DVB Teletext and Closed Captions Support for DeckLink SDI Input in Nimble Streamer – Softvelum: efficient tools to build your streaming networks

Nimble Streamer can now extract subtitle and caption data from DeckLink SDI input and make it available for further processing and delivery. The feature is intended for live SDI ingest workflows where caption or teletext data must remain available after Nimble Streamer receives the signal from a DeckLink-compatible device.

Supported Subtitle and Caption Formats

Nimble Streamer supports the following subtitle and caption data from DeckLink SDI input for this workflow:

DVB teletext.
Closed captions carried as CC 608/708/A53 data.

Set up SDI input first

Before processing SDI teletext or closed captions, you need to set up the SDI input itself.

Please refer to SDI input setup article for full procedure.

Here’s an example of Live Transcoder scenario ready for further adjustment for teletext.

Make sure you test the output live stream via designated protocol before moving forward.

Capturing DVB Teletext from DeckLink SDI Input

The teletext processing requires the following steps.

Enable subtitle processing

Before using DVB teletext from DeckLink SDI input, you need to enable DVB subtitle processing in Nimble Streamer.

Add the following line to nimble.conf:

dvb_subtitles_processing_enabled = true

Restart Nimble Streamer after changing the configuration file as described in this reference page.

This setting enables the internal subtitle processing required for the DVB subtitle handling.

DeckLink exposes DVB teletext packets only when teletext lines capture is enabled for the input. In WMSPanel, open the transcoder scenario and configure the DeckLink’s Video source.

Video Source Parameter

Set the teletext_lines parameter to “all” to the Video source settings:

This tells the DeckLink input to capture all teletext lines from the SDI signal. Once enabled, Nimble Streamer can receive DVB teletext packets from the DeckLink source and pass them further through the processing chain.

Use this setting only when the SDI input actually carries DVB teletext and that data must be preserved or processed downstream.

Configuring the MPEG-TS teletext descriptor in WMSPanel

You also need to specify respective MPEG2TS teletext descriptor settings for the languages carried as the DVB teletext. This descriptor is required for any further processing, but SDI input does not provide it as part of its data, so it needs to be set manually by the user.

If the MPEG-TS teletext descriptor is not configured, then “eng” (English) with page “801” will be used.

You can configure the teletext descriptor in the Video source settings. Click on “Add teletext language” to show respective option and define the language:

The UI allows you to specify multiple languages that are present in teletext. This is useful when the source contains teletext pages for several languages and downstream components need the descriptor information to identify them correctly.

Capturing Closed Captions from DeckLink SDI Input

If the incoming DeckLink SDI signal carries CC 708/608/A53 data, Nimble Streamer will extract that data automatically.

You only need teletext_lines parameter set to “all” in SDI source. Having SDI input configured, you’ll get closed captions in your output right away.

FFmpeg DeckLink fork for testing

During development and debugging, Softvelum created a fork of FFmpeg for testing DVB scenarios.

This fork adds behavior needed for the test workflow: when MPEG-TS input file contains teletext or closed captions, FFmpeg can pass that data to a DeckLink output device. This allowed our engineers to generate SDI test signals with teletext and validate how Nimble Streamer receives and processes the data from DeckLink.

You can run it like this:

./ffmpeg -hide_banner -loglevel error -nostats -re -i teletext-eng.ts -map 0:v:0 -map 0:s:0 -map 0:a:0 -c:v wrapped_avframe -pix_fmt uyvy422 -c:s copy -c:a pcm_s16le -ar 48000 -ac 2 -f decklink -teletext_lines all "DeckLink Duo (1)"

And send proper teletext and closed captions into SDI output which you specified (in our case it’s “DeckLink Duo (1)“).

You can run ffprobe to make sure this file has the teletext:

ffprobe teletext-eng.ts

The output will look like this:

Input #0, mpegts, from 'teletext-eng.ts':
  Duration: 00:01:00.62, start: 1.400000, bitrate: 4401 kb/s
  Program 1 
    Metadata:
      service_name    : Service01
      service_provider: FFmpeg
  Stream #0:0[0x100]: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p(tv, top first), 1920x1080 [SAR 1:1 DAR 16:9], 25 fps, 25 tbr, 90k tbn, 50 tbc
  Stream #0:1[0x101](eng): Audio: aac (LC) ([15][0][0][0] / 0x000F), 48000 Hz, stereo, fltp, 130 kb/s
  Stream #0:2[0x102](eng): Subtitle: dvb_teletext ([6][0][0][0] / 0x0006), 492x250

You can see “Subtitle: dvb_teletext” element there which means you’ll have teletext on your SDI device after running ffmpeg command.

Summary

Nimble Streamer now supports DVB teletext and closed captions from DeckLink SDI input, both DVB teletext and CC 708/608/A53 closed captions.

Feel free to install Nimble Streamer, set up SDI input as described above and try this in action.