CTCSS Tones and DCS Codes Explained for Repeater Users

The first time you program a handheld for a local repeater, the frequency and offset are the easy part. Then comes the box labelled tone. Enter the wrong value and you hear the repeater but it ignores you, or you key up and nothing happens. Enter the right one and everything works, yet you may still not know what the tone does. This guide explains CTCSS tones and their digital cousin DCS: what they are, why repeaters use them, how they differ from other kinds of squelch and what to check when something does not work.

The technical details come from the public encyclopedia articles on CTCSS and on squelch, which are linked below, and which cite the industry standards. Your own repeater’s rules always come first, so check the local repeater list for the correct values.

Squelch in One Minute

A receiver’s squelch circuit mutes the audio when there is no useful signal, so you do not listen to hiss all day. The squelch article lists the basic types. Carrier squelch works on signal strength alone. Noise squelch is more reliable on FM, because it listens to the noise that disappears when a real carrier is present. Both have one weakness: they open for any strong enough signal, including interference.

Selective calling solves that by adding a lock. As the article describes, where several users share a channel, the receiver audio opens only when the correct code is present. A carrier squelch is unlocked and lets any signal in, while selective calling locks out everything except signals carrying the correct key. Four techniques are commonly used: CTCSS, tone burst or Selcall, DCS and the newer XTCSS.

What CTCSS Is

The encyclopedia article on CTCSS explains that the Continuous Tone-Coded Squelch System adds a low-frequency audio tone to the voice. A receiver set for that tone unmutes only when it hears the tone, and ignores users on a different tone or on none. It is also called tone squelch or PL, short for Private Line, a trademark of Motorola. General Electric called its version Channel Guard, and many manufacturers used their own names for compatible features.

The tones range from 67 to 254.1 Hz. They are usually called sub-audible, although the article points out they are not literally below the range of human hearing; they are poorly reproduced by most communications speakers and are filtered out before reaching the speaker. In a typical FM system the tone level is set to about 15 percent of the system deviation, with engineered systems using 10 to 20 percent.

Two common misunderstandings need correcting. First, CTCSS tones are sometimes called sub-channels, but no additional channels are created. Everyone is on the same frequency and their transmissions still interfere with each other, although the interference is often masked. Second, CTCSS is not security. A receiver without CTCSS enabled hears all traffic, and the system gives no protection against interception.

Why Repeaters Use a Tone

Amateur repeaters use CTCSS for access, not privacy. The article on squelch explains that a carrier squelch receiver cannot tell a valid carrier from a spurious signal, so tone squelch is often added to avoid false keyups. That is especially helpful on congested frequencies and on bands that suffer from skip or band openings. A repeater with an access tone ignores the many signals that happen to land on its input frequency. The same idea hides intermodulation from poor receivers. The CTCSS article notes that receivers that cannot reject strong signals in urban environments will still suffer the interference, but the decoder prevents the user from hearing it. For the underlying receiver limits, see our article on receiver dynamic range, blocking and IMD.

There are three things you can set on most amateur radios, though the menu names vary: a tone sent when you transmit, which opens the repeater, a tone required when you receive, which mutes everything not carrying it, and the choice of no tone at all. For ordinary repeater use you need only the transmit tone. Requiring a receive tone as well is useful if you want to silence other users on a shared channel, but it can hide a repeater that is sending a different tone than you expected. Always read your radio’s manual for the exact terms.

Comparing the Squelch Systems

Decoding Speed and Why Low Tones Clip

A subtle engineering point explains why some repeaters seem to cut off the first syllable. A CTCSS decoder is a very narrow filter, and the lower the tone frequency, the longer it takes to recognise. The article says receivers on 67.0 Hz can take noticeably longer to decode than those on 203.5 Hz. In a repeater, the receiver must first sense the carrier, then decode the tone, and only then does the transmitter start, so a low tone can clip a syllable or two. Controller makers offer an audio delay option, a fixed delay of the repeated audio, which gives the decoder time to recognise the tone without having to use higher frequencies. Engineered systems often prefer tones between 127.3 and 162.2 Hz as a compromise between speed and keeping the tone out of audible audio.

The article also mentions a classic trap: avoid adjacent tones such as 97.4 and 100.0 Hz on the same channel, because some decoders may false trigger. And in some regions certain tones are avoided entirely. The tone of 100.0 Hz is avoided in the United Kingdom, for example, because it is twice the mains frequency, so an inadequately smoothed power supply might open the squelch. The same applies in many other 50 Hz countries.

What DCS Is

Digital-Coded Squelch was designed as the digital replacement for CTCSS. The squelch article says that a group of radios share the same DCS code, much as they would share a CTCSS tone. DCS goes by different names: generically CDCSS, Digital Private Line or DPL from Motorola, Digital Channel Guard from General Electric and DTCS from Icom. It adds a sub-audible bitstream at 134.4 bits per second to the transmitted audio. The code word is a 23-bit Golay code with 12 data bits followed by 11 check bits, which can detect and correct errors of three or fewer bits. After the fixed bits, nine code bits remain, written conventionally as a three-digit octal number. Only 83 of the 512 possible codes are used in the standard, to prevent false decoding caused by alignment collisions.

