Film - A Practical Guide to Video and Audio Compression - page 62

Illumination created by a
diffuse light source.

Filtered light converted
to an electrical signal by
a CCD imaging detector.

Figure 4-17 Modern CCD scanner.

calibrated regularly. The beam has a halo that reduces the focus and some blurring is evi-
dent if this is not correctly calibrated. This defocusing effect or softening of the beam edges
is an artifact of CRT tubes and is called the Kell effect. Spot shape is adjustable in some TV
receivers, and making the beam oval prevents some twittering artifacts when the beam
shape is vertically oriented in an interlaced system. This allows the beam edges to overlap
slightly on successive lines. Flattening the beam horizontally sacrifices resolution across
the screen.

4.11.2 Pulldown—60 Hz Frame Rate

In the United States, the frame rate of the film must be altered from 24 to 30 fps. Running
the film at a faster frame rate is not a viable solution. The change in motion is too extreme
and everything will appear to be over-cranked .
The trick is to introduce an extra frame every now and then. By keeping an occa-
sional frame in the gate slightly longer, the time base is stretched.
To attain a smooth pulldown and an integer relationship between the frame rate of
the film and video, the film is run a little slower than normal (at 23.97 fps instead of
24 fps). The first frame is transferred to the first 2 fields and the second frame to the next
3 fields. At this point, the film is out of synchronization with the interlaced field structure of