Extract Audio from CAVS to J2B — Free Online Tool
Extract audio from CAVS (Chinese Audio Video Standard) video files and save it as J2B, the Jazz Jackrabbit 2 game audio format built on the ASYLUM Music Format. The conversion uses the LAME MP3 encoder to transcode the AAC audio stream from the CAVS container into an MP3-based J2B file, discarding the H.264 video entirely.
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FFmpeg Command
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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
CAVS files carry an AAC audio stream alongside H.264 video inside a container designed for Chinese broadcast and distribution. During this conversion, FFmpeg strips the video stream entirely using the -vn flag, then decodes the AAC audio and re-encodes it using the libmp3lame encoder at 128k bitrate. The resulting MP3-encoded audio is wrapped in the J2B container structure — a simple header originally developed for the Jazz Jackrabbit 2 game engine that wraps ASYLUM-format module audio. Because AAC and MP3 are both lossy codecs, this is a lossy-to-lossy transcode, meaning there will be a small generation of quality loss compared to the original AAC source. The output J2B file contains no video, no subtitles, and no chapter data — just the re-encoded audio stream.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg program, the open-source multimedia processing engine that handles reading the CAVS container, decoding AAC audio, re-encoding to MP3, and writing the J2B output. |
-i input.cavs
|
Specifies the input file in CAVS format — the Chinese Audio Video Standard container carrying an H.264 video stream and an AAC audio stream that FFmpeg will parse. |
-vn
|
Disables video output entirely, telling FFmpeg to ignore the H.264 video stream from the CAVS file so that only the audio track is processed and written to the J2B output. |
-c:a libmp3lame
|
Selects the LAME MP3 encoder to re-encode the decoded AAC audio from the CAVS file into MP3, which is the codec used inside the J2B container for Jazz Jackrabbit 2 audio compatibility. |
-b:a 128k
|
Sets the MP3 audio bitrate to 128 kilobits per second — a balance between file size and audio quality suitable for most game audio applications. Increase to 192k or 320k if the source CAVS audio quality warrants it. |
output.j2b
|
Defines the output filename and tells FFmpeg to use the J2B container format — the Jazz Jackrabbit 2 audio wrapper — determined by the .j2b file extension. |
Common Use Cases
- Extracting background music or dialogue from Chinese broadcast CAVS recordings to retarget audio assets for use in a Jazz Jackrabbit 2 custom level or mod.
- Converting CAVS-format video audio tracks into J2B so that retro game modders can test MP3-based audio in JJ2 without needing to source separate audio files.
- Pulling the audio commentary or narration from a CAVS broadcast clip and packaging it as a J2B file for playback in JJ2-compatible engines that support the format.
- Archiving the audio portion of CAVS media separately from the video for lightweight storage, where J2B is the required delivery format for a game audio pipeline.
- Testing the compatibility of Chinese-standard broadcast audio (originally AAC in CAVS) when re-encoded to MP3 inside a J2B wrapper for legacy game engine integration.
- Providing a local FFmpeg command for batch-processing large collections of CAVS files over 1GB into J2B audio assets on a desktop workstation.
Frequently Asked Questions
Yes — this is a lossy-to-lossy transcode. The original AAC audio in the CAVS file is already compressed, and re-encoding it to MP3 via libmp3lame introduces a second generation of compression artifacts. At the default 128k bitrate the loss is subtle for most listeners, but audiophiles or audio engineers working with critical material may notice it. If quality is a priority, increase the bitrate in the FFmpeg command to 192k or 320k.
J2B was created specifically for the Jazz Jackrabbit 2 game to wrap ASYLUM Music Format module data with a simple identifying header. Outside of JJ2 and its modding community, virtually no mainstream media players or operating systems natively support J2B. If you need the audio for general playback, a standard MP3 or AAC file would be a far more compatible choice — J2B output is only practical if you are specifically targeting the JJ2 game engine or a compatible tool.
The H.264 video stream is completely discarded by the -vn flag and does not appear in the output at all. FFmpeg reads the CAVS container, identifies the audio track, and ignores all video data entirely. The resulting J2B file is audio-only with no embedded video, thumbnails, or visual metadata.
Adjust the -b:a value in the command to control the MP3 output bitrate. Replace 128k with 64k for a smaller file at lower quality, or with 192k, 256k, or 320k for progressively better fidelity. For example: ffmpeg -i input.cavs -vn -c:a libmp3lame -b:a 192k output.j2b. Note that increasing the bitrate beyond the quality ceiling of the original AAC source will not recover detail that was already lost in the CAVS file.
The displayed command processes a single file, but you can batch it on Linux or macOS using a shell loop: for f in *.cavs; do ffmpeg -i "$f" -vn -c:a libmp3lame -b:a 128k "${f%.cavs}.j2b"; done. On Windows, a similar for loop works in Command Prompt or PowerShell. This is especially useful for large collections over 1GB that exceed the browser tool's per-file limit.
CAVS files do not have robust metadata support for audio tracks, and J2B's ASYLUM-based format similarly has no standard metadata container for tags like title, artist, or album. Any incidental stream metadata from the CAVS source is unlikely to carry through to the J2B output, so do not rely on this conversion to preserve descriptive tags.
Technical Notes
CAVS (Chinese Audio Video Standard) encodes video with H.264 (libx264) and audio with AAC, making it a lossy format designed for Chinese broadcast distribution. The audio stream is standard AAC and is fully decodable by FFmpeg without special licensing concerns. J2B is an unusual output target — it is structurally a thin header wrapping ASYLUM Music Format data, but FFmpeg implements it as an MP3-carrying container using libmp3lame, which is how the format is practically used in the Jazz Jackrabbit 2 modding ecosystem. Because both AAC (source) and MP3 (destination) are lossy codecs, users should be aware that transcoding between them compounds compression artifacts. The J2B format does not support multiple audio tracks, chapter markers, or subtitles, so any of those features present in the CAVS source are silently dropped. File sizes for J2B output will be significantly smaller than the original CAVS file since all video data is removed and the audio is re-compressed at 128k MP3 by default. Users needing higher fidelity should increase the -b:a bitrate, though 320k is the practical ceiling for MP3.