NTSC and PAL video signals are analog in nature. Computers, however, display information
digitally. So NTSC and PAL video signals must be digitized, or sampled, before they
can be used by the computer. The process of digitizing video is commonly called capturing.
A video-graphics adapter, often called a frame grabber or video capture board, is
used to digitize an analog video signal and convert it into a computer graphics signal.
Digital recording of a video signal requires substantial amounts of disk storage
because the color and brightness information for each pixel in every image frame
must be stored. A full-screen image on a 13-inch computer monitor measures 640 pixels
by 480 pixels. Thus, each full-screen frame of video contains 307,200 (640 by 480)
pixels. To display the full-screen image in 24-bit color, each pixel must represent
24 bits of information (or 8 bits per RGB component). Twenty-four bits of information
are equal to 3 bytes. That figure multiplied by a full-screen, 307,200-pixel image
results in a storage requirement of 921,600 bytes for each frame of digitized video.
At a frame rate of 30 fps, storing 1 second of digitized NTSC video requires more
than 27 megabytes!
