Digital Signal Processing Reference
In-Depth Information
This makes the weave algorithm significantly more expensive
in logic elements and external RAM bandwidth than either of the
bob algorithms. As mentioned before, the results of the weave
algorithm can sometimes be perfect, in the instance where pairs
of interlaced fields have been created from original progressive
frames or when there is little-to-no motion.
Motion-adaptive deinterlacing provides the best quality but
requires greater logic and memory resources. In the simple
motion adaptive scenario shown above, we have to store the four
fields in external memory, store the value of M (motion) in
memory, and dedicate logic for calculating the sum and then the
difference between the sums.
Ideally, deinterlacers are implemented in hardware, and
FPGAs are used to implement sophisticated high-definition
deinterlacers. Memory is the most important hardware resource
required to build a highly efficient deinterlacer. This applies to
both on-chip memory to store the m
n block of pixels across
the different fields (the calculated and previous motion value
matrices), as well as the external (generally DDR) memory to
store multiple-input video fields and the calculated frames.
Table 6.1 shows the resources used to implement a motion-
adaptive deinterlacing algorithm on a PAL video source in Altera
Cyclone
III and Stratix
III FPGAs.
The table contrasts the resources used for a motion-adaptive
deinterlacing technique with the resources used for a simple
weave technique. Notice the drop-off in the amount of memory
used even when the weave technique is applied to a higher
resolution image.
The single biggest resource that must be carefully considered
for implementing motion-adaptive deinterlacing is external
memory bandwidth. Fields, motion values and output frames
have to be moved into and out of the external memory at the
video frame rate.
Thus an important consideration in the design of a dein-
terlacer is the expected memory bandwidth. To buffer one field of
a 480i video source requires 165.7 Mbps:
(720
240 pixels / field)
(16 bits / pixel)
(59.94 fields / sec)
ΒΌ
165 Mbps
The bandwidth doubles for a progressive frame and increases
even more for HD video. To calculate the bandwidth, calculate
the number of frame accesses that a deinterlacer has to execute
and then add the total bandwidth. Compare this to the expected
bandwidth of the DDR memory interface, which depends on the
throughput and the width of the memory interface.
