CAVS to ASF Converter

CAVS to ASF: The Default Codec Is H.264, Which Is the Opposite of Why You Want ASF

ASF is Microsoft's Advanced Systems Format — the container behind Windows Media, and the wrapper that .wmv and .wma files are built on. Almost nobody chooses ASF for its own sake; people choose it because a Windows-era player, a capture card's software, or an old media-server ingest path insists on it.

Which makes the default on this page worth flagging up front. Video Codec opens on H.264, not on a Windows Media codec. The ASF muxer will write H.264 without complaint and any modern player reads the result back — a test encode with these settings produced a perfectly valid single-stream ASF file — but H.264 is not one of the codecs ASF was built around, and a Windows Media-era player expecting WMV will not decode it. If Windows compatibility is the reason you are here, change Video Codec to WMV 2 before converting. If you just need the picture out of an orphaned AVS stream and any container will do, CAVS to MP4 is a better destination than ASF.

How to Convert CAVS to ASF

1. Upload Your CAVS File: Drag the .cavs onto the page or click "+ Add Files". A .cavs is a bare AVS1 video bitstream with no container around it, which is why ordinary players refuse to open one; several can be queued and all convert with the same settings.
2. Choose the Video Codec Deliberately: The list holds twenty entries and opens on H.264. Pick WMV 2 for a file a Windows Media-era player will actually decode, WMV 1 for very old software, or leave H.264 alone only if you know the consumer is a modern player that happens to want the .asf extension.
3. Set File Compression and Video resolution: File Compression opens on Quality Preset at "Very High (Recommended)", alongside Specific file size, Constant Bitrate, Variable Bitrate, Constant Quality and Constraint Quality. Constant Quality is the most useful of these, but its scale changes with your codec choice — see below. Video resolution opens on Keep original; Trim takes a Start time and a Duration.
4. Convert and Download: Click Convert and save the .asf. Files are uploaded over an encrypted connection, decoded and re-encoded on our servers, and deleted automatically after a few hours — no sign-up, no watermark, never shared or made public.

Which Video Codec: The Decision That Defines the File

Twenty entries render, but the real decision is a two-way one: a Windows Media codec, or a modern one. Everything else on the list is a codec you would be better off putting in a container that suits it.

The Quality Slider Changes Scale When You Change Codec

Switching codecs does not just swap the encoder — it re-ranges the Constant Quality control, because the two families use different quality scales and the pipeline follows the codec rather than the container.

The 1-31 scale on WMV is a real, working control, and its range is wide. On a 704x576 six-second test clip, WMV 2 encodes came out at 4,394,359 bytes at quantiser 1, 2,602,359 bytes at 5 (the value the page opens on), 1,139,959 bytes at 15 and 477,559 bytes at 31. Setting it to 5 and leaving it alone is a sensible high-quality default for standard-definition material; push it toward 15 if you need the file substantially smaller and the source is not detailed enough to punish you for it.

One thing not to do is carry a number across between the two scales. A value of 23 means "good quality" on the H.264 CRF scale and "quite poor" on a 1-31 quantiser, so if you change codec, re-set the slider rather than assuming the number still means what it meant.

What a .cavs File Actually Is

Frequently Asked Questions

Why does the converted ASF have no sound?

Because a .cavs file has none to begin with. It is a video elementary stream: coded picture data with no container, no audio track and no timing index. In Chinese AVS broadcast and optical-media workflows the picture was carried as one stream and the sound as another, and the two were multiplexed together only when the final container — a transport stream, an MP4, an MKV — was built. The bare .cavs you have is the picture half. The Audio Codec dropdown still renders and still nominates WMA v2, but there is nothing for it to encode, so the ASF comes out with a single video stream. If you need the soundtrack, convert the original container rather than the demuxed stream.

Will Windows Media Player open the ASF this page produces?

Only if you change the codec first. Left on its default the file contains H.264, which the ASF muxer accepts and modern players read, but which is outside the set of codecs Windows Media was built around — a Windows Media-era player will open the container and then fail to decode the video. Select WMV 2 under Video Codec and you get Windows Media Video 8 inside an ASF, which is the pairing that container exists for. WMV 1 is there for software older still.

Is .cavs the same thing as a .avs AviSynth script?

No, and the two are easy to confuse. A .cavs file is a raw AVS1 video bitstream — actual coded picture data from the Chinese Audio Video Standard. A .avs file is an AviSynth script: a short text file of frameserving instructions that references video stored elsewhere, with no picture data inside it at all. This converter expects the bitstream. If you upload an AviSynth script by mistake there is nothing in it to encode.

Is AVS1 the same as AVS2 or AVS3?

No — they are three generations of the same Chinese standards family, and a .cavs file is the first one. AVS1 was adopted as national standard GB/T 20090.2 in 2006 by the AVS Workgroup, which was established in June 2002. AVS2 followed as a Chinese national standard at the end of 2016 and was later published as IEEE 1857.4. AVS3 is the current generation and was added to DVB's media-delivery toolbox in 2022. This page's decoder handles the AVS1 elementary stream that the .cavs extension denotes; the later generations use different bitstream syntax and normally arrive inside a real container rather than as a loose stream.

Is this a rewrap, or is the video re-encoded?

Re-encoded, always. ASF does not carry AVS1 under any circumstances, so there is no version of this conversion that moves your existing compressed frames into a new container untouched — the decoder reconstructs the pictures and an encoder compresses them again. That is one lossy generation, and no setting recovers what it costs. Keep Constant Quality low (which means high quality on both scales) if the AVS stream is your only copy of the material.

Why does the ASF report an odd duration or frame rate?

Because a bare .cavs has no container-level timing information — no duration field, no frame-rate declaration, no seek index. The decoder infers timing from the bitstream itself, and the ASF muxer writes whatever it is given. If the resulting file reports something unexpected, that is the missing metadata showing through rather than a fault in the encode. It is also why trimming by time on a .cavs source can land slightly off where you expected.

Should I change Video resolution, or leave it on Keep original?

Leave it on Keep original unless you have a specific target. AVS1 material is typically standard-definition broadcast content, and scaling it up only enlarges what the original encoder already discarded — the re-encode then has to spend bits describing interpolated pixels. Scaling down is legitimate if you need a smaller file, and the Preset Resolutions list sets a height with the width following the source aspect ratio. Note that any resize also flattens the pixel aspect ratio to square, so if your source is anamorphic standard-definition, set an explicit Width x Height that already accounts for the display shape.

What else can I put a raw AVS stream into?

Any of the site's video targets, and the container matters more than you might think for a stream nobody's player will open loose. CAVS to MP4 is the safest general answer — H.264 in MP4 plays on phones, browsers, televisions and editors. CAVS to AVI is the choice for legacy Windows editing tools that predate MP4 support. CAVS to WMV gets you Windows Media Video under the extension Windows users expect, which for most people is a more useful destination than .asf even though the underlying container is the same.

How large a file can I upload, and how are my files handled?

There is no fixed per-file cap; upload size and connection speed are the practical limit, and raw elementary streams are usually modest by video standards because they carry no audio and no container overhead. Everything you send travels over an encrypted connection, is decoded and re-encoded on our servers, and both the upload and the output are deleted automatically after a few hours. Files are never shared or made public, there is no account to create and no watermark is added.