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While there are many digital signals that are easy to identify, many are not. There are just as many modes that most people have never heard of. This [http://www.udxf.nl/list%20of%20digital%20systems.pdf article (PDF)] is a list of those modes that have been identified, but there are likely more that have never been identified or classified. It's quite possible that these modes are military in nature. These modes could be encrypted and very difficult, if not outright impossible, to read.
 
While there are many digital signals that are easy to identify, many are not. There are just as many modes that most people have never heard of. This [http://www.udxf.nl/list%20of%20digital%20systems.pdf article (PDF)] is a list of those modes that have been identified, but there are likely more that have never been identified or classified. It's quite possible that these modes are military in nature. These modes could be encrypted and very difficult, if not outright impossible, to read.
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==So how can we identify the users of these modes?==
 
==So how can we identify the users of these modes?==

Revision as of 21:39, 26 October 2014

What are all those strange sounds?

There are many digital modes that you will find outside the amateur service, that would defeat most ham-level decoders ability to decode. Let's mention just a few of them


  • ALE. This mode is often used to test the propagation path between 2 stations, and to sometimes pass short messages. The link provides more information on this mode as well as compatible software. Note that this one of the newer modes that hams, as well as non-hams, utilize.


  • Digital Selective Calling (DSC). This website has a great deal of information, as well as links to software that can read this mode.


  • FAX (Facsimile) - Most often used for weather charts, and on rare occasions, news items. Happily this is one mode that is easy to both recognize and decode.


  • HF Datalink (HFDL), also known as HF ACARS. See the linked article for more information.


  • SITOR-B - Also known as AMTOR Mode B, FEC or NAVTEX, this mode is often used for maritime weather and warnings from various organizations. This website also has a great deal of information on this mode. While not used by the amateur community much anymore (apart from W1AW broadcasts), it still enjoys wide popularity in the maritime community.


  • STANAG 4285 - Most often used by the French and NATO forces. See this article (PDF) from the Utility DXers Forum homepage on identifying and decoding this mode. Often the traffic is encrypted, but the IDs are not, making it easy to at least identify the users.


While there are many digital signals that are easy to identify, many are not. There are just as many modes that most people have never heard of. This article (PDF) is a list of those modes that have been identified, but there are likely more that have never been identified or classified. It's quite possible that these modes are military in nature. These modes could be encrypted and very difficult, if not outright impossible, to read.


So how can we identify the users of these modes?

In some cases, the frequencies used by these modes are well known and widely published (such as HFDL). This is, of course, but one clue in identifying a user. We need another tool to help narrow the field.

Some older modems and TNCs (for example the Universal M-7000) had the ability to attempt to guess what mode a particular signal was sending (some very expensive soundcard decoders also have this ability). Unfortunatly these classifiers can be fooled by a poor signal level or noise - so it's not always the best choice for determining the mode of a signal

Many digital decoding packages have the ability to display an audio representation of the signal being received. This ability comes by many names; sonogram, waterfall display or audio spectrum display, just to name a few. This article (PDF) describes in some detail how to use a sonogram and how to record the signal. Both are important - you may need to play the signal back several times before you get a clear picture of what the signal looks like. The accuracy of the sonogram is going to be greatly influences by many factors; the quality of the signal being received, your receiving setup and the PC you are using will all play a role here.

It should be noted that new modes show up from time to time that take considerable knowledge and skill to properly analyze.

Now what about identifying the patterns shown in the sonogram?

These links have pictures of sonograms (and in some cases audio samples) of numerous kinds of digital signals. You can use these as a guide, but remember, it's another clue as to the mode (and by extension, sometimes the potential user) of the signal


What about decoding?

There are numerous packages that can decode many of the non-encrypted modes that you will find on HF. The Utility Monitoring Central website lists packages for PCs, Macs and even a few for IPADs. Some are free, some not. Msst require a simple connection between your PC and your receiver.

In the case of connecting a receiver to the PC soundcard, a clean connection is a must. This article from our wiki discusses some of the pitfall and solutions. It's strongly encouraged to make the connection with shielded audio cable to restrict the possibility of stray noise pickup. However, you will need to experiment with the soundcard settings on the PC to see which works for you. There's no way to present a guide for each and every instance, as there is wide variation in models and quality, depending on the soundcard installed on the PC.

Connecting a PC soundcard to your Software Defined Radio (SDR) may require the use of some Dynamic Link Libraries (DLLs) that must be installed. With others you can use software like Virtual Audio Cable (VAC) to route the audio from the SDR to the PC. Those packages that can directly interface with SDRs are shown in green on the Utility Monitoring Central website.


Are there other resources?

Print magazines are not always the best place to go for this kind of information. Their long lead times (sometimes as much as 30-45 days) means that the data gets stale very quickly.

The Utility DXers Forum (UDXF) Yahoo group is an excellent place to help identify the mode and user of a particular signal. There are also specialized Yahoo groups (such as the one for HFDL) that can supply lists of known frequencies and transmit sites for this mode. There are also ircchat sites where you can ask questions, and get nearly immediate responses. These are often your best sources of information.