Magnetic receivers use the electromagnetic principle to receive and analyze signals. An electromagnetic principle can be defined as electric currents running along a wire that produce magnetic forces and rotating magnets around a wire produce electricity. The simplest example of a magnetic receiver is the telephone. A magnetic receiver in a telephone converts the analog electrical signals back into acoustical energy. Signal receivers are used in television transmissions, and are used to receive cable and satellite transmissions. A signal receiver device receives broadcast signals and detects rating or category codes therein. The signal receiver compares the detected code to a setting set in the device. If the comparison indicates that the incoming signal has a rating greater than that set or is of an undesired category the broadcast signal is blocked or an alert is given. Magnetic receivers and signal receivers may be tamper-resistant in certain devices to prevent disabling of the device.