In conventional two-way analog radio systems, the standard radio has a squiber noise or squelch carrier, which allows the radio to receive all transmissions. Selective calls are used to overcome parts of all two-way radios on a single radio frequency channel. Where more than one user is on the same channel, (co-channel users,) selective calls can handle some of the receivers or can redirect calls to a single radio. The selective call feature corresponds to two main categories - individual calls and group calls . Individual calls generally have longer time constants: it takes more time to call a radio unit than calling a large group of radios.
Selective calling is similar to the use of a lock on the door. A radio with operator dismissal is opened and will let the signal in. Selective calling locks all signals except those with the correct "key", in this case a particular digital code. Selective calling systems can overlap; radio may have (group call) and individual DTMF calls.
Selective calls prevent users from hearing others on shared channels. It does not remove interference from co-channel users (other users on the same radio channel). If two users try to talk at the same time, the signal will be affected by the other party using the channel.
Some selective calling systems experience forgery. In other words, the decoder activates when a valid signal is not present. Counterfeiting may stem from poor maintenance or engineering problems.
Video Selective calling
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In conventional two-way radio FM systems, the most common form of selective calling is CTCSS, which is based on a sub-sound tone. One of these system implementations is by Motorola and is called Private Path , or PL . Radio made by almost all manufacturers will work with existing systems using CTCSS. This system allows the radio group to remain silenced when other users are talking on the channel. In business and industry systems, as many as 50 sets of users can share the same channel without having to listen to calls for their respective staff. In government systems, users can avoid having to listen to users outside of their own agency. (Government channels are usually separated by distance between user groups Only one local group of users is assigned to a channel.)
In use where missed calls are allowed, selective calls can also hide the presence of disturbing signals such as the intermodulations produced by the receiver. Beneficiaries with poor specifications - such as low cost mobile scanners or radio - can not reject unwanted signals in nearby channels in urban environments. Interference will still exist and will still degrade system performance but by using selective calls the user does not need to hear the noise generated by receiving interference.
In the United States, Federal Communications Commission rules require selective callers to monitor channels, yes. , switch to the extinguish operator, before transmission. In other words, the user must monitor (listen) to make sure the channel is not used by someone in other selective calling code before transmission. To enforce this rule, BTSs usually have a monitor switch in the microphone. The push-to-talk button is divided into two segments. One segment changes selective calling. Other segments of that button are transmitting. The mechanical interlock prevents the transmission button from being pressed until the monitor button turns off. This is called, "mandatory monitor before sending". On mobile radio, the microphone is stored in a hang-up box. When the microphone is pulled out of the hang-up, the radio back to the operator extinguishes and the selective call feature is disabled. Users automatically monitor - verify no one else is using the channel - by pulling the microphone out of the hang-up box. Handheld radios sometimes have LED indicators that indicate when the channel is used.
CTCSS
CTCSS (Continuous Tone-Coded Squelch System) superimposes one of about 50 continuous audio tones on transmitted signals, ranging from 67 to 254 Hz. Whenever the transmitter is on, the tone is encoded on the signal. CTCSS is often called PL tone (for SelCall
Selcall (Selective Call) transmits five in-band audio tones to start a conversation. This feature is common in European systems. In the simplex system, the 5-tone only opens the loudspeakers of the desired partner. In a repeater system, CTCSS or tone-burst or 5-tone is required to activate the company repeater, depending on the system design. If the radio you are calling is within reach of the sender, it answers an incoming call with a stored receipt tone. Sometimes a system using Selcall is referred to as CCIR or ZVEI , the specific tone coding scheme used in the Selcall system. On the continent, people use the temporary ZVEI scheme in Great Britain, the CCIR is very common.
In the same way that a single CTCSS tone will be used on the entire radio group, a single five-tone sequence is used in the radio group. All radios also have private phone numbers stored, to be contacted for individual conversations, not group calls. Either way, the radio speakers will light up as soon as the fifth note of a valid sequence is decoded. In case of a group call, a short announcement tone is generated on the radio speaker. In the case of a private call, an acceptance tone is sent back to the sender and then the receive line is open. The speaker stays on until the operator's dismissal detects that the operator is no longer accepted. At that moment, the speakers will be silent and the decoder is reset. The receiver speaker dies and remains silenced until another order of five valid tones is decoded.
