SILVERLINE VTX 03/09/08 User Guide Issue1
Page 5
4. ANALOGUE VIDEO DISTRIBUTION SYSTEMS
Apart from the normal terrestrial video broadcast systems; PAL, NTSC and SECAM the
Cove Communications
SILVERLINE
VTX Fibre Optic Video Transmitter may be used with
other distribution systems, provided that the signal has a composite format, i.e. it has a
mixed sync part to its waveform.
4.1 Levels and Impedance
The input impedance of the Cove Communications
SILVERLINE
VTX Fibre Optic video
transmitter is 75 Ohms. Should the application require a video looping facility, the
transmitter can be supplied with a high input impedance suitable for looping.
4.2 Bandwidths
The SILVERLINE VTX has a bandwidth that extends beyond 15MHz and over the
broadcast specified frequency response for PAL, NTSC and SECAM, when used in
conjunction with the Cove Communications Fibre Optic Video Receiver the frequency
response will be within ±1dB from D.C. to 5.5MHz.
5. OPERATION
5.1 General
The
SILVERLINE
VTX is intended for use with the range of Fibre Optic Video receivers.
This range includes the
SILVERLINE
VRX miniature receiver and the
STARLINE
19 inch
rack mountable range of receivers, VRX 1 Channel, VRX 2 Channel, and the VRX 3
Channel version.
The video transmitter is small enough to be connected directly to the video source and
may either draw its power from the host equipment or from an external D.C. supply. The
video signal is processed in the analogue domain and converts the analogue video input
signal into a linearly modulated infra-red light source for transmission down the fibre optic
cable.
5.2 AGC (Automatic Gain Control)
The AGC (Automatic Gain Control) featured in the range of Cove Communications Fibre
Optic Video Receivers uses the mixed sync portion of the video signal to set the through
gain. If the mixed sync signal presented to the transmitter is 0.3V and the peak white level
is 0.7V then the output signal from the receiver will be the same. However, if the input
video mixed sync amplitude is low compared to the peak white level, at the transmitter the
video output level will be high from the receiver because the mixed sync level will be
amplified up to the standard 0.3V level. Conversely if the input mixed sync level is too high
compared to the peak white at the transmitter the output video level at the receiver will be
too low because the mixed syncs will be attenuated back to the correct 0.3V level. If the
amplitude relationship between the mixed syncs and the peak white video is correct at the
transmitter but happens to be overall either too high or too low the AGC in the receiver will
correct for this and output a perfectly correct signal.
