Digital Signal Processing Reference
In-Depth Information
1
VIDEO IN THE MODERN WORLD
Video began as a purely analog technology. Successive images
were captured on film streaming through the camera. The movie
was played by using flashes of light to illuminate each frame on
the moving film, at rates sufficient to show continual motion.
Flicker, however, was easily seen.
An improved system for early broadcast television utilized the
luminance (or light intensity) information represented as an
analog signal. To transmit an image, the luminance information
was sent in successive horizontal scans. Sufficient horizontal
scans built up a two-dimensional image. Televisions andmonitors
used cathode ray guns that shot a stream of electrons to excite
a phosphorus-coated screen. The slowly fading phosphorus ten-
ded to eliminate flicker. The cathode gun scanned in successive
rows, each row just below the previous row, guided by magnetic
circuits. This happened so rapidly that images were “painted” at
a rate of 25 to 30 frames per second (fps). The luminance signal
was used to control the intensity of the electron stream.
A horizontal synchronization signal is used to separate the
horizontal scan periods. The horizontal sync period is a short
pulse at the end of each scan line. This has to be long enough to
allow the electron gun to move back to the left side of the screen,
in preparation for the next scan line. Similarly, the vertical
synchronization signal occurs at the end of the last or bottom
scan, and is used to separate each video frame. The vertical
synchronization interval is much longer, and allows the electron
gun to move back up from the lower right corner of the screen to
the upper left corner, to begin a new frame.
Later, color information in the form of red and blue hues was
added, known as chrominance information. This was super-
imposed on the luminance signal, so that the color system is
backwards compatible to the black-and-white system.
Modern video signals are represented, stored and trans-
mitted digitally. Digital representation has opened up all sorts
of new usages of video. Digital video processing is of grow-
ing importance in a variety of markets such as video surveil-
lance; video conferencing; medical imaging; military imaging
