#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;
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.
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.
#include "gsm.h"
gsm handle;
gsm_frame buf;
gsm_signal sample[160];
play() {
if (!(handle = gsm_create())) exit(1);
while (read(1, (char *)buf, sizeof(buf)) == sizeof(buf)) {
if (gsm_decode(handle, buf, sample) < 0) exit(2);
write(soundfd, sample, sizeof(sample));
}
gsm_destroy(handle);
}
record() {
if (!(handle = gsm_create())) exit(1);
while (read(1, sample, sizeof(sample)) == sizeof(sample)) {
gsm_encode(handle, sample, buf);
write(s, (char *)buf, sizeof(buf));
}
gsm_destroy(handle);
}