Convert 3G2 to J2B — Free Online Tool
Convert 3G2 mobile video files to J2B audio format, extracting the AAC or MP3 audio track and re-encoding it as MP3 using the LAME encoder — the codec required by the Jazz Jackrabbit 2 ASYLUM Music Format container. This tool runs entirely in your browser via FFmpeg.wasm, so no files leave your device.
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FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your 3G2 file here
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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
3G2 is a CDMA-era mobile container that wraps video (typically H.264) and audio (typically AAC or MP3) streams together. J2B is an audio-only format used by the Jazz Jackrabbit 2 game engine, built on the ASYLUM Music Format with a simple wrapper header. During this conversion, FFmpeg discards the video stream entirely and re-encodes the audio track using the LAME MP3 encoder (libmp3lame) at 128k bitrate. If the 3G2 file's audio is already MP3, it still must be re-encoded because FFmpeg needs to construct the J2B container structure. The result is a compact, mono-or-stereo MP3 stream wrapped in a J2B-compatible file.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg tool, which handles all demuxing, decoding, encoding, and muxing steps for this conversion from 3G2 to J2B. |
-i input.3g2
|
Specifies the input file — a 3G2 container typically carrying an H.264 video stream and an AAC or MP3 audio stream recorded for CDMA mobile playback. |
-c:a libmp3lame
|
Selects the LAME MP3 encoder for the audio stream, which is required because J2B files store their audio as MP3 data in the ASYLUM Music Format used by Jazz Jackrabbit 2. |
-b:a 128k
|
Sets the MP3 audio bitrate to 128 kilobits per second, providing a reasonable balance between file size and audio quality for the J2B output, especially given that 3G2 source audio is often already low-bitrate. |
output.j2b
|
Specifies the output filename with the .j2b extension, producing the Jazz Jackrabbit 2 audio file containing the re-encoded MP3 audio extracted from the 3G2 source. |
Common Use Cases
- Extracting a music or sound clip recorded or stored on an old CDMA phone (in 3G2 format) for use as a custom audio asset in Jazz Jackrabbit 2 modding projects
- Converting ringtones or short audio recordings from 3G2 format to J2B so they can be imported into JJ2 level editors that expect J2B music files
- Archiving audio from legacy 3G2 video files into a format compatible with retro game audio toolchains that use the ASYLUM Music Format
- Testing how a specific audio track from a 3G2 source sounds when transcoded through the LAME MP3 pipeline that J2B depends on, before incorporating it into a game mod
- Batch-preparing audio samples captured on CDMA network devices for inclusion in a Jazz Jackrabbit 2 custom episode soundtrack
Frequently Asked Questions
No. J2B is a strictly audio-only format with no provision for video streams — it is designed solely for the ASYLUM Music Format used by Jazz Jackrabbit 2. FFmpeg automatically drops the video track during this conversion, and only the audio content from the 3G2 file is re-encoded and written to the J2B output.
Yes, there is a generation of lossy transcoding involved. The original AAC audio in the 3G2 file is decoded to raw PCM and then re-encoded as MP3 at 128k using the LAME encoder. Both AAC and MP3 are lossy formats, so going from one lossy codec to another always introduces some additional quality degradation. For short clips or speech, the difference is usually minor at 128k, but for music with high-frequency content, the artifacts from double-encoding may be noticeable.
Yes. In the command 'ffmpeg -i input.3g2 -c:a libmp3lame -b:a 128k output.j2b', you can replace '128k' with a higher value such as '192k', '256k', or '320k' to improve audio fidelity. Keep in mind that the source audio from 3G2 files is often recorded at low bitrates for mobile transmission, so exceeding the original source quality will not recover information that was already lost during initial encoding.
On Linux or macOS, you can use a shell loop: 'for f in *.3g2; do ffmpeg -i "$f" -c:a libmp3lame -b:a 128k "${f%.3g2}.j2b"; done'. On Windows Command Prompt, use: 'for %f in (*.3g2) do ffmpeg -i "%f" -c:a libmp3lame -b:a 128k "%~nf.j2b"'. The browser-based tool processes one file at a time, so the FFmpeg command line is especially useful when batch converting large collections.
The J2B format is a minimal wrapper around the ASYLUM Music Format and does not have a standardized metadata schema the way formats like MP3 ID3 tags or MP4 atoms do. Any metadata embedded in the 3G2 container (title, artist, creation date) will not be meaningfully carried over to the J2B output, as the format was designed for game engine playback rather than general media tagging.
J2B files are based on the ASYLUM Music Format, which stores audio as MP3 data — making libmp3lame (the LAME MP3 encoder) the only appropriate codec for writing valid J2B output. Using a different encoder like AAC or Vorbis would produce a file that game engines and tools expecting the ASYLUM format would reject or misplay. The libmp3lame encoder is also widely regarded as the highest-quality open-source MP3 encoder available.
Technical Notes
3G2 files were designed for CDMA mobile networks (3GPP2 standard) and typically contain H.264 video alongside AAC audio, both encoded at low bitrates suited for constrained mobile bandwidth — often 64k or below for audio. J2B is an obscure, game-specific audio container used exclusively by Jazz Jackrabbit 2, wrapping an MP3 stream in a small binary header that the game's ASYLUM Music Format engine expects. Because FFmpeg does not have a native J2B muxer in the same sense as mainstream formats, the output file is essentially an MP3 stream with a J2B extension, which is consistent with how J2B files are structured in practice. There is no chapter, subtitle, or secondary audio track support in either format. The '-movflags +faststart' flag relevant to 3G2 playback optimization is not applicable on the output side since J2B has no such concept. Users should be aware that 3G2 audio sources often have a limited frequency ceiling due to original mobile encoding constraints, meaning the J2B output at 128k MP3 may still sound bandwidth-limited even at that bitrate.