What is DCS, CTCSS?

With all the new digital and programmable radios out there you will need to know some of the features and options that appear when you are programming them.

First a little history: Back in the late 50's and early 60's when two-way radios were becoming more popular, the channels were getting crowded and there needed to be a way to "filter" unwanted signals or allow wanted signals. Motorola came up with a plan to send a low frequency tone along with the voice modulation when transmitting. This sub-audible tone was filtered out of the voice modulation so it wouldn't interfere with the tone detector. Also the tone was filtered out of the audio in the receiver so it wasn't easily heard. All receivers that were authorized to hear their transmission had a common tone decoder built into the receiver. This was early referred to as a "tone squelch". Motorola patented it as "Private Line". The encoder was a small resonant mechanical reed that vibrated at a specific frequency between 67 hz and 250 hz. GE came up with their own system using the same tones and called it "Channel Guard". This enabled many users such as businesses to share the same frequency (not at the same time) without interfering with each other. If they used a repeater, they were called "Community Repeaters". When atmospheric conditions on the VHF/UHF band was good, you could hear other areas of the country on the same frequency. With tone decoder squelches, this eliminated interference from other areas.

In the late 70's these tone systems became crowded and another method came to light. A unique "digital tone" was created and Motorola called it Digital Private Line or "DPL". This added many more privacy tones to be used.

Since Motorola held the patents to the designation PL and DPL, the industry started referring it to TCS and DCS or CTCSS. TCS being "tone coded squelch" and DCS being "digital coded squelch", and CTCSS as "continuous tone coded squelch system". Of course no tones is designated CS or "carrier squelch". You will see these options when programming your new digital radios.

Back in the early days Motorola used plug-in reeds like those shown below.

Then they became smaller

Then the digital ones were little thick film plug in modules like these. They were referred to as "code plugs".

Today, all digital radios with software programming of frequency channels, various options, and squelches are contained into a software file, referred to again as "code plugs" and don't need a physical plug-in module.

When programming the DCS options there are about 40 codes. There are also inverted codes. As an example, one is code is D023N meaning it's digital, code 023 and normal code. You may also see D023I which is code 023 with an inverted digital code. With the inverted code this gives double the codes available.

So what codes do you use and do you need to set the receive and transmit codes?

This will all depend on your specific purpose. If you are a ham radio operator and need to access a "closed" repeater, meaning a repeater that requires a code to bring the repeater up, determine what code the repeater uses and that you have permission to access the repeater, and program in the transmit code. It is not necessary to program in the receive code but there are reasons for doing it either way. If the repeater transmits the tone code too, then you can program in the receive code in your radio. This will prevent any other on frequency repeaters opening the squelch which becomes annoying. Another reason is that some cheaper handhelds and mobiles that don't have good selectivity are plagued with the squelch "chattering" which becomes annoying. Also strong off frequency services can cause the receiver to "chatter". A reason not to program in a tone code, called "carrier squelch", is that someone else that can't make it through the repeater but is close by to you can call you direct can call you on the repeater output frequency. But as long as they were transmitting the same code it won't be a problem. If they weren't sending a code they wouldn't be heard. This is also called "talk around" or simplex.

Other than that, you and your group of operators can use any code available. Remember, FRS radios have about 32 codes called "privacy codes" (these are the same tones as the Motorola codes). It doesn't hide or encrypt your communications from anyone. It just keeps other users on the same channel as you from being heard.

For an in-depth explanation of CTCSS and DPL you can read it here.


2022 Rick C. Ver 0.67 Mar 9, 2022