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)