The same tone format is used for one-way voice-and-voice radio calling in the US. This is informally known as the Reach format.
DCS
DCS or Digital Kodak Squelch superimposes the continuous stream of FSK digital data, at 134.5 baud, on the transmitted signal. In the same way that a single CTCSS tone will be used on the entire radio group, the same DCS code is used in a group of radios. DCS is also referred to as DPL tone (for Digital Private Line , Motorola trademark), and likewise, GE implementation of DCS is referred to Digital Guard Channel (or DCG ).
Some equipment uses a 136 Hz square wave turn off code . The turn-off signal is sent for one to three tenths of a second (100-300Ã,Âμm) at the end of the transmission to turn off the audio so that the squelch crash is not heard. Radio with DCS option is generally compatible if the radio decoders will use the same code as the radio in the existing system. The code is usually described as a three-digit octal, (eg, 054). Some DCS codes are inverse data from others: one code with an inverted sign and spacing can form a different valid DCS code (413 equivalent to 054 inverted).
XTCSS
XTCSS is the latest signaling technique and provides 99 code with the added advantage of 'silent operation'. XTCSS equipped radio aims to enjoy more privacy and operating flexibility. XTCSS is implemented using a combination of CTCSS and in-band signaling.
Tone exploded or single note
Tone blast is an outdated selective calling method in which the radio transmits an audio tone of 0.5 to 1.5 seconds at the start of each transmission. This scheme existed before the circuit for CTCSS had been developed. This method is widely used in the United States from the 1950s to the 1980s. Manned spacecraft operations often use this method.
In the same way that a single CTCSS tone will be used on the entire radio group, a single blast tone is used in a group of radios. The radio loudspeaker lights up as soon as the tone is decoded and the speakers remain lit until the carrier bearer detects that the carrier is no longer acceptable. At that moment, the speakers will be silent and the decoder is reset. The receiver speaker dies and remains silenced until another decoded burst tone is decoded.
In some cases, a burst tone is used to select a repeater. By changing the tone, the mobile radio will move a different repeater site. Typical tone schemes may use 1,800 Hz, 2,000 Hz, 2,200 Hz, 2,400 Hz, and 2,552 Hz tones. This is a scheme used by most California State agencies during an era when exploding tones were used. Some systems have been observed to use tones as low as 800 Hz. The standard or five-tone Motorola is used for single tone formats in the 1980s: 1,350 Hz, 1,500 Hz, 1,650 Hz, 1,800 Hz, 1,950 Hz. These are identified in the system documentation for a number of remote control equipment models as well as sales brochures for Motorola Syntor and Micor mobile radio Systems 90 accessories. The common tone burst frequencies used by many amateur radio systems in Europe are 1,750 Hz.
In the German public service radio network calltone 1,750 Hz (Tone I) and 2,135 Hz (Tone II) are used to activate different repeaters or call operators. To duplicate call features, tones are used in short (1,000 ms) and old calls (& gt; 2,000 ms)
In well-designed systems, repeaters or radios usually include an audio notch filter that reduces the tone volume of the speakers.
Single-tone schema variations are seen in one-way paging receivers. In some two-tone sequential systems, it sends 4-8 seconds from the second tone page all the receivers that have the code include the second tone. This is sometimes referred to as B long note . The receivers are made by Plectron and are often used for volunteer firefighting pages using a single long tone. A typical Plectron receiver decoder will not break the tone code as a valid call unless there are at least two to four seconds, (a very long variation of blast tone).
Maps Selective calling
Conventional analog individual calling
In an individual call, a particular radio is called. Most individual calling schemes involve a tone sequence. Most schemes have dozens to thousands of possible individual codes. As a practical matter, more than two hundred radios in one channel make the traffic level unusable. So, 1,000 individual calls will usually be more than needed.
Individual calls are usually based on events. For example, a crane truck can be called to give the driver a duty or the ambulance can be called with an emergency call.
Some Motorola pagers can decode four individual 5-tone signals (see SelCall above). Some fire departments use this feature to implement individual signals (using the first of four signals), station-based signals (eg, each person's paging from one fire station, using a second signal), regionally-based signals (ie, everyone in the region northwest, using a third signal), and all calls (each firefighter, using the fourth signal).
