mixer(4) - OpenBSD manual pages

NAME

audio, mixer — device-independent audio driver layer

SYNOPSIS

audio* at ...

#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/audioio.h>
#include <string.h>

The audio 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.

There are four device files available for audio operation: /dev/audio, /dev/sound, /dev/audioctl, and /dev/mixer. /dev/audio and /dev/sound are used for recording or playback of digital samples. /dev/mixer is used to manipulate volume, recording source, or other audio mixer functions. /dev/audioctl accepts the same ioctl(2) operations as /dev/sound, but no other operations. In contrast to /dev/sound, which has the exclusive open property, /dev/audioctl can be opened at any time and can be used to manipulate the audio device while it is in use.

SAMPLING DEVICES

When /dev/audio or /dev/sound is opened, it attempts to maintain the previous audio sample format and record/playback mode. In addition, if it is opened read-only (write-only) the device is set to half-duplex record (play) mode with recording (playing) unpaused. In all respects /dev/audio and /dev/sound are identical.

Only one process may hold open a sampling device at a given time (although file descriptors may be shared between processes once the first open completes).

On a half-duplex device, either reads or writes are allowed, but not both. On a full-duplex device, reads and writes may operate concurrently without interference.

If a writing process does not call write(2) frequently enough to provide samples at the pace the hardware consumes them silence is inserted. If a reading process does not call read(2) frequently enough, it will simply miss samples.

The audio device is accessed with read(2) or write(2).

The audio device, like most devices, can be used in poll(2),

The following ioctl(2) commands are supported on the sample devices:

AUDIO_RERROR int *

AUDIO_PERROR int *

Obsolete. These commands fetch the count of dropped input or output samples into the int * argument, respectively. There is no information regarding when in the sample stream they were dropped.

AUDIO_GETDEV audio_device_t *

This command fetches the current hardware device information into the audio_device_t * argument.

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;

AUDIO_GETFD int *

This command returns 1 if in full-duplex mode, else 0.

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.

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 bps;        /* value for bps parameter */
	int msb;        /* value for msb parameter */
} audio_encoding_t;

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.

AUDIO_SETFD int *

Does nothing, left for compatibility; argument must point to a non-zero integer if the device is opened in read-write mode.

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:

AUDIO_PROP_FULLDUPLEX: The device admits full-duplex operation.
AUDIO_PROP_INDEPENDENT: The device can set playing and recording channel counts independently.

AUDIO_GETIOFFS audio_offset_t *

AUDIO_GETOOFFS audio_offset_t *

Obsolete. These commands fetch the number of bytes played or recorded. The information is returned in the audio_offset structure.

typedef struct audio_offset {
	u_int	samples;   /* Total number of bytes transferred */
} audio_offset_t;

AUDIO_GETINFO audio_info_t *

AUDIO_SETINFO audio_info_t *

Get or set audio information as encoded in the audio_info structure.

typedef struct audio_info {
	struct	audio_prinfo play;   /* info for play (output) side */
	struct	audio_prinfo record; /* info for record (input) side */
	u_int	hiwat;		/* blocks count in play buffer */
	u_int	mode;		/* current device mode */
#define AUMODE_PLAY	0x01
#define AUMODE_RECORD	0x02
} audio_info_t;

When setting the current state with AUDIO_SETINFO, the audio_info structure should first be initialized with

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 AUDIO_GETINFO first.

The mode field is read-only and set to AUMODE_PLAY, AUMODE_RECORD, or a bitwise OR combination of the three. Only full-duplex audio devices support simultaneous record and playback.

hiwat contains the number of blocks in the kernel play buffer. Writes to the audio devices will queue blocks until the play buffer is full, at which point any more write calls will block until space for at least one byte is available.

struct audio_prinfo {
	u_int	sample_rate;	/* sample rate in bit/s */
	u_int	channels;	/* number of channels, usually 1 or 2 */
	u_int	precision;	/* number of bits/sample */
	u_int	bps;		/* number of bytes/sample */
	u_int	msb;		/* data alignment */
	u_int	encoding;	/* data encoding (AUDIO_ENCODING_* below) */
	u_int	block_size;     /* size a block */
	/* Current state of device: */
	u_char	pause;		/* non-zero if paused, zero to resume */
	u_char	active;		/* non-zero if I/O is currently active */
};

The audio driver requires identical playback and recording sample rates, sample encodings, and block durations.

