Klaus Merkel (rbb)

HbbTV is certainly not the only technology for delivering accessibility services, but it provides an extraordinarily rich toolbox for broadcasters to implement a wide range of services, including those addressing accessibility requirements. It’s worth noting that in many European countries HbbTV has the highest market reach among all smart TV platforms.

A recent EBU report (tech.ebu. ch/publications/tr065) examines how HbbTV can be used to provide accessibility services for those with sensory differences. It focuses on four important features, namely subtitling for the deaf and hard of hearing, sign language interpretation, audio description, and accessible audio experiences.


The report details several options for implementing subtitles. One of the basic decisions is whether to use native rendering on the TV set (and there are three standardized rendering engines available in many sets: teletext, DVB subtitling and TTML/EBU- TT-D) or to let the HbbTV application do the rendering of the subtitles. Whereas native rendering is easier to implement, as it does not need dedicated rendering libraries to be integrated into the HbbTV offering, the main benefit of the app-based rendering is that individual rendering options can be offered, like adjusting font sizes and colours and the position of the subtitles.

Additional degrees of freedom can be found in delivery and syncing modes for both the video stream and the subtitles. HbbTV allows, for example, to sync IP-delivered subtitles to broadcast-delivered video. This enables the provision of subtitles in multiple languages without consuming any additional broadcast bandwidth. The report provides a matrix with an overview of all options plus decision trees.


Sign language interpretation, in contrast, has only one efficient implementation option. Unless the decision is to have the interpreter burnt into the broadcast video, we must go for a separate IP video. A picture-in- picture rendering option that would place the signer video on top of the broadcast is not available in typical HbbTV devices. Thus, a video containing the combination of the regular TV programme plus the signer must be rendered at the back end and launched on the device as one single video. This IP- streamed video with signing can be announced and launched via an HbbTV application.


Audio description and accessible audio experiences, like enhanced dialogue prominence, rely on technical features for the provision of additional audio tracks. In this context too, the HbbTV toolbox offers several approaches that may be smarter and more flexible than just adding more broadcast soundtracks. Syncing mechanisms in HbbTV 2.0 allow audio streams delivered via IP to be synchronized with the broadcast video. This means an unlimited number of additional audio channels can be delivered without consuming extra bandwidth on the broadcast. Also, for catch-up services the preferred audio track can simply be chosen in the player settings.

If the TV is equipped accordingly, Next Generation Audio codecs can be used to offer immersive and/or personalized experiences and mixing capabilities in the receiver. Finally, mobile devices can also be paired with HbbTV 2.0 devices for the individual use of audio variants.

HbbTV thus provides a very broad and versatile base for the accessibility tower (see David Wood’s article in issue 51 of tech-i), letting people build on the upper layers with confidence!


This article was first published in issue 51 of tech-i magazine.

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