Interlace Scanning
The process of this field-by-field scanning is known as interlace scanning because the lines in each field interlace with the alternate lines of the other field. There are two fields for each frame.
Because the images are appearing at the rate of 1/60 of a second, the eye does not see the interval between the two fields. Therefore, the eye perceives continuous motion.
An interesting experiment that illustrates the concept of interlace scanning is to follow a similar scanning pattern as the electron beam would on a frame of video. Look at the paragraph below and first read just the boldfaced, odd lines. Then go back to the top of the paragraph and read the non-boldfaced, even lines. Notice the
way the eyes retrace from the end of a line back to the left margin to begin scanning the next odd line. At the end of the paragraph, the eyes retrace from the last line back to the top again to read or scan the even lines. This is what the electron beam does during its blanking periods.
A television image is created through Interlace scanning.
Interlace scanning is the process of scanning every other line from top to bottom. The beam first scans the odd lines top to bottom,and then it scans the even lines top to bottom. Each scan from top to bottom is a field. It is the combination of the two successive fields that make up an entire frame of a video image.
It is not until both sets (or fields) of odd lines and even lines are interlaced together that the full meaning of the paragraph (or full-frame image) becomes clear. This holds true especially with electronic graphics. When viewing only one field, the letters look ragged and uneven. Only when viewing a complete interlaced frame do the letters look smooth and even.
Black and White Specifications
Dividing the video image into two fields, each with 2621/2 lines, provides an advantage when broadcasting a video signal. Since there is much less information in 2621/2 lines than there is in 525 lines, the video signal does not require as much bandwidth or spectrum space for transmission. For black and white video images, the original NTSC standards
were as follows:
• 525 lines per frame
• 480 lines per frame of active video
• 30 frames per second
• 15,750 lines per second (line frequency)
• 2621/2 lines per field
• 2 fields per frame
• 60 fields per second
• Horizontal blanking before each line
• Vertical blanking between successive fields
The process of this field-by-field scanning is known as interlace scanning because the lines in each field interlace with the alternate lines of the other field. There are two fields for each frame.
Because the images are appearing at the rate of 1/60 of a second, the eye does not see the interval between the two fields. Therefore, the eye perceives continuous motion.
An interesting experiment that illustrates the concept of interlace scanning is to follow a similar scanning pattern as the electron beam would on a frame of video. Look at the paragraph below and first read just the boldfaced, odd lines. Then go back to the top of the paragraph and read the non-boldfaced, even lines. Notice the
way the eyes retrace from the end of a line back to the left margin to begin scanning the next odd line. At the end of the paragraph, the eyes retrace from the last line back to the top again to read or scan the even lines. This is what the electron beam does during its blanking periods.
A television image is created through Interlace scanning.
Interlace scanning is the process of scanning every other line from top to bottom. The beam first scans the odd lines top to bottom,and then it scans the even lines top to bottom. Each scan from top to bottom is a field. It is the combination of the two successive fields that make up an entire frame of a video image.
It is not until both sets (or fields) of odd lines and even lines are interlaced together that the full meaning of the paragraph (or full-frame image) becomes clear. This holds true especially with electronic graphics. When viewing only one field, the letters look ragged and uneven. Only when viewing a complete interlaced frame do the letters look smooth and even.
Black and White Specifications
Dividing the video image into two fields, each with 2621/2 lines, provides an advantage when broadcasting a video signal. Since there is much less information in 2621/2 lines than there is in 525 lines, the video signal does not require as much bandwidth or spectrum space for transmission. For black and white video images, the original NTSC standards
were as follows:
• 525 lines per frame
• 480 lines per frame of active video
• 30 frames per second
• 15,750 lines per second (line frequency)
• 2621/2 lines per field
• 2 fields per frame
• 60 fields per second
• Horizontal blanking before each line
• Vertical blanking between successive fields
These specifications are different when color is added to the video signal.
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