Introduction to compression - The MPEG - page 1

Chapter 1: Introduction to compression
1.1 What is MPEG?
MPEG is actually an acronym for the Moving Pictures Experts Group which was formed by the ISO (International
Standards Organization) to set standards for audio and video compression and transmission.
Compression is summarized in Figure 1.1 . It will be seen in (a) that the data rate is reduced at source by the
compressor . The compressed data are then passed through a communication channel and returned to the original
rate by the expander . The ratio between the source data rate and the channel data rate is called the compression
factor . The term coding gain is also used. Sometimes a compressor and expander in series are referred to as a
compander . The compressor may equally well be referred to as a coder and the expander a decoder in which case
the tandem pair may be called a codec .
Figure 1.1: In (a) a compression system consists of compressor or coder, a transmission channel and a matching
expander or decoder. The combination of coder and decoder is known as a codec. (b) MPEG is asymmetrical since
the encoder is much more complex than the decoder.
Where the encoder is more complex than the decoder, the system is said to be asymmetrical. Figure 1.1 (b) shows
that MPEG works in this way. The encoder needs to be algorithmic or adaptive whereas the decoder is 'dumb' and
carries out fixed actions. This is advantageous in applications such as broadcasting where the number of
expensive complex encoders is small but the number of simple inexpensive decoders is large. In point-to-point
applications the advantage of asymmetrical coding is not so great.
The approach of the ISO to standardization in MPEG is novel because it is not the encoder which is standardized.
Figure 1.2 (a) shows that instead the way in which a decoder shall interpret the bitstream is defined. A decoder
which can successfully interpret the bitstream is said to be compliant . Figure 1.2 (b) shows that the advantage of
standardizing the decoder is that over time encoding algorithms can improve yet compliant decoders will continue
to function with them.
It should be noted that a compliant decoder must correctly be able to interpret every allowable bitstream, whereas
an encoder which produces a restricted subset of the possible codes can still be compliant.