Tones are used in many facets of two way radio communication. Audible tones may be used in paging, remote control and signalling applications. Subaudible tones are often used for discrete functions, like controlling receivers or repeater stations.

CTCSS, also known as Private Line (Motorola), Channel Guard (General Electric), Quiet Channel (RCA), and a host of other names, refers to a continuous sub-audible tone transmitted with the voice when the station is on the air. All units in a radio system use the same tone. This allows each receiver to block out other stations on the same frequency and reduce listener fatigue.

For example, the Smithtown Police Department uses 45.50 MHz for dispatching. Another town 80 miles away uses the same frequency and can occasionally be heard by the Smithtown base station and mobile units. If Smithtown installs a subaudible tone on all radios, they will no longer receive the other town unless both CTCSS tones are the same.

The FCC does not regulate CTCSS tones. Two way radio shops can switch these tones around as needed to prevent interference between users. Older two way radios needed a special circuit board and plug in reeds to add CTCSS. Almost every new radio contains CTCSS as a standard programmable feature. Scanner users will still hear everyone on the channel. However, some newer high tech scanners and monitor receivers now have CTCSS, so it's possible to block out licensees and interference you don't want to hear. These are also devices that "listen" to a received signal and decode the CTCSS tone for you.

The Reed Codes are a system designed by Motorola to identify CTCSS tones. The use of reed codes has become generic, especially among Motorola radio shops and equipment users. This information is posted is case you learn a licensee "uses 155.505 Mhz with a 2A PL tone". Now you know what it is. Older Motorola radio equipment did not support all the CTCSS tones now available, and that's why some CTCSS tones don't have a reed code assigned.

Digital Coded Squelch, also known as Digital Private Line or DPL (Motorola) and Digital Channel Guard or DCG (General Electric), uses a digital code, rather than a continuously emitted subaudible tone to keep a receiver quiet. The purpose of digital coded squelch is the same as for CTCSS.

In urban areas, all CTCSS tones may be in use. Digital tones allow for additional options, and since DCS is newer, many of the codes are not used. CTCSS has been around since the early 1960's. Digital Coded Squelch was available only on high end radios in the 1980's, and did not receive widespread use until the late 1990's.

Today, almost every radio is capable of being programmed with either CTCSS and/or Digital Coded Squelch on each frequency. Most scanners cannot decode Digital Coded Squelch, but I'm sure that's coming soon. Some hard core scanner listeners and Amateur Radio operators buy old commercial radio gear and program it to monitor certain frequencies of interest. This list is provided with those users in mind.