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Railfan Technology > Status of narrowbanding?

Date: 05/19/21 10:14
Status of narrowbanding?
Author: MattW

Broadly speaking, what is the current status of narrowband communications? I'm talking with a friend from a railfan group out here on CSX near Atlanta and he's trying to figure out why he can't pick up audio from the trains nearly as well anymore. He hooked up an SDR to an NS road frequency and showed me the waterfall display and the defect detector looked to be transmitting only 6.25KHz wide. He also said that on his setup, detector and train audio were comparable. I've also noticed that train transmissions are a lot quieter than they used to be, almost like using a wideband receiver to pickup the original 12.5KHz narrowband transmissions. I won't be able to hook up my own SDR and investigate until this weekend or next week, but I was wondering does anyone know if the railroads have gone ahead to 6.25KHz bandwidth?

Date: 05/19/21 10:24
Re: Status of narrowbanding?
Author: 41bridge

Please with the acronyms. For the non tech, what’s an SDR?

Date: 05/19/21 10:41
Re: Status of narrowbanding?
Author: MattW

Sorry, SDR is software defined radio, sometimes also called "dongles" since the inexpensive ones are about the size of a big USB dongle or old thumb drive. Most of the signal processing for them is handled by the computer they're plugged into so you can do some interesting things with them. In this case, my friend and are are using them as a poor man's signal analyzer.

Date: 05/19/21 12:14
Re: Status of narrowbanding?
Author: qed479

Thanks. Please keep us posted. I vaguely remember seeing something about new channels coming along with PTC, but that could just be faulty memory on my part.

Date: 05/20/21 14:05
Re: Status of narrowbanding?
Author: WW

Here is the current status, from what I know.  Most railroads are still using the "original" 97 AAR frequencies, but have been using those narrow-band since 2013, when narrow-banding became federally required.  The AAR channel nos. are the same, except with a "0" prefix-- in other words, wide-band channel 23 was replaced with channel 023, for example. Narrow-banding allowed additional channels to be created between the existing 97 channels (these new channels are the "splinter" analog channels.  Railroads can use these, if authorized, but relatively few currently are, as far as I know.  NXDN ,which is very narrow-band digital is, as of now, only used in a few places, but the Class 1 railroads pretty much have the infrastructure and radios in place to "cut over" to it at any point that they decide to.  The channel spacing for wide-band was 25 kHz, narrow-band is 12.5 kHz, and very narrow band (NXDN) is 6.25 kHz.  As I've noted many times previously, very few amateur radios will tune the splinter AAR narrow-band channels, and only a very few (very expensive) scanners will tune and decode NXDN signals.

PTC is a totally different animal, and it runs in the 220 mHz band, not in the 160-162 mHz range of current railroad voice communications.  

Date: 05/27/21 07:07
Re: Status of narrowbanding?
Author: ironmtn

Thanks to MattW for starting this thread and further posting to it about SDR (Software Defined Radio), and to WW for additional comments on narrow-banding and NXDN. As it happens, this is all very timely for me, and your posts also raised and answered some questions I had -- and covered a few issues that I hadn't previously thought about.

I'll ask the follow-on question first, then give some background for them. Sorry for getting into the weeds for the non-technical folks, so in asking questions I'll try to give some context to give you some idea (maybe with some additional reading on your own) of what is being asked. And for the radio and tech folks, apologies in advance if I address any of this incorrectly or improperly. I'm a longtime IT guy, but radio in general is a bit of a reach for me, and I am brand-new to Software Defined Radio (SDR).


