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+.\"	$NetBSD: audio.4,v 1.106 2024/05/27 02:39:15 nia Exp $
+.\"
+.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" This code is derived from software contributed to The NetBSD Foundation
+.\" by John T. Kohl.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd May 27, 2024
+.Dt AUDIO 4
+.Os
+.Sh NAME
+.Nm audio
+.Nd device-independent audio driver layer
+.Sh SYNOPSIS
+.In sys/audioio.h
+.Sh DESCRIPTION
+The
+.Nm
+driver provides support for various audio peripherals.
+It provides a uniform programming interface layer above different
+underlying audio hardware drivers.
+The audio layer provides full-duplex operation if the
+underlying hardware configuration supports it.
+.Pp
+There are four device files available for audio operation:
+.Pa /dev/audio ,
+.Pa /dev/sound ,
+.Pa /dev/audioctl ,
+and
+.Pa /dev/mixer .
+.Pp
+.Pa /dev/audio
+and
+.Pa /dev/sound
+are used for recording or playback of digital samples.
+.Pp
+.Pa /dev/mixer
+is used to manipulate volume, recording source, or other audio mixer
+functions.
+.Pp
+.Pa /dev/audioctl
+accepts the same
+.Xr ioctl 2
+operations as
+.Pa /dev/sound ,
+but no other operations.
+It can be opened at any time and can be used to manipulate the
+audio device while it is in use.
+.Sh SAMPLING DEVICES
+When
+.Pa /dev/audio
+is opened, it automatically sets the track to manipulate
+monaural 8-bit mu-law 8000Hz.
+When
+.Pa /dev/sound
+is opened, it maintains the audio format and pause/unpause
+state of the most recently opened track.
+In all other respects
+.Pa /dev/audio
+and
+.Pa /dev/sound
+are identical.
+.Pp
+On a full-duplex device, reads and writes may operate concurrently
+without interference.
+.Pp
+On a half-duplex device, if there are any recording descriptors already,
+opening with write mode will fail.
+Similarly, if there are any playback descriptors already,
+opening with read mode will fail.
+If both playback and recording are requested on a half-duplex device,
+it will be treated as playback mode.
+.Pp
+On either type of device, opening with write mode will start in playback mode,
+opening with read mode will start in recording mode.
+.Pp
+If the playback mode is paused then silence is
+played instead of the provided samples, and if recording is paused then
+the process blocks in
+.Xr read 2
+until recording is unpaused.
+.Pp
+If a writing process does not call
+.Xr write 2
+frequently enough to provide samples at the pace the hardware
+consumes them silence is inserted.
+If a reading process does not call
+.Xr read 2
+frequently enough, it will simply miss samples.
+.Pp
+The audio driver supports track multiplexing.
+All sampling devices can be opened at any time without interference.
+For playback, all tracks opened simultaneously are mixed,
+even if their specified format is different.
+For recording, recorded data is distributed to all opened tracks,
+even if their specified format is different.
+To achieve this, the audio driver has a small efficient encoding converter,
+a channel mixer, and a frequency converter.
+The frequency conversion adapts the simplest way
+(interpolation method for upward, and simple thinning method for downward)
+due to restrictions in kernel resources and processing time.
+It will work well in most cases but don't expect excessive quality.
+.Pp
+The audio device is normally accessed with
+.Xr read 2
+or
+.Xr write 2
+calls, but it can also be mapped into user memory with
+.Xr mmap 2 .
+Once the device has been mapped it can no longer be accessed
+by read or write; all access is by reading and writing to
+the mapped memory.
+The mmap'ped buffer appears as a block of memory of size
+.Va buffersize
+(as available via
+.Dv AUDIO_GETINFO
+or
+.Dv AUDIO_GETBUFINFO ) .
+The audio driver will continuously move data from this buffer
+from/to the mixing buffer, wrapping around at the end of the buffer.
+To find out where the hardware is currently accessing data in the buffer the
+.Dv AUDIO_GETIOFFS
+and
+.Dv AUDIO_GETOOFFS
+calls can be used.
