ffmpeg -i input.ttml output.vtt If you have seg_1.ttml , seg_2.ttml , etc.:

import glob from lxml import etree combined = etree.Element("tt", nsmap={None: "http://www.w3.org/ns/ttml"}) body = etree.SubElement(combined, "body") div = etree.SubElement(body, "div")

But respect the source. Don’t hammer APIs. And always test your converted output—TTML’s richness is both its strength and its curse.