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The tone is transmitted at a low level simultaneously with the voice. This is called CTCSS encoding. CTCSS tones range from 67 to 257 Hz. The tones are usually referred to as sub-audible tones. In an FM two-way radio system, CTCSS encoder levels are usually set for 15% of system deviation. For example, in a 5 kHz deviation system, the CTCSS ...
(7) National GMRS calling channel (CTCSS tone 141.3 Hz). Note: Some inexpensive GMRS mobiles and portables do not fully comply with FCC permissible modulation bandwidth for GMRS and thus have weak transmitter audio and reduced range. Conditions: Effective February 16, 1999, the GMRS rules have been amended and one may operate on any of the ...
Systems generally use tones off of a single, designed tone plan. Individual tone plans are engineered to avoid overlapping or nearby tone frequencies that may cause falsing. Some systems use CTCSS subaudible tones as the tones composing the two-tone sequence. For example, a two tone sequence might consist of 123.0 Hz followed by 203.5 Hz.
CTCSS (Continuous Tone-Coded Squelch System) continuously superimposes any one of about 50 low-pitch audio tones on 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.
Most analog two-way radios utilize a technology called CTCSS or DCS that helps block out unwanted transmissions. To make MURS two-way radios work together, they must have matching CTCSS or DCS tones. This can usually be done via basic programming which almost all MURS two-way radios support.
The crudest and oldest of these is called CTCSS, or Continuous Tone-Controlled Squelch System. This consists of superimposing a precise very low frequency tone on the audio signal. Only the receiver tuned to this specific tone turns the signal into audio: this receiver shuts off the audio when the tone is not present or is a different frequency.
Another widely used system is the Continuous Tone-Coded Squelch System (CTCSS) also referred to as subaudible tones or PL tones (a Motorola trademark). This uses single audio tones in the range from 67 to 257 Hz to address each message to a specific radio or a group (or fleet) of radios. Each radio or group is assigned a different tone frequency.
As with FRS/GMRS and PMR446, the use of tone squelch systems such as CTCSS/DCS is encouraged. Like the PMR446, LPD433, Japan's 421–422 MHz SLPR service and KDR444 services, use of these frequencies in countries such as the United States is illegal without an amateur radio license as they fall within the 420–450 MHz 70 cm ham radio allocation.