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PCM(4)			 BSD Kernel Interfaces Manual			PCM(4)

     pcm, snd -- FreeBSD PCM audio device infrastructure

     For a card	with bridge driver support, and	a PnP card:
     device pcm

     For a card	without	bridge driver support, and a non-PnP card, the follow-
     ing lines may be required in /boot/device.hints:"isa"

     The pcm driver provides support for PCM audio play	and capture.  This
     driver also supports various PCI, WSS/MSS compatible, ISA sound cards,
     and AC97 mixer.  Once the pcm 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 du-
     plex operation is available on most cards.

     If	the sound card is supported by a bridge	driver,	the pcm	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	works best with	WSS/MSS	cards, which have a very clean archi-
     tecture and an orthogonal set of features.	 They also happen to be	among
     the cheapest audio	cards on the market.

     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 foo and can be
     loaded by the boot	loader via boot/loader.conf 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.

     Each device can optionally	support	more playback channels that 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
     The following sysctl(8) tunables are available:

	   hw.snd.pcm%d.buffersize     Configure the amount of DMA bufferspace
				       available for a device.

	   hw.snd.targetirqrate	       Set the default block size such that
				       continuous playback will	achieve	this
				       IRQ rate.  This value can be tuned to
				       improve application performance.	 In-
				       crease this value when the sound	lags
				       and decrease it if sound	stutters or
				       breaks up.

	   hw.snd.unit		       When using devfs(5), the	default	device
				       for /dev/dsp.  Equivalent to a symlink
				       from /dev/dsp to

	   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 de-
				       vice natively supports.

	   hw.snd.verbose	       Level of	verbosity for the /dev/sndstat
				       device.	Higher values include more
				       output and the highest level, three,
				       should be used when reported problems.
				       Other options include: 0	- Installed
				       devices and their allocated bus re-
				       sources.	 1 - The number	of playback,
				       record, virtual channels, and flags per
				       device.	2 - Channel information	per
				       device including	the channel's current
				       format, speed, and pseudo device	sta-
				       tistics such as buffer overruns and
				       buffer underruns.  3 - File names and
				       versions	of the currently sound loaded

	   hw.snd.maxautovchans	       Global VCHAN setting that only affects
				       devices that have only one playback
				       channel.	 The sound system will dynami-
				       cally create up this many VCHANs.  Set
				       to "0" if no VCHANS are desired.

	   hw.snd.pcm%d.vchans	       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 disable
				       VCHANs for this device.

   Recording Channels
     On	devices	that have more than one	recording source (ie: mic and line),
     there is a	corresponding /dev/dspr%d.%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-

     Below we include a	list of	supported codecs/cards.	 If your sound card is
     not listed	here, it may be	supported by a bridge driver.

     CS4237, CS4236, CS4232, CS4231 (ISA)
	 All these cards work perfectly	in full	duplex using the MSS mode.
	 This chipset is used, among others, on	the A/Open AW35	and AW32, on
	 some Intel motherboards, and (the CS4231) on some non-PnP cards.

	 The CS4232 is reported	as buggy in the	Voxware	documentation but I am
	 not sure if this is true.  On one of my Intel motherboards, capture
	 does not work simply because the capture DMA channel is not wired to
	 the ISA DMA controller.

     Yamaha OPL-SAx (ISA)
	 Works perfectly in all	modes.	This chip is used in several PnP
	 cards,	but also (in non-PnP mode) on motherboards and laptops (e.g.
	 the Toshiba Libretto).

     OPTi931 (ISA)
	 The chip is buggy, but	the driver has many workarounds	to make	it
	 work in full duplex because for some time these were the only full
	 duplex	cards I	could find. u-law formats uses U8 format internally
	 because of a bug in the chip.

     Trident 4DWave DX/NX (PCI)

     ENSONIQ AudioPCI ES1370/1371 (PCI)
	 Creative Labs SoundBlaster PCI	is supported as	well.

     ESS Solo-1/1E (PCI)

     NeoMagic 256AV/ZX (PCI)

     The pcm 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/dspr%d.%d	Should be connected to a record	codec.
     /dev/sndstat	Current	pcm status, including all channels and driv-

     The first number in the device node represents the	unit number of the PCM
     device.  All pcm PCM devices are listed in	/dev/sndstat. Additional mes-
     sages are sometimes recorded when the device is probed and	attached,
     these messages can	be viewed with the dmesg(8) utility.

     ac97: dac not ready
	 AC97 codec is not likely to be	accompanied with the sound card.

     unsupported subdevice XX
	 A device node is not created properly.

     Some features of your cards (e.g. global volume control) might not	be
     supported on all devices.

     The pcm device driver first appeared in FreeBSD 2.2.6 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 standard.

     csa(4), gusc(4), sbc(4), devfs(5),	loader.conf(5),	dmesg(8), kldload(8),

     The OSS API,

     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.

BSD			       November	14, 2003			   BSD


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