Systems transmitted a continuous tone (at 3051.9 Hz by default), called Busy Tone. A low-pass filter eliminated most of the tone from speaker audio. The tone was present through the transmission, but cut off just before the repeater dropped in order to eliminate squelch tail. There were two possible continuous tones: all radios on a single system used the same tone. These were used to distinguish between sites in the same way SAT tones were used on AMPS cellular systems. The first system in an area typically used the default tone of 3051.9 Hz, while 2918.7 Hz could be used for an additional system on the same channel(s) that might have some coverage overlap. Radios using the system had to have their continuous tone set to match the desired system and have a compatible two-tone sequence for their group.
