---
name: "gsm"
section: "3"
platform: "linux"
package: "gsm"
version: "1.0.24-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/gsm"
---

# gsm(3)

GSM 06.10 lossy sound compression

## 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@pobox.com and cabo@tzi.org.

## SEE ALSO

[toast(1)](/toast/1.md), [gsm\_print(3)](/gsm_print/3.md), [gsm\_explode(3)](/gsm_explode/3.md), [gsm\_option(3)](/gsm_option/3.md)