MPEG applications - The MPEG - page 214

Chapter 7: MPEG applications
7.1 Introduction
It should be borne in mind that however exciting it may be, MPEG is only a technology. Technology per se is not
useful and only becomes useful when it is incorporated into an appropriate and affordable product or service. If the
use is not appropriate or affordable, there is no point in blaming the technology.
In this chapter a number of useful applications of MPEG are outlined. In each case designers have followed the
rules of high technology to design systems which maximize the potential of the technology whilst protecting against
its weaknesses.
MPEG is an information technology. It cannot deliver anything physical: you get the movie but no popcorn. The
strength of MPEG is that it delivers audio-visual material with economy of bandwidth. Its weaknesses are sensitivity
to bit errors and the requirement for near-real- time transmission. The variable-length coding of MPEG can lose
synchronization in the presence of a single bit error. Decoders can take a considerable time to recover from a
buffer underflow or overflow.
Wherever bandwidth or storage capacity is in short supply, MPEG is useful. Where bandwidth is plentiful, as in
optical fibre networks, MPEG is pointless. As the capacity and economics of storage devices continue to increase,
MPEG becomes less important.
MPEG will remain important in any area where there will be a permanent shortage of bandwidth; this includes any
mobile equipment or service which must use radio communication.
Figure 7.1 shows that well-engineered applications of MPEG will always include an error-correction system
appropriate for the characteristics of the channel and suitable mechanisms to ensure that decoder buffers neither
overflow nor underflow. Constant-bit rate channels having constant delay are easy to implement because the
buffering mechanisms of the standard MPEG encoders and decoders are able to operate with no further
assistance. Digital TV transmitters provide essentially constant bit-rate because this is related to the standardized
(and regulated) channel bandwidth allowed.
Figure 7.1: Any real-time MPEG delivery system must include means to maintain a low error rate and buffering to
deliver data within the decoder's time window.
In packet networks the delivery delay is not constant because other types of messages are multiplexed with the
audio-visual message of interest. Variable delay channels cause MPEG decoders difficulty. Not only is there a
greater danger of buffer overflow or underflow, but the program clock reference information is no longer correct if a
packet is received with a time shift. In this case additional buffering is needed at both ends of the channel so that
the variable delay characteristic of the channel is invisible to the MPEG layer.