Higher values include more output and the highest level,
four, should be used when reporting problems.
Other options include:
.Bl -tag -width 2n
.It 0
Installed devices and their allocated bus resources.
.It 1
The number of playback, record, virtual channels, and
flags per device.
.It 2
Channel information per device including the channel's
current format, speed, and pseudo device statistics such as
buffer overruns and buffer underruns.
.It 3
File names and versions of the currently loaded sound modules.
.It 4
Various messages intended for debugging.
.El
.It Va hw.snd.vpc_0db
Default value for
.Nm
volume.
Increase to give more room for attenuation control.
Decrease for more amplification, with the possible cost of sound clipping.
.It Va hw.snd.vpc_autoreset
When a channel is closed the channel volume will be reset to 0db.
This means that any changes to the volume will be lost.
Enabling this will preserve the volume, at the cost of possible confusion
when applications tries to re-open the same device.
.It Va hw.snd.vpc_mixer_bypass
The recommended way to use the VPC feature is to teach applications to use the
correct
.Fn ioctl :
.Dv SNDCTL_DSP_GETPLAYVOL , SNDCTL_DSP_SETPLAYVOL ,
.Dv SNDCTL_DSP_SETRECVOL , SNDCTL_DSP_SETRECVOL .
This is however not always possible.
Enable this to allow applications to use their own existing mixer logic
to control their own channel volume.
.It Va hw.snd.vpc_reset
Enable to restore all channel volumes back to the default value of 0db.
.It Va dev.pcm.%d.bitperfect
Enable or disable bitperfect mode.
When enabled, channels will skip all dsp processing, such as channel
matrixing, rate converting and equalizing.
The pure
.Nm
stream will be fed directly to the hardware.
If VCHANs are enabled, the bitperfect mode will use the VCHAN format/rate as
the definitive format/rate target.
The recommended way to use bitperfect mode is to disable VCHANs and enable this
sysctl.
Default is disabled.
.It Va dev.pcm.%d.[play|rec].vchans
Enable (1) or disable (0) VCHANs.
Default is enabled.
.It Va dev.pcm.%d.[play|rec].vchanformat
Format for VCHAN mixing.
All playback paths will be converted to this format before the mixing
process begins.
By default only 2 channels are enabled.
Available options include:
.Bl -tag -width 2n
.It s16le:1.0
Mono.
.It s16le:2.0
Stereo, 2 channels (left, right).
.It s16le:2.1
3 channels (left, right, LFE).
.It s16le:3.0
3 channels (left, right, rear center).
.It s16le:4.0
Quadraphonic, 4 channels (front/rear left and right).
.It s16le:4.1
5 channels (4.0 + LFE).
.It s16le:5.0
5 channels (4.0 + center).
.It s16le:5.1
6 channels (4.0 + center + LFE).
.It s16le:6.0
6 channels (4.0 + front/rear center).
.It s16le:6.1
7 channels (6.0 + LFE).
.It s16le:7.1
8 channels (4.0 + center + LFE + left and right side).
.El
.It Va dev.pcm.%d.[play|rec].vchanmode
VCHAN format/rate selection.
Available options include:
.Bl -tag -width 2n
.It fixed
Channel mixing is done using fixed format/rate.
Advanced operations such as digital passthrough will not work.
Can be considered as a
.Dq legacy
mode.
This is the default mode for hardware channels which lack support for digital
formats.
.It passthrough
Channel mixing is done using fixed format/rate, but advanced operations such
as digital passthrough also work.
All channels will produce sound as usual until a digital format playback is
requested.
When this happens all other channels will be muted and the latest incoming
digital format will be allowed to pass through undisturbed.
Multiple concurrent digital streams are supported, but the latest stream will
take precedence and mute all other streams.
.It adaptive
Works like the
.Dq passthrough
mode, but is a bit smarter, especially for
multiple
.Nm
channels with different format/rate.
When a new channel is about to start, the entire list of virtual channels will
be scanned, and the channel with the best format/rate (usually the
highest/biggest) will be selected.
This ensures that mixing quality depends on the best channel.
The downside is that the hardware DMA mode needs to be restarted, which may
cause annoying pops or clicks.
.El
.It Va dev.pcm.%d.[play|rec].vchanrate
Sample rate speed for VCHAN mixing.
All playback paths will be converted to this sample rate before the mixing
process begins.
.It Va dev.pcm.%d.polling
Experimental polling mode support where the driver operates by querying the
device state on each tick using a
.Xr callout 9
mechanism.
Disabled by default and currently only available for a few device drivers.
.El
.Ss Statistics
Channel statistics are only kept while the device is open.
So with situations involving overruns and underruns, consider the output
while the errant application is open and running.
.Ss IOCTL Support
The driver supports most of the OSS
.Fn ioctl
functions, and most applications work unmodified.
A few differences exist, while memory mapped playback is
supported natively and in Linux emulation, memory mapped recording is not due
to VM system design.
As a consequence, some applications may need to be recompiled
with a slightly modified audio module.
