AUDIO(4)

NAME
audio - device-independent audio driver layer

SYNOPSIS
#include <sys/types.h>
#include <sys/audioio.h>

DESCRIPTION
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 three device files available for audio operation: /dev/audio, /dev/sound, 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.

SAMPLING DEVICES
When /dev/audio is opened, it automatically directs the underlying driver to manipulate monaural 8-bit mulaw samples. In addition, if it is opened read-only (write-only) the device is set to half-duplex record (play) mode with recording (playing) unpaused and playing (recording) paused. When /dev/sound is opened, it maintains the previous audio sample mode and record/playback mode. In all other 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).

Reads and writes to a sampling device should be in multiples of the current audio block size which can be queried and set using the interfaces described below. Writes which are not multiples of the block size will be padded to a block boundary with silence. Reads which are not multiples of the block size will consume a block from the audio hardware but only return the requested number of bytes.

On a half-duplex device, writes while recording is in progress will be immediately discarded. Similarly, reads while playback is in progress will be filled with silence but delayed to return at the current sampling rate. If both playback and recording are requested on a half-duplex device, playback mode takes precedence and recordings will get silence. On a full-duplex device, reads and writes may operate concurrently without interference. On either type of device, 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 read(2) until recording is unpaused.

If a writing process does not call write(2) frequently enough to provide audio playback blocks in time for the next hardware interrupt service, one or more audio silence blocks will be queued for playback. The writing process must provide enough data via subsequent write calls to ``catch up'' in time to the current audio block before any more process-provided samples will be played. [Alternatively, the playing process can use one of the interfaces below to halt and later restart audio playback.] If a reading process does not call read(2) frequently enough, it will simply miss samples.

The following ioctl(2) commands are supported on the sample devices:
AUDIO_FLUSH
    This command stops all playback and recording, clears all queued buffers, resets error counters, and restarts recording and playback as appropriate for the current sampling mode.
AUDIO_RERROR (int)
    This command fetches the count of dropped input samples into its integer argument. There is no information regarding when in the sample stream they were dropped.
AUDIO_WSEEK (int)
    This command fetches the count of samples are queued ahead of the first sample in the most recent sample block written into its integer argument.
AUDIO_DRAIN
    This command suspends the calling process until all queued playback samples have been played by the hardware.
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_GETENC (audio_encoding_t)

        typedef struct audio_encoding {
        	int index;	/* input: nth encoding */
        	char name[MAX_AUDIO_DEV_LEN];
        	int format_id;
        } audio_encoding_t;

    This command is used iteratively to fetch sample encoding names and format_ids into the input/output audio_encoding_t argument. To query all the supported encodings, start with an index field of zero and continue with successive encodings (1, 2, ...) until the command returns an error.
AUDIO_GETFD (int)
    This command fetches a non-zero value into its integer argument if the hardware supports full-duplex operation, or a zero value if the hardware only supports half-duplex operation.
AUDIO_SETFD (int)
    This command sets the device into full-duplex operation if its integer argument has a non-zero value, or into half-duplex operation if it contains a zero value. If the device does not support full-duplex operation, attempting to set full-duplex mode returns an error.
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	__spare;
        	/* BSD extensions */
        	u_int	blocksize;	/* input blocking threshold */
        	u_int	hiwat;		/* output high water mark */
        	u_int	lowat;		/* output low water mark */
        	u_int	backlog;	/* samples of output backlog to gen. */
        	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 should be set to AUMODE_PLAY, AUMODE_RECORD, or their bitwise OR.

    hiwat and 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. hiwat and lowat set those high- and low-water marks (in audio blocks).

    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. Upon return from an AUDIO_SETINFO call, the actual blocksize set is returned in this field.

    backlog is currently unused.

        struct audio_prinfo {
        	u_int	sample_rate;	/* sample rate in samples/s */
        	u_int	channels;	/* number of channels, usually 1 or 2 */
        	u_int	precision;	/* number of bits/sample */
        	u_int	encoding;	/* data encoding (AUDIO_ENCODING_* above) */
        	u_int	gain;		/* volume level */
        	u_int	port;		/* selected I/O port */
        	u_long	seek;		/* BSD extension */
        	u_int	ispare[3];
        	/* Current state of device: */
        	u_int	samples;	/* number of samples */
        	u_int	eof;		/* End Of File (zero-size writes) counter */
        	u_char	pause;		/* non-zero if paused, zero to resume */
        	u_char	error;		/* non-zero if underflow/overflow ocurred */
        	u_char	waiting;	/* non-zero if another process hangs in open */
        	u_char	cspare[3];
        	u_char	open;		/* non-zero if currently open */
        	u_char	active;		/* non-zero if I/O is currently active */
        };

    [Note: many hardware audio drivers require identical playback and recording sample rates, sample encodings, and channel counts. The recording information is always set last and will prevail on such hardware.]

    The gain and port settings provide simple shortcuts to the richer mixer interface described below. The gain should be in the range [AUDIO_MIN_GAIN,AUDIO_MAX_GAIN]. The port value is hardware-dependent and should be selected (if setting with AUDIO_SETINFO) based upon return values from the mixer query functions below or from a prior AUDIO_GETINFO.

    The seek and samples fields are only used for AUDIO_GETINFO. seek represents the count of samples pending; samples represents the total number of samples recorded or played, less those that were dropped due to inadequate consumption/production rates.

    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.

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)

        #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;

    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. For a mixer value, the value field specifies both the number of channels and the values for each of the channels. 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 encodings, start with an index field of zero and continue with successive encodings (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;
        		} 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).

    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. For example, a mixer level controlling the input gain on the ``line in'' circuit would be a class that matches an input class device with the name ``Inputs'' (AudioCInputs). 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.

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

By convention, all the mixer device indices for generic class grouping are at the end of the index number space for a particular hardware device, and can be distinguished from other mixer controls because they use a name from one of the AudioC* string values.

FILES
/dev/audio /dev/mixer /dev/sound

SEE ALSO
ioctl(2).
For ports using the ISA bus: gus(4), pas(4), pss(4), sb(4), wss(4).

BUGS
Some of the device-specific manual pages do not yet exist.
The device class conventions are just a wish and not yet reality.
Audio playback can be scratchy with pops and crackles due to the audio layer's buffering scheme. Using a bigger blocksize will help reduce such annoyances.