1.\" $NetBSD: audio.4,v 1.104 2020/12/09 05:48:56 isaki Exp $ 2.\" 3.\" Copyright (c) 1996 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by John T. Kohl. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.Dd March 28, 2020 31.Dt AUDIO 4 32.Os 33.Sh NAME 34.Nm audio 35.Nd device-independent audio driver layer 36.Sh SYNOPSIS 37.In sys/audioio.h 38.Sh DESCRIPTION 39The 40.Nm 41driver provides support for various audio peripherals. 42It provides a uniform programming interface layer above different 43underlying audio hardware drivers. 44The audio layer provides full-duplex operation if the 45underlying hardware configuration supports it. 46.Pp 47There are four device files available for audio operation: 48.Pa /dev/audio , 49.Pa /dev/sound , 50.Pa /dev/audioctl , 51and 52.Pa /dev/mixer . 53.Pp 54.Pa /dev/audio 55and 56.Pa /dev/sound 57are used for recording or playback of digital samples. 58.Pp 59.Pa /dev/mixer 60is used to manipulate volume, recording source, or other audio mixer 61functions. 62.Pp 63.Pa /dev/audioctl 64accepts the same 65.Xr ioctl 2 66operations as 67.Pa /dev/sound , 68but no other operations. 69It can be opened at any time and can be used to manipulate the 70audio device while it is in use. 71.Sh SAMPLING DEVICES 72When 73.Pa /dev/audio 74is opened, it automatically sets the track to manipulate 75monaural 8-bit mu-law 8000Hz. 76When 77.Pa /dev/sound 78is opened, it maintains the audio format and pause/unpause 79state of the most recently opened track. 80In all other respects 81.Pa /dev/audio 82and 83.Pa /dev/sound 84are identical. 85.Pp 86On a full-duplex device, reads and writes may operate concurrently 87without interference. 88.Pp 89On a half-duplex device, if there are any recording descriptors already, 90opening with write mode will fail. 91Similarly, if there are any playback descriptors already, 92opening with read mode will fail. 93If both playback and recording are requested on a half-duplex device, 94it will be treated as playback mode. 95.Pp 96On either type of device, opening with write mode will start in playback mode, 97opening with read mode will start in recording mode. 98.Pp 99If the playback mode is paused then silence is 100played instead of the provided samples, and if recording is paused then 101the process blocks in 102.Xr read 2 103until recording is unpaused. 104.Pp 105If a writing process does not call 106.Xr write 2 107frequently enough to provide samples at the pace the hardware 108consumes them silence is inserted. 109If a reading process does not call 110.Xr read 2 111frequently enough, it will simply miss samples. 112.Pp 113The audio driver supports track multiplexing. 114All sampling devices can be opened at any time without interference. 115For playback, all tracks opened simultaneously are mixed, 116even if their specified format is different. 117For recording, recorded data is distributed to all opened tracks, 118even if their specified format is different. 119To achieve this, the audio driver has a small efficient encoding converter, 120a channel mixer, and a frequency converter. 121The frequency conversion adapts the simplest way 122(interpolation method for upward, and simple thinning method for downward) 123due to restriction in kernel resources and processing time. 124It will work well in most case but don't expect excessive quality. 125.Pp 126The audio device is normally accessed with 127.Xr read 2 128or 129.Xr write 2 130calls, but it can also be mapped into user memory with 131.Xr mmap 2 . 