gsm_decode (3)
Leading comments
Copyright 1992 by Jutta Degener and Carsten Bormann, Technische Universitaet Berlin. See the accompanying file "COPYRIGHT" for details. THERE IS ABSOLUTELY NO WARRANTY FOR THIS SOFTWARE.
NAME
gsm_create, gsm_destroy, gsm_encode, gsm_decode --- GSM 06.10 lossy sound compressionSYNOPSIS
#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);
}