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These tones have many names:
Tones are often attached to a repeater's uplink frequency to control access or limit noise/interference. In the commercial radio applications and invented by Motorola, the term is known as PL, which was short for Motorola Private Line. In the amateur radio community, the non-propriety term of Continuous Tone-Coded Squelch System, or CTCSS, if often used.
These tones were originally analog only (see chart below). Later, a digital method was also introduced known as Digital Coded Squelch, or DCS. These methods are only used on analog systems. Other methods for controlling access are available for different transmission modes and are discussed on their respective pages.
The tone system can also be applied to the downlink tone to limit interference and noise on the user's radio. There are many sources of noise that can break the set squelch threshold, even though the repeater is not transmitting. Although the radio will still internally “hear” the noise, it will not open the squelch and transfer the audio to the speaker until the proper tone has been received.
Not all repeaters use a downlink tone. If a user sets tone squelch in their radio on a tone that is not being transmitted by the repeater, the squelch will never open and the user will never hear a transmission. Also, repeaters may not transmit the same tone that is used to receive (uplink). This is called “split-toning.” One should not make any assumptions as to whether or not a repeater is transmitting a tone and what the tone could.
Downlink tone is not a term that is recognized by most amateurs and you probably won't see it in many radio manuals. Although, Yaesu tends to use the term. It's even been discussed on Reddit. To reduce confusion as to which tone is being referenced in terms of a paired frequency set (duplex with separate input and output frequencies, as repeaters, typically use, the downlink tone refers to the tone that the repeater is transmitting on the downlink frequency. If a user programs this feature into their radio, it is often known as “Tone Squelch.” It is not required that a user radio program this tone to receive the repeater's downlink frequency.
Confusion sometimes occurs when it is not known from which radio, the user's or the repeater's, that is being requested. Usually, it seems to get turned around when a repeater owner enters that data. Many repeater coordination councils request the repeater's tones in terms of the repeater's transmit and receive tones, which are opposite from the user radio's on a duplex system.
RepeaterBook enters and displays data from the perspective of the remote (user) radio.
All tones are in Hz.