Continuous Tone-Coded Squelch System - Theory of Operation

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 as many as 50 (depending on which "standard" is used) precise, very low distortion, low-pitched 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 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 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.