Advantages of using Digital Code Squelch

Question

I am trying to decide whether or not I should use a CTCSS (Continuous Tone-Coded Squelch System) or a DCS (Digital Coded Squelch) for my mini repeater I just set up.

What are the advantages of using a DCS over a CTCSS?

I am in an area with a lot of RF noise, and I think DCS may help keep some of that noise out.

Which method will pick up a weaker signal?

What factors should I consider when choosing the squelch method?

Answer 1

DCS (aka DSQ/DPL) provides a slightly larger range of codes to pick from compared to CTCSS. This means less chance that a nearby station will accidentally overlap with yours.

Specifically, DCS gives you 83 codes, whereas CTCSS gives you somewhere between 26-50 squelch tones depending on the radio -- manufacturers have added extra codes over time. For compatibility, you're probably best sticking to the original 26. Here's a list:

http://www.repeater-builder.com/tech-info/pl+dpl.html

(Fun fact: Technically DCS has enough address space to support 512 codes, but the majority of those are disallowed in order to prevent false positives because of the way the DCS protocol works. So 83 is what we're stuck with.)

So, those 26-ish tones vs 83 codes are the biggest difference. Practically speaking though, there's a few other considerations:

• CTCSS is compatible with older (cheaper) equipment. DCS didn't come around until much later, so it's not uncommon to find radios on eBay that only support CTCSS.
• CTCSS is a bit more forgiving to implement, since it requires less bandwidth. Again, mostly an issue with older radios.
• CTCSS is purely analog, so if you're building a radio from scratch it would probably be easier to implement. Decoding a DCS signal obviously requires a digital processor.
• DCS is less likely to trigger accidentally. Some CTCSS tones share harmonics with the local power grid, for example. Also, some old/cheap radios with noisy/slow circuits can accidentally trigger adjacent CTCSS tones, so some guides recommend only using ever-other tone.
• DCS is much less common on the amateur bands, which further reduces the chances of a collision if you use DCS.

That said, the range is about the same for both. And neither will behave differently in the face of RF noise, since the squelch is either completely open or closed at any given time.

Answer 2

DCS uses a rotating series of codes like 257257257257 which in this case is 257 repeated continually. CTCSS uses a sub audible single tone like say 100 hz. Once a CTCSS squelch is open, wide band noise like intermod or computer hash can keep the squelch open after the transmitted carrier when its sub audible tone stops, resulting in unwanted noise at the receiver. Sometimes the noise can have enough of the required sub audible tone to open the squelch and keep it open, resulting in jammed repeaters etc.

However with DCS when the rotating code stops, the squelch can not be kept open by outside interference. Also at the end of the transmission the transmitting radio sends a "close squelch" command which causes the receiving radio to close its squelch before the transmitting radio ceases its transmission. This eliminates the common squelch tail resulting in a much cleaner end of transmission. The voice just stops without any noise or tail.

DCS works much better in urban high noise environments and in my opinion produces a much nicer "sound environment". There is voice, then no voice, without the CTCSS hiss and crash at the end.

The 3 digit rotating code like 257 means that code 572 and 725 will give the same 57257257257 sequence so only some of the codes are listed. If 257 is the code, then although 725 and 572 will generate the same sequence the listed 257 should be used so as not to confuse the user into thinking they are distinct codes.

Try it on simplex. Set up two memories on the same frequency. One with CTCSS and one with DCS. Then have both operators change together between the memories and do a "side by side" comparison. I have been using DCS on repeaters and simplex for years and really like it.

Answer 3

Digital Coded Squelch will not cut any noise out of analog FM transmissions, and it will NOT "add an extra layer of image and interference rejection", DCS is not some kind of RF magic - it is simply a 'digital number sequence' imposed on the audio and it itself is a form of audio which is filtered out of the speech part of the audio.

Yes, DCS is potentially harder to be tripped accidentally but if there is so much noise and interference that an analog CTCSS fails to work properly, you have other seriously severe problems that should be addressed first and not merely covered up with DCS. Attempting to use DCS to cut/fight interference is exactly like putting lipstick on a pig - i.e. under the lipstick it's still the same pig and same smell.

Fortunately, digital does always mean better, it is all about the proper implementation. Furthermore using DCS will exclude a whole bunch of equipment from pre-DCS years - equipment often with superior RF performance and real serviceability. Unless of course that is exactly what you want to do: exclude more stations and prevent communications by further separating yourself from the ultimate spirit and purpose of Ham Radio.

Answer 4

If you use DCS instead of CTCSS then it adds an extra layer of image and interference rejection But either way should be good for your situation