13.2 – MPEG-2

MPEG-2 was a more complex beast to deal with. A digitised TV channel can yield 20-24 Mbit/s, depending on the delivery system (terrestrial/satellite broadcasting or cable TV). Digital stereo audio can take 0.2 Mbit/s and standard resolution 4 Mbit/s (say a little less with more compression). Audio could be multichannel (say, 5.1) and hopefully consume less bitrate for a total bitrate of a TV program of 4 Mbit/s. Hence the bandwidth taken by an analogue TV program can be used for 5-6 digital TV programs.

Figure  52 – Model of the MPEG-2 standard

The fact that digital TV programs part of a multiplex may come from independent sources and that digital channels in the real world are subject to errors force the design of an entirely different Systems layer for MPEG-2. The fact that users need to access other data sent in a carousel, that in an interactive scenario (with a return channel) there is a need for session management and that a user may interact with a server forced MPEG to add a new stream for user-to-network and user-to-user protocols.

In conclusion the MPEG-2 model is a natural extension of the MPEG-1 model (superficially, the DSM-CC line, but the impact is more pervasive).

The official title of MPEG-2 is Generic coding of moving pictures and associated audio information. It was originally intended for coding of standard definition television (MPEG-3 was expected to deal with coding of High Definition Television). As the work progressed, however, it became clear that a single format for both standard and high definition was not only desirable but possible. Therefore, the MPEG-3 project never took off.

The standard is not specific of a video resolution (this was already the case for MPEG-1 Video) but rationalises the notion of profiles, i.e. assemblies of coding tools and levels a notion that applies to, say, resolution, bitrate etc. Profiles and levels have subsequently adopted in most MPEG standardisation areas.

The standard is composed of 10 parts, some of which are

  • Part 1 – Systems specifies the Systems layer to enable the transport of a multichannel digital TV stream on a variety of delivery media (link)
  • Part 2 – Video specifies the video coding algorithm. Video is interlaced and may have a wide range of resolutions with support to scalability and multiview in appropriate profiles (link)
  • Part 3 – Audio specifies a MPEG-1 Audio backward-compatible multichannel audio coding algorithm. This means that an MPEG-1 Audio decoder is capable of extracting and decoding an MPEG-1 Audio bitstream (link)
  • Part 6 – Extensions for DSM-CC specifies User-to-User and User-to-Network protocols for both broadcasting and interactive applications. For instance DSM-CC can be used to enable such functionalities as carousel or session set up (link)
  • Part 7 – Advanced Audio Coding (AAC) specifies a non-backward compatible multichannel audio coding algorithm. This was done because backward compatibility imposes too big a penalty for some applications, e.g. those that do not need backward compatibility (link), the first time MPEG was forced to develop two standards for apparently the same applications.

 

Table of contents 13.1 MPEG-1 13.3 MPEG-4