Convert J2B to AMR — Free Online Tool
Convert J2B game audio files from Jazz Jackrabbit 2 into AMR format using the libopencore_amrnb codec — entirely in your browser, with no file uploads required. This tool decodes the ASYLUM Music Format wrapped in J2B's header and re-encodes it as Adaptive Multi-Rate audio optimized for speech and mobile use.
to
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
J2B files are a proprietary audio container used exclusively by Jazz Jackrabbit 2, wrapping the ASYLUM Music Format (AMF) in a custom header. FFmpeg first strips the J2B header and decodes the underlying module music data using libmp3lame as an intermediate decoder, then the raw PCM audio is re-encoded into AMR Narrow Band format using the libopencore_amrnb codec. Because AMR-NB was designed for speech telephony, it operates at a fixed 8 kHz sample rate — meaning the full stereo, wide-frequency game music in the J2B source will be downmixed to mono and low-pass filtered to fit AMR's constraints. The result is a small, mobile-compatible .amr file at 12,200 bps, though significant audio quality is lost relative to the original game soundtrack.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg multimedia processing tool, which handles decoding the J2B file's ASYLUM Module Format content and re-encoding it to AMR. |
-i input.j2b
|
Specifies the input J2B file — the Jazz Jackrabbit 2 audio container that wraps ASYLUM Module Format data in a proprietary header FFmpeg knows how to parse. |
-c:a libopencore_amrnb
|
Sets the audio encoder to libopencore_amrnb, the open-source implementation of the Adaptive Multi-Rate Narrow Band codec, which is required to produce a valid .amr output file compatible with mobile telephony systems. |
-b:a 12200
|
Sets the AMR-NB encoding mode to 12,200 bits per second (the MR122 mode), which is the highest quality available in the AMR-NB standard and gives the best possible reproduction of the J2B source audio within AMR's strict telephony-focused constraints. |
output.amr
|
Defines the output filename with the .amr extension, which signals FFmpeg to write the file using the AMR container format including the standard file header recognized by mobile devices and media players. |
Common Use Cases
- Extracting Jazz Jackrabbit 2 background music to use as ringtones or notification sounds on older mobile phones that natively support AMR playback
- Archiving J2B game audio in a widely documented open format for long-term preservation when ASYLUM Module playback tools are unavailable
- Repurposing short looping J2B game music segments as voice-memo placeholders or test audio for AMR codec development and testing
- Preparing J2B game audio for integration into mobile applications or embedded systems that require AMR-NB encoded audio for minimal storage footprint
- Converting J2B soundtrack files for playback on feature phones or legacy VoIP systems that only support AMR audio streams
- Quickly inspecting the tonal and melodic content of J2B module music in a universally playable format without needing a game engine or tracker software
Frequently Asked Questions
Not particularly — and this is a fundamental codec mismatch to be aware of. AMR-NB was engineered specifically to encode human speech at 8 kHz mono, so the rich stereo, multi-instrument module music of Jazz Jackrabbit 2 will be heavily downsampled and downmixed during this conversion. High frequencies, stereo separation, and instrument layering that define the J2B soundtrack will be largely lost. If audio quality matters, consider converting to a format like MP3 or OGG instead.
AMR-NB uses a fixed set of codec modes defined by the GSM and 3GPP telephony standards, each corresponding to a specific bitrate in bits per second rather than kilobits. These modes (ranging from 4,750 bps to 12,200 bps) were designed to balance speech intelligibility against bandwidth on mobile networks. The 12,200 bps mode used in this tool is the highest quality AMR-NB mode, known as AMR 12.2 or MR122, but even this is far below what a 128k MP3 would deliver for music content.
No. The ASYLUM Music Format inside J2B stores sequencing data, instrument samples, and loop points as tracker-style module instructions rather than standard audio metadata. When FFmpeg decodes and re-encodes to AMR, only the rendered PCM audio stream is passed through — all tracker metadata, loop markers, and instrument definitions are discarded. The AMR file will contain a flat, linear audio recording with no looping capability.
Replace the value after -b:a in the command with one of the valid AMR-NB mode bitrates: 4750, 5150, 5900, 6700, 7400, 7950, or 10200 (instead of 12200). For example, use '-b:a 7950' for a smaller file at slightly lower speech quality. Note that AMR does not accept arbitrary bitrate values — FFmpeg will snap to the nearest valid mode, so sticking to the official mode values listed above is recommended to avoid unexpected behavior.
The single-file command shown on this page can be adapted for batch processing in a shell script. On Linux or macOS, you can run: for f in *.j2b; do ffmpeg -i "$f" -c:a libopencore_amrnb -b:a 12200 "${f%.j2b}.amr"; done. On Windows Command Prompt, use a for loop: for %f in (*.j2b) do ffmpeg -i "%f" -c:a libopencore_amrnb -b:a 12200 "%~nf.amr". This is especially useful if you have the full Jazz Jackrabbit 2 soundtrack as separate J2B files.
The libopencore_amrnb codec is not included in all FFmpeg distributions due to patent licensing considerations. Official FFmpeg builds available from ffmpeg.org often exclude it, but builds from third-party sources like gyan.dev (Windows) or Homebrew's ffmpeg with --with-opencore-amr (macOS) typically include it. If you see an 'encoder libopencore_amrnb not found' error, you'll need to use a build compiled with AMR support enabled. The browser-based tool on this page uses FFmpeg.wasm which includes the codec, so no installation issues apply there.
Technical Notes
J2B is a niche format exclusive to Jazz Jackrabbit 2, and very few modern tools support direct playback outside of the game engine itself. FFmpeg handles J2B by decoding the ASYLUM Module Format data into PCM, treating the file similarly to other tracker formats. The critical technical constraint in this conversion is AMR-NB's fixed 8,000 Hz sample rate and mandatory mono channel requirement — FFmpeg will automatically resample and downmix the J2B audio regardless of its original sample rate or channel count. This makes the conversion inherently lossy in ways beyond the bitrate reduction: stereo width is collapsed and all frequency content above 4 kHz (the Nyquist limit at 8 kHz) is cut off. The libopencore_amrnb encoder operates in compliance with 3GPP TS 26.071 and produces .amr files with the standard '#!AMR
' file magic header, making them compatible with most mobile devices and AMR-aware media players. No ID3 tags or other metadata fields are preserved in the output, as the AMR container format has no standardized metadata block for music-oriented fields like title or artist.
' file magic header, making them compatible with most mobile devices and AMR-aware media players. No ID3 tags or other metadata fields are preserved in the output, as the AMR container format has no standardized metadata block for music-oriented fields like title or artist.