Free online ISMV to OPUS converter

What the ISMV format is

ISMV is a fragmented MP4 variant used by Microsoft IIS Smooth Streaming. Files commonly contain H.264 video and AAC audio divided into media fragments for adaptive streaming. Microsoft Expression Encoder, IIS Media Services workflows, and compatible streaming encoders create ISMV files; you may encounter them in archived web-streaming content or old media delivery systems.

What the OPUS format is

OPUS is a lossy audio codec normally stored in an Ogg container with the .opus extension. It supports speech and music, handles low bitrates efficiently, and is widely supported by browsers, VoIP software, media players, and current mobile platforms. Choose it when the destination accepts Opus or when smaller audio files are preferable to AAC or uncompressed audio.

How to convert ISMV to OPUS

Using FFmpeg

FFmpeg is the most direct desktop method because it can demux fragmented MP4 media and encode the selected audio stream:

ffmpeg -i input.ismv -map 0:a:0 -vn -c:a libopus -b:a 128k output.opus

Replace input.ismv with the source filename. The -map 0:a:0 option selects the first audio stream, -vn excludes video, and -b:a 128k sets the Opus bitrate. Use a different output name if required.

To inspect the streams before converting, run:

ffprobe input.ismv

If the file has several audio tracks, select another track with -map 0:a:1, -map 0:a:2, and so on. For speech, -b:a 48k to 64k is often sufficient; for general music, 96k to 160k is a typical range.

Using 101convert.com

On 101convert.com, upload the ISMV file, select OPUS as the output format, start the conversion, and download the resulting .opus file. This option requires no local software installation.

Quality and compatibility limits

ISMV files commonly contain AAC audio, so converting to Opus performs a second lossy encode. It cannot restore quality already lost in the source; for archival preservation, keep the original ISMV or extract the original audio without re-encoding when the destination supports AAC.

An ISMV containing only video, encrypted media, or an unsupported audio codec cannot produce a normal OPUS file. Some older players identify .opus files incorrectly even though the audio is valid; use a current Opus-capable player or verify the output with ffprobe output.opus.