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If it's video switchers
As well as functioning as a simple manual video switcher, these units can also be set to run on Auto so that they skip sequentially through each connected camera, for a preset period of time.
Some units need to be manually programmed to indicate to the controlling circuitry, precisely how many cameras are present, whilst others automatically sense the presence of a connected video signal, and then incorporate that camera into the switching cycle.
Whilst all units have a manual selection button for each camera input, there is also a secondary 'Auto' button to instruct the unit to change from one to another, the timing of which is normally controlled by an externally accessible variable resistor or potentiometer.
This control can either require a small bladed screwdriver to be used, or preferably a simple finger adjustable knob. Some of the more expensive units will actually allow different time settings for each connected camera.
The main changeover technique which is used is technically described as vertical interval switching, which simply means that when instructed to switch by the internal timer circuitry, the unit will operate between snatched frames, to reduce vertical rolling in the displayed picture. Another term that is commonly used, is 'low bounce' switching, which means that when a change over does take place, the image settles immediately, without any residue jumping.
In most basic CCTV systems, particularly those used in a simple retail environment, the sequential switcher has been the tool of choice for many years, although increasingly, their use is being replaced somewhat by the 'Quad' multiplexer.
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