Someone once said "Knowledge is the key to success". This rule
also applies to the installation and maintenance of CCTV camera
equipment. Have you ever installed a CCTV camera system and then had
to go back to solve a problem that was overlooked? A basic
understanding of CCTV video signals, can save you hundreds of man
hours, improve customer relations and increase job profitability all
at the same time. This section will discuss problems and solutions
for CCTV camera installations.
To discuss video let's start with the unit of measure, the
I.R.E. unit. I.R.E. stands for Institute of Radio Engineers, this
regulating body set the standards of measure for the video industry.
This standard has been adopted by all industries in the United States
and other parts of the world. 140 I.R.E. units is equal to 1 Volt
Peak to Peak. I.R.E. units are easier to use because they divide into
a video signal evenly.
For example proper Sync on a camera is 40 I.R.E. units, the
Voltage equivalent would be 0.2857143 Volts. Unfortunately this
voltage cannot be measured on the Volt Ohm Milliamp Meter.
SYNC PULSE AMPLITUDE, AND HOW IT EFFECTS CCTV INSTALLATIONS.
A CCTV video camera creates synchronization pulses to lock
the viewing monitor on the picture. These pulses occur at a rate of
15,750 times a second. There is one synchronization pulse or (sync
pulse) for each line in the picture frame. The sync pulse tells the
video monitor to start drawing a video line across the picture
screen. When it gets to the end of the screen another sync pulse
begins the next line, and so forth until the screen has been filled
with lines. It take 262 and a half lines to form a frame, and two
frames to form the video picture we see on the
monitor.
The proper level for sync is 40 I.R.E. units. If the sync
signal from the camera is too small in amplitude the picture will
break up or roll. If the sync pulse is too big, any black portion
of the picture will be more gray and the dynamic range of the picture
will be degraded. Peak white level will also be compressed causing a
blooming effect (loss of picture definition).
WHITE LEVEL IRIS SETTING, HOW MUCH IS ENOUGH?
There is a standard for Iris setting, or white level and it is
100 I.R.E. units. When setting a manual iris, or an automatic iris
the level should be the same, 100 I.R.E. units. If you set the iris
below 100 I.R.E. units, the picture will be dim with less than
desired dynamic range and the white picture elements will not be pure
white. If you set the iris for more than 100 I.R.E. units, the
picture can be washed out causing loss of picture definition.
Some cameras can be set to 120 I.R.E. units, but it should be
noted that the standard is 100 I.R.E. units and in any case all
camera's in the system should be set to the same level of white. This
will ensure that the white portion of the picture will be the same
brightness when a monitor is switched between them.
CM-2.ISB PAGE 8 of 10
PEAK TO PEAK MEASUREMENT OF THE CCTV SIGNAL.
