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GSM(3)                DragonFly Library Functions Manual                GSM(3)


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)

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