man/man4/pcm.4

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.Dd June 23, 2009
.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
.Pp
Non-PnP sound cards require the following lines in
.Xr device.hints 5 :
.Bd -literal -offset indent
hint.pcm.0.at="isa"
hint.pcm.0.irq="5"
hint.pcm.0.drq="1"
hint.pcm.0.flags="0x0"
.Ed
.Sh DESCRIPTION
The
.Nm
driver provides support for
.Tn PCM
audio play and capture.
This driver also supports various
.Tn PCI ,
.Tn ISA ,
.Tn WSS/MSS
compatible
sound cards, AC97 mixer and High Definition Audio.
Once the
.Nm
driver attaches, supported devices provide audio record and
playback channels.
The
.Fx
sound system provides dynamic mixing
.Dq VCHAN
and rate conversion
.Dq soft formats .
True full duplex operation is available on most sound cards.
.Pp
If the sound card is supported by a bridge driver, the
.Nm
driver works in conjunction with the bridge driver.
.Pp
Apart from the usual parameters, the flags field is used to specify
the secondary
.Tn DMA
channel (generally used for capture in full duplex cards).
Flags are set to 0 for cards not using a secondary
.Tn DMA
channel, or to 0x10 + C to specify channel C.
.Pp
The driver does its best to recognize the installed hardware and drive
it correctly so the user is not required to add several lines in
.Pa /boot/device.hints .
For
.Tn PCI
and
.Tn ISA
.Tn PnP
cards this is actually easy
since they identify themselves.
For legacy
.Tn ISA
cards, the driver looks for
.Tn MSS
cards at addresses 0x530 and 0x604 (unless overridden
in
.Pa /boot/device.hints ) .
.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_emu10k1_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_emu10k1_load
.Pq Dq Li NO
If set to
.Dq Li YES ,
only the SoundBlaster 5.1 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 VCHANs
Each device can optionally support more playback and recording channels
than physical hardware provides by using
.Dq virtual channels
or
.Tn VCHANs .
.Tn VCHAN
options can be configured via the
.Xr sysctl 8
interface but can only be manipulated while the device is inactive.
.Ss Runtime Configuration
There are a number of
.Xr sysctl 8
variables available.
.Va hw.snd.*
tunables are global settings and
.Va dev.pcm.*
are device specific.
.Bl -tag -width ".Va hw.snd.report_soft_formats" -offset indent
.It Va hw.snd.compat_linux_mmap
Enable to allow PROT_EXEC page mappings.
All Linux applications using sound and
.Xr mmap 2
require this.
.It Va hw.snd.default_auto
Enable to automatically assign default sound unit to the most recent
attached device.
.It Va hw.snd.default_unit
Default sound card for systems with multiple sound cards.
When using
.Xr devfs 5 ,
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_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.feeder_polyphase_max
Adjust to set the maximum number of allowed polyphase entries during the
process of building resampling filters.
Only applicable when the SINC interpolator is used.
Default value is 183040.
Set to 0 to disable polyphase resampling.
.It Va hw.snd.feeder_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 inteprolator, with 100dB stopband, 164
taps and 97% bandwidth as quality values.
.El
.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.maxautovchans
Global
.Tn VCHAN
setting that only affects devices with at least one playback or recording channel available.
The sound system will dynamically create up to this many
.Tn VCHANs .
Set to
.Dq 0
if no
.Tn VCHANS
are desired.
Maximum value is 256.
.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
.Tn VCHANs
are enabled, the bitperfect mode will use the
.Tn VCHAN
format/rate as the definitive format/rate target.
The recommended way to use bitperfect mode is to disable
.Tn VCHANs
and enable this sysctl.
Default is disabled.
.It Va dev.pcm.%d.[play|rec].vchans
The current number of
.Tn VCHANs
allocated per device.
This can be set to preallocate a certain number of
.Tn VCHANs .
Setting this value to
.Dq 0
will disable
.Tn VCHANs
for this device.
.It Va dev.pcm.%d.[play|rec].vchanformat
Format for
.Tn VCHAN
mixing.
All playback paths will be converted to this format before the mixing
process begins.
.It Va dev.pcm.%d.[play|rec].vchanmode
.Tn 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
.Tn 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 Recording Channels
On devices that have more than one recording source (ie: mic and line),
there is a corresponding
.Pa /dev/dsp%d.r%d
device.
.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
.Tn OSS
.Fn ioctl
functions, and most applications work unmodified.
A few differences exist, while memory mapped playback is
supported natively and in
.Tn Linux
emulation, memory mapped recording is
not due to
.Tn 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/audio%d.%d" -compact
.It Pa /dev/audio%d.%d
Sparc-compatible audio device.
.It Pa /dev/dsp%d.%d
Digitized voice device.
.It Pa /dev/dspW%d.%d
Like
.Pa /dev/dsp ,
but 16 bits per sample.
.It Pa /dev/dsp%d.p%d
Playback channel.
.It Pa /dev/dsp%d.r%d
Record channel.
.It Pa /dev/dsp%d.vp%d
Virtual playback channel.
.It Pa /dev/dsp%d.vr%d
Virtual recording channel.
.It Pa /dev/sndstat
Current
.Nm
status, including all channels and drivers.
.El
.Pp
The first number in the device node
represents the unit number of the
.Nm
device.
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.
.Pp
The above device nodes are only created on demand through the dynamic
.Xr devfs 5
clone handler.
Users are strongly discouraged to access them directly.
For specific sound card access, please instead use
.Pa /dev/dsp
or
.Pa /dev/dsp%d .
.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 snd_ad1816 4 ,
.Xr snd_als4000 4 ,
.Xr snd_atiixp 4 ,
.Xr snd_audiocs 4 ,
.Xr snd_cmi 4 ,
.Xr snd_cs4281 4 ,
.Xr snd_csa 4 ,
.Xr snd_ds1 4 ,
.Xr snd_emu10k1 4 ,
.Xr snd_emu10kx 4 ,
.Xr snd_envy24 4 ,
.Xr snd_envy24ht 4 ,
.Xr snd_es137x 4 ,
.Xr snd_ess 4 ,
.Xr snd_fm801 4 ,
.Xr snd_gusc 4 ,
.Xr snd_hda 4 ,
.Xr snd_ich 4 ,
.Xr snd_maestro 4 ,
.Xr snd_maestro3 4 ,
.Xr snd_mss 4 ,
.Xr snd_neomagic 4 ,
.Xr snd_sbc 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 devfs 5 ,
.Xr device.hints 5 ,
.Xr loader.conf 5 ,
.Xr dmesg 8 ,
.Xr kldload 8 ,
.Xr sysctl 8
.Rs
.%T "Cookbook formulae for audio EQ biquad filter coefficients, by Robert Bristow-Johnson"
.%O "http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt"
.Re
.Rs
.%T "Julius O'Smith's Digital Audio Resampling"
.%O "http://ccrma.stanford.edu/~jos/resample/"
.Re
.Rs
.%T "Polynomial Interpolators for High-Quality Resampling of Oversampled Audio, by Olli Niemitalo"
.%O "http://www.student.oulu.fi/~oniemita/dsp/deip.pdf"
.Re
.Rs
.%T "The OSS API"
.%O "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 luigi@iet.unipi.it
initially wrote the
.Nm pcm
device driver and this manual page.
.An Cameron Grant Aq gandalf@vilnya.demon.co.uk
later revised the device driver for
.Fx 4.0 .
.An Seigo Tanimura Aq 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.