36 Digital CCTV
vertical camera resolution. Next, divide this number by the
compression factor (if compression is 10 : 1 use the number 10).
Multiply the result by the frame-capture rate (30 frames per
second multiply by 30). The fi nal number equals the amount of
bytes you will need the capacity to store per second. 720 times 480
= 345600 divided by 10 = 34560 times 30 = 1,036,800 Bytes per
second (Bps) needed storage capacity per second.
Speeds at which images can be transmitted are signifi cantly
increased by digital compression, but many variables still affect
the update rates. These variables include such factors as image
color, movement within the image, and bandwidth of the trans-
mission medium.
THE ELECTROMAGNETIC SPECTRUM
In order for us to see, there must be light. What we perceive as
color is really a refl ection from the surface of what we are looking
at. The colors we perceive are actually electromagnetic radiation
of various frequencies. The visible light spectrum is part of a total
electromagnetic spectrum that ranges from low frequencies like
radio waves in thousands of cycles per second to high frequencies
like gamma rays in multi-trillions of cycles per second.
The electromagnetic spectrum is comprised of the complete
range of electromagnetic frequencies from 3 kHz to beyond
300,000 THz. The electromagnetic spectrum includes, from longest
wavelength to shortest: radio waves, microwaves, infrared, optical,
ultraviolet, X-rays, and gamma rays. See Figure 2-12. The visible
spectrum contains all the colors between infrared and ultraviolet.
Infrared and ultra violet are invisible to the human eye.
Figure 2-12 Electromagnetic Spectrum
What Exactly is Digital Video? 37
NTSC standards have not changed signifi cantly since their
establishment, except for the addition of new strictures for color
signals. NTSC signals are not directly compatible with computer
systems. The NTSC standard for television defi nes a composite
video signal with a refresh rate of 60 half-frames per second. Each
frame contains 525 lines and can contain 16 million different colors.
Composite video is the format of an analog television signal before
it is modulated onto an RF carrier. It is a composite of three source
signals called Y, U, and V (together referred to as YUV). Compos-
ite video is sometimes called CVBS for color, video, blanking, and
sync, or composite video baseband signal.
When NTSC television standards were introduced the frame
rate was set at 30 Hz (1/2 the 60 Hz line frequency). Then, the rate
was moved to 29.97 Hz to maintain 4.5 MHz between the visual
and audio carriers. Movies fi lmed at 24 frames per second are
simply converted to 29.97 frames per second on television
broadcasting.
There are three dominating video standards in use around
the world, NTSC, PAL, and SECAM. These three formats have
developed in different parts of the world for historic reasons. The
PAL standard involves the scanning of 625 lines per frame and
utilizes a wider channel bandwidth than NTSC. PAL, which stands
for Phase Alternating Line, was introduced in the early 1960’s and
was implemented in most European countries except for France
where the standard is SECAM (Sequential Couleur Avec Memoire
or Sequential Color with Memory), also introduced in the 1960’s.
SECAM uses the same bandwidth as PAL but transmits color
information sequentially. American engineers have been known
to jokingly explain that SECAM stands for “System Essentially
Contrary to the American Method.” Generally these formats are
not compatible with each other because they differ in aspects like
specifi c scanning frequencies, number of scan lines, and color
modulation techniques.
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