Much controversy swirls whenever a repeater operator decides to "go PL" or add Continuous Tone Controlled Squelch System encoding and decoding to their repeater(s). So here is a quick explanation on CTCSS and why it is used on the W4RAT repeaters.
CTCSS is a system used in many professional telecomunications systems to reduce the problems of listening to other users on the same frequency. The 2 meter repeater input is on 146.28MHz and it transmits on 146.88MHz. There are repeaters in both Raleigh, NC and Silver Spring, MD on the same pair of frequencies. Now why would SERA coordinate the same frequency pairs on systems so close together? Well, the reality of the situation is that coordination was not always used when repeaters were put on the air. Informal coordination was the norm ("Hey, does anyone know of a repeater on X/Y pair around here?") So many systems came on the air before formal, scientifically designed coordination efforts became necessary. Also, there is a very high density of repeaters in the mid-Atlantic area - try to find an open 2 meter pair...it's nearly impossible. So, some overlap is likely to occur on almost any frequency, particularly if the systems are wide coverage systems as ours are.
What happens is that atmospheric conditions can enhance propogation to the point that users in the Raleigh and Silver Springs area can actually reach the input of our repeater, thus keying W4RAT and transmitting their traffic (usually noisy and sounding off frequency for many reason) through our system. Other systems can cause intermodulation distortion and also key up our repeaters and re-transmit objectionable noise.
The way this is prevented is by requiring OUR users to transmit a sub-audible tone continuously while keying their transmitter (called encoding). These tones are actually within our hearing range, but the design of our FM transmitting systems limit the bandpass of our voice audio to about 300 to 3000 Hz. This subaudible tone is decoded by the repeater receiver, and it uses that signal, rather than a carrier or lack of noise, to open the squelch and pass the audio on to the transmitter. It also triggers the push-to-talk (PTT) circuit on the transmitter. If an interference signal, whether a far-away repeater and its users, or mixed-frequency signals caused by intermodulation does not have this particular frequency on its signal, our receiver does not open its squelch, and we do not hear the offending signal at all. If you have your CTCSS engaged on your receiver, watch the S meter when the receiver is sqelched. On days when there would be objectionable interference on our repeater, you can often see your S meter bouncing up and down while you listen to no noise.
This brings us to the second part of the CTCSS system. If the receiver in your station, mobile or fixed, is equipped with CTCSS decoding, then by turning it on, you will not hear the noise generated by intermod distortion or long-distance repeaters on our transmitter frequency that use our CTCSS tone. Our repeater transmits the same tone on the re-transmitted audio it receives. If your receiver has CTCSS decoding enabled, then our transmitter is the only signal that will open your squelch on that channel.
The only downside is that very old transmitters and receivers may not have built-in CTCSS capabilities. Some units can be retrofitted with manufacturer supplied systems or third-party CTCSS boards. Fortunately, there are not many hams using equipment that old any more, and the enhancement to everyone else makes it almost mandatory to use CTCSS today.
If you want more information about CTCSS, check out Wikipedia:
CTCSS