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NAME
     audiofile - file format for audio data

SYNOPSIS
     #include <multimedia/libaudio.h>

DESCRIPTION
     An audio file is composed of three parts:  a 24-byte header,
     a variable-length annotation block, and a contiguous segment
     of audio data.  Audio header and data fields are  stored  in
     network  (big-endian)  byte  order, regardless of the native
     byte-order of the machine architecture on which an  applica-
     tion  may  be running.  Therefore, multi-byte audio data may
     require byte reversal in order  to  operate  on  it  by  the
     arithmetic unit of certain processors.

     The header is defined by the following structure:

          typedef unsigned long    u_32;          /* unsigned 32-bit integer */

          typedef struct {
               u_32      magic;         /* magic number */
               u_32      hdr_size; /* byte offset to start of audio data */
               u_32      data_size;     /* data length, in bytes (optional) */
               u_32      encoding; /* data encoding enumeration */
               u_32      sample_rate;   /* samples per second */
               u_32      channels; /* number of interleaved channels */
          } Audio_filehdr;

     The magic field always contains the following constant:

          AUDIO_FILE_MAGIC         ( 0x2e736e64 ) /* ".snd" */

     The hdr_size field contains the length of the  fixed  header
     plus the variable-length annotation field.  Consequently, it
     may be interpreted as an offset from the start of  the  file
     to the beginning of the audio data.

     The data_size field contains the length,  in  bytes  of  the
     audio  data  segment.   If this length is not known when the
     header is written, it should be set  to  AUDIO_UNKNOWN_SIZE,
     defined as follows:

          AUDIO_UNKNOWN_SIZE  ( ~0 )         /* (unsigned) -1 */

     When the data_size field  contains  AUDIO_UNKNOWN_SIZE,  the
     length  of  the  audio data may be determined by subtracting
     hdr_size from the total length of the file.

     The encoding field contains one of the following  enumerated
     keys:

          AUDIO_ENCODING_MULAW_8        (1)  /* 8-bit ISDN  u-law
          */  AUDIO_ENCODING_LINEAR_8       (2)  /*  8-bit linear
          PCM  */  AUDIO_ENCODING_LINEAR_16      (3)  /*   16-bit
          linear   PCM  */  AUDIO_ENCODING_LINEAR_32      (5)  /*
          32-bit            linear             PCM             */
          AUDIO_ENCODING_FLOAT          (6)  /*    32-bit    IEEE
          floating point */ AUDIO_ENCODING_DOUBLE         (7)  /*
          64-bit        IEEE        floating       point       */
          AUDIO_ENCODING_ADPCM_G721     (23) /* CCITT g.721 4-bit
          ADPCM  */  AUDIO_ENCODING_ADPCM_G723_3   (25) /*  CCITT
          g.723            3-bit             ADPCM             */
          AUDIO_ENCODING_ALAW_8         (27) /*  8-bit ISDN A-law
          */

     All of the linear formats are signed integers,  centered  at
     zero.   Another  common linear format, 8-bit unsigned linear
     PCM, is not currently supported.  The floating-point formats
     are  signed, zero-centered, and normalized to the unit value
     ( -1.0 <= x <= 1.0 ).

     The sample_rate field contains the audio sampling  rate,  in
     samples  per  second.   Common  sample  rates  include 8000,
     11025, 16000, 22050, 32000, 44100,  and  48000  samples  per
     second.

     The channels field contains the number of  interleaved  data
     channels.  For monaural data, this value is set to one.  For
     stereo data, this value is set to two.  More than  two  data
     channels  may  be  interleaved,  but such formats may not be
     supported by all audio applications.

     Following the header structure is a variable-length  annota-
     tion  field.  The contents of this field are currently unde-
     fined, except that its length must be a non-zero multiple of
     eight bytes and it must be terminated with at least one null
     (zero) byte.

     The audio data segment  begins  on  an  eight-byte  boundary
     immediately  following  the  annotation field. Audio data is
     encoded in the format identified by  the  file  header.  The
     current  implementation  supports  only  a single audio data
     segment per file.

NOTES
     Applications are encouraged NOT to decode the fields of  the
     audio  file  header  directly.  Instead, applications should
     use        the         routines         described         in
     /usr/demo/SOUND/man/man3/audio_filehdr.3 to read, write, and
     decode audio file headers.

     The variable-length annotation field is currently ignored by
     most  audio  applications.   It  is  present for interchange
     compatibility.  In the future,  applications  may  use  this
     field  to  store structured data annotation, such as markers
     and edit points.

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