Selective calling
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. Selective calling is used to address a subset of all two-way radios on a single radio frequency channel. Where more than one user is on the same channel, selective calling can address a subset of all receivers or can direct a call to a single radio. Selective calling features fit into two major categories—individual calling and group calling. Individual calls generally have longer time-constants: it takes more air-time to call an individual radio unit than to call a large group of radios.
Selective calling is akin to the use of a lock on a door. A radio with carrier squelch is unlocked and will let any signal in. Selective calling locks out all signals except ones with the correct "key", in this case a specific digital code. Selective calling systems can overlap; e.g. a radio may have CTCSS and DTMF calling.
Selective calling prevents the user from hearing others on a shared channel. It does not eliminate interference from co-channel users. 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 falsing. In other words, the decoder activates when a valid signal is not present. Falsing may come from a maintenance problem or poor engineering.
Group calling
In conventional FM two-way radio systems, the most common form of selective calling is CTCSS, which is based on a sub-audible tone. One implementation of this system is by Motorola and is called Private Line, or PL. Radios made by nearly any manufacturer will work acceptably with existing systems using CTCSS. The system allows groups of radios to remain muted while other users are talking on the channel. In business and industrial systems, as many as 50 sets of users could share the same channel without having to listen to calls for each other's staffs. In government systems, users can avoid having to hear users outside their own agency.In uses where missed calls are allowable, selective calling can also 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 unwanted signals on nearby channels 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.
In the United States, Federal Communications Commission rules require users of selective calling to monitor the channel, i.e. switch to carrier squelch before transmitting. In other words, the user must monitor to make sure the channel is not in use by someone on another selective calling code before transmitting. To enforce this rule, base stations often have a monitor switch on the microphone. The push-to-talk button is split into two segments. One segment turns the selective calling off. The other segment of the button transmits. A mechanical interlock prevents the transmit button from being pressed until the monitor button is down. This is called "compulsory monitor before transmit". In mobile radios, microphones are stored in a hang-up box. When the microphone is pulled out of the hang-up, the radio reverts to carrier squelch and the selective calling feature is disabled. The user automatically monitors—verifies no one else is using the channel—by pulling the microphone out of the hang-up box. Hand-held radios sometimes have LED indicators that show when the channel is in use.
CTCSS
CTCSS superimposes any one of about 50 continuous audio tones on the transmitted signal, ranging from 67 to 254 Hz. At any time when the transmitter is on, the tone is encoded on the signal. CTCSS is often called PL tone, or simply tone squelch. General Electric's implementation of CTCSS is called Channel Guard. When RCA was in the land mobile radio business, their brand name was Quiet Channel. Tone codes may universally be described by their tone frequency.SelCall
Selcall transmits a burst of five in-band audio tones to initiate the conversation. This feature is common in European systems. In a simplex system, the 5-tone just opens the speaker of the desired partner. In a repeater system, another CTCSS or tone-burst or 5-tone is needed to activate the company's repeater, depending on the systems design. If the called radio is within reach of the sender, it answers the incoming call with its stored receipt tone. Sometimes systems using Selcall are referred to as CCIR or ZVEI, specific tone encoding schemes used in Selcall systems. On the continent, people use the ZVEI scheme while in Great Britain the CCIR is very common.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. All radios also have their own private callnumber stored, to be reached for an individual conversation instead of a group call. In either way the radio speaker turns on as soon as the fifth tone of a valid sequence is decoded. In case of a group call, a short announcement tone is generated on the radios speaker. In case of a private call, the receipt tone is transmitted back to the sender and then the receive path is open. The speaker stays on until the carrier squelch detects that the carrier is no longer being received. At that point, the speaker mutes and the decoder resets. The receiver speaker turns off and remains muted until another valid five-tone sequence is decoded.
A similar tone format is used for one-way tone-and-voice radio paging in the US. It is informally known as Reach format.
DCS
DCS or Digital-Coded Squelch superimposes a continuous stream of FSK digital data, at 134.5 baud, 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 DPL tone, and likewise, GE's implementation of DCS is referred to a Digital Channel Guard.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 at the end of a transmission to mute the audio so that a squelch crash is not heard. Radios with DCS options are generally compatible provided the radio's encoder-decoder will use the same code as radios in the existing system. Codes are usually described as three octal digits. Some DCS codes are inverted data of others: one code with the marks and spaces inverted may form a different valid DCS code. Because of the use of the 136 Hz code, many receivers will decode a DCS signal when tuned to the CTCSS tone of 136.5 Hz.
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 using a combination of CTCSS and in-band signaling.Tone burst or ''single tone''
Tone burst is an obsolete method of selective calling where the radio transmits a single 0.5- to 1.5-second audio tone at the beginning of each transmission. This scheme existed before circuitry for CTCSS had been developed. This method was in wide use in the United States from the 1950s through the 1980s. Human spaceflight operations made frequent use of this method.In the same way that a single CTCSS tone would be used on an entire group of radios, a single burst tone is used in a group of radios. The radio speaker turns on as soon as the tone is decoded and the speaker stays on until the carrier squelch detects that the carrier is no longer being received. At that point, the speaker mutes and the decoder resets. The receiver speaker turns off and remains muted until another valid burst tone is decoded.
In some cases, burst tones were used to select repeaters. By changing tones, the mobile radio would actuate a different repeater site. A typical tone scheme might use the tones 1,800 Hz, 2,000 Hz, 2,200 Hz, 2,400 Hz, and 2,552 Hz. This was the scheme used by most State of California agencies during the era when tone burst was in use. Some systems have been observed to use tones as low as 800 Hz. The default or standard five Motorola tones used for single tone format as of the 1980s: 1,350 Hz, 1,500 Hz, 1,650 Hz, 1,800 Hz, 1,950 Hz. These were identified in 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. A common tone burst frequency used by many amateur radio systems in Europe is 1,750 Hz.
In German public service radio networks the calltone 1,750 Hz and 2,135 Hz are used to activate different repeaters or call an operator. To double the calling features, tones are used in short call and long call.
In well-designed systems, repeaters or radios usually included an audio notch filter that reduced the volume of the tone at the speaker.
A variation to the single tone scheme was seen in one-way paging receivers. In some two-tone sequential systems, sending 4–8 seconds of the second tone pages all receivers which have a code including the second tone. This is sometimes referred to as long tone B. Receivers made by Plectron and often used to page volunteer firefighters use a long single tone. The decoder in the typical Plectron receiver would not decode the tone as a valid call unless it was present for at least two to four seconds.
Conventional analog individual calling
In individual calling, a specific radio is called. Most individual calling schemes involve a sequence of tones. Most schemes have a dozen to thousands of possible individual codes. As a practical matter, more than about two hundred radios on a single channel make an unusable level of traffic. So 1,000 individual calls will usually be more than needed.Individual calls are usually event-based. For example, a tow truck may be called to give the driver an assignment or an ambulance may be called with an emergency call.
Some Motorola pagers could decode four different individual 5-tone signals. Some fire departments used this feature to implement an individual signal, a station based signal, a region-based signal, and an all-call.