Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
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):"isa"

     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 conver-
     sion ``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	chan-
     nel 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	con-
				       version 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	work-
				       load.  The default value	is 1, which is
				       considered a moderate/safe latency pro-

	   hw.snd.latency	       Configure the buffering latency.	 Only
				       affects applications that do not
				       explicitly request blocksize / frag-
				       ments.  This tunable provides finer
				       granularity than	the
				       hw.snd.latency_profile tunable.	Possi-
				       ble 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 map-
				       pings.  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
				       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 include:

				       0   Installed devices and their allo-
					   cated 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 cur-
					   rently 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

				       The current number of VCHANs allocated
				       per device.  This can be	set to preal-
				       locate a	certain	number of VCHANs.
				       Setting this value to ``0'' will	dis-
				       able VCHANs for this device.

				       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	drivers.

   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 situ-
     ations 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 applica-
     tions 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 appli-
     cations may need to be recompiled with a slightly modified	audio module.
     See <sys/soundcard.h> for a complete list of the supported	ioctl()	func-

     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 driv-

     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	hard-
     ware 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 stan-

     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	9.2			 June 23, 2007			   FreeBSD 9.2


Want to link to this manual page? Use this URL:

home | help