Repeaters have one or more receivers and a transmitter, with a controller. The controller activates the repeater when it detects a carrier on one of its incoming channels, representing a user talking. The repeater receives the radio signal, demodulates it to an audio signal which is filtered to remove noise, and retransmits it on a second channel to avoid interference with the first signal. This is received by a second two way radio in the repeater's expanded listening area. When the second user replies on the second channel, representing the other half of the half-duplex conversation, his signal is received by the repeater and similarly translated and retransmitted on the first channel back to the first user. Most controllers also decode Continuous Tone-Coded Squelch System tones, which allows the repeater to activate only if the station is sending a particular pre-programmed code, preventing unauthorized stations from using the repeater. Additionally, as repeaters are placed on high locations it also prevents distant stations on the same frequency from interfering. A Morse code or a synthesized voice module may produce station ID to comply with station identification regulations; this is most common on amateur radio repeaters, some LMR stations must also identify to be legal in some areas.