MPEG-2 Systems 1. What is MPEG-2 Systems? 1. What is MPEG-2 Systems?MPEG-2 Systems is an ISO/IEC standard (13818-1) that defines the syntax and semantics of bitstreams in which digital audio and visual data are multiplexed. Such bitstreams are said to be MPEG-2 Systems compliant. This specification does not mandate, however, how equipment that produces, transmits, or decodes such bitstreams should be designed. As a result, the specification can be used in a diverse array of environments, including local storage, broadcast (terrestrial and satellite), as well as interactive environments. 2. Who developed this standard?This standard was industry driven, and complemented the MPEG-2 activities in audio and video coding. The consumer TV industry actively participated in the definition of MPEG-2 Systems, to ensure that a low-complexity receiver can be built at a reasonable cost. 3. Which industries have adopted this standard?The list is extensive and continuously growing: consumer TV (cable, satellite and terrestrial broadcast), Video on Demand, Digital Video Disc, personal computing, card payment, test and measurement, etc. 4. Which standard bodies have adopted this standard?In Europe, DVB (Digital Video Broadcast). In the USA, FCC (Federal Communications Commission), ATSC, SCTE. In Japan, MITI/JISC. The DAVIC consortium. DVD. 5. Why do I need to be MPEG-2 Systems compliant?In the design of equipment, you have to be MPEG-2 Systems compliant for several reasons. First, if your equipment has to be compliant with DVB, FCC, JISC, ATSC, SCTE, DVD, or DAVIC requirements, as these bodies require MPEG-2 Systems compliance. Secondly, because your design can rely on Integrated Circuits developed in that area. Finally, your application will be opened to all the MPEG-2 Systems world, and will be usable over a large number of networks. The MPEG-2 Systems standard enables the widest interoperability in digital video and audio applications and services. 6. Is there a reference implementation?Yes, reference bitstreams are described in the MPEG-2 document TR 13813-5. 7. What are the different MPEG-2 Systems components?MPEG-2 Systems provides a two layer multiplexing approach. The first layer is dedicated to ensure tight synchronisation between video and audio. It is a common way for presenting all the different materials which require synchronisation (video, audio, and private data). This layer is called Packetized Elementary Stream (PES). The second layer is dependent on the intended communication medium. The specification for error free environments such as local storage is called MPEG-2 Program Stream, while the specification addressing error prone environments is called MPEG-2 Transport Stream. 8. What is the difference between MPEG-1 Systems and MPEG-2 Systems?MPEG-2 Systems mandated compatibility with MPEG-1 Systems. The MPEG-2 Program Stream is designed for that purpose. MPEG-2 Systems also addresses error prone environments, and provides all the hooks for Conditional Access systems. 9. What is the difference between an MPEG-1 Systems stream and an MPEG-2 Systems Program stream?The major difference lies on the signalling which is present in MPEG-2 Program Streams and was absent in MPEG-1 Systems. A minor difference also exists in the PES format. 10. Is the MPEG-2 Transport Stream a transport multiplex?No, MPEG-2 transport is rather a service multiplex. No mechanism, within the syntax, exists to ensure the reliable delivery of the transported data. MPEG-2 transport relies on underlying layers for such services. MPEG-2 transport requires the underlying layer to identify the transport packets and to indicate in the transport packet header, when a transport packet has been erroneously transmitted. The MPEG-2 Transport Stream is so named to signify that it is the input to the Transport layer in the OSI seven layer network model. It is not, in itself, the Transport layer. 11. What is carried in MPEG-2 Transport Streams?MPEG-2 Transport Streams carry transport packets. These packets carry two types of information: the compressed material and the associated signalling tables. A transport packet is identified by its PID (Packet Identifier). Each PID is assigned to carry data belonging either to one particular compressed data source (and only this data source) or one particular signalling table. The ordered sequence of packets with a given PID may be considered as one data stream. The compressed data source may be derived from either video, audio or data elementary streams. These elementary streams may be tightly synchronized (as it is usually necessary for Digital TV programs, or for Digital Radio programs), or not synchronised (in the case of programs offering downloading of software or games, as an example). The associated signalling tables consist of the description of the elementary streams which are combined to build programs, and in the description of those programs. Tables are carried in sections. The signalling tables are called PSI (Program Specific Information). 12. Why are Transport packets 188 bytes long?Because MPEG-2 wanted these packets to be carried over ATM. At that time, according to the AAL which was envisaged, ATM cells were supposed to have a payload of 47 bytes. 188 = 4 * 47. 13. What about the programs carried within the MPEG-2 Transport Stream?There is a description of each program carried within the MPEG-2 Transport Stream. This description usually requires a particular table, the Program Map Table, with one table per program. This table is only sent periodically. On the other hand, the elementary streams which make up a program are continuously carried in PES streams. In that sense it could be said that an MPEG-2 Transport Stream does not carry programs, but only carries elementary streams and the instructions required to associate particular elementary streams into particular programs. 14. Why do so many application use MPEG-2?One of the attractive aspects of MPEG-2 comes from its fundamental requirement to be generic. Another reason is that in all the syntax and signalling provisions are made to allow applications to develop their own private syntaxes and signalling. A lot of private needs may be satisfied. 15. What is the main assumption made by MPEG-2 Systems?That the network is ideal, and that each byte is transmitted with a constant delay. 16. What is a syntax?Generally speaking, a syntax specifies the structure of a bitstream: how different parameters, tags, etc., are mapped and laid out on the bitstream. For multiplexing purposes, it is important for the syntax to provide patterns which can be recognized with an extremely high degree of confidence. These patterns are called synchronisation patterns. In addition, an indication of time and of the bit rate of the bitstream may also be provided. Equipped with such elements, a bitstream corresponding to an MPEG-2 syntax is a self contained bitstream on which a receiver can slave itself, in order to acquire that bitstream exactly synchronised with the production of that bitstream. However, time indication and bit rate indication are not mandatory. 17. What is a time stamp?They are two types of time stamps:
