Free online ISMV to FLV converter

What the ISMV format is

ISMV is a fragmented MP4 media format used by Microsoft IIS Smooth Streaming. Files commonly contain H.264 video and may be accompanied by a separate .isma file containing AAC audio.

Microsoft Expression Encoder, IIS Media Services, and other Smooth Streaming packaging tools create ISMV segments. You may encounter them in archived streaming output, server publishing directories, or media packages that also include an .ism manifest and multiple bitrate tracks.

What the FLV format is

FLV is a Flash Video container designed for progressive web playback and older streaming systems. It commonly stores H.264 video with AAC audio or older Sorenson Spark and MP3 streams.

FLV remains useful when a legacy application specifically requires the container. H.264/AAC FLV files are generally more compatible than files using obsolete Flash codecs, but modern browsers and players usually prefer MP4, WebM, or Matroska.

How to convert ISMV to FLV

Using 101convert online

1. Open the ISMV-to-FLV converter on 101convert.com.
2. Select the .ismv file and start the conversion.
3. Download the resulting .flv file after processing completes.

This option requires no software installation and is suitable when the ISMV file contains the complete media stream. If the package stores audio separately in an .isma file, use FFmpeg so both tracks can be supplied explicitly.

Using FFmpeg

Install FFmpeg, place the source file in a working folder, and run:

ffmpeg -i input.ismv -c:v libx264 -preset medium -crf 23 -c:a aac -b:a 128k -f flv output.flv

This decodes the source and creates an H.264 video/AAC audio FLV. Replace input.ismv and output.flv with the actual filenames.

If the Smooth Streaming package has separate video and audio files, combine them with:

ffmpeg -i video.ismv -i audio.isma -map 0:v:0 -map 1:a:0 -c:v libx264 -preset medium -crf 23 -c:a aac -b:a 128k -f flv output.flv

To inspect available streams before converting, run:

ffprobe input.ismv

If the source already contains H.264 video and AAC audio, a stream-copy attempt avoids re-encoding:

ffmpeg -i input.ismv -c copy -f flv output.flv

Use re-encoding instead if FFmpeg reports an unsupported codec, timestamp error, incompatible audio format, or invalid FLV stream.

Quality, compatibility, and limitations

Re-encoding reduces quality slightly; lower CRF values increase quality and file size, while higher values reduce both. A CRF of 18–23 is a practical range for H.264 video, but the correct value depends on the source resolution and bitrate.

FLV does not support every codec or media feature found in fragmented MP4. H.264 video and AAC audio provide the safest combination. Subtitles, multiple alternate tracks, Smooth Streaming metadata, and adaptive-bitrate behavior may be lost during conversion.

An ISMV file may be only one bitrate fragment or one track rather than a complete recording. If the output is short, silent, or lacks video, locate the matching .isma audio file, other media fragments, and the .ism manifest before converting.