A solution for consistent subtitle quality and positioning
Screen has developed a method for served web video playback that ensures that subtitles are always displayed in the style intended regardless of the player – while allowing the viewer to select from several language streams (and off) without the need to re-encode the video or host multiple copies.
Screen’s MediaMate file-based open subtitle encoder converts a standard subtitle file (per language) into timing data and images for the target resolution(s) applying an EDL if necessary to adjust for web-specific edits or commercial breaks. The resultant PNG and XML files are hosted on a web-server ready for use; for extra flexibility this web-server need not be the same as that hosting the video.
- producer has full creative control over style and position
- text is clear (not affected by the video compression)
- allows viewer to select language or turn captions off
- uses standard subtitle file formats
- can automatically adjust broadcast version for the web via EDL
Subtitles for live web streams are not easily supported by web-video players and the variable latencies in web delivery make synchronisation very difficult. If subtitles for live streams are required then hitherto they have been burnt-in as open subtitles prior to encoding as a web-stream. However Screen has now developed its own synchronisation method to enable accurately timed subtitles to be delivered on live web-video feeds – as closed subtitles in one or more languages, without the need to encode multiple streams.
Screen’s innovative solution (a TVB Europe Best in Show award winner at IBC 2014), uses Polistream to add a time-stamp to the video as it’s encoded – then using code added to the player to analyse the timing data and display the correct subtitle (using the same image-based approach described above – except that the images are also created by Polistream). This technique has initially been successfully demonstrated using an Elemental HLS encoder with output to PC, Android and iOS – integration with encoders from other manufacturers is also under consideration.
A broadcaster’s Polistream system can now be easily upgraded to add an interface to their streaming video encoder so that real-time subtitling on any live broadcast is also conveyed on the simulcast web output. The same method can also be used with “live TV on the web” to deliver multi-language subtitles for pre-recorded programmes.
This approach ensures reliable subtitles that are always in sync with the video regardless of the video player or platform – even when live video rewind or pause/resume is used.