Convert AU to J2B — Free Online Tool
Convert Sun AU audio files to J2B format using the LAME MP3 encoder, transcoding raw PCM audio (typically 16-bit big-endian) from Unix's classic audio format into the ASYLUM Music Format container used by Jazz Jackrabbit 2. Runs entirely in your browser — no uploads required.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your AU file here
or click to browse
Free — no uploads, no signups. Your files never leave your browser.
Settings
Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
AU files store uncompressed PCM audio (most commonly 16-bit big-endian signed integers, as defined by Sun Microsystems) with a minimal header identifying sample rate, channel count, and encoding. During conversion, FFmpeg decodes the raw PCM stream from the AU container and re-encodes it using the libmp3lame encoder at 128kbps, producing a lossy MP3 bitstream wrapped in the J2B container header used by Jazz Jackrabbit 2. Because AU audio is lossless PCM and J2B uses lossy MP3 compression, this is a one-way destructive conversion — the original uncompressed audio is permanently compressed. The J2B wrapper itself is minimal, adding a small identifying header around the MP3 data to make it recognizable to the Jazz Jackrabbit 2 game engine.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg binary — the open-source multimedia processing engine that handles AU decoding and J2B/MP3 encoding. In the browser version, this runs as a WebAssembly (FFmpeg.wasm) instance with no server involvement. |
-i input.au
|
Specifies the input file in Sun AU format. FFmpeg reads the AU header to determine the audio codec (e.g., pcm_s16be, pcm_mulaw), sample rate, and channel count before decoding the raw audio stream. |
-c:a libmp3lame
|
Selects the libmp3lame encoder to transcode the decoded PCM audio from the AU file into an MP3 bitstream — the audio codec required by the J2B container used by Jazz Jackrabbit 2. |
-b:a 128k
|
Sets the MP3 output bitrate to 128 kilobits per second, the default quality level for J2B output. This produces a reasonable balance between file size and audio fidelity for game audio use; increase to 192k or 320k for higher quality, or decrease to 64k for smaller files. |
output.j2b
|
Specifies the output filename with the .j2b extension. FFmpeg uses this extension to write the MP3 audio stream into a J2B container — the ASYLUM Music Format wrapper recognized by the Jazz Jackrabbit 2 game engine. |
Common Use Cases
- Importing custom sound effects or music originally sourced from Unix/NeXT audio archives into Jazz Jackrabbit 2 via mods or level editors that accept J2B files.
- Repurposing classic Unix workstation audio samples — such as those from early Sun Microsystems systems — for use in retro game modding communities centered around JJ2.
- Converting AU-formatted sound assets from legacy Unix software or early internet audio downloads into a format compatible with Jazz Jackrabbit 2's audio engine.
- Batch-preparing a library of PCM AU audio clips recorded on Unix systems for use as J2B game music or sound replacement files in JJ2 custom levels.
- Testing and comparing how raw PCM audio from an AU source sounds after MP3 compression at various bitrates before finalizing a J2B asset for a Jazz Jackrabbit 2 mod.
Frequently Asked Questions
Yes. AU files with PCM encoding (such as pcm_s16be) are lossless — they store audio without any compression artifacts. J2B uses MP3 compression via the libmp3lame encoder, which is inherently lossy. At the default 128kbps bitrate, most listeners won't notice a difference for typical sound effects, but the conversion is irreversible — you cannot recover the original lossless PCM data from the resulting J2B file.
J2B is a container used by Jazz Jackrabbit 2 that wraps the ASYLUM Music Format, and its audio layer relies on MP3 encoding via libmp3lame. Since AU files contain standard PCM audio that FFmpeg fully decodes, the source audio is compatible — the AU codec (e.g., pcm_s16be, pcm_alaw, pcm_mulaw) doesn't matter as long as FFmpeg can decode it, which it can for all common AU variants. The output is a valid MP3 bitstream in a J2B wrapper.
Modify the -b:a flag in the command. For example, use -b:a 64k for a smaller file size with lower fidelity, or -b:a 320k for the highest MP3 quality supported by libmp3lame. For J2B use in Jazz Jackrabbit 2, 128k is generally sufficient for game audio, but 192k or 256k may be preferable if the source AU file contains high-quality music rather than simple sound effects.
The command as shown processes one file at a time, but you can batch convert using a shell loop. On Linux or macOS: for f in *.au; do ffmpeg -i "$f" -c:a libmp3lame -b:a 128k "${f%.au}.j2b"; done. On Windows Command Prompt: for %f in (*.au) do ffmpeg -i "%f" -c:a libmp3lame -b:a 128k "%~nf.j2b". The browser-based tool processes one file per conversion.
AU files sometimes use pcm_mulaw (µ-law) or pcm_alaw (A-law) encoding, which are compressed 8-bit telephony codecs common in early Unix and internet audio. FFmpeg automatically detects and decodes these codecs from the AU header before re-encoding to MP3, so no special handling is required. However, µ-law and A-law audio is typically 8kHz mono phone-quality audio, which will result in noticeably low-quality J2B output regardless of the MP3 bitrate chosen.
No. The AU format has an extremely minimal header — it stores only technical parameters (sample rate, encoding type, channel count) and an optional plain-text annotation field, not structured ID3-style metadata. J2B similarly does not carry rich metadata. Any annotation text in the AU header is not transferred to the J2B output by default, so the resulting file will contain no embedded title, artist, or comment information.
Technical Notes
The AU format's default codec (pcm_s16be — 16-bit signed big-endian PCM) is straightforward for FFmpeg to decode, making this conversion reliable across all standard AU files. The key technical constraint is the one-way quality reduction: PCM audio has effectively infinite dynamic range compared to a fixed-bitrate MP3 stream, so any AU file containing high-frequency content or fine transients will exhibit some compression artifacts in the J2B output. The J2B container is narrowly specific to Jazz Jackrabbit 2 and is not a general-purpose audio format — files produced here are intended solely for use within that game's ecosystem. libmp3lame is the gold-standard open-source MP3 encoder and produces high-quality output at 128kbps and above. Because AU does not support multiple audio tracks, subtitles, chapters, or video, and neither does J2B, there are no stream-selection complications in this conversion — it is strictly a single audio stream transcode. If your AU file contains audio sampled at an unusual rate (e.g., 8000Hz from a telephony source), the MP3 output will preserve that sample rate, which may sound noticeably degraded compared to standard 44100Hz or 48000Hz audio.