Scanning
When looking at a picture, such as a photograph or a drawing, the human eye takes the scene in all at once. The eye can move from spot to spot to examine details, but in essence, the entire picture is seen at one time. Likewise, when watching a film, the eye sees moving images go by on the screen. The illusion of motion is created by projecting many pictures or frames of film each second. The eye perceives motion, even though the film is made up of thousands of individual still pictures. Video is different from film in that a complete frame of video is broken up into component parts when it is created.
Video Lines
The electron beam inside a video camera transforms a light image into an electronic signal. Then, an electron beam within a video receiver or monitor causes chemicals called phosphors to glow so they transform the electrical signal back into light.
The specifications for this process were standardized by the NTSC (National Television System Committee) when the television system was originally conceived in the late 1930s. The NTSC standard is used in North America and parts of Asia and Latin America. As other countries developed their own television systems, other video
standards were created. Eastern and Western Europe use a system called PAL (Phase Alternate Line). France and the countries of the former Soviet Union use a system known as SECAM (Séquential Colour Avec Mémoire, or Sequential Color with Memory).
For each NTSC video frame, the electron beam scans a total of 525 lines. There are 30 frames scanned each second, which means that a total of 15,750 lines (black and white video) are scanned each second (30 frames 525 lines per frame). This rate is called the line frequency. The NTSC line frequency and frame rate changed with the addition of color. Both PAL and SECAM use 625 lines per frame at 25 frames per second. These two systems were developed after the introduction of color television and consequently did not require any additional changes. There are variations and combinations that attempt to combine the best elements of all of these standards.
Scanning 15,750 lines per second is so fast that the eye never notices the traveling beam. The video image is constantly refreshed as the electron beam scans the 525 lines in each frame. As soon as one frame is completely displayed, scanning begins on the next frame, so the whole process appears seamless to the viewer.
The electron beam in a video camera is made to scan by electronic signals called drive pulses. Horizontal drive pulses move the beam back and forth; and vertical drive pulses move the horizontally scanning beam up the face of the pickup tube. These drive pulses are generated inside the camera.
Blanking An electron beam scanning a picture tube is like an old typewriter.
It works in only one direction. When it reaches the end of a line of video, it must retrace or go back to the other side of the screen to start the next line. Likewise, when it reaches the bottom of the image, it must retrace or go back to the top of the image to begin scanning the next frame (Figure 3.1)..bmp)
When looking at a picture, such as a photograph or a drawing, the human eye takes the scene in all at once. The eye can move from spot to spot to examine details, but in essence, the entire picture is seen at one time. Likewise, when watching a film, the eye sees moving images go by on the screen. The illusion of motion is created by projecting many pictures or frames of film each second. The eye perceives motion, even though the film is made up of thousands of individual still pictures. Video is different from film in that a complete frame of video is broken up into component parts when it is created.
Video Lines
The electron beam inside a video camera transforms a light image into an electronic signal. Then, an electron beam within a video receiver or monitor causes chemicals called phosphors to glow so they transform the electrical signal back into light.
The specifications for this process were standardized by the NTSC (National Television System Committee) when the television system was originally conceived in the late 1930s. The NTSC standard is used in North America and parts of Asia and Latin America. As other countries developed their own television systems, other video
standards were created. Eastern and Western Europe use a system called PAL (Phase Alternate Line). France and the countries of the former Soviet Union use a system known as SECAM (Séquential Colour Avec Mémoire, or Sequential Color with Memory).
For each NTSC video frame, the electron beam scans a total of 525 lines. There are 30 frames scanned each second, which means that a total of 15,750 lines (black and white video) are scanned each second (30 frames 525 lines per frame). This rate is called the line frequency. The NTSC line frequency and frame rate changed with the addition of color. Both PAL and SECAM use 625 lines per frame at 25 frames per second. These two systems were developed after the introduction of color television and consequently did not require any additional changes. There are variations and combinations that attempt to combine the best elements of all of these standards.
Scanning 15,750 lines per second is so fast that the eye never notices the traveling beam. The video image is constantly refreshed as the electron beam scans the 525 lines in each frame. As soon as one frame is completely displayed, scanning begins on the next frame, so the whole process appears seamless to the viewer.
