Continuous Tone-Coded Squelch System - Interference and CTCSS

Interference and CTCSS

In non-critical uses, CTCSS can also be used to hide the presence of interfering signals such as receiver-produced intermodulation. Receivers with poor specifications — such as scanners or low-cost mobile radios — cannot reject the strong signals present in urban environments. The interference will still be present and may block the receiver, but the decoder will prevent it from being heard. It will still degrade system performance but the user will not have to hear the noises produced by receiving the interference.

CTCSS is very commonly used in amateur radio for this purpose. Wideband and extremely sensitive radios are common in the amateur radio field, which imposes limits on achievable intermodulation and adjacent-channel performance. Often all repeaters in a geographical region share the same CTCSS tone as a method of reducing co-channel interference from adjacent regions and increasing frequency reuse. This is a practice linked back to an old FCC practice of coordinating CTCSS tones for business services. In many rural areas of the USA where no coordination is necessary, a default of 100 Hz has become a de facto standard.

Family Radio Service (FRS), PMR446 and other "bubble pack" radios often use from 10 to 38 different CTCSS tones (the number depends on the manufacturer), usually erroneously called "sub-channels", or "privacy codes" in the sales literature. While these do not add to the available number of conversations which can take place at once in a given area, they do reduce annoying interference experienced by users. However they do NOT afford any privacy or security, no matter what the sales literature says. A receiver with the tone squelch turned off (i.e. in carrier squelch mode) will hear everything.

It is a bad idea to use any coded squelch system to hide interference issues in systems with life-safety or public-safety uses such as police, fire, search and rescue or ambulance company dispatching. Adding tone or digital squelch to a radio system doesn't solve interference issues, it just covers them up. The presence of interfering signals should be corrected rather than masked. Interfering signals masked by tone squelch will produce apparently random missed messages. The intermittent nature of interfering signals will make the problem difficult to reproduce and troubleshoot. Users will not understand why they cannot hear a call, and will lose confidence in their radio system. In a worst case scenario in a life safety environment a missed message, or a misunderstood message, may result in casualties.