RS4 Codici
The term is an abbreviation of Ripetizione Segnali a 4 codici (signal repetition system with 4 codes).It helps the driver to reliably determine the aspect of the next signal even in poor visibility and at high velocities.The information is conveyed via amplitude modulation of a 50 Hz alternating current of the Blocco Automatico a Correnti Codificate (BAcc) in the rails.Receiver coils in front of the first axle of a locomotive or control car are used to detect the signal.As trains got faster an additional set of codes sent with a 178 Hz carrier were added.