Information Technology Reference
In-Depth Information
Figure 1.2:
(a) MPEG defines the protocol of the bitstream between encoder and decoder. The decoder is defined
by implication, the encoder is left very much to the designer. (b) This approach allows future encoders of better
performance to remain compatible with existing decoders. (c) This approach also allows an encoder to produce a
standard bitstream while its technical operation remains a commercial secret.
The MPEG standards give very little information regarding the structure and operation of the encoder. Provided the
bitstream is compliant, any coder construction will meet the standard, although some designs will give better picture
quality than others. Encoder construction is not revealed in the bitstream and manufacturers can supply encoders
using algorithms which are proprietary and their details do not need to be published. A useful result is that there
can be competition between different encoder designs which means that better designs can evolve. The user will
have greater choice because different levels of cost and complexity can exist in a range of coders yet a compliant
decoder will operate with them all.
MPEG is, however, much more than a compression scheme as it also standardizes the protocol and syntax under
which it is possible to combine or multiplex audio data with video data to produce a digital equivalent of a television
program. Many such programs can be combined in a single multiplex and MPEG defines the way in which such
multiplexes can be created and transported. The definitions include the metadata which decoders require to
demultiplex correctly and which users will need to locate programs of interest.
As with all video systems there is a requirement for synchronizing or genlocking and this is particularly complex
when a multiplex is assembled from many signals which are not necessarily synchronized to one another.
1.2 Why compression is necessary
Compression, bit rate reduction, data reduction and source coding are all terms which mean basically the same
thing in this context. In essence the same (or nearly the same) information is carried using a smaller quantity or
rate of data. It should be pointed out that in audio
compression
traditionally means a process in which the dynamic
range of the sound is reduced. In the context of MPEG the same word means that the bit rate is reduced, ideally
leaving the dynamics of the signal unchanged. Provided the context is clear, the two meanings can co-exist without
a great deal of confusion.
There are several reasons why compression techniques are popular:
(a) Compression extends the playing time of a given storage device.