+Note that
+.Xr mmap 2
+no longer maps hardware buffers directly.
+Now it is achieved by emulation, so don't expect significant improvements 
+over normal
+.Xr write 2 .
+For historical reasons, only encodings that are not set
+.Dv AUDIO_ENCODINGFLAG_EMULATED
+are able to
+.Xr mmap 2 .
+.Pp
+The audio device, like most devices, can be used in
+.Xr select 2 ,
+can be set in non-blocking mode and can be set (with a
+.Dv FIOASYNC
+ioctl) to send a
+.Dv SIGIO
+when I/O is possible.
+The mixer device can be set to generate a
+.Dv SIGIO
+whenever a mixer value is changed.
+.Pp
+The following
+.Xr ioctl 2
+commands are supported on the sample devices:
+.Bl -tag -width indent
+.It Dv AUDIO_FLUSH
+This command stops all playback and recording, clears all queued
+buffers, resets error counters on this track,
+and restarts recording and playback as
+appropriate for the current sampling mode.
+.It Dv AUDIO_PERROR (int)
+.It Dv AUDIO_RERROR (int)
+This command fetches the count of dropped output (input)
+bytes into its integer argument.
+There is no information regarding when in the sample stream
+they were dropped.
+.It Dv AUDIO_WSEEK (u_long)
+This command fetches the count of bytes that are queued ahead of the
+first sample in the most recent sample block written into its integer argument.
+.It Dv AUDIO_DRAIN
+This command suspends the calling process until all queued playback
+samples have been played.
+.It Dv AUDIO_GETDEV (audio_device_t)
+This command fetches the current hardware device information into the
+.Vt audio_device_t
+argument.
+.Bd -literal
+typedef struct audio_device {
+        char name[MAX_AUDIO_DEV_LEN];
+        char version[MAX_AUDIO_DEV_LEN];
+        char config[MAX_AUDIO_DEV_LEN];
+} audio_device_t;
+.Ed
+.It Dv AUDIO_GETENC (audio_encoding_t)
+This command is used iteratively to fetch sample encoding names and
+format IDs into the input/output audio_encoding_t argument.
+The encoding returned by the command is the user-accessible encoding,
+not the hardware-supported encoding.
+.Bd -literal
+typedef struct audio_encoding {
+	int index;      /* input: nth encoding */
+	char name[MAX_AUDIO_DEV_LEN]; /* name of encoding */
+	int encoding;   /* value for encoding parameter */
+	int precision;  /* value for precision parameter */
+	int flags;
+#define AUDIO_ENCODINGFLAG_EMULATED 1 /* software emulation mode */
+} audio_encoding_t;
+.Ed
+.Pp
+To query
+all the supported encodings, start with an index field of 0 and
+continue with successive encodings (1, 2, ...) until the command returns
+an error.
+.It Dv AUDIO_GETFD (int)
+This command is obsolete.
+.It Dv AUDIO_SETFD (int)
+This command is obsolete.
+.It Dv AUDIO_GETPROPS (int)
+This command gets a bit set of hardware properties.
+If the hardware
+has a certain property the corresponding bit is set, otherwise it is not.
+The properties can have the following values:
+.Pp
+.Bl -tag -width AUDIO_PROP_INDEPENDENT -compact
+.It Dv AUDIO_PROP_FULLDUPLEX
+the device admits full duplex operation.
+.It Dv AUDIO_PROP_MMAP
+the device can be used with
+.Xr mmap 2 .
+.It Dv AUDIO_PROP_INDEPENDENT
+the device can set the playing and recording encoding parameters
+independently.
+.It Dv AUDIO_PROP_PLAYBACK
+the device is capable of audio playback.
+.It Dv AUDIO_PROP_CAPTURE
+the device is capable of audio capture.
+.El
+.It Dv AUDIO_GETIOFFS (audio_offset_t)
+.It Dv AUDIO_GETOOFFS (audio_offset_t)
+This command fetches the current offset in the input(output) buffer where
+the track mixer will be putting(getting) data.
+It mostly useful when the device buffer is available in user space via the
+.Xr mmap 2
+call.
