In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver in the absence of a sufficiently strong desired input signal. Squelch is widely used in two-way radios and radio scanners to suppress the sound of channel noise when the radio is not receiving a transmission. Squelch can be 'opened', which allows all signals entering the receivers discriminator tap to be heard. This can be useful when trying to hear distant, or otherwise weak signals.
A carrier squelch or noise squelch is the most simple variant of all. It operates strictly on the signal strength, such as when a television mutes the audio or blanks the video on "empty" channels, or when a walkie talkie mutes the audio when no signal is present. In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. Receivers in base stations or repeaters at remote mountain top sites are usually not adjustable remotely from the control point.
In devices such as two-way radios (also known as radiotelephones), the squelch on a local receiver can be adjusted with a knob, others have push buttons or a sequence of button presses. This setting adjusts the threshold at which signals will open (un-mute) the audio channel. Backing off the control will turn on the audio, and the operator will hear white noise (also called "static" or squelch noise) when there is no signal present. The usual operation is to adjust the control until the channel just shuts off - then only a small threshold signal is needed to turn on the speaker. However, if a weak signal is annoying, the operator can set the control a little higher thereby adjusting the squelch to open only when stronger signals are received.
A typical FM two-way radio carrier squelch circuit is noise operated. It takes out the voice components of the receive audio by passing the detected audio through a high-pass filter. A typical filter might pass frequencies over 4,000 Hz (4 kHz). The squelch control adjusts the gain of an amplifier which varies the level of noise coming out of the filter. The audio output of the filter and amplifier is rectified and produces a DC voltage when noise is present. The presence of continuous noise on an idle channel creates a DC voltage which turns the receiver audio off. When a signal with little or no noise is received, the noise-derived voltage goes away and the receiver audio is unmuted. Some applications have the receiver tied to other equipment that uses the audio muting control voltage as a "signal present" indication, for example in a repeater the act of the receiver unmuting will switch on the transmitter.
Tone squelch and selective calling
Tone squelch, or other forms of selective calling, is sometimes used to solve interference problems. Where more than one user is on the same channel (co-channel users), selective calling addresses a subset of all receivers. Instead of turning on the receive audio for any signal, the audio turns on only in the presence of the correct selective calling code. This is akin to the use of a lock on a door. A carrier squelch is unlocked and will let any signal in. Selective calling locks out all signals except ones with the correct key to the lock (the correct code).
In non-critical uses, selective calling can also be used to hide the presence of interfering signals such as receiver-produced intermodulation. Receivers with poor specifications—such as inexpensive police scanners or low-cost mobile radios—cannot reject the strong signals present in urban environments. The interference will still be present. 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.
Four different techniques are commonly used. Selective calling can be regarded as a form of in-band signalling.
CTCSS (Continuous Tone-Coded Squelch System) continuously superimposes any one of about 50 low-pitch audio tones on the transmitted signal, ranging from 67 to 254 Hz. The original tone set was 10, then 32 tones, and has been expanded even further over the years. CTCSS is often called PL tone (for Private Line, a trademark of Motorola), or simply tone squelch. General Electric's implementation of CTCSS is called Channel Guard (or CG). RCA Corporation used the name Quiet Channel, or QC. 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.
Selcall (Selective Calling) transmits a burst of up to five inband audio tones at the beginning of each transmission. This feature (sometimes called "tone burst") is common in European systems. Early systems used one tone (commonly called "Tone Burst"). Several tones were used, the most common being 1,750 Hz, which is still used in European amateur radio repeater systems. The addressing scheme provided by one tone was not enough, so a two tone system was devised - one tone followed by a second tone (sometimes called a "1+1" system). Later on, Motorola marketed a system called "Quik-Call" that used two simultaneous tones followed by two more simultaneous tones (sometimes called a "2+2" system) that was heavily used by fire department dispatch systems in the USA. Later selective call systems used paging system technology that made use of five sequential tones. In the same way that a single CTCSS tone would be used on an entire group of radios, a single five-tone sequence is used in a group of radios.
DCS (Digital-Coded Squelch), generically known as CDCSS (Continuous Digital-Coded Squelch System), was designed as the digital replacement for CTCSS. In the same way that a single CTCSS tone would be used on an entire group of radios, the same DCS code is used in a group of radios. DCS is also referred to as Digital Private Line (or DPL), another trademark of Motorola, and likewise, General Electric's implementation of DCS is referred to a Digital Channel Guard (or DCG). DCS is also called DTCS (Digital Tone Code Squelch) by Icom, other names by other manufacturers. Radios with DCS options are generally compatible provided the radio's encoder-decoder will use the same code as radios in the existing system.
DCS adds a 134.4 bps (sub-audible) bitstream to the transmitted audio. The code word is a 23-bit Golay (23,12) code which has the ability to detect and correct errors of 3 or fewer bits. The word consists of 12 data bits followed by 11 check bits. The last 3 data bits are a fixed '001', this leaves 9 code bits (512 possibilities) which are conventionally represented as a 3-digit octal number. Note that the first bit transmitted is the LSB, so the code is "backwards" from the transmitted bit order. Only 83 of the 512 possible codes are available, to prevent falsing due to alignment collisions.
XTCSS is the newest signalling technique and it provides 99 codes with the added advantage of "silent operation". XTCSS fitted radios are purposed to enjoy more privacy and flexibility of operation. XTCSS is implemented as a combination of CTCSS and in-band signalling.
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Squelch was invented first and is still in wide use in two-way/three-way radio, especially in the amateur radio world. Squelch of any kind is used to indicate loss of signal, which is used to keep commercial and amateur radio repeaters from transmitting continually. Since a carrier squelch receiver cannot tell a valid carrier from a spurious signal (noise, etc.), CTCSS is often used as well, as it avoids false keyups. Use of CTCSS is especially helpful on congested frequencies or on frequency bands prone to skip and during band openings.
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 does not 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.
Professional wireless microphones use squelch to avoid reproducing noise when the receiver does not receive enough signal from the microphone. Most professional models have adjustable squelch, usually set with a screwdriver adjustment on the receiver.