Convert FLV to J2B — Free Online Tool
Convert FLV video files to J2B audio format, extracting the audio stream from Flash Video containers and encoding it as MP3 — the only codec supported by the Jazz Jackrabbit 2 ASYLUM Music Format. This tool is ideal for repurposing FLV audio content into a format compatible with the J2B game audio ecosystem.
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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
FLV (Flash Video) is a container that typically holds video encoded with H.264 (libx264) or the legacy FLV codec, paired with AAC or MP3 audio. J2B is a purely audio format used by Jazz Jackrabbit 2, based on the ASYLUM Music Format with a proprietary header — it carries no video whatsoever. During this conversion, FFmpeg discards the video stream entirely and re-encodes the audio track from its source codec (commonly AAC inside FLV) to MP3 using the LAME encoder at 128k bitrate. Because AAC and MP3 are both lossy codecs, this is a lossy-to-lossy transcode, meaning some generation loss is introduced. The output .j2b file wraps the resulting MP3 data in a J2B-compatible structure, making it readable by Jazz Jackrabbit 2 and compatible tools.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg tool, which handles all media parsing, stream selection, decoding, encoding, and muxing for this FLV-to-J2B conversion. |
-i input.flv
|
Specifies the input Flash Video file. FFmpeg reads the FLV container, identifying its video stream (typically H.264) and audio stream (typically AAC or MP3) for processing. |
-c:a libmp3lame
|
Instructs FFmpeg to encode the audio stream using the LAME MP3 encoder — the only audio codec supported by the J2B format. If the source FLV audio is AAC, this triggers a full decode-and-reencode to MP3. |
-b:a 128k
|
Sets the MP3 audio bitrate to 128 kilobits per second, which is the default quality level for J2B output. This is a reasonable balance between file size and audio fidelity for game audio use in Jazz Jackrabbit 2. |
output.j2b
|
Defines the output filename with the .j2b extension. FFmpeg uses this extension to mux the encoded MP3 audio into a J2B-compatible file structure, wrapping it in the ASYLUM Music Format header expected by Jazz Jackrabbit 2. The video stream from the FLV is automatically discarded since J2B is audio-only. |
Common Use Cases
- Replacing or modding in-game music in Jazz Jackrabbit 2 using audio sourced from FLV recordings of gameplay videos or cutscenes
- Extracting a commentary or speech audio track from an FLV screen recording to create a J2B-format audio asset for a JJ2 level mod
- Converting FLV-format music downloads (common from older Flash-era sites) into J2B files for use in custom Jazz Jackrabbit 2 levels
- Archiving or porting audio from Flash-era video content into J2B for retro game preservation projects involving Jazz Jackrabbit 2
- Creating custom ambient or background music tracks for JJ2 level editors by sourcing audio from FLV video files
- Batch-extracting audio from a collection of FLV files to build a library of J2B assets for a Jazz Jackrabbit 2 fan-made episode
Frequently Asked Questions
Yes — this is a lossy-to-lossy transcode. FLV files commonly store audio as AAC, which is then re-encoded to MP3 using the LAME encoder. Each lossy encode introduces some quality degradation, so the output MP3 will not be identical to the original audio. At the default 128k bitrate, the result is generally acceptable for game audio use, but if you have access to a lossless source (such as WAV or FLAC), converting from that instead would yield a better-quality J2B file.
No. J2B is a purely audio format based on the ASYLUM Music Format used by Jazz Jackrabbit 2 — it has no concept of video streams, subtitles, or multiple audio tracks. During conversion, FFmpeg automatically discards the video stream and any secondary audio tracks, keeping only the primary audio track for encoding to MP3.
FLV files contain both a video stream and an audio stream, whereas J2B contains only audio encoded as MP3. Removing the video stream — which accounts for the vast majority of data in most video files — dramatically reduces the file size. The final J2B file size depends almost entirely on the audio duration and the chosen bitrate (default 128k).
Replace the value after '-b:a' with your desired bitrate. For example, use '-b:a 192k' for higher quality or '-b:a 64k' for a smaller file. The J2B format supports MP3 bitrates ranging from 64k up to 320k. For game audio in Jazz Jackrabbit 2, 128k is typically sufficient and keeps file sizes small, but 192k or 256k may be preferable if you want noticeably better fidelity.
Yes, on the command line you can use a shell loop to process multiple files. On Linux or macOS: 'for f in *.flv; do ffmpeg -i "$f" -c:a libmp3lame -b:a 128k "${f%.flv}.j2b"; done'. On Windows Command Prompt: 'for %f in (*.flv) do ffmpeg -i "%f" -c:a libmp3lame -b:a 128k "%~nf.j2b"'. This browser-based tool processes one file at a time, but the displayed FFmpeg command is exactly what you'd use locally for batch scripting.
No. J2B is a minimal game audio format based on ASYLUM Music Format and does not support standard metadata tags such as title, artist, or album. Any metadata embedded in the FLV container (such as stream info or XMP tags) will be lost during conversion. If metadata preservation matters, consider keeping the original FLV or converting to a format like MP3 or OGG that supports ID3 or Vorbis comment tags.
Technical Notes
J2B is an obscure, game-specific audio format tied to Jazz Jackrabbit 2, wrapping MP3 audio (ASYLUM Music Format) in a simple proprietary header. Because J2B's only supported codec is MP3 via libmp3lame, there is no option to copy the audio stream directly even if the source FLV contains MP3 audio — FFmpeg must re-encode to ensure the output conforms to J2B's structure. FLV containers most commonly carry AAC audio when paired with H.264 video (the libx264 default), making a full AAC-to-MP3 transcode the typical path. The video stream in the FLV is fully discarded since J2B has no video support. One known limitation is that the .j2b extension is not natively recognized by most general-purpose media players, so playback testing should be done with Jazz Jackrabbit 2 or a compatible ASYLUM module player. File sizes are predictable: a 1-minute audio track at 128k bitrate produces approximately 960KB of output. Since J2B does not support chapters, multiple audio tracks, or subtitles, any such tracks in the source FLV are silently dropped.