Making sure television is accessible to older people and people with disabilities

New audio description standard will make access easier

Tuesday, 21 April 2009 16:29

Differences and limitations in the way audio description (AD) is broadcast have, until now, made it difficult for digital television receivers to provide a user-friendly way for viewers to access it. Digital Video Broadcasting, the international industry consortium, has devised a new standard that will overcome these difficulties.

The problem

The problems are due to the different ways AD is broadcast by different broadcasters and the limitations in how broadcasters can identify different audio streams.

There are two different types of AD – broadcast mixed and receiver mixed. In broadcast mixed AD, the description track and the main audio track are mixed together into a single stream. Viewers can then switch between the standard audio stream (without AD) or the audio described stream. In receiver mixed AD, the description is broadcast as a separate stream on its own and is combined with the main audio by the receiver (the set top box or digital television) when the viewer requests AD.

The MPEG coding standard used for digital television provides four audio types for identifying different streams. These are described as follows:

0x00: Undefined, generally used for the main audio.

0x01: Clean effects.

0x02: For hearing impaired.

0x03: Commentary for visually impaired.

Even if the AD has audio type 0x03, the receiver doesn’t know whether this is a broadcast mix (including the main audio) or a receiver mix (just the description on its own). Although the international Digital Video Broadcasting (DVB) standard is to use type 0x03 for receiver mixed AD, some broadcasters use 0x03 for broadcast mixed AD. An alternative option, used by some broadcasters, is to use 0x00 for broadcast mix AD as well as for the main audio stream. To distinguish the two, the AD stream is labelled as a different language. For example, in Finland the AD stream is labelled as Danish and viewers who want to hear the AD select Danish. This isn’t very user friendly because the viewer has to know in advance which language is used by the broadcaster to signify AD and it’s just plain weird having to select Danish to get AD.

Because of these problems, receiver manufacturers across Europe have not been able to offer a universally compatible user friendly interface for viewers to set their preferences and select the appropriate audio service.

The solution

The international industry consortium DVB has developed a solution that is backwards compatible and is expected to become official in late 2009. It involves a new descriptor, called the supplementary audio descriptor, which gives each audio stream an editorial classification and a mix type.

The editorial classification states the purpose of the stream as either Main Audio, Audio Description, Clean Audio or Spoken Subtitles.

The mix type indicates whether the audio stream is intended to be mixed with another or is standalone. This tells the receiver whether an AD stream is receiver mixed or broadcast mixed.

There should be no effect on existing receivers, but new receivers which understand the descriptor will be able to offer the viewer the choice of supplementary services in a more user friendly manner.

TV Access will be encouraging Irish broadcasters to adopt the new standard to give consumer equipment manufacturers the commercial incentive to implement it in their receivers.