Introduction to Digital Terrestrial Television (DTT)
Last Updated (Friday, 01 May 2009 10:54)
Along with many other countries in Europe and elsewhere around the world, Ireland is about to begin switching its free-to-air TV broadcasts – RTÉ1, RTÉ 2, TV3 and TG4 – from analogue to digital. The new Digital Terrestrial Television (DTT) broadcasts will be phased in across the country area by area, over a three year period starting in autumn 2009. They will run alongside the existing analogue broadcasts until 2012 when analogue will be permanently switched off.
Viewers will then have a number of choices of how to watch television:
- Purchase and install DTT receiver equipment (a set-top-box or integrated TV) and receive the free-to-air channels;
- Pay for a subscription to the new commercial DTT service provided by Boxer;
- Pay for a subscription to a paid satellite TV service, such as Sky;
- Pay for a subscription to a paid cable TV service, such as UPC (formerly Chorus/NTL).
In all cases, a person switching from the current analogue free-to-air TV will see the following differences:
- Different equipment, in the form of the new set-top-box or integrated TV with a new remote control;
- Different interaction methods, in the form of on-screen menus, electronic programme guides and interactivity;
- Different content, including supplementary programme information, enhanced services and possibly Internet access.
These changes, whilst undoubtedly providing a richer television experience, will also present a number of accessibility and affordability challenges for older people and people with disabilities, particularly those on low incomes.
How DTT works
In the way it works for consumers, Digital Terrestrial Television (DTT) is not very different from the current analogue television. With analogue television, you have a transmitter which broadcasts the analogue television signal using radio waves. This is picked up by an aerial, typically on the roof of the viewer’s house. The signal then travels down a wire to the television which contains an analogue television receiver. This receiver converts the analogue signal into pictures and sound.
How analogue terrestrial television works:
With DTT, the same radio transmitter broadcasts the signal, but in a digital form. The same roof aerial picks up the digital signal and sends it down the same wire to the same television. The difference is that instead of an analogue television receiver, it passes through a digital receiver. This receiver may be integrated into the television like the analogue receiver but nowadays is more likely to take the form of a separate set-top-box. The digital receiver converts the signal into pictures and sound and provides access to the supplementary information, Electronic Programming Guide, interactivity, etc.
How digital terrestrial television works:
Benefits of digital television
The digital switchover is happening because digital television brings a number of benefits for both broadcasters and consumers. For consumers, the most important of these are more programmes, extra information and interactivity.
Content
Using digital signals allows more channels to be broadcast. The radio spectrum is used for a huge number of different purposes but there is a limited amount of it available, so it has to be used as efficiently as possible. Going digital allows about five times as much programming to be broadcast in the same ‘bandwidth’. This means that where a broadcaster could broadcast four channels in the past, now they have enough space to broadcast 20 channels. This allows for more programmes or a choice of multiple ‘views’ of the same programme – a football match for example.
Extra information
Digital also allows extra information to be sent alongside the programmes. This includes programme descriptions, ‘now’ and ‘next’ listings and full Electronic Programme Guides (EPGs) showing programme listings and information on all channels for the coming days or weeks.
Interactivity
Digital technologies allows interactivity so that viewers can become more involved in the television experience, directly influencing the content of programmes or the outcomes of dramas for example. Although this has yet to be explored to any great extent, it has the potential to completely change the way we think of television, from a passive to an active medium.
Additional benefits for people with sensory impairments
As well as the above benefits that all consumers can enjoy, there are particular benefits for viewers with sensory impairments because the digital technologies make it possible to include closed audio descriptions and closed sign language as well as the closed subtitles which are already possible with analogue television.
The term ‘closed’ means that the subtitles, sign language or audio description can be switched on or off by individual viewers. This is necessary because not all viewers want to see the subtitles or sign language or hear the audio description. Most just want the basic programme with the regular audio. In contrast, open subtitles, sign language and audio descriptions are mixed into the programme’s video or audio stream prior to broadcast so individual viewers cannot remove them. This is not usually what is wanted.
Closed subtitles can already be provided on analogue television using teletext. The subtitles are written as the content of a specific teletext page which is refreshed at the required rate. The viewer then goes to this page (for example page 888 for RTÉ 1 subtitles) and shows it superimposed over the programme. This method does not work for sign language or audio description however, because it is not possible to put audio or video content on a teletext page. But digital television allows the sign language or audio description to be sent as a separate stream alongside the programme and combined by the digital receiver at the viewer’s request. This makes it possible to provide sign language and audio description without forcing everyone to receive them.