Scanner Tales: PL or Not to PL, that is the obsession
So we all know what PL Tones are, right? Well, if you don’t, a PL tone is basically a subaudible tone superimposed on a radio signal. This tone, typically between 67 and 250 Hz., is supposed to be below the frequencies detectable by the normal human ear but easily detected by radios. They are used for several purposes in radio communications, such as blocking access to repeaters unless you are transmitting the proper tone, allowing a radio to ignore traffic not transmitting that proper tone on shared channels and even to count access time for systems that charge by the minute or key-up.
PL tones have been around for over 70 years, the concept was invented by Motorola in the 1950’s. a couple decades later Motorola came up with “Digital Coded Squelch”, more commonly notated as “DPL”. DPL serves pretty much the same function as PL tones but goes about it a little differently. It also adds to the number of codes available.
There are several names for PL and DPL codes but that is beyond the scope of today’s Tale. For our purposes we will just use the PL and DPL nomenclature. The Repeater Builder website has a great article on the origination on PL Tones at A historical and technical overview of tone squelch (PL, CG, QC, CTCSS, etc.) systems. if you are interested in the history of the codes and why they have such odd names like “1A”, “7Z” etc. and why the tones are in weird increments.
So I tell you all that to talk about this: How I got obsessed with PL tones and how I would go about searching for them. It all started when I was just a young lad of 18 working as a police and fire dispatcher in a mid-sized suburb of Chicago. We had a UHF police repeater and a simplex VHF fire channel as well as a couple other simplex channels for Public Works and police car to car. At first only the police repeater and PW channels had PL capabilities, but I didn’t know that yet. I had no idea what PL codes were, all I knew was we only heard our town (and the town we shared the channel with) on the police repeater at work but on the scanner, I could hear other towns occasionally.
A friend, Scott, started to explain PL tones to me a year or so later after we met but I really didn’t understand the concept for a while. Eventually our radio guy (it wasn’t me then) added PL tones to the fire channel as well as the police car-car channel so we didn’t have to listen to outside agencies on these channels. While it was fun for a scanner dude like me when things were slow to listen to agencies a hundred miles or more away at night it got annoying when work got in the way of my fanboying.
Eventually I started understanding the concept of PL tones from the lessons Scott gave me as well as from other RCMA members who took me into their confidence and shared the forbidden “PL List”. Soon I was pretty well hooked on PL tones and trying to find them. At the time (early 1980’s) RCMA did not allow discussions of PL tones as there was no scanner need for them. Scanners did not have the ability to display or utilize them so the only reason one would need a PL tone was to interfere with the legitimate traffic was their reasoning. We had a subset of club members who enjoyed collecting PL tone information, most notably our infamous friend, the late Brandt. (See “The Tactical Cabbie” at Scanner Characters: The Tactical Cabbie)
At the time, well before the first PL-capable scanner that I am aware of (the BC760XLT) came out we used many methods to find PL tones. We continued these methods after as they were usually easier than programming a scanner over and over to find a transmitted PL. Eventually, they came out with scanners that would instantly display a received PL tone and the need for other methods was reduced. These days even cheap scanners like the BCT15X or BC125AT can instantly display PL and DPL codes.
We had a few devices to decode PL’s. Some of use used frequency counters connected to scanners that would display the decoded PL code. While it worked, it didn’t work well. The freqs displayed were often inaccurate, especially when there was voice traffic. To get a clean decode you had to have a dead key for a second or two.
There were some commercial products that used this method, they were equally ineffective, but it was what we had… Soon enough someone discovered the Zetron Model 8. This was actually intended as a repeater programmer and timekeeper, but it could decode PL codes as well as DPL and other type of codes instantly and display them on a huge red LED display. You could view this from across the room with no problems. The Zetron was highly effective, but it was too big to be really practical in mobile situations and impossible with a portable.
Optoelectronics then introduced the DC440, this was a much smaller decoder that instantly displayed the received PL and DPL codes. It worked well but had a small dot-matrix LCD display that was hard to read unless you were right on top of it.
Soon thereafter we found the ideal companion. This was the CSI CD-1. This little gem connected to the scanner discriminator and instantly displayed the PL or DPL tone on a nice sized LED display that was easily read from within a car or on the desk. It was cheaper than the DC440 and, in my opinion, it was the ideal accessory for the PL hunter in me. I liked it so much that I had 4 of them, one at work, two at home (on an R7000 and a scanner) and another in the car. I still have one in the shop and every once in a while, I pull it out and use it, it still works great after 30 years.
Nowadays most scanners decode PL and DPL codes instantly and display it right next to the frequency. You can program scanners to receive only those stations with the proper code or even to ignore a specific code if they are particularly annoying. With software like ARC-xx or ProScan you can log decoded codes along with hit counts.
