While the Discriminator Tap mod has been a staple for many scanner users over the last couple of decades or so the need for it has mostly passed these days. The stuff you did with it can usually be performed either directly on the radio or via a USB port. Before this however, it was a handy mod used for all kinds of nasty things, legal and otherwise. Things like decoding PL and DPL codes, trunking data, paging and data modes were all possible with a discriminator tap and perhaps a dataslicer.
When a new scanner came out it was often a race to see who could find the discriminator tap point first. I was able to get a hold of a couple scanners very early and was able to find these tap points, and provide the info to readers of Radio Reference, CARMA and other forums.
Some of you young-ins out there are probably asking what the heck is a discriminator or a dataslicer and why one would want to bother with it. Well, stick around kid and I will learn ya a little knowledge from us old folk. The discriminator basically takes unfiltered audio (“Base band”) from the radio and ports it out so it can be used to feed dataslicers or decoders. The scanner’s audio amp will filter out portions of the audio spectrum, making it unusable for some decoding functions. Having that unfiltered audio allows one to feed it into a computer or stand-alone decoder for proper decoding.
Scanners do not normally present this unfiltered audio so if you want to utilize it you need to add a jack to the radio and connect that jack to the proper point on the scanner. Uniden and GRE/RS/Whistler designed scanners usually provide an internal test point that makes it easy for us to find. GRE designed scanners usually have a “TP4” labeled on the circuit board, some Uniden scanners have a point labeled as “DISC”. Not all scanners are so easy to find but if needed, it is usually pretty easy to find. Chances are someone already has found it for your model scanner, and it is listed in the Radio Reference Wiki or some other web search found by using Mr. Google.
I found that if a point in the radio, when connected to a PL decoder, was able to decode PL codes lower than 90 Hz. or any DPL code it was useful for almost any data or tone decoding. While some higher frequency PL codes and DTMF could be decoded off the Record Jack or even the external speaker jack, these will not normally work for DPL, low frequency PL or data.
A Dataslicer is an interface device used to convert raw discriminator audio from a receiver to a format that is readable by a computer serial port. Basically, it is a level converter. There is more to it than that of course, but chances are these days it really isn’t needed.
One can build a simple 2-level dataslicer with just a few components, including an integrated circuit, a couple resistors and capacitors and a couple diodes. It can be built small enough to fit inside a DB-9 serial port connector. There is also a 4-level dataslicer for more complicated projects that involves more components to build.
There used to be a couple of cottage industry type places that built and sold dataslicers, but as far as I can tell they are long gone now. Read on about Bill Cheek, he was probably the best known source.
OK, so you tapped the scanner and built a dataslicer, now what? Well, there were several programs, usually run in DOS, that would decode various forms of data one would hear on a scanner. “Trunker” and its several derivatives, would decode and display data from trunking control data signals. Back when Motorola 3600 baud control data formats ruled the airwaves for businesses and public safety users, these systems were ubiquitous around North America. Tune in a trunked system’s control channel and Trunker would provide the System ID, Connect Tone and frequency information, along with things like Talkgroups and radio ID’s.
Dataslicers and discriminators also were used to decode other data signals like paging and mobile data terminals (MDT’s). Back then these signals used common and unencrypted data modes and were easily decoded with a tapped scanner, a dataslicer and free software available on the Internet.
Some things worked with discriminator audio but did not require a dataslicer. Most common were PL/DPL/DTMF decoders. OptoElectronics and CSI made standalone decoders that would connect directly to the discriminator and read out these codes as they were transmitted. These were great for us back in the day since scanners did not have PL decoders on them at the time. This allowed us to “fingerprint” users. If you saw 155.250 and a PL of 192.8 it was Mayberry PD, a PL of 127.3 identified it as Mt. Pilot. Once you figured out who the users were a quick glance at the PL, and you could easily see if it was one or the other.
The late, great Bill Cheek published how-to guides on modifying your scanner and building the dataslicers as well as other interfaces. He and his wife were even arrested for this in the late 1990’s. It seems someone using such a device posted decoded MDT traffic on the internet. While it is unknown whether the equipment used came from the Cheeks, they were arrested as they were the most prominent sellers of books, equipment and software that could be used for this. While all this stuff had legitimate (legal) uses as well, it could also be used to intercept police MDT’s, which were not encrypted at the time. While Bill died of cancer before the case could be tried the whole thing sent a chill up the spine of many scanner hobbyists.
I made use of many scanners with discriminator taps over the years; during the 1990-2010 time period I had almost all my desktop scanners tapped as well as a couple handhelds. For the most part this was for PL decoders, I had 4 of the CSI CD-1 and CD-2 decoders (Still have one of them!) as well as a couple others. For years I also had a Zetron Model 8 repeater programmer. While intended to program repeaters (duh!) it also was great at decoding transmitted PL and DPL codes as well as DTMF (Touch Tones). With the huge display one could see it from across the room. I had one at home in the shack and even had one set up in the car before it was replaced by the much smaller CSI CD-1 decoder.
I also used dataslicers for Trunker as well as eTrunker (for EDACS systems) and used them extensively collecting data on trunked systems around the Midwest. I was the source for much of the data in the old TrunkedRadio.net database from the Midwest, this eventually became the Radio Reference Database we all know and love. Almost all of those systems are long gone, replaced by P25, DMR and NXDN (or cell phones). I had a blast running around sponging up all this data back in the day.
These days I still have a couple scanners that have discriminator taps on them, including a BCT15X and a BCD996XT. Occasionally I drag out my trusty CD-1 decoder and watch the PL tones as they pop up on local analog conventional channels. When skip is up this comes in really handy as the display on the scanner is kind of small and hard to read. I really wish my Icom R8600 had a way to tap that unfiltered audio, but since it is an SDR-based receiver it lacks that ability.
