This page describes how FAB Subtitler BCAST can transmit subtitles for HLS internet live streams.

Introduction to transmitting subtitles for live internet streams

FAB Subtitler BCAST can generate data that is necessary in order to add subtitles to the internet video/audio streams in HLS format. FAB Subtitler monitors the video .m3u8 playlist from the video encoder and the video chunks and generates the subtitle .m3u8 playlist and the subtitle chunks which are then distributed through a CDN in the same way as video/audio and the master playlist. The master playlist has to be modified in a way that it includes additional data necessary in order for the players to be able to identify and display the subtitles.

The following diagram shows the system design:

FAB Subtitler is monitoring the video HLS playlist and is generating the subtitle playlist and subtitle chunks in real time. Those chunks and playlist for subtitles are then distributed preferably through the same CDN where video and audio are distributed.

FAB Subtitler BCAST can monitor the video playlist in two ways:

• Solution 1 (PUSH MODE): By accepting the playlist.m3u8 and video chunks over http which are POSTed by a video encoder to the IP address of FAB Subtitler
• Solution 2 (PULL MODE): By downloading the playlist.m3u8 and video chunks from an URL

Configuration of FAB Subtitler BCAST

To activate real time generation of subtitle HLS playlist and chunks, one HLS Live Streaming output has to be added in the System/Network part of the configuration of FAB Subtitler BCAST for every video stream as can be seen on the following picture:

The configuration of HLS Live Streaming output is shown on the following picture:

• For solution 1 (PUSH MODE): accepting the playlist.m3u8 and video chunks over http POSTed by a video encoder

• For solution 2 (PULL MODE): downloading the playlist.m3u8 and video chunks

“Video playlist URL” is the URL that the FAB Subtitler will use to fetch the video playlist. This can either be the master playlist or the variant video playlist with the lowest bitrate. FAB subtitler requires unencrypted video, so a playlist without DRM has to be specified here. If necessary, an additional non-DRM output has to be configured on the HLS encoder that would only be accessible for the FAB Subtitler but not from other networks.

One possibility to find the correct variant video playlist is to look for it in the master playlist:

Look for the one with the smallest bandwidth value or for the one with the smallest resolution if the bandwidth is not specified. Use the URL path of the master playlist and append the name of the video playlist as shown in the example above. Instead of using direct link to the playlist you might consider using another link that would enable you to direct the request to the correct HLS encoder.

“Advance subt. playlist by” allows defining how many video segments subtitles can be advanced in time. This can help reducing delay for live subtitles. Generally 1 shall be used, however you have to verify that all targeted players display subtitles correctly. If there are problems with players, use value 0. Even Higher values than 1 may cause some players not to display the subtitles at all.

“URL suffix for subtitle playlist” is a string that will be appended to the URL on the host on which FAB Subtitler is running. The full resulting URL is displayed next to the entry field. Subtitle playlist and chunks are accessible through that URL.

“Language” is the language of the subtitle track.

“Name” is an optional, human readable name of the subtitle track.

“Subtitle format” can be TTML- Timed Text XML or WebVTT (.m4s) or WebVTT (.webvtt). All three formats are supported by most HLS players.

“Timed text format TTML” enables you to select which type of TTML you would like to use. Not all HLS players support all TTML features so depending on the players used, you might have to use a different TTML type.

The following TTML formats can be generated for live streaming: Simple Timed Text, Flash JW Player (with and without colors), ARD TT, iTunes TT, Amazon Video, Netflix, NBC Universal, Redbull TT, IMSC1 (20th Century Fox), IMSC1.1 (Warner Bros), Paramount IMSC1.1, SMPTE-TT and Rosetta IMSC1.2. The formats NPO Open Profile, NPO Hearing Impaired, NPO EBU-TT-D and RTV Slovenia TTML are file formats only. They will currently not produce any HLS output.

“Remove tt namespace”: The EBU has specified that tt namespace shall be used for EBU-TT compliant TTML subtitles. There are almost no players that support using namespace, so you might consider removing tt namespace from the TTML used.

FAB Subtitler can publish the subtitle playlist and subtitle chunks in the following ways:

• Push subtitles using HTTP(s) POST/PUT to a configurable URL
• Push subtitles using file copy to a network folder
• Only make subtitles available over internal web server so that they can be downloaded

Please note that FAB Subtitler will synchronize files on the network folder with locally produced subtitle playlist and chunks. Therefore all files starting with the configured prexix (subt in examples above) will be deleted on the destination.

“Public base URL of this folder” is the address under which the players reach the folder where FAB Subtitler publishes the subtitle playlist and chunks, normally the address on the CDN, for example https://cdn.example.com/live/channel1/subt/. When it is set, it is put in front of the chunk and initialization segment names written into the subtitle playlist. When it is empty, relative names are written. Players do not reach FAB Subtitler directly, so never enter the address of the FAB Subtitler computer here, but the address of the CDN or leave it empty when relative addressing is working.

When subtitles are transmitted in several languages, the same HLS output is added to all desired language groups in the language definitions. There, “DASH/HLS Language” sets the language that is used for the output (when empty, the language of the language definition is used) and “DASH/HLS Name” the name of the track.

Adapting the master playlist

There are some additional entries that have to be added to the master playlist in order to enable the players to find and display subtitles. If you are not able to modify the master playlist that is coming from the HLS encoder, just make a copy of that master playlist, modify that one and let the players use that one instead of the one that is generated by the HLS encoder.

In the following picture the necessary modifications are marked yellow:

The #EXT-X-VERSION should be at least 6.

It’s recommended to include the line #EXT-X-INDEPENDENT-SEGMENTS.

There should be one line #EXT-X-MEDIA:TYPE=SUBTITLES… for each subtitling language or type. Set name and language as required. The URI should point to the place where the subtitle playlist and chunks are accessible from the outside. Normally this URI will point to CDN.

For each video stream add the codec of the subtitles to the CODECS list and at the end of the line append ,SUBTITLES=“subs”. The codec depends on the subtitle format that you selected:

For example CODECS="avc1.4d401f,mp4a.40.2,stpp.ttml.im1t". For WebVTT (.webvtt) no codec is added to the CODECS list.

Checklist for setting up subtitles for HLS live stream

• Decide whether you will use solution 1 (PUSH MODE) or solution 2 (PULL MODE)
• Define the URL where FAB Subtitler must push the m3u8 playlist and segments with subtitles
• Configure FAB Subtitler as described above
• For solution 1 configure the HLS live streaming encoder to push the m3u8 playlist and video segments to the IP/URL of FAB Subtitler
• For solution 2 configure the correct PULL URL for the m3u8 playlist and video chunks
• Adapt the master playlist to include subtitles so that the player will be able to find and display them
• Run the HLS live stream and FAB Subtitler to transmit subtitles and check if the player displays them correctly.

This page was last updated on 2026-10-03