132Once the device has been mapped it can no longer be accessed 133by read or write; all access is by reading and writing to 134the mapped memory. 135The mmap'ped buffer appears as a block of memory of size 136.Va buffersize 137(as available via 138.Dv AUDIO_GETINFO 139or 140.Dv AUDIO_GETBUFINFO ) . 141The audio driver will continuously move data from this buffer 142from/to the mixing buffer, wrapping around at the end of the buffer. 143To find out where the hardware is currently accessing data in the buffer the 144.Dv AUDIO_GETIOFFS 145and 146.Dv AUDIO_GETOOFFS 147calls can be used. 148Note that 149.Xr mmap 2 150no longer maps hardware buffers directly. 151Now it is achieved by emulation so don't expect any improvements excessively 152rather than normal 153.Xr write 2 . 154For historical reasons, only encodings that are not set 155.Dv AUDIO_ENCODINGFLAG_EMULATED 156are able to 157.Xr mmap 2 . 158.Pp 159The audio device, like most devices, can be used in 160.Xr select 2 , 161can be set in non-blocking mode and can be set (with a 162.Dv FIOASYNC 163ioctl) to send a 164.Dv SIGIO 165when I/O is possible. 166The mixer device can be set to generate a 167.Dv SIGIO 168whenever a mixer value is changed. 169.Pp 170The following 171.Xr ioctl 2 172commands are supported on the sample devices: 173.Bl -tag -width indent 174.It Dv AUDIO_FLUSH 175This command stops all playback and recording, clears all queued 176buffers, resets error counters on this track, 177and restarts recording and playback as 178appropriate for the current sampling mode. 179.It Dv AUDIO_PERROR (int) 180.It Dv AUDIO_RERROR (int) 181This command fetches the count of dropped output (input) 182bytes into its integer argument. 183There is no information regarding when in the sample stream 184they were dropped. 185.It Dv AUDIO_WSEEK (u_long) 186This command fetches the count of bytes that are queued ahead of the 187first sample in the most recent sample block written into its integer argument. 188.It Dv AUDIO_DRAIN 189This command suspends the calling process until all queued playback 190samples have been played. 191.It Dv AUDIO_GETDEV (audio_device_t) 192This command fetches the current hardware device information into the 193.Vt audio_device_t 194argument. 195.Bd -literal 196typedef struct audio_device { 197 char name[MAX_AUDIO_DEV_LEN]; 198 char version[MAX_AUDIO_DEV_LEN]; 199 char config[MAX_AUDIO_DEV_LEN]; 200} audio_device_t; 201.Ed 202.It Dv AUDIO_GETENC (audio_encoding_t) 203This command is used iteratively to fetch sample encoding names and 204format ids into the input/output audio_encoding_t argument. 205The encoding returned by the command is user accessible encoding and 206is not hardware supported encoding. 207.Bd -literal 208typedef struct audio_encoding { 209 int index; /* input: nth encoding */ 210 char name[MAX_AUDIO_DEV_LEN]; /* name of encoding */ 211 int encoding; /* value for encoding parameter */ 212 int precision; /* value for precision parameter */ 213 int flags; 214#define AUDIO_ENCODINGFLAG_EMULATED 1 /* software emulation mode */ 215} audio_encoding_t; 216.Ed 217.Pp 218To query 219all the supported encodings, start with an index field of 0 and 220continue with successive encodings (1, 2, ...) until the command returns 221an error. 222.It Dv AUDIO_GETFD (int) 223This command is obsolete. 224.It Dv AUDIO_SETFD (int) 225This command is obsolete. 226.It Dv AUDIO_GETPROPS (int) 227This command gets a bit set of hardware properties. 