Video - A Practical Guide to Video and Audio Compression - page 82

Interlace

Reconstruction

Odd
Even
Odd
Even
Odd
Even

Odd

Even

Odd

Figure 5-3 Interlaced field rasters.

Whether the odd or even field is painted first depends on what is called the field domi-
nance. It is very important that this is maintained identically to how the footage was
filmed because if it is not, a strange “twittering” effect is observed and all the motion
seems very jerky. That is because every 25th of a second, the time base goes back by a 50th
of a second—a “two steps forward and one step back” scenario—and it is not pleasant to
watch. Figure 5-4 plots the time base for a correctly clocked field presentation and one that
has its field dominance reversed.

5.6.1 Persistence of Vision

The time base of the picture movement is in a 50th of a second increment, based on the
field times and not the frame times. The field rate is twice the frame rate, but none of the
lines in the odd field are displaying a part of the image that exists in the even field. It is as
if there are two separate but simultaneous and totally synchronized movies playing as far
as any motion is concerned.
Because the tube phosphor glows for a little while after the electron beam has passed
over it and our eyes have a “persistence of vision,” you don't see this as two alternate
scans. Instead your eyes aggregate it into a single image.