See
.In sys/soundcard.h
for a complete list of the supported
.Fn ioctl
functions.
.Sh FILES
The
.Nm
drivers may create the following device nodes:
.Pp
.Bl -tag -width ".Pa /dev/sndstat" -compact
.It Pa /dev/dsp%d
Audio device.
The number represents the unit number of the device.
.It Pa /dev/dsp
Alias of
.Pa /dev/dsp${hw.snd.default_unit} .
Available only if
.Pa hw.snd.basename_clone
is set.
.It Pa /dev/sndstat
Current
.Nm
status, including all channels and drivers.
.El
.Pp
All
.Nm
devices are listed
in
.Pa /dev/sndstat .
Additional messages are sometimes recorded when the
device is probed and attached, these messages can be viewed with the
.Xr dmesg 8
utility.
.Sh EXAMPLES
Use the sound metadriver to load all
.Nm
bridge device drivers at once
(for example if it is unclear which the correct driver to use is):
.Pp
.Dl kldload snd_driver
.Pp
Load a specific bridge device driver, in this case the Intel
High Definition Audio driver:
.Pp
.Dl kldload snd_hda
.Pp
Check the status of all detected
.Nm
devices:
.Pp
.Dl cat /dev/sndstat
.Pp
Change the default sound device, in this case to the second device.
This is handy if there are multiple
.Nm
devices available:
.Pp
.Dl mixer -d pcm1
.Sh DIAGNOSTICS
.Bl -diag
.It pcm%d:play:%d:dsp%d.p%d: play interrupt timeout, channel dead
The hardware does not generate interrupts to serve incoming (play)
or outgoing (record) data.
.It unsupported subdevice XX
A device node is not created properly.
.El
.Sh SEE ALSO
.Xr devfs 4 ,
.Xr snd_ai2s 4 ,
.Xr snd_als4000 4 ,
.Xr snd_atiixp 4 ,
.Xr snd_cmi 4 ,
.Xr snd_cs4281 4 ,
.Xr snd_csa 4 ,
.Xr snd_davbus 4 ,
.Xr snd_emu10k1 4 ,
.Xr snd_emu10kx 4 ,
.Xr snd_envy24 4 ,
.Xr snd_envy24ht 4 ,
.Xr snd_es137x 4 ,
.Xr snd_fm801 4 ,
.Xr snd_hda 4 ,
.Xr snd_hdsp 4 ,
.Xr snd_hdspe 4 ,
.Xr snd_ich 4 ,
.Xr snd_maestro3 4 ,
.Xr snd_neomagic 4 ,
.Xr snd_solo 4 ,
.Xr snd_spicds 4 ,
.Xr snd_t4dwave 4 ,
.Xr snd_uaudio 4 ,
.Xr snd_via8233 4 ,
.Xr snd_via82c686 4 ,
.Xr snd_vibes 4 ,
.Xr device.hints 5 ,
.Xr loader.conf 5 ,
.Xr dmesg 8 ,
.Xr kldload 8 ,
.Xr mixer 8 ,
.Xr sysctl 8
.Rs
.%T "Cookbook formulae for audio EQ biquad filter coefficients (Audio-EQ-Cookbook.txt), by Robert Bristow-Johnson"
.%U "https://www.musicdsp.org/en/latest/Filters/197-rbj-audio-eq-cookbook.html"
.Re
.Rs
.%T "Julius O'Smith's Digital Audio Resampling"
.%U "http://ccrma.stanford.edu/~jos/resample/"
.Re
.Rs
.%T "Polynomial Interpolators for High-Quality Resampling of Oversampled Audio, by Olli Niemitalo"
.%U "http://yehar.com/blog/wp-content/uploads/2009/08/deip.pdf"
.Re
.Rs
.%T "The OSS API"
.%U "http://www.opensound.com/pguide/oss.pdf"
.Re
.Sh HISTORY
The
.Nm
device driver first appeared in
.Fx 2.2.6
as
.Nm pcm ,
written by
.An Luigi Rizzo .
It was later
rewritten in
.Fx 4.0
by
.An Cameron Grant .
The API evolved from the VOXWARE
standard which later became OSS standard.
.Sh AUTHORS
.An -nosplit
.An Luigi Rizzo Aq Mt luigi@iet.unipi.it
initially wrote the
.Nm pcm
device driver and this manual page.
.An Cameron Grant Aq Mt gandalf@vilnya.demon.co.uk
later revised the device driver for
.Fx 4.0 .
.An Seigo Tanimura Aq Mt tanimura@r.dl.itc.u-tokyo.ac.jp
revised this manual page.
It was then rewritten for
.Fx 5.2 .
.Sh BUGS
Some features of your sound card (e.g., global volume control) might not
be supported on all devices.
.Pp
Some audio devices might refuse to work properly unless the sample rate is
configured the same for both recording and playback, even if only simplex is
used.
See the
.Va dev.pcm.%d.[play|rec].vchanrate
sysctls.