DAB datacasting in the Netherlands

ing. J.C.W. van der Waal


Netherlands Broadcasting Transmission Company, NOZEMA, Netherlands.




INTRODUCTION

NOZEMA is the Dutch networkprovider for the public and private broadcasters in the Netherlands. At the moment NOZEMA is actively promoting datacasting through the existing analogue radio and television networks. The coming DAB and DVB-T networks are very promising in audio and video quality, flexibility and in datacasting capacities. NOZEMA expects to be the network operator for DAB and started in 1993 with an experimental DAB transmitter. In 1994 another transmitter was added and a Single Frequency Network (SFN) was set up. By adding more transmitters the network will grow into the final nation-wide network. Most of the commercial broadcasters have already signed a letter of intent with NOZEMA to start with DAB as soon as possible. NOZEMA is a leading member of the Dutch DAB foundation to promote and demonstrate the need and possibilities of DAB.

INTERPRETATION OF OSI LAYERED MODEL

To explain the different DAB aspects in a few words it is helpful to use the OSI layered model. The following table gives the layered model for DAB as presented by the ITU-R (1). In the physical layer the actual radio transmission of the OFDM signal takes place. The signal has been convolutionally coded, time- and frequency interleaved and modulated. The second layer or datalink layer provides the means for the receiver to synchronise to the transmission frames. The network layer identifies the groups of data as programmes. The transport layer identifies the audio and ancillary dataservices and multiplexes them. The session layer deals with the selection of, and access to, broadcast information. The presentation layer finally converts and presents the broadcast information to the listener, so he can select a programme and see some service information.

Table 1 - OSI layered model for DAB

Name of Layer Description Features specific to the System
Application layer Practical use of the system System facilities
Audio quality
Transmission modes
Presentation layer Conversion for presentation Audio encoding & decoding
Audio presentation
Service information
Session layer Data selection Programme selection
Conditional access
Transport layer Grouping of data Programme services
Main service multiplex
Ancillary data
Association of data
Network layer Logical channel ISO audio frames
Programme associated data
Datalink layer Format of the transmitted signal Transmission frames and Synchronisation
Physical layer Physical (radio) transmission Energy dispersal
Convolutional encoding
Time interleaving
Frequency interleaving
Modulation by 4-DPSK OFDM
Radio transmission


As we can see in the layer model there are several layers where ancillary data is handled. There are three areas where this data may be carried within the system multiplexer:

THE DAB NETWORK IN THE NETHERLANDS

At the moment the DAB network in the Netherlands consist of three transmitters and one gapfiller. This network covers the densely populated west-part of the Netherlands, which equals about 50% of the population, see figure 2 at the end of this document. In Lopik the 4th generation ensemble multiplexer is situated, multiplexing several audio programmes, the programme related data and the general data services. From Lopik the OFDM baseband signal is transported to the other two transmitters via a UHF channel. Lateron in the experiment the UHF channel will be replaced by 2 Mbit/s leased lines or satellite distribution. Between the multiplexer and the COFDM coder the multiplex signal is transported over the ensemble transport interface to the transmitters. The frequency reference used for synchronising the transmitters in the single frequency network is derived from the Global Positioning System.


Figure 1 : Distribution network
The distribution network which NOZEMA is building is shown in figure 1. The figure shows a services multiplexer in a studiocomplex or another central point where several broadcasters are located. The signal from the service multiplex is transported to the ensemble multiplex via a 2 Mbit/s leased line with the J.52 standard or the service transport interface (STI), which is still under development. The ensemble multiplexer can handle the STI-signal and some local audiochannels. The data input for the data subchannels is also inserted at the ensemble multiplexer. For these local inputs the X.21 interface is best suited. The output of the ensemble multiplex is transported to the COFDM coder via a 2 Mbit/s leased line via the ensemble transport interface (ETI) as defined by the EUREKA-147 project. The problems on delay variance in the network will be solved in a network adapter (NA).

DIFFERENT TYPES OF DATASERVICES

These days the electronic highway is mentioned in every paper; innovating television programmes have items about it and governments are financially supporting projects. Datacasting through broadcastsystems will play an important role in the electronic highway for those applications providing big amounts of data in point to multi-point configuration and in mobile applications. The unique attributes of datacasting are: real time reception at all locations, simultaneous reception at all locations and distribution costs independent of the number of receivers. The datacapacity in datacasting using present techniques on TV and FM ends at about 19 kbps, this however is where the capacity of DAB starts. This makes DAB a medium well suited for transporting data for dataservices.

