Selective calling

Selective calling locks out all signals except ones with the correct "key", in this case a specific digital code.

In conventional FM two-way radio systems, the most common form of selective calling is CTCSS, which is based on a sub-audible tone.

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.

Receivers with poor specifications—such as scanners or low-cost mobile radios—cannot reject the unwanted signals on nearby channels in urban environments.

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.

When the microphone is pulled out of the hang-up, the radio reverts to carrier squelch and the selective calling feature is disabled.

[2] When RCA was in the land mobile radio business, their brand name was Quiet Channel (or QC).

Selcall (Selective Calling) transmits a burst of five in-band audio tones to initiate the conversation.

In case of a private call, the receipt tone is transmitted back to the sender and then the receive path is open.

DCS or Digital-Coded Squelch superimposes a continuous stream of FSK digital data, at 134.5 baud, on the transmitted signal.

The turn-off signal is sent for one- to three-tenths of a second (100–300 ms) at the end of a transmission to mute the audio so that a squelch crash is not heard.

In well-designed systems, repeaters or radios usually included an audio notch filter that reduced the volume of the tone at the speaker.

As a practical matter, more than about two hundred radios on a single channel make an unusable level of traffic.

On FM two-way radios, digits are usually sent at a level that equals two-thirds, (66%,) of system deviation.

Keeping the DTMF tone modulation below 2⁄3 system maximum preserves the clean sine wave produced by the encoder.

Sending digits at higher levels causes the transmitter's circuits that are designed to prevent over-modulation to distort or clip the waveform of the tones.

Generally, any radio that is equipped to decode the digit string 0-1-2-3 would be compatible with any system using DTMF.

Two-tone sequential, also known as 1+1, is a selective calling method originally used in one-way, tone-and-voice paging receivers.

Keeping the tone modulation below 2⁄3 system maximum preserves the clean sine wave produced by the encoder.

Sending digits at higher levels causes the transmitter's circuits that are designed to prevent over-modulation to distort or clip the waveform of the tones.

When the radio receives the correct tones in the proper sequence, it may momentarily buzz or sound a Sonalert.

The 1970s television show, Emergency!, depicted its use for base station ringdowns in the Los Angeles County Fire Department.

In Motorola mobile equipment, the decoders were housed in a box that bolted onto the radio control head.

When the radio receives the correct tone pairs in the proper sequence, it may momentarily buzz or sound a Sonalert.

MDC, also known as MDC-1200 and MDC-600, is a low-speed Motorola data system using audio frequency shift keying, (AFSK).

This delays the start of the tone sequence to allows systems with long time constants in CTCSS decoders or voting comparators to open an audio path.

In addition to Motorola, other companies make add-on encoders that can modify a different brand of radio to work with a Modat system.

Trunked radio systems have built-in unit ID and selective calling features.

Two-way radio systems using digital modulation schemes such as TDMA can embed unit ID and selective calling into the data stream multiplexed in parallel with the voice.