+The information is returned in the
+.Vt audio_offset_t
+structure.
+.Bd -literal
+typedef struct audio_offset {
+	u_int	samples;   /* Total number of bytes transferred */
+	u_int	deltablks; /* Blocks transferred since last checked */
+	u_int	offset;    /* Physical transfer offset in buffer */
+} audio_offset_t;
+.Ed
+.It Dv AUDIO_GETINFO (audio_info_t)
+.It Dv AUDIO_GETBUFINFO (audio_info_t)
+.It Dv AUDIO_SETINFO (audio_info_t)
+Get or set audio information as encoded in the audio_info structure.
+For historical reasons, the audio_info structure has three different
+layer's parameters: track, track mixer, and hardware rich mixer.
+.Bd -literal
+typedef struct audio_info {
+	struct	audio_prinfo play;   /* info for play (output) side */
+	struct	audio_prinfo record; /* info for record (input) side */
+        u_int	monitor_gain;			/* input to output mix [HWmixer] */
+	/* BSD extensions */
+	u_int	blocksize;	/* read/write block size [track] */
+	u_int	hiwat;		/* output high water mark [track] */
+	u_int	lowat;		/* output low water mark [track] */
+	u_int	_ispare1;
+	u_int	mode;		/* current operation mode [track] */
+#define AUMODE_PLAY	0x01
+#define AUMODE_RECORD	0x02
+#define AUMODE_PLAY_ALL 0x04	/* Not used anymore */
+} audio_info_t;
+.Ed
+.Pp
+When setting the current state with
+.Dv AUDIO_SETINFO ,
+the audio_info structure should first be initialized with
+.Li AUDIO_INITINFO(&info)
+and then the particular values to be changed should be set.
+This allows the audio driver to only set those things that you wish
+to change and eliminates the need to query the device with
+.Dv AUDIO_GETINFO
+or
+.Dv AUDIO_GETBUFINFO
+first.
+.Pp
+The
+.Va mode
+field indicates current operation mode, either one of
+.Dv AUMODE_PLAY
+or
+.Dv AUMODE_RECORD .
+These two flags can not be changed once this descriptor is opened.
+For playback mode, the obsolete
+.Dv AUMODE_PLAY_ALL
+flag can be set but has no effect.
+.Pp
+.Va hiwat
+and
+.Va lowat
+are used to control write behavior.
+Writes to the audio devices will queue up blocks until the high-water
+mark is reached, at which point any more write calls will block
+until the queue is drained to the low-water mark.
+.Va hiwat
+and
+.Va lowat
+set those high- and low-water marks (in audio blocks).
+The default for
+.Va hiwat
+is the maximum value and for
+.Va lowat
+75% of
+.Va hiwat .
+.Pp
+.Va blocksize
+sets the current audio blocksize.
+The generic audio driver layer and the hardware driver have the
+opportunity to adjust this block size to get it within
+implementation-required limits.
+Normally the
+.Va blocksize
+is calculated to correspond to the value of the
+.Em hw.audioX.blk_ms
+sysctl and is recalculated when the encoding parameters change.
+If the descriptor is opened for read only,
+.Va blocksize
+indicates the blocksize for the recording track.
+Otherwise,
+.Va blocksize
+indicates the blocksize for the playback track.
+.Bd -literal
+struct audio_prinfo {
+	u_int	sample_rate;	/* sample rate in samples/s [track] */
+	u_int	channels;	/* number of channels, usually 1 or 2 [track] */
+	u_int	precision;	/* number of bits/sample [track] */
+	u_int	encoding;	/* data encoding (AUDIO_ENCODING_* below) [track] */
+	u_int	gain;		/* volume level [HWmixer] */
+	u_int	port;		/* selected I/O port [HWmixer] */
+	u_long	seek;		/* BSD extension [track] */
+	u_int	avail_ports;	/* available I/O ports [HWmixer] */
+	u_int	buffer_size;	/* total size audio buffer [track] */
+	u_int	_ispare[1];
+	u_int	samples;	/* number of samples [track] */
+	u_int	eof;		/* End Of File (zero-size writes) counter [track] */
+	u_char	pause;		/* non-zero if paused, zero to resume [track] */
+	u_char	error;		/* non-zero if underflow/overflow occurred [track] */
+	u_char	waiting;	/* non-zero if another process hangs in open [track] */
+	u_char	balance;	/* stereo channel balance [HWmixer] */
+	u_char	cspare[2];
+	u_char	open;		/* non-zero if currently open [trackmixer] */
+	u_char	active;		/* non-zero if I/O is currently active [trackmixer] */
+};
+.Ed
+.Pp
+Note: many hardware audio drivers require identical playback and
+recording sample rates, sample encodings, and channel counts.