Now the question many people have is why? Why would I spend that kind of money, time and effort to decode and catalog PL and DPL codes? Well, basically it was fun. I enjoy collecting data like that. PL and DPL codes are kind of a fingerprint, when you see a specific code on a frequency you can often figure out the used if it has been logged as in use by some user. In my own files, the RadioReference database and other sources, these codes can help identify an unknown user quickly.
In skip season for low-band or during temperature inversions on higher frequencies PL and DPL codes are invaluable to identify the distant users. I recall one of the biggest inversion events, during the late 80’s or early 90’s when I was picking up VHF High Band stations from hundreds of miles away and most were easily identified by the PL or DPL code they were using.
The weird thing though was that I was not the most ardent searcher of PL and DPL codes. Our “Tactical Cabbie” friend Brandt made me look like a rank amateur when it came to finding PL codes. We had about a dozen of us in RCMA-Chicago (later known as CARMA) that actively searched out PL and DPL codes. I even have a friend that can tell by ear what PL tone is in use with amazing accuracy. If he were a musical guy, I would suspect him to have “perfect pitch”.
Even now, I almost always have my scanners set to “Search” mode for PL and DPL codes, even when I am listening to services not known for heavy use of these codes like railroads and marine traffic. You never know when you might find a new code. Even on channels programmed that I know the codes used I set it to search in case they change the tone or perhaps I am hearing out-of-town traffic on a warm summer night.
When I became “the radio guy” at work one of the first things I did was to add PL to our police car-to-car and fire channels. This was 15 years after the agency I had been a dispatcher for, and a different community. We had to coordinate it with several other towns we shared the channels with, but they were happy to do it as the channels were getting more crowded every day.
One thing that was common back in the 80’s and 90’s, especially in the Midwest, was each county having a standard PL tone in use. The common method involved the Sheriff, city police and most of the FD’s all having the same PL code. This was due to many radios of the day only having the ability to transmit or receive a single PL code. This then allowed a sheriff’s car to use the city police channel or fire channel etc. For some reason most of the highway departments had differing PL codes, I guess they didn’t participate in communications with police and fire. Eventually these older radios were replaced by programmable radios that could use any code on any channel, even different codes for receive and transmit. While we take that for granted today, it just wasn’t practical then or required specialized equipment.
So we all know what PL Tones are, right? Well, if you don’t, a PL tone is basically a subaudible tone superimposed on a radio signal. This tone, typically between 67 and 250 Hz., is supposed to be below the frequencies detectable by the normal human ear but easily detected by radios. They are used for several purposes in radio communications, such as blocking access to repeaters unless you are transmitting the proper tone, allowing a radio to ignore traffic not transmitting that proper tone on shared channels and even to count access time for systems that charge by the minute or key-up.
PL tones have been around for over 70 years, the concept was invented by Motorola in the 1950’s. a couple decades later Motorola came up with “Digital Coded Squelch”, more commonly notated as “DPL”. DPL serves pretty much the same function as PL tones but goes about it a little differently. It also adds to the number of codes available.
There are several names for PL and DPL codes but that is beyond the scope of today’s Tale. For our purposes we will just use the PL and DPL nomenclature. The Repeater Builder website has a great article on the origination on PL Tones at A historical and technical overview of tone squelch (PL, CG, QC, CTCSS, etc.) systems. if you are interested in the history of the codes and why they have such odd names like “1A”, “7Z” etc. and why the tones are in weird increments.
So I tell you all that to talk about this: How I got obsessed with PL tones and how I would go about searching for them. It all started when I was just a young lad of 18 working as a police and fire dispatcher in a mid-sized suburb of Chicago. We had a UHF police repeater and a simplex VHF fire channel as well as a couple other simplex channels for Public Works and police car to car. At first only the police repeater and PW channels had PL capabilities, but I didn’t know that yet. I had no idea what PL codes were, all I knew was we only heard our town (and the town we shared the channel with) on the police repeater at work but on the scanner, I could hear other towns occasionally.
A friend, Scott, started to explain PL tones to me a year or so later after we met but I really didn’t understand the concept for a while. Eventually our radio guy (it wasn’t me then) added PL tones to the fire channel as well as the police car-car channel so we didn’t have to listen to outside agencies on these channels. While it was fun for a scanner dude like me when things were slow to listen to agencies a hundred miles or more away at night it got annoying when work got in the way of my fanboying.
