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Teletext and Closed Captions

Passing teletext to HLS and DASH

Flussonic allows passing DVB teletext from MPEG-TS:

  • To HLS — teletext in the WebVTT format
  • To DASH — teletext in the WebVTT and TTML format.

No additional options needed to configure the conversion. If there is teletext in the input stream, then Flussonic will automatically convert it to WebVTT and TTML.

Configuration example (no additional options):


stream example_stream1 {
  url tshttp://EXAMPLE-IP/STREAM_NAME/mpegts;
}

To check for dvb_teletext in the input stream, you can run the following command:


ffprobe http://EXAMPLE-IP/STREAM_NAME/mpegts

Stream #0:0[0x447]: Video: h264 (Main) ([27][0][0][0] / 0x001B), yuv420p(tv, bt470bg, top first), 704x576 [SAR 16:11 DAR 16:9], 25 fps, 25 tbr, 90k tbn, 50 tbc
Stream #0:1[0xc12](eng): Audio: mp2 ([4][0][0][0] / 0x0004), 48000 Hz, stereo, fltp, 192 kb/s
Stream #0:2[0x17e2](swe,nor,dan,fin): Subtitle: dvb_teletext ([6][0][0][0] / 0x0006)

After the conversion, an output stream has teletext:

Flussonic teletext

Passing closed captions in HLS and DASH

Signaling CEA-608/708 caption service metadata

In order for HLS and DASH players to display closed captions its presence should be explicit in the so called manifest files or just manifests. These files contain the URLs and names for the streams as well as adaptive bitrate information, closed captions, etc.
Flussonic can inform you of the presence of the closed captions in the stream.

To enable this feature add the following parameter to the configuration file (/etc/flussonic/flussonic.conf) next to the stream's URL:

cc.{608|708}.{INSTREAM-ID}.{lang|name}=VALUE

Parameters:

  • 608/708 — standard of the closed captions. The value is either 608 or 708 for CEA-608 and CEA-708 standards respectively.
  • INSTREAM-ID — channel number that has closed captions. It's an integer between 1 and 4 for CEA-608 and between 1 and 63 for CEA-708.
  • lang — language of the closed captions.
  • name — under what name the audio track will be displayed on the player.
  • VALUE — specified language value. Depending on whether it is lang or name the value may differ.

For example, let's specify English as language of our closed captions. Then the configuration will look as follows:

  • with lang option: cc.708.1.lang=eng (ISO 639.2/B standard)
  • with name option: cc.708.1.name=English (name of the track, that will be displayed on the player's closed captions option)

Usage example:


stream example_stream2 {
  url tshttp://EXAMPLE-IP/STREAM_NAME/mpegts cc.708.12.lang=fr cc.608.1.lang=eng;
}

In the example above there are 2 closed captions tracks:

  1. 12th channel contains CEA-708 standard closed captions in French
  2. 1st channel contains CEA-608 standard closed captions in English

You can view the presence of closed captions in DASH and HLS manifests.

  • for DASH it is achieved with the help of the Accessibility tag in the playlist file. You have to run the curl command to download it first:

curl http://FLUSSONIC-IP/example_stream2/index.mpd

Based on the previous example of the stream example_stream2 you can check that closed captions are present:


<Accessibility schemeIdUri="urn:scte:dash:cc:cea-608:2015" value="CC1=eng;CC1=eng"/>
<Accessibility schemeIdUri="urn:scte:dash:cc:cea-708:2015" value="12=lang:fr;12=lang:fr"/>

  • the same steps for HLS, except that the playlist file format is slightly different:

curl http://FLUSSONIC-IP/example_stream2/index.m3u8


#EXT-X-MEDIA:TYPE=CLOSED-CAPTIONS,GROUP-ID="v1cc",LANGUAGE="fr",NAME="fr12",INSTREAM-ID="SERVICE12"
#EXT-X-MEDIA:TYPE=CLOSED-CAPTIONS,GROUP-ID="v1cc",LANGUAGE="eng",NAME="eng1",INSTREAM-ID="CC1",AUTOSELECT=YES,DEFAULT=YES  

Extracting and converting closed captions

Flussonic allows the extraction of CEA-608 closed captions from the input stream and its further conversion:

  • in WebVTT format — for HLS
  • in WebVTT and TTML formats — for DASH.

After transcoding a stream that has embedded closed captions, Flussonic keeps the closed captions in the output stream.

For Flussonic to perform the extraction and further conversion add the option cc.extract to the stream's URL.

For MPEG-TS streams:


stream example_stream3 {
  url tshttp://EXAMPLE-IP/STREAM_NAME/mpegts cc.extract;
}

cc.extract option is available on MPEG-TS sources.

Subtitles positioning

To position subtitles on the video set the parameter substyle valign=top|middle|bottom align=left|center|right, for example:


stream example_stream5 {
  url tshttp://EXAMPLE-IP/STREAM_NAME/mpegts cc.extract;
  substyle valign=top align=left;
}

Subtitle settings can be added to the Output tab of stream settings:

Flussonic closed captions

After the conversion, an output stream has closed captions:

Flussonic closed captions

Choosing subtitles format for DASH playback

As two formats of subtitles are included in a DASH manifest, you can choose one of them when playing an output stream:

https://FLUSSONIC-IP/STREAM_NAME/index.mpd?text=wvtt

or

https://FLUSSONIC-IP/STREAM_NAME/index.mpd?text=ttml (TTML is the default format)

Passing subtitles to MSS

Flussonic passes any type of subtitles (subtitles, closed captions, or teletext) in the TTML format to output MSS streams. No special configuration is required, the only requirement is that the incoming stream must have TTML subtitles.

You can configure the position of subtitles by adding substyle valign=top|middle|bottom align=left|center|right in the incoming stream settings:


stream example_stream6 {
  url tshttp://EXAMPLE-IP/STREAM_NAME/mpegts cc.extract;
  substyle valign=top align=left;
}

About TTML subtitles

TTML (Timed Text Markup Language) is a standard for closed captioning and subtitling that is widely supported by media players, streaming platforms and other software, and also used in the television industry. The TTML standard offers rich features for positioning, alignment, styling, multiple languages, and so on. TTML subtitles are passed as an XML-based text file with the .ttml or .xml file extension.

Flussonic passes TTML subtitles to MSS and DASH streams.