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APPENDIX
397
How analog video is displayed
Analog video is displayed on an NTSC or PAL compatible device, such as
a television set. The tube of a television set is made up of a series of lines
which display the pictures that we see. The number of lines determines the
resolution and quality of the video. (NTSC devices have 525 lines and PAL
have 625.) When the video signal is received, it moves across and down
these lines, with each pass referred to as a scan, and one complete pass as
a field. Once it has reached the end of the scan, it repeats the process,
updating the display each time. This all happens very quickly, with the
tube being scanned 60 times a second for NTSC devices and 50 for PAL
devices.
When television was first developed, the sets at the time had trouble
scanning so many lines for each field. To compensate for this, it was
decided to interlace the display, (scan alternate lines rather than every line).
For example, the first scan would pass over every even line, the second
every odd. (As such it took two fields of information to display an actual
image.) This was still quick enough to fool the human eye into believing
that it was seeing a continuous and uninterrupted picture.
The world of digital video
Instead of having the video represented by a series of waves and signals,
digital video's data is made up of a series of zeros and ones. Converting
analog signals into digital data involves digitizing or capturing data. This is
done using a special hardware card called the video capture board. There
are several varieties of video capture boards in the marketplace. Some are
designed to perform professional broadcast videos, and others, to process
the more consumer-orientated home video.
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