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Tape  Format  System
Hi8 Track System

Photography (Basic) - Introduction to photography and other graphic techniques
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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







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