+The playing information is always set last and will prevail on such hardware.
+If the hardware can handle different settings the
+.Dv AUDIO_PROP_INDEPENDENT
+property is set.
+.Pp
+The encoding parameter can have the following values:
+.Pp
+.Bl -tag -width AUDIO_ENCODING_SLINEAR_BE -compact
+.It Dv AUDIO_ENCODING_ULAW
+mu-law encoding, 8 bits/sample
+.It Dv AUDIO_ENCODING_ALAW
+A-law encoding, 8 bits/sample
+.It Dv AUDIO_ENCODING_SLINEAR
+two's complement signed linear encoding with the platform byte order
+.It Dv AUDIO_ENCODING_ULINEAR
+unsigned linear encoding
+with the platform byte order
+.It Dv AUDIO_ENCODING_ADPCM
+ADPCM encoding, 8 bits/sample
+.It Dv AUDIO_ENCODING_SLINEAR_LE
+two's complement signed linear encoding with little endian byte order
+.It Dv AUDIO_ENCODING_SLINEAR_BE
+two's complement signed linear encoding with big endian byte order
+.It Dv AUDIO_ENCODING_ULINEAR_LE
+unsigned linear encoding with little endian byte order
+.It Dv AUDIO_ENCODING_ULINEAR_BE
+unsigned linear encoding with big endian byte order
+.It Dv AUDIO_ENCODING_AC3
+Dolby Digital AC3
+.El
+.Pp
+The
+.Nm
+driver accepts the following formats.
+.Va encoding
+and
+.Va precision
+are one of the values obtained by
+.Dv AUDIO_GETENC ,
+regardless of formats supported by underlying driver.
+.Va frequency
+ranges from 1000Hz to 192000Hz,
+regardless of frequency (ranges) supported by underlying driver.
+.Va channels
+depends your underlying driver.
+If the underlying driver only supports monaural (1 channel)
+or stereo (2 channels), you can specify 1 or 2 regardless of
+number of channels supported by underlying driver.
+If the underlying driver supports three or more channels, you can specify
+the number of channels supported by the underlying driver or fewer.
+.Pp
+The
+.Va gain ,
+.Va port
+and
+.Va balance
+settings provide simple shortcuts to the richer mixer
+interface described below and are not obtained by
+.Dv AUDIO_GETBUFINFO .
+The gain should be in the range
+.Bq Dv AUDIO_MIN_GAIN , Dv AUDIO_MAX_GAIN
+and the balance in the range
+.Bq Dv AUDIO_LEFT_BALANCE , Dv AUDIO_RIGHT_BALANCE
+with the normal setting at
+.Dv AUDIO_MID_BALANCE .
+.Pp
+The input port should be a combination of:
+.Pp
+.Bl -tag -width AUDIO_MICROPHONE -compact
+.It Dv AUDIO_MICROPHONE
+to select microphone input.
+.It Dv AUDIO_LINE_IN
+to select line input.
+.It Dv AUDIO_CD
+to select CD input.
+.El
+.Pp
+The output port should be a combination of:
+.Pp
+.Bl -tag -width AUDIO_HEADPHONE -compact
+.It Dv AUDIO_SPEAKER
+to select speaker output.
+.It Dv AUDIO_HEADPHONE
+to select headphone output.
+.It Dv AUDIO_LINE_OUT
+to select line output.
