Action item. definition and use of Clean audio in European television

Here is a summary of the European digital television definition for clean audio [adapted from a forthcoming edition of TS 101 154 from the DVB]. The system relies on client side mixing, under control of the programme-maker.

Clean Audio refers to audio providing improved intelligibility. It is targeted for viewers with hearing impairments, but can as well serve as improvement for listening in noisy environments.... an important requirement is to adjust, on a passage by passage basis, the relative level of programme sound in the mix which the secondary audio user hears. The programme maker is best able to determine the level under controlled conditions when authoring the secondary audio information to modulate the level of programme sound in the secondary audio capable receiver so suitable secondary audio information is thus transmitted within the secondary audio stream.

Individual secondary audio users will have different aural acuity, and there are, in practice, differences in audio signal level for different home receivers. An essential requirement is for the user to be able to adjust the volume to suit his/her condition.

A descriptor is sent in a service to indicate a clean audio service. The level by which the main audio service should be attenuated is sent in the metadata (on a per channel basis). The client should accentuate the levels of the channel containing the dialogue and attenuate the other channels, accordingly. The level of the centre channel added should additionally be under user control to allow individual tailoring of the sound for audibility.

Background:  http://www.rnid.org.uk/VirtualContent/84923/background_noise_PDF.pdf
European HD TV requirements: http://tech.ebu.ch/docs/tech/tech3333.pdf [p15]