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250 Chapter 10. Error Concealment Using Motion Field Interpolation

within the long-term memory block-motion $eld. Note that at low frame skips,
this simple ZR method is suNcient, whereas at high frame skips the more
complex method of BM has to be employed. This may be due to the fact
that at high frame skips, the zero-biased distribution becomes more spread
(see Property 6:3:1:2 and Figure 6.3). In other words, d t = 0 becomes less
probable, and longer temporal components start to appear more frequently in
the motion $eld. Such components need to be recovered using BM. Both AV
and MFI provide poor temporal-component recovery compared to BM and ZR.

10.5.2 Spatial-Components Recovery
This set of experiments investigates the best technique for recovering the spa-
tial components (d x ;d y ) of a damaged long-term motion vector. In this case,
the temporal recovery technique T, in the combination S-T, was kept con-
stant at ZR, whereas the spatial recovery technique S was varied over ZR,
AV, BM, and MFI. In other words, four S-T combinations were considered:
ZR-ZR, AV-ZR, BM-ZR, and MFI-ZR.
Figures 10.14, 10.15, and 10.16 show the results for the QSIF sequences
AKIYO,FOREMAN, and TABLE TENNIS, respectively. Part (a) of each $gure shows
the performance with a frame skip of 3 over a range of macroblock error rates,
whereas part (b) shows the performance with a macroblockerror rate of 20%
over a range of frame skips.
In general, the best spatial-components recovery is achieved by MFI
followed by BM. This is similar to the single-reference results reported in
Section 10.4. Thus, moving from a single-reference system to a multiple-
reference system does not signi$cantly inOuence the spatial-components

Spatial-components recovery, Akiyo, M=10, QP=10, Skip=3 Spatial-components recovery, Akiyo, M=10, QP=10, Macroblock error rate=20%
31 31
ZR-ZR
AV-ZR 30.5
BM-ZR
30 MFI-ZR
30
29
29.5
29
PSNR Y (dB) 28 PSNR Y (dB) 28.5
27
28
27.5
26
27
ZR-ZR
25
26.5 AV-ZR
BM-ZR
MFI-ZR
24 26
10 20 30 40 50 1 2 3
4
Macroblock error rate (%) Frame skip
(a) Performance over a range of error rates (b) Performance over a range of frame skips
Figure 10.14: Spatial-components recovery for QSIF AKIYO with M =10 and QP =10