1993 - von Volker Steiner - DBP Telekom

The application of digital technology is more and more implemented for the transmission and recording of (HD)TV programs. In closer examination of all the system proposals having appeared up to now in the digital (HD)TV scene, one is faced with some questions:

• What are the relative advantages of a digital system?
• Is it adavantageous to study the digitalization of TV production and transmission at first, and the extendibility to HDTV in a second independent step?
• Is it the best approach to focus on a terrestrial transmission system, or should cable and satellite transmission be integrated in the investigation for a common system?

It is highly important, in designing the new system, to meet the potential market requirements. As we have learnt from the experience with D2-MAC and HD-MAC, both standards perfor-med quite well for a high percentage of source picture, but a rapid market success of both was not achievable.

The technical advantages of a digital method are obvious,

• possibility of data rate reduction combined with effective utilization of the frequency spectrum
• adaptability of the error correction process to the propagation conditions
• optimized utilization of the frequency spectrum due to smaller protection ratio required than for analogue systems
• flexible signal and data-multiplexing

In addition there are advantages expected for the TV viewer:

• improved picture and sound quality
• utilization of small portable receivers, as well as
• telepresence due to wide screen technology
• additional TV programs available

In the following text, after a short overview of the European approach to digital TV, basic elements of a digital (HD)TV system are outlined and a practical implementation of a single frequency network implemented by TELEKOM in Berlin is described.

The aim of the European activity, having started in 1992 with the establishment of a European Launching Group and its Working Group for Digital Television Broadcasting (WGDTB), is the design of a common system for Digital Television Broadcasting.

It was concluded that digital television broadcasting should be commonly usable for terrestrial, cable and satellite transmission: Moreover, it should provide different levels of service performance and should be applicable for stationary receivers with fixed antennas, as well as for portable and mobile receivers. The concept of conventional definition digital simulcasting (16:9 aspect ratio) during a transition period is part of the group's proposal. However, due to the situation that little terrestrial frequency spectrum is available, there are some doubts that terrestrial simulcast HDTV will be possible in all European countries.

The potential options for digital terrestrial television in terms of picture quality and target vie-wing environments have been analyzed. It is recommended to concentrate the studies on sy-stems which are able to provide some of the services simultaneously in the same channel. This procedure causes so-called multi-layer systems, where one layer will provide not only different quality levels, but may also be applicable for different reception environments.

Four quality levels for digital (HDTV) terrestrial broadcasting services have been defined and are quoted below:

• HDTV quality: "should be essentially transparent to the HDI standard for 99,7% of the content of typical TV programs"
  - applicable to fixed roof-top aerial installations
• EDTV quality: "should be essentially transparent to the 4:2:2 level of CCIR Rec. 601 for 99,7% of the content of typical TV programs"
  -applicable to fixed roof-top aerial installations
• SDTV quality: "approximately equivalent to that of current PAL or SEC AM"
  -applicable to portable receivers
• LDTV quality: "equivalent to that obtainable from the MPEG 1 system which operates on a source resolution approximately 1/4 of 4:2:2 level of Rec. 60 (about VHS quality)"
  -applicable to mobile receivers

The following table shows the associated service characteristics with a rough estimate of the total bit rate necessary per service. These definitions, primarily intended for terrestrial applications, may easily be applied to cable and satellite services, too.

Advantages of a multilayer system are:

• the ability to provide reception in environments where propagation conditions do not permit reception at a level of higher quality
• receivers could be provided of different quality and cost, which can receive the same transmitted signal.

The main problem for terrestrial implementation of a system suggested above in Europe is the spectrum availability. There is no new spectrum seen, which could be allocated to the broadcasting service. And all the spectra already allocated to the service are heavily used.

Therefore, there is only the possibility to operate the digital terrestrial television in the existing frequency bands. The new services have to coexist with the analogue television services for a transition period of about 10 to 15 years.

There may be a chance to introduce digital (HD)TV more rapidly via cable and satellite, where we don't have the constraints of such a high degree like in the terrestrial environment. If it is possible to find a system of high commonality of the technical parameters given by the different media, the implementation process could be made easier.

Therefore, in the following it is investigated whether the channel characteristics of the trans-mission media could meet the requirements for digital (HD)TV.

The main parameters of terrestrial, cable and satellite channels which could be considered for digital services are as follows:

As a target for terrestrial transmission UHF channels with an 8 MHz bandwidth are taken into account, only because they should allow data rates of 30 to 34 Mbit/s by application of highly sophisticated modulation techniques.