1) A Software Defined Radio (SDR) setup, as I understand it and plan to use it, consists of four principal components::
  • a) The SDR dongle, which I understand to have receiver and processor functions. The dongle, a small metal or plastic case with internal electronics (a bit larger than some typical USB memory devices) typically attaches to a Microsoft Windows computer via a standard USB port on the computer. An example (there are others): https://www.nooelec.com/store/nesdr-smartee-sdr.html
  • b) An appropriate antenna for the frequencies to be monitored, with the antenna connected to the SDR dongle to feed it the radio signals to be processed / monitored by the SDR dongle. The antenna is essentially functioning for the SDR dongle in the same way as an antenna does for our familiar scanner radio receivers -- capturing the radio signals, and moving them to the SDR dongle via a cable. An example: https://www.nooelec.com/store/ratlsnake-m6.html
  • c) Appropriate SDR software, to tune the desired frequency, do some processing of that signal, and feed it on a Windows-compatible laptop computer or tablet for audio play on the computer's speakers, or further processing by other software for other functions. The SDR dongle by itself does not have a user-visible tuner, like the keypad on our current scanners, or a tuning dial on an AM-FM radio receiver. So, appropriate software must be used to tune-in the desired frequency captured by the antenna and routed through the SDR dongle to the computer for audio listening, or for other use (for example, by ATCS Monitor software -- more on that below).
  • d) A compatible laptop or desktop computer with standard USB port connectors for the SDR dongle. The antenna feed is directly to the dongle, not to the computer. My computer is a Windows 10 laptop, and compatibility of all components (listed below) was fine. I am not a Mac user, and paid no attention to Mac compatibility, so you would have to check on that yourself if you are a Apple Mac user.
With all of that said: Is such a setup capable of reasonably good use for NXDN railroad radio monitoring / listening by railfans? -- and in a more or less comparable way to the way we currently use a scanner or other radio receiver for current non-NXDN railroad radio receivers? The discussion above suggest that the answer to this question is, broadly, "yes" -- but perhaps with some limitations.
  • This question recognizes that such a setup is not as simple or convenient as a single scanner or other radio receiver and antenna, such as we use them now for non-NXDN railroad radio monitoring / listening. It involves a much less convenient setup of multiple devices, connections and software -- that's understood and accepted. I put that aside in asking this question. I'd prefer the convenience of a single radio receiver device, or scanner and antenna. But until the price points for NXDN-capable scanners or radios drops substantially, I do not see a purchase of such gear on my personal horizon. So, I'm asking about a possible alternative, albeit one that is somewhat less convenient (and also perhaps also less capable, but hopefully still reasonably capable).

2) Are the NXDN railroad radio transmissions utilized on a single narrowband frequency for both transmit and receive by the railroad, or are multiple frequencies used for transmit and receive by the railroad (thus requiring railfans to monitor multiple frequencies for a given railroad and territory to hear both sides of a conversation)?

3) If SDR can be used for listening to NXDN railroad radio transmissions, would multiple SDR dongles (one per frequency) be needed to monitor multiple railroad frequencies (for example, for separate NXDN transmit - receive frequencies for a given railroad if used, or for two different NXDN frequencies for two different railroads)?

4) Is there SDR software for use with SDR dongle(s) and antenna(s) which is capable of "scanning" multiple NXDN frequencies in a similar way to our current non-NXDN scanner radio receivers? I've searched for such software and haven't found it, or I missed that "scanning" capability in the descriptions (which for that class of software applications can get pretty technical).

5) If SDR can be used to listen to NXDN railroad radio transmissions, is there any technical reason why this setup could not be used to simultaneously also monitor signals for ATCS Monitor software? (Such software is used to view track occupancies by trains, and routing setups for trains on a dispatcher panel-like computer display).
  • If yes, could this be done using a single SDR antenna, single SDR dongle, and single instance of SDR software?
  • Or would it require multiple devices and software instances? For example, one SDR antenna - dongle - software instance setup to monitor / listen to NXDN railroad radio transmissions, and another separate SDR antenna - dongle - software instance to acquire ATCS Monitor datafeeds? My guess is that this second configuration is more likely.
6) Are there preferred SDR dongles, antennas, and software applications for monitoring railroad NXDN radio transmissions (assuming that is possible)?


I recently acquired an SDR dongle (a Nooelec NESDR SMArtee -- https://www.nooelec.com/store/nesdr-smartee-sdr.html) and related antennas (Nooelec RaTLsnake M6, with three different screw-on antenna elements for a magnet-mount base -- https://www.nooelec.com/store/ratlsnake-m6.html).

The SDR software I am currently using is Cubic SDR (https://cubicsdr.com), which was recommended for this SDR dongle and antenna combination. The dongle and antennas were purchased from Amazon at very reasonable cost, and the Cubic SDR software was a free download. I'm not particularly thrilled with the design and end-user convenience of the Cubic SDR software, and am still getting used to using it. For now, I'm monitoring / listening to other types of radio transmissions such as air traffic control (ATC) and even commercially broadcast FM music and news, just for learning and practice with the Cubic SDR software, and the SDR dongle and antennas. There are no railroads in my area currently using NXDN radio, so I cannot test such usage without a trip to another part of my home state of Michigan.

