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CTCSS or DCS? And why?


dwmitchell61
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Sub-audible tones (CTCSS) or digital (DCS)?

 

Which do you use and why? Is one better than the other? is the digital coding more reliable than the sub-audible tones? I see that the repeater operators seem to always use CTCSS.

 

I am hoping this isn't one of those "Which engine oil is best?" topics?

 

I am trying to determine what channel and method of squelch to use between my base and the HT.

 

Thank you in advance...

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CTCSS is older and more prone to spurious noise opening the squelch - weather or ambient. Additionally, there are relatively few usable tones. DCS is newer was was designed to address those shortcomings.

 

Both came out of Motorola, who initially was one of the only manufacturers of radios with enough frequency stability to use DCS (though others caught up quickly). Even today, some Baofeng as they age have trouble with a DCS system. Also, many older radios can't use DCS at all, and a few transitional models needed modules to enable it (usually you'd swap our the CTCSS module to a dual mode module).

 

So, yes, DCS is better, but CTCSS more compatible.

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Both are meant as one thing, an access code to open up the squelch on the radio to hear traffic. No code means you will hear everything on that channel, with a code you will only hear traffic with a matching code.

 

Not all repeaters use CTCSS, The WRAK968 repeaters are set up with DCS codes instead. To my knowledge it is user preference as to which code group they wish to use with no real advantage to ether side.

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I thought the DCS (digital) would be less susceptible to noise than the CTCSS (analog tones). I know why they are used, just curious as to why the two different standards came about, and if there was an advantage of one over the other. Or is it a case of engineers demonstrating their cerebellar prowess?

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Note that on an FM or Phase modulated radio all transmissions are analog. Digital data is actually represented by the presence or absence of an audible or sub-audible tone. IIRC 131.4hz is used for DCS. If the signal frequency is not stable, the DCS sub-tone will also vary, making detection of the bitstream difficult or impossible.

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DCS is transmitted at a rate of 134.4 bits/s.  The DCS waveform directly modulates the FM carrier, with a logic 1 represented by a positive carrier shift and a logic 0 represented by a negative carrier shift.  The FM deviation is about the same as CTCSS/PL.  The DCS waveform's edge rates are carefully controlled to limit harmonic content to avoid generating audible harmonics.

 

A DCS word consists of a 23 bit cyclic Golay code with 12 bit codeword (23,12) formed from the 12 least significant bits.  The 11 most significant bits are error correcting code bits generated by the Golay algorithm from the 12 codeword bits.  The 12 bit codeword consists of a fixed Octal 4 (100 binary) plus the 3 Octal digits that you can program.  A DCS word plays out backwards over the air due to the least significant bit being transmitted first.  A DCS word is sent repeatedly as a continuous low baud rate data stream.

 

DCS/DPL has a specific turn-off code or reverse burst at the end of a transmission which resembles a 134.4Hz sine wave.  In radio systems with a mix of CTCSS/PL and DCS/DPL usage, it is recommended to avoid using a CTCSS/PL tone of 136.5Hz due to the DCS/DPL kerchunk problem caused by the 134.4Hz DCS reverse burst.

 

The time required to reliably decode DCS is slightly longer than CTCSS and is about 350ms or less.  CTCSS typically decodes in about 250ms or less.  When decoding DCS on very weak signals, the DCS decode threshold can be as much as 2 dB worse than CTCSS.  From an end user perspective, these differences are way down in the noise (LOL) and it's hard to tell any difference.  The average end user probably won't notice any difference in DCS/DPL vs. CTCSS/PL performance.

 

DCS has its own set of issues to deal with.  The long high and low bit times of the NRZ data can cause problems with baseline wander and cause decode problems.  Distortion in the transmitted DCS waveform itself can cause decoding problems in the receiver.  DCS is also sensitive to frequency errors in the transmitter and receiver.  A signal transmitted with DCS received a few kHz off-frequency in the receiver impresses a DC offset on the recovered DCS waveform which causes the decoder to see a step function instead of the DCS data.  This causes very slow decoding or failure to decode.  Good low frequency response down to about 3 Hz in the modulator in the transmitter and in the discriminator output in the receiver are necessary for reliable DCS operation. IIRC phase modulated systems have trouble with DCS.  True FM modulated systems tend to work better.

 

Motorola trademarked the Digital Private Line (DPL) name.  Other manufacturers have to call it something else.  Digital Coded Squelch (DCS) is the generic name.  You'll see mixed usage of the two names as I have used them above. If it's not a Motorola radio, it's not DPL. It's DCS instead even though it's exactly the same thing as DPL.  Motorola originally implemented 83 DPL codes and manufacturers have expanded the set of codes to 104 codes.  Motorola's 83 DPL codes have been implemented by all manufacturers but not all manufacturers have implemented the full set of 104 codes.  Motorola's original set of 83 DPL codes have been fully vetted and are known to work well.  Some of the extra codes making up the set of 104 codes may not work as well.  YMMV.

 

I've also heard DPL sometimes referred to as "Definitely Prevents Losers". LOL.  I successfully thwarted a GMRS repeater jammer years ago with DCS/DPL so the jammer couldn't key my repeater.  The jammer didn't have a clue about DCS/DPL nor did he have DCS/DPL capability in his antiquated equipment and only had CTCSS/PL.  The jammer eventually gave up.  This trick won't work anymore given that practically all manufacturers now offer DCS capability in their radios.

 

DCS information:

https://wiki.radioreference.com/index.php/DCS

http://onfreq.com/syntorx/dcs.html

http://www.repeater-builder.com/tech-info/ctcss/ctcss-overview.html

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Thank you all for the information. I haven't heard of the Golay code/algorithm.... I will have to look that up. LoL.... I am acquainted with Gray code, BCD code, and binary coded octal.... Used to work with come navigation equipment that was nearly as ancient as I. The telemetry monitoring unit we used during preflights had nixie tube displays. We used thumbwheels to dial in hexadecimal addresses to display. Some outputs were in octal and we had to convert the octal numbers to decimal forms of radians and then convert the radians to degrees.... Good ole days...

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