Digital Television in Australia

[2]Why Digital?[2]

To overcome problems experienced with Analog Television

Quoting from the final report of the Australian Broadcasting Authority Digital Terrestial Television Specialist Group:

[Over the air] television broadcasting is a demanding engineering challenge. The signals are subject to reflection from and obstruction by buildings and the terrain features. They are subject to interference from sources of electrical noise (e.g. motor vehicle ignition, overhead power lines, electric traction systems, florescent lamps, fax machines and digital mobile telephones and other transmissions). They are also subject to fluctuations in signal strength as they pass through the atmosphere.

Many viewers will have experienced TV reception problems such as ghosting, "snowy" pictures and interference. These are all hard for viewers to overcome. By its very nature, digital television should have a dramatic effect on these problems.

One method that has been used to improve the situation has been the provision of "translator" services to repeat broadcast signals into "hidden" areas. These are limited by cost and available radio frequency spectrum (as all repeat services need to be broadcast on seperate channels). Digital Television allows the use of a Single Frequency Network (SFN) which could potentially allow all the translators within each service area to use the same frequency, freeing up large amounts of spectrum and making installation of receivers easier.

At the time of writing, a "1+1" approach is intended to be used in Australia. This is to say that the main broadcasts in each area will be broadcast on their own channel as they are now, while all "translator" services for a given channel will form a SFN. SBS intends to just use a SFN across a given service area, including for the main service.

The effect of "ghosting" (caused by reception of a direct signal and a slightly delayed reflected signal) is eliminated when the receiver is successful in recognising the digital data.

The European DVB system is very good at dealing with this "multipath interference" - indeed Single Frequency Networks are not far off from deliberately causing multipath interference. The US-based system is not very good at dealing with it and this is the subject of continuing debate in the US.

Traditional Transmission Problems Diagram

To provide enhanced television services

Again quoting from the ABA Report:

Features such as wide screen formats and higher definition can provide a pathway for the consumer to experience the full benefits of digital transmission.

The most well known of the new capabilities is High Definition Television (HDTV), which can provide pictures of much higher quality than conventional broadcasts. It will also be possible to transmit multiple Standard Definition Television (SDTV) programs within the same bandwidth. Some data capacity could also be allocated to provide closed captions more effectively, or multiple language soundtracks. Other program-related data could also be sent in this way (such as sports scores or news headlines).

Possible Bandwidth Uses Diagram

How Digital Television can be used has been heavily legislated by the Howard Government. Purchasers of the new "Datacasting" licences face heavy restrictions on what video content they can carry, while incumbant broadcasters have been forced to always carry SDTV content, meaning there is insufficient space for the quality of HDTV intended to be used here. They will still have quotas for the lower-quality HDTV and face restrictions on what material can be carried when multiple SDTV video streams are offered.

This situation is unlikely to continue in the present form. It was born out of an attempt to please everyone as there were many varied interests involved and is not really satisfactory for anyone.

I will finish this section with another quote from the aforementioned report:

There does not seem to be any argument that digital transmission will eventually replace analog transmission for all broadcast delivery, but no one is certain how long this might take.

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