The scalability tools specified by MPEG II are designed to support applications beyond that supported by single layer video. In a scaleable video coding, it is assumed that given an encoded bitstream, decoders of various complexities can decode and display appropriate reproductions of coded video. The basic scalability tools offered are: data partitioning, SNR scalability, spatial scalability and temporal scalability. Combinations of these basic scalability tools are also supported and are referred to as hybrid scalability. In the case of basic scalability, two layers of video referred to as the lower layer and the enhancement layer are allowed. Whereas in hybrid scalability up to three layers are supported.
MPEG Extensions include:
This involves generating two spatial resolution video layers from a single video source such that the lower layer is coded by itself to provide the basic spatial resolution and the enhancement layer employs the spatially interpolated lower layer and carries the full spatial resolution of the input video source.
This involves generating two video layers of same spatial resolution but different video qualities from a single video source. The lower layer is coded by itself to provide the basic video quality and the enhancement layer is coded to enhance the lower layer. The enhancement layer when added back to the lower layer regenerates a higher quality reproduction of the input video.
This involves generating two video layers whereas the lower one is encoded by itself to provide the basic temporal rate and the enhancement layer is coded with temporal prediction with respect to the lower layer. These layers when decoded and temporally multiplexed yield full temporal resolution of the video source.
This involves the partitioning of the video coded bitstream into two parts. One part will carry the more critical parts of the bitstream such as headers, motion vectors and DC coefficients; the other part will carry less critical data such as the higher DCT coefficients.
Profiles and levels provide a means of defining subsets of the syntax and semantics and thereby the decoder capabilities to decode a certain stream. A profile is a defined sub-set of the entire bitstream syntax that is defined by MPEG II. A level is a defined set of constraints imposed on parameters in the bit stream.
To constrain the choice pragmatically, five MPEG II profiles are defined and four levels of quality. However, It is important to realize specification is of an encoded stream rather than of an actual compression and decompression mechanism. This leaves lots of options open to the implementor Profiles allow us to scope these choices (as in other standards, e.g. in telecommunications) This is important, as the hard work (expensive end) is the encoder, while the stream as specified, is generally easy however it Is implemented, to decode. The diagram below (4.16) shows a comparison of the data rate out of an H.261 and an MPEG coder.
MPEG II is now an ISO standard. Due to the forward and backward temporal compression used by MPEG, a better compression and better quality can be produced. As MPEG does not limit the picture resolution, high resolution data can still be compressed using MPEG. The scaleable extensions defined by MPEG can map neatly on the hierarchical scheme explained in 2.1. The out-of- order processing which occurs in both encoding and decoding side can introduce considerable latencies. This is undesirable in video telephony and video conferencing.
Prices for hardware MPEG encoders are quite expensive at the moment though this should change over the near future. The new SunVideo board (see below) does support MPEG I encoding. Software implementation of MPEG I DECoders are already available.
