Digital Coded Squelch (DCS)

DCS or DPL, Ditial Private Line which Motorola calls itis not new, it has been around since the late '80s in commercial equipment but has only started to become available in Amateur rigs over the last few years. It may not be a great advantage to Amateur Radio as the mode is more particular to the setup of the radio for proper operations.

Some of the Amateur rigs, like the Yaesy VX7-R also support mixed mode PL/DCS or DCS/PL in addition to PL, PL/PL and DCS. Most rigs will ONLY support DCS encode/decode and not just encode.

Digital Coded Squelch (DCS) is digital data or code word that is transmitted with the voice audio. This data is sub-audible with most of it's energy below 300Hz. However is does have a wide bandwidth from 2 to 300 Hz. Unlike Continuous Tone Coded Squelch System (CTCSS) which uses continuous tones below 300 Hz., DCS uses digital data or code words. Each code word is unique and all code words may be used on the same channel without interference. At the end of the radio transmission and about 1/2 second before the transmitter un-keys, the radio will encode a 134 Hz tone that serves as a turn off code. The FM deviation level of DCS data should be in the range of 500 to 800 Hz.

Unlike CTCSS, DCS signal spectrum occupies considerable more bandwidth. A poor low frequency response in the transmitter or receiver may not seriously distort a single frequency tone signal but may seriously degrade a wide band signal containing multiple frequency components. The distortion risk is especially high if the frequency response delays the wide band frequency components.

DCS is operated at a low baud rate (137 bits per second) and because DCS may have extended periods of all ones and zeros almost all components in the transmitter and receiver chain must be coupled down to at lease 2 Hz or lower. This requirement means that certain transmitters and receivers must be modified before they are capable of DCS operation. Phase modulators, in particular, need special consideration because they theoretically are incapable of being directly modulated by dc, unlike direct FM modulation methods. Low frequency response is the primary requirement for DCS systems.

You will fined that it is extremely important for the receiver and transmitter to be on frequency to achieve maximum performance of the DCS function. Errors in the transmitter and receiver frequencies show up a the discriminator output as a step function. Because of the long time constant required for the low frequency response, a step function can block the decoder momentarily. With DCS, error correction is necessary. But if too many errors occur, you may experience some blocking out of the decoder. Errors can occur because of unwanted low frequency energy. The DCS decoders can be effected by voice energy that falls below 300 Hz. Some radios do not remove this energy before transmission and can cause voice blocking of the decoder. A sub audio filter that removes this low frequency energy before the audio is re-transmitted is necessary for reliable DCS operation.

Before you start modifying your radio to operate DCS, make sure your service monitor is DCS capable. Some older monitors require modifications to obtain the low frequency audio response needed for DCS operation. If in doubt, contact the manufacturer.

The following is a table of standard 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  

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