228If the hardware 229has a certain property the corresponding bit is set, otherwise it is not. 230The properties can have the following values: 231.Pp 232.Bl -tag -width AUDIO_PROP_INDEPENDENT -compact 233.It Dv AUDIO_PROP_FULLDUPLEX 234the device admits full duplex operation. 235.It Dv AUDIO_PROP_MMAP 236the device can be used with 237.Xr mmap 2 . 238.It Dv AUDIO_PROP_INDEPENDENT 239the device can set the playing and recording encoding parameters 240independently. 241.It Dv AUDIO_PROP_PLAYBACK 242the device is capable of audio playback. 243.It Dv AUDIO_PROP_CAPTURE 244the device is capable of audio capture. 245.El 246.It Dv AUDIO_GETIOFFS (audio_offset_t) 247.It Dv AUDIO_GETOOFFS (audio_offset_t) 248This command fetches the current offset in the input(output) buffer where 249the track mixer will be putting(getting) data. 250It mostly useful when the device 251buffer is available in user space via the 252.Xr mmap 2 253call. 254The information is returned in the 255.Vt audio_offset_t 256structure. 257.Bd -literal 258typedef struct audio_offset { 259 u_int samples; /* Total number of bytes transferred */ 260 u_int deltablks; /* Blocks transferred since last checked */ 261 u_int offset; /* Physical transfer offset in buffer */ 262} audio_offset_t; 263.Ed 264.It Dv AUDIO_GETINFO (audio_info_t) 265.It Dv AUDIO_GETBUFINFO (audio_info_t) 266.It Dv AUDIO_SETINFO (audio_info_t) 267Get or set audio information as encoded in the audio_info structure. 268For historical reasons, the audio_info structure has three different 269layer's parameters: track, track mixer and hardware rich mixer. 270.Bd -literal 271typedef struct audio_info { 272 struct audio_prinfo play; /* info for play (output) side */ 273 struct audio_prinfo record; /* info for record (input) side */ 274 u_int monitor_gain; /* input to output mix [HWmixer] */ 275 /* BSD extensions */ 276 u_int blocksize; /* read/write block size [track] */ 277 u_int hiwat; /* output high water mark [track] */ 278 u_int lowat; /* output low water mark [track] */ 279 u_int _ispare1; 280 u_int mode; /* current operation mode [track] */ 281#define AUMODE_PLAY 0x01 282#define AUMODE_RECORD 0x02 283#define AUMODE_PLAY_ALL 0x04 /* Not used anymore */ 284} audio_info_t; 285.Ed 286.Pp 287When setting the current state with 288.Dv AUDIO_SETINFO , 289the audio_info structure should first be initialized with 290.Li AUDIO_INITINFO(&info) 291and then the particular values to be changed should be set. 292This allows the audio driver to only set those things that you wish 293to change and eliminates the need to query the device with 294.Dv AUDIO_GETINFO 295or 296.Dv AUDIO_GETBUFINFO 297first. 298.Pp 299The 300.Va mode 301field indicates current operation mode, either one of 302.Dv AUMODE_PLAY 303or 304.Dv AUMODE_RECORD . 305These two flags can not be changed once this descriptor is opened. 306For playback mode, the obsolete 307.Dv AUMODE_PLAY_ALL 308flag can be set but has no effect. 309.Pp 310.Va hiwat 311and 312.Va lowat 313are used to control write behavior. 314Writes to the audio devices will queue up blocks until the high-water 315mark is reached, at which point any more write calls will block 316until the queue is drained to the low-water mark. 317.Va hiwat 318and 319.Va lowat 320set those high- and low-water marks (in audio blocks). 321The default for 322.Va hiwat 323is the maximum value and for 324.Va lowat 32575% of 326.Va hiwat . 327.Pp 328.Va blocksize 329sets the current audio blocksize. 330The generic audio driver layer and the hardware driver have the 331opportunity to adjust this block size to get it within 332implementation-required limits. 