Convert AVI to J2B — Free Online Tool
Convert AVI video files to J2B audio format, extracting and encoding the audio stream as MP3 using the LAME encoder — the only codec supported by the Jazz Jackrabbit 2 game's ASYLUM-based music format. This tool runs entirely in your browser via FFmpeg.wasm, so no files ever leave your device.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your AVI file here
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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
AVI is a Microsoft container that interleaves audio and video streams, often encoding audio as MP3 (libmp3lame), AAC, or Vorbis. J2B is a game-specific audio format used by Jazz Jackrabbit 2, built around the ASYLUM Music Format with a simple binary header, and it exclusively supports MP3-encoded audio. During this conversion, FFmpeg discards the AVI video stream entirely and re-encodes (or transcodes) the audio stream using the LAME MP3 encoder at 128k bitrate, then wraps the output in the J2B container structure. Because J2B supports only a single audio track and no video, subtitles, or chapters, any of those extra streams in the source AVI are silently dropped.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg binary — in this browser-based tool, this runs via FFmpeg.wasm compiled to WebAssembly, executing entirely client-side without any server upload. |
-i input.avi
|
Specifies the input AVI file. FFmpeg reads the RIFF-based AVI container and identifies all interleaved streams — typically at least one video stream and one audio stream — for processing. |
-c:a libmp3lame
|
Sets the audio encoder to LAME, which produces MP3-encoded audio. This is mandatory because J2B exclusively supports MP3 audio; no other codec is valid for this output format. |
-b:a 128k
|
Sets the MP3 audio bitrate to 128 kilobits per second, the default quality level for J2B output. This is a constant bitrate (CBR) setting balancing file size and audio fidelity suitable for in-game music use. |
output.j2b
|
Defines the output filename and triggers FFmpeg to write a J2B-format file. The .j2b extension causes FFmpeg to use the appropriate muxer, discarding the AVI's video stream since J2B is audio-only. |
Common Use Cases
- Modding Jazz Jackrabbit 2 by replacing or adding in-game background music with audio sourced from AVI video recordings or cutscenes
- Extracting a recorded gameplay or demo AVI's audio commentary track and packaging it as a J2B file for use in custom JJ2 levels
- Converting AVI-format video soundtracks from 1990s CD-ROM games into J2B so they can be played back within the Jazz Jackrabbit 2 engine or compatible mod tools
- Producing test audio assets in J2B format from AVI source material while prototyping custom Jazz Jackrabbit 2 level packs
- Archiving or converting retro game music stored in AVI containers into the J2B format for preservation in JJ2 mod repositories
- Quickly generating a playable J2B file from an AVI recording of a chiptune or tracker music performance for use in fan-made JJ2 episodes
Frequently Asked Questions
No — J2B is a pure audio format and has no ability to store video data. FFmpeg will strip the video stream from the AVI entirely and only process the audio track. If your goal is to keep the video, J2B is not an appropriate target format; it is designed exclusively for in-game music playback in Jazz Jackrabbit 2.
No. Even if your AVI contains AAC or Vorbis audio, FFmpeg will re-encode it to MP3 using the LAME encoder before writing the J2B output. There is no stream-copy shortcut here because J2B strictly requires MP3 — the transcoding step is always performed regardless of the source audio codec.
Replace the '128k' value in the '-b:a 128k' flag with your desired bitrate. J2B supports 64k, 128k, 192k, 256k, and 320k. Higher bitrates like 192k or 256k will produce better audio fidelity but larger file sizes, while 64k saves space at the cost of noticeable quality loss — particularly relevant since JJ2 levels have strict size budgets for music files.
J2B supports only a single audio track, so FFmpeg will default to the first audio stream found in the AVI and discard all others. If you need a specific secondary track — for example, a different language dub or a stereo mix from a multi-track AVI — you should add '-map 0:a:1' (or the appropriate stream index) to the command before the output filename to select it explicitly.
This tool outputs a file with the .j2b extension encoded with libmp3lame, which matches the audio codec requirement of the format. However, authentic J2B files used by JJ2 are originally based on ASYLUM module tracker files — not MP3 streams — and some JJ2 engine builds or mod tools may not recognize an MP3-wrapped J2B as a valid music asset. Compatibility depends on your specific JJ2 version or level editor. Using this output as a music replacement in custom levels may require additional tooling.
The single-file command shown here processes one file at a time. To batch convert on a desktop, you can wrap it in a shell loop — for example, in bash: 'for f in *.avi; do ffmpeg -i "$f" -c:a libmp3lame -b:a 128k "${f%.avi}.j2b"; done'. On Windows Command Prompt, use a 'for' loop with 'ffmpeg -i "%f" -c:a libmp3lame -b:a 128k' syntax. The in-browser tool processes files individually and is best suited for single conversions.
Technical Notes
J2B is an obscure, game-specific container format tied to Jazz Jackrabbit 2, and its only supported codec in any FFmpeg-compatible rendering is MP3 via libmp3lame — making codec choice non-negotiable for this conversion. The source AVI may use any of its supported audio codecs (AAC, MP3, or Vorbis), but all will be transcoded to MP3, which introduces a generational quality loss if the source was already lossy. AVI metadata such as title tags or artist fields embedded via RIFF INFO chunks will not be carried over to the J2B output, as the format's header structure does not include equivalent metadata fields. AVI's support for multiple audio tracks is irrelevant here since J2B is single-track only. Because AVI does not support subtitles or chapters natively and J2B does not either, no stream-mapping conflicts arise from those features. One practical concern: AVI files containing uncompressed PCM audio or high-bitrate audio will produce a notably larger J2B file at 128k than expected if you assumed the audio was already compressed — in those cases, consider raising or lowering the bitrate deliberately based on your file size target.