Convert CAVS to M4B — Free Online Tool
Convert CAVS video files to M4B audiobook format by extracting the AAC audio stream and packaging it into an MPEG-4 container with chapter and bookmarking support. Ideal for repurposing Chinese broadcast video content into portable audiobook or podcast files.
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FFmpeg Command
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Estimated output:
Conversion Complete!
DownloadHow It Works
CAVS (Chinese Audio Video Standard) files typically carry AAC audio alongside their video streams. During this conversion, the video stream is discarded entirely — M4B is a pure audio format — and the AAC audio is re-encoded at the target bitrate (128k by default) and wrapped in an MPEG-4 container with the .m4b extension. The -movflags +faststart flag reorganizes the file's metadata to the beginning of the output, which enables progressive playback in podcast apps and audiobook players before the full file has loaded. Because both the source and destination use AAC as their audio codec, quality loss is minimal, but since the audio is re-encoded rather than stream-copied, there is a small generational quality cost.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg program, the open-source multimedia processing engine that performs all format reading, audio decoding, re-encoding, and container writing for this CAVS-to-M4B conversion. |
-i input.cavs
|
Specifies the input file in CAVS (Chinese Audio Video Standard) format. FFmpeg reads both the AVS video stream and the AAC audio stream from this container, though only the audio will be used in the output. |
-c:a aac
|
Sets the audio codec for the output to AAC using FFmpeg's native AAC encoder. Since M4B requires AAC audio and the CAVS source already carries AAC, this re-encodes the audio into the new container at the specified bitrate. |
-b:a 128k
|
Sets the audio bitrate to 128 kilobits per second. This is a reasonable default for CAVS broadcast audio repurposed as spoken-word M4B content; increase to 192k or 256k if the source contains music or high-fidelity audio. |
-movflags +faststart
|
Moves the MP4/M4B 'moov' metadata atom to the beginning of the output file. This is required for proper M4B playback in audiobook and podcast apps, allowing them to start playing and display accurate progress bars before the entire file is read. |
output.m4b
|
Specifies the output filename with the .m4b extension, which signals to both FFmpeg and media players that this MPEG-4 audio file is an audiobook container supporting chapters and bookmarking rather than a generic audio file. |
Common Use Cases
- Extracting lecture or educational audio from Chinese broadcast CAVS recordings to create portable M4B audiobooks for offline listening on iPhone or iPod
- Converting CAVS-encoded language learning videos into M4B podcast episodes that support bookmarking so learners can resume exactly where they left off
- Repurposing CAVS broadcast content into M4B files for distribution on podcast platforms like Apple Podcasts, which natively supports the M4B container
- Archiving the audio commentary track from a CAVS-format recording into an M4B file that supports chapter markers for easy navigation across long content
- Stripping the audio from CAVS video files captured from Chinese digital television for use in audiobook reader apps that support chapter-level bookmarking
Frequently Asked Questions
M4B is a pure audio container format designed specifically for audiobooks and podcasts — it does not support video streams. This conversion intentionally discards the CAVS video track and retains only the audio. If you need to preserve the video, you should convert to a container like MP4 or MKV instead.
There is a small quality loss because the audio is re-encoded from AAC (as found in CAVS) to AAC in the M4B container, rather than being stream-copied. This is a generational encode — two lossy passes on the same codec. At the default 128k bitrate the difference is generally imperceptible for speech-heavy content like lectures or broadcasts, but for music-heavy audio you may want to increase the bitrate to 192k or 256k.
CAVS does not support embedded chapter metadata, so there are no source chapters to carry over. The output M4B file will be a single continuous audio track. If you want chapter markers in the M4B file, you would need to add them manually using a tool like mp4chaps or Hindenburg after the conversion is complete.
Replace the value after -b:a with your preferred bitrate. For example, use '-b:a 192k' for higher quality or '-b:a 96k' for a smaller file size. For spoken-word content from CAVS broadcasts, 96k or 128k is generally sufficient, while 192k or above is better suited for music or high-fidelity audio content.
Yes. On Linux or macOS, you can run a shell loop such as: for f in *.cavs; do ffmpeg -i "$f" -c:a aac -b:a 128k -movflags +faststart "${f%.cavs}.m4b"; done. On Windows Command Prompt, use: for %f in (*.cavs) do ffmpeg -i "%f" -c:a aac -b:a 128k -movflags +faststart "%~nf.m4b". The browser-based tool processes one file at a time, so the FFmpeg command is particularly useful for bulk conversions of large libraries.
Yes. M4B with AAC audio and the +faststart flag is the native audiobook format for Apple's ecosystem, including Apple Books, the iPhone Music app (legacy), and third-party apps like Bound and BookPlayer. The file will support bookmarking and resume playback automatically. Note that Apple Books may require the file to have proper ID3/MP4 metadata tags (title, author) to be correctly catalogued in your library.
Technical Notes
CAVS files encode video using the AVS1 (or AVS2) video codec and typically pair it with AAC audio, which is why this conversion can target M4B's AAC audio without a codec mismatch. However, because FFmpeg re-encodes the audio rather than performing a direct stream copy (the CAVS container encapsulates the audio in a way that requires unwrapping and re-muxing), there is a minor quality penalty. The -movflags +faststart flag is essential for M4B compatibility: it moves the 'moov' atom to the front of the file so that podcast and audiobook apps can begin playback immediately and display accurate seek bars. M4B does not support subtitles, multiple audio tracks, or video — all of which CAVS theoretically supports on the source side — so any such tracks in the source file are silently dropped. Metadata from the CAVS container is generally not preserved in the output M4B file; you should tag the output file with an MP4 metadata editor if the audiobook is intended for library or podcast distribution.