+.El
+.Pp
+The available ports can be found in
+.Va avail_ports
+.Dv ( AUDIO_GETBUFINFO
+only).
+.Pp
+.Va buffer_size
+is the total size of the audio buffer.
+The buffer size divided by the
+.Va blocksize
+gives the maximum value for
+.Va hiwat .
+Currently the
+.Va buffer_size
+can only be read and not set.
+.Pp
+The
+.Va seek
+and
+.Va samples
+fields are only used by
+.Dv AUDIO_GETINFO
+and
+.Dv AUDIO_GETBUFINFO .
+.Va seek
+represents the count of
+bytes pending;
+.Va samples
+represents the total number of bytes recorded or played, less those
+that were dropped due to inadequate consumption/production rates.
+.Pp
+.Va pause
+returns the current pause/unpause state for recording or playback.
+For
+.Dv AUDIO_SETINFO ,
+if the pause value is specified it will either pause
+or unpause the particular direction.
+.It Dv AUDIO_QUERYFORMAT (audio_format_query_t)
+This command enumerates formats supported by the hardware.
+Similarly to
+.Dv AUDIO_GETENC ,
+to query all the supported formats,
+start with an index field of 0 and continue with successive formats
+(1, 2, ...) until the command returns an error.
+.Bd -literal
+typedef struct audio_format_query {
+	u_int	index;
+	struct audio_format fmt;
+} audio_format_query_t;
+.Ed
+.It Dv AUDIO_GETFORMAT (audio_info_t)
+This command fetches the current hardware format.
+Only the following members in audio_info_t are used.
+Members which are not listed here or belong in invalid direction are
+filled by \-1.
+.Bl -bullet
+.It
+mode
+.It
+play.encoding
+.It
+play.precision
+.It
+play.channels
+.It
+play.sample_rate
+.It
+record.encoding
+.It
+record.precision
+.It
+record.channels
+.It
+record.sample_rate
+.El
+.Pp
+.Va mode
+indicates which direction is valid.
+.It Dv AUDIO_SETFORMAT (audio_info_t)
+This command sets the hardware format.
+It will fail if there are any opened descriptors.
+So obviously, it must be issued on
+.Pa /dev/audioctl .
+Similarly to
+.Dv AUDIO_GETFORMAT ,
+only above members in audio_info_t are used.
+Members which is not listed or belong in invalid direction are ignored.
+The parameters can be chosen from the choices obtained by
+.Dv AUDIO_QUERYFORMAT .
+.It Dv AUDIO_GETCHAN (int)
+This command is obsolete.
+.It Dv AUDIO_SETCHAN (int)
+This command is obsolete.
+.El
+.Sh MIXER DEVICE
+The mixer device,
+.Pa /dev/mixer ,
+may be manipulated with
+.Xr ioctl 2
+but does not support
+.Xr read 2
+or
+.Xr write 2 .
+It supports the following
+.Xr ioctl 2
+commands:
+.Bl -tag -width indent
+.It Dv AUDIO_GETDEV (audio_device_t)
+This command is the same as described above for the sampling devices.
+.It Dv AUDIO_MIXER_READ (mixer_ctrl_t)
+.It Dv AUDIO_MIXER_WRITE (mixer_ctrl_t)
+These commands read the current mixer state or set new mixer state for
+the specified device
+.Va dev .
+.Va type
+identifies which type of value is supplied in the
+.Vt mixer_ctrl_t
+argument.
+.Bd -literal
+#define AUDIO_MIXER_CLASS  0
+#define AUDIO_MIXER_ENUM   1
+#define AUDIO_MIXER_SET    2
+#define AUDIO_MIXER_VALUE  3
+typedef struct mixer_ctrl {
+	int dev;			/* input: nth device */
+	int type;
+	union {
+		int ord;		/* enum */
+		int mask;		/* set */
+		mixer_level_t value;	/* value */
+	} un;
+} mixer_ctrl_t;
+
+#define AUDIO_MIN_GAIN  0
+#define AUDIO_MAX_GAIN  255
+typedef struct mixer_level {
+        int num_channels;
+        u_char level[8];               /* [num_channels] */
+} mixer_level_t;
+#define AUDIO_MIXER_LEVEL_MONO  0
+#define AUDIO_MIXER_LEVEL_LEFT  0
+#define AUDIO_MIXER_LEVEL_RIGHT 1
+.Ed
+.Pp
+For a mixer value, the
+.Va value
+field specifies both the number of channels and the values for each
+channel.
