DVB Update: Service Information Martin Gold NDS

DVB SI Update: Overview What is Service Information? Features of DVB Service Information Specification Status and Developments

DVB SI Update: What is SI? Operates in conjunction with PSI (defined by MPEG): SI data provides information for: Automatic tuning to transport stream User Information for Service, “Event”, and “Component” selection PSI data provides information for: Configuration of decoder for selected Service DVB extensions for non-MPEG components DVB has added data on top of the PSI to complement it, so you get an environment which supports automatic tuning and information for user selection. So relative to the analogue environment where the user is very coupled to what is going on in the technical sense, choosing physical channels, and then if you want some program information you need to be aware there is a teletext system which you need to select. You go to that and select a particular page, which is all very primitive compared to what you can do with the PSI and SI data streams. They allow a pretty seamless environment for a user interface on a digital TV system. DVB extensions to the PSI have been defined for teletext and subtitling.

Data structured as several “Tables” DVB SI Features Data structured as several “Tables” Structures use “fixed format” for essential data, and descriptors for optional or variable-length data (similar to PSI) Efficient data transmission Extensible while maintaining compatibility Support for “private” extensions Can provide standard EPG data-stream “Look and Feel” determined by receiver software Resident or Downloaded We have items called tables. Combination of “Fixed format” and “variable length” descriptors. Fixed format - particular bytes in a table have a pre-determined meaning. The structure is particularly efficient as the definitions are already pre-known to the decoder. The variable length descriptors are a sequence known as TLV. Tag Length Value. Where the tag identifies the data that is to follow, the length indicates the length of the data. You have a very flexible format with a sequence of descriptors which allows you to carry almost anything, which is extremely useful when you want flexibility, but obviously not as efficient as fixed format because of the overhead of the T and the L instead of just the V. The combination is also used for PSI transmission. The descriptors give the right combination of efficiency and we use descriptors to achieve extensibility. It also gives support for additions to the specification whether defined by DVB or in particular implementations which add their own extensions without interfering with the basic specification. DVB-SI can be regarded as a standard EPG format, in terms of the transmitted data but the presentation, look and feel is determined by the receiver software. That software could be a fixed or temporary application downloaded as part of the broadcasters bitstream. This allows particular broadcasters can have their own look and feel for the EPG.

DVB SI Model Satellite Cable Terrestrial Networks Bouquet Transponder 1 2 3 T Channel C Service S Video Audio 1 Audio 2 Data Bouquet Networks Transport Streams Services Components To describe the model for DVB Service Information we have a fairly straightforward hierarchy here. The networks which are the physical delivery media, which typically provides you several transport streams. Within a transport stream you have one or more services and the service it’s self is mad up of several components. If it is a TV service it is at least Audio and Video, you may have several audio components for different languages, you may have subtitling on top of the video or data components. You can view the services in the time domain when they are made up of a sequence of events for the individual TV programs.

Network Information Table SI Features: NIT Network Information Table Identification of transmission as a member of a group of multiplexes - “Network” Network Name Tuning parameters with support for various delivery media List of additional frequencies for terrestrial transmission Designed for simple transcoding of transport streams This provides the description of the transport streams that is carried by the physical media. This is known as a Network. The user can select the Network name. A special challenge for terrestrial media is that you typically have a number of signals carrying the same transmission by virtue of relay transmissions. To allow this the specification added a list of additional frequencies. NTI designed to allow moving the transport stream from one delivery medium to another, like from satellite to cable or terrestrial to cable. The NIT is relatively small and is easy to replace in the transport stream, without having to modify the whole transport stream. That is an operation you can do on a transport stream at relatively low cost.

Service Description Table SI Features: SDT & BAT Service Description Table Identifies Service names and Service types Linked Services Pointer to MPEG Program in PSI Service_id = MPEG Program Number Bouquet Association Table (Optional) Groupings of Services May convey “logical channel number” SDT Linkages between services can be identified. If one service is a part time service then when it stops running you can identify another service that would be good for the receiver to select automatically. BAT This is optional. You do not have to transmit it but in some situations it could be useful. Logical channel numbers This is a way of identifying particular service by a number which is the way we view analogue services today. When DVB developed the Service Information specification they very much thought there would be so many channels, you would never remember the numbers, and obviously you would want to select them by the service names. In practice where every system has been implemented it actually makes use of logical channel numbers.

SI Features: EIT Present/Following Schedule (optional) Information on current and next events Schedule (optional) Up to 64 days ahead - ordered by service and time Event Information Title, short description Start time & duration Content classification & parental rating Longer text description Information on components The EIT comes in two flavors. The present / following information which tells you what is on now and what is on next The EIT Schedule which allows you to give the information for further ahead.

SI Features: TDT,TOT, RST Time and Date Table Transmission of current time for automatic setting of receiver clock Time Offset Table (optional) Transmission of time offset by zone - both current, and next offset values, with date at which next occurs Running Status Table (optional) Mechanism for signalling status transitions with greater timing precision The transmission of the time and date is done in UTC, and the SI event times are also done in UTC which avoids the complications and confusions which always arise in software at daylight saving transitions. Daylight savings transitions are handled by the Time Offset Table. The RST is probably a rather futuristic table, with current implementations not having the speed of operation in the decoder to take advantage of the greater timing precision. It is rather difficult to generate a data stream that achieves the greater timing accuracy that this could offer.

Example EPG using DVB SI As an illustration this is a screen shot of an electronic program guide (EPG) implemented entirely using DVB-SI. We have services down the side and time across the page. The services are identified by their service name and also here by the logical channel number. The event name is displayed from the EIT, with a sequence of event names with the start time and duration reflected by their position relative to the top scale. The current time is shown, taken from the TDT. Where the item does not fit in the space, it is not part of the transmission to determine how you truncate it. The receiver software determines this which is part of the look and feel of the box. What you see on the screen from a different implementation might have more or less time, a different number of channels or the whole appearance could be completely different. The basic information is all conveyed by the DVB-SI specification.

DVB SI Specification Status October 95: Edition 1 ETS 300 468 January 97: Edition 2 ETS 300 468, added Time Offset Table Multilingual and Unicode Support Extension to content description Support for DVB Subtitling Incorporated from SI Guidelines Private Data Specifier Mechanism Service Move Service bit-rate signalling One of the key issues for additions to the specification was achieving compatibility with earlier editions. This was made quite straight forward to do because of the sensible format of the SI structures.

DVB SI Specification Status (2) January 98: V1.3.1 EN 300 468, added Support for Terrestrial transmission SI for recorded “partial” bitstreams Support for Data Broadcasting Agreed additions for next edition (V1.4.1) HD Video component description Support for AC-3 audio Use of Transport Stream Description Table PSI descriptor for Data Broadcasting Support for 6MHz terrestrial channels Support for VBI data extensions