CTCSS and DCS
| CTCSS and DCS | |
|---|---|
| What they are | Tone squelch systems for FM radios |
| CTCSS | Continuous Tone-Coded Squelch System: a sub-audible tone, 67.0–254.1 Hz |
| CTCSS also called | PL tone (Motorola's "Private Line"), Channel Guard, tone |
| DCS | Digital-Coded Squelch: a slow digital code, written as three digits such as 023 |
| On the radio | |
| Encode | Transmitting the tone or code; often labeled TONE or T |
| Decode | Opening the squelch only for the tone; often TSQL or CT |
| Main uses | Repeater access; keeping a receiver quiet |
| Not | A privacy feature; anyone can still listen |
CTCSS and DCS are two ways of adding an inaudible signal to an FM transmission so that a receiver can tell wanted signals from unwanted ones. Together they are known as tone squelch. Most amateur repeaters use one or the other to decide whether to retransmit what they hear, and any FM radio can use them to stay silent until a station with the right tone or code comes on. A wrong or missing tone setting is one of the most common reasons a new ham can't get into a repeater.
CTCSS
CTCSS, the Continuous Tone-Coded Squelch System, mixes a steady, low-level audio tone with the transmitted voice. The standard tones run from 67.0 to 254.1 Hz, and most radios offer a set of about 50 of them, unevenly spaced, such as 88.5, 100.0, 123.0 and 146.2 Hz. Because most receivers filter out audio below about 300 Hz, listeners don't hear the tone, which is why it is described as sub-audible. Hams usually call it a PL tone, after Motorola's trade name Private Line; General Electric's name for the same system was Channel Guard.
Encode and decode
A radio can use a tone in two separate ways, and its menus keep them apart:
- Encode means transmitting the tone. This is what a repeater listens for before it will key up, so it is the setting that matters when programming a repeater channel. Radios often label it TONE or T.
- Decode means the receiver stays muted until it hears the tone. Most radios combine encode and decode in a setting called tone squelch, labeled TSQL, CT or CTCSS depending on the maker.
| Setting | Sends a tone | Needs a tone to unmute | Typical use |
|---|---|---|---|
| Off | No | No | Simplex; repeaters with no tone access |
| Tone (encode) | Yes | No | Most repeater channels |
| Tone squelch (encode and decode) | Yes | Yes | Busy or shared channels; mixed analog and digital repeaters |
| DCS | Yes (a code) | Yes (a code) | Repeaters and channels that use DCS |
Decode on a repeater channel works only if the repeater sends a tone on its output. Many do, often the same tone as their input, but not all, and a radio set to tone squelch on a repeater that sends no tone will hear nothing at all. Tone squelch is not privacy: anyone whose radio has decode turned off hears every transmission.
DCS
DCS, Digital-Coded Squelch, which Motorola calls Digital Private Line (DPL), does the same job with a continuous low-speed digital code instead of a tone. The code repeats a 23-bit word at 134.4 bits per second, again below the range of the speaker audio. Codes are written as three-digit octal (base-8) numbers such as 023 or 754, so the digits 8 and 9 never appear, and radios offer a standard list of a little over 100. A code can be sent with normal or inverted polarity; some radios offer both, and transmitter and receiver must agree. DCS is much less common than CTCSS on amateur repeaters, though some use it, and it is widespread in commercial and public-safety radio.
Why repeaters use them
A repeater with no tone access opens for anything on its input frequency: a distant station on the same frequency pair during tropospheric ducting, intermodulation (spurious signals produced when strong nearby transmitters mix), electrical noise, or a user aiming at a different repeater. Requiring a tone means that only users who intend to use that repeater key it up. Neighboring repeaters that share a frequency pair usually use different tones, so a user in the overlap reaches only the intended one.
The same idea keeps a listener's radio quiet. In a city full of strong transmitters, or on a channel shared with other users, tone squelch lets a radio stay silent except for the station or repeater it cares about. Analog users of mixed-mode System Fusion repeaters use it so that digital transmissions, which carry no tone, aren't heard as noise. Digital voice systems have their own equivalents, such as the color code in DMR.
Finding a tone
Repeater directories and club websites list each repeater's tone. If a listing is missing or out of date, many radios have a tone search (or tone scan) function: while a signal is being received, the radio steps through the standard tones or codes until it finds the one present, then displays it. On a repeater's output this works only if the repeater transmits a tone. If it doesn't, listening directly on the repeater's input frequency while a nearby user transmits can reveal the tone that user is sending.