EBU • UER - Technical Review

No. 288 (September 2001)

Digital TV and the Internet

The “digital divide” worries politicians throughout the world. At a global level, there are huge disparities in access to modern communications: for example, there are more telephones in Tokyo than in the whole of Africa. Even within developed countries, the Internet tends to be the province of richer and better-educated people.

Various initiatives are being made to redress such imbalances. Earlier this year, the OECD published a report on “Understanding the Digital Divide” which contains the statement:

“Interactive television services could potentially bring the Internet to poor, less well-educated, elderly and other low-access groups and thus help to bridge the digital divide”.

Politicians in many countries have made similar statements about the role of digital TV. Politicians have widely embraced the concept of convergence, which is based on the fact that “a bit is a bit” whether it represents audio, video, data, software, etc. However, such politicians may not understand the differences between broadcast networks and telecommunications networks: the former are designed to deliver one-to-many services, whilst the latter are designed to deliver one-to-one services. This distinction has been blurred by broadcasters delivering encrypted data to specific individuals (e.g. data delivered via teletext on analogue TV) and by telecommunications operators contemplating new services (e.g. “broadcast” services via 3rd generation mobile telephones).

Broadcasters investing in digital TV naturally welcome the support of senior politicians, but it would be immensely embarrassing if these political endorsements were based on misunderstandings about the potential of digital TV. We need to address the fundamental question “Can digital TV deliver the Internet?”

Obviously, e-mail and surfing the Internet are examples of two-way, one-to-one services. How can a one-way, one-to-many broadcasting network accommodate such activities?

The broadcast transmission network could be used as the “downstream” channel, delivering web pages in response to individual requests submitted via telephone lines. Digital terrestrial and satellite TV transmissions can deliver data at, say, 24 Mbit/s. At first sight, this data rate seems impressive in comparison with 28.8 or 56 kbit/s telephone modems – but the broadcast data transmission must be shared with other users. If a digital terrestrial transmitter serves 1 million people and if only 1% of the potential audience (i.e. 10,000 people) requests data via the digital transmitter, each individual would receive an average data rate of 2.4 kbit/s. At this rate, users would not be happy to find that many web pages would take 100 seconds to appear! In practice, even this average data rate would be difficult to achieve as few broadcasters have sufficient spectrum to reserve an entire digital terrestrial TV multiplex for such “non-broadcast” services. Terrestrial broadcasters might decide to allocate 2 Mbit/s for such services, instead of 24 Mbit/s, thus further restricting the available data rate.

Data capacity is less of a constraint for satellite broadcasters: some already offer “high-speed” surfing to their customers, by delivering 20 – 30 Mbit/s per transponder for the downstream links. Again, the success of these services depends on the number of simultaneous users: if there are 1,000 users, the average data rate per user will be 20 – 30 kbit/s. Practical experience of such systems confirms that the download speed depends on the number of simultaneous users: excellent results can be achieved in the middle of the night, but the results can be disappointing during busier periods. Of course, data rate is not the only criterion to judge the success of such services: one satellite beam can cover much of Europe, thus covering areas beyond the reach of wired “broad-band” services, such as ADSL or cable modems.

Broadcast networks can be used to deliver data to individual users, but users (and system designers) need to recognize that the available data rate is shared between many users. Such constraints also apply to wired connections to the Internet: congestion “somewhere” on the Internet generally prevents us achieving even the nominal data rates offered by 56 kbit/s telephone modems. The links between Internet Service Providers and the Internet are often overloaded at peak times, when the limited bandwidth must be shared amongst many users.

Broadcast networks can be used to deliver “walled garden” versions of the Internet, in which broadcasters deliver a selection of, say, 100 web pages via the digital transmission channel. The entire contents of the 100 pages might be transmitted via a data carousel every 20 seconds, thus giving an average waiting time of about 10 seconds. Such services do not permit users to “surf the web”. One of the great attractions of the Worldwide Web is that users embark on a voyage of discovery – using hyperlinks to jump from one web site to another – and often reaching interesting but unexpected destinations. As there are currently about 1.6 billion publicly-accessible web pages, it is obvious that a 100-page subset of the Worldwide Web cannot hope to recreate that type of experience. Nevertheless, we must recognize that teletext has been a great success in Europe – despite offering only a very limited service of news, weather, sports results, etc.

Many TV broadcasters often announce that further information about their programmes is available at a specific web address. Such announcements causes viewers to scramble to find paper and a pen to record the web address before it disappears from the screen – with the objective of accessing the web site via their computer at a convenient time. Unfortunately, these URLs are often noted down incorrectly or, perhaps, are lost. Even worse, you may find that the web site is inaccessible because too many of the TV audience are trying to visit it simultaneously! This whole process can be very frustrating, especially as it is obvious that the best way to deliver this supplementary content would be to use the over-air transmission channel.

It is debatable whether such content is best delivered in the form of digital teletext or in the form of “cut down” web pages. If web pages are intended to be displayed on TV sets, rather than on computer monitors, they need to be re-formatted to ensure legibility at typical viewing distances (e.g. at 6 H where H is picture height). As digital TV receivers do not have the full functionality of computers, it is also necessary to remove many features of web pages, such as embedded scripts, attachments, etc. Ongoing developments in interactive TV (such as DVB-MHP 1.1 which adds HTML functionality to DVB-MHP 1.0) will address such issues.

Another possibility is that digital TV can be used to deliver e-mail to viewers. The digital TV transmissions can be used to provide the downstream channel, with the e-mails being shown on their TV screens. This process is possible because e-mails contain very small amounts of data in comparison with typical web pages and because the broadcaster can dynamically regulate the amount of over-air capacity being used to deliver e-mails. Viewers can also send e-mails, but this is achieved via the telephone modem in their digital TV or set-top box. In practice, such e-mail services are relatively primitive – for example, there are severe limitations on the size of each e-mail, no facilities for attached files (such as images) and no spell-checker. However, they begin to address the issue of the “digital divide” by providing e-mail facilities to digital TV viewers, who otherwise might not have any access to the Internet.

Similar services have long been available using analogue teletext. Users can use their telephones to request private information, such as the latest statement of their bank account. The statements are transmitted via teletext and displayed on their TV screens.

One crucial issue is whether such systems can cope with increased demand for downstream data capacity. It is easy to imagine that broadcasters would not want such services to be hugely successful because they would not have the data capacity to satisfy the demand.

In summary, we must ensure that politicians understand that digital TV cannot deliver the full experience of the Internet – but it can offer many valuable multimedia enhancements.

Philip Laven
Director
EBU Technical Department