Digital terrestrial television

The innumerable networks through which information circulates at unthinkable speeds reduce the spatial dimensions of the world and make time understood in terms of immediacy. In this panorama of global communication, which will already be defined before the end of the millennium, we are particularly interested in focusing on the technologies that will transform the functioning of television.

The fact that the geostationary orbit is one of the most finite goods we possess – added to the fact that video transmission occupies a space almost four hundred times larger in the spectrum than used in the transmission of value-added services – was fundamental for the development of new systems such as video digitization and compression technology. These not only make better use of the frequency spectrum and available power, but also distribute information more effectively.

Digitization transforms complex analog signals over time such as audio and video, into binary numerical signals, which occupy an infinitely smaller space than the first and being compatible with the language of the computer, achieve the most powerful combination and radically transform the possibilities of video production.

Research on current current television (ATV) for terrestrial transmissions is moving towards full digital implementation. In the United States, a major alliance has been formed to design a single ATV digital terrestrial transmission system. In Europe, some projects are on the way to develop these same systems, such as the HD-DIVINE project, from the Nordic countries, the ITC and NTL SPECTRE from the United Kingdom, the Diamond from Thomson-CSFLER from France and the STERNE from CCETT also french.

The average aggregate information required for an ATV transmission system, which includes compressed video, compressed audio, conditional access, and auxiliary channel information, is around 18 to 20 Mbps. In the United States, that average information must be compressed to a 6 MHz channel for terrestrial transmission. In the case of other countries, the same average information should be transmitted to a channel with a band length of 7 to 9 MHz. This difference in available bandwidth could lead to the selection of different transmission patterns in different parts of the world.

The coverage of a television station and its network will change with the introduction of digital television service. The power required by a DTTB (Digital Television Terrestrial Broadcasting) transmitter will be significantly lower than that used for a conventional television transmitter. This power reduction can be up to 10 to 20 Db. The interference level of an ATV station would be much lower than that of conventional television stations, caused by noise, as a feature of the digital television signal and its low transmission power. The separation of channels could thus be reduced. Moreover, some channels that cannot be used to transmit NTSC signals, due to the presence of intermodulation products and other interference, could be used for ATV transmission. It is expected that by then sufficient channels will be available to provide each conventional station with an ATV system.

The behavior of digital transmission techniques will cause a sharp transition from perfect reception to unacceptable image quality, with only a small increase in time between locations.

Two different solutions have been proposed to solve this problem. The first is to use a multi-strip transmission system where some of the transmitted image is more robust. You get some kind of degradation, but you reduce the transmission of information. The second solution is to use channel repeaters to provide a clean signal in areas where the signal coming from the main transmitter is insufficient to offer an acceptable quality service. This procedure is common for conventional television and can also be used on ATV service.

So far, not much consideration has been given to the digital television broadcasting model, and yet it could help reduce costs of new digital television equipment, eliminate the requirement that people have to change their digital equipment when they leave from one continent to another, and make it more difficult for governments to get around. control of television reception from other countries by selecting an unsupported national system.

Although technology in television broadcasting has advanced enormously in recent years, the progress of methods for evaluating the viewer's opinion of image quality has failed in its pace, so that today it is not easy to know how significant it is to evaluate, characterize or optimize new systems. And this is not a merely academic concern, but also commercial, because the competition between the digital medium will soon be influenced by image quality factors.

A fundamental property of digital television, which is not well characterized in terms of the viewer, concerns the way in which systems respond to the presence of errors that occur in the channel. Without sophisticated error protection, noises produced by electrical impulses in the home can occasionally result in momentary distortions that are sure to be more unpleasant than in analog systems. System failures due to atmospheric conditions will also be very different. They will not result in a gradual increase in noise, but instead a sudden and total loss of image will be observed.

Unlike the analog signal traditionally used, which degrades to noise but continues to generate images – although of lower quality – when talking about the digital one can no longer depend on the quality of the signal, but on the reliability of the system used to transmit it.

In analog signals the interference caused by the proximity of the antenna to high voltage power lines, for example, produces sparks, while in the digital one the screen turns black – there is a total absence of images.

Errors caused by noises of electrical impulses can be particularly noticed in systems that are used under satellite or terrestrial transmission power and their effects are dilated on the screen by fractions of a second in the most efficient compression schemes.

Sometimes, when the atmospheric phenomenon pushes the level of errors beyond its limit, it will give way to a quick and total failure. This situation will be the focus of many complaints from users to service operators. Hence, some designers are trying to avoid such a situation, implementing digital systems that fail in the outdated way – step by step – as an attempt to match the failure characteristics of analog systems. The discomfort can also be mitigated with the new chips, which will be distributed free of charge to the cables, which improve the visual process. If the data is destroyed, they try to reorder it, but if the cut is abrupt, they freeze the image.

The need is also perceived to establish the influence that the quality of the audio has on the quality of perception of the image. In general, it seems that sound intervenes by distracting the viewer and making it more tolerable to imperfections. This effect is significant and may well have economic consequences, influencing the estimates of the service coverage limit by area.

One of the most immediate applications of video compression and digitization is the previously unusual possibility of transmitting television over a telephone cable.

This would free up the electromagnetic spectrum opening up the field to a greater circulation of data and voice. The American company AT&T already has a project in this regard. The union between television and the telephone is a reality that began to be realized in 1993 when Bell Atlantic obtained the first cable television operator in the United States.

"What is observed is the progressive merger of all sectors, but undoubtedly, says electrical engineer Daniel Rosas, "as long as incompatible equipment continues to proliferate and companies do not join together to find a regulatory model to which telecommunications are subject, it will not be possible to implement the planetary possibilities of technology."

In the United States, a large alliance formed by the proponents of digital systems seeks to design a unique digital broadcasting system. Several European projects, including ITC's SPECTRE, aim at the same goal. Either way, everyone agrees that this system will include compressed video, compressed audio, conditional access, a security band to avoid interference and a data channel of 18 to 20 Mbps.

As for the infrastructure needed to implement the dizzying combination of video with the interactive capabilities of computers, the role of fiber optics is irreplaceable. With a minimum of energy expenditure, video servers – which replace the usual mail – will be able to dispatch through networks all kinds of visual projects. There is talk that opportunities in video distribution will emerge as phone and cable companies, software producers and databases are linked. Thus, the dream of a comprehensive planetary communication network is a reality that only requires the will of large companies.

If this is achieved, we will not have more parallel networks but a single one that will express a total convergence in the world of information.