Continuous Tone-Coded Squelch System
In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is one type of circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel. (See squelch.) It is sometimes referred to as tone squelch. It does this by adding a low frequency audio tone to the voice. Where more than one group of users is on the same radio frequency (called co-channel users), CTCSS circuitry mutes those users who are using a different CTCSS tone or no CTCSS. It is sometimes referred to as a sub-channel, but this is a misnomer because no additional channels are created. All users with different CTCSS tones on the same channel are still transmitting on the identical radio frequency, and their transmissions interfere with each other, however the interference is masked under most (but not all) conditions. The CTCSS feature also does not offer any security.
A receiver with just a carrier or noise squelch unmutes for any sufficiently strong signal; in CTCSS mode it unmutes only when the signal also carries the correct sub-audible audio tone. The tones are not actually below the range of human hearing, but are poorly reproduced by most communications-grade speakers and in any event are usually filtered out before being sent to the speaker or headphone. CTCSS can be regarded as a form of in-band signaling.
As a simple example, suppose a two-way radio frequency is shared by a pizza delivery service and a landscape maintenance service. Conventional radios without CTCSS would hear all transmissions from both groups. The landscapers have to listen to the pizza shop and the pizza shop has to hear about landscape activity. With CTCSS and a different tone for each group, radios hear only the activity from their own group. This is supposed to reduce missed messages and the distraction of unnecessary radio chatter for the other users.
Note that in the example above there are only two co-channel users. In dense two-way radio environments, many separate groups may co-exist on a single radio channel.
The disadvantage of using CTCSS in shared frequencies is that users cannot hear transmissions from other groups. They may erroneously assume that the frequency is idle and then transmit at the same time as another user, thus interfering with the other group's transmissions. For example, in the above situation, a landscaper might be communicating with another landscaper. Meanwhile, a pizza delivery driver—not hearing any transmissions—assumes that the frequency is clear and calls their dispatch office. The two simultaneous transmissions could interfere with each other—resulting in one or both not being understood. The more groups that share a single frequency and the more frequently they transmit, the more likely that this accidental interference will occur. Radios equipped with the "Busy Channel Lockout" feature will prevent transmitting in this case.
Theory of operation
Radio transmitters using CTCSS always transmit their own tone code whenever the transmit button is pressed. The tone is transmitted at a low level simultaneously with the voice. This is called CTCSS encoding. CTCSS tones range from 67 to 257 Hz. The tones are usually referred to as sub-audible tones. In an FM two-way radio system, CTCSS encoder levels are usually set for 15% of system deviation. For example, in a 5 kHz deviation system, the CTCSS tone level would normally be set to 750 Hz deviation. Engineered systems may call for different level settings in the 500 Hz to 1 kHz (10–20%) range.
The ability of a receiver to mute the audio until it detects a carrier with the correct CTCSS tone is called decoding. Receivers are equipped with features to allow the CTCSS "lock" to be disabled. On USA licensed systems, Federal Communications Commission rules require CTCSS users on shared channels to disable their receiver's CTCSS to check if co-channel users are talking before transmitting. On a base station console, a microphone may have a split push-to-talk button. Pressing one half of the button, (often marked with a speaker icon or the letters "MON", short for "MONitor") disables the CTCSS decoder and reverts the receiver to hearing any signal on the channel. This is called the monitor function. There is sometimes a mechanical interlock: the user must push down and hold the monitor button or the transmit button is locked and cannot be pressed. This interlock option is referred to as compulsory monitor before transmit (the user is forced to monitor by the hardware design of the equipment itself). On mobile radios, the microphone is usually stored in a hang-up clip or a hang-up box containing a microphone clip. When the user pulls the microphone out of the hang-up clip to make a call, a switch in the clip (box) forces the receiver to revert to conventional carrier squelch mode ("monitor"). Some designs relocate the switch into the body of the microphone itself. In hand-held radios, an LED indicator may glow green, yellow, or orange to indicate another user is talking on the channel. Hand-held radios usually have a switch or push-button to monitor. Some modern radios have a feature called "Busy Channel Lockout", which will not allow the user to transmit as long as the radio is receiving another signal.
