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AMATEUR RADIO BY GARRY CRATT, VK2YBX Selective tone calling in commercial & amateur radio Signalling systems designed & used in commercial communications are now finding their way into amateur communica­tions. This article sets out to explain the difference between the various commonly used systems & why different systems exist. The use of a signalling system can greatly improve the effi­ ciency of a communications system, by allowing many users to operate on the same communications frequency without causing interference. A signalling system thus allows greater utilisation of the RF spec­trum. A multitude of signalling systems already exist around the world, ranging from very simple single tone or sub-audible tone systems to sophisticated computer-controlled proprietary sys­tems. However, most commonly encountered systems in use in Aus­tralia fall into the following categories. CTCSS signalling The Continuous Tone Controlled Squelch System, or CTCSS as it is commonly known, is a system where­ by an RF carrier is modulated with a continuous audio tone, in addition to speech. Originally known as Tone Squelch, modern systems now utilise tones in the low frequency range, from 67Hz to 250.3Hz (see Table 1). When the modulated carrier signal is received, a decoder detects the particular tone in use and uses it to unmute the receiver. In this way, transmissions using a different tone, or no tone, are ignored by a CTCSS equipped receiving station. The CTCSS tone is 84  Silicon Chip filtered out in the receiver, prior to the audio stage. This system is widely used in simplex commercial radio systems, where it may be necessary to selectively call base stations, other mobiles, or operate various remote receiver functions. As the system is operating on a shared frequency basis, it is important to ensure that the channel is free before transmitting, in order to minimise interference caused by simul­taneous transmissions from a number of mobile stations. One of the disadvantages of CTCSS is that, due to the very nature of the tones used, repeater operation in CTCSS mode is often unreliable or impossible. This is because most transceivers are designed to deliberately Table 1: CTCSS Codes 67.0 94.8 131.8 171.3 203.5 69.4 97.4 136.5 173.8 206.5 71.9 100.0 141.3 177.3 210.7 74.4 103.5 146.2 179.9 218.1 77.0 107.2 151.4 183.5 225.7 79.7 110.9 156.7 186.2 229.1 82.5 114.8 159.8 189.9 233.6 85.4 118.8 162.2 192.8 241.8 88.5 123.0 165.5 196.6 250.3 91.5 127.3 167.9 199.5 roll off the audio response outside the speech range, particularly below 300Hz. Modern commercial repeaters specifically designed for CTCSS operations employ special decoding circuitry for this purpose. In addition, CTCSS tones are not compatible with the DTMF tones used in phone patch operations. Nevertheless, some form of signalling through repeaters is necessary, and a signalling system known as SELCALL is often used. This system comprises a range of discrete audio frequen­ cies, each corresponding to a digit from 0-9, plus two extra tones for “repeat”, where adjacent tones are identical, and “group” signalling, where a number of mobiles are to be called simultaneously. 5-tone sequential tone sets The 5-tone principle has been accepted internationally by a number of leading standard associations such as CCIR, EIA, ZVEI, NATEL and EEA. As can been seen from Table 2, while the broad principle has been adopted, there are a number of sequential tone sets in use around the world, as dictated by the appropriate standard association. Selective calling also facilitates ANI and Group calling through repeaters. Automatic number identification (ANI) is used to indicate that a call has been received at an unattended mobile radio. As the transceiver initiating the call is equipped with a unique 5-tone code, it is a relatively simple matter to store and display the code, revealing the identity of the sender and time of transmission at the unattended end of the radio link. In addition, the signalling system can be used to activate an Table 2: Tone Sequential Standard Table 3: DTMF Signalling Frequencies Tone CCIR EEA EIA ZVE-1 ZVEI-2 ZVEI-3 0 1981 1981 600 2400 2400 2200 1 1124 1124 741 1060 1060 970 2 1197 1197 882 1160 1160 1060 3 1275 1275 1023 1270 1270 1160 4 1358 1358 1164 1400 1400 1270 5 1446 1446 1305 1530 1530 1400 6 1540 1540 1446 1670 1670 1530 7 1640 1640 1587 1830 1830 1670 8 1747 1747 1728 2000 2000 1830 