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CTCSS, PL, Tone Squelch, and other Necessary Evils
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Introduction
Meat and Potatoes of CTCSS
70 Trades in its Hum
Dual Mode Squelch: How it Works
Tone Squelch bugs effect some Yaesu HT's
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Introduction
Long ago and not so far away, Motorola came up with a way to get more than one Land Mobile customer on the same frequency at almost the same time. They figured that different customers could coexist on the same frequency if they did not have to listen to each other routinely. They invented Continuous Tone Coded Squelch System or CTCSS for short and patented it as "PL" short for "Private line". Other manufacturers, finding that the system was absolutely necessary to stay competitive came up with "Channel Guard," "Quiet Channel," "Call Guard," and many other names for the same thing to avoid lawsuits for marketing a patented system.
The manufacturers of amateur equipment seem to have settled on "tone" for encode only and "tone squelch" for encode/decode. Most of the amateur VHF and UHF equipment manufactured in the last ten years has at least encode capability (standard or optional) and many have decode capability (standard or optional).
When it is available, it is simply a plug in circuit board. Aftermarket encoders and encoder/decoders can be added to virtually any transceiver since they have now been developed smaller than a postage stamp.
The system is based on a "subaudible" tone injected after the audio stages into the transmitter during encode and the tone is detected before the audio circuits in the receiver. The decoder switch is then used to perform some function, usually to unmute the receiver when the tone is decoded.
In the commercial equipment, the audio bandwidth tends to be narrower than our amateur equipment and there are circuits installed to filter out the tones so they are truly subaudible. Most of our amateur equipment transmits and receives a much broader audio bandwidth and has no special tone filters, so most hear the tones. The lower the tone frequency the less audible it tends to be.
Meat and Potatoes of CTCSS
The system is designed around a set of relatively low frequency tones (32 or 38 depending on which "standard" you use) ranging from 67.0 Hz to 250.3 Hz. The tones are a perfect sine wave and the frequency tolerance is very tight, typically +/- 0.5 Hz. The tone is encoded and injected into the transmitter after the audio shaping circuits. The frequency deviation (level) is typically 0.4 to 0.8 kHz which is rather insignificant when compared to the typical 5 kHz voice deviation.
In the receiver, the tone is detected right off the discriminator before any audio processing and decoded allowing the receiver to unmute. Commercial radios filter out the tone, but our amateur radios do not so the tone is usually noticeable. It is sometimes mistaken for a power supply hum.
CTCSS does not alleviate RF interference. If two FM signals are on the same frequency at the same time, there will still be a heterodyne or beat note (unless one is 6 dB stronger than the other). But if CTCSS is being utilized and both systems use different CTCSS tones, they will not have to listen to the other system's traffic.
With the advent of commercial repeater stations, several customers can use the same repeater without listening to each others' transmissions. In a commercial installation, the microphone hanger is grounded and when the mic is hung up, the decoder is turned on, thus muting the receiver. When the operator picks up the mic, the decoder is disabled and the receiver becomes "carrier squelch," hearing everything within range. If nothing is heard, the call is made. If another user is heard, they are supposed to monitor until the traffic clears and then make their call. Base station mics have a "monitor" button next to the PTT button to disable the decoder, allowing the operator to check for traffic.
Amateur radios do not have this automatic feature since the CTCSS system is used to allow users to restrict what they want to listen to, not to allow several fleets of radios to operate on the same frequency. Most handheld radios that can be factory equipped for full CTCSS encode and decode have a monitor button, usually around the PTT bar. Unfortunately, amateur mobiles have to manually turn off the tone to monitor the channel in the carrier squelch mode.
Some repeaters use CTCSS decode to keep from being keyed up by distant stations using other co-channel repeaters. Others use CTCSS to keep all but subscribing members off the repeater. If the intent of the repeater operator is to restrict the access to members only, it is called a closed repeater.
There are many repeaters that require CTCSS tones to activate them who welcome any and all users regardless of membership status. They have CTCSS access for some other reason, usually to cut down the needless traffic and static caused when a distant station working another repeater keys up two or three repeaters at the same time. These usually list the tone frequency in the remarks section of the Repeater Directory.
Some repeaters have part time CTCSS access turned on by the Control Operator only when he or she gets tired of listening to stations keying up their repeater while trying to access others. The 224.220 MHz KA3TWG repeater on the Delaware Memorial Bridge is an example of this philosophy. When there is band enhancement and weak signals working other repeaters start coming in, they turn on the decoder in the repeater receiver to keep their sanity.
