Converting .undf files to .mkv format.
Occasionally, the VLC Media Player will generate an error message stating that the .undf multimedia format is not supported. This prompts some users to attempt converting their media files to other format, despite them being absolutely fine. Such error is displayed if the VLC Media Player doesn't have a suitable decoder installed for the video format.
Videos in the .mkv format can occasionally trigger such error messages. No need for a conversion in that case. All you need to do is to look for a newer VLC Media Player version, or its codecs. This issue can occasionally be caused by partially downloaded files; in such case, you should either complete the download process or download the file again.
The .undf file type is not recognized by any file conversion tool.
In a word, direct undf to mkv conversion is not possible.
So don't expect any so-called undf to mkv converter or undf to mkv free online conversion tool available.
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