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All the bolt on bits of equipment ...
To be honest, the use of a Test Signal Generator is really only ever going to be a subject of interest to a CCTV engineer.
The unit is designed to output a standardised range of signal test patterns, which can be used to common up a room full of monitors, or to check linearity in a picture (i.e. make sure all the bendy bits are straightened out!), or indeed any number of other important functions.
They are particularly useful for testing and to assist in measuring, the efficiency or imperfections in any given transmission system.
However, you do need to to be something of an expert to use this specialist tool to best effect.
The RF Modulator
The vast majority of industry standard CCTV equipment is designed to output signals as base level Composite Video (sometimes referred to as CVBS). However, there may occasionally be a requirement to inject a signal into a TV distribution network, for example, displaying the front door to a block of flats, on each tenants own individual television.
In order to do this, the video signal must first be encoded as a Radio Frequency (or RF) signal, and this is achieved using a device called an RF Modulator. They are generally manufactured to work within a specific band of output frequencies (depending on the country), and the trick is to adjust the output to a spare channel, which will not interfere with any other broadcast stations.
As relatively inexpensive devices, they are useful for specific domestic applications, but are rarely if ever used by hardened CCTV professionals!
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