AUDIO(4) | Device Drivers Manual | AUDIO(4) |
audio
, mixer
— device-independent audio driver layer
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.
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 *AUDIO_GETDEV
audio_device_t *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 *AUDIO_GETENC
audio_encoding_t *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 *AUDIO_GETPROPS
int *AUDIO_PROP_FULLDUPLEX
AUDIO_PROP_INDEPENDENT
AUDIO_GETIOFFS
audio_offset_t *AUDIO_GETOOFFS
audio_offset_t *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 *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
AUDIO_ENCODING_ALAW
AUDIO_ENCODING_SLINEAR_LE
AUDIO_ENCODING_SLINEAR_BE
AUDIO_ENCODING_ULINEAR_LE
AUDIO_ENCODING_ULINEAR_BE
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 *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:
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 *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; #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 *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.
aucat(1), audioctl(1), cdio(1), mixerctl(1), ioctl(2), sio_open(3), ac97(4), uaudio(4), audio(9)
July 28, 2015 | OpenBSD-5.9 |