audio_intro.3

操作系统SunOS 4.1.3版本的源码

.\" @(#)audio_intro.3 1.1 92/07/30 SMI
.ds Dc 7/30/92
.TH AUDIO_INTRO 3 "\*(Dc" "" "Audio Library"
.SH NAME
audio_intro, libaudio.a
\- audio library functions
.SH DISCLAIMER
.LP
The manual pages, header files, and object library associated with
.B /usr/demo/SOUND/libaudio.a
are furnished on an AS IS basis as a preliminary interface to several
useful audio data-processing capabilities.  A future release of the
operating system may redefine the syntax or semantics of some of the
functions described herein.
.SH SYNOPSIS
.nf
.B #include <multimedia/libaudio.h>
.fi
.SH DESCRIPTION
.B libaudio.a
provides a preliminary interface to many of the common operations
necessary to access and manipulate audio files and devices.
By embodying the specifics of file formats and device
.B ioctl
commands in library routines, programs may be insulated, in part, from
future changes to the interface and data storage formats.
.SH File Header
.LP
Audio files contain a header that has a magic number (identifying
the file as an audio file) and fields that describe
the encoding format used to store the audio data.
Immediately following the header is a variable-length information field
in which, for instance,
.SM ASCII
annotation may be stored.
.LP
The format of the audio file header is compatible with a subset of the
NeXT\(tm
sound file header.  Sun has adopted NeXT's sound file header ideas
to facilitate the transparent sharing of audio files in a networked
environment.
[The NeXT Nested and Indirect sound file types are not currently supported.]
.LP
In order that audio files may be successfully shared amongst multiple
machine architectures, the header and data fields are written using
.I big-endian
byte-ordering.  Though this means that byte-swapping may be necessary
to read and write some data encodings on
.I little-endian
machines such as the Sun386i, this overhead is largely overwhelmed
by all other signal manipulation operations.  The benefits of transparent
file access far outweigh the costs.
.LP
Routines to support the reading and writing of audio file headers
are documented in the
.BR audio_filehdr (3)
manual page.  These routines convert between the file header format used for
file interchange and the in-core audio header format used by most audio
applications.
.SH Audio Header
.LP
Audio data is described by an
.B Audio_hdr
structure that is similar, but
.I not
identical, to the audio file header.  The audio header structure is
described in the
.BR audio_hdr (3)
manual page.
The
.BR audio_misc (3)
manual page describes routines that perform some simple transformations
based on the contents of the audio header structure.
.SH Audio Device Control
.LP
The
.BR audio (4)
manual page describes the direct interface to the audio i/o device
(\fB/dev/audio\fP) and the audio control pseudo-device (\fB/dev/audioctl\fP).
For the most part, however, programs may use the routines documented in the
.BR audio_device (3)
manual page to access and control the device.
.SH Audio Data Conversions
.LP
The SPARCstation audio device operates on data that has been encoded
in \(*m-law format, a quasi-logarithmic compression.
Since most signal-processing algorithms perform arithmetic on linearly
encoded data, it is convenient to be able to convert to and from a
linear \s-1PCM\s0 representation.  The
.BR audio_ulaw2linear (3)
manual page describes macros that will perform these transformations.
.SH FUTURE DIRECTIONS
.LP
Although access to audio file headers are partly concealed in the file header
routines, the data formats themselves are not properly hidden.
Routines to demultiplex interleaved audio channels, and to transparently
convert between data encodings, should be provided.
.LP
Access to many of the audio device status and control fields has been
concealed in the audio device routines.  As audio server technology emerges,
these interfaces may remain stable while the underlying implementation changes.
However, access to the data streams of the audio device should be similarly
concealed.
.SH SEE ALSO
.BR audio_convert (3),
.BR audio_filehdr (3),
.BR audio_hdr (3),
.BR audio_misc (3),
.BR audio_ulaw2linear (3),
.BR audio (4).
.SH NOTES
NeXT is a trademark of NeXT, Inc.