CTCSS

In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is a circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel. It is sometimes called tone squelch. Where more than one user group is on the same channel (called co-channel users), CTCSS filters out other users if they are using a different CTCSS tone or no CTCSS.

In a CTCSS radio circuit, instead of opening the receive audio for any signal, the two-way radio receiver's audio opens only in the presence of the normal RF signal AND the correct sub-audible audio tone (sub-audible meaning that the receiver circuitry can detect it, but is not apparent to the users in the audio output). This is akin to the use of a lock on a door. A carrier squelch or noise squelch receiver not configured with CTCSS is unlocked and will let any signal in. A receiver with CTCSS locks out all signals except ones encoded with the correct tone. CTCSS can be regarded as a form of in-band signaling

Example

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 would have to listen to the pizza shop. The pizza shop would have to hear about landscape customer complaints. If both installed CTCSS, units from each group would only hear radios from their own group. This is supposed to reduce missed messages and the distraction of unnecessary radio chatter.

Note that in the example above there are only two co-channel users. In dense two-way radio environments a large number of groups may be present on a single radio channel.

Theory of operation

Radios in a professional two-way radio system using CTCSS always transmit their own tone code whenever the transmit button is pressed (simultaneously with the voice). This is called CTCSS encoding. CTCSS continuously superimposes any one of 32, 38 or 40 (depending on which "standard" is used) precise, low distortion, low-pitch audio tones on the transmitted signal, ranging 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. In professional 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 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 equipment design.) On mobile radios, the microphone is usually stored in a hang-up clip or hang-up box. 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 slide 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 or unmute the receiver's audio stages. When the tone is present, the receiver is unmuted, when it is not present the receiver is silent.

In a professional 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 take longer to decode. Receivers in a system using 67.0 Hz will take noticeably longer to decode than ones using 203.5 Hz, and they will 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 tone encoder 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 high 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 system was developed by Motorola and is called Digital Private Line, or DPL. General Electric responded with the same system under the name of Digital Channel Guard. 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 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 standard RS-220; the CTCSS tones also may be listed in equipment manuals. Some systems use non-standard tones^[1]. The US Military uses 150.0 Hz, a tone that is unique to them. 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 38 tones including all tones with Motorola PL codes, except for the tones WZ, 8Z, 9Z, and 0Z. ^[2] The lowest series has adjacent tones that are roughly in the harmonic ratio of 2^0.05 to 1 (≈1.035265), while the other two series have adjacent tones roughly in the ratio of 10^0.015 to 1 (≈1.035142).

One concern on tone numbers

Vendor names

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. Kenwood radios call the feature Quiet Talk or QT. Johnson used "TG" for "ToneGuard", and later "CG" for "CallGuard". Zetron literature refers to "ToneLock". There are many other company-specific names used by radio vendors to describe compatible options. Any CTCSS system that has compatible tones 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 used most 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 your 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.

One caveat about all CTCSS being interchangeable is that 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. The CTCSS tone does a phase shift for about 200 milliseconds at the end of a transmission. In old systems, decoders used mechanical reeds to decode CTCSS tones. When audio at a resonant pitch was fed into the reed, it would vibrate on a set of springs, turning on the speaker audio. The end-of-transmission phase reversal (called "reverse burst" by Motorola and "squelch tail elimination" or "STE" by GE ^[3]) caused the reed to abruptly stop vibrating and the receive audio would 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 non-Motorola 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").

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 by using selective calling the user will not have to hear the noises produced by receiving the interference.

CTCSS is very commonly used in amateur radio 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 "bubble pack" radios often use from 10 to 38 different CTCSS tones (the number depends on the manufacturer), usually erroneously called "sub-channels", or "privacy codes" in the sales literature. While these do not add to the available number of conversations which can take place at once in a given area, they do reduce annoying interference experienced by users. However they do NOT afford any privacy, no matter what the sales literature says. A receiver with the tone squelch turned off (i.e. in carrier squelch mode) hears everything.

It is a bad idea to use any coded squelch system to hide interference issues in systems with life-safety or public-safety uses such as police, fire, search and rescue or ambulance company dispatching. Adding tone or digital squelch to a radio system doesn't solve interference issues, it just covers them up. The presence of interfering signals should be corrected rather than masked. 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, will result in a death.