Information Technology Reference
In-Depth Information
As an alternative,
Figure 1.12
(b) shows that by coding only the lower spatial frequencies in a HDTV picture a base
bitstream can be made which an SDTV receiver can decode. If the lower definition picture is locally decoded and
subtracted from the original picture, a 'definition- enhancing' picture would result. This can be coded into a helper
signal. A suitable decoder could combine the main and helper signals to re- create the HDTV picture. This is the
principle of spatial scaleability.
The High Profile supports both SNR and spatial scaleability as well as allowing the option of 4:2:2 sampling (see
Section 2.11
).
The 4:2:2 Profile has been developed for improved compatibility with existing digital television production
equipment. This allows 4:2:2 working without requiring the additional complexity of using the High Profile. For
example a HP ML decoder must support SNR scaleability which is not a requirement for production.
MPEG-2 increased the number of audio channels possible to five whilst remaining compatible with MPEG-1 audio.
MPEG-2 subsequently introduced a more efficient audio coding scheme known as MPEG-2 AAC (advanced audio
coding) which is not backwards compatible with the earlier audio coding schemes.
1.10 Introduction to MPEG-4
MPEG-4 introduces a number of new coding tools as shown in
Figure 1.13
. In MPEG-1 and MPEG-2 the motion
compensation is based on regular fixed-size areas of image known as
macroblocks
. Whilst this works well at the
designed bit rates, there will always be some inefficiency due to real moving objects failing to align with macroblock
boundaries. This will increase the residual bit rate. In MPEG-4, moving objects can be coded as arbitrary shapes.
Figure 1.14
shows that a background can be coded quite independently from objects in front of it. Object motion
can then be described with vectors and much-reduced residual data. According to the Profile, objects may be two-
dimensional, three- dimensional and opaque or translucent. The decoder must contain effectively a layering vision
mixer which is capable of prioritizing image data as a function of how close it is to the viewer. The picture coding of
MPEG-4 is known as texture coding and is more advanced than the MPEG-2 equivalent, using more lossless
predictive coding for pixel values, coefficients and vectors.
Figure 1.13:
MPEG-4 introduces a number of new coding tools over those of earlier MPEG standards. These
include object coding, mesh coding, still picture coding and face and body animation.
Figure 1.14:
(a) In MPEG-1 and MPEG-2, computer graphic images must be rendered to video before coding. (b)
