Page 238 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 9.6. Forward Techniques 215

moving
object in the
frame

resynchronization
codeword

spatial domain bit domain
(a) Resynchronization codewords at regular intervals in the spatial-domain

spatial domain bit domain
(b) Resynchronization codewords at regular intervals in the bit-domain

Figure 9.4: Resynchronisation codewords at regular intervals

9.6.3.2 The Error-Resilience Entropy Code (EREC)
An interesting alternative to inserting resynchronisation codewords is the error
resilience entropy code (EREC) [177, 185]. The EREC takes variable-length
blocks of data and rearranges them into xed-length slots. For example, assume
that there are N variable-length blocks with lengths b i ;i =1 :::N. The encoder
�
rst chooses a total data size T¿ b i , which is su$cient to encode all the
data. This total data size is split into N slots of xed lengths s i ;i =1 :::N.
An N-stage algorithm is then used to place the data from the variable-length
blocks into the xed-length slots. At each stage n, a block i with data left
unplaced searches slot j = i+
n (mod N) for space to place some or all of the
remaining data. Here,
n is an o set sequence that is usually pseudo-random.
Figure 9.5 shows an example of the EREC algorithm. In this case, there are
N = 6 variable-length blocks, with lengths 11, 9, 4, 3, 9, and 6 bits. The total
data size is chosen as T = 42 and is divided into N = 6 slots, with a length
of s i = 7 bits each. The o set sequence is
n = {0; 1; 2; 3; 4; 5; 6}. In stage 1 of