Extract Audio from CAVS to M4B — Free Online Tool
Extract audio from CAVS video files and convert it directly to M4B format — ideal for turning Chinese broadcast content into audiobook or podcast-compatible files. The AAC audio stream from the CAVS container is re-encoded into an MPEG-4 audio file with chapter and bookmarking support, optimized for playback in audiobook apps like Apple Books and podcast clients.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your CAVS 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
CAVS (Chinese Audio Video Standard) files typically carry AAC audio alongside their proprietary video stream. This tool strips the video entirely using the -vn flag and re-encodes the AAC audio into an M4B container — an MPEG-4 audio format built on the same MP4 foundation but designed specifically for audiobooks and podcasts. Because the source audio is already AAC and the target codec is also AAC, the re-encoding pass is minimal in scope, though it does transcode at the specified bitrate to ensure clean container compliance and proper M4B framing. The -movflags +faststart flag repositions the MP4 metadata atom to the beginning of the file, enabling streaming playback to begin before the entire file is downloaded — a key requirement for podcast clients and audiobook apps.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg tool — the same open-source engine running in your browser via WebAssembly (FFmpeg.wasm) that processes your CAVS file entirely locally without any server upload. |
-i input.cavs
|
Specifies the input file in CAVS format. FFmpeg reads the CAVS container, identifying the video stream (typically H.264) and the audio stream (typically AAC) for selective processing. |
-vn
|
Disables video output entirely, stripping the CAVS video stream from the result. Since M4B is a pure audio format designed for audiobooks and podcasts, no video data is carried to the output. |
-c:a aac
|
Encodes the extracted audio as AAC using FFmpeg's native AAC encoder. AAC is the required codec for M4B files and the standard audio codec used by Apple's audiobook and podcast ecosystem. |
-b:a 128k
|
Sets the AAC audio bitrate to 128 kilobits per second, the default used by this tool. For spoken-word content extracted from CAVS broadcasts, 128k provides clean intelligibility at a compact file size; increase to 192k or 256k for music-heavy content. |
-movflags +faststart
|
Moves the MP4 metadata (moov atom) to the beginning of the M4B file after encoding. This is essential for audiobook and podcast apps — it allows playback to begin immediately during streaming or network access without waiting for the entire file to load. |
output.m4b
|
Defines the output filename with the .m4b extension, signaling to media players and audiobook apps that this is a bookmarkable, chapter-capable audio file rather than a generic audio track. |
Common Use Cases
- Converting a recorded CAVS broadcast of a Chinese-language lecture or educational program into an M4B audiobook for offline listening in Apple Books with bookmarking support
- Extracting narration or commentary audio from CAVS broadcast recordings to create podcast episodes distributed through platforms that prefer M4B for chapter-enabled content
- Repurposing audio from CAVS-format Chinese TV dramas or radio theater recordings into M4B files so listeners can resume playback exactly where they left off on their iPhone or iPad
- Archiving spoken-word content from CAVS broadcast sources into M4B format, which is more widely supported by audiobook library apps like Libby or Audiobookshelf than raw CAVS files
- Preparing Chinese-language audio content sourced from CAVS recordings for distribution as structured podcast episodes that support per-chapter navigation in compatible players
- Stripping the video from a CAVS-format language-learning program to create a portable M4B audio lesson file compatible with audiobook and language-learning apps
Frequently Asked Questions
Yes, there is a generation of quality loss because the audio is transcoded — even though both the source CAVS file and the target M4B file use AAC, the audio is decoded and re-encoded at the specified bitrate (default 128k). If the original CAVS file was encoded at a higher bitrate, some fidelity is lost in the process. For spoken-word content like lectures or narration, 128k AAC is generally indistinguishable from the source; for music or complex audio, bumping to 192k or 256k is advisable.
CAVS files do not support embedded chapter data, so there are no chapters to carry over from the source. However, M4B natively supports chapters, meaning you can add chapter markers to the output file after conversion using tools like mp4chaps or Chapter and Verse. This makes M4B a meaningful upgrade in structure over the CAVS source for long-form audio content.
Replace the value after -b:a in the command. For example, change -b:a 128k to -b:a 192k for better quality, or -b:a 96k to reduce file size. For audiobooks and spoken-word content, 96k or 128k AAC is typically sufficient and keeps file sizes manageable for long recordings. For music extracted from broadcast CAVS content, 192k or 256k is a better choice.
Yes. On Linux or macOS, you can loop over files with a shell command: for f in *.cavs; do ffmpeg -i "$f" -vn -c:a aac -b:a 128k -movflags +faststart "${f%.cavs}.m4b"; done. On Windows PowerShell, use: Get-ChildItem *.cavs | ForEach-Object { ffmpeg -i $_.FullName -vn -c:a aac -b:a 128k -movflags +faststart ($_.BaseName + '.m4b') }. This is especially useful when processing large CAVS broadcast archives that exceed the browser tool's 1GB limit.
Yes. M4B with AAC audio and the +faststart flag is the native format for Apple's audiobook ecosystem and is broadly supported by iOS, macOS, and popular third-party audiobook and podcast apps including Overcast, Pocket Casts, and Audiobookshelf. The faststart optimization ensures the file begins playing before it is fully loaded, which is a requirement for streaming in many of these apps.
M4B and M4A are technically very similar — both are MPEG-4 audio containers with AAC audio — but M4B signals to media players that the file is an audiobook or podcast, enabling features like per-file bookmarking, chapter navigation, and variable playback speed in apps that support the format. If you're extracting audio from a CAVS recording specifically to listen to it as a long-form spoken-word file, M4B is the more appropriate target because it unlocks resume and chapter functionality that M4A alone does not trigger in audiobook apps.
Technical Notes
CAVS is a Chinese national broadcast standard whose container structure is not universally supported by all media tools, but FFmpeg handles it reliably. The audio in CAVS files is typically AAC, which aligns well with M4B's default and most compatible codec. Despite both formats using AAC, this conversion is a transcode rather than a stream copy because the audio must be repackaged with M4B-compliant framing and the bitrate must be explicitly set for container compliance — a direct stream copy (-c:a copy) may work in some cases and would avoid any quality loss, but compatibility with audiobook apps is less guaranteed without explicit re-encoding. The -movflags +faststart flag is non-negotiable for M4B files intended for streaming or audiobook app use, as it moves the moov atom to the file header so players can begin playback without buffering the entire file. CAVS files do not carry subtitle tracks or multiple audio tracks, so no data is lost beyond the video stream itself. Metadata such as title or artist tags from the CAVS source is generally not preserved automatically and should be added to the M4B output using an ID3/MP4 tagging tool after conversion if needed for library organization.