Service Information

Introduction

The DVB has prepared a specification for Sl to accompany broadcast DVB signals, intended to assist the receiver/decoder and the viewer to navigate through the array of services offered. (ETSI draft standard prETS 300 468 and associated Guidelines Document ETR 211.)

The data necessary for a receiver/decoder to configure itself automatically to decode the received bitstream is included in the Program Specific Information (PSI) which is specified by the MPEG-2 systems standard (ISO/IEC 13818-1). The DVB-SI specifies additional data which complements the PSI by providing data to aid automatic tuning of receiver/decoders and additional information intended for display to the user.

The DVB-SI describes the technical attributes of each service offered by a broadcaster and delivers information about present and future programme events which can be used to generate a simple ("Level-1") EPG in the receiver/decoder. It is based on a set of four mandatory data tables plus a series of optional tables. Each table contains descriptors outlining the characteristics of the programme services/programme events being described.

The four mandatory Sl tables are:

NIT The Network Information Table describes the delivery system parameters of a multiplex. The NIT for the actual delivery system currently delivering the transport stream is mandatory; it is also possible to send NlTs for other multiplexes and these are optional. In the terrestrial situation, we propose the use of NIT_other in two different ways:

  • to give information about other multiplexes transmitted from the same site (NIT_other_local).
  • to give information about the same multiplex transmitted from other sites, which may be receivable in overlap areas (NIT_other_global).

Networks are assigned individual network_id values which serve as unique identification codes for networks. Each NIT_actual must list:

  • Network_ID (ie the current network)
  • Original_Network_ID
  • Transport_Stream_ID
  • for I = 1 to n : Service_ID

NITs may contain the tuning information that can be used during the set-up of a receiver/decoder, including alternative frequencies for each unique multiplex.

SDT The Service Description Table lists the names and other parameters associated with each service referred to. Service descriptors for other transport streams are optional. The service_id is a unique identifier of a service within a transport stream. It is thought that the ITC will want to manage / regulate the allocation of Sevice IDs. The service_ID contains a Running Status field which, for example, coupled with a linkage descriptor, can be used to implement BBC opt-outs and may be used to describe the change from GMTV to ITV Companies on ITV1.

EIT The Event Information Table (EIT) is used to transmit information relating to the events (programmes) in the MPEG transport stream. It is mandatory to transmit an EIT for present-and-following events ("now-and-next") in the actual transport stream and optionally covers future events in the same transport stream. EITs can also be transmitted for events in other transport streams, thus allowing the viewer to select programmes on other services. EITs also contain a Running Status field which can be used to initiate a vcr record function shortly before a selected programme starts.

TDT The Time and Date Table is used to update the receiver/decoder's internal clock.

The optional tables are as follows:

Other NITs, SDT, EITs are used to give details of other multiplexes, services and events.

RST At a programme junction, the change of present and following information may be delayed due to the cycle time of the SI information. If required a Running Status Table may be used more accurately signal the status of an event, although this information is continually available from EIT_p/f.

BAT A Bouquet Association Table provides a means of grouping services to allow a service provider to point to other services on other multiplexes, including cable or satellite if desired. For example, Granada might use a bouquet association table on its terrestrial transmissions to point to GSkyB services on satellite. The use of multiple bouquets is allowed.

Particular Problems of Terrestrial

(a) Digital terrestrial television is fundamentally different to satellite in the large number of transmitters in a network. The SI received by the IRD has to take account of widespread overlap area conditions, such that an individual receive aerial installation may well offer a choice of frequencies for a network, which may or may not contain regional variations. It thus seems desirable for the SI for a particular main station to include information about adjoining regions which may be receivable by a particular aerial installation. In the design of the SI system, there is the twin risk of either not providing pointers to available multiplexes, or of pointing to multiplexes which are not available. The only solution is for the IRD to correlate SI information with its own local knowledge of available digital services derived during installation.

The Scheme Proposed for DTT in the UK

In course of preparation.