The pros and cons of CTCSS
Earlier this week, we began a discussion of CTCSS, an acronym for Continuous Tone Coded Squelch System. Today we will examine the practical application of CTCSS. Before the concept of trunked radio user groups and push-to-talk (PTT) over cellular, many users purchased portable radios and shared airtime over a given RF channel, which usually was owned and licensed by a local radio shop. Plumbers, electricians and livery services all could operate on the same channel with each company employing a different CTCSS tone.
This concept commonly was known as a community repeater, with CTCSS used to separate the various users. This same concept is seen today in the increasing popularity of the Family Radio Service (FRS) band. Some FRS radio literature calls these CTCSS tones “privacy codes,” though there is no encryption employed and conversations over these radios are overheard simply by pressing the monitor button. In any event, CTCSS works well in separating user groups.
It allows users to operate their radios in somewhat RF-noisy environments that, without CTCSS employed, would cause the radio’s receiver to inadvertently unsquelch, which would annoy the operator. This is particularly a problem for radio users who utilize open carrier squelch scan. If the squelch constantly opens and hangs while the radio is scanning, the user is more prone to disable the scan function, use scan nuisance delete, or change channels — thereby increasing the likelihood of the user missing a call.
However, radio administrators must use caution when implementing CTCSS in high RF-noise and -interference environments, because while CTCSS allows the receiver to remain quiet, the problems of channel noise and interference still are present and can affect adversely the operation of the subscriber’s receiver. Additionally, adding CTCSS to a receiver requires the radio to decode the received tone. Many radios currently on the market are noticeably slow to decode lower frequency tones; thus, higher frequency tones are decoded faster, allowing for a faster receiver response and minimizing the risk of missed radio traffic. Many of the newer radios also can receive more than one CTCSS tone to open the speaker on a given channel.
CTCSS can be used in encode-only mode to provide the subscriber access to a repeater or network. Some repeaters will pass CTCSS tones to selectively call individual users, while other repeaters will re-generate a clean CTCSS tone when they receive a particular tone. Additionally, administrators can choose to use a separate CTCSS tone for the repeater input, as opposed to the output, in order to minimize the possibility of the repeater hearing itself in high RF-interference environments — which happens more often than realized.
Reverse burst. In a series of transmissions between a mobile or portable subscriber unit and the base station or repeater, there normally is a loud burst of audio at the end of each transmission as the subscriber unit quits transmitting, but the receiver squelch is still open. This typically is referred to as a “squelch tail.” In a series of short transmissions, squelch tail becomes quite annoying.
Fortunately, the radio equipment manufacturers have devised a method to eliminate the squelch tail. This is accomplished by sending the same tone frequency, but 180 degrees out of phase, for a period of approximately 250 milliseconds upon the release of the subscriber unit’s PTT button. When the base station or repeater station senses this reverse burst, it immediately mutes the receiver audio and there no longer is a squelch tail; instead, what one experiences is a quiet, abrupt end of the transmission. Some radios can be set for a 180-degree reverse burst, while others utilize a 120-degree reverse burst. The latter was an attempt by one of the major manufacturers to make their radios sound superior to those of its competitors, but the other manufacturers soon figured out this scheme and they too made the 120-degree reverse burst feature available.
Digital coded squelch. In the 1970s, before there was cellular, trunking, 700 MHz, 800 MHz and 900 MHz radio systems, all of the land-mobile radio systems were on low band (30-50 MHz), high band (136-174 MHz) or UHF (400-470 MHz) frequencies, and these bands were very crowded. As a result, the 38 CTCSS tones were insufficient to eliminate interference in the large cities. Motorola then came out with a digital version of the tones, and there were more than 100 of these codes. Motorola called this digital private line, or DPL. Because of patent protection, only Motorola subscriber units could be used on these systems. Once the patents ran out in the late 1980s or early 1990s, then the other manufacturers did include the DCS codes in their radios.
The DCS codes also had the inverted code for each corresponding main code, and the inverted code was used as the squelch-tail eliminator on these systems.
Conclusion. CTCSS has been employed in conventional communications systems for decades and with modern synthesized radios, there is no longer the need to add a module for CTCSS encode and decode functionality. While the concept of CTCSS is not difficult to understand, it can be a great asset to the radio technician, engineer and end-user if employed properly. As with any implemented radio feature, ensure that all your users and those agencies that they may need to communicate with have the proper CTCSS tones enabled on the proper channels. For interoperability channels and associated CTCSS tones, a good resource to reference is the National Interoperability Field Operations Guide produced by the Department of Homeland Security. Lastly, ensure that radio technicians are present at interoperability training exercises to ensure that the proper CTCSS tones and frequencies are programmed into the respective agencies’ radio gear, so when that emergency need to communicate arises, your agency will be ready.
Ira Wiesenfeld, P.E., has been involved with commercial radio systems since 1966, and has experience with land-mobile-radio, paging and military communications systems. He holds an FCC general radiotelephone operator’s license and is the author of Wiring for Wireless Sites, as well as many articles in various magazines. Wiesenfeld can be reached at firstname.lastname@example.org.
Christopher Dalton has designed, staged and implemented virtually every kind of LMR system in his two-decade-long career, including conventional, trunked, simulcast, Project 25, single-site and multisite. He holds an FCC general radiotelephone operator’s license. Dalton can be reached at email@example.com.