Page 239 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

216 Chapter 9. Error-Resilience Video Coding Techniques

7 bits 7 bits 7 bits 7 bits
Slot 1
Slot 2
Slot 3
Slot 4
Slot 5
Slot 6 offset 1 offset 2 offset 3

Stage 1 Stage 2 Stage 3 Stage 6
Empty bit Block 1 bit Block 2 bit Block 3 bit Block 4 bit Block 5 bit Block 6 bit

Figure 9.5: Example of the EREC algorithm

the algorithm, blocks 3, 4, and 6 are completely placed into the corresponding
slots, with some leftover space in those slots. Blocks 1, 2, and 5, however,
are only partially placed in the corresponding slots and have some bits left
to be placed in empty spaces in other slots. According to the o set sequence,
block 1 searches slot 2 for empty space, block 2 searches slot 3, and block
5 searches slot 6. Both blocks 2 and 5 nd empty spaces. Thus, in stage 2,
all the remaining bits from block 2 are placed in slot 3, whereas some of
the remaining bits of block 5 are placed in slot 6. Since block 1 did not nd
empty spaces in slot 2, then, according to the o set sequence, it searches slot
3, and so on. By the end of stage 6, all data bits are placed in the slots. The
decoder operates in a similar manner. Thus bits in a slot are decoded and
placed in a block until an end-of-block codeword is encountered.
In the presence of errors, the resilience provided by the EREC algorithm
is due to two factors. First, each block starts at a known position in the
bitstream (i.e., the start of the corresponding slot). Thus, in the case of loss of
synchronization, the decoder simply jumps to the start of the next slot without
the need for resynchronization codewords. Second, subjectively less important
data (e.g., high-frequency DCT coe$cients) are usually placed in later stages
of the algorithm. With the EREC algorithm, most error propagation e ects
(due, for example, to missing or falsely detecting end-of-block codewords) hit
data placed at later stages of the algorithm rather than the more important data
at the start of the slots.

9.6.3.3 Reversible Variable-Length Coding (RVLC)

Reversible VLC codewords are designed to be decoded both in the forward
and backward directions. As already described, when an error is detected in