I have some DFXP caption files that I need to convert to ismt files for playout using microsoft smooth streaming.
Does anyone know how I can get these files converted without using expression encoder?
I've looked everywhere but can't find any details on the ismt format other than it's an mp4 fragment file.
As you said, IISMT files are mp4 fragmented files that have captions in the mdat boxes. Hence converting dfxp files to iismt files involves:
1.chunking the dfxp files and placing them into mdat boxes
2.constructing the trfa box with the offsets to each fragment.
3.adding the relevant timing information to the ismc manifest.
You can do this using mp4split
mp4split -o video.ismt video.dfxp
For specifics, refer to http://docs.unified-streaming.com/documentation/package/subtitles.html.