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The Ovation System employs three specific noise rejection measures for digital signal plant interconnections: • Low pass filtering • Substantial signal levels (48 VDC or 115 VAC) • Isolation or optical coupling Low pass filtering and the use of large signal level techniques provide frequency and energy level discrimination, respectively. Isolation of the digital signal receiver from ground is important as a means for rejecting noise which causes both wires in a signal pair to change voltage-to-ground potentials. An example of this type of isolation is a signal source (transmitter) which is grounded at a point remote from the receiver, where transmitter and receiver grounds are not at the same voltage. In this case, ground potential difference appears as a voltage on both wires of the corresponding signal pair. Another example in which isolation may be required to reject ground potential difference noise would be in circuits where coupling exists between signal wires, inducing a potential in both wires. Induced potentials can occur when signal wires are present in environments with changing electromagnetic or electrostatic fields. Isolation may be required in this case. An optical isolator (also known as an opto-isolator) may be used to bring digital signals into the receiver. No receiver response to noise can occur unless signal line noise current flows. Low frequency current, which may flow as a result of equal noise voltage-to-ground potentials on both wires of the signal pair, is eliminated if the signal wires are not grounded at more than one point. This is called the common-mode voltage.