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. A telecommunication circuit is defined as follows The complete path between two terminals over which one-way or two-way communications may be provided Video is the technology of electronically capturing, Recording, processing storing transmitting and reconstructing a sequence of Still images Output is the term denoting either an exit or changes which exit a System and which activate/modify a Process. This article is about a radio receiver for other uses see Radio (disambiguation. Input is the term denoting either an entrance or changes which are inserted into a System and which activate/modify a Process. In Telecommunication, signalling (UK spelling or signaling (US spelling has the following meanings The use of signals for controlling communications
Squelch excludes undesired lower-power input signals that may be present at or near the frequency of the desired signal. In Physics, power (symbol P) is the rate at which work is performed or energy is transmitted or the amount of energy required or expended for Frequency is a measure of the number of occurrences of a repeating event per unit Time. Squelch is a noise gate that only allows signals at a specified strength over a threshold to be played through the speaker. A Noise Gate or gate is an Electronic device or Software logic that is used to control the volume of an audio signal As an example, general static (with no transmission) is squelched out. (Contrast with noise suppression. Active noise control (ANC (also known as noise cancellation, active noise reduction (ANR or antinoise) is a method for reducing unwanted Sound )[1]
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Carrier squelch
A simple carrier squelch or noise squelch operates strictly on the signal strength of the signal, 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 Telecommunications, particularly in Radio, signal strength refers to the magnitude of the Electric field at a reference point that is a significant distance Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic Silence is a relative or total lack of audible Sound you can not hear a thing it is quiet Channel, in communications (sometimes called communications channel) refers to the medium used to convey Information from a A walkie-talkie (more formally known as a handheld transceiver) is a hand-held portable Two-way radio Transceiver. In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. Receivers in base stations at remote mountain top sites are usually not adjustable remotely from the control point. The term base station can be used in the context of Land surveying, Wireless computer networking, and Wireless communications.
In devices such as radiotelephones (also known as two-way radios), the squelch threshold is set with an adjustable knob marked squelch. A radiotelephone is a communications device that allows two or more people to talk using Radio. two-way radio is a radio that can both transmit and receive (a Transceiver) unlike a broadcast receiver which only receives content This setting adjusts the threshold at which signals will open the audio channel. Backing off the control will turn on the audio, and the operator will hear white noise if there is no signal present. White noise is a random signal (or process with a flat Power spectral density. 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.
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 high-pass filter is a filter that passes high frequencies well but attenuates (reduces the amplitude of frequencies lower than the Cutoff frequency 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. In Electronics, gain is a measure of the ability of a circuit (often an Amplifier) to increase the power or Amplitude of a Generally an amplifier or simply amp, is any device that changes usually increases the amplitude of a signal. The audio output of the filter and amplifier is rectified and produces a DC voltage when noise is present. A rectifier is an electrical device that converts Alternating current (AC to Direct current (DC a process known as rectification. Direct current ( DC) is the unidirectional flow of Electric charge. Electronic noise is an unwanted signal characteristic of all electronic circuits. 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, are sometimes used to solve interference problems. In a conventional analog two-way radio system a standard radio has ''noise squelch'' or ''carrier squelch'' which allows a radio to receive all transmissions 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 signaling. See also Signalling (telecommunications In Telecommunications, in-band signaling is the sending of Metadata and control information in
CTCSS
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. 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 Pitch represents the perceived Fundamental frequency of a sound The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. 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. Motorola Inc ( is an American, multinational Fortune 100, Telecommunications company based in Schaumburg Illinois. 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
Selcall (Selective Calling) transmits a burst of five inband audio tones at the beginning of each transmission. For the similarly-named system used in aviation see SELCAL. Selcall ( Selective calling) is a type of Squelch protocol This feature 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 stream of FSK digital data, at about 134 bits per second, on the transmitted signal. A stream is a body of Water with a current, confined within a bed and stream-banks Frequency-shift keying (FSK is a Frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a Carrier wave A digital system uses discrete (discontinuous values usually but not always Symbolized Numerically (hence called "digital" to represent information for Debt AIDS Trade in Africa (or DATA) is a Multinational non-government organization founded in January 2002 in London by U2 's In Telecommunications and Computing, bitrate (sometimes written bit rate, data rate or as a Variable R or f b 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 DPL tone (for Digital Private Line, another trademark of Motorola), and likewise, GE's implementation of DCS is referred to a Digital Channel Guard (or DCG). A trademark or trade mark, represented by the symbols ™ and ®, or mark is a distinctive sign or indicator used by an individual DCS is also called DTCS (Digital Tone Code Squelch) by Icom. ( is an international Manufacturer of Radio transmitting and receiving equipment founded in 1954 by Tokuzo Inoue. 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.
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, especially in the amateur radio world. Amateur radio, often called ham radio, is both a Hobby and a service in which participants called "hams" use various types of Radio communications Squelch of any kind is used to indicate loss of signal, which is used to keep repeaters and commercial repeaters from transmitting continually. 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 Since a receiver cannot tell a valid carrier signal 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. In Radio propagation, skip is when a Radio signal is reflected or Refracted by the atmosphere or Ionosphere, and
Family Radio Service (FRS) and PMR446 radios often use 38 different CTCSS tones, usually erroneously called "sub-channels". The Family Radio Service (FRS is an improved Walkie talkie radio system authorized in the United States since 1996 PMR446 (Personal Mobile Radio 446 MHz is a Radio frequency part of the UHF range that is open without licensing for personal usage in most countries of the Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. 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. Banter redirects here for the Radio 4 panel show see Banter (radio show A conversation is Communication by two or more people or by one's self In physics interference is the addition ( superposition) of two or more Waves that result in a new wave pattern However they do NOT afford any privacy, no matter what the sales literature says. A receiver in carrier squelch mode hears everything.
Professional wireless microphones use squelch to avoid reproducing noise when the receiver does not receive enough signal from the 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 Most professional models have adjustable squelch, usually set with a screwdriver adjustment on the receiver.
Notes
- ^ Source: modified from Federal Standard 1037C and from the NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management in support of MIL-STD-188. Federal Standard 1037C, entitled Telecommunications Glossary of Telecommunication Terms is a United States Federal Standard issued by the General Services Administration The NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management, also known as the Red Book, is a publication of the United States Office of Spectrum MIL-STD-188 is a series of US military standards relating to Telecommunications Purpose Faced with “past technical deficiencies in telecommunications
See also
External links
A Noise Gate or gate is an Electronic device or Software logic that is used to control the volume of an audio signalDictionary
squelch
-verb
- (transitive, US) to halt, stop, eliminate, stamp out, or put down, often suddenly or by force
- (transitive, radio technology) to suppress the unwanted hiss or static between received transmissions by adjusting the gain of your receiver.
- (intransitive, UK) to make a sucking, splashing noise as when walking on muddy ground
- (intransitive, UK) to walk or step through a substance such as mud
-noun
- A squelching sound.
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