We have instituted a different idea on the 146.70 repeater in Newark. We think it is the best of all worlds and allows us to have our cake and eat it too.
70 Trades In its Hum
For several months and two moves, the 146.70 repeater in Newark has had a hum on it. Very few people could stand to monitor it for any length of time due to this "hum," almost constant weak signals on both the input and output coming from other co-channel repeaters and the mysterious WNRK interference keying it up. The signals from other repeaters on 70 are often so strong that they can't be squelched out in most of New Castle County if your receiver is hooked to any reasonable antenna.
The power supply problem never quite made it onto the priority list since Dave WA3U was in the process of putting together a new repeater package to replace the old repeater and figured the problem would go away anyway. At about 7:00 PM on September 21, Dave threw the switch and the new 70 was on the air.
With insignificantly more power, a new controller complete with courtesy beeps and voice ID, and a significantly more sensitive receiver, 70 was back and better than ever. Part of the upgrade was to put a "PL" tone on the transmitter full time so that those of us who are equipped with CTCSS decode, or "tone squelch" as the amateur manufacturers call it, can mute our receivers and not have to listen to unending static from other repeaters.
The best new feature is a three mode squelch system on the repeater coupled with three different courtesy beeps to tell which mode the receiver is in.
Dual Mode Squelch: How it Works
When you hear a two sequential tone like the 73 (only backwards) the repeater is full carrier squelch and requires about 0.18 uV of signal to key it up. This is the normal mode and we will leave it in normal mode unless conditions warrant changing to one of the other modes.
The dual mode squelch is evidenced by a relatively long (but still very short) single tone courtesy beep. In the dual mode you will not have to have "PL" on your signal if you can put 0.4 uV into the repeater at least every 2.5 seconds. If you have "PL" and have it turned on, you can key the repeater up with as little as 0.18 uV. The dual mode will only be used when we start to get a lot of weak signals working other repeaters keying up ours.
Only in special instances will we turn on the full "PL" mode where you need tone to key the repeater regardless of your signal strength. The courtesy tone changes to an almost imperceptible blip instead of a beep when it is in the full "PL" decode mode.
The night Dave turned the new repeater on, we had a tremendous inversion. 73 stayed timed out for long periods and I was keying fivr different repeaters up on 70 with 5 watts on my base station. Our 70 was constantly keyed in normal mode. We put the dual mode squelch on and didn't hear another peep all night long. It took a useless repeater and made it a nice easy to monitor channel.
The buzz you hear whenever the repeater is keyed up is the "PL" sine wave of 77.0 Hz and those of you who have tone capability on transmit are encouraged to turn the tone on at all times just in case we have it in dual or full "PL" mode. The same tone of 77.0 Hz is used on both transmit and receive. If you have CTCSS or "tone squelch," you may want to turn it on to keep the other repeater signals out.
Other upgrades are in the works for 70 and everyone is encouraged to join in and see how the new repeater sounds and check out the new coverage area since it went on the water tower. I think if you listen at a time when the other repeaters are active you will agree that the addition of the "PL" tone to the transmitter is well worth the aggravation of the buzz on the signal and if you listen long enough you won't even notice it.
The next time we have enhanced conditions and 73 remains timed out for long periods of time from being bombarded by signals from all over the east coast or you just get tired of listening to the chatter from our friends around Frederick, MD, consider this: Both 70 and 73 use the same model controller and similar circuits are on the WA3U drawing board for those of you who prefer 73.
Tone Squelch bugs effect some Yaesu HT's
Since the installation of the new repeater on 70, many of us are using tone squelch when we use the 70 machine. Most are not experiencing any problems and all report easy listening.
The only reported trouble with any radio in tone squelch mode is with my Yaesu FT-530 handhelds and Yaesu is aware of the problem. Not all FT-530s will have the problem and there is a modification for it if you have a problem with your receiver muting during voice peaks while in the "T-SQ." mode. The good news is I have the mod sheet and will give anyone a copy who requests it-the bad news is I don't recommend anyone try to do it themselves since it requires changing an SMD (surface mount device) chip resistor on the 144 MHz IF board that is smaller than a pinkie fingernail.
I asked the nice young lady in Yaesu's service department if they would fix the problem under warranty even if the radio is out of warranty and she stopped short of saying yes. I got the impression that if you complain that the radio has always been like that, they're more likely to do a freebie for you. FT-530 Radios with a "26" after the letter in the serial number are the most likely to be affected units.
My Kenwood TM-641, TM-741 and TM742 have no problems nor do any of the FT-470s we have tried. The HTX-202 from Radio Shack works fine as does the TH-77 we tested.
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