DTMF
In the optional call dual-tone multi-frequency (DTMF), radio is alerted by a series of digits. Systems typically use 2- to 7 digits. These can be dialed from traditional dial telephones connected to the radio or can be generated as DTMF digit strings by automatic encoders. In some systems, the sending computer is connected to a DTMF encoder via a serial cable (RS-232): the computer sends a command to an encoder that generates a predefined string of digits that is then transmitted to the transmitter.
In FM two-way radio, the digit is usually sent at the same level as two-thirds, (66%,) of the system deviation. For example, in a deviation system  ± 5 kHz, the DTMF encoder is set to produce 3.3 kHz transmitter deviation (modulation), or less. In systems with strongly received signals, the tone level is sometimes set so low that the radio user is not forced to listen to it at a high level. Keeping DTMF tone modulation below 2 / 3 the maximum system maintains the clean sine wave generated by the encoder. Sending digits at a higher level causes a transmitter circuit designed to prevent excessive modulation to distort or bypass the waveform from the tone. Distorted waveforms can not decode correctly or may include harmonics that cause counterfeiting. Digits are usually sent at least 55 milliseconds (ms) in length with at least 55Ã, ms silent between each digit. Some decoders may require longer lasting digits. The DTMF digit consists of a tone pair: line tone and column tone. The row and column tone levels should be the same so the decoder can interpret them reliably.
Radio with DTMF decoder can monitor all system traffic or stay muted until called, depending on system design. When the radio receives the correct digits of a string, it may be a moment of buzz or Sonalert sound. The indicator light can be on and stay on. On most systems, the radio receiving audio will stick after receiving a valid digit string if it is usually muted.
Many companies have trademarked names for their DTMF features. For example, Motorola calls their DTMF option, Touch Calls . Because DTMF is the standard format, most features are interchangeable. Generally, any radio equipped to decode the 0-1-2-3 digit string will be compatible with any system using DTMF.
Some systems use DTMF for push-to-talk unit ID. Each time push-to-talk is pressed, the radio sends a DTMF digit string. Each radio has a unique string of numbers. This allows the base station to know who was last called or last press push-to-talk.
Two-tone sequence
The two-tone sequence , also known as 1 1 , is a selective calling method originally used in one-way pitch-and-sound paging receivers. Many companies have their own names for the two-tone sequential option. General Electric Mobile Radio calls it Type 99 . Motorola calls it Quik-Call II . For example, the encoder transmits a single note followed by 50 to 1,000 milliseconds of silence and then a second note. The decoder looks for the first valid tone followed by a valid second tone within the specified timeframe, (time window ). For example, a decoder that detects the first legitimate tone can allow up to 2 seconds for a legitimate second note to decode. If no valid second tone is decoded in 2 seconds, the decoder will be reset and wait for the first valid tone.
A series of tone plans or schemes are widely used for this system. Some tone plans use tone frequencies that are close or overlap with the tone used by other coding plans. For example, one plan might use a very narrow filter and set a 702.3 Hz tone. Others might use a simple filter capacitor and inductor and specify 700 Hz tone. Decoders may not be able to distinguish between these two notes because they are very close. Systems generally use the tone of a single designed tone plan. Individual tone plans are engineered to avoid overlapping or nearby tone frequencies that can lead to counterfeiting. Some systems use the tone of CTCSS subctract as a tone composing a two-tone sequence. For example, a two-tone sequence may consist of 123.0 Hz followed by 203.5 Hz.
In a two-way FM radio, the tone is usually sent at the same level as two-thirds of the system deviation. For example, in a deviation system  ± 5 kHz, the tone encoder is set to produce 3.3 kHz deviation deviation, (modulation,) or less. Because the tone is audible, in a system with a strong received signal, the tone level is sometimes set lower so that the radio user is not forced to listen to it at a high level. Keeping the tone modulation below 2 / 3 the maximum system maintains the clean sine wave generated by the encoder. Sending digits at a higher level causes a transmitter circuit designed to prevent excessive modulation to distort or bypass the waveform from the tone. Distorted waveforms can not decode correctly or may include harmonics that cause counterfeiting. Tones are usually sent at least 500 milliseconds (ms) up to 3 seconds (3,000 ms) in length.
Radio with a two-tone sequential decoder can monitor all system traffic or remain muted until called, depending on the system design. When the radio receives the correct tone in the right sequence, it may soon be heard or heard Sonalert. The indicator light can be on and stay on. On most systems, the radio receiving audio will stick if it is usually muted. In systems that use a combination of audible tone sequences and CTCSS, it is common practice to turn off CTCSS encoding while a two-tone sequence is sent. This means system users with CTCSS decoders do not have to listen to paging tones.
