MPEG applications - The MPEG - page 215

Figure 7.2: There are a large number of conflicting demands in the design of a compression system. Here the
effect of a number of choices is shown to alter the balance between economy and bandwidth.
There are now many members of the MPEG family and it can be difficult to decide which type of MPEG coding to
use. Figure 7.2 shows some general guidelines. As a decoder becomes more complex, the bit rate which can be
used for a given quality will fall. However, this will be accompanied by an increase in power consumption at the
encoder and decoder as well as an increased delay. The designer will have to balance the cost of the bandwidth
with the cost of the encoding and decoding equipment.
7.2 Video phones
The telephone is a classic example of limited bandwidth. If there is a market for video phones, then compression is
the only way forward. The facial animation coding developed for MPEG-4 is an obvious contender as it allows a
reasonable reproduction of the caller's face with a data rate of only a few kilobits per second. This is considerably
less than the data rate needed for the speech. Video telephony is relatively easy between desk-based equipment,
where the camera can be mounted on top of the display, but it is not clear how this can be implemented in a
cellular telephone which is held against the user's ear. Another important issue for cellular telephones is the
amount of signal processing needed to decode and display facial animation data. Essentially the decoder is a real-
time rendering engine and this is only practicable in a portable product if the current consumption is within the
range of available battery technology
7.3 Digital television broadcasting
Digital television broadcasting relies on the combination of a number of fundamental technologies. These are:
MPEG-2 compression to reduce the bit rate, multiplexing to combine picture and sound data into a common
bitstream, digital modulation schemes to reduce the RF bandwidth needed by a given bit rate and error correction
to reduce the error statistics of the channel down to a value acceptable to MPEG data.
MPEG is a compression and multiplexing standard and does not specify how error correction should be performed.
Consequently a transmission standard must define a system which has to correct essentially all errors such that the
delivery mechanism is transparent.
DVB (digital video broadcasting) [ 1 ] is a standard which incorporates MPEG-2 picture and sound coding but which
also specifies all the additional steps needed to deliver an MPEG transport stream from one place to another. This
transport stream will consist of a number of elementary streams of video and audio, where the audio may be coded
according to MPEG audio standard or AC-3. In a system working within its capabilities, the picture and sound
quality will be determined only by the performance of the compression system and not by the RF transmission
channel. This is the fundamental difference between analog and digital broadcasting. In analog television
broadcasting, the picture quality may be limited by composite video encoding artifacts as well as transmission
artifacts such as noise and ghosting. In digital television broadcasting the picture quality is determined instead by
the compression artifacts and interlace artifacts if interlace has been retained.