+If the channel count does not match the current channel count, the
+attempt to change the setting may fail (depending on the hardware
+device driver implementation).
+Audio levels may be adjusted in increments of the
+.Dv delta
+value returned by
+.Dv AUDIO_MIXER_DEVINFO .
+This field is optional for hardware drivers to specify - devices with a
+delta of 0 may allow arbitrary adjustment of levels.
+.Pp
+For an enumeration value, the
+.Va ord
+field should be set to one of the possible values as returned by a prior
+.Dv AUDIO_MIXER_DEVINFO
+command.
+.Pp
+The type
+.Dv AUDIO_MIXER_CLASS
+is only used for classifying particular mixer device
+types and is not used for
+.Dv AUDIO_MIXER_READ
+or
+.Dv AUDIO_MIXER_WRITE .
+.It Dv AUDIO_MIXER_DEVINFO (mixer_devinfo_t)
+This command is used iteratively to fetch audio mixer device information
+into the input/output
+.Vt mixer_devinfo_t
+argument.
+To query all the supported devices, start with an index field of
+0 and continue with successive devices (1, 2, ...) until the
+command returns an error.
+.Bd -literal
+typedef struct mixer_devinfo {
+	int index;		/* input: nth mixer device */
+	audio_mixer_name_t label;
+	int type;
+	int mixer_class;
+	int next, prev;
+#define AUDIO_MIXER_LAST	-1
+	union {
+		struct audio_mixer_enum {
+			int num_mem;
+			struct {
+				audio_mixer_name_t label;
+				int ord;
+			} member[32];
+		} e;
+		struct audio_mixer_set {
+			int num_mem;
+			struct {
+				audio_mixer_name_t label;
+				int mask;
+			} member[32];
+		} s;
+		struct audio_mixer_value {
+			audio_mixer_name_t units;
+			int num_channels;
+			int delta;
+		} v;
+	} un;
+} mixer_devinfo_t;
+.Ed
+.Pp
+The
+.Va label
+field identifies the name of this particular mixer control.
+The
+.Va index
+field may be used as the
+.Va dev
+field in
+.Dv AUDIO_MIXER_READ
+and
+.Dv AUDIO_MIXER_WRITE
+commands.
+The
+.Va type
+field identifies the type of this mixer control.
+Enumeration types are typically used for on/off style controls (e.g., a
+mute control) or for input/output device selection (e.g., select
+recording input source from CD, line in, or microphone).
+Set types are similar to enumeration types but any combination
+of the mask bits can be used.
+.Pp
+The
+.Va mixer_class
+field identifies what class of control this is.
+The
+.Pq arbitrary
+value set by the hardware driver may be determined by examining the
+.Va mixer_class
+field of the class itself, a mixer of type
+.Dv AUDIO_MIXER_CLASS .
+For example, a mixer controlling the input gain on the line in circuit
+would have a
+.Va mixer_class
+that matches an input class device with the name
+.Dq inputs
+.Dv ( AudioCinputs ) ,
+and would have a
+.Va label
+of
+.Dq line
+.Dv ( AudioNline ) .
+Mixer controls which control audio circuitry for a particular audio
+source (e.g., line-in, CD in, DAC output) are collected under the input class,
+while those which control all audio sources (e.g., master volume,
+equalization controls) are under the output class.
+Hardware devices capable of recording typically also have a record class,
+for controls that only affect recording, and also a monitor class.
+.Pp
+The
+.Va next
+and
+.Va prev
+may be used by the hardware device driver to provide hints for the next
+and previous devices in a related set (for example, the line in level
+control would have the line in mute as its
+.Dq next
+value).
+If there is no relevant next or previous value,
+.Dv AUDIO_MIXER_LAST
+is specified.
