CTCSS/PL

CTCSS stands for Continuous Tone Coded Squelch System, often referred to as "PL" for Private Line (Motorola's trade name), GE calls it Channel Guard. CTCSS is a sub audible tone in the range of 67 to 254.1 Hz. These tones are encoded with the voice audio at all times during the transmission. Many repeaters require the use of a PL tone to access the repeater.

CTCSS is designed around a set of relatively low frequency tones, 32 or 42 depending on which "standard" you use. The tones are a perfect sine wave and the frequency tolerance is very tight, typically +/- 0.5 Hz. The tone is encoded and injected into the transmitter after the audio shaping circuits. The frequency deviation (level) is typically 0.4 to 0.8 kHz, which is rather insignificant when compared to the typical 5 kHz voice deviation. In the receiver, the tone is detected right off the discriminator before any audio processing and decoded allowing the receiver to unmute.

Contrary to popular belief, many repeaters that require the use of a specific PL tone to access the repeater are NOT closed repeaters. There are many repeaters that require CTCSS tones to activate them who welcome any and all users regardless of membership status. PL is often used as a means of solving an interference problem, or preventing one in the first place. PL is a minor inconvenience when you consider how many potential problems it can eliminate. The use of PL may be required for a repeater coordination to be granted if conditions so warrant, such as proximity to a co-channel repeater, or in an area where band openings frequently aggravate co-channel interference problems. Some repeaters may also generate a PL tone on the repeater output so that repeater users who are equipped with a radio capable of decoding PL will not hear other interference sources on the channel that would otherwise open the user's radio's squelch.

The table below displays the PL frequencies and their associated EIA code.