Eventually I started understanding the concept of PL tones from the lessons Scott gave me as well as from other RCMA members who took me into their confidence and shared the forbidden “PL List”. Soon I was pretty well hooked on PL tones and trying to find them. At the time (early 1980’s) RCMA did not allow discussions of PL tones as there was no scanner need for them. Scanners did not have the ability to display or utilize them so the only reason one would need a PL tone was to interfere with the legitimate traffic was their reasoning. We had a subset of club members who enjoyed collecting PL tone information, most notably our infamous friend, the late Brandt. (See “The Tactical Cabbie” at Scanner Characters: The Tactical Cabbie)
At the time, well before the first PL-capable scanner that I am aware of (the BC760XLT) came out we used many methods to find PL tones. We continued these methods after as they were usually easier than programming a scanner over and over to find a transmitted PL. Eventually, they came out with scanners that would instantly display a received PL tone and the need for other methods was reduced. These days even cheap scanners like the BCT15X or BC125AT can instantly display PL and DPL codes.
We had a few devices to decode PL’s. Some of use used frequency counters connected to scanners that would display the decoded PL code. While it worked, it didn’t work well. The freqs displayed were often inaccurate, especially when there was voice traffic. To get a clean decode you had to have a dead key for a second or two.
There were some commercial products that used this method, they were equally ineffective, but it was what we had… Soon enough someone discovered the Zetron Model 8. This was actually intended as a repeater programmer and timekeeper, but it could decode PL codes as well as DPL and other type of codes instantly and display them on a huge red LED display. You could view this from across the room with no problems. The Zetron was highly effective, but it was too big to be really practical in mobile situations and impossible with a portable.
Optoelectronics then introduced the DC440, this was a much smaller decoder that instantly displayed the received PL and DPL codes. It worked well but had a small dot-matrix LCD display that was hard to read unless you were right on top of it.
Soon thereafter we found the ideal companion. This was the CSI CD-1. This little gem connected to the scanner discriminator and instantly displayed the PL or DPL tone on a nice sized LED display that was easily read from within a car or on the desk. It was cheaper than the DC440 and, in my opinion, it was the ideal accessory for the PL hunter in me. I liked it so much that I had 4 of them, one at work, two at home (on an R7000 and a scanner) and another in the car. I still have one in the shop and every once in a while, I pull it out and use it, it still works great after 30 years.
Nowadays most scanners decode PL and DPL codes instantly and display it right next to the frequency. You can program scanners to receive only those stations with the proper code or even to ignore a specific code if they are particularly annoying. With software like ARC-xx or ProScan you can log decoded codes along with hit counts.
Now the question many people have is why? Why would I spend that kind of money, time and effort to decode and catalog PL and DPL codes? Well, basically it was fun. I enjoy collecting data like that. PL and DPL codes are kind of a fingerprint, when you see a specific code on a frequency you can often figure out the used if it has been logged as in use by some user. In my own files, the RadioReference database and other sources, these codes can help identify an unknown user quickly.
In skip season for low-band or during temperature inversions on higher frequencies PL and DPL codes are invaluable to identify the distant users. I recall one of the biggest inversion events, during the late 80’s or early 90’s when I was picking up VHF High Band stations from hundreds of miles away and most were easily identified by the PL or DPL code they were using.
The weird thing though was that I was not the most ardent searcher of PL and DPL codes. Our “Tactical Cabbie” friend Brandt made me look like a rank amateur when it came to finding PL codes. We had about a dozen of us in RCMA-Chicago (later known as CARMA) that actively searched out PL and DPL codes. I even have a friend that can tell by ear what PL tone is in use with amazing accuracy. If he were a musical guy, I would suspect him to have “perfect pitch”.
Even now, I almost always have my scanners set to “Search” mode for PL and DPL codes, even when I am listening to services not known for heavy use of these codes like railroads and marine traffic. You never know when you might find a new code. Even on channels programmed that I know the codes used I set it to search in case they change the tone or perhaps I am hearing out-of-town traffic on a warm summer night.
When I became “the radio guy” at work one of the first things I did was to add PL to our police car-to-car and fire channels. This was 15 years after the agency I had been a dispatcher for, and a different community. We had to coordinate it with several other towns we shared the channels with, but they were happy to do it as the channels were getting more crowded every day.
One thing that was common back in the 80’s and 90’s, especially in the Midwest, was each county having a standard PL tone in use. The common method involved the Sheriff, city police and most of the FD’s all having the same PL code. This was due to many radios of the day only having the ability to transmit or receive a single PL code. This then allowed a sheriff’s car to use the city police channel or fire channel etc. For some reason most of the highway departments had differing PL codes, I guess they didn’t participate in communications with police and fire. Eventually these older radios were replaced by programmable radios that could use any code on any channel, even different codes for receive and transmit. While we take that for granted today, it just wasn’t practical then or required specialized equipment.