18. Are time stamps mandatory?No, they are not mandatory. Some applications like Digital TV broadcast, where tight synchronisation is required, will make an extensive use of time stamps. In that case both reference time stamp and DTS/PTS are used. In other cases (game or software downloading for example) neither reference nor DTS/PTS time stamps are necessary. DTS and PTS time stamps are not relevant if reference time stamps are not present. 19. Where are the PTSs and DTSs inserted?They are inserted as close as possible to the portions of compressed video, audio, or data to which they apply. Precisely, this means that they are inserted in the PES packet headers, a syntax which is common to all data sources. 20. What is PSI?PSI (Program Specific Information) carries the signalling. PSI has no synchronisation pattern in the section headers. 21. What is the difference between a PES packet and a PSI section?A PES packet is a way to uniformly packetize elementary streams. Embedded in PES packets, elementary streams may be synchronized with time stamps. They are not protected. The PES packets may be of variable length, which allows them to be also of fixed length. PES packets may be rather long packets. As elementary streams are continuous streams, it is also possible to know that a PES packet is finished because the next PES packet arrives. Therefore, sometimes a length indication is not even relevant (for video PES packets). A PSI section is a way to carry a portion of a PSI table. A PSI section is a way to uniformly represent signalling. PSI sections are not synchronized. They may be protected by a CRC. The sections are of a variable length. They are rather small. The length is always relevant. It is the only mechanism to go from one section to the next section when they are carried in the same packet. An update mechanism is also supported which allows association of a version number to a section. 22. What will be found embedded in a PES packet?Continuous streams are to be found within the PES packets: They consist of either video, audio or data material. The video may be of different kinds (MPEG-1, MPEG-2), and the same for audio. There is no assumption about elementary data streams. One of the first uses is for subtitling. 23. What will be found in PSI sections?Signalling is carried in PSI sections. Conditional Access messages are usually also carried in PSI sections. Downloading of data will almost certainly use the PSI section mechanism. 24. Did MPEG-2 do research on Conditional Access methods?MPEG-2 only provided hooks for the Conditional Access systems:
25. Why is PSI information not synchronised?Because tight synchronisation between signalling and elementary streams was not required. It is enough, generally speaking, to signal an event a little in advance. In some cases, however, that makes dynamic changes quite tricky, especially when elementary streams are scrambled, or when a program changes from an "in the clear" state to a scrambled state. 26. Why is Video or Audio material not protected at MPEG-2 systems level?The error concealment techniques are implemented within the audio and the video layers. For error protection MPEG-2 Systems relies on the underlying layer to deliver transport packets with a BER rate around 10-10. 27. Is the MPEG-2 transport stream a two or a three level multiplex?It is a two layer multiplex, as the pure audio and video material are first packetized in PES packets and secondly packetized in transport packets. Transport packets are multiplexed. It is not a three level multiplex as there is no packetization related to programs. 28. Is the MPEG-2 program stream a two or a three level multiplex?It is a one layer multiplex, as the pure audio and video material are only packetized in PES packets. PES packets are then multiplexed. It is not a two level multiplex as there is no packetization related to the carried program. 29. What is the use of NULL packets?They are usually used as a provision for rate stuffing in order to avoid the bottleneck of insufficient resources. Usually transport packets have to be declared in the PSI information tables. A NULL packet is a particular undeclared transport packet that belongs to nobody. Its payload is undefined. Some applications use NULL packets in order to insure a good and quick synchronization mechanism, as their modulation scheme is not aligned on the start of transport packets. 30. Is the bitstream rate always explicitly carried?Yes for the PES syntax and the MPEG-2 program syntax. No for the MPEG-2 transport syntax. For that syntax, the transport rate may appear in the PSI information. 31. How do DVB and ATSC use MPEG-2 transport?In defining their own operational rules and implementation guidelines. They specified also their own signalling (Service Information) using the already defined MPEG-2 private sections. Modulation schemes adapted to satellite, to cable and to terrestrial broadcast have been adopted. Physical interfaces between equipment have been specified. Parameters, relevant for real-time and offline measurements, have been specified. 32. Are there any error detection mechanisms?Two CRCs are to be found:
That is why sections have to be short, in order for the CRC to be efficient. 33. What is a system target decoder model?A system target decoder (STD) model is a virtual decoder. There are two models, one within the MPEG-2 program syntax (the P-STD), the other within the MPEG-2 transport syntax (The T-STD). A model defines buffer sizes, their input and output rates, and timing constraints related to time stamps values. 34. Why have the STD models been invented?Not to be implementation dependent. The first model comes from MPEG-1 Systems. Some of the assumptions in the T-STD are even not realistic at all: buffers, for instance, when decoding occurs are supposed to be emptied instantaneously. 35. What are the constraints imposed by the T-STD model?They apply to different elements:
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