HOW PL / TONE SQUELCH / CTCSS SYSTEMS WORK
What follows is very important information - many serious problems occur when helicopter ambulances do not understand how their PL systems work. Additionally, badly designed PL systems have a horrendous effect on many public safety radio systems.
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In case I have lost you already - PL stands for Private Line - CTCSS stands for Continuous Tone Coded Squelch System - and Tone Squelch is the same as PL which is the same as CTCSS - they are all different names for the same thing. Please allow me to explain what the "thing" is.
Let us say that you and I have two walkie-talkies. We talk back and forth all day with no problem. Then - trouble in paradise - someone else starts using our channel. Well this really is a bummer - we dont want to listen to those other fools all day long - so what can we do about it?
How about if I put a little device in my radio which is called a "encoder". This encoder puts a soft hum on my radio signal. Then we put a little device called a "decoder" on your radio. What the decoder does is - it listens for that hum - if it doesn't hear the hum - then it does not allow any messgaes to come out of your speaker on your radio. Voila - we have a system for cutting out all the garbage from those other clowns. Of course - we will have to put an encoder on your radio also - and we will also have to put a decoder on my radio.
So by putting a decoder and a encoder on each radio, and by setting them up to send and listen for that same hum, then we have devised a system to eliminate a major hassle from our life.
Of course - if your encoder breaks and stops sending that hum along with your messages, then I will never hear anything that you say. Or alternately, if my decoder breaks, then I will not hear any of your messages.
This is the basic workings of a PL system. So if you have an ambulance helicopter coming to your town and they want to know what your radio frequency and your PL tone is, now you know what they are talking about and why they are asking. If they do not have the correct PL info, they can transmit all day on your frequency and never hear you. Or you will never hear them.
Pop quiz time -
1. What happens if a strong skip signal interferes with your radio system? How will it affect your PL system?
2. What if the strong skip signal has the same Pl tone as your radios?
3. What if someone transmits on your frequency with a Digital Squelch Tone?
Never heard of a Digital Squelch Tone? Well - actually nobody had heard of one of these before about 10 years ago. "Digital squelch" uses a morse-code-like signal to serve the same purpose as the hum which I described previously. The same basic cautions apply with a digital squelch system - if you do not have the correct numbers programmed into your radios in the correct manner, then you are going to run into major difficulties. (Note - DPL = Digital Private Line)
These tone squelch systems are used extensively by any agency which has multiple repeater sites operating on the same frequencies. Can you figure out why? Answer - if you have 10 repeaters in your system - all working on the same frequencies - then anytime that some mobile keys up his radio, he would activate all 10 of the repeater sites at the same time unless a tone squelch system was used. If all 10 of the repeaters came on at the same time, then many people would get many headaches listening to the resulting squeals and squawks on the repeater output channel. If each repeater site requires a different PL on the input frequency in order to activate - then we can cut out all the resulting interference.
Does this make sense? Can you picture how the piece of this puzzle fit together?
Try this little quiz - If repeater A require a PL of 100 on the input channel and transmits with a PL of 200, then what PL should Car 1 transmit and what PL should Car 1 listen for when he uses repeater A?
Answer - Car 1 needs to transmit a PL of 100 and to listen for a PL of 200 - in other words - the encoder in Car 1's radio should transmit a PL of 100 and the decoder in Car 1's radio should be looking for a PL of 200. Is everyone clear on this?
I hope that this info helps people to better understand how radio systems operate.
The URL for this item is http://www.geocities.com/CapeCanaveral/3327/pl.html
By Peter Szerlag July 13, 1998
Please feel free to share this info with anyone that might benefit from it.
IMPORTANT Additional Info - a very wise person reminds me to tell you that PL/CTCSS systems do NOT eliminate interference, they just cover it up. This is not totally a bad thing. Covering up distant stations will make things much easier on your ears. However, if there is a near-by agency who is using your radio frequency, then they will still give you interference and cover up some of your transmissions: BUT - you will not hear them doing it. Kind of a "good news/bad news" kind of situation. July 20, 1998
Approximately 100 digital PLs are available with todays radio equipment and about 36 tone PLs. I have to dig out the exact numbers for you sometime. But the general idea is - the newer radios with the newer digital PLs will probably get less interference from people on skip who might be using the same PL or DPL.
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Here are a couple of links with some well written explainations of PL systems.
Reviewed - 6/29/99