What audio format does Asterisk use?

Asterisk supports several native audio formats: G.711 u-law (.ulaw) for North American telephony, G.711 a-law (.alaw) for European systems, GSM (.gsm) for compressed audio, WAV 8kHz 16-bit (.wav) for uncompressed audio, signed linear (.sln) for internal processing, signed linear 16 kHz (.sln16) and G.722 (.g722) for wideband on supported endpoints. All narrowband formats use 8 kHz mono.

How do I convert MP3 to ulaw for Asterisk?

Use sox on the command line: sox input.mp3 -r 8000 -c 1 -t ul output.ulaw. This resamples to 8 kHz, converts to mono, and encodes as G.711 u-law. Or use this online converter to upload your MP3 and download the converted files without installing anything.

What is the difference between ulaw and alaw?

Both are G.711 codecs that encode audio at 64 kbps. U-law (mu-law) is the standard in North America and Japan (T1/PRI lines). A-law is the standard in Europe and most other regions (E1 lines). They sound nearly identical but are not interchangeable. Use whichever matches your trunk or endpoint codec.

Should I use the telephone bandpass filter when converting audio for Asterisk?

Yes, for voice prompts and IVR recordings. The telephone network carries frequencies between 300 Hz and 3400 Hz. Audio content outside this range cannot be transmitted and wastes codec bandwidth. The bandpass filter removes inaudible frequencies, resulting in cleaner audio on the receiving end. For music on hold, skipping the filter preserves more of the original character.

What are rotated hold-music variants and why would I use them?

By default, Asterisk plays hold-music files from the beginning every time a caller is placed on hold. If you only have one or two tracks, repeat callers hear the same intro over and over. Rotated variants are the same track with the start point shifted — the tool clips the beginning and appends it to the end. When you place multiple variants in your MOH directory, Asterisk picks one at random for each hold session, giving the illusion of variety even with a single source track. This is especially valuable for small offices with low call volume, where the same few callers would otherwise memorize your hold music.

Asterisk Audio Converter: Convert MP3/WAV to ulaw, alaw, GSM, G.722

1 Upload

Drop an audio file here or click to browse

MP3, WAV, OGG, FLAC, AIFF, M4A (max 10 MB)

Output filename (optional)

Lowercase, spaces become dashes, special characters removed. If left blank, the original filename is used.

Clean filenames for Asterisk (lowercase, spaces to dashes, remove special characters)

2 Output Formats

G.711 u-law (.ulaw)

G.711 a-law (.alaw)

GSM (.gsm)

WAV 8kHz 16-bit (.wav)

Signed Linear (.sln)

Signed Linear 16kHz (.sln16) (wideband)

G.722 wideband (.g722) (wideband)

Include original file in archive downloads (ZIP / tar.gz will contain the source + converted files)

3 Processing Options

Quick presets

Soft limiter (dynamic range compression) (prevents codec clipping on loud music)

Normalize peak level to

Trim leading and trailing silence (good for IVR prompts)

Apply telephone bandpass filter (300 Hz to 3400 Hz) (voice prompts; skip for music to preserve bass)

Generate rotated hold-music variants (prevents callers from hearing the same start point)

Uploading...

About Asterisk audio formats

Not sure which format to use or why providing multiple formats avoids real-time transcoding? The Asterisk Audio Formats Explained guide covers ulaw vs alaw, when G.722 wideband makes sense, and how to convert files on the command line with sox.

How it works

Asterisk plays audio files from the Music on Hold directory, IVR prompts, and voicemail greetings. For best performance, provide files in the codec that matches the caller's channel so Asterisk does not need to transcode in real time.

Recommended presets

Voice / IVR: normalize to -3 dB, trim silence, apply 300–3400 Hz bandpass.
Music on Hold: soft limiter, normalize to -8 dB, no bandpass (preserves bass).

Format	Extension	Use case
G.711 u-law	`.ulaw`	North American telephony (T1/PRI lines). Most common Asterisk format.
G.711 a-law	`.alaw`	European and international telephony (E1 lines).
GSM	`.gsm`	Good compression, widely compatible. Default for many Asterisk installations.
WAV (PCM)	`.wav`	Uncompressed 8 kHz 16-bit. Highest quality, largest file size.
Signed Linear	`.sln`	Raw PCM with no header. Asterisk internal format. Smallest overhead.
Signed Linear 16kHz	`.sln16`	Raw wideband PCM (16 kHz). Pairs with G.722 for wideband-capable endpoints.
G.722	`.g722`	Wideband (16 kHz). Higher quality on PJSIP endpoints that support it.