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);
}