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The output is typically a computer
monitor, but it could be a printer
or television. When the input or
output is
interlaced (such
as
NTSC or
VHS video),
special consideration should be
given to the interlaced
construction. Many parts of video technology were originally specified more than 50 years ago. The design criteria of those days was met with interlaced video. In this century, things have changed, but we still retain some of the legacy. When interlaced video is captured with a charged coupled device ( CCD), it is usually captured as two pictures taken 1/60th of a second apart. If the image moves in that time (a common occurence), the borders in a video frame can become very distorted. The zigzagging from one to several pixels makes segmentation rather subjective. For this and other reasons, we want to separate the video frame (frames are the way most video is digitally captured) into video fields. We do this by putting the odd lines and columns in one field and the even lines and columns in the other field. After segmentation, these fields can be recombined into a frame. Since video is often smaller on computers (for bandwidth reasons), it may be desirable to to skip the recombination. The following data flow diagram shows how VHS or NTSC format video would be changed to HDTV resolution. Since an hour of video contains 216,000 video fields, the macro language will be needed to allow the computer to run in the batch mode. |
Video Compression Data Flow
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