The electron beam in a video camera is made to scan by electronic signals called drive pulses. Horizontal drive pulses move the beam back and forth; and vertical drive pulses move the horizontally scanning beam up the face of the pickup tube. These drive pulses are generated inside the camera.
Blanking An electron beam scanning a picture tube is like an old typewriter.
It works in only one direction. When it reaches the end of a line of video, it must retrace or go back to the other side of the screen to start the next line. Likewise, when it reaches the bottom of the image, it must retrace or go back to the top of the image to begin scanning the next frame (Figure 3.1).
The period of time during which the electron beam retraces to beginscanning or tracing the next line is part of a larger time intervalcalled horizontal blanking. The period of time that the electron gunis retracing to the top of the image to begin scanning another frameis called vertical blanking. During horizontal or vertical blanking,the beam of electrons is blanked out or turned off, so as not tocause any voltage to flow. This way the retrace is not visible.
The horizontal blanking interval is the separation between consecutivelines. The vertical blanking interval is the separation betweenconsecutive frames. As the video image is integrated with otherimages, using equipment such as video editing systems or videoswitchers, the change from source to source occurs during thevertical blanking interval after a complete image has been drawn.
This can be compared to splicing on the frame line of a film frame.Horizontal blanking actually occurs slightly before the beginning ofeach line of video information. Vertical blanking occurs after eachframe. The video picture itself is referred to as active video. In the NTSC system, active video uses 480 out of the 525 lines contained in one frame. PAL and SECAM use 580 active linesout of the 625 total lines. Blanking functions as the picture frame around the active video. It is a necessary component of the TV signal, even though the electron beam is shut off. Blanking specifications are an important part of the picture specifications.
Persistence of Vision Film is shot at 24 frames per second. However, if it were projected at that rate, a flickering quality to the moving image would be noticeable. The flickering is a result of the phenomenon that lets us perceive motion in a movie in the first place. That phenomenon
is called persistence of vision.
Persistence of vision means that the retina, the light-sensitive portion of the human eye, retains the image exposed to it for a certain period of time. This image then fades as the eye waits to
receive the next image. The threshold of retention is 1/30 to 1/32 of a second. If the images change on the retina at a rate slower than that, the eye sees the light and then the dark that follows. If the images change at a faster rate, the eye sees the images as continuous motion and not as individual images. This concept was the basis of a device developed in the 19th century called the Zoetrope
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The horizontal blanking interval is the separation between consecutivelines. The vertical blanking interval is the separation betweenconsecutive frames. As the video image is integrated with otherimages, using equipment such as video editing systems or videoswitchers, the change from source to source occurs during thevertical blanking interval after a complete image has been drawn.
This can be compared to splicing on the frame line of a film frame.Horizontal blanking actually occurs slightly before the beginning ofeach line of video information. Vertical blanking occurs after eachframe. The video picture itself is referred to as active video. In the NTSC system, active video uses 480 out of the 525 lines contained in one frame. PAL and SECAM use 580 active linesout of the 625 total lines. Blanking functions as the picture frame around the active video. It is a necessary component of the TV signal, even though the electron beam is shut off. Blanking specifications are an important part of the picture specifications.
Persistence of Vision Film is shot at 24 frames per second. However, if it were projected at that rate, a flickering quality to the moving image would be noticeable. The flickering is a result of the phenomenon that lets us perceive motion in a movie in the first place. That phenomenon
is called persistence of vision.
Persistence of vision means that the retina, the light-sensitive portion of the human eye, retains the image exposed to it for a certain period of time. This image then fades as the eye waits to
receive the next image. The threshold of retention is 1/30 to 1/32 of a second. If the images change on the retina at a rate slower than that, the eye sees the light and then the dark that follows. If the images change at a faster rate, the eye sees the images as continuous motion and not as individual images. This concept was the basis of a device developed in the 19th century called the Zoetrope
(Figure 3.3). By viewing a series of still images through a small slit in a spinning wheel, the images appeared to move.In film, this concept is exploited by simply showing each frame twice. The picture in the gate of the film projector is held, and the shutter opens twice. Then the film moves to the next frame and the shutter again reveals the picture twice. In this way, 48 frames per second are shown while the projector runs at 24 frames per second, and the eye perceives smooth, continuous motion.
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