Free online KUX to ASF converter

What the KUX format is

KUX is a proprietary video container used by Youku’s desktop software for downloaded or offline-playback video. It commonly contains video and audio streams in a format derived from FLV, and some files include Youku-specific protection or metadata. You typically encounter .kux files in Youku download or cache folders rather than as files exported by standard video-editing applications.

Support depends on the decoder. A current FFmpeg build can read many KUX files through its KUX demuxer, but protected files may fail if they require Youku’s client or account authorization.

What the ASF format is

ASF, or Advanced Systems Format, is a Microsoft media container designed for streaming and storing synchronized audio and video. It is commonly associated with WMV video and WMA audio, uses the .asf extension, and is supported by Windows Media software and some older network or hardware players.

ASF is appropriate when the receiving application specifically requests ASF or expects WMV/WMA streams. It is not usually the best preservation container for modern H.264/AAC video, because compatibility with ASF depends on the player’s codec support.

How to actually convert KUX to ASF

The most practical desktop method is FFmpeg. Use a recent build, then open a terminal in the folder containing the KUX file.

1. Check whether FFmpeg can read the file and identify its streams:

ffprobe "input.kux"

2. Try stream copying first. This avoids quality loss when the source codecs are accepted by the ASF muxer:

ffmpeg -i "input.kux" -map 0:v:0 -map 0:a? -c copy "output.asf"

If FFmpeg reports an unsupported codec, invalid packet format, or ASF muxing error, transcode to the codecs most broadly associated with ASF:

ffmpeg -i "input.kux" -map 0:v:0 -map 0:a? -c:v wmv2 -b:v 1500k -c:a wmav2 -b:a 128k "output.asf"

Adjust -b:v to match the source resolution and desired size. For example, 800–1200 kbit/s is often sufficient for lower-resolution video, while 2000–4000 kbit/s may be needed for 720p material. The transcode command decodes the KUX streams and re-encodes them; it therefore takes longer and can reduce quality compared with stream copying.

If you do not want to install software, the 101convert.com website can convert the file online for free. Uploading only works when the KUX file is accepted by the service and is not restricted by Youku protection or account encryption.

Quality and compatibility limitations

A successful container conversion does not remove encryption or access restrictions. If ffprobe cannot inspect the KUX file or FFmpeg reports encrypted data, obtain an unprotected video through Youku’s supported download or export function before converting.

ASF compatibility is strongest with WMV2 video and WMAV2 audio. H.264, AAC, variable-frame-rate video, subtitles, chapters, and embedded metadata may be discarded or behave inconsistently in older ASF players. Check the resulting file with ffprobe "output.asf" and test it in the application or device that requires ASF.