Convert 3GP to J2B — Free Online Tool
Extract and convert the audio track from a 3GP mobile video file into J2B format, the ASYLUM Music Format-based audio used by Jazz Jackrabbit 2. The conversion decodes the 3GP's AAC or MP3 audio stream and re-encodes it using the LAME MP3 encoder at 128k bitrate, producing a J2B-wrapped MP3 file compatible with JJ2 game audio tooling.
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FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
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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
3GP files are multimedia containers designed for 3G mobile networks, typically carrying H.264 video alongside AAC audio. J2B is a game-specific audio format used exclusively by Jazz Jackrabbit 2, consisting of an ASYLUM Music Format payload wrapped in a proprietary header — and it only supports MP3 audio via the LAME encoder. During this conversion, FFmpeg discards the 3GP video stream entirely and extracts the audio, decoding whatever codec was used (AAC is most common in 3GP files) and re-encoding the audio as MP3 at 128k bitrate. The resulting bitstream is then written into the J2B container. Because both the source audio codec (AAC) and destination codec (MP3) are lossy, this is a lossy-to-lossy transcode, meaning some audio generation loss is unavoidable.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg binary — the open-source multimedia processing engine that powers this conversion both in the browser via WebAssembly and on your local desktop command line. |
-i input.3gp
|
Specifies the input file, a 3GP container typically containing H.264 video and AAC audio as used on 3G mobile devices. FFmpeg will demux both streams but only the audio will be used in this conversion. |
-c:a libmp3lame
|
Sets the audio codec to LAME MP3 encoder, which is the only audio codec supported by the J2B output format. This forces a full decode-and-reencode of the 3GP's source audio (commonly AAC) into MP3. |
-b:a 128k
|
Sets the MP3 audio bitrate to 128 kilobits per second, which is the default for J2B output. This is a reasonable quality target for game audio, though you can raise it to 192k or 256k if your source 3GP audio has sufficient fidelity to benefit. |
output.j2b
|
The output filename with the .j2b extension, which tells FFmpeg to mux the encoded MP3 audio into a J2B container — the ASYLUM Music Format wrapper used by Jazz Jackrabbit 2 for in-game music playback. |
Common Use Cases
- Importing a custom music track recorded or received on an old 3G mobile phone into Jazz Jackrabbit 2 as a level soundtrack replacement
- Converting field recordings made on a legacy 3GP-capable device into J2B format for use with JJ2 mod communities that accept custom music contributions
- Extracting and reformatting the audio from a 3GP mobile game cutscene or ringtone clip to repurpose as a J2B background track in a fan-made JJ2 level pack
- Batch-preparing a library of 3GP audio clips collected from early 2000s mobile content into J2B format for archival or retro game modding projects
- Testing how a voice memo or ambient sound recorded in 3GP survives the AAC-to-MP3 transcode before committing it to a JJ2 level distribution
Frequently Asked Questions
Yes, some quality loss is expected because this is a lossy-to-lossy transcode. The 3GP file most likely contains AAC audio, which is decoded to raw PCM and then re-encoded as MP3 at 128k by the LAME encoder. Each generation of lossy encoding introduces new compression artifacts. The audible impact depends on the original AAC bitrate — if the source was already at 64k or lower (common in bandwidth-constrained 3GP files), the resulting J2B MP3 may sound noticeably degraded. For game audio purposes, 128k MP3 is generally acceptable quality.
No. The J2B format is tied specifically to the ASYLUM Music Format used by Jazz Jackrabbit 2, and FFmpeg's J2B muxer only supports the libmp3lame encoder. This means regardless of what audio codec your 3GP file uses — AAC, AMR, or MP3 — it will always be re-encoded as MP3 in the output. There is no option to pass through audio without re-encoding when targeting J2B.
The video track is dropped entirely. J2B is a pure audio format with no video container support, so FFmpeg automatically strips the H.264 or MJPEG video stream from the 3GP file and only processes the audio. No explicit flag is needed in the command to discard video — FFmpeg infers this because the output format has no video codec defined.
Modify the value after the -b:a flag in the command. The default is 128k, but J2B via FFmpeg supports 64k, 128k, 192k, 256k, and 320k. For example, use -b:a 192k for higher fidelity or -b:a 64k for a smaller file. Since 3GP source audio is often recorded at low bitrates for mobile delivery, pushing beyond 192k may not yield audible improvement because the detail lost during 3GP's original encoding cannot be recovered.
The displayed command processes one file at a time, but you can batch-process on your desktop using a shell loop. On Linux or macOS: for f in *.3gp; do ffmpeg -i "$f" -c:a libmp3lame -b:a 128k "${f%.3gp}.j2b"; done. On Windows Command Prompt: for %f in (*.3gp) do ffmpeg -i "%f" -c:a libmp3lame -b:a 128k "%~nf.j2b". The browser-based tool processes files individually.
J2B is essentially only useful in the context of Jazz Jackrabbit 2 modding. The practical scenario is a JJ2 level creator who recorded a custom melody or ambient sound on an old mobile device — which saved it as 3GP — and needs to package it as a J2B file to embed it in a custom JJ2 level. Outside of JJ2 modding, J2B has no mainstream software support, so this is a niche but legitimate workflow for retro game content creators.
Technical Notes
The 3GP container format was developed for 3G mobile networks and typically encodes audio at low bitrates (32k–128k AAC or even AMR) to minimize bandwidth and storage. When transcoding to J2B, FFmpeg must fully decode the source audio before re-encoding it with libmp3lame, since there is no codec passthrough possible between AAC and MP3. The J2B format itself is not a general-purpose container — it wraps the ASYLUM Music Format used internally by the Jazz Jackrabbit 2 game engine, and playback outside of JJ2-aware tools or emulators is not guaranteed. Metadata from the 3GP file (such as title, artist, or mobile device timestamps) is not preserved in the J2B output, as J2B carries no standard metadata fields. There are no subtitle, chapter, or multi-track considerations for either format. One important limitation: if your 3GP file uses AMR audio (common in very old 3G phones for voice calls), FFmpeg will still decode and transcode it correctly, but the quality ceiling is very low given AMR's speech-optimized, narrow-band encoding.