Quik-Call I
Quik-Call I , also known as 2 2 , is a selective call method originally used in one-way paging receivers. The name Quik-Call is a trademark of Motorola. It sends a pair of tones followed by 50 to 1,000 milliseconds of silence and then a second pair of tones. The decoder searches for the first valid pitch pair followed by a valid second pitch pair within the specified timeframe, (time window ). For example, a decoder that detects a valid first pitch pair may allow up to 2 seconds for a valid second pair of tones to be decoded. If no valid second tone is decoded in 2 seconds, the decoder will be reset and wait for the first valid tone pairs. This system is less susceptible to counterfeiting because it uses pairs of tone decoders that must detect valid tone pairs simultaneously.
Quik-Call me most famous for use in fire service. 1970's television show, Emergency! , describes its use for the base station base at the Los Angeles County Fire Department. In some systems, cellular radio has built-in decoder options. In Motorola mobile devices, the decoder is placed in a box that is bolted to the radio control head. In the 1960s, it was also used to drive the tube receivers used to call in volunteer firefighters or to trigger a siren used to summon volunteers.
Radio with Quik-Call I decoder can monitor all system traffic or keep silenced until called, depending on system design. When the radio receives the correct pairs of pairs in the proper sequence, it may be momentarily buzz or sound Sonalert . The indicator light can be on and stay on. On most systems, the radio receiving audio will stick if it is usually muted. In Emergency Television show! , the decoder turns on the lights, activates the loudspeaker above the head, activates the horn/horn, and possibly turns off the cooking utensils.
MDC-600 and MDC-1200
MDC , also known as MDC-1200 and MDC-600 , is a low-speed Motorola data system using audio frequency shift locking (AFSK). MDC-600 uses 600 baud data rate. MDC-1200 uses 1,200 baud rate data. The system uses one of two baud rates. Mark and spaces are 1,200 Hz and 1,800 Hz. Data is sent in bursts through the radio system sound channels.
The Motorola radio with MDC option has an option that allows the radio to filter out bursts of data from the recipient's audio. Instead of hearing AFSK data, users hear short chirps from radio speakers whenever a data blast occurs. (Users must enable this feature in the radio program selection settings).
MDC signaling includes a number of features: unit ID, status button, emergency button, and selective call. These features can be programmed and can be used in whatever combination the user wants. They are usually included in high-end analog FM radio made by Motorola. In addition to Motorola, two other companies make a compatible base station decoder for the MDC-1200.
Other in-band signaling
Modat
Modat , also written MODAT , is an outdated Motorola data system using a sequence of seven audio tones similar to the five-tone Selcall format. Some systems still use Modes today. Modes are used for the unit ID and emergency buttons, not for selective calls. In a typical installation, every radio in the system is coded seven unique tones. Each time the push-to-talk radio button is pressed, the radio transmits a sequence of seven tones at the beginning of the transmission. To prevent users from speaking when the tone sequence is broadcast, a seven-tone sequence is played on the two-way radio receiver speaker.
The sequence of modate tones is described as a six-digit string or seven characters. For example, a single Moda code can be described as 698R124 or 6988124 (where the "R" tone indicates "repeat the last digit"). The data format that comes from Modod's decoder is not clear.
The Modem feature can be programmed and can be used in whatever combination the user wants. For example, some systems only use push-to-talk unit IDs or only emergency buttons. Others can use both. One adjustable setting is the length of time from push to talk tap until the pitch order starts. This delayed the start of the tone sequence to allow a system with long time constants in the CTCSS decoder or a voting comparator to open the audio path. In addition to Motorola, other companies create additional encoders that can modify different radio brands to work with the Modem system.
Modular unit ID systems are often heard from radio on Barbour television production, such as the Cops television show, which depicts South California law enforcement agencies in the 1980s.
Out-of-band individual calls
Embedded radio system has integrated unit ID and selective call feature. Each trunked system has its own unique features. See articles for specific systems to learn more.
Two-way radio systems using digital modulation schemes such as TDMA can embed ID units and selective calls into multiplexed data streams in parallel with sound. See articles related to specific systems to learn more.
References
Source of the article : Wikipedia
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