Here's my first cut at an audio(4) manual page.  We'll still need device
specific pages for ISA audio devices, and I'm sure there's stuff here
that could use further explaining or code samples.  Suggestions would be
most welcome.

I'd like to submit this fairly soon as a PR to get it into the manual
page sections for the 1.1 release, so comment sooner rather than later.

==John

.\" Copyright (c) 1995 John T. Kohl
.\" All rights reserved.
.\"
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.\" 3. The name of the author may not be used to endorse or promote products
.\"    derived from this software without specific prior written permission.
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.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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.\"	
.\"	$Id: audio.4,v 1.2 1995/11/06 01:23:49 jtk Exp $
.\"
.Dd November 5, 1995
.Dt AUDIO 4
.Os 
.Sh NAME
.Nm audio
.Nd
device-independent audio driver layer
.Sh SYNOPSIS
.Fd #include <sys/audioio.h>
.Pa /dev/audio
.br
.Pa /dev/sound
.br
.Pa /dev/mixer
.Sh DESCRIPTION
The
.Nm 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.
.Pp
There are three device files available for audio operation:
.Pa /dev/audio ,
.Pa /dev/sound ,
and
.Pa /dev/mixer .
.Pa /dev/audio
and
.Pa /dev/sound
are used for recording or playback of digital samples.
.Pa /dev/mixer
is used to manipulate volume, recording source, or other audio mixer
functions.

.Sh SAMPLING DEVICES
When
.Pa /dev/audio
is opened, it automatically directs the underlying driver to manipulate
monaural 8-bit mulaw sample.  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
.Pa /dev/sound
is opened, it maintains the previous audio sample mode and
record/playback mode.  In all other respects
.Pa /dev/audio
and
.Pa /dev/sound
are identical.
.Pp
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).
.Pp
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.
.Pp
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
.Xr read 2
until recording is unpaused.
.Pp
If a writing process does not call
.Xr 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
.Xr read 2
frequently enough, it will simply miss samples.
.Pp
The following
.Xr ioctl 2
commands are supported on the sample devices:
.Bl -tag -width indent -compact
.It Dv 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.
.It Dv 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.
.It Dv 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.
.It Dv AUDIO_DRAIN
This command suspends the calling process until all queued playback
samples have been played by the hardware.
.It Dv AUDIO_GETDEV (audio_device_t)
This command fetches the current hardware device information into the
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)
.Bd -literal
typedef struct audio_encoding {
	int index;
	char name[MAX_AUDIO_DEV_LEN];
	int format_id;
} audio_encoding_t;
.Ed
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.
.It Dv 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.
.It Dv 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.
.It Dv AUDIO_GETINFO (audio_info_t)
.It Dv AUDIO_SETINFO (audio_info_t)
Get or set audio information as encoded in the audio_info structure.
.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	__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;
.Ed
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.
.Pp
The
.Va mode
field should be set to AUMODE_PLAY, AUMODE_RECORD, or their bitwise OR.
.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).
.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.  Upon return from an
AUDIO_SETINFO call, the actual blocksize set is returned in this field.
.Pp
.Va backlog
is currently unused.
.Bd -literal
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 */
};
.Ed
.Pp
[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.]
.Pp
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.
.Pp
The
.Va seek
and
.Va samples
fields are only used for AUDIO_GETINFO.
.Va seek
represents the count of
samples pending;
.Va samples
represents the total number of samples 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 AUDIO_SETINFO, if the pause value is specified it will either pause
or unpause the particular direction.
.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 -compact
.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)
.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;
	int type;
	union {
		int ord;		/* enum */
		int mask;		/* set */
		mixer_level_t value;	/* value */
	} un;
} mixer_ctrl_t;
.Ed
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 mixer_ctrl_t
argument.  
For a mixer value, the
.Va 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
.Va 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.
.It Dv 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.
.Bd -literal
typedef struct mixer_devinfo {
	int index;
	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;
.Ed
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 AUDIO_MIXER_READ and 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).
.Pp
The
.Va 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.
.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 "next" value).  If there is
no relevant next or previous value, AUDIO_MIXER_LAST is specified.
.Pp
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_LEVEL mixer control types, the channel count is
returned; the units name specifies what the level controls (typical
values are AudioNvolume, AudioNtreble, AudioNbass).
.\" For AUDIO_MIXER_SET mixer control types, what is what?
.El
.Pp
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.
.Sh SEE ALSO
.Xr ioctl 2 .
.br
For ports using the ISA bus:
.Xr gus 4 ,
.Xr pas 4 ,
.Xr pss 4 ,
.Xr sb 4 ,
.Xr wss 4 .
.Sh BUGS
Some of the device-specific manual pages do not yet exist.
.Pp
The device class conventions are just a wish and not yet reality.