333Normally the 334.Va blocksize 335is calculated to correspond to the value of the 336.Em hw.audioX.blk_ms 337sysctl and is recalculated when the encoding parameters change. 338If the descriptor is opened for read only, 339.Va blocksize 340indicates the blocksize for the recording track. 341Otherwise, 342.Va blocksize 343indicates the blocksize for the playback track. 344.Bd -literal 345struct audio_prinfo { 346 u_int sample_rate; /* sample rate in samples/s [track] */ 347 u_int channels; /* number of channels, usually 1 or 2 [track] */ 348 u_int precision; /* number of bits/sample [track] */ 349 u_int encoding; /* data encoding (AUDIO_ENCODING_* below) [track] */ 350 u_int gain; /* volume level [HWmixer] */ 351 u_int port; /* selected I/O port [HWmixer] */ 352 u_long seek; /* BSD extension [track] */ 353 u_int avail_ports; /* available I/O ports [HWmixer] */ 354 u_int buffer_size; /* total size audio buffer [track] */ 355 u_int _ispare[1]; 356 u_int samples; /* number of samples [track] */ 357 u_int eof; /* End Of File (zero-size writes) counter [track] */ 358 u_char pause; /* non-zero if paused, zero to resume [track] */ 359 u_char error; /* non-zero if underflow/overflow occurred [track] */ 360 u_char waiting; /* non-zero if another process hangs in open [track] */ 361 u_char balance; /* stereo channel balance [HWmixer] */ 362 u_char cspare[2]; 363 u_char open; /* non-zero if currently open [trackmixer] */ 364 u_char active; /* non-zero if I/O is currently active [trackmixer] */ 365}; 366.Ed 367.Pp 368Note: many hardware audio drivers require identical playback and 369recording sample rates, sample encodings, and channel counts. 370The playing information is always set last and will prevail on such hardware. 371If the hardware can handle different settings the 372.Dv AUDIO_PROP_INDEPENDENT 373property is set. 374.Pp 375The encoding parameter can have the following values: 376.Pp 377.Bl -tag -width AUDIO_ENCODING_SLINEAR_BE -compact 378.It Dv AUDIO_ENCODING_ULAW 379mu-law encoding, 8 bits/sample 380.It Dv AUDIO_ENCODING_ALAW 381A-law encoding, 8 bits/sample 382.It Dv AUDIO_ENCODING_SLINEAR 383two's complement signed linear encoding with the platform byte order 384.It Dv AUDIO_ENCODING_ULINEAR 385unsigned linear encoding 386with the platform byte order 387.It Dv AUDIO_ENCODING_ADPCM 388ADPCM encoding, 8 bits/sample 389.It Dv AUDIO_ENCODING_SLINEAR_LE 390two's complement signed linear encoding with little endian byte order 391.It Dv AUDIO_ENCODING_SLINEAR_BE 392two's complement signed linear encoding with big endian byte order 393.It Dv AUDIO_ENCODING_ULINEAR_LE 394unsigned linear encoding with little endian byte order 395.It Dv AUDIO_ENCODING_ULINEAR_BE 396unsigned linear encoding with big endian byte order 397.It Dv AUDIO_ENCODING_AC3 398Dolby Digital AC3 399.El 400.Pp 401The 402.Nm 403driver accepts the following formats. 404.Va encoding 405and 406.Va precision 407are one of the values obtained by 408.Dv AUDIO_GETENC , 409regardless of formats supported by underlying driver. 410.Va frequency 411ranges from 1000Hz to 192000Hz, 412regardless of frequency (ranges) supported by underlying driver. 413.Va channels 414depends your underlying driver. 415If the underlying driver only supports monaural (1channel) 416or stereo (2channels), you can specify 1 or 2 regardless of 417number of channels supported by underlying driver. 418If the underlying driver supports three or more channels, you can specify 419the number of channels supported by the underlying driver or less. 420.Pp 421The 422.Va gain , 423.Va port 424and 425.Va balance 426settings provide simple shortcuts to the richer mixer 427interface described below and are not obtained by 428.Dv AUDIO_GETBUFINFO . 