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PCM(4)                 FreeBSD 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
     following 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
     duplex 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
     architecture 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

           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.
                                              Increase 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 device 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 resources.  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 statistics
                                              such as buffer overruns and
                                              buffer underruns.  3 - File
                                              names and versions of the
                                              currently sound loaded modules.

           hw.snd.maxautovchans               Global VCHAN setting that only
                                              affects devices that have only
                                              one playback channel.  The sound
                                              system will dynamically 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 preallocate 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
     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.

     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

     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
     messages 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.

FreeBSD 11.0-PRERELEASE        November 14, 2003       FreeBSD 11.0-PRERELEASE


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