A CTCSS decoder is based on a very narrow bandpass filter which passes the desired CTCSS tone. The filter's output is amplified and rectified, creating a DC voltage whenever the desired tone is present. The DC voltage is used to turn on, enable or unmute the receiver's speaker audio stages. When the tone is present, the receiver is unmuted, when it is not present the receiver is silent.
In a communications receiver designed for CTCSS, a high-pass audio filter is supposed to block CTCSS tones (below 300 Hz) so they are not heard in the speaker. Since audio curves vary from one receiver to another, some radios may pass an audible level of the CTCSS tone to the speaker. Lower tone frequencies generally are less audible. If the magenta audio curve shown at right were plotted from a CTCSS-equipped receiver, it would drop nearly straight down below 300 Hz.
Because period is the inverse of frequency, lower tone frequencies can take longer to decode (depends on the decoder design). Receivers in a system using 67.0 Hz can take noticeably longer to decode than ones using 203.5 Hz, and they can take longer than one decoding 250.3 Hz. In some repeater systems, the time lag can be significant. The lower tone may cause one or two syllables to be clipped before the receiver audio is unmuted (is heard). This is because receivers are decoding in a chain. The repeater receiver must first sense the carrier signal on the input, then decode the CTCSS tone. When that occurs, the system transmitter turns on, encoding the CTCSS tone on its carrier signal (the output frequency). All radios in the system start decoding after they sense a carrier signal then recognize the tone on the carrier as valid. Any distortion on the encoded tone will also affect the decoding time.
Engineered systems often use tones in the 127.3 Hz to 162.2 Hz range to balance fast decoding with keeping the tones out of the audible part of the receive audio. Most amateur radio repeater controller manufacturers offer an audio delay option—this delays the repeated speech audio for a selectable number of milliseconds before it is retransmitted. During this fixed delay period (the amount of which is adjusted during installation, then locked down), the CTCSS decoder has enough time to recognize the right tone. This way the problem with lost syllables at the beginning of a transmission can be overcome without having to use higher frequency tones.
In early systems, it was common to avoid the use of adjacent tones. On channels where every available tone is not in use, this is good engineering practice. For example, an ideal would be to avoid using 97.4 Hz and 100.0 Hz on the same channel. The tones are so close that some decoders may periodically false trigger. The user occasionally hears a syllable or two of co-channel users on a different CTCSS tone talking. As electronic components age, or through production variances, some radios in a system may be better than others at rejecting nearby tone frequencies.
CTCSS is an analog system. A later Digital-Coded Squelch (DCS) system was developed by Motorola under the trademarked name Digital Private Line (DPL). General Electric responded with the same system under the name of Digital Channel Guard (DCG). The generic name is CDCSS (Continuous Digital-Coded Squelch System). The use of digital squelch on a channel that has existing tone squelch users precludes the use of the 131.8 and 136.5 Hz tones as the digital bit rate is 134.4 bits per second and the decoders set to those two tones will sense an intermittent signal (referred to in the two-way radio field as "falsing" the decoder).
List of tones
CTCSS tones are standardized by the EIA/TIA. The full list of the tones can be found in their original standard RS-220A, and the most recent EIA/TIA-603-E; the CTCSS tones also may be listed in manufacturers instruction, maintenance or operational manuals. Some systems use non-standard tones. The NATO Military radios use 150.0 Hz, and this can be found in the user manuals for the radios. Some areas do not use certain tones. For example, the tone of 100.0 Hz is avoided in the United Kingdom since this is twice the UK mains power line frequency; an inadequately smoothed power supply may cause unwanted squelch opening (this is true in many other areas that use 50 Hz power). Squelch tones typically come from one of three series as listed below along with the two character PL code used by Motorola to identify tones. The most common set of supported squelch tones is a set of 39 tones including all tones with Motorola PL codes, except for the tones 8Z, 9Z, and 0Z (zero-Z). The lowest series has adjacent tones that are roughly in the harmonic ratio of 20.05 to 1 (≈1.035265), while the other two series have adjacent tones roughly in the ratio of 100.015 to 1 (≈1.035142). An example technical description can be found in a Philips technical information sheet about their CTCSS products.