429The gain should be in the range 430.Bq Dv AUDIO_MIN_GAIN , Dv AUDIO_MAX_GAIN 431and the balance in the range 432.Bq Dv AUDIO_LEFT_BALANCE , Dv AUDIO_RIGHT_BALANCE 433with the normal setting at 434.Dv AUDIO_MID_BALANCE . 435.Pp 436The input port should be a combination of: 437.Pp 438.Bl -tag -width AUDIO_MICROPHONE -compact 439.It Dv AUDIO_MICROPHONE 440to select microphone input. 441.It Dv AUDIO_LINE_IN 442to select line input. 443.It Dv AUDIO_CD 444to select CD input. 445.El 446.Pp 447The output port should be a combination of: 448.Pp 449.Bl -tag -width AUDIO_HEADPHONE -compact 450.It Dv AUDIO_SPEAKER 451to select speaker output. 452.It Dv AUDIO_HEADPHONE 453to select headphone output. 454.It Dv AUDIO_LINE_OUT 455to select line output. 456.El 457.Pp 458The available ports can be found in 459.Va avail_ports 460.Dv ( AUDIO_GETBUFINFO 461only). 462.Pp 463.Va buffer_size 464is the total size of the audio buffer. 465The buffer size divided by the 466.Va blocksize 467gives the maximum value for 468.Va hiwat . 469Currently the 470.Va buffer_size 471can only be read and not set. 472.Pp 473The 474.Va seek 475and 476.Va samples 477fields are only used by 478.Dv AUDIO_GETINFO 479and 480.Dv AUDIO_GETBUFINFO . 481.Va seek 482represents the count of 483bytes pending; 484.Va samples 485represents the total number of bytes recorded or played, less those 486that were dropped due to inadequate consumption/production rates. 487.Pp 488.Va pause 489returns the current pause/unpause state for recording or playback. 490For 491.Dv AUDIO_SETINFO , 492if the pause value is specified it will either pause 493or unpause the particular direction. 494.It Dv AUDIO_QUERYFORMAT (audio_format_query_t) 495This command enumerates formats supported by the hardware. 496Similarly to 497.Dv AUDIO_GETENC , 498to query all the supported formats, 499start with an index field of 0 and continue with successive formats 500(1, 2, ...) until the command returns an error. 501.Bd -literal 502typedef struct audio_format_query { 503 u_int index; 504 struct audio_format fmt; 505} audio_format_query_t; 506.Ed 507.It Dv AUDIO_GETFORMAT (audio_info_t) 508This command fetches the current hardware format. 509Only the following members in audio_info_t are used. 510Members which are not listed here or belong in invalid direction are 511filled by \-1. 512.Bl -bullet 513.It 514mode 515.It 516play.encoding 517.It 518play.precision 519.It 520play.channels 521.It 522play.sample_rate 523.It 524record.encoding 525.It 526record.precision 527.It 528record.channels 529.It 530record.sample_rate 531.El 532.Pp 533.Va mode 534indicates which direction is valid. 535.It Dv AUDIO_SETFORMAT (audio_info_t) 536This command sets the hardware format. 537It will fail if there are any opened descriptors. 538So obviously, it must be issued on 539.Pa /dev/audioctl . 540Similarly to 541.Dv AUDIO_GETFORMAT , 542only above members in audio_info_t are used. 543Members which is not listed or belong in invalid direction are ignored. 544The parameters can be chosen from the choices obtained by 545.Dv AUDIO_QUERYFORMAT . 546.It Dv AUDIO_GETCHAN (int) 547This command is obsolete. 548.It Dv AUDIO_SETCHAN (int) 549This command is obsolete. 550.El 551.Sh MIXER DEVICE 552The mixer device, 553.Pa /dev/mixer , 554may be manipulated with 555.Xr ioctl 2 556but does not support 557.Xr read 2 558or 559.Xr write 2 . 