30 September 1988
a. To achieve an accurate reproduction of a scene, the scanning of the
beam must be done in an organized way. In both the studio camera and the
home television receiver, the scanning of the target of screen is done by
the electron beam moving in horizontal lines across the target plate of a
screen. At the same time, the electron beam gradually moves down the scene.
When the beam reaches the bottom of the scene, the beam is sent back to the
top. There are 525 horizontal lines in a complete picture.
b. The horizontal lines are alternately scanned. That is, all the odd-
numbered lines are scanned first, then the beam returns to the top of the
scene and scans all the even-numbered lines.
This is called "interlaced"
scanning (fig 1-2).
c. Each scan of the scene is called a field and only involves half of
the total 525 lines, or 262.5 lines. Two complete scans of the scene (525
lines) is called a frame. Because the fields are scanned at 60 frames per
second, the viewer only perceives the completed picture.
The accurate reproduction of the images of both the studio camera and
the television receiver must be synchronized to scan the same part of the
scene at the same time. At the end of each line the beam must return to the
left side of the scene.
This is called "horizontal retrace".
coordination of the horizontal retrace is handled by the horizontal sync
pulse (fig 1-3).