Continuous Tone-Coded Squelch System - Reverse CTCSS

Reverse CTCSS

Some professional systems use a phase-reversal of the CTCSS tone at the end of a transmission to eliminate the squelch crash or squelch tail. This is common with General Electric Mobile Radio and Motorola systems. The CTCSS tone does a phase shift for about 200 milliseconds at the end of a transmission. In old systems, decoders used mechanical reeds to decode CTCSS tones. When audio at a resonant pitch was fed into the reed, it would resonate / vibrate, whhich would turn on the speaker audio. The end-of-transmission phase reversal (called "reverse burst" by Motorola and "squelch tail elimination" or "STE" by GE ) caused the reed to abruptly stop vibrating which would cause the receive audio to mute. Initially, a phase shift of 180 degrees was used, but experience showed that a shift of ±120 to 135 degrees was optimal in halting the mechanical reeds. These systems often have audio muting logic set for CTCSS only. If a non-Motorola transmitter, (without the phase reversal feature,) is used, the squelch can remain unmuted for as long as the reed continues to vibrate — up to 1.5 seconds at the end of a transmission as it coasts to a stop (sometimes referred to as the "flywheel effect" or called "freewheeling"). Thus, there is one caveat about all CTCSS being interchangeable. Note that the "reverse burst" / "squelch tail elimination" system is all contained in the transmiter.