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If it's video switchers
Based on a standard Auto / Manual sequential switcher, an alarm input varient allows various trigger devices to be connected to an individual camera terminal; so if for example a PIR movement sensor detects someone approaching a Fire Escape door, the switcher will then automatically change over to that specific camera.
Some auto switchers can be easily upgraded to alarm input by the simple addition of a small daughterboard that plugs straight on to the main PCB.
In whichever configuration, most switchers are available as 2, 4, 6, or 8 channel operation, with some models even being able to support remote operation through the addition of a small 'hard wired' desk top control box.
Doktor Jons advice is that if you are looking to obtain a switcher, allow for the number of cameras to be used immediately, plus a further input if video recording is to be included (for viewing playback on the same monitor), and then step up by at least two spare channels, so theres room for possible future expansion.
Also, if you are only using two cameras, dont be tempted to buy a limiting 2 channel switcher, when the four input
If the unexpected happens and you need to add a further camera, you'll be very pleased you did.
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