Processing for compression - The MPEG - page 94

Figure 3.61: (a) The optic flow axis of the original scene is distorted by the double projection of each frame. (b) A
tracking eye sees a double image in the presence of motion.
When 25 Hz film is displayed with a simple MPEG decoder each frame of a film is generally output twice to produce
a flicker frequency of 50 Hz. The result with a moving object is that the motion is not properly portrayed and there is
judder. Figure 3.61 (b) shows the origin of the judder.
In 60 Hz areas the film frames are decoded at 24 Hz, but odd frames are output as three fields, even frames result
in two fields. This is the well- known 3/2 pulldown system used to convert film rate to 60 Hz video. Motion portrayal
(or lack of it) in this case is even worse.
In fact the 24/25 Hz frame rate output of an MPEG decoder is a perfect application for motion compensation. As
Figure 3.62 shows, a motion- compensated standards conversion process is used to output whatever frame rate is
required without judder, leading to much-improved subjective quality.
Figure 3.62: A film with a frame rate of 24 Hz cannot be displayed directly because of flicker. Using a motion-
compensated standards conversion process extra frames can be synthesized in which moving objects are correctly
positioned. Any television picture rate can then be obtained from film.
3.18 Camera-shake compensation
As video cameras become smaller and lighter, it becomes increasingly difficult to move them smoothly and the
result is camera shake. This is irritating to watch, as well as requiring a higher bit rate in compression systems.
There are two solutions to the problem, one which is contained within the camera, and one which can be used at
some later time on the video data.
Figure 3.63 shows that image-stabilizing cameras contain miniature gyroscopes which produce an electrical output
proportional to their rate of turn about a specified axis. A pair of these, mounted orthogonally, can produce vectors
describing the camera shake. This can be used to oppose the shake by shifting the image. In one approach, the
shifting is done optically. Figure 3.64 shows a pair of glass plates with the intervening spaced filled with transparent
liquid. By tilting the plates a variable-angle prism can be obtained and this is fitted in the optical system before the
sensor. If the prism plates are suitably driven by servos from the gyroscopic sensors, the optical axis along which
the camera is looking can remain constant despite shake. Shift is also possible by displacing some of the lens
elements.