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sound, pcm, snd - FreeBSD PCM audio device infrastructure
To compile this driver into the kernel, place the following line in your
kernel configuration file:
Non-PnP sound cards require the following lines in device.hints(5):
Note: There exists some ambiguity in the naming at the moment (sound,
pcm, snd). It will be resolved soon by renaming device sound to device
snd, and doing associated changes.
The sound driver provides support for PCM audio play and capture. This
driver also supports various PCI, ISA, WSS/MSS compatible sound cards,
AC97 mixer and High Definition Audio. Once the sound driver attaches,
supported devices provide audio record and playback channels. The
FreeBSD sound system provides dynamic mixing “VCHAN” and rate conversion
“soft formats”. True full duplex operation is available on most sound
If the sound card is supported by a bridge driver, the sound driver works
in conjunction with the bridge driver.
Apart from the usual parameters, the flags field is used to specify the
secondary DMA channel (generally used for capture in full duplex cards).
Flags are set to 0 for cards not using a secondary DMA channel, or to
0x10 + C to specify channel C.
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
/boot/device.hints. For PCI and ISA PnP cards this is actually easy
since they identify themselves. For legacy ISA cards, the driver looks
for MSS cards at addresses 0x530 and 0x604 (unless overridden in
In general, the module snd_foo corresponds to device snd_foo and can be
loaded by the boot loader(8) via loader.conf(5) or from the command line
using the kldload(8) utility. Options which can be specified in
snd_driver_load (“NO”) If set to “YES”, this option loads all
snd_emu10k1_load (“NO”) If set to “YES”, only the SoundBlaster 5.1
driver and dependent modules will be loaded.
snd_foo_load (“NO”) If set to “YES”, load driver for
To define default values for the different mixer channels, set the
channel to the preferred value using hints, e.g.: hint.pcm.0.line="0".
This will mute the input channel per default.
Each device can optionally support more playback and recording channels
than physical hardware provides by using “virtual channels” or VCHANs.
VCHAN options can be configured via the sysctl(8) interface but can only
be manipulated while the device is inactive.
There are a number of sysctl(8) variables available. hw.snd.* tunables
are global settings and dev.pcm.* are device specific.
hw.snd.latency_profile Define sets of buffering latency
conversion tables for the
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.
hw.snd.latency Configure the buffering latency. Only
affects applications that do not
explicitly request blocksize /
fragments. This tunable provides finer
granularity than the
Possible values range between 0 (lowest
latency) and 10 (highest latency).
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.
hw.snd.compat_linux_mmap Enable to allow PROT_EXEC page
mappings. All Linux applications using
sound and mmap(2) require this.
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
hw.snd.feeder_rate_max Maximum allowable sample rate.
hw.snd.feeder_rate_min Minimum allowable sample rate.
hw.snd.verbose Level of verbosity for the /dev/sndstat
device. Higher values include more
output and the highest level, four,
should be used when reporting problems.
Other options include:
0 Installed devices and their
allocated bus resources.
1 The number of playback, record,
virtual channels, and flags per
2 Channel information per device
including the channel’s current
format, speed, and pseudo device
statistics such as buffer overruns
and buffer underruns.
3 File names and versions of the
currently loaded sound modules.
4 Various messages intended for
hw.snd.maxautovchans Global VCHAN setting that only affects
devices with at least one playback or
recording channel available. The sound
system will dynamically create up this
many VCHANs. Set to “0” if no VCHANS
are desired. Maximum value is 256.
hw.snd.default_unit Default sound card for systems with
multiple sound cards. When using
devfs(5), the default device for
/dev/dsp. Equivalent to a symlink from
hw.snd.default_auto Enable to automatically assign default
sound unit to the most recent attached
The current number of VCHANs allocated
per device. This can be set to
preallocate a certain number of VCHANs.
Setting this value to “0” will disable
VCHANs for this device.
Sample rate speed for VCHAN mixing.
All playback paths will be converted to
this sample rate before the mixing
Format for VCHAN mixing. All playback
paths will be converted to this format
before the mixing process begins.
dev.pcm.%d.polling Experimental polling mode support where
the driver operates by querying the
device state on each tick using a
callout(9) mechanism. Disabled by
default and currently only available
for a few device drivers.
On devices that have more than one recording source (ie: mic and line),
there is a corresponding /dev/dsp%d.r%d device.
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.
The driver supports most of the OSS 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 #include <sys/soundcard.h>
for a complete list of the supported ioctl() functions.
The sound drivers may create the following device nodes:
/dev/audio%d.%d Sparc-compatible audio device.
/dev/dsp%d.%d Digitized voice device.
/dev/dspW%d.%d Like /dev/dsp, but 16 bits per sample.
/dev/dsp%d.p%d Playback channel.
/dev/dsp%d.r%d Record channel.
/dev/dsp%d.vp%d Virtual playback channel.
/dev/dsp%d.vr%d Virtual recording channel.
/dev/sndstat Current sound status, including all channels and
The first number in the device node represents the unit number of the
sound device. All sound devices are listed in /dev/sndstat. Additional
messages are sometimes recorded when the device is probed and attached,
these messages can be viewed with the dmesg(8) utility.
The above device nodes are only created on demand through the dynamic
devfs(5) clone handler. Users are strongly discouraged to access them
directly. For specific sound card access, please instead use /dev/dsp or
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.
unsupported subdevice XX A device node is not created properly.
snd_ad1816(4), snd_als4000(4), snd_atiixp(4), snd_audiocs(4), snd_cmi(4),
snd_cs4281(4), snd_csa(4), snd_ds1(4), snd_emu10k1(4), snd_emu10kx(4),
snd_envy24(4), snd_envy24ht(4), snd_es137x(4), snd_ess(4), snd_fm801(4),
snd_gusc(4), snd_hda(4), snd_ich(4), snd_maestro(4), snd_maestro3(4),
snd_mss(4), snd_neomagic(4), snd_sbc(4), snd_solo(4), snd_spicds(4),
snd_t4dwave(4), snd_uaudio(4), snd_via8233(4), snd_via82c686(4),
snd_vibes(4), devfs(5), device.hints(5), loader.conf(5), dmesg(8),
The OSS API, http://www.opensound.com/pguide/oss.pdf.
The sound device driver first appeared in FreeBSD 2.2.6 as pcm, written
by Luigi Rizzo. It was later rewritten in FreeBSD 4.0 by Cameron Grant.
The API evolved from the VOXWARE standard which later became OSS
Luigi Rizzo 〈firstname.lastname@example.org〉 initially wrote the pcm device driver
and this manual page. Cameron Grant 〈email@example.com〉 later
revised the device driver for FreeBSD 4.0. Seigo Tanimura
〈firstname.lastname@example.org〉 revised this manual page. It was then
rewritten for FreeBSD 5.2.
Some features of your sound card (e.g., global volume control) might not
be supported on all devices.