Initially the system had one channel that was transmitted as direct audio. The wires and plugs for the system were the same as for standard power wires and plugs which could cause receivers to burn out by attaching to mains socket. During World War II, all RF receivers were confiscated, but cable radio continued operating and, in particular, was used to transmit warnings of aerial bombing. The 1960s saw an enhancement with the addition of two additional channels, using AM on carrier frequencies of 78 and 120 kHz. The installation of this system became mandatory for all new buildings. The system, along with usual broadcasting, was created to inform people of emergencies.