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)