CTCSS (Continuous Tone-Coded Squelch System) Tone Squelch, Pilot Tone. Glossary of radio communication terms and abbreviations

A method of access control in radio communication systems based on the presence of audio tones of specific frequencies within the useful signal, located outside the modulation frequency range and below the audible spectrum at frequencies under 300 Hz. The receiver of a radio station is activated only when a pre-programmed CTCSS tone is detected. This is a standard function in most modern radio equipment. A more advanced access control method is DCS.

The tone encoding function is used to divide correspondents (users) into groups operating on the same radio channel. Only those with the same CTCSS code (tone) can listen and transmit within their group. Those not tuned to the correct CTCSS code will have such transmissions suppressed as unwanted noise, resulting in complete silence.
When transmitting, a sub-tone signal (below 300 Hz) of a specific frequency (determined by the CTCSS code) is sent. Upon reception, the CTCSS squelch instantly recognizes the signal as “own” or “foreign.” If the code is “own,” the radio activates for reception and plays the message; if “foreign,” it remains muted and the correspondent hears nothing.

In other words, when you receive a signal from a subscriber whose code differs from that set on your radio, you will not hear that subscriber. Moreover, the signals you transmit will only be heard by a subscriber whose radio tone code matches yours.

CTCSS is also used for more effective noise suppression in the 40 MHz (Low Band) range. In such cases, the noise level may be high enough to completely block the receiver path; however, an active CTCSS decoder will prevent such noise from being output to the speaker.
It should be noted that using CTCSS slightly reduces the reception sensitivity of a radio, as low-level transmissions may be interpreted by the CTCSS decoder as noise, causing the squelch to constantly “click” or remain closed entirely, potentially leading to partial or complete loss of received information.

In practice, when communicating between different types of radios, it is preferable to match not the sub-tone number but its frequency — according to the user manual. The best frequency choice is neither too low (to reduce tone detection time) nor too high (to avoid conflicts with voice signals in the cheapest models), optimally from 120 to 200 Hz. At the same time, frequencies that are multiples of the mains frequency (e.g., 50 Hz in Russia) — such as near 100, 150, and 200 Hz — should be avoided.

(!) Note: Different manufacturers use different names for CTCSS. For example, Motorola labels CTCSS as "PL" (Private Line), GE/Ericsson as "CG" (Channel Guard), Icom as "C.Tone", Johnson as "TG" (ToneGuard) or later "CG" (CallGuard), Zetron as "ToneLock", Ritron Inc. as "QC" (Quiet Call), and Kenwood as "QT" (Quiet Talk).

Table of 64 CTCSS Tones

Related articles: DCS, DTMF.