Also known as: CTCSS, PL, tone squelch, Continuous Tone-Coded Squelch System
CTCSS (Continuous Tone-Coded Squelch System, and known by trade names such as Motorola’s PL / Private Line) is a scheme that transmits a continuous sub-audible tone — a single pure tone between about 67 and 254 Hz — underneath FM voice so that a receiver’s squelch opens only when it detects the matching tone.1 It lets several user groups share one radio channel without hearing each other’s traffic.
How it works
The transmitting radio adds a low-level continuous tone to the audio that frequency-modulates the carrier, at a modest deviation (typically ~500-800 Hz, well below the voice peaks). Because the tone sits below the 300 Hz low end of communications voice, the receiver can split it off with a simple high-pass/low-pass pair: voice above 300 Hz goes to the speaker, and the sub-audible region below it goes to a tone detector. If the detected tone matches the programmed frequency from the standardized set (there are around 50 defined tones, e.g. 100.0, 141.3, 203.5 Hz), the receiver’s squelch opens; otherwise the audio stays muted even though a signal is fully present and quieting the noise squelch. Being continuous, the tone is present the whole time the carrier is keyed, so the squelch can close promptly when it disappears.
CTCSS does not provide privacy — anyone with carrier squelch or a scanner hears everything on the channel — it only manages who your radio bothers you with. It is also widely used as a repeater access tone: a repeater keys up only for users transmitting the correct CTCSS, rejecting interference and distant co-channel signals.
To avoid a burst of noise when a transmission ends, many radios send a reverse burst (also called a turn-off phase shift): just before unkeying, the transmitter shifts the CTCSS tone’s phase by about 120-180°, which the receiver detects as a loss of valid tone and uses to close its squelch a fraction of a second early, muting the “squelch tail” before the carrier actually drops. This is the analog-tone equivalent of the turn-off codeword that DCS uses for the same purpose. The standardized tone set is chosen so no tone is a harmonic of another, which keeps a strong low tone from falsely triggering a detector tuned to a higher one.
Relevance to SDR
For a scanner, CTCSS is a useful sorting and identification tool. A software receiver that FM-demodulates a channel can run a narrow tone detector (a Goertzel or a short bank of bandpass filters) over the sub-audible region to read out which tone is present, letting the user log or filter transmissions by group even on a shared channel. This is exactly analogous to the digital DCS code that serves the same purpose with a low-speed data burst rather than a tone. CTCSS belongs to the analog FM world; GopherTrunk targets digital trunking, where group separation is handled at the protocol layer by talkgroup IDs rather than sub-audible tones, so it does not decode CTCSS in its trunking path — but the same tone-detection technique applies when working with conventional FM audio.
In practice
Because CTCSS tones are so close together (100.0 vs 103.5 Hz, for instance) a software detector needs good frequency resolution, which in turn needs a fair-length observation window — a fraction of a second of audio. That is not a problem for a continuous tone but it does mean a decoder cannot report the tone instantly at the start of a transmission. A common implementation runs several Goertzel detectors, one per candidate tone, and declares a match when one clearly dominates for long enough to rule out a transient. Note that CTCSS is orthogonal to squelch quality: it only decides whether to unmute, so a receiver still needs a good signal-to-noise ratio for the audio itself to be usable once the tone gate opens.
Sources
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Continuous Tone-Coded Squelch System — Wikipedia, for the sub-audible tone set, squelch gating, and repeater-access use. ↩