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NAME

       gsm_create, gsm_destroy, gsm_encode, gsm_decode — GSM 06.10 lossy sound
       compression

SYNOPSIS

       #include "gsm.h"

       gsm gsm_create();

       void gsm_encode(handle, src, dst)
       gsm handle;
       gsm_signal src[160];
       gsm_frame dst;

       int gsm_decode(handle, src, dst)
       gsm handle;
       gsm_frame src;
       gsm_signal dst[160];

       void gsm_destroy(handle)
       gsm handle;

DESCRIPTION

       Gsm is an implementation of the final  draft  GSM  06.10  standard  for
       full-rate speech transcoding.

       gsm_create()  initializes  a  gsm pass and returns a ’gsm’ object which
       can  be  used  as  a  handle  in  subsequent  calls  to   gsm_decode(),
       gsm_encode() or gsm_destroy().

       gsm_encode()   encodes  an  array  of  160  13-bit  samples  (given  as
       gsm_signal’s, signed integral values  of  at  least  16  bits)  into  a
       gsm_frame  of 33 bytes.  (gsm_frame is a type defined as an array of 33
       gsm_bytes in gsm.h.)

       gsm_decode() decodes a gsm_frame into an array of  160  13-bit  samples
       (given  as  gsm_signals),  which  sound  rather like what you handed to
       gsm_encode() on the other side of the wire.

       gsm_destroy() finishes a gsm pass and frees all storage associated with
       it.

   Sample format
       The following scaling is assumed for input to the algorithm:
          0  1                             11 12
          S..v..v..v..v..v..v..v..v..v..v..v..v..*..*..*
       Only the top 13 bits are used as a signed input value.
       The output of gsm_decode() has the three lower bits set to zero.

RETURN VALUE

       gsm_create()  returns  an  opaque  handle  object  of type gsm, or 0 on
       error.  gsm_decode() returns -1 if the passed frame is invalid, else 0.

EXAMPLE

       #include "gsm.h"

       gsm handle;
       gsm_frame buf;
       gsm_signal sample[160];
       int cc, soundfd;

       play() {  /* read compressed data from standard input, write to soundfd */

            if (!(handle = gsm_create())) error...
            while (cc = read(0, (char *)buf, sizeof buf)) {
                 if (cc != sizeof buf) error...
                 if (gsm_decode(handle, buf, sample) < 0) error...
                 if (write(soundfd, sample, sizeof sample) != sizeof sample)
                      error...
            }
            gsm_destroy(handle);
       }

       record() {     /* read from soundfd, write compressed to standard output */

            if (!(handle = gsm_create())) error...
            while (cc = read(soundfd, sample, sizeof sample)) {
                 if (cc != sizeof sample) error...
                 gsm_encode(handle, sample, buf);
                 if (write(1, (char *)buf, sizeof buf) != sizeof sample)
                      error...
            }
            gsm_destroy(handle);
       }

BUGS

       Please  direct  bug  reports  to  jutta@cs.tu-berlin.de and cabo@cs.tu-
       berlin.de.

SEE ALSO

       toast(1), gsm_print(3), gsm_explode(3), gsm_option(3)

                                                                        GSM(3)