- 1 Non-standard numerical codes. Many radios use a matching set of numerical codes to represent corresponding tones; however, there is no published standard and only partial industry adoption.
- 2 Some radios use 69.4 Hz instead, which better fits the harmonic sequence, and this tone is often omitted as a choice.
- 3 Also known by the code SP.
- 4 Not actually in this harmonic sequence, but an average of the ZA and 1Z tones used to fill the gap between the lower and middle sequences. 98.1 Hz would be the tone after ZA, and the tone before 1Z would be 96.6 Hz, assuming the same harmonics were used.
- 5 Many NATO (military) radio have a switchable 150.0 Hz tone. The list includes the following radios: AN/PRC-68, AN/PRC-117F, AN/PRC-117G, AN/PRC-77, AN/PRC-113, AN/PRC-137, AN/PRC-139, AN/PRC-152, AN/PRC-119, AN/VRC-12, AN/PSC-5, and Thales 148 MBITR.
- 6 The 8Z, 9Z, and 0Z ("zero-Z") tones are often omitted from radios that use the M1–M7 series of tones.
- 7 Non-standard tone not included in current TIA-603-E.
CTCSS is often called PL tone (for Private Line, a trademark of Motorola), or simply tone. General Electric's and Bendix King's implementation of CTCSS is called Channel Guard (or CG). Vintage RCA radios called their implementation Quiet Channel. Icom radios call this feature C.Tone. Kenwood radios call the feature Quiet Talk or QT. E. F. Johnson Corp. used "TG" for "ToneGuard", and later "CG" for "CallGuard". Zetron literature refers to "ToneLock", and Ritron, Inc. labels their implementations "Quiet Call" (QC) and "Digital Quiet Call" (DQC). There are many other company-specific names used by radio vendors to describe compatible options. Any CTCSS system that has compatible tones and levels is interchangeable. Old and new radios with CTCSS and radios across manufacturers are compatible.
In amateur radio, the terms PL tone, PL and simply tone are still used somewhat commonly. Often, there is a distinction between the terms tone and tone squelch, in which the former refers to the use of transmitting a CTCSS tone while using standard carrier squelch on the receiver. Use of transmit-only CTCSS allows stations to communicate with repeaters and other stations using CTCSS while the link is marginal and the CTCSS tones may not be properly decoded. The term tone squelch most often includes tone and the radio will not only transmit a CTCSS tone to the distant station or repeater, but will squelch all incoming signals that do not also include the CTCSS tone. This is helpful in areas where multiple repeaters may be sharing the same output frequency but have different CTCSS tones, or where local interference is too strong for the front-end of your radio.
Some professional systems use a phase-reversal of the CTCSS tone at the end of a transmission to eliminate the squelch crash or squelch tail. This is common with General Electric Mobile Radio and Motorola systems. When the user releases his push-to-talk button the CTCSS tone does a phase shift for about 200 milliseconds. In older systems, the tone decoders used mechanical reeds to decode CTCSS tones. When audio at a resonant pitch was fed into the reed, it would resonate / vibrate, which would turn on the speaker audio. The end-of-transmission phase reversal (called "Reverse Burst" by Motorola (and trademarked by them) and "Squelch Tail Elimination" or "STE" by GE ) caused the reed to abruptly stop vibrating which would cause the receive audio to instantly mute. Initially, a phase shift of 180 degrees was used, but experience showed that a shift of ±120 to 135 degrees was optimal in halting the mechanical reeds. These systems often have audio muting logic set for CTCSS only. If a transmitter without the phase reversal feature is used, the squelch can remain unmuted for as long as the reed continues to vibrate—up to 1.5 seconds at the end of a transmission as it coasts to a stop (sometimes referred to as the "flywheel effect" or called "freewheeling"). Thus, there is one caveat about all CTCSS being interchangeable — the shift angle must match if the phase changing system is to be effective. Note that the hardware used to implement the "reverse burst" / "squelch tail elimination" system is all contained in the transmitter.
