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CTCSS or "PL" "Sub Channels?"

by
John Huggins, at al

For many years two-way radios have had a feature called by many names including Motorola's Private Line ("PL"), GE/Ericsson's "Channel Guard", E. F. Johnson's "Call Guard", RCA's "Quiet Channel." Others simply call it tone activated squelch. A few call it by the most correct name (probably) of Continuous Tone Coded Squelch System (CTCSS).

DCS and digital RANs come later- read on...

All this CTCSS stuff is just a simple low frequency audio tone that is transmitted along with your voice when you talk, and a receiver with a filter or logic gate that keeps the receiver silent unless it hears the correct tone. When the receiver hears the correct tone, it opens the squelch gate and turns on the speaker, otherwise it keeps the speaker silent even though the receiver may be hearing a strong, clear radio signal.

Human speach seldom drops below 300Hz. Communications engineers realized this long ago and don't even transmit the un-necessary audio spectrum; in fact both telephones and 2-way radios only handle audio between 300Hz and 3,000Hz. CTCSS takes advantage of the lower unused bandwidth by mixing one of the many tones into the audio. If you were to listen to this raw tone/voice combination on a good stereo, you would hear the voice along with a very annoying low hum. But before the audio gets to the receiver's speaker, the radio filters out the annoying low frequency tones and only plays the voice portion. 2-way fidelity really does stink compared to what you expect when you play music, but for passing the human voice over a radio system it is perfectly acceptable.


There are three very important points to remember about squelch codes:

---CTCSS DOES NOT keep your communications private.

---CTCSS WILL keep your intended receiver from hearing you unless both radios are programmed properly.

---CTCSS DOES keep you from hearing unwanted noises or voices unless the correct code is also received.


So...

To start with, always create (or get) the master plan before you hand out the first radio. If every radio is a particular system has the correct squelch codes properly programmed from the get-go, the end users will have no problems. And, always verify that the code selected is the right one before deploying a system. Reprogramming a fleet after the fact is unhandy at best. Been there, done that...

So, why bother? IF your radio has CTCSS receive filtering turned on, it will keep the radio silent unless the correct code is present. It will keep you from hearing foreign signals IF the foreigners do not transmit YOUR code. This also means that while your home or office computer (normally a very strong source of radio interference) may be sending a RF signal strong enough the "break the squelch" and open up your radio filling the room with a lovely hissing noise, the likelihood of it ALSO sending a valid CTCSS is slim making your radio a much quieter companion to have around.

This also means that a crowded frequency can have multiple users... sort of. If your organization uses one code, and another uses a different code, each group's radios will not open up when the other group transmits. However, if two transmitters happen to key up at the same time, your receiver will only hear garbage since both transmitters are on the same RF frequency. But unless the frequency is very busy in a particular location or a distant transmitter has a very high power this is seldom a problem.

There's another advantage. An open (non-CTCSS) receiver may need to have the squelch tightened WAY up to keep noise and unwanted signals from comming out of the speaker. This also has the tendency to keep out weak signals you really need to hear. Applying a squelch receive filter means that the radio can have a "looser" squelch. This both filters out unwanted noise and if the right code is present allows weak desired signals to come through. This is especially important where 2-way radios are used in paging systems. Modern radios in scan look at the squelch code also, and simply skip on by signals (noises OR voices) that may be strong enough to open the squelch, but don't have the right code.

Most radios have some means to turn off the CTCSS receive filter. Portables usually have the top left side button (not the talk button) programmed to manually toggle the filter on and off. Some radios can be programmed to automatically open the filter gate for a period of time after you transmit. And, many mobile radios turn the receive filter off when the microphone is lifted from the hanger.

Squelch codes are a NECESSITY in repeater systems. The receive side of the repeater uses a squelch code to keep foreigners or distant base stations on the same channel - and believe me, they're out there - from keying up your repeater's transmitter. Simply put, the repeater uses the squelch code to identify which signals it should re-transmit, and ignore interference or foreign users. Modern repeaters also allow multiple codes- if the repeater hears code A, then it transmits code A. If it hears code F, it transmits code F, and so on. This allows different groups to access the same repeater but not have their traffic normally heard by other user groups.

