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NAME
     g721_encode g721_decode g721_init_state - CCITT G.721  audio
     compression and decompression.

SYNOPSIS
     #include <multimedia/libaudio.h>

     int g721_encode(in_buf, in_size, header, out_buf, out_size, state_ptr)
     void *in_buf;
     int in_size;
     Audio_hdr *header;
     char *out_buf;
     int *out_size;
     struct audio_g72x_state *state_ptr;

     int g721_decode(in_buf, in_size, header, out_buf, out_size, state_ptr)
     char *in_buf;
     int size;
     Audio_hdr *header;
     void *out_buf;
     int *out_size;
     struct audio_g72x_state *state_ptr;

     void g721_init_state(state_ptr)
     struct audio_g72x_state *state_ptr;

DESCRIPTION
     These routines implement the CCITT G.721 standard for encod-
     ing  8-bit  u-law  or A-law coded or 16-bit linear PCM audio
     data as 4-bit Adaptive Differential  Pulse  Coded  Modulated
     (ADPCM)  data.   They  provide 32 Kbps coding for 8KHz audio
     data with little degradation in sound quality.   The  encode
     and  decode  routines  use  a  audio_g72x_state structure to
     store state information between calls.  An  audio_g72x_state
     structure  must be initialized by calling g721_init_state().
     This routine should also be used whenever there is a discon-
     tinuity  in the compressed data stream (for instance, if one
     input/output file  is  closed  and  another  is  opened  for
     decoding/encoding).

     The g721_encode() function encodes size bytes of input  data
     from the buffer pointed by in_buf.  The encoding type of the
     input data is given in the header structure.  The compressed
     ADPCM  data  is returned in out_buf, whose length (in bytes)
     is  returned  in  out_size.    state_ptr   points   to   the
     audio_g72x_state  structure  for  preserving encoding state.
     The first 4-bit code is returned in  the  least  significant
     nibble  of  the  first  byte in the output buffer, while the
     succeeding code is packed into the high  order  nibble.   In
     the  event  that  the  total number of nibbles in out_buf is
     odd, the last unpairable nibble is saved in the state struc-
     ture  until  the next invocation of g721_encode() - at which
     time it is packed with the code from the first value in  the
     new  input  buffer.   The output buffer specified by out_buf
     must then contain at least (in_size + 1) / 2 bytes for u-law
     or  A-law  input  data  and (in_size / 2) + 1) / 2 bytes for
     linear PCM input data to accomodate the encoded output.

     After the entire input signal is  processed,  the  remaining
     state  structure  should be flushed by calling g721_encode()
     one more  time  with  the  in_size  argument  set  to  zero.
     out_buf should refer to a valid output buffer to which up to
     one byte of data may be written.  If data is  returned,  the
     final  nibble of encoded data was paired with zero, which is
     harmless when decoded.

     The g721_decode() function expands  4-bit  ADPCM  data  into
     16-bit  linear  PCM  or 8-bit u-law or A-law format.  in_buf
     specifies the address of a buffer containing  in_size  bytes
     of  ADPCM data.  The data type of the decoded data is speci-
     fied in the header structure.  Decoded data is stored in the
     buffer  whose  address  is  given by out_buf.  The number of
     decoded samples (not bytes!) is returned in  out_size.   The
     output  buffer  must contain enough room to store (2 * size)
     samples.  state_ptr points to the audio_g72x_state structure
     for preserving decoding state.

RETURN VALUE
     g721_encode() and g721_decode() return the audio error codes
     or AUDIO_SUCCESS.

SEE ALSO
     audio_g711(3), audio_g723(3).

NOTES
     These routines comprise an implementation of the CCITT G.721
     ADPCM  encoding  algorithm.   This implementation has passed
     the test vectors furnished by CCITT (Dec. 20, 1988)  for  u-
     law and A-law data.

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