The CTCSS article adds a practical warning: DCS on a channel that already has tone squelch users often rules out the 131.8 and 136.5 Hz tones, because the digital bit rate of 134.4 bits per second can falsely trigger decoders set to those tones.

Reverse Burst and the Squelch Tail

You may have heard the short burst of noise at the end of a transmission through a repeater. This squelch tail happens because the decoder needs a moment to realise that the tone has gone away. Professional systems reduce it with a phase reversal of the CTCSS tone for about 200 milliseconds at the end of the transmission, known as reverse burst or squelch tail elimination, which makes the decoder mute instantly. If your radio does not send it, the receiver may stay open slightly longer. Many amateur repeaters instead use a courtesy tone and a controlled hang time, which is a matter for the repeater owner.

The Standard Tone List and Its Quirks

CTCSS tones are standardised by the EIA and TIA, in the original RS-220A and the later TIA-603 documents, and they are also listed in manufacturers’ manuals. The most common set of supported tones contains 39 values, and the lists are built from three series. In the lowest series adjacent tones are spaced by a ratio of roughly 1.035, which is why the values look irregular: 67.0, 71.9, 74.4, 77.0, 79.7, 82.5, 85.4, 88.5 and so on. Some radios use 69.4 Hz instead of 69.3. Some systems use non-standard tones, such as the 150.0 Hz tone found on many NATO military radios. Because the values are so close together, the numbers in a repeater list have to be typed exactly, and a radio that offers only a numbered list of tones may use different numbers from another make. The numbered codes printed in one manual are not a published standard, so always enter the frequency in hertz when you can.

Consumer Radios Use the Same Tones

You will also meet these systems outside amateur radio. Licence-free consumer radios such as FRS and PMR446 models often include a CTCSS feature under a name such as privacy code or interference eliminator. The squelch article reproduces the code table for PMR446 radios with 38 codes, where code 0 is off and codes 1 to 38 map to the standard tones, and for radios with 121 codes, where codes 39 to 121 are DCS codes. The lesson for amateurs is that the same tone may carry different numbers on a consumer radio and on an amateur one, and that a code on a children’s walkie-talkie does not make the channel private.

Setting Up Your Radio

1. Find the repeater’s values. Use a current local repeater list or the club’s page. The tone is part of the repeater’s access information, along with the frequency, offset and any other requirements. Our guide to VHF and UHF simplex and repeater operating covers the rest of the settings.
2. Check your band plan. Repeater inputs, outputs and offsets differ by region. See our overview of the IARU band plan regions.
3. Set the transmit tone. Choose the value exactly as listed, since 100.0 and 103.5 are different tones and neighbours can confuse a decoder.
4. Test on a quiet moment. Key briefly, give your callsign and listen for the repeater’s response.
5. Use a tone or DCS code for simplex only when it helps. On a busy simplex channel, a receive tone can mute neighbours but also hide stations that do not use it.
6. If something fails, change one setting at a time: tone, offset, then frequency.

If you operate from a rooftop or a vehicle, a good antenna matters as much as the correct tone. Our article on 2 metre antennas describes simple designs.

Common Problems

• You hear the repeater but it ignores you. The transmit tone is wrong, missing or on the wrong decimal.
• You cannot hear the repeater. You may have a receive tone enabled that the repeater does not send.
• Clipped first syllable. The repeater may be using a low tone, or you started speaking too early. Pause before speaking.
• Random noise opens the speaker. That is normal on carrier squelch. A receive tone can mute it, but it will not remove the interference itself.
• DCS and CTCSS mixed up. They are separate settings, and a repeater uses one or the other.

The Bottom Line

CTCSS tones and DCS codes are locks, not secrets. They stop repeaters and receivers from reacting to the wrong signals, and they make crowded channels easier to live with. Learn which one your repeater uses, set the transmit tone exactly, and use a receive tone only when you understand its trade-offs. Remember that nothing about a tone protects your conversation from being heard.

Frequently Asked Questions

What is a CTCSS tone?

It is a low-frequency audio tone, from 67 to 254.1 Hz, added to a transmission so that a receiver set for the same tone will open its squelch and others will stay muted.

Is CTCSS the same as PL?

Yes. PL, or Private Line, is the Motorola name for CTCSS. Other makers used names such as Channel Guard, and compatible equipment works together.

Does CTCSS make my conversation private?

No. A receiver without CTCSS enabled hears everything on the channel, and the system offers no protection against interception.

What is the difference between CTCSS and DCS?

CTCSS uses an analog tone, and DCS sends a 134.4 bit/s digital code written as a three-digit octal number. A given system uses one or the other.

Why do some repeaters clip the start of my transmission?

The repeater must detect the carrier and decode the tone before it transmits, and low tones take longer. Waiting a moment after pressing the key helps.

Why is 100.0 Hz avoided in some countries?

In 50 Hz mains countries such as the UK it is twice the power line frequency, so a poorly smoothed power supply can accidentally open the squelch.