Information Technology Reference
In-Depth Information
MPEG-1 data. In a sense an MPEG-1 bitstream is an MPEG-2 bitstream which has a restricted vocabulary and so
can be readily understood by an MPEG-2 decoder.
MPEG-2 has too many applications to solve with a single standard and so it is subdivided into Profiles and Levels.
Put simply a Profile describes a degree of complexity whereas a Level describes the picture size or resolution
which goes with that Profile. Not all Levels are supported at all Profiles.
Figure 1.11
shows the available
combinations. In principle there are twenty-four of these, but not all have been defined. An MPEG-2 decoder
having a given Profile and Level must also be able to decode lower Profiles and Levels.
Figure 1.11:
Profiles and Levels in MPEG-2. See text for details.
The simple Profile does not support bidirectional coding and so only
I
and
P
pictures will be output. This reduces
the coding and decoding delay and allows simpler hardware. The simple Profile has only been defined at Main
Level (SP ML).
The Main Profile is designed for a large proportion of uses. The Low Level uses a low resolution input having only
352 pixels per line. The majority of broadcast applications will require the MP ML (Main Profile at Main Level)
subset of MPEG which supports SDTV (standard definition television). The High-1440 Level is a high-definition
scheme which doubles the definition compared to Main Level. The High Level not only doubles the resolution but
maintains that resolution with 16:9 format by increasing the number of horizontal samples from 1440 to 1920.
In compression systems using spatial transforms and requantizing it is possible to produce scaleable signals. A
scaleable process is one in which the input results in a main signal and a 'helper' signal. The main signal can be
decoded alone to give a picture of a certain quality, but if the information from the helper signal is added some
aspect of the quality can be improved.
Figure 1.12
(a) shows that in a conventional MPEG coder, by heavily requantizing coefficients a picture with
moderate signal-to-noise ratio results. If, however, that picture is locally decoded and subtracted pixel by pixel from
the original, a 'quantizing noise' picture would result. This can be compressed and transmitted as the helper signal.
A simple decoder only decodes the main 'noisy' bitstream, but a more complex decoder can decode both
bitstreams and combine them to produce a low- noise picture. This is the principle of SNR scaleability.
Figure 1.12:
(a) An SNR scaleable encoder produces a 'noisy' signal and a noise cancelling signal. (b) A spatially
scaleable encoder produces a low-resolution picture and a resolution-enhancing picture.