Listening to railroad NXDN radio audio was not the purpose for acquiring this setup. In fact, before this thread I had never even thought about the possible capability for NXDN railroad radio audio reception. But this thread "turned the lightbulb on" to this possible use and led to my questions.

My original plan for the SDR setup was to use it with my Windows 10 convertible laptop (a Lenovo Yoga) that has the ability to fold the screen over to utilize in tablet mode, a configuration that is easier for field use than in usual laptop "clamshell" configuration . The laptop has two USB ports capable of interfacing with SDR dongles, and my new Nooelec NESDR SMArtee dongle fits them both, and works just fine. The initial primary intended use for the SDR dongle - antenna - software setup was to acquire radio ATCS signalling datastreams for use with ATCS Monitor software to view train movements and lineups with ATCS Monitor software.

Like many of us who use ATCS Monitor (quite a few of us, I think), I can monitor most track territories I'm interested in via an internet connection data feed. But there are some territories of interest to me that do not have feeds on the internet, and therefore require that you capture the non-audio radio transmissions with their ATCS data at a lineside location using a specially-modified scanner radio receiver (modified to have a "phase discriminator tap"), or other capable radio receiver. Older second-hand Motorola Maxtrac units retired from public safety agencies, and sometimes available on eBay, also seem to be favored for such use -- they have the "phase discriminator tap" built-in. Or possibly, as I hope, an SDR setup like the one described above in lieu of the specially modified scanner, or the Motorola Maxtrac unit.

When I'm lineside, I plan to use the SDR setup to acquire the ATCS radio signals in order to feed that datastream to the laptop and ATCS Monitor software in order to view those track territories and trains. I haven't yet been able to visit those non-internet-feed territories to try out my new SDR setup, so I can't yet report actual results. But based on online reading and study, I believe it will work okay, and give me visibility to train movements and track lineups in those territories.

But, any use of the SDR dongle - antenna - software setup ALSO for NXDN railroad radio audio (either by itself, or in some combination with ATCS Monitor use), would therefore be an unexpected bonus. I might have thought of doing so at some point, but this thread certainly "turned the lightbulb on" to possibly doing so. My thanks again for starting and adding to the thread, and thanks in advance for any response to my further questions. I also hope this these questions and description may prove useful to others here on TO with regard to the use of Software Defined Radio (SDR) for railfanning.

Lots of revisions in trying to state this all usefully and somewhat clearly (I hope) -- sorry. But I'm done now....I think. Anyway...Onward!


Edited 18 time(s). Last edit at 05/27/21 10:18 by ironmtn.

Date: 05/28/21 11:42
Re: Status of narrowbanding?
Author: WW

I will try to answer the "2)" bullet above.  With one significant exception, most railroad radio transmission are "simplex"--that is, one frequency (AAR Channel) used for both transmission and receiving.  In AAR lexicon, a railroad channel is referred to by its transmission/receiving channel no., so, for example, a railroad simplex frequency of 161.5650 mHz would be referred to as Channel 097 097.  The use of simplex channels is likely to continue with NXDN.  The significant exception is when railroad communication uses semi-duplex channels (different transmit/receive frequencies) to utilize repeaters (usually in mountainous areas).  Let's say a railroad is using 160.3050 mHz transmit and 161.5650 mHz receive  to use a repeater, the AAR channel designation would be Channel 013 097. By the way, "full duplex" is like your telephone, where both the transmit and receive frequencies can be used simultaneously by the phone or radio.

Where NXDN may change some of this is its ability to use "voting."  Voting is pretty much what your cell phone does.  As you move through the environment, the cellular system and radio (your cell phone is nothing more than full-duplexing digital radio with voting capability) can jump from tower to tower as the signal strength changes  It may do that on a single channel or jump to other channels.  Will railroad voice radio migrate to a "cellular" type system?  Maybe, but I think that is a long way off.  What IS happening is that more and more railroad communication is being done by non-voice communication.  I've been a multiple-week business trip across much of the Plains States and Midwest.  Compared with even just a few years ago, railroad voice radio communications are but a fraction of what they were. Railfans should recognize that, no matter how sophisticated their radio voice monitoring equipment may be, there is just going to be an ever-shrinking volume of voice radio communications on the railroads.  More and more, data transmission is not becoming the norm.      

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