GSM file format (.gsm)

What is a GSM file?

In short: A GSM file (.gsm) is an audio file in the GSM format. GSM is the codec that carried the first generation of digital mobile calls, at 13 kbps. It can be converted to MP3, WAV or FLAC for free in a web browser with iHateFiles, without uploading it.

The original GSM 06.10 full-rate speech codec, from the GSM mobile standard of 1987.

GSM is the codec that carried the first generation of digital mobile calls, at 13 kbps. It is built entirely around a model of the human vocal tract, which is why speech survives it and everything else does not.

What GSM is good at

• Speech at 13 kbps
• Simple enough for 1980s handset hardware
• Still used in telephony systems and voicemail

Where GSM falls short

• Music is unrecognisable
• Narrow band, roughly 300 Hz to 3.4 kHz
• Obsolete outside telephony

When to use GSM

Convert GSM to MP3 or WAV to play a voicemail or telephony recording on a computer.

How to open a GSM file

Telephony software and VLC.

The thing that usually goes wrong

The quality ceiling is the codec, not the file. Converting to a higher bitrate cannot add back a frequency range that was never captured.

Inside a GSM file

GSM 06.10 is the speech codec that ran the original digital mobile phone network, and a .gsm file is a raw stream of its frames with no container at all. It compresses to 13 kbit/s — about a hundredth of CD audio — by modelling the human vocal tract rather than the sound itself.

That model is why it sounds the way it does. Speech comes through intelligibly at a very low bitrate; music comes through as a warbling mess, because a guitar is not a vocal tract. It survives in telephony systems, Asterisk voicemail in particular, where the format is still a default.