Digital Broadcasting

Digital Broadcasting, transmission of digital data made up of a series of zeros and ones, representing the sounds of a radio broadcast, or the moving pictures and sounds of a television broadcast, or even pure data in the form of words and numbers. The advantages of digital technology are, first, that it can create broadcasting space for hundreds of new channels and, secondly, that higher definition signals can deliver CD-quality sound and, in the case of television, much greater picture clarity.

In digital broadcasting, as in traditional broadcasting, information is transmitted by means of electromagnetic radiation emitted by a transmitter, which is picked up by a receiver that transforms the broadcast signal back into a useful form (see Radio and Television). The distinction is that whereas the current form of radio and television uses varying information, digital broadcasting transmits digital data in binary form. The immediate output of microphones and television cameras in the television and radio studios is still in the form of continuously varying (analogue) voltages, but these are converted into digital form by analogue-to-digital converters. For the past several years, television and radio signals have usually been handled in digital form for recording and manipulation inside broadcasting companies. While ordinary analogue broadcasting depends on the changing (modulation) of the amplitude or frequency of a carrier wave by a continuously varying signal, the broadcasting of digital signals uses instantaneous changes in the phase of the carrier waves. Each one or zero transmitted makes a change to the phase of the carrier wave. Different methods of achieving this depend on exactly what form the broadcast transmission takes.

For satellite broadcasting of digital data, two carrier waves at the same frequency, but 90 degrees out of phase, are used. The phase of each of these carriers can be shifted through 180 degrees, which gives four signal states that can represent 00, 01, 10, and 11 in binary two-bit code. For digital television carried by cable to the viewers, sixteen steps of amplitude modulation are added to the four phase shifts, so that more data can be packed into the same time interval.

For traditional (“terrestrial” television) digital television broadcasting (DTT), intended to replace the existing analogue broadcasting, the system is similar to that for cable transmission, but instead of one carrier, a band of several thousand carrier frequencies spaced very closely together are used, and all the information to be transmitted is spread among them. This band of carrier frequencies is called a multiplex, and it can be fitted in the gaps in the electromagnetic UHF (ultra-high frequency) spectrum between the existing analogue television broadcast frequencies. For digital audio broadcasting (DAB) in the United Kingdom, the broadcasts will occupy the old VHF (very-high frequency) television frequency band, which is now empty. As well as the transmitting arrangements described, the transmission of high-quality digital television requires compression of the digital data. In Europe, this will be done using the MPEG-2 system of encoding and data compression, as already used for some television and computer recording purposes.

There are six multiplex bands available in the United Kingdom, and three of these have been allocated for the use of the existing terrestrial broadcasting organizations. The other three were allocated to the company ONdigital (previously British Digital Broadcasting, and relaunched in 2001 as ITV Digital), owned by the two British independent television companies Carlton and Granada, to begin transmission services for new programme providers. Since more than one television channel can be accommodated in one multiplex, depending on the quality of picture provided, new television services can be carried, along with the digital versions of the programmes on existing British television channels. There is also space on the multiplexes for other transmissions, such as purely text-based ones, or interactive ones. The interactive ones include those for buying advertised goods and services; the viewer will communicate with the seller through a special link using the telephone system.

If required at some point in the future, a single high-definition wide-screen television channel would almost fill the capacity of one multiplex. On the other hand, it is possible to have several television channels with lower-quality images transmitted by a single multiplex, as is projected at the moment. There will be a strong commercial incentive to do this, if the public accepts it, since it appears that the quality of the digital television picture will not be regulated by any statutory body.

The United Kingdom, like other countries, is gradually moving over to digital signals, and the government plans to phase out analogue signals from 2006. Following the launch in the United Kingdom of Sky's digital satellite service on October 1, 1998, the first digital terrestrial television (DTT) service in the world was launched in the same country on November 15, 1998, by ONdigital. The financial collapse of ONdigital’s successor ITV Digital in 2002 led to a reallocation of DTT licences. A new network, Freeview, backed by a consortium of the BBC, Crown Castle, and BSkyB, took over the services previously run by ITV Digital, offering up to 30 free-to-air digital channels from October 2002. Unlike satellite services, digital television can be received through an existing aerial, although customers have to purchase a set-top box to decode the transmission.

Digital television transmissions from satellites began in 1997 in a few countries, notably France and the United States. The United States and major European countries began transmitting terrestrial digital television in 1998. Other countries such as Japan and Australia started broadcasting in 1999.