Specifications

HOW FIBER SYSTEMS WORK:
To understand the problem it will help you to know how a video fiber optic
transmitter and receiver work. When you connect your video signal onto the fiber
transmitter that signal is first put through a buffer amplifier to control the input
impedance and then the video signal is clamped to the tip of White, the most
positive part of the video signal. After that the signal can be directly modulated
onto the fiber using the AM (Amplitude Modulated) system or it can be introduced
onto the fiber using the FM (Frequency Modulated) system.
Both systems will work quite well in most applications, but both systems use
a "TIP OF WHITE CLAMP" which can create a problem. In most video equipment
normal clamping is accomplished by using the "Back Porch" part of the video
signal. This type of clamp holds the minimum luminance or black level in one
place so that overall video level changes do not affect the relationship between
the Sync Pulse and the Luminance level in the video signal. Next the signal goes
through shaping circuitry and then on the light source. This is usually a Light
Emitting Diode LED or LASER diode.
The receiver works in very much the same way but in the reverse order, with
light from the fiber hitting a light sensitive receiver element or photo detector which
converts the light signal into an electrical signal, then the signal goes into an AGC
(Automatic Gain Control) to offset any light amplitude losses incurred in the fiber
run. The AGC is a required part of the system but it combined with the Tip of
White clamp creates a problem. Next the signal is demodulated using either the
AM or FM process back into a video signal.
THE CAUSE OF THE PROBLEM:
Remember the "tip of white clamp" in the transmitter, it uses electronic
circuitry to make the most positive part of the video waveform stay at a particular
voltage no matter what size or shape of the waveform. This type of "clamp" is
used to keep the maximum white level of the video at the maximum illumination
point for the LED or LASER light source.
Then in the receiver there is an AGC to make up the loss of light in the fiber,
this is necessary to improve the signal to noise ratio in the fiber optic system, but it
also creates a problem. The AGC raises or lowers the video signal to keep it at
140 I.R.E. units peak to peak regardless of the input level. This type of clamp and
AGC is quite effective at maintaining good operation of the fiber optic system, but
it requires a correct input video signal to operate properly.
CCTV TRAINING MANUAL PAGE 26 OF 28