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Barrier-free AV services

The participation of all people, whether old or young, with or without disability, in its audio-visual offers is the declared goal of the public broadcasters. One focus of barrier-free television offers is subtitling and audio descriptions, which are successively being expanded for individuals with hearing or visual disabilities. The IRT is increasingly researching the use of the new technological possibilities in the era of convergence of television and the Internet to expand the barrier-free offers and thus optimize access for all people and make it future-proof.

The work of the IRT is focused, among other things, on unifying the production and transmission of subtitles for different end devices such as PC, tablet and SmartTV. Furthermore, possibilities for the improvement of speech intelligibility of audio-visual content are being researched. The HbbTV technology, which enables the personalisation of barrier-free offers via the Internet, plays an important role.  For this purpose, the IRT is developing solutions in collaboration with various partners, for example as part of the EU project HBB4ALL, some of which are also being tried out in pilot tests. 

Subtitles in EBU-TT-D format

The IRT supports broadcasters with, for example, the distribution of their subtitle offers in catch-up TV portals. One focus is the uniform implementation of the subtitle formats EBU-TT and EBU-TT-D for the production or distribution of subtitles. As part of its EBU activities, the IRT plays a decisive role in the development of these formats, which should also secure a smooth delivery of subtitles via the Internet. For the conversion of existing subtitle formats the IRT has developed an open source solution available under the Apache 2.0 license at Github. Beyond subtitle formats, the IRT also deals with the distribution of subtitles for Internet live streams on desktops and mobile devices. The IRT ensures that European and international developments are considered and is committed to the issue in international standardization committees for television and the Internet, including DVB, EBU and W3C.

Clean Audio

The speech intelligibility of television programs is not only an issue for people with normal hearing but especially for people with impaired hearing. This concerns users of hearing aids with medium to serious hearing impairment as well as people with hearing impairments who are not necessarily in need of a hearing aid (e.g. in the case of age-related hearing loss). For these target groups, the IRT has developed software that automatically creates a “clean audio” version from existing audio mixes (in 5.1 or stereo format). The speech is enhanced as much as possible with relation to the other audio elements, thus increasing the intelligibility of speech but without any discomforting effect on the entire audio experience. Such a clean audio track could be offered via the Internet, for example as an extra service for HbbTV, and selected by the end user instead of being embedded in the original audio channel.

The first studies with test subjects as part of the EU project HBB4ALL produced positive results: improved speech intelligibility was confirmed for all participants. A field test for the potential extra service was realized in collaboration with the rbb innovation projects.

Technical guideline for improved speech intelligibility on television

Many television viewers seem to not always be content with what they hear – as many viewer complaints prove. Therefore, ARD and ZDF decided to investigate these complaints. A cross-channel audio working group of the broadcasters was commissioned to prepare a “recommendation for programme and technology – speech intelligibility on television”, aiming to achieve effective improvements. The technical guideline promotes more (speech) intelligibility and includes recommendations for production planning, production, processing and transmission, and deals with issues such as audio design, noises, equipment in production control rooms and the hearing capacity of the viewers. The German guideline is available online at

Speech intelligibility in “voice-over-voice” situations

Voice-over-voice describes the simultaneous occurrence of two voice signals, as is the case with translated passages in magazine contributions. Such passages often trigger complaints from television viewers with regard to speech intelligibility. As part of several audio tests, the IRT investigated possible influencing factors such as the monitoring conditions in a room or the monitoring loudness. The goal was to come up with a practice-oriented recommendation for a more comfortable mix. It is not possible to give a generally valid recommendation for an ideal level offset since mixing two voice tracks is a complex process that is influenced by several factors. Based on the results of the investigation, a range was compiled in which the distance between off-speaker and original audio was to be ideally selected. The results of the research were also integrated in the technical guideline for improved speech intelligibility on television presented on these pages. 

Your contact

Dr. Markus Ludwig
Head of Department
Tel. +49 (0)89 32399-321
Sebastian Goossens
Head of Department
Tel. +49 (0)89 32399-358