In cable networks, 8 MHz or 12 MHz channels can be made available in the so-called Hyper-band. 34 Mbit/s or 45 Mbit/s transmission rates could be realized. With respect to the satellite transmission, there are different initial positions, which have to be taken into account. As shown in the table below, available channels have to be classified in terms of the EERP and the bandwidth.

As suggested in some papers already published, sensible transmission rates could be 34 or 45 Mbit/s.

It seems to be a possibility to chose 34 Mbit/s as the general figure for all transmission media envisaged, although it might be problematic to achieve 34 Mbit/s in the terrestrial case. In addition for cable and satellite, 45 Mbit/s is an interesting alternative.

The elements of a digital (HD)TV system outlined here are based on the results of the work done by ISO/MPEG, with respect to the MPEG-2 standard. The figure below shows a functio-nal diagram of the system with multi-program and multi-service features.

Video and audio signals of the programs of different quality status (HDTV, EDTV, SDTV, LDTV) are source coded and are multiplexed in a service component multiplexer together with the associated data. The resulting signal is protected with an external error correction code, and encrypted if necessary. Several programs derived from different sources are multiplexed into one channel by means of a program multiplex (Flex-Mux or Super-Mux). The resultant data are fed to a channel through an adaptation layer to meet the specific requirements of the physical transmission medium. The adaptation layer may include the inner error correction code, an interleaver to regenerate data packets lost and the modulator (e.g. in case of a satellite link).
As could be derived from the service characteristics indicated in a former chapter, bit rates from 1.5 Mbit/s for LTDV up to 30 Mbit/s for HDTV have to be achieved by applying appro-priate source coding techniques. In order to simplify the necessary hardware, these coding schemes have to be "generic", i.e. application-independent.

For TV applications, the aspect of scalability plays an important role: Scalability means, for example, that from an encoded HDTV signal, the bit stream of an encoded TV signal can be extracted by simple demultiplexing. This would allow to display an image in reduced format without processing the complete image. Scalability would also allow a graceful degradation of the HDTV picture at the boundary of a broadcasting service area by reducing the resolution from HDTV to EDTV to SDTV respectively.

The main work in the field of coding and multiplexing process is done in the frame of MPEG. It is also a basis for a practical application provided by TELEKOM for demonstration during the "International Radio Show", coming up in Berlin.

In order to elaborate the practical consequences of the application of digital (HD)TV in the network, Deutsche Bundespost TELEKOM have implemented a research project VTDINET (Video on Digital Networks). It comprises the implementation of a single frequency network in Berlin with up to 5 transmitter stations. Investigations in cable and satellite transmission could also be carried out.

Already 3 stations shall be in operation at the IFA 93 (Internationale Funkausstelung Berlin) to show single frequency network multiprogram experiments in an 8 MHz UHF channel. The digitally encoded multiplexing scheme of 2 to 4 programs shall also be fed into the hyperband of the Berlin CATV network.

The main target of the project is to push the development of key components of the transmis-sion chain.

Topical interests of the field trial are:

• In the shorter term, to demonstrate and to gain experience with the multi-programming via cable, satellite and terrestrial media by using a 34 Mbit/s-transmission rate.
• In the longer term, services other than television shall be added and different levels of resolution shall be investigated, too.

The key components to be used are:

• Flex-Mux according to the MPEG syntax and meeting the list of requirements.
  
  
• COFDM-Modems with an input data rate of 34 Mbit/s and a RF output signal which could be tuned from 30 up to 900 MHz. In addition 32 QAM-modems for cable applications shall be used, as well as QPSK-modems for the satellite transmission.
• Codecs with variable bit rates.
  During the IFA demonstration, a codec which is capable of encoding the source signal with 3.6/5.6/7.6/12.6 Mbit/s (without error correction) will be applied.

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In designing a digital broadcasting system, there are some reasons to go the way of highest commonality for the technical parameters of the different types of services to be broadcast. It has been shown that there are common elements with respect to the different transmission media.

Outside of the technical area, there are the market requirements to be observed if a new system should be introduced. The implementation of a digital (HD)TV system presupposes that the TV viewers are willing to buy a new TV set. This step could be made easier if

• there will be enough program material meeting the requirements of the new system
• new features could be made available to the customer (e.g. flat screen)

The various digital projects like the German TELEKOM VEDINET support this development.

References
WGDTB 1063 "Report to the European Launching Group on the Prospects for digital terrestrial television"