Most manufacturers will let you program a radio with a transmit-only code and an open ("CS" or "QSC") receiver with no code. You will transmit your group's code for those who have receive CTCSS enabled, but hear everything, near or far, real or interference, good or bad. It is also possible to transmit one code, and receive another, a useful feature in large repeater systems.

Don't confuse squelch codes with "Selective Calling". While sometimes very similar, they are two different features and technologies.

Not all radios have all codes in their software, and some (/\/\)anufacturers can't seem to speak plain English. They give their CTCSS codes a proprietary name, mostly to make life difficult and make them seem more special. Other manufacturers simply use the frequency of the tone; the most simple and fool-proof method. For completeness I have included a table containing the available tone frequencies and a cross reference to what others name them. Since most FRS radios seem to have adopted a 1 - 38 sub-channel system we at last have a way to express what CTCSS tone we all wish to utilize with a simple number.

Midian Electronics has an excellent wall chart and downloadable reference for all industry standard (and non-standard) tones. You can get it at http://www.midians.com/pdf/tone_signaling.pdf.

The moral of the story is that CTCSS sub-channels are a good idea for all users, but don't expect them to provide you with a private channel; You are still sharing the same RF frequency with others just like a party line telephone system with all the same issues. You just can't hear them.

Commonly used squelch codes.

DCS and RAN codes.

There's one more wrinkle: Over the past dozen years, radio manufacturers have started using a variation called Digital Coded Squelch ("DCS"). DCS (or DPL) works in a similar way to CTCSS: It is a digital code of ones and zeros that is sent at a low audio frequency with each transmission. Just like CTCSS, receivers that do not recognize the code remain silent. And like CTCSS, the low frequency code is filtered out of what you hear.

The DCS codeword consists of a 23 bit frame which is transmitted at 134.4 bits/sec. As this data is send on audio frequencies below that normally used for speech, and like CTCSS the radio filters out the code so the user doesn't hear it.

There are a total of 104 codes available but only 52 codes are normally assigned for use on radio bands. This is because the other 52 are the inverted codes or complementary codes, e.g. a codeword such as 10010 is the complement of 01101. Simply, an inverted code sequence can look like a mirror image of normal code sequence and cause false audio.

DCS codes can be a bit finnicy- if the audio of the radios is sloppy or either is off-frequency, the low frequency code isn't received properly, and the receiver doesn't open up when required.

Digital radios do not use either of the above but have a system that provides the same function. For digital radios, engineers have developed a third system that works the same as CTCSS or DCS for filtering out unwanted signals, but instead of using a sub-audible tone puts the code directly in the string of "1's" and "0's" that make up digital transmissions. APCO P-25 radios use a feature called a Network Access Code ("NAC"), while the newest real 6.25 bandwidth radios from Icom, Kenwood, Bendix-King and others use a similar cross-platform feature called a Radio Access Number ("RAN"). The code appears in more than one place in each data packet, allowing for "late entry". Motorola "Trbo" radios use a unique proprietary scheme not available to other manufacturers.

A few final notes. For public safety, DHS has standardized on a CTCSS of "156.7" and the APCO P-25 NAC of "$293" for nearly all interoperable radios nationwide. For most users, any particular locality should decide on one code and use it in all of their equipment across many frequencies whenever possible. This makes it easier to remember what code is used.

Lower tones (CTCSS below 150hz) seem to have less background noise than higher ones. And while the FCC clearly sets out the frequency, bandwidth and emission of every license, there is no rhyme or reason to selection of squelch codes. Doing your code homework before setting up a system is mandatory, but it is often a matter of trial and error.

INSIST that your radio vendor tells you what codes he put in your radios and write it down on the license. It it your information, not his, and having the codes makes interoperability easier and keeps any one vendor from having a lock on your future business

A final note: ALWAYS ALWAYS ALWAYS transmit a squelch code, even if you like hearing all the static and foreign voices and don't use it on the receive side. A mutual aid agency may require it to hear you. And somewhere down the road, it may become handy to have it, and pre-sending the code on all transmitters makes the transition smoother.