The encoding parameter can have the following values:

AUDIO_ENCODING_ULAW: mu-law encoding, 8 bits/sample
AUDIO_ENCODING_ALAW: A-law encoding, 8 bits/sample
AUDIO_ENCODING_SLINEAR_LE: two's complement signed linear encoding with little endian byte order
AUDIO_ENCODING_SLINEAR_BE: two's complement signed linear encoding with big endian byte order
AUDIO_ENCODING_ULINEAR_LE: unsigned linear encoding with little endian byte order
AUDIO_ENCODING_ULINEAR_BE: unsigned linear encoding with big endian byte order

The precision parameter describes the number of bits of audio data per sample. The bps parameter describes the number of bytes of audio data per sample. The msb parameter describes the alignment of the data in the sample. It is only meaningful when precision / NBBY < bps. A value of 1 means the data is aligned to the most significant bit.

block_size is the block size in bytes, which determines the frequency at which blocking read(2), write(2), or poll(2), wake up. 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 block_size is recalculated when other parameters changes.

It is recommended to set block_size at the same time as, or after, all other parameters have been set.

pause returns the current pause/unpause state for recording or playback. For AUDIO_SETINFO, if the pause value is specified it will either pause or unpause the particular direction. In full-duplex the pause values for both directions must be equal.

AUDIO_GETPOS struct audio_pos *

Fetch an atomic snapshot of device timing information in the audio_pos structure.

struct audio_pos {
	unsigned int play_pos;	/* total bytes played */
	unsigned int play_xrun;	/* bytes of silence inserted */
	unsigned int rec_pos;	/* total bytes recorded */
	unsigned int rec_xrun;	/* bytes dropped */
};

The properties have the following meaning:

play_pos: Total number of bytes played by the device since playback started (a.k.a the device wall clock).
play_xrun: The number of bytes corresponding to silence played because write(2) wasn't called fast enough.
rec_pos: Total number of bytes recorded by the device since recording started (a.k.a the device wall clock).
rec_xrun: The number of bytes dropped because read(2) wasn't called fast enough.

MIXER DEVICE

The mixer device, /dev/mixer, may be manipulated with ioctl(2) but does not support read(2) or write(2). It supports the following ioctl(2) commands:

AUDIO_GETDEV audio_device_t *

This command is the same as described above for the sampling devices.

AUDIO_MIXER_READ mixer_ctrl_t *

AUDIO_MIXER_WRITE mixer_ctrl_t *

These commands read the current mixer state or set new mixer state for the specified device dev. type identifies which type of value is supplied in the mixer_ctrl_t * argument.

#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

For a mixer value, the 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). For an enumeration value, the ord field should be set to one of the possible values as returned by a prior AUDIO_MIXER_DEVINFO command. The type AUDIO_MIXER_CLASS is only used for classifying particular mixer device types and is not used for AUDIO_MIXER_READ or AUDIO_MIXER_WRITE.

AUDIO_MIXER_DEVINFO mixer_devinfo_t *

This command is used iteratively to fetch audio mixer device information into the input/output 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.

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;

The label field identifies the name of this particular mixer control. The index field may be used as the dev field in AUDIO_MIXER_READ and AUDIO_MIXER_WRITE commands. The 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.

The mixer_class field identifies what class of control this is. This value is set to the index value used to query the class itself. The (arbitrary) value set by the hardware driver may be determined by examining the mixer_class field of the class itself, a mixer of type AUDIO_MIXER_CLASS. For example, a mixer level controlling the input gain on the “line in” circuit would have a mixer_class that matches an input class device with the name “inputs” (AudioCinputs) and would have a label of “line” (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.

The next and 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 “next” value). If there is no relevant next or previous value, AUDIO_MIXER_LAST is specified.

For 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 AudioNon and AudioNoff. For the AUDIO_MIXER_VALUE and AUDIO_MIXER_SET mixer control types, the channel count is returned; the units name specifies what the level controls (typical values are AudioNvolume, AudioNtreble, and AudioNbass).

By convention, all the mixer devices can be distinguished from other mixer controls because they use a name from one of the AudioC* string values.

FILES

/dev/audio
/dev/audioctl
/dev/sound
/dev/mixer

OpenBSD manual page server

NAME

SYNOPSIS

DESCRIPTION

SAMPLING DEVICES

MIXER DEVICE

FILES

SEE ALSO