A continuous tone controlled squelch system, CTCSS, is a method used in two-way radio communication systems to control the activation of a receiver’s squelch. Squelch is a circuit that mutes the audio output of a receiver when no signal is being received, in order to eliminate background noise. CTCSS is a sub-audible tone that is added to the transmitted audio signal and used to selectively open the squelch of a receiver that is programmed to recognize that particular tone.
The tone is usually in the range of 67 to 254 Hz and is added to the audio signal before it is transmitted. The receiver on the other end is set to open its squelch only when it detects the specific CTCSS tone that is being transmitted. If the received tone matches the programmed tone in the receiver, the squelch opens and the audio is heard. If the tone does not match, the squelch remains closed and no audio is heard.
The purpose of using CTCSS is to prevent unwanted interference from other radio users on the same frequency. It allows multiple users to share the same frequency without hearing each other’s transmissions, unless they are using the same CTCSS tone. This can be especially useful in busy radio environments where multiple users are transmitting simultaneously.
CTCSS used in amateur radio:
If you use amateur radio repeaters, you will normally need to set your radio to use either CTCSS or DCS (digital controlled squelch). In the good old days, we would use a 1750Hz tone to open repeaters. It was known as tone burst and, if your radio was an old PMR rig with no tone burst, whistling in the microphone would often fire up the repeater.
CTCSS Letter codes:
These letters are often sent as part of a repeater ident. They represent the CTCSS frequency required to access the repeater. For example, an ident in Morse might read: GB3RW E
| A = 67.0Hz | B = 71.9Hz | C = 77.0Hz |
| D = 82.5Hz | E = 88.5Hz | F = 94.8Hz |
| G = 103.5Hz | H = 110.9Hz | J = 118.8Hz |
Carrier accessed repeaters:
Some repeaters are carrier accessed. All you do to open the repeater is press your PTT and simply transmit a carrier on the input frequency Some repeaters on the ten metre amateur band use this simple access method.
Why use a squelch system?
Lets look at a real life example. GB3RW located in Sussex has an output frequency of 145.600MHz. GB3WR in Somerset also has an output frequency of 145.600MHz. Under normal conditions, the two repeaters are too far apart to interfere with each other. Under lift conditions, when the two metre band is open, it might be possible to hear both repeaters at the same time. This might be annoying but something worse can happen. Let’s say that I’m working through GB3RW, transmitting on its input frequency which is 145.000MHz. GB3WR uses the same input frequency so I might also be accessing that repeater. Every time I open my local repeater, I also open the Somerset repeater. Someone in Somerset opening their local repeater might also open my local repeater. This could present a real problem.
CTCSS is the answer:
Let’s add some electronics to my local repeater so that it requires a unique tone to access it. The repeater is listening on 145.000MHz for an 88.5Hz tone. While I’m chatting into the microphone, my radio will transmit a continuous 88.5Hz tone. This will hold the repeater open until I stop transmitting.
The repeater in Somerset is also listening on 145.000MHz. But it’s listening for a different tone, which is 94.8Hz. If propagation conditions are right, the Somerset repeater will hear my tone on its input frequency… and it won’t open. If my local repeater picks up someone opening the Somerset repeater with a tone of 94.8Hz, my repeater won’t open. Several repeaters around the country might use the same input and output frequencies but they will use different access tones. A repeater might pick up your carrier but, unless it hears it’s unique tone, it won’t open.
DCS is another system:
Rather than use a continuous tone to hold a repeater open, it is possible to use a digital controlled squelch. The Digital Coded Squelch system was developed by Motorola. DCS, works by encoding a specific digital code or tone at the transmitting end of a radio communication system. This code is then decoded at the receiving end, allowing only radios programed with the matching code to open the squelch and hear the transmitted audio. Radios without the correct DCS code will not be able to open the squelch and you will not hear the transmitted audio, effectively blocking unwanted signals. DCS is used in professional two-way radio systems, such as commercial applications, where multiple users need to share a limited number of frequencies. It provides an additional layer of privacy and security by allowing users to establish a private code that must be matched by other radios in order to communicate on the same channel.
DCS is also known by other names, such as Digital Private Line (DPL), Digital Channel Guard (DCG), Continuous Tone-Coded Squelch System (CTCSS), or simply “PL tones” or “PL codes” in some radio systems. It is important to note that DCS is a proprietary technology, and different manufacturers may have their own implementations and terminology for similar functionality.
CTCSS Codes:
| Number | Frequency Hz | Number | Frequency Hz |
| 1 | 67.0 | 20 | 131.8 |
| 2 | 71.9 | 21 | 136.5 |
| 3 | 74.4 | 22 | 141.3 |
| 4 | 77.0 | 23 | 146.2 |
| 5 | 79.7 | 24 | 151.4 |
| 6 | 82.5 | 25 | 156.7 |
| 7 | 85.4 | 26 | 162.2 |
| 8 | 88.5 | 27 | 167.9 |
| 9 | 91.5 | 28 | 173.8 |
| 10 | 94.8 | 29 | 179.9 |
| 11 | 97.4 | 30 | 186.2 |
| 12 | 100.0 | 31 | 192.8 |
| 13 | 103.5 | 32 | 203.5 |
| 14 | 107.2 | 33 | 210.7 |
| 15 | 110.9 | 34 | 218.1 |
| 16 | 114.8 | 35 | 225.7 |
| 17 | 118.8 | 36 | 233.6 |
| 18 | 123.0 | 37 | 241.8 |
| 19 | 127.3 | 38 | 250.3 |
DCS Codes:
6 50 125 174 255 343 445 526 703
7 51 131 205 261 346 446 532 712
15 53 132 212 263 351 452 546 723
17 54 134 214 265 356 454 565 731
21 65 141 223 266 364 455 606 732
23 71 143 225 271 365 462 612 734
25 72 145 226 274 371 464 624 743
26 73 152 243 306 411 465 627 754
31 74 155 244 311 412 466 631
32 114 156 245 315 413 503 632
36 115 162 246 325 423 506 654
43 116 165 251 331 431 516 662
47 122 172 252 332 432 523 664
Tone Burst:
A 1750Hz tone burst is a sound signal that consists of a single frequency tone at 1750Hz that is abruptly turned on and off, creating a burst of sound at that frequency. Now outdated, it was used in amateur radio for opening local repeaters.
DTMF:
DTMF, dual tone multiple frequency, is a signalling system used in telecommunications for transmitting digits or tones over analog telephone lines. DTMF is generated by pressing keys on a telephone keypad or other DTMF enabled devices. Each key corresponds to a unique combination of two frequencies, one from a low-frequency group and one from a high-frequency group. The combination of these two frequencies represents the specific digit or function associated with the pressed key.
The low-frequency group consists of the frequencies 697 Hz, 770 Hz, 852 Hz, and 941 Hz, while the high-frequency group consists of the frequencies 1209 Hz, 1336 Hz, 1477 Hz, and 1633 Hz. When a key is pressed, the corresponding pair of frequencies is transmitted over the phone line, and the receiving end decodes the DTMF tones to identify the pressed key or command.
DTMF has been widely used in telecommunications systems for many years due to its simplicity, reliability, and ease of implementation. However, with the advent of digital communication technologies, DTMF has been largely replaced by other signaling methods such as in-band signaling or digital signaling protocols. Nevertheless, DTMF remains in use for various applications and is still supported by most modern telecommunication systems and devices.