560It supports the following 561.Xr ioctl 2 562commands: 563.Bl -tag -width indent 564.It Dv AUDIO_GETDEV (audio_device_t) 565This command is the same as described above for the sampling devices. 566.It Dv AUDIO_MIXER_READ (mixer_ctrl_t) 567.It Dv AUDIO_MIXER_WRITE (mixer_ctrl_t) 568These commands read the current mixer state or set new mixer state for 569the specified device 570.Va dev . 571.Va type 572identifies which type of value is supplied in the 573.Vt mixer_ctrl_t 574argument. 575.Bd -literal 576#define AUDIO_MIXER_CLASS 0 577#define AUDIO_MIXER_ENUM 1 578#define AUDIO_MIXER_SET 2 579#define AUDIO_MIXER_VALUE 3 580typedef struct mixer_ctrl { 581 int dev; /* input: nth device */ 582 int type; 583 union { 584 int ord; /* enum */ 585 int mask; /* set */ 586 mixer_level_t value; /* value */ 587 } un; 588} mixer_ctrl_t; 589 590#define AUDIO_MIN_GAIN 0 591#define AUDIO_MAX_GAIN 255 592typedef struct mixer_level { 593 int num_channels; 594 u_char level[8]; /* [num_channels] */ 595} mixer_level_t; 596#define AUDIO_MIXER_LEVEL_MONO 0 597#define AUDIO_MIXER_LEVEL_LEFT 0 598#define AUDIO_MIXER_LEVEL_RIGHT 1 599.Ed 600.Pp 601For a mixer value, the 602.Va value 603field specifies both the number of channels and the values for each 604channel. 605If the channel count does not match the current channel count, the 606attempt to change the setting may fail (depending on the hardware 607device driver implementation). 608For an enumeration value, the 609.Va ord 610field should be set to one of the possible values as returned by a prior 611.Dv AUDIO_MIXER_DEVINFO 612command. 613The type 614.Dv AUDIO_MIXER_CLASS 615is only used for classifying particular mixer device 616types and is not used for 617.Dv AUDIO_MIXER_READ 618or 619.Dv AUDIO_MIXER_WRITE . 620.It Dv AUDIO_MIXER_DEVINFO (mixer_devinfo_t) 621This command is used iteratively to fetch audio mixer device information 622into the input/output 623.Vt mixer_devinfo_t 624argument. 625To query all the supported devices, start with an index field of 6260 and continue with successive devices (1, 2, ...) until the 627command returns an error. 628.Bd -literal 629typedef struct mixer_devinfo { 630 int index; /* input: nth mixer device */ 631 audio_mixer_name_t label; 632 int type; 633 int mixer_class; 634 int next, prev; 635#define AUDIO_MIXER_LAST -1 636 union { 637 struct audio_mixer_enum { 638 int num_mem; 639 struct { 640 audio_mixer_name_t label; 641 int ord; 642 } member[32]; 643 } e; 644 struct audio_mixer_set { 645 int num_mem; 646 struct { 647 audio_mixer_name_t label; 648 int mask; 649 } member[32]; 650 } s; 651 struct audio_mixer_value { 652 audio_mixer_name_t units; 653 int num_channels; 654 int delta; 655 } v; 656 } un; 657} mixer_devinfo_t; 658.Ed 659.Pp 660The 661.Va label 662field identifies the name of this particular mixer control. 663The 664.Va index 665field may be used as the 666.Va dev 667field in 668.Dv AUDIO_MIXER_READ 669and 670.Dv AUDIO_MIXER_WRITE 671commands. 672The 673.Va type 674field identifies the type of this mixer control. 675Enumeration types are typically used for on/off style controls (e.g. a 676mute control) or for input/output device selection (e.g. select 677recording input source from CD, line in, or microphone). 678Set types are similar to enumeration types but any combination 679of the mask bits can be used. 680.Pp 681The 682.Va mixer_class 683field identifies what class of control this is. 684The 685.Pq arbitrary 686value set by the hardware driver may be determined by examining the 687.Va mixer_class 688field of the class itself, 689a mixer of type 690.Dv AUDIO_MIXER_CLASS . 