Interference and CTCSS
In non-critical uses, CTCSS can also be used to hide the presence of interfering signals such as receiver-produced intermodulation. Receivers with poor specifications—such as scanners or low-cost mobile radios—cannot reject the strong signals present in urban environments. The interference will still be present and may block the receiver, but the decoder will prevent it from being heard. It will still degrade system performance but the user will not have to hear the noises produced by receiving the interference.
CTCSS is commonly used in VHF and UHF amateur radio operations for this purpose. Wideband and extremely sensitive radios are common in the amateur radio field, which imposes limits on achievable intermodulation and adjacent-channel performance.  Often all repeaters in a geographical region share the same CTCSS tone as a method of reducing co-channel interference from adjacent regions and increasing frequency reuse. This is a practice linked back to an old FCC practice of coordinating CTCSS tones for business services. In many rural areas of the USA where no coordination is necessary, a default of 100 Hz has become a de facto standard.
Family Radio Service (FRS), PMR446 and other consumer-grade "bubble pack" radios often include a feature called "Interference Eliminator Codes", "sub-channels", or "privacy codes". These do not afford privacy or security, but serve only to reduce annoying interference by other users or other noise sources; a receiver with the tone squelch turned off will hear everything on the channel.  GMRS/FRS radios offering CTCSS codes typically provide a choice of 38 tones, but the tone number and the tone frequencies used may vary from one manufacturer to another (or even within product lines of one manufacturer) and should not be assumed to be consistent (i.e. "Tone 12" in one set of radios may not be "Tone 12" in another). When a radio offers more than 50 codes (121 is becoming common), the higher ones (e.g. 39–121) are usually DCS codes.
Adding tone or digital squelch to a radio system doesn't solve interference issues, it just covers them up. Interfering signals masked by tone squelch will produce apparently random missed messages. The intermittent nature of interfering signals will make the problem difficult to reproduce and troubleshoot. Users will not understand why they cannot hear a call, and will lose confidence in their radio system. In a worst-case scenario in a life safety environment a missed message, or a misunderstood message, may result in fatalities.
Using coded squelch systems can prevent weak signals from being received, for example, when the person transmitting or receiving is in an area obstructed by buildings or terrain or is a long distance away. A well tuned receiver that isn't configured to require CTCSS or DCS tones to open the squelch might still carry the weak message along with static noise, while a coded squelch enabled receiver may not perceive the tones and will ignore the message entirely.
- Mike Morris WA6ILQ (29 January 2013), A Historical and Technical Overview of Tone Squelch Systems, retrieved 2013-03-06
- EIA Standard RS-220-A, Continuous Tone-Controlled Squelch Systems (CTCSS). March 1979.
- Land Mobile FM or PM – Communications Equipment – Measurement and Performance Standards, TIA-603-E (Technical report). Telecommunications Industry Association (TIA). March 2, 2016. p. 10.
- List of non-standard CTCSS codes
- "CTCSS Compatibility in FRS Radios". Archived from the original on 2001-09-24.
- "Information Sheet: Continuous Tone Controlled Squelch System" (PDF). Retrieved 2019-07-08.
- Explanation of Reverse Burst & "And Squelch" Archived 2008-01-19 at the Wayback Machine - Kevin K. Custer W3KKC
- Mark J. Wilson (ed.), The ARRL Operating Manual for Radio Amateurs , American Radio Relay League, 2007, ISBN 0872591093 pp. 2-12 - 2-13
- H. Ward Silver, Two-Way Radios and Scanners For Dummies, John Wiley & Sons, 2011, ISBN 1118054601, pp.62-64
- Robert H. Eisner (23 July 1999), CTCSS Compatibility in FRS Radios, retrieved 2014-08-06