Viewers can use a slider button in addition to the volume control, which can be moved to reduce background noise, including music, making the dialogue crisper.

According to Lauren Ward, project lead on the A&E Audio Project Team, work on the special episode of Casualty began in post production, during the audio mix, when each sound, or group of sounds, was given an importance level (stored in metadata) by the dubbing mixer or producer.

The slider, or ‘Narrative Balancer’ as the BBC calls it, is then added to the online media player. “At one end of the slider all the objects are the same level as the original broadcast mix. At the other end is a simplified mix with louder speech and only the most important sounds to the narrative. The viewer is then able to adjust between these two mixes to find the balance of dialogue and other sounds that they prefer,” Ward told the BBC Taster website.