When a new scanner came out it was often a race to see who could find the discriminator tap point first. I was able to get a hold of a couple scanners very early and was able to find these tap points, and provide the info to readers of Radio Reference, CARMA and other forums.
Some of you young-ins out there are probably asking what the heck is a discriminator or a dataslicer and why one would want to bother with it. Well, stick around kid and I will learn ya a little knowledge from us old folk. The discriminator basically takes unfiltered audio (“Base band”) from the radio and ports it out so it can be used to feed dataslicers or decoders. The scanner’s audio amp will filter out portions of the audio spectrum, making it unusable for some decoding functions. Having that unfiltered audio allows one to feed it into a computer or stand-alone decoder for proper decoding.
Scanners do not normally present this unfiltered audio so if you want to utilize it you need to add a jack to the radio and connect that jack to the proper point on the scanner. Uniden and GRE/RS/Whistler designed scanners usually provide an internal test point that makes it easy for us to find. GRE designed scanners usually have a “TP4” labeled on the circuit board, some Uniden scanners have a point labeled as “DISC”. Not all scanners are so easy to find but if needed, it is usually pretty easy to find. Chances are someone already has found it for your model scanner, and it is listed in the Radio Reference Wiki or some other web search found by using Mr. Google.
I found that if a point in the radio, when connected to a PL decoder, was able to decode PL codes lower than 90 Hz. or any DPL code it was useful for almost any data or tone decoding. While some higher frequency PL codes and DTMF could be decoded off the Record Jack or even the external speaker jack, these will not normally work for DPL, low frequency PL or data.
A Dataslicer is an interface device used to convert raw discriminator audio from a receiver to a format that is readable by a computer serial port. Basically, it is a level converter. There is more to it than that of course, but chances are these days it really isn’t needed.
One can build a simple 2-level dataslicer with just a few components, including an integrated circuit, a couple resistors and capacitors and a couple diodes. It can be built small enough to fit inside a DB-9 serial port connector. There is also a 4-level dataslicer for more complicated projects that involves more components to build.
There used to be a couple of cottage industry type places that built and sold dataslicers, but as far as I can tell they are long gone now. Read on about Bill Cheek, he was probably the best known source.
OK, so you tapped the scanner and built a dataslicer, now what? Well, there were several programs, usually run in DOS, that would decode various forms of data one would hear on a scanner. “Trunker” and its several derivatives, would decode and display data from trunking control data signals. Back when Motorola 3600 baud control data formats ruled the airwaves for businesses and public safety users, these systems were ubiquitous around North America. Tune in a trunked system’s control channel and Trunker would provide the System ID, Connect Tone and frequency information, along with things like Talkgroups and radio ID’s.
Dataslicers and discriminators also were used to decode other data signals like paging and mobile data terminals (MDT’s). Back then these signals used common and unencrypted data modes and were easily decoded with a tapped scanner, a dataslicer and free software available on the Internet.
Some things worked with discriminator audio but did not require a dataslicer. Most common were PL/DPL/DTMF decoders. OptoElectronics and CSI made standalone decoders that would connect directly to the discriminator and read out these codes as they were transmitted. These were great for us back in the day since scanners did not have PL decoders on them at the time. This allowed us to “fingerprint” users. If you saw 155.250 and a PL of 192.8 it was Mayberry PD, a PL of 127.3 identified it as Mt. Pilot. Once you figured out who the users were a quick glance at the PL, and you could easily see if it was one or the other.
The late, great Bill Cheek published how-to guides on modifying your scanner and building the dataslicers as well as other interfaces. He and his wife were even arrested for this in the late 1990’s. It seems someone using such a device posted decoded MDT traffic on the internet. While it is unknown whether the equipment used came from the Cheeks, they were arrested as they were the most prominent sellers of books, equipment and software that could be used for this. While all this stuff had legitimate (legal) uses as well, it could also be used to intercept police MDT’s, which were not encrypted at the time. While Bill died of cancer before the case could be tried the whole thing sent a chill up the spine of many scanner hobbyists.
I made use of many scanners with discriminator taps over the years; during the 1990-2010 time period I had almost all my desktop scanners tapped as well as a couple handhelds. For the most part this was for PL decoders, I had 4 of the CSI CD-1 and CD-2 decoders (Still have one of them!) as well as a couple others. For years I also had a Zetron Model 8 repeater programmer. While intended to program repeaters (duh!) it also was great at decoding transmitted PL and DPL codes as well as DTMF (Touch Tones). With the huge display one could see it from across the room. I had one at home in the shack and even had one set up in the car before it was replaced by the much smaller CSI CD-1 decoder.
I also used dataslicers for Trunker as well as eTrunker (for EDACS systems) and used them extensively collecting data on trunked systems around the Midwest. I was the source for much of the data in the old TrunkedRadio.net database from the Midwest, this eventually became the Radio Reference Database we all know and love. Almost all of those systems are long gone, replaced by P25, DMR and NXDN (or cell phones). I had a blast running around sponging up all this data back in the day.
These days I still have a couple scanners that have discriminator taps on them, including a BCT15X and a BCD996XT. Occasionally I drag out my trusty CD-1 decoder and watch the PL tones as they pop up on local analog conventional channels. When skip is up this comes in really handy as the display on the scanner is kind of small and hard to read. I really wish my Icom R8600 had a way to tap that unfiltered audio, but since it is an SDR-based receiver it lacks that ability.
