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 (co-channel users), 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 (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 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. (Government channels are usually separated by distance between user groups. Only one local user group is assigned to a channel.) 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 (listen) 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 (Continuous Tone-Coded Squelch System) 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 (for Private Line, a trademark of Motorola), or simply tone squelch. General Electric's implementation of CTCSS is called Channel Guard (or CG). When RCA was in the land mobile radio business, their brand name was Quiet Channel (or QC). Tone codes may universally be described by their tone frequency (for example, 131.8 Hz).
SelCall
Selcall (Selective Calling) 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.
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