Squelch
Encyclopedia
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.
, such as when a television
mute
s the audio or blank
s 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 station
s at remote mountain top sites are usually not adjustable remotely from the control point.
In devices such as two-way radio
s (also known as radiotelephone
s), the squelch 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 squelch noise) if 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 adjust the squelch to open only when stronger signals are received. If you hold the squelch open you will also get a lot of noise.
A typical FM
two-way radio carrier squelch circuit 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 noise creates a DC voltage which turns the receiver audio off. When a signal with little or no noise is received, the 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.
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 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.
s on the transmitted signal, ranging from 67 to 254 Hz
. The original tone set was 32 tones, and has been expanded 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.
of FSK
digital
data
, at 134.4 bits per second, on the transmitted signal. 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, and 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.
Be aware that the same 23-bit DCS word can, for example, produce three different valid DCS codes due to the encoding architecture.
world. Squelch of any kind is used to indicate loss of signal, which is used to keep commercial and amateur radio repeater
s 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 bands prone to skip and during band openings.
Family Radio Service
(FRS) and PMR446
radio
s often use a number of different CTCSS tones, usually erroneously called "sub-channels" (the actual number of tones depends on the manufacturer). While these do not add to the available number of conversation
s which can take place at once in a given area, they do reduce annoying interference
experienced by users. However they do not afford any protection from eavesdropping, as the voice signal is not encrypted. The squelch system relies on the receiver to comply with the squelch signal, but if a receiver chooses to listen in, the signal is not protected.
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.
Professional wireless microphone
s 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.
Telecommunication circuit
A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted.A dedicated circuit, private circuit, or leased line is a line that is dedicated to only one use...
function that acts to suppress the audio (or video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
) output
Output
Output is the term denoting either an exit or changes which exit a system and which activate/modify a process. It is an abstract concept, used in the modeling, system design and system exploitation.-In control theory:...
of a receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
in the absence of a sufficiently strong desired input signal.
Carrier squelch
A carrier squelch or noise squelch is the most simple variant of all. It operates strictly on the signal strengthSignal strength
In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
, such as when a television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
mute
Silence
Silence is the relative or total lack of audible sound. By analogy, the word silence may also refer to any absence of communication, even in media other than speech....
s the audio or blank
Blank
Blank may refer to:*Blank, a space where there is nothing at all.*Blank , a thick, shaped stone biface of suitable size and configuration for refining into a stone tool...
s the video on "empty" channels
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
, 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 station
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
s at remote mountain top sites are usually not adjustable remotely from the control point.
In devices such as two-way radio
Two-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...
s (also known as radiotelephone
Radiotelephone
A radiotelephone is a communications system for transmission of speech over radio. Radiotelephone systems are not necessarily interconnected with the public "land line" telephone network. "Radiotelephone" is often used to describe the usage of radio spectrum where it is important to distinguish the...
s), the squelch 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
White noise
White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
(also called squelch noise) if 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 adjust the squelch to open only when stronger signals are received. If you hold the squelch open you will also get a lot of noise.
A typical FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
two-way radio carrier squelch circuit takes out the voice components of the receive audio by passing the detected audio through a high-pass filter
High-pass filter
A high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...
. A typical filter might pass frequencies over 4,000 Hz (4 kHz). The squelch control adjusts the gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
of an amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
which varies the level of noise coming out of the filter. The audio output of the filter and amplifier is rectified
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
and produces a DC voltage
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
when noise
Electronic noise
Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects...
is present. The presence of noise creates a DC voltage which turns the receiver audio off. When a signal with little or no noise is received, the 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.
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 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 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
CTCSS (Continuous Tone-Coded Squelch System) continuously superimposes any one of about 50 low-pitch audio tonePitch (music)
Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...
s on the transmitted signal, ranging from 67 to 254 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
. The original tone set was 32 tones, and has been expanded over the years. CTCSS is often called PL tone (for Private Line, a trademark of Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
), or simply tone squelch. General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
'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
Selcall (Selective Calling) transmits a burst of five inband audio tones at the beginning of each transmission. This feature (sometimes called "tone burst") is common in European systems. 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
DCS (Digital-Coded Squelch) superimposes a continuous streamStream
A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...
of FSK
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
, at 134.4 bits per second, on the transmitted signal. 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
Trademark
A trademark, trade mark, or trade-mark is a distinctive sign or indicator used by an individual, business organization, or other legal entity to identify that the products or services to consumers with which the trademark appears originate from a unique source, and to distinguish its products or...
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, and 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.
Be aware that the same 23-bit DCS word can, for example, produce three different valid DCS codes due to the encoding architecture.
XTCSS
XTCSS is the newest signaling 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.Uses
Carrier squelch was invented first and is still in wide use in two-way radio, especially in the amateur radioAmateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
world. Squelch of any kind is used to indicate loss of signal, which is used to keep commercial and amateur radio repeater
Amateur radio repeater
An amateur radio repeater is an electronic device that receives a weak or low-level amateur radio signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation...
s 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 bands prone to skip and during band openings.
Family Radio Service
Family Radio Service
The Family Radio Service is an improved walkie talkie radio system authorized in the United States since 1996. This personal radio service uses channelized frequencies in the ultra high frequency band. It does not suffer the interference effects found on citizens' band at 27 MHz, or the...
(FRS) and PMR446
PMR446
PMR446 is a part of the UHF radio frequency range that is open without licensing for personal usage in most countries of the European Union. It has roughly the same use as FRS or GMRS in the United States and Canada. Depending on surrounding terrain range can vary from a few hundred metres to a...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
s often use a number of different CTCSS tones, usually erroneously called "sub-channels" (the actual number of tones depends on the manufacturer). While these do not add to the available number of conversation
Conversation
Conversation is a form of interactive, spontaneous communication between two or more people who are following rules of etiquette.Conversation analysis is a branch of sociology which studies the structure and organization of human interaction, with a more specific focus on conversational...
s which can take place at once in a given area, they do reduce annoying interference
Interference (communication)
In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
experienced by users. However they do not afford any protection from eavesdropping, as the voice signal is not encrypted. The squelch system relies on the receiver to comply with the squelch signal, but if a receiver chooses to listen in, the signal is not protected.
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.
Professional wireless microphone
Wireless microphone
A wireless microphone, as the name implies, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated...
s 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.