Basic Photography Course

Figure 13-4 illustrates how this scanning process takes
place. The electron beam first scans all odd-numbered
lines, from left to right (a). When all odd-numbered lines
have been scanned, it makes up a field. One field
consists of 262.5 lines. After the odd-numbered ones are
scanned, the beam jumps back to the top of the screen.

At this point, the beam is so weak that it does not affect

the screen. Back at the top of the screen the beam starts
scanning the even-numbered lines (b). When all
even-numbered lines are scanned a second field is
formed. The two fields make up a frame (c) or one
complete television picture. A frame consists of
525 lines. After completing a frame, the beam returns to
the top to start with another first field.

This charge-forming-and-scanning is a fast,

continuous process. The complete camera tube screen

(frame) is scanned 30 times per second.

The motion-video camera picks up reflections of

light from the scene while the microphone picks up

sound. At the same time, the camera changes the light
reflections into electrical impulses, and the microphone

changes the sound into electrical impulses.

This is basically the way a black-and-white video

camera works. A color video camera works on the same
principle; however, a color video camera has three
tubes. Through the use of a beam splitting device and
filters, one tube forms a red image, a second tube forms
a green image, and the third tube forms a blue image.
The three tubes have identical scanning patterns, so the
picture signals they produce are identical, except they
differ in color.

During a video recording, the videotape moves past

a rotating head that "writes" the video and audio signals
on the videotape. During playback, the rotating head
"reads" the magnetically stored information off the tape
Some VTRs use two or four heads for their record/play
(write/read) functions. Digital VTRs have even more

Figure 13-5.Helical scanning system.

read/write heads. For explanation purposes a VTR with
two record/play heads is discussed in this chapter.

Helical, or Slant-Track, System

The two heads are mounted opposite each other,

either on a rapidly spinning head drum or on a bar that
spins inside a stationary head drum. When the bar spins
inside a stationary head drum, the heads contact the tape
through a slot in the head drum. The tape is wound
around the head drum in a slanted, spiral-like manner.
This permits more tape area to contact the head,
allowing the transfer of large amounts of video
information (fig. 13-5). If the head contacted only the
width of the tape, extreme tape or drum speed would be
necessary. Because the Greek word for spiral is helix,
this tape wrap, and often the whole video-recording
system, is called the helical scan, or slant track

13-7

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DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> Figure 13-4.Video scanning. Figure 13-4 illustrates how this scanning process takes place. The electron beam first scans all odd-numbered lines, from left to right (a). When all odd-numbered lines have been scanned, it makes up a field. One field consists of 262.5 lines. After the odd-numbered ones are scanned, the beam jumps back to the top of the screen. At this point, the beam is so weak that it does not affect the screen. Back at the top of the screen the beam starts scanning the even-numbered lines (b). When all even-numbered lines are scanned a second field is formed. The two fields make up a frame (c) or one complete television picture. A frame consists of 525 lines. After completing a frame, the beam returns to the top to start with another first field. This charge-forming-and-scanning is a fast, continuous process. The complete camera tube screen (frame) is scanned 30 times per second. The motion-video camera picks up reflections of light from the scene while the microphone picks up sound. At the same time, the camera changes the light reflections into electrical impulses, and the microphone changes the sound into electrical impulses. This is basically the way a black-and-white video camera works. A color video camera works on the same principle; however, a color video camera has three tubes. Through the use of a beam splitting device and filters, one tube forms a red image, a second tube forms a green image, and the third tube forms a blue image. The three tubes have identical scanning patterns, so the picture signals they produce are identical, except they differ in color. During a video recording, the videotape moves past a rotating head that "writes" the video and audio signals on the videotape. During playback, the rotating head "reads" the magnetically stored information off the tape Some VTRs use two or four heads for their record/play (write/read) functions. Digital VTRs have even more Figure 13-5.Helical scanning system. read/write heads. For explanation purposes a VTR with two record/play heads is discussed in this chapter. Helical, or Slant-Track, System The two heads are mounted opposite each other, either on a rapidly spinning head drum or on a bar that spins inside a stationary head drum. When the bar spins inside a stationary head drum, the heads contact the tape through a slot in the head drum. The tape is wound around the head drum in a slanted, spiral-like manner. This permits more tape area to contact the head, allowing the transfer of large amounts of video information (fig. 13-5). If the head contacted only the width of the tape, extreme tape or drum speed would be necessary. Because the Greek word for spiral is helix, this tape wrap, and often the whole video-recording system, is called the helical scan, or slant track 13-7 contents Basic Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
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