Dataservices for broadcasters of course can be song-, artist- and programme information. Radiogames and hit parades will get a new dimension using programme associated data to display questions or artists nominated for the hit parade. Traffic messages can be transmitted in speech, utilising a 32 kbps subchannel, together with regional labels for the listener to recognise. The traffic messages can be stored into a memory for later play-back. Other applications can be : File-transfer, software updates, real-time distribution of financial information, facsimile applications, database updates for public and semi-public spaces or even interactive multi-media applications with or without a returnchannel.

THE DIGITAL FUTURE STARTS TODAY

NOZEMA already offers services for datacasting through analogue broadcast systems. We offer our video- line, RDS and FM-subcarrier capacity to service providers. One of the successful applications on our datacasting facilities is the Dutch National Railroad Company. In the past, whenever a train was delayed or other relevant messages had to be sent to the traveller information offices throughout the country, the railroad company used to use a computer connected to several faxmachines. This solution was not very satisfactory because of timedelay between the first and the last to receive the message (this was around 45 minutes) and the costs involved by dialling all the offices via the PTT lines. The main features of the facsimile via datacasting for the railroad company are real time reception at all locations (without a timedelay between the offices) at much lower costs.
Another application is the newspaper for visually disabled people. Every night four major Dutch newspapers transmit their paper to the pc’s of the blind people. The information is stored on harddisk and can be retrieved whenever they like. The newspaper can be read out loud with the aid of a speech synthesiser or read on a Braille reader. This application gets the blind out of there isolation and the interest from the newspapers is great, more papers are expected to participate in the near future. Because of the need for datacapacity for advanced applications it is our opinion that the digital future of tomorrow starts today.

DAB DATACASTING IN THE NETHERLANDS

The first datacasting experiments within DAB in the Netherlands consisted of transmissions of general data coded in the ITTS format in the PAD. Simulcasting the existing radio programmes, data was added from a file on a pc, representing a sort of testpage showing the name of NOZEMA. It was also possible to have a dcc-player provide both audio and data. In this way the songtext can be shown on a pc synchronously with the audio. Future application with commercial capabilities will occur in close co-operation with serviceproviders. As mentioned above some Dutch publishers have showed an interest in distributing an electronic newspaper, more advanced than the paper which is being broadcast at the moment. The newspaper together with pictures will take up quite a bit of capacity. A second application of importance is Differential GPS with submeter accuracy for car navigation. The DGPS data for submeter accuracy with good mobile reception utilising FM is under development, the 2 to 10 meter accuracy application is already running in the Netherlands. In both cases, the newspaper and DGPS, DAB seems to be the most suited way to transport the signal to the mobile receiver with respect to capacity and error-free reception. Some technical issues on interfacing, interoperability and interworking with other services, applications and networks still have to be solved, this mainly due to the premature stage of the DAB networks in Europe and other continents.

FUTURE DEVELOPMENTS

NOZEMA participates in several projects of the European ACTS programme. At the moment of writing this paper the EU still has to approve these projects, but one of the projects in which NOZEMA participates is MOTIVE (Multimedia Oriented and Television services In Vehicles). MOTIVE will develop system requirements and implement demonstrations through pilotnetworks for mobile reception of multimedia oriented and television services .
The distribution of these services to vehicles will use the DAB or DVB-T. The services can consume just a small amount of datacapacity (e.g. order-information for CD's and concerts) up to the full capacity for mobile television to vehicles via DAB.
In addition to this NOZEMA is a member of the Digital Audio-Visual Council, "DAVIC" (2). The purpose of this council is to promote emerging digital audio-visual applications and services, initially broadcast and interactive types. By securing the timely availability of international agreed specifications of open interfaces and protocols that maximise interoperability across countries and services. At the moment more than 125 companies are a member of this council. By participating in international development projects NOZEMA’s future started today.

REFERENCES

1. ITU-R special report on Digital Sound Broadcasting, Draft Version 2.0.
2. Statutes of the Digital Audio-Visual Council, DAVIC/108.