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SOUND(4)               FreeBSD Kernel Interfaces Manual               SOUND(4)

     sound, pcm, snd - FreeBSD PCM audio device infrastructure

     To compile this driver into the kernel, place the following line in your
     kernel configuration file:

           device sound

     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 cards.

     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

   Boot Variables
     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
     /boot/loader.conf include:

           snd_driver_load       (``NO'') If set to ``YES'', this option loads
                                 all available drivers.

           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
                                 card/chipset foo.

     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.

   Runtime Configuration
     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
                                           hw.snd.latency_profile tunable.
                                           Possible values range between 0
                                           (lowest latency) and 10 (highest

           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

           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.

           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

                                           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

                                           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 /dev/dsp to

           hw.snd.default_auto             Enable to automatically assign
                                           default sound unit to the most
                                           recent attached device.

           dev.pcm.%d.[play|rec].vchans    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

                                           Sample rate speed for VCHAN mixing.
                                           All playback paths will be
                                           converted to this sample rate
                                           before the mixing process begins.

                                           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

   Recording Channels
     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.

   IOCTL Support
     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 <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),
     kldload(8), sysctl(8)

     The OSS API,

     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 <> initially wrote the pcm device driver
     and this manual page.  Cameron Grant <> later
     revised the device driver for FreeBSD 4.0.  Seigo Tanimura
     <> 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.

FreeBSD 11.0-PRERELEASE          June 23, 2007         FreeBSD 11.0-PRERELEASE


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