9 1860 1860 1869 2200 2200 2000 R* 2110 2110 459 2600 970 2400 G 2400 2400 2151 2800 885 2600 ITPS* 100ms 40ms 33ms 70ms 70ms 70ms High Group Frequencies (Hz) Low Group Frequencies (Hz) Note: R = Repeat tone; G = Group tone; ITPS = International Tone Period Standard audio alarm, indicating “call received” status. Group calling allows a base operator to call a group of mobiles, without disturbing other mobiles on the same frequency, and preserves some degree of security. DTMF signalling DTMF or “touch tone” signalling is commonly used in amateur circles to gain or restrict access to repeaters. In addition, the use of the standard DTMF (dual tone multi frequency) tones, as shown in Table 3, allows easy interconnection to the PSTN tele­phone network, a great advantage for those utilising phone patch interconnect equipment. In fact, commercial trunked radio tran­sceivers use DTMF signalling so that the very basis for their existence (interconnection to the PSTN as a competitor to the AMPS cellular network) is easily achieved. However, there are particular disadvantages in using DTMF signalling in the mobile radio environment. Table 3 shows the combinations of two tones required to produce a DTMF “digit”. The difference in level between the two tones must be held to specif­ic limits to ensure accurate signalling. The maximum allowable “twist” of these tones is 4dB (AUSTEL standard TS-002) in Austra­lia and 3dB in New Zealand. In an RF environment, this twist level can normally only be guaranteed to within 6dB, making the system unreliable. Also, the minimum achievable signal to noise ratio in a DTMF system can be mathematically calculated to be at least 6dB worse than that achieved by a 5-tone sequential system, because in a DTMF system, the deviation per tone is only half that used in a 5-tone system. There are a number of other disadvantages with DTMF radio signalling in areas such as speed, timing, dynamic range and intermodulation products. Digital coded squelch A further development in signalling technology is the DCS or “digital coded squelch” system, where each of a group of codes corresponds to a digital TTL data stream sent typically as an 8-bit word, either in bursts or contin- Table 4: DCS Codes 023 131 251 371 532 025 132 252 411 546 026 134 255 412 565 031 143 261 413 606 032 145 263 423 612 036 152 265 431 624 043 155 266 432 627 047 156 271 445 631 051 162 274 446 632 053 165 306 452 654 054 172 311 454 662 065 174 315 455 664 071 205 325 462 703 072 212 331 464 712 073 223 332 465 723 074 225 343 466 731 114 226 346 503 732 115 243 351 506 734 116 244 356 516 743 122 245 364 523 754 125 246 365 526 1209 1336 1477 1633 697 1 2 3 A 770 4 5 6 B 852 7 8 9 C 941 * 0 # D uously. Unlike analog tone systems, the advantage of a digital system is the enormous number of mobiles that can be operated on the one frequency. Some com­mercial users are currently operating 250 DCS mobile transceivers on the one frequency! Table 4 shows a list of the commonly used 104 DCS codes. Each code corresponds to a specific data stream. Pocket pagers Although not a signalling system used in 2-way communica­tions, the POCSAG code system used in pocket pagers is certainly worth mentioning as an advanced one-way signalling system. In 1975, the British Post Office established the Post Office Code Standardisation Advisory Group to study and design a digital radiopaging message format. Two years later, that group recom­ mended what is today called POCSAG. Understanding this system requires a good working knowledge of Boolean algebra and is beyond the scope of this article. Suffice to say, POCSAG is the predominant paging signalling system in use in the world today. One interesting piece of test equipment we discovered whilst working on this article is the “CD-1 Communications Decod­er Unit”, a stand-alone tone decoder capable of displaying CTCSS, DCS and DTMF signalling tones received by radio. Designed as an accessory for a service monitor, this unit can easily be wired to any receiver and used to display signalling codes in use. The unit is available from Raedale Pty Ltd in Queensland. Telephone (075) 76 3000. The most obvious use is the monitoring of unauthorised users of commercial repeaters. No doubt there is a similar appli­cation to which the unit could be put in amateur radio. Signalling Technology Pty Ltd of Melbourne (phone 03 786 0077) also stock a useful range of encoding and SC decoding products. December 1993  85