Relays, controllers, etc.

The category includes all devices that provide the ability to control audio and video signals, and relays that allow the transmission of these signals.

An example of such a device would be a signal detector, which detects a signal and makes modifications depending on the function of the device in question.
 

Another device is a level controller, which allows automatic control of the volume level. Such devices are used in systems where a constant level of amplification is required.