B-K Electric Tone Squelch Page

CTCSS or "PL" "Sub Channels?"

by
John Huggins

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. Some call it by the most correct name (probably) of Continuous Tone Coded Squelch System (CTCSS).

All this CTCSS stuff really is is a tone that is sent along with your voice to the receiving radio. If you were to listen to this combination on a good stereo, you would hear your voice along with a very annoying low tone. However, radio engineers long ago realized that communications via voice did not need superb audio fidelity. So the audio that is allowed through is between about 300Hz - 3,000Hz. This really does stink compared to what you expect when you play music, but for passing the human voice, this is fine.

So...

CTCSS takes advantage of the unused bandwidth below 300Hz by mixing one of the many tones listed below into the audio.

CTCSS DOES NOT keep others from hearing you. It only keeps you from hearing them unless they transmit the correct code. IF the radio has CTCSS turned on, it can keep the radio audio circuit silent unless a particular tone is found. It will keep them from hearing you ONLY IF they have CTCSS turned on AND you transmit no code, or the wrong code. This also means that while your computer (normally a very bad 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.

What this means for you is that a crowded frequency can have multiple users... sort of. If your organization chooses one tone, and another chooses a different tone, each group's radios will not open the squelch when the other group transmits. However, if two transmitters key up at the same time, your receiver will only play garbage since both transmitters are on the same RF frequency.

Squelch codes are a NECESSITY in repeater systems. The receive side of the fixed repeater has a squelch code programmed in to keep distant users (or unauthorized users) on the same channel - and believe me, they're out there - from keying up your transmitter. Simply put, the repeater uses the squelch code to help identify the signals it should re-transmit, and reject interference or non-repeater users.

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

Most manufacturers will let you program a radio with a transmit-only code and an open ("CS" or "QSC") receive with no code, so it is not necessary to have any particular channel silent until it hears the correct tone. 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 feature useful in repeaters.

Not all radios have all codes, and some Manufacturers can't 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 an 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 they provide you with a real channel; You are still sharing the same RF frequency with others just like a party line telephone system with all the same issues.

"Meet me on channel 16, sub-channel 5."

 

And, there's one more wrinkle: Over the past dozen years, radio manufacturers have started using a variation called Digital Coded Squelch ("DCS"). DCS works in a similar way to CTCSS: It is an inaudible digital code sent with each transmission. The code is recognized by the selected receivers enabling them to receive the transmission. Receivers that do not recognize the code remain silent.

The DCS codeword consists of a 23 bit frame which is transmitted at 134.4 bits/sec. As this frequency is below that normally used for speech, it is a fairly simple matter to filter out the tone in the receiver so the user is unaware of 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.

The newest digital radios do not use either of the above. Engineers have developed a third system that functions the same as CTCSS or DCS for filtering out unwanted signals, but is included directly in the string of "1's" and "0's" that make up digital transmissions. APCO P-25 radios use a signal called a Network Access Code ("NAC"), while the newest real 6.25 bandwidth digitals use a similar feature called a Radio Access Number ("RAN")

 In public safety, the industry has settled on 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. Lower tones 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, so 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 your vendor from having a lock on your future business

A final note: ALWAYS ALWAYS ALWAYS transmit a squelch code, even if you don't use it on the receive side. Somewhere down the road, it may become handy to have it, and pre-sending the code on all transmitters makes the transition smoother.

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