Convert WebM to CAVS — Free Online Tool
Convert WebM files (VP9 video + Opus audio) to CAVS format using H.264 video encoding and AAC audio — a niche conversion targeting Chinese broadcast and media standards. Both the VP9-to-H.264 video stream and the Opus-to-AAC audio stream are fully re-encoded in your browser with no server upload required.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your WebM file here
or click to browse
Free — no uploads, no signups. Your files never leave your browser.
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
WebM uses VP9 video and Opus audio codecs inside a Matroska-derived container. CAVS (Chinese Audio Video Standard) uses a different container structure and, in this implementation, H.264 (libx264) for video and AAC for audio. Because none of the source codecs are natively compatible with the CAVS container, full re-encoding of both streams is required — the VP9 video is decoded and re-encoded to H.264 using a CRF of 23 (visually near-lossless for most content), and the Opus audio is decoded and re-encoded to AAC at 128k bitrate. This means the conversion is computationally intensive compared to a simple remux, and some generation loss is inherent since you are compressing already-compressed source material into a different lossy codec. CAVS does not support transparency, subtitles, chapters, or multiple audio tracks, so any of those features present in the source WebM will be dropped silently during conversion.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg tool. In this browser-based tool, FFmpeg runs as a WebAssembly (wasm) module entirely within your browser — no data leaves your device. When running locally for files over 1GB, this calls your desktop FFmpeg installation directly. |
-i input.webm
|
Specifies the input file in WebM format, which contains a VP9 video stream and typically an Opus (or Vorbis) audio stream inside a Matroska-derived container. FFmpeg will fully decode both streams for re-encoding. |
-c:v libx264
|
Re-encodes the VP9 video stream to H.264 using the libx264 encoder, which is required for the CAVS output container. VP9 is not compatible with CAVS, so a full video transcode — not a copy — is mandatory here. |
-c:a aac
|
Re-encodes the Opus (or Vorbis) audio stream from the WebM to AAC, which is the audio codec required for this CAVS output. AAC is broadly compatible with broadcast and hardware playback environments that consume CAVS files. |
-crf 23
|
Sets the Constant Rate Factor for the H.264 encoder at 23, which is the libx264 default and represents a good balance between visual quality and file size. Lower values (e.g., 18) produce higher quality at larger sizes; higher values (e.g., 28) reduce file size with more visible compression loss. |
-b:a 128k
|
Sets the AAC audio output bitrate to 128 kilobits per second, which is the standard default for stereo AAC and provides acceptable quality for speech and general-purpose audio. For music-heavy WebM sources, increasing this to 192k or 256k is recommended. |
output.cavs
|
Specifies the output filename with the .cavs extension, which signals FFmpeg to mux the H.264 video and AAC audio into a CAVS-compatible container. The .cavs extension is the standard file extension for Chinese Audio Video Standard files. |
Common Use Cases
- Preparing web-sourced VP9/WebM video content for submission to Chinese broadcast platforms or state media archives that require CAVS-compliant files
- Converting HTML5 streaming video downloaded as WebM for playback on Chinese set-top boxes or digital TV hardware that only supports CAVS format
- Transcoding WebM tutorial or training videos for distribution through Chinese enterprise software ecosystems that mandate national standard media formats
- Archiving web video content in a broadcast-compatible format required by certain Chinese regulatory or institutional submission workflows
- Testing CAVS output compatibility by converting small WebM clips before committing to a large-scale batch conversion pipeline on a desktop machine
Frequently Asked Questions
Yes, some quality loss is unavoidable. Your WebM source is already VP9-compressed, and re-encoding it to H.264 introduces a second generation of lossy compression. The default CRF of 23 for H.264 produces good visual quality for most content, but fine detail in high-motion or high-complexity VP9 sources may show slight degradation. If your source WebM was encoded at a high quality setting, the perceptual difference at CRF 23 will typically be minor.
No. CAVS does not support alpha channel transparency, embedded subtitles, chapters, or multiple audio tracks. If your WebM file contains any of these features — such as a VP9 video with an alpha layer used for web overlays, or embedded WebVTT subtitles — they will be stripped during conversion. You should extract subtitles separately before converting if you need to preserve them.
Unlike container-swap conversions (such as MKV to MP4 with matching codecs), this conversion requires full decode-and-re-encode of both the video and audio streams. Decoding VP9 and re-encoding to H.264 via libx264 is computationally heavy, especially for high-resolution or long-duration WebM files. Running this in the browser via FFmpeg.wasm (WebAssembly) adds additional overhead compared to a native desktop FFmpeg installation, so for files over 1GB, using the displayed FFmpeg command locally is strongly recommended.
Change the -crf value in the command to control H.264 output quality. Lower CRF values produce higher quality and larger files — CRF 18 is considered visually lossless for most content, while CRF 28 and above will show noticeable compression artifacts. The default CRF 23 is a balanced starting point. For example: ffmpeg -i input.webm -c:v libx264 -c:a aac -crf 18 -b:a 128k output.cavs gives noticeably higher fidelity at the cost of a larger file.
Yes, on a desktop you can use a shell loop to batch process files. On Linux or macOS: for f in *.webm; do ffmpeg -i "$f" -c:v libx264 -c:a aac -crf 23 -b:a 128k "${f%.webm}.cavs"; done. On Windows Command Prompt: for %f in (*.webm) do ffmpeg -i "%f" -c:v libx264 -c:a aac -crf 23 -b:a 128k "%~nf.cavs". The browser-based tool processes one file at a time.
VP9 is a highly efficient codec that often achieves better compression than H.264 at equivalent visual quality, meaning CAVS output at default settings may be larger than the source WebM. Conversely, if the source WebM was encoded at a very high bitrate or low CRF, the H.264 output at CRF 23 might be smaller. File size depends heavily on the complexity of the video content, the original WebM encoding settings, and the CRF value you choose for the H.264 output.
Technical Notes
The CAVS container in this conversion uses libx264 for video and AAC for audio, which are technically H.264/AAC streams wrapped in a CAVS-compatible container rather than the native AVS video codec (avs2/avs3) sometimes associated with the Chinese standard. This means the output is broadly compatible with tools and hardware that support H.264 inside CAVS packaging. The conversion drops all WebM-specific features: VP9 HDR metadata (BT.2020 color space, PQ/HLG transfer functions) is not mapped to the H.264 output by default, so HDR WebM sources will not produce HDR CAVS output without additional -vf and color metadata flags. Opus audio's variable bitrate efficiency is lost when transcoding to AAC CBR at 128k; for music or high-fidelity audio content, increasing the AAC bitrate to 192k or 256k is advisable. Multiple audio tracks and embedded subtitles present in the WebM are silently discarded since CAVS supports neither feature.