Continuous Tone Coded Squelch System (CTCSS) is a common way of coding two-way radio gear to allow selective calling of radios in the same organization. ie: Units with the same CTCSS code will be able to communicate together. CTCSS is also known as "PL" in Motorola gear, "Channel Guard" in General Electric gear, and "Call Guard" in E.F. Johnson gear. How it works: The transmitting unit generates a tone that is broadcasted along with the voice signal of the radio. The receiving unit recieves the tone and opens the radios squelch circuitry if the tones match each other. Most radios put a high pass filter in the audio output statge to filter the CTCSS tone out of the audio. In urban areas and on commercial bands there is usually more than one organization on an assigned frequency. The CTCSS encoding method keeps the two organizations from hearing each other's transmissions. Since there is still only one radio frequency that these two groups are sharing, if both groups key their radios at the same time and are geographically close to each other, they will cause interference and usually nobody hears anything. Motorola assigns a 2 letter code for ease of remembering the different codes. Here is a list of the CTCSS tones in Hz and the Motorola "PL" code. 67.0 XZ 69.3 WZ 71.9 XA 74.4 WA 77.0 XB 79.7 WB 82.5 YZ 85.4 YA 88.5 YB 91.5 ZZ 94.8 ZA 97.4 ZB 100.0 1Z 103.5 1Z 107.2 1B 110.9 2Z 114.8 2A 118.8 2B 123.0 3Z 127.3 3A 131.8 3B 136.5 4Z 141.3 4A 146.2 4B 151.4 5Z 156.7 5A 162.2 5B 167.9 6Z 173.8 6A 179.9 6B 186.2 7Z 192.8 7A 203.5 M1 206.5 8Z 210.7 M2 218.1 M3 225.7 M4 229.1 9Z 233.6 M5 241.8 M6 250.3 M7 254.1 OZ Motorola also has a Digital "PL" system that uses a digital code instead of an audible tone. Last I heard, Motorola will guarentee that their Digital PL will keep other groups out of your radio system but they won't guarentee the older "PL" tone system. -------------------------------------------------------------------------- Lawrence Roney - Department of Telecommunications KC6HQR Santa Monica Community College District 1900 Pico Blvd. Santa Monica, CA 90405-1628 -------------------------------------------------------------------------- DOWNLOADED FROM P-80 SYSTEMS 304-744-2253