+.Pp
+For
+.Dv AUDIO_MIXER_ENUM
+mixer control types,
+the enumeration values and their corresponding names are filled in.
+For example, a mute control would return appropriate values paired with
+.Dv AudioNon
+and
+.Dv AudioNoff .
+For
+.Dv AUDIO_MIXER_VALUE
+and
+.Dv AUDIO_MIXER_SET
+mixer control types, the channel count is
+returned; the units name specifies what the level controls (typical
+values are
+.Dv AudioNvolume ,
+.Dv AudioNtreble ,
+.Dv AudioNbass ) .
+.\" For AUDIO_MIXER_SET mixer control types, what is what?
+.El
+.Pp
+By convention, all the mixer devices can be distinguished from other
+mixer controls because they use a name from one of the
+.Dv AudioC*
+string values.
+.Sh FILES
+.Bl -tag -width /dev/audioctl -compact
+.It Pa /dev/audio
+.It Pa /dev/audioctl
+.It Pa /dev/sound
+.It Pa /dev/mixer
+.El
+.Sh SEE ALSO
+.Xr audiocfg 1 ,
+.Xr audioctl 1 ,
+.Xr audioplay 1 ,
+.Xr audiorecord 1 ,
+.Xr mixerctl 1 ,
+.Xr ioctl 2 ,
+.Xr ossaudio 3 ,
+.Xr acorn32/vidcaudio 4 ,
+.\" .Xr amiga/aucc 4 ,
+.\" .Xr amiga/melody 4 ,
+.Xr arcofi 4 ,
+.Xr aria 4 ,
+.Xr auacer 4 ,
+.Xr audiocs 4 ,
+.Xr auich 4 ,
+.Xr auixp 4 ,
+.\" .Xr ausoc 4 ,
+.Xr autri 4 ,
+.Xr auvia 4 ,
+.Xr bba 4 ,
+.Xr btsco 4 ,
+.Xr clcs 4 ,
+.Xr clct 4 ,
+.Xr cmpci 4 ,
+.Xr dreamcast/aica 4 ,
+.Xr eap 4 ,
+.Xr emuxki 4 ,
+.Xr esa 4 ,
+.Xr esm 4 ,
+.Xr eso 4 ,
+.Xr ess 4 ,
+.\" .Xr evbarm/aaci 4 ,
+.\" .Xr evbarm/digfilt 4 ,
+.\" .Xr evbarm/pxaacu 4 ,
+.\" .Xr evbarm/udassio 4 ,
+.Xr fms 4 ,
+.Xr gcscaudio 4 ,
+.Xr gus 4 ,
+.Xr guspnp 4 ,
+.Xr hdafg 4 ,
+.Xr hdaudio 4 ,
+.Xr hppa/harmony 4 ,
+.Xr macppc/awacs 4 ,
+.Xr macppc/snapper 4 ,
+.Xr midi 4 ,
+.Xr neo 4 ,
+.Xr pad 4 ,
+.Xr pas 4 ,
+.\" .Xr paud 4 ,
+.Xr radio 4 ,
+.\" .Xr repluse 4 ,
+.Xr sb 4 ,
+.Xr sgimips/haltwo 4 ,
+.Xr sgimips/mavb 4 ,
+.Xr sparc/audioamd 4 ,
+.Xr sparc/dbri 4 ,
+.Xr sv 4 ,
+.\" .Xr toccata 4 ,
+.Xr uaudio 4 ,
+.\" .Xr vaudio 4 ,
+.\" .Xr vcaudio 4 ,
+.\" .Xr vraiu 4 ,
+.\" .Xr vsaudio 4 ,
+.Xr wss 4 ,
+.Xr x68k/vs 4 ,
+.Xr yds 4 ,
+.Xr ym 4
+.\" .Xr zaudio 4
+.Sh HISTORY
+Support for virtual channels and mixing first appeared in
+.Nx 8.0 .
+.Sh BUGS
+If the device is used in
+.Xr mmap 2
+it is currently always mapped for writing (playing) due to VM system weirdness.