691For example, a mixer controlling the input gain on the line in circuit 692would have a 693.Va mixer_class 694that matches an input class device with the name 695.Dq inputs 696.Dv ( AudioCinputs ) , 697and would have a 698.Va label 699of 700.Dq line 701.Dv ( AudioNline ) . 702Mixer controls which control audio circuitry for a particular audio 703source (e.g. line-in, CD in, DAC output) are collected under the input class, 704while those which control all audio sources (e.g. master volume, 705equalization controls) are under the output class. 706Hardware devices capable of recording typically also have a record class, 707for controls that only affect recording, 708and also a monitor class. 709.Pp 710The 711.Va next 712and 713.Va prev 714may be used by the hardware device driver to provide hints for the next 715and previous devices in a related set (for example, the line in level 716control would have the line in mute as its 717.Dq next 718value). 719If there is no relevant next or previous value, 720.Dv AUDIO_MIXER_LAST 721is specified. 722.Pp 723For 724.Dv AUDIO_MIXER_ENUM 725mixer control types, 726the enumeration values and their corresponding names are filled in. 727For example, a mute control would return appropriate values paired with 728.Dv AudioNon 729and 730.Dv AudioNoff . 731For 732.Dv AUDIO_MIXER_VALUE 733and 734.Dv AUDIO_MIXER_SET 735mixer control types, the channel count is 736returned; the units name specifies what the level controls (typical 737values are 738.Dv AudioNvolume , 739.Dv AudioNtreble , 740.Dv AudioNbass ) . 741.\" For AUDIO_MIXER_SET mixer control types, what is what? 742.El 743.Pp 744By convention, all the mixer devices can be distinguished from other 745mixer controls because they use a name from one of the 746.Dv AudioC* 747string values. 748.Sh FILES 749.Bl -tag -width /dev/audioctl -compact 750.It Pa /dev/audio 751.It Pa /dev/audioctl 752.It Pa /dev/sound 753.It Pa /dev/mixer 754.El 755.Sh SEE ALSO 756.Xr audiocfg 1 , 757.Xr audioctl 1 , 758.Xr audioplay 1 , 759.Xr audiorecord 1 , 760.Xr mixerctl 1 , 761.Xr ioctl 2 , 762.Xr ossaudio 3 , 763.Xr acorn32/vidcaudio 4 , 764.\" .Xr amiga/aucc 4 , 765.\" .Xr amiga/melody 4 , 766.Xr arcofi 4 , 767.Xr aria 4 , 768.Xr auacer 4 , 769.Xr audiocs 4 , 770.Xr auich 4 , 771.Xr auixp 4 , 772.\" .Xr ausoc 4 , 773.Xr autri 4 , 774.Xr auvia 4 , 775.Xr bba 4 , 776.Xr btsco 4 , 777.Xr clcs 4 , 778.Xr clct 4 , 779.Xr cmpci 4 , 780.Xr dreamcast/aica 4 , 781.Xr eap 4 , 782.Xr emuxki 4 , 783.Xr esa 4 , 784.Xr esm 4 , 785.Xr eso 4 , 786.Xr ess 4 , 787.\" .Xr evbarm/aaci 4 , 788.\" .Xr evbarm/digfilt 4 , 789.\" .Xr evbarm/pxaacu 4 , 790.\" .Xr evbarm/udassio 4 , 791.Xr fms 4 , 792.Xr gcscaudio 4 , 793.Xr gus 4 , 794.Xr guspnp 4 , 795.Xr hdafg 4 , 796.Xr hdaudio 4 , 797.Xr hppa/harmony 4 , 798.Xr macppc/awacs 4 , 799.Xr macppc/snapper 4 , 800.Xr midi 4 , 801.Xr neo 4 , 802.Xr pad 4 , 803.Xr pas 4 , 804.\" .Xr paud 4 , 805.Xr radio 4 , 806.\" .Xr repluse 4 , 807.Xr sb 4 , 808.Xr sgimips/haltwo 4 , 809.Xr sgimips/mavb 4 , 810.Xr sparc/audioamd 4 , 811.Xr sparc/dbri 4 , 812.Xr sv 4 , 813.\" .Xr toccata 4 , 814.Xr uaudio 4 , 815.\" .Xr vaudio 4 , 816.\" .Xr vcaudio 4 , 817.\" .Xr vraiu 4 , 818.\" .Xr vsaudio 4 , 819.Xr wss 4 , 820.Xr x68k/vs 4 , 821.Xr yds 4 , 822.Xr ym 4 823.\" .Xr zaudio 4 824.Sh HISTORY 825Support for virtual channels and mixing first appeared in 826.Nx 8.0 . 827.Sh BUGS 828If the device is used in 829.Xr mmap 2 830it is currently always mapped for writing (playing) due to VM system weirdness. 831