Prepare subtitles for an HLS stream on Mac

Agree on what the viewer should be able to select

Suppose you are releasing a recorded ceramics demonstration. Viewers should be able to choose Dutch subtitles, turn them off to watch the potter’s hands, and turn them back on later. Ask the streaming team for the final on-demand master, including any opening ident, and confirm that their packager accepts a continuous WebVTT file. A six-second ident added after caption review can leave every instruction six seconds early.

Write down the cut identifier, spoken language, track language and purpose. A Dutch dialogue translation and an English accessibility track serve different viewers: on the latter, include a meaningful kiln timer sound if it explains why the potter turns away. Clear the recording, included material and translation for this streaming audience. A private review link is a separate permission from distribution.

Pick checks a reviewer can recognize without a stopwatch

Open the approved recording in Polyglot on an Apple Silicon Mac and transcribe it locally after model setup, or import the matching SRT or WebVTT and attach the master for review. Compare the source words with playback before translating. Ask a fluent reviewer to check the Dutch instructions, especially numbers and the difference between opening and closing the vent. Export the reviewed track as WebVTT and retain the original project.

Send the file, its language and the exact master filename together. Add three landmarks: the words just as the potter closes the vent, a longer sentence that remains visible while the wheel slows, and the first words after a silent firing sequence. Include the actual cue times and expected wording from your cut. That last landmark catches a subtitle playlist that stops during the silence even though all its early lines look fine.

Keep the film clock when the stream is split

Polyglot supplies timed text. The streaming team creates the HLS playlists and media segments. For WebVTT on Apple devices, Apple specifies a timestamp map that relates subtitle time to the media clock. The subtitle playlist must cover the full program, including stretches with no speech. Ask the packager to supply that mapping and duration rather than shifting a reviewed subtitle file until one playback happens to look right.

For example, suppose the packager uses a subtitle segment boundary at 00:12 and a line runs from 00:10 to 00:14. Under the HLS specification, both subtitle segments covering that line carry the cue with its full start and end. Do not cut it into two sentences at 00:12. A viewer seeking to 00:13 should still see the line already in progress. Use an actual subtitle segment boundary from the delivered stream for this test; video and subtitle segments need not have the same length.

Accept the player link, then file the fault at the right stage

Open the packaged on-demand stream in the player your audience uses. Select Dutch and read an actual line. Seek into the crossing cue and past the silent firing sequence. Switch video quality if the player allows it, or ask the streaming team to exercise another rendition. Confirm that the selected language stays available and in sync. Turn it off and verify that the words disappear.

If Dutch is missing only at one quality, ask the packager to compare that variant’s subtitle group with the working one. If every cue has the same offset on the stream but matches the master locally, send the two playback times and ask for the cut and timestamp map to be checked. If the same incorrect temperature appears in both places, correct the text, re-export and repeat the affected checks. Record the player, version, selected language and failing scene so the next reviewer can reproduce it.

Keep exploring.