Page 242 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 9.7. Postprocessing (or Concealment) Techniques 219

damaged
blocks
1 2 3 4 5 6 7 8 9 10 11 1 13 3 15 5 17 7 19 9 21 11
12 13 14 15 16 17 18 19 20 21 22 12 2 14 4 16 6 18 8 20 10 22
23 24 25 26 27 28 29 30 31 32 33 23 35 25 37 27 39 29 41 31 43 33
34 35 36 37 38 39 40 41 42 43 44 34 24 36 26 38 28 40 30 42 32 44

(a) without interleaving (b) with interleaving
Figure 9.6: Coding and transmission order with and without interleaving

There are also other multiple description techniques, as detailed in Refs. 179
and 180.

9.6.6 Interleaved Coding
In normal coding, the blocks of a given frame are encoded in raster scan
order, as illustrated in Figure 9.6(a). In this case, when an error occurs in
one block, spatial error propagation results in the loss of a contiguous set of
blocks. In the example shown, an error in block 12 results in the loss of all
2
blocks to its right. As is discussed later, the concealment of a damaged block
depends heavily on the availability of its four neighboring blocks. In this case,
a damaged block will have only its top and bottom neighbors intact.
Interleaved coding attempts to separate the information of neighboring
blocks as far as possible. As a result, an error in a block will propagate
to nonadjacent blocks. Figure 9.6(b) shows the even=odd interleaving scheme
adopted in Ref. 189. The numbers here indicate the encoding and transmis-
sion order. Thus, the rst block in the rst row (block 1) is encoded and
transmitted rst, followed by the second block in the second row (block 2),
and so on. Note that in this case, when an error occurs in block 12, the lost
set of blocks is not contiguous. Thus, a damaged block will have all its four
neighbors intact and this will help the error concealment process considerably.

9.7 Postprocessing (or Concealment) Techniques

The second category of error-resilience techniques are postprocessing (or
concealment) techniques. In postprocessing techniques, the decoder plays the

2 This example assumes that resynchronization codewords are inserted at the beginning of each
row of blocks.