Convert 3GPP to J2B — Free Online Tool
Convert 3GPP mobile video files to J2B audio format, extracting the audio track and encoding it as MP3 using the LAME encoder — the codec native to the Jazz Jackrabbit 2 game audio format. The video stream is discarded entirely, producing a standalone J2B file compatible with Jazz Jackrabbit 2 audio tooling.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your 3GPP 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
3GPP files are mobile-optimized multimedia containers that typically carry H.264 video and AAC audio. During this conversion, the video stream is completely dropped and the AAC audio track is decoded and re-encoded as MP3 using the libmp3lame encoder at 128k bitrate, then written into a J2B container. J2B is a niche game audio format rooted in the ASYLUM Music Format — it wraps MP3-compatible audio data in a simple header used by Jazz Jackrabbit 2. Because the AAC audio must be transcoded to MP3 (two different lossy codecs), there is a generation loss in audio quality, though at 128k the result is generally acceptable for game audio use.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg program, the open-source multimedia processing engine that powers this conversion. In the browser this runs as FFmpeg.wasm compiled to WebAssembly, with no server involvement. |
-i input.3gp
|
Specifies the input file: a 3GPP container (.3gp), the mobile multimedia format standardized for 3G devices, which typically contains H.264 video and AAC audio streams. |
-c:a libmp3lame
|
Selects the LAME MP3 encoder for the audio stream, transcoding the 3GPP file's AAC audio into MP3 — the audio codec that the J2B format wraps inside its ASYLUM-derived container header. |
-b:a 128k
|
Sets the MP3 audio bitrate to 128 kilobits per second, the default quality level for J2B output. This is a reasonable balance between file size and audio fidelity for game audio use, though the effective quality is also constrained by whatever bitrate the original 3GPP AAC track used. |
output.j2b
|
Defines the output filename with the .j2b extension, signalling to FFmpeg to write the result in the J2B format — an audio container associated with Jazz Jackrabbit 2, containing the freshly encoded MP3 audio from the 3GPP source. |
Common Use Cases
- Extracting audio from a 3GPP video recording of a live musical performance to create a J2B asset for a Jazz Jackrabbit 2 mod or fan project
- Converting mobile-captured sound effects stored in 3GPP format into J2B files for use in custom Jazz Jackrabbit 2 level design
- Pulling the audio track from a 3GPP clip recorded on an older mobile phone to supply background music for a retro game project targeting the J2B format
- Preparing audio content from 3G mobile recordings for archival or analysis using tools that specifically read J2B files
- Stripping the audio from a 3GPP promotional or tutorial video to repurpose as a looping in-game music track in J2B format for a JJ2 community level pack
Frequently Asked Questions
Yes, there will be a small but measurable quality loss. The 3GPP file's audio is typically encoded in AAC, and converting it to MP3 (the codec inside J2B) requires a full decode-and-re-encode cycle between two lossy formats. This is known as generation loss. At the default 128k bitrate the result is generally fine for game audio use, but if your source 3GPP file had a very low audio bitrate to begin with, the output may sound degraded.
The video stream is completely discarded. J2B is a purely audio format with no support for video data, so FFmpeg drops the H.264 or MJPEG video track entirely and only processes the audio. The resulting J2B file contains only the transcoded MP3 audio from your original 3GPP recording.
In principle yes, provided the J2B file conforms to the format's header expectations and the audio is valid MP3. However, Jazz Jackrabbit 2 was originally designed around tracker-style module music in ASYLUM format, not arbitrary MP3 recordings. Compatibility depends on the version of the game and any mods or patches being used — community tools like J2AS or custom players may be more permissive than the original engine.
Replace the value after '-b:a' with your preferred bitrate. For higher quality use '192k', '256k', or '320k'; for smaller files use '64k'. For example: 'ffmpeg -i input.3gp -c:a libmp3lame -b:a 192k output.j2b'. Keep in mind that increasing the bitrate cannot recover detail lost in the original 3GPP AAC encoding, so there is a practical ceiling on quality improvement.
FFmpeg is smart enough to omit the video stream automatically when the output format (J2B) has no video codec defined. Since J2B is an audio-only format, FFmpeg infers that no video mapping is possible and skips it without needing an explicit '-vn' flag. You can add '-vn' manually if you want to be explicit, and the output will be identical.
Yes, on the command line you can loop over files using a shell script. 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"'. This browser-based tool processes one file at a time, so the command-line approach is recommended for batch jobs.
Technical Notes
J2B is an extremely niche format tied specifically to Jazz Jackrabbit 2, wrapping audio in a header derived from the ASYLUM Music Format originally used for tracker/module music. When producing a J2B from a real-world audio source like a 3GPP recording, the libmp3lame encoder handles the MP3 encoding, which is the audio layer J2B relies on. The 3GPP container's AAC audio must be fully decoded before re-encoding as MP3, meaning two lossy compression steps occur in sequence. Metadata from the 3GPP file — such as title tags, artist information, or creation timestamps — is not preserved in the J2B output, as the format has no standardized metadata fields for such data. There is no transparency, subtitle, chapter, or multiple audio track support in either format. File size will vary depending on the source audio duration and the chosen bitrate; at 128k a one-minute audio track produces roughly 960KB. Because J2B is not a widely recognised format outside the Jazz Jackrabbit 2 community, standard media players will not open the output file — dedicated tools or the game itself are needed to use it.