· MPEG-1: "Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbit/s"
· MPEG-1+: MPEG-1 at MPEG-2 (CCIR) resolution. It has a higher bit-rate and enhanced picture quality than MPEG-1.
· MPEG-2: "Generic Coding of Moving Pictures and Associated Audio"
· MPEG-3: no longer exists (it has been incorporated within MPEG-2)
· MPEG-4: "Very Low Bitrate Audio-Visual Coding"

MPEG-1: ISO/ IEC 11172 Information technology "Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbit/s"

The MPEG-1 standard consist of five parts [Gadegast, 1995]:

Part 1: Systems
This defines the syntax for the synchronisation of audio and video bitstreams over digital channels. It also defines the syntax for transporting packets of combined audio/video bitstreams over the same channels.

Part 2: Video
This defines the syntax (header and bitstream elements) and describes the semantics for a compressed bitstream. The bitstream is comprised of a series of nested layers; sequence, picture, slice, macroblock, and block (see section 2.7.4.3). By defining a compressed bitstream, a decompressor is implicitly defined. Compression tools are suggested by MPEG and algorithms are presented for each layer which, when used in combination, achieve efficient compression. These algorithms do not have to be used; providing that the result is a compliant bitstream, proprietary methods may be developed separately and used in the encoding process. The syntax also provides methods for aiding synchronisation, random access, buffer regulation and error recovery, all of which are functions that are valuable to the correct decoding of the audio and video signals.

Part 3: Audio
This defines the syntax and semantics for three compression methods, known as layers I, II and III. Each layer varies in the coding complexity and coding efficiency it can achieve. Layer I is the simplest, a sub-band coder with a psycho-acoustic model, Layer II adds more advanced bit allocation techniques and greater accuracy, and Layer III adds a hybrid filterbank and non-uniform quantisation [Crowcroft, 1995]. With improving quality/compression ratio comes increased demands for processing power, which means that cheaper systems can be brought out initially until market penetration increases and then upgrades can be offered. 2-D images have much more redundancy than a speech signal and therefore yield much higher compression ratios [Grant, 1994]. This results in the audio part of an MPEG stream being a significant (up to 364kbit/s) part of the total bitrate.

Part 4: Conformance
This defines what exactly is required to be MPEG compliant. It also defines two sets of test guidelines to ensure that a given bitstream is MPEG compliant in all three parts (system, video and audio). Encoder compliance is not addressed.

Part 5: Software Simulation
Provides an example software encoder and MPEG compliant decoder for video and audio. The example software is in ANSI C. Also included is software to enable system streams to be constructed from video and audio streams held in storage, and also to separate out these separate video/audio streams from a system stream.

MPEG-2: ISO/ IEC 13818 Information Technology "Generic Coding of Moving Pictures and Associated Audio"

The MPEG-2 standard consists of the five above plus four additional parts [Gadegast, 1995]:

Part 6: Digital Storage Medium Command and Control ( DSM-CC)
This defines the syntax for emulation of VCR controls and random access to bitstreams stored using digital storage mediums, such as CD-ROM's. VCR-like commands include still frame, fast forward, advance and goto. Having VCR-like commands is almost a necessity in order that this new technology is widely accepted. The more people that can use video on demand systems, the greater the potential market penetration. Also there is a desire amongst service providers [Livingstone, 1996] to provide continuity in their user interfaces and to try and avoid the interface feeling too ‘artificial’.

Part 7: Non-Backwards Compatible Audio (NBC)
This defines the syntax for including discrete multichannel surround-sound audio within a compressed bitstream. The additional channels are coded as ancillary data to the earlier MPEG-1 'left' and 'right' channels, allowing exising MPEG-1 decoders to recover the two main channels, ignore the rest and as a result be compatible with the new standards. The sampling frequencies and audio channels available in MPEG-1 and MPEG-2 are compared in table 4.

Standard	MPEG-1	MPEG-2
No. of audio channels	2 (left/right)	5 (left/right, centre, and two surround).
Audio sampling frequencies supported	32kHz, 44.1kHz (CD), 48kHz	16kHz, 22.05kHz, 24kHz, 32kHz, 44.1kHz (CD), 48kHz

Table 5. Comparison of MPEG-1 and MPEG-2 audio capabilities.

Part 8: 10-bit video extension
This defines the syntax and semantics for compressing video with sample precision of 10 bits. This increased quality is intended to be used for studio applications (distribution, editing & archiving). This extension is not expected until 1997 or 1998 [Gadegast, 1995].

Part 9 Real-time Interface (RTI)
This defines the syntax for video on demand control signals between set-top boxes and head-end servers. These signals have very low bandwidth requirements, therefore the transfer rate from head-end (service provider) to subscriber-end (known as the downstream transfer rate) is very much greater than the transfer rate in the opposite direction (the upstream rate). This allows the use of asymmetric technologies such as ADSL (see section 3.4.2).

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