Subtitling and captions are an increasingly important feature of online video streaming (OTT) services. In the US, supporting captions is a legal obligation for all TV and VOD providers.
The following table shows the subtitling formats supported by OpenTV Player.
|TXT to SMPTE-TT over ID3|
|DVB Bitmap to SMPTE-TT over ID3|
|DVB Bitmap over ID3|
|DVB Bitmap passthrough2.|
- 2.Legacy functionality.
- Restricted by platform
- Working on support
- Not applicable
Subtitling and caption options
There are numerous formats and standards for subtitles and captions, which makes selecting the right solution a challenge for many broadcasters. When selecting subtitles and captions, consider the entire chain:
- The subtitle format contained within the content source.
- The subtitle formats the transcoder/packager supports.
- The subtitle formats that the video player can consume and display.
For other markets, and in particular the European market, it is more challenging to offer consistent subtitling support throughout the different end user platforms. Most of the time, subtitle formats that are found within content provider sources are in DVB format, in teletext, or in bitmap formats. A conversion is therefore necessary at transcoder level.
An elegant solution for achieving a consistent output format is to transform DVBBitmap and TXT subtitles into a single normalised format such as SMPTE-TT. As proposed by the ietf.org spec that can be found here, the resulting SMPTE-TT subtitle data can then be packaged within ID3 timed metadata, thus ensuring a precise timing of when subtitles need to be displayed.
As all of our secure players support ID3 demuxing, we have chosen this option to extract the subtitle data as it offers a streamlined and consistent approach for all of the platforms we support. However, transformation of DVB Bitmap to SMPTE-TT packaged to ID3 metadata must be supported at the transcoder/packager level.
For the US market, it is more straightforward, as closed captioning CC 608 and CC 708 have been widely adopted by content producers and video transcoder vendors. Our OpenTV Player offers support of CC 608/708 on all platforms, provided that our premium player is selected for Android.