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34-455: DSC was developed to replace a voice call in older procedures. Because a DSC signal uses a stable signal with a narrow bandwidth and the receiver has no squelch , it has a slightly longer range than analog signals , with up to twenty-five percent longer range and significantly faster. DSC senders are programmed with the ship's Maritime Mobile Service Identity (MMSI) and may be connected to the ship's Global Positioning System (GPS), which allows

68-483: A "2+2" system) that was heavily used by fire department dispatch systems in the US. Later selective call systems used paging system technology that made use of a burst of five sequential tones. 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,

102-489: A "signal present" indication; for example, in a repeater the act of the receiver unmuting will switch on the transmitter. Squelch can be opened (turned off), which allows all signals to be heard, including radio frequency noise on the receiving frequency. This can be useful when trying to hear distant or otherwise weak signals, for example in DXing . Carrier squelch is the most simple variant of all. It functions strictly on

136-610: 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. DCS codes are standardized by the Telecommunications Industry Association with the following 83 codes being found in their most recent standard, however, some systems use non-standard codes. For those PMR446 radios with 121 codes,

170-406: A few minutes until the ship's batteries are under water. As the distress message can only be sent on one of the bands, many ships and coast stations may be listening to a band without the message, and will after five minutes relay the distress signal to a coast station. Distress calls can be both non designated and designated. The latter allows one of ten predefined designations to be sent along with

204-416: A transmission on the channel designated by the distress signal. A coast station which receives a DSC distress alert will wait 2.75 minutes prior to sending an acknowledgment to allow other vessels in the nearby area to receive the alert. This helps shore stations narrow down where a vessel without GPS is located. The sending device will then both stop repeating the alert, and tune to the designated channel for

238-405: Is a circuit function that acts to suppress the audio (or video ) output of a receiver in the absence of a strong input signal . Essentially, squelch is a specialized type of noise gate designed to suppress weak signals. Squelch is used in two-way radios and VHF/UHF radio scanners to eliminate the sound of noise when the radio is not receiving a desired transmission. In some designs,

272-512: Is interchangeable. Old and new radios with CTCSS and radios across manufacturers are compatible. For those PMR446 radios with 38 codes, the codes 0 to 38 are CTCSS Tones: Selcall (Selective Calling) transmits a burst of up to five in-band 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,

306-589: Is no prohibition against broadcasting non-distress, "routine" calls on other DSC-designated frequencies, which are defined in ITU M.541 as: There is a general consensus for routine calls to use 2177.0, 4208.0, 6312.5, 8415.0, 12577.5, and 16805.0 kHz (the first frequency listed above in each band). The DSC is a synchronous system using characters composed from a ten-bit error detecting code . The bits are encoded using frequency-shift keying . For High Frequency and Medium Frequency two tones 170 Hz apart either side of

340-475: Is noise-operated and can be used in AM or FM receivers, and relies on the receiver quieting in the presence of an AM or FM carrier. To minimize the effects of voice audio on squelch operation, the audio from the receiver's detector is passed through a high-pass filter , typically passing 4,000 Hz (4kHz) and above, leaving only high frequency noise. The squelch control adjusts the gain of an amplifier which varies

374-492: Is used to indicate loss of signal, which is used to keep commercial and amateur radio repeaters from continually transmitting . 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. Professional wireless microphones use squelch to avoid reproducing noise when

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408-431: 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. Carrier squelch uses receiver Automatic gain control (AGC) to determine the squelch threshold. Single-sideband modulation (SSB) typically uses carrier squelch . Noise squelch is more reliable than carrier squelch. A noise squelch circuit

442-419: The 2, 4, 6, 8, 12 and 16 MHz marine bands. At minimum, controllers will monitor 2187.5 kHz and 8414.5 kHz and one more band. However for automated monitoring a second, receive-only antenna is often needed (especially on non-commercial leisure boats) since a separate tuner is used apart from the main one; this is separate from programming radios to monitor user-defined DSC frequencies (which would use

476-475: The ability to send distress, distress relay, all ships urgency, all ships safety, individual, group, geographic area and telephone alerts/announcements on DSC channel 70 (Digital channel reserved for DSC only.). Class D VHFs, used for most leisure vessels, can send distress, all ships urgency, all ships safety and individual alerts/announcements on VHF channels on DSC channel 70. On both class A and D you can be directed to Ch 06, 08, 72, 77 or other simplex channel for

510-522: The allocated frequency with 100 Baud symbol rate are used. For VHF the two tones used are 1300 and 2100 Hz with a symbol rate of 1200 Baud . Each character is transmitted twice with a time delay. The detailed specification is published in the International Telecommunication Union recommendation ITU-R M.493, revision 15 published in 2019 being the most recent. Squelch In telecommunications , squelch

544-529: The apparatus to know who it is, what time it is and where it is. This allows a distress signal to be sent very quickly. Often, ships use separate VHF DSC and MF/HF DSC controllers. For VHF, DSC has its own dedicated receiver for monitoring Channel 70, but uses the main VHF transceiver for transmission . However, for the user, the controller is often a single unit. MF/HF DSC devices monitor multiple bands for distress, urgency and safety sécurité transmissions in

578-399: The codes 39 to 121 are DCS codes: XTCSS is the newest signalling technique, and 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. Squelch was invented first and is still in wide use in two-way radio. Squelch of any kind

612-421: The distress message to be sent. Ships receiving a distress alert who are outside coast station range or do not receive an acknowledgment, are required to relay the distress alert by any means to land. The priorities for communication are, Distress, Urgency, Safety and Routine. A Distress DSC call is called an Alert. Urgency, Safety and Routine are called Announcements. Class A VHFs, used on commercial ships, have

646-477: The distress signal. These are "abandoning ship", "fire or explosion", "flooding", " collision ", "grounding", "listing", "sinking", "disabled and adrift", "piracy or attack" and " man overboard ". To avoid false distress alerts, distress buttons normally have protective covers, often with a spring-loaded cover so two hands need to be used simultaneously. Alternatively, some devices have two-button systems. Operators are required to cancel falsely sent distress alerts with

680-410: The follow-up RT (radio telephony [voice]) call. Class D has only one antenna and thus can only watch Channel 70 when not transmitting. For routine alerts, which are used to establish communication with another station on a working channel, the receiver acknowledges to confirm that communication can be done on the appropriate channel. While there are reserved frequencies for distress HF DSC calls , there

714-407: The high-pass band. For this reason, many receivers with noise squelch will also use a carrier squelch set at a higher threshold than the noise squelch . Tone squelch, or another form 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

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748-423: The level of the noise coming out of the filter. This noise is rectified , producing 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 is reduced and the receiver audio is unmuted. Noise squelch can be defeated by intermodulation present in

782-471: The main antenna). When sending a distress signal, the DSC device will at minimum include the ship's MMSI number. It may also include the coordinates if radio is connected to GPS system and, if necessary, the channel for the following radiotelephony or radiotelex messages. The distress can be sent either as a single-frequency or multi-frequency attempt. In the former, a distress signal is sent on one band and

816-405: 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). Motorola later marketed a system called "Quik-Call" that used two simultaneous tones followed by two more simultaneous tones (sometimes called

850-415: The operator will adjust the control until noise is heard, and then adjust in the opposite direction until the noise is squelched. At this point, a weak signal will unsquelch the receiver and be heard by the operator. Further adjustment will increase the level of signal required to unsquelch the receiver. Some applications have the receiver tied to other equipment that uses the audio muting control voltage, as

884-469: The radio's encoder-decoder will use the same code as radios in the existing system. DCS adds a 134.4  bit/s (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

918-578: The receiver 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

952-416: The receiver does not receive enough signal from the microphone. Most professional models have adjustable squelch, usually set with a screwdriver adjustment or front-panel control on the receiver. International distress frequency An international distress frequency is a radio frequency that is designated for emergency communication by international agreement. For much of the 20th century, 500 kHz

986-453: 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 as Digital Channel Guard (or DCG ). Despite the fact that it is not a tone, 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

1020-438: 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 two-way radios (also known as radiotelephones ), the received signal level required to unsquelch (un-mute) the receiver may be fixed or adjustable with a knob or a sequence of button presses. Typically

1054-482: The strong signals present in urban environments. The interference will still be present, and 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 . CTCSS (Continuous Tone-Coded Squelch System) continuously superimposes any one of about 50 low-pitch audio tones on

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1088-533: The system will wait up to four minutes for a DSC acknowledgment from a coast station . If none is received, it will repeat the distress alert up to five times. In a multi-frequency attempt, the distress signal is sent on the MF and all the HF distress frequencies in turn. As this requires retuning the antenna for each sending, without waiting for an acknowledgment, a multi-frequency attempt should only be done if there are only

1122-538: 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

1156-559: Was the primary international distress frequency. Its use has been phased out in favor of the Global Maritime Distress Safety System . Use of some distress frequencies is permitted for calling other stations to establish contact, whereupon the stations move to another frequency. Such channels are known as distress, safety and calling frequencies . Satellite processing from all 121.5 or 243 MHz locators has been discontinued. Since February 1, 2009,

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