The Really Useful Neat Stuff Page

CTCSS

Continuous Tone Coded Squelch System, also known by various trade names such as Private Line or PL (Motorola), Channel Guard (GE) and Quiet Channel (E.F.Johnson), is a method of superimposing a subaudible audio tone on a carrier in order to eliminate co-channel interference. CTCSS tones are known by various designators in addition to the tone frequency; the most common of these are the Motorola codes. The original 32 tones ended at 203.5 Hz; there are now 50 tones, with new tones having been added above 203.5 and in between other tones.

The Wolfsburg (or "Flex") designators are commonly used by EMS helicopters; the Wolfsburg radio is a rugged continuous-coverage VHF-UHF radio that can be set to almost any frequency between 30 MHz and 512 MHz. It is generally used by EMS helicopters for communicating with agencies that have requested helicopter transport of trauma victims or other seriously ill/injured persons.

The RELM (Regency Land Mobile) codes are those commonly used in Regency/RELM land mobile transceivers and the new HS-200 and MS-200 scanners.

Note:
The 150.0 Hz PL tone is used by the U.S military on VHF low-band frequencies to reduce or eliminate interference. It is often referred to by military communications personnel as "New Squelch".

DCS

Digital Coded Squelch, also known by the Motorola trade name DPL (Digital Private Line), is similar to CTCSS except that it superimposes a subaudible digital code on a carrier instead of a subaudible audio tone. DCS codes can be normal or inverted; this gives a total of 206 digital codes.

PAGING TONES

Fire and EMS departments use paging tones constantly. Various combinations of tones are used for alerting individual firehouses, off-duty full-time personnel, volunteer personnel, for activating lights and doors, and a whole variety of other purposes.

There are two common sets of tones used for pager alerting, Plectron and Motorola. Plectron was an early manufacturer of paging equipment (mostly for public safety), and while the company has since gone through several changes of ownership and are now owned by Federal Signal Corporation, Plectron receivers are still available (and remain to this day excellent single-channel monitor receivers). Motorola paging equipment is essentially the same as Plectron, with the major difference being the available tone frequencies. General Electric also manufactured a line of pagers and alerting receivers geared to the public safety market, with yet another set of unique tone frequencies, but the GE line was not nearly as popular as Plectron or Motorola. The tones most commonly heard today are Motorola and Plectron.

Plectron is an early paging system commonly used by many volunteer fire departments; in fact, the name "Plectron" has almost become a generic phrase for alerting pagers and receivers, and is so common that many departments refer to their pagers and monitor receivers as "Plectrons", even if their system was made by Motorola or another manufacturer.

Plectron uses a long single tone, or two tones sent sequentially with various timing cycles.

There are various formats used for Motorola tone paging; the most common are Quik-Call I ("2+2") and and Quik-Call II ("1+1"). Quik-Call II is the most common, and sends 2 individual tones sequentially. Long single tones can also be sent to activate pagers set up for group as well as individual paging; this is common with volunteer fire departments.

Quik-Call I is not quite as common; it sends 2 tones sequentially, but each tone is made up of 2 individual tones (similar to DTMF; see below). The advantage to this is that it is more difficult for pagers and monitor receivers to "false" (be activated by a false tone, or atmospheric noise, etc.).

DTMF

Dual-Tone Multi-Frequency tones are the ones you hear when dialing a call on your touch-tone phone. Originally designed for use on the military AUTOVON telephone network, DTMF tones are used in radio systems for remote control of various devices like fire and tornado sirens, firehouse lights and doors, and so on; activation of monitor receivers; radio identification; and almost any other purpose that requires remote tone control.

Each tone that is heard is a combination of the high and low tones shown along the top and right side of the diagram below; for example, when '1' is pressed, the tone you hear is a combination of 1209 Hz and 697 Hz. Not usually seen on telephones or most commercial radios, the A, B, C and D characters were originally designated as FO (Flash Override), F (Flash), I (Immediate), and P (Priority); these were used on the military AUTOVON network to establish the priority of a call and bump lower-priority calls from the network if necessary (Flash Override is the highest priority). A, B, C and D are most commonly used today by amateur radio operators for repeater control.

Note:
Most of the above information came from the Zetron Model 25 Programmable Encoder Technical and Programming manuals I use at work. There are other paging tones and formats, but the ones listed above are those most commonly used in public safety.