Page 52 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 2.6. Intraframe Coding 29

s e e · e · s ˆ
Symbol Symbol
+ Quantizer +
- encoder decoder
~ ~
s s ˆ s
Predictor + Predictor

(a) Encoder (b) Decoder
Figure 2.8: Block diagram of a predictive coding system

signal and the more eGcient the coding system. At the decoder, the same
prediction is produced using previously decoded pels, and the received error
signal is added to reconstruct the current pel. A block diagram of a predic-
tive coding system is depicted in Figure 2.8. Predictive coding is commonly
referred to as di)erential pulse code modulation (DPCM). A special case of
this method is delta modulation (DM), which quantizes the error signal using
two quantization levels only.
Predictive coding can take many forms, depending on the design of the
predictor and the quantizer blocks. The predictor can use a linear or a nonlinear
function of the previously decoded pels, it can be 1-D (using pels from the
same line) or 2-D (using pels from the same line and from previous lines), and
it can be xed or adaptive. The quantizer also can be uniform or nonuniform,
and it can be xed or adaptive.
The minimal storage and processing requirements were partly responsible
for the early popularity of this method, when storage and processing devices
were scarce and expensive resources. The method, however, provides only
a modest amount of compression. In addition, its performance is highly de-
pendent on the statistics of the input data, and it is very sensitive to errors
(feedback through the prediction loop can cause error propagation). As pro-
cessing and storage devices became more available, more complex, more eG-
cient methods like transform coding have become more popular. Despite this,
predictive coding is still used in video coding, as, for example, in the lossless
coding of motion vectors.

2.6.2 Transform Coding

Transform coding, developed more than two decades ago, has proven to be
a very e6ective video coding method. Today, it forms the basis of almost all
video coding standards. Figure 2.9 shows a block diagram of a typical